Light controlled receptors for heavy metal ions

Light controlled receptors for heavy metal ions

Coordination Chemistry Reviews 357 (2018) 18–49 Contents lists available at ScienceDirect Coordination Chemistry Reviews journal homepage: www.elsev...

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Coordination Chemistry Reviews 357 (2018) 18–49

Contents lists available at ScienceDirect

Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr

Review

Light controlled receptors for heavy metal ions Priya Ranjan Sahoo, Kunal Prakash, Satish Kumar ⇑ Department of Chemistry, St. Stephen’s College, University Enclave, Delhi 110007, India

a r t i c l e

i n f o

Article history: Received 13 July 2017 Accepted 7 November 2017 Available online 5 December 2017 Keywords: Photoreversible Photochromic switches Heavy metal ions Sensors Optical sensors

a b s t r a c t This review focuses on the recent growth in the photo triggered molecular receptors for heavy metal ions. The photochromic unit in such molecular receptors performs as a trigger unit to allow control over a range of the properties through an external stimulus. This review opens up with the new opportunities for the development of host molecular motifs for heavy metal ion sensing applications. The photochromic switches based on a chemical structure like spiropyran, chromenes and spirooxazines tagged with appropriate chromogenic or fluorogenic unit reported till date have been summarized and categorized by the selectivity of metal ion to achieve the suitable optical response. The review has relevance for designing new photoreversible switches with the interesting optical response for environmentally important heavy metal ions. Ó 2017 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification of photoactive receptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advantages of photoactive receptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of photoactive receptors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Photoactive Zn2+ ion sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Photoactive Cu2+ ion sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Photoactive Co2+ ion sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Photoactive Hg2+ ion sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Photoactive Fe3+ ion sensors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6. Photoactive Pd2+ ion sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7. Photoactive lead (Pb2+) ion sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.8. Photoactive Ni2+ ion sensors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.9. Photoactive Gd3+ ion sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.10. Photoactive Eu3+ ion sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.11. Photoactive Tb3+ ion sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.12. Photoactive sensor for La3+ ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.13. Photoactive sensors for multiple metal ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.13.1. Photoactive sensor for Zn2+ and Cu2+ ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.13.2. Fe3+/Fe2+ ion sensitive photoactive sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.13.3. Photoactive sensor for Zn2+, Co2+ and Ni2+ ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.13.4. Photoactive sensor for Zn2+, Co2+, Cu2+ ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.13.5. Photoactive sensor for Co2+, Cu2+, Ni2+ ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.13.6. Photoactive Cu2+, Fe2+, and Al3+ ion sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.13.7. Photoactive sensor for Pb2+ and Ba2+, Mg2+ ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.13.8. Photoactive sensor for Zn2+, Ni2+, Fe3+, Cu2+, Ce3+ ions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.13.9. Photoactive sensor for Cu2+, Zn2+, Mn2+, Co2+ and Ni2+ ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.13.10. Photoactive sensor for Gd3+, Eu3+, Zn2+, Ni2+, Mn2+, Cd2+ ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. E-mail address: [email protected] (S. Kumar). https://doi.org/10.1016/j.ccr.2017.11.010 0010-8545/Ó 2017 Elsevier B.V. All rights reserved.

19 19 21 21 22 25 28 29 29 31 32 33 33 34 34 35 35 36 36 37 37 37 38 38 39 39 39

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5. 6.

4.13.11. Photoactive sensor for Zn2+, Fe2+, Co2+, Mn2+, Ni2+, Cu2+ ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.13.12. Photoactive sensor for Zn2+,Co2+, Hg2+,Ni2+,Cu2+ along with Pb2+ or Mg2+ ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.13.13. Photoactive sensor for multiple lanthanide ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Role of substituents over the photochrome in metal ion selectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Photoswitches, in general, are tools that function in accordance with light sources [1–4]. The mechanism that plays a bigger role, while working at the interface of switches and sensors, is normally based on photochromism [5–7]. It was Fritzsche, who first reported the phenomenon of photochromism in 1867 [8]. He observed the disappearance of an orange color solution of tetracene in broad daylight and appearance of color in the dark. Later, Hirshberg and Hebd in 1950 explored the synthesis of photochromic molecules and provided an insight into the mechanistic aspects of photochromism (The Greek term ‘‘phos” means ‘light’ and ‘‘chroma” means ‘color’) [9]. Since then, photochemistry has gone through major advancement, particularly during last six decades. The initial exploration by Hirshberg led to the development of many new classes of photochromic compounds with reversible response [10,11]. In most of the photochromic processes, rearrangement between two different species occurs with the induction of suitable light sources. Reversibility serves as the important role model for most of the photochromic phenomena. The comprehensive definition of photochromism deals mainly with the reaction processes, where progress in forward or backward directions can be triggered by an external agent called ‘‘light” or ‘‘metal ion” (Scheme 1). The observance of significantly different absorption or emission or color enables clear identification of different species [12]. The photochromic phenomena in systems such as spiropyrans, spirooxazines, oxazines, benzopyrans etc. generally deals with the photoinduced conversion of the closed form (colorless) into an open form (colored) or vice versa. The open form is generally referred to as the merocyanine form, while the closed form is also called as spiro form. The conversion of open form to closed form occurs thermally or sometimes photochemically [13,14]. The open merocyanine form further isomerizes into several different stereoisomeric forms through cis–trans isomerization of the double bonds [15]. However, in case of both photochromic and thermochromic systems such as naphthopyran, the photochemical reaction at low temperature yields a single product instead of several photoproducts. The decrease in temperature often transforms the dual response (thermo- and photo-reversible) of naphthopyran into only photoreversible nature [16]. In most instances, systems like spiropyrans, chromenes or spirooxazines proceeds through thermal induced reverse reaction [17,18]. Another type of photoswitchable frameworks developed by Favaro et al. on irradiation with suitable light source produced two different colored forms via rotation around CAC bond (cisoid and transoid). A photoswitchable system developed by Favaro et al. also displayed thermoreversible response [19]. It has been demonstrated that the rate of thermal bleaching increases with an increase in the temperature, which in turn decreases the light-induced coloration [16]. The systems like fulgides or diarylethenes proceed through the photochemical bleaching reaction in most cases [20–22]. The photochromism in the systems described here can be classified as positive or negative photochromism depending upon the coloration and decoloration response in different solvents [23].

40 40 43 46 47 47 47

One of the chief rationales behind photochromic (or photoactive) systems to display photochromism is the competency at which the physical and chemical properties can be reversibly controlled at the behest of an external agent. Light as an external input seems practical in this case since its action is convenient to monitor precisely, substantially flexible for specific action and can be switched with regard to our convenience [24,25]. In photochromic molecules like naphthopyrans, spiropyrans, spirooxazines, oxazine etc. the photocleavable Csp3-pyranAO bond plays a crucial role in determining the photochromic index [15]. The interconversion processes involve structural as well as electronic modifications. The absorption signature difference can be modulated at the molecular and supramolecular level. Similarly, the sensitivity of the systems can be enhanced by attaching fluorescent or chromogenic tags to the original molecule.

2. Classification of photoactive receptors The structural alteration in the photoactive receptors proceeds through a single step electrocyclic transformation during light irradiation (Fig. 1). Several examples of photoactive molecules exist in the literature till date [26]. Out of several photochromic species; naphthopyrans, spiropyrans, and spirooxazines display enhanced activation in response to metal chelation [27–30]. Another class of photoswitches has also been reported in the literature such as acylhydrazones developed by Hecht and co-workers [31]. Such small molecule switches exhibit excellent fatigue resistance with more than 100 nm band separation between E and Z isomers. Hecht and co-workers have also revamped the fatigue resistance of diarylethenes by introducing electron withdrawing groups such as trifluoro methyl substituents into the core system [32]. Bragg and Katz and co-workers have developed visible light triggered 1,2-dicyanoethene substituted blinkers, which can undergo reversible transformation between cis and trans geometries [33,34]. The excellent electronic nature of reversible dicyanoethenes bears the potential to act as an electron carrier in novel sunlight responsive semiconductor materials.

hv Colorless

dark

Mn+

Colored Mn+

hv’ Mn+ Colored

Scheme 1. A typical transformation in the photochromic systems induced by metal ion or light.

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Naphthopyran

UV Vis

O

O

Closed form

Open form

Spirooxazine O N

N

UV

N O

N

Vis

Open form

Closed form

Spiropyran -O

UV N O

NO2

N

Vis

NO2

Open form

Closed form

Azobenzene N N

UV

N N

Vis cis

trans Diarylethenes

F

F F

F

F

F

UV Vis

S

S

S Open

S

Closed

Acylhydrazones O O

N N

N

N

H

N

D

Dicyanoethene

S

S

Blue light or heat

CN S

cis

N N

Z

E

NC

N H

hn

white light (sunlight)

S

CN S

S

S Fig. 1. Various type of photoactive molecule.

CN trans

S

P.R. Sahoo et al. / Coordination Chemistry Reviews 357 (2018) 18–49

3. Advantages of photoactive receptors Unlike other sensors, photoactive receptors provide the viable structural design to induce metal ion binding and consequent release with the visible light [35]. Owing to their swift switching response under the influence of light, photoactive molecules operated directly in many practical fields with huge turnover worldwide, mainly memory storage, nonlinear optics, rewritable inkjet appliances, sensitive optical fibers etc. [36–46].

4. Applications of photoactive receptors Applications of photoactive molecules can be divided into two main categories: (a) On the basis of color or change in absorption spectra [43,47– 52]. It includes photochromic lenses, digital data storage, inks etc. (b) On the basis of physical or chemical transformation, i.e., change in dielectric constant, conductance, viscosity, etc. [53–58]. It includes optical semiconductors, reversible holographic systems. Albeit much discussed heavy metal ion detection using different types of receptors reported in the literature [59–65], there are nevertheless many advanced forms of photoreversible study that reflects the heavy metal ions sensing consciousness of the past [66,67]. This review is far from static, bringing freshness in highlighting elaborative binding mechanism and the fundamental principle behind tracking. It includes metal ions from diverse arena starting from transition metal ions like Fe3+ to lanthanide metal ions notably Eu3+, Gd3+ and Tb3+ etc. This review will address the photo-reversibility feature as an advanced tool for sensing heavy metal ions. Here, we are focused on receptor design based on well-known photochromic molecules such as spirooxazines, spiropyrans, and naphthopyrans. Nature’s design towards the elemental composition is something unique, although, the presence of heavy metal ions in small quantities are essential to many organisms, if they are present in excess, affect the environment and pose a serious threat to human health [68–72]. Universalized use of industrial components, excessively in universal life have created new sources of heavy metal pollution that pushed the scientific community to broaden the detection technique in a new horizon, keeping in mind the growing environmental concern [73]. The technique incorporating supramolecular chemistry provided a tool for more sensitive and selective sensors to achieve lower limits of detection with higher accuracy in a lesser time, minute scale trapping, utmost economy favored, larger coverage and easily operated photoreversible sensor [74]. Photoreversible sensors have increasingly found feasible to tackle the adverse atmospheric remedies that bear such features on a single platform with added flexibility. The highlights of photoreversible sensors involve alternation in structure between two different forms of a system, which results in different absorption peaks on the screen of a UV recorder in a reversible manner; reversible in a sense the later one (MC) can easily come back to the initial one (SP) within short timescale [75]. The pivotal function of the light-mediated sensor is the authentic selection of metal species. The so-called lacunae in theory and practice make it difficult to launch an all-rounder inbuilt encapsulator. By changing the appropriate functionality around the base structure, the target species may exhibit a new twist to the interesting phenomena of detection [66]. That’s why probably today’s scientists and technologists are gearing up for more advanced sensors. The mandate is to concep-

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tualize sensors surveillance capacity that can be enhanced by manipulating crown ethers [76], calix[n]arene [77], fullerene activated functionality etc. [78]. More often, light sensitivity makes such receptors conspicuous, while binding to the metal ions. Photochromic receptors notably spirooxazines, spiropyrans, chromenes, diarylethenes, fulgides, diazastilbenes, dihydroindolizines etc. are increasingly used now-a-days for metal ions capturing purposes [67]. One of the major advantages of applying these photochromic systems in sensing arena is that they exhibit beautiful color visible to naked eye either triggered by light or induced by selective metal ions. The coupling of fluorophores to the photochromic system has shown to produce remarkable emission intensity along with a distinct change in absorption spectra [79]. It is easy, convenient and practicable to utilize such techniques in real time monitoring applications on the basis of naked eye detection or luminescence behavior. The photoreversible encapsulators can form complexes with the heavy metal ions present in the atmosphere and in the deep sea or river bodies; following which the respective metal ions can be released through a channelized light controlled process and can be disposed thereof. Reversible light-responsive receptors are very convenient to use for practical applications due to no requirement of external additive for regeneration [1]. The exposure to the light of a suitable wavelength that corresponds to the absorption maxima of the complex determines the release of the encapsulated metal ion. Therefore, the time and the location of metal ion release by the receptors can be controlled using the light of suitable wavelength, taking a perfect chance of accuracy. Photochromic molecules can be modified with suitable functional groups to bind specific metal ions present in low concentrations. Although many receptors were designed in the past, still there are challenges surrounding reversible behavior. The approach of the metal ion towards host receptor exposes all possible trapping sites for coordination with the guest metal ion. When a light-induced source approaches the photochromic receptor, the vulnerable Csp3-pyranAO single bond begins to fragment and the highly conjugated molecule rotates on its own with different possible conformers accordingly [67]. Ultimately, the host–guest complex yields a response in the form of a colored substance through charge transfer [80]. The photochromic behavior of receptors like spirooxazines, spiropyrans etc. has shown to be largely dependent on Csp3-pyranAO bond length. A value of Csp3-pyranAO bond length more than (or equal to) 1.454 Å usually impart a good photochromic index in the molecule [15]. The reversible equilibrium is more prone to external light, polarized solvent, functional groups over the base structure. Specifically, the nitro group plays a key role in extraordinary quantum yield output in the transformation of spiro to merocyanine form and cooperates in stabilizing the later zwitterionic form [81]. The photostability of the corresponding merocyanine can be experienced by introducing suitable substituents with heteroatoms. However, the nitro group also enhanced the rate of decomposition of merocyanine form [82–84]. It must be pointed out that the stability of the merocyanine form with respect to the spiro (closed) form can be investigated by exposing the spiro form to the suitable wavelength of light followed by recording the decrease in the absorption of the merocyanine form with time using UV–Visible spectroscopy [15,85,86]. The thermal conversion of the merocyanine form to the spiro form that also implies the conversion of the open form into the closed form, generally follows first reaction kinetics Eq. (1) (Abst, Abseq and Abso are the absorbances of the merocyanine form at time t, equilibrium and time t = 0, respectively) [87]. The open merocyanine form with an oxygenated chromophore with suitable substituents provides an expected host for

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metal ion chelation. In some cases, the metal coordination yields a stable merocyanine form, while in other cases there is a reduction in the rate of thermal closure of the merocyanine form to the closed spiro form [87,88]. This reduced thermal rate of ring closure can be investigated at different concentration of metal ion using first-order reaction kinetics methods (Scheme 2 and Eq. (2)) to access the strength of the association between the metal ion and photoreversible receptor. The photo studies of Taylor et al. suggested that the complexation process increases many folds with UV light exposure [89]. Factors like atomic radius, oxidation state, and hard-soft mechanism gather wealthier information that commensurates with ligating sites. There are plenty of parameters notably pH, solvent medium, the ionic strength of aqueous solution that help in unraveling the mystery of photochelation. Once the signal is induced by the chromophore with the help of substituent, it transforms into a photochromic signal that can be received by the suitable metal ion. From the admired handiwork of Stela et al., it can be strongly cited that the suppression of thermal ring closure can be observed just by attaching a benzothiazole moiety to the 51-position of the naphthooxazine nucleus [90]. Owing to the substitution effect, the rotational movement is decreased grossly with cyclization. As a polar form of merocyanine is highly stable vis-à-vis non-polar spiro form, the effect is enlarged in polar solvents. The study shows a more pronounced effect of these particular substituents upon photochromic behavior. The fatigue resistance of chelated species is largely dependent on the typical nature of heteroatom and ring size [91]. The coordination of metal species with the photoreversible chelator leads to stable complex formation followed by color change as the open form, which usually forms the complex is planar and highly conjugated. The brightness can be increased subsequently through UV light exposure, which increases the rate of formation of merocyanine form (polar, colored open form). Due to the facts mentioned in this review related to photochromic systems, a number of receptors selective for different metal ions were reviewed, which are described in the following sections.

Fig. 2. Absorbance change of open merocyanine of 1 with varying zinc ions [111]. Reproduced with permission of the publisher [111]. Copyright 2016 American Chemical Society.

larger amount will land us in a situation with lungs disease, anemia and the like [93]. The Zn2+ ions also induce neuronal cell death, which ultimately causes epilepsy and brain injury [94]. The toxicity associated with zinc ions necessitates the detection and determination of trace amount of zinc ion to save both aquatic and non-aquatic life [95]. The development of selective and sensitive optical sensors for the detection of zinc ions, which are easily available and portable for field application, are extremely important. Different types of organic sensors for the detection of zinc ions were reported in the literature [96–110]. However, sensors based on photochromic molecules represent a unique family of receptors for zinc ions due to the potential control of light over the binding with an intense optical signal. Tian et al. have reported a spirooxazine receptor (1) for the detection of zinc ions. The association of zinc ion with the receptor unveiled an isomerized structure, which provided an overwhelming response to zinc ions (Fig. 2). The increase in the concentration of the zinc ions shifted the absorption maxima of the merocyanine form towards 500 nm. The receptor exhibited thermally reversible backward reaction (i.e., open merocyanine to closed spiro form) at a very low temperature 30.0 °C in acetone [111].

4.1. Photoactive Zn2+ ion sensors The zinc ion is considered as the second most abundant among the toxic heavy metal ions [92]. Ingestion of zinc ion even in small concentration badly affects the human immune system, loss of appetite etc. and intake of a

R

UV

N O

R

D

N

O

N

N

Abst- Abseq

= -kdct

ln

Eq = 1

4: R = H 5: R = NO2

Zn2+

Abso - Abseq kop

MC + Mn+

SP kspiro MC-M

N

O

Ke

kdec

ksobd =

R

N 2+

Zn

n+

N

O

2Cl-

N

kspiro Ke[Mn+]

+ kop + kdec

Eq = 2

Scheme 2. The typical graphical model to investigate the association between the photoreversible receptor and the metal ion.

R Scheme 3. Complex formation of receptors (4, 5), R = H, –NO2 with Zn2+ in aqueous solution.

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Fig. 3. (A) UV–visible spectrum of the receptor 4 in CH3CN with (a) No Zn2+ (b) 1.2  105 M Zn2+ (c–g) after exposure to visible light for different time intervals [114]. (B) Switching response of the receptor 4 at 564 nm with visible light on at 8 s (h) and off at 48 s (i). Reproduced from [114] with the permission of The Royal Society of Chemistry.

HOOC

N

HOOC N O

NO2

O

Vis

N

N

NO2

Zn2+

N N

N

Zn2+

N

6-Zn2+

6

Scheme 4. The Zn2+ ion induced ring opening of spiropyran receptor 6.

Fig. 5. The reversible response of spiropyran 7 and merocyanine–Zn2+ complex [117]. UV–Visible absorption spectra of spiropyran before (black line) and after addition of one equivalent Of Zn(ClO4)2 (red line) and after irradiation with visible light (green line). Reproduced from [117] with the permission from Elsevier.” Fig. 4. Fluorescence response of receptor 6 with increasing Zn2+ ion concentration [116]. Reprinted with permission from Ref. [116]. Copyright 2016 American Chemical Society.

NO2 MeO

F hn2

Zn2+ N O O N

N

O

NO2 Visible light

N

O

O

Zn2+

7

Scheme 5. The reversible interconversion of closed spiropyran receptor 7 and a hypothetical merocyanine–zinc complex.

MeO

Me

HO 8

F

Me

hn1, D O OH

Scheme 6. Photoinduced transformation of naphthopyran receptor 8.

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N N O R

N

2

N

S

N

N O 1

N O R = CH3 (CH2 )2 CO2CH 3

3

HOOC

Alhashimy et al. have successfully developed a photochromic 3,3-dimethyl-5-(2-benzothiazolyl)-spiro indoline-2, 31-[3H]naphthooxazine receptor 2 [112]. The most interesting fact is that by substituting carboxylic group to the indole nucleus, the merocyanine–Zn2+ complex life time was enhanced by almost 20 times in comparison to the methylated indole. The phenolate chromophore moiety of the receptor was able to bind with Zn2+ ions producing blue coloration in acetone with response in the visible region of the electromagnetic spectrum. From kinetic experiments in acetone, it was shown that the substituents (R = CH3(CH2)2CO2CH3 and CH3(CH2)2COOH) on heterocyclic indole nitrogen have a significant effect on the kinetics of the merocyanine–spirooxazine (MC–SO) thermal relaxation and interaction of Zn2+ ion to the receptor 2 enhanced its photostability by three times [112]. Kim et al. have reported the high photostability of spirooxazine receptor 3, which responded to different concentrations of Zn2+ ions due to the presence of extra coordinated site such as ACOOH group. The unique characteristic of discoloration kinetics rate via carboxylic acid substituted spirooxazine 3 has been observed at varying concentrations of Zn(NO3)2. The enhanced chelation ability has been attributed to the presence of an extra oxygen atom of ACOOH group in comparison to the unsubstituted spironaphthooxazine [113]. Collins et al. have observed a red color complex while dissolving Zn2+ ions into a solution of spiropyran receptor 4 (R = H) in dark (Scheme 3, Fig. 3) [114]. The process of thermal reversion, in general, competes with complex formation between the merocyanine form and metal ions. The complex formation of the receptor with zinc ion, in this case, reduced the conversion of the ring opened merocyanine form into colorless spiro form (closed form). However, when the solution was exposed to visible light, the metal ion was released, resulting in a colorless solution in acetonitrile due to the conversion of the merocyanine form complexed to zinc ion into the closed form. The distinct color change and the reversibility in aqueous condition can be witnessed through naked eyes. However, a very slow release of metal ion has been observed, when the authors replaced hydrogen with a nitro group (receptor 5). The nitro group through electron withdrawing ability facilitated the stabilization of phenolate ions resulting in slow response. The receptor 4 responded well enough in the presence of ZnCl2, which has been confirmed by the 14 times enhancement in emission intensity. The enhancement in fluorescence intensity has been attributed to the effective complexation. The sensitivity of receptor 4 towards Zn2+ ion touched the parts per billion mark (3.3 ppb) in 50% aqueous solution (ethanol–water) [115]. Heng et al. have reported Zn2+ ions sensing in dying cells using carboxyl substituted spiropyran receptor 6 [116]. The carboxyl group improved the solubility of the spiropyran receptor 6 in aqueous solution. The spiropyran–liposome conjugate was observed to have the enhanced chelating ability for Zn2+ ions (Scheme 4). The most interesting fact regarding spiropyran coated liposome was its activity in the aqueous medium (20% CH3CN in water). It can also detect trace quantities of Zn2+ ions present in biological

samples in nano-liter volumes with the help of a small optical fiber. An increase in the intensity of fluorescence has also been observed with an increase in Zn2+ ion concentration (Fig. 4) [116]. Natali et al. have also reported a Zn2+ ion sensor using a substituted spiropyran based receptor 7 (Scheme 5) [117]. The transformation of spiro form to merocyanine form induced by Zn2+ ions has been observed using 1H NMR. An absorption peak at 504 nm has been observed (Fig. 5) due to Zn2+ ion complexation and the absorption peak disappeared as soon as the solution has been exposed to the visible light, which indicated that the opening–closing equilibrium was a light dependent phenomenon. No absorption band at 504 nm has been observed for receptor 7 upon addition of other transition metal ions like Ni2+, Co2+, Mn2+, Cd2+ and alkali, alkaline earth metal ions like Mg2+, Ca2+, Na+, and K+. A 100 times increase in the absorbance intensity has been observed for Zn2+ ion in comparison to other metal ions. For Zn2+, 1:1 stoichiometry has been determined using Job’s plot. A significantly different absorption maximum has been observed in the presence of Zn2+ ions. No such emission spectra have been observed in the presence of other metal ions [117]. Stauffer and Weber have also synthesized a Zn2+ ion selective naphthopyran receptor 8 (Scheme 6) [118]. The receptor 8 produced interesting strong and reversible binding response with the Zn2+ ion. The thermal equilibrium data suggested that the sluggish conversion of open form to closed form. The Zn2+ ion bind strongly in the presence of 306–416 nm of the light source, whereas the binding of the receptor with Zn2+ ion is slow in the dark condition. The complex formation can be disrupted using visible light, which highlights light controlled nature of the metal ion binding. The Job’s plot indicated a 1:1 binding stoichiometry between the receptor and Zn2+ ions with an association constant of 1.1  109 M1.

O(CH2)12S-S(CH2)12O N O

O N

O 2N

NO2

9

Wen et al. have developed and reported the metal ion binding properties of spiropyran dimer 9 with thiol subunit. The merocyanine form of the receptor 9 coordinated the Zn2+ ions on the Au electrode surface. Capture and release of Zn2+ ions by the open form have been achieved with light stimuli [119]. The spiropyran 9 responded to a ZnCl2 solution and displayed an absorption band at 486 nm on UV light irradiation. The binding constant between the open form of the receptor and Zn2+ ion has been observed to be K = 633 M1. The polar merocyanine form came back to nonpolar spiro form with visible light irradiation. However, no absorption band has been observed with alkali metal ions. The experimental outcome ideally matched with that of expected electronic transduction and the receptor 9 can also function as a logic gate.

R 1 R2 R 3

N O O HN N

HN

N O 10

NO2

F3C

MeO 11: R1=R2=R3=CH3 12: R1=R2=Cyclohexyl,R3=H

N O 13

MeO

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Zhu et al. have developed a fluorescent near-IR sensor (10) [120]. The receptor 10 exhibited 16-fold emission enhancement with Zn2+ ion by displaying an intense red color in aqueous ethanol [120]. The fluorescence resonance energy transfer (FRET) has been observed between quinoline and the open merocyanine isomer during the interaction of receptor 10 with the Zn2+ ion. Roxburgh et al. have developed a variety of spiropyran receptors 11, 12, 13, 14 and evaluated them for utility towards the reversible transportation of Zn2+ ions [121].

2+

N N O

Cu2+ N N

O N Cu2+

O

N

O

O

O 20

Scheme 7. The Cu2+ complexation mechanism using spiropyran receptor 20.

N O O

F3C

N NH

N

N O 14

4.2. Photoactive Cu2+ ion sensors

O 15

Copper ion, being a heavy metal sometimes does not get metabolized by our body, thereby depositing in the tissue cavities and hence comes under the purview of toxicity [126]. Accumulation of excess copper ion causes Wilson’s disease [127]. The free radical inducing behavior of copper ions augment toxicity levels inside the cells [128,129]. Due to the toxicity associated with copper ions, it is imperative to develop optical sensors for the detection and monitoring of copper ions in the environment and biological samples.

MeO

O N N H 3C

O

N

16

N

N O

Zhu et al. have synthesized a spiropyran derivative (15) in a three-step procedure [122]. The spiropyran derivative (15) detected the sub-nanomolar concentration of Zn2+ ions in aqueous solution with an optical response in NIR region and large stokes shift of 110 nm. The fluorescence titration of 15 versus Zn2+ ion provided a red shift with 36-fold fluorescence enhancement, which proceeds through an intramolecular charge transfer (ICT) process. The results of Job’s plot analysis revealed 1:1 binding stoichiometry between spiropyran derivative 15 and Zn2+ ions. The metal ion complexation process can also be reversed by the addition of EDTA. Bao et al. have synthesized a bipyridine substituted spiropyran (16) and obtained its Zn2+ complex, which has been characterized using single crystal X-ray crystallography [123]. The synthesized receptor 16–metal complex responded in the presence of light of a different wavelength in a reversible manner.

R 19: R = -N(CH 3) 2 20: R =

N

O

Shao et al. have synthesized a fluorescent copper ion sensor 20 by attaching a morpholine unit to a spiropyran architecture (Scheme 7) [130]. When a solution of receptor 20 was titrated against a solution of Cu2+ ions, an emission band with enhanced intensity at a longer wavelength (640 nm) has been observed in aqueous solution (Fig. 6). The addition of nitrate salts of Cd2+, Co2+, Zn2+, Hg2+, Ni2+, Pb2+ ions resulted in no response, whereas

H3C CH 3 N O

N O 17

N

O

NH

O

NO2

H3CO

C CH 2 CH3 x O

18 CH3 C CH2

y

O CH2CH2O CH3 9

Zhu et al. have also synthesized a fluorescent spiropyran sensor 17 in a multistep protocol [124]. The sensor 17 detected Zn2+ ion in ethanol with turn-on fluorescence response. Kobayashi et al. have also reported a spiropyran–polymer conjugate (18), which produced reversible ionic conduction with Zn2+ ion under light irradiation [125].

Fig. 6. Fluorescence emission change with various concentration of Cu2+ ion [130]. Reprinted with permission from Ref. [130]. Copyright 2016 American Chemical Society.

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P.R. Sahoo et al. / Coordination Chemistry Reviews 357 (2018) 18–49

the receptor 20 produced a dramatic response in color (turned red) when Cu2+ ions were added. The receptor 20 can be used to detect Cu2+ ions in blood serum without any interference from Zn2+ or Fe2+/Fe3+ ions. Similarly, a highly Cu2+ ion selective merocyanine receptor 21 has been developed by Guo et al. [131]. The merocyanine form based receptor 21 has been characterized by NMR spectroscopic technique. Theoretical calculations performed using Gaussian 03 program also corroborated the experimental observations, where the merocyanine isomer is 18.4 kcal/mol lower in energy than its spiro form, which suggested that its merocyanine form is present in a major amount at the equilibrium [131]. The intramolecular hydrogen bonding present in the merocyanine isomer provided stability to its conformation. A detection limit of less than 1.0 mM has been determined for Cu2+ ion using UV–Visible spectroscopy along with an association constant value of 1.0 ± 0.1 5  1010 M2 between the merocyanine isomer 21 and Cu2+ ions. Job’s plot indicated a 1:2 (Guest to Host) binding stoichiometry between Cu2+ ions and merocyanine form of the receptor 21.

28: R =

N O

NO2 Cu(II)

O

Cu(0) H

O OH

30: R =

N O R

O

NO2

dimerization H H O N R

N O R

NO2

-2H+

NO2

O

O

N

NI

N

22

N O

NH HN O

21

NO2

23

O

N O

O N R

O 2N

Liu et al. have synthesized a spiropyran receptor 22 by utilizing dioxipolyamine [132]. The receptor 22 displayed a selective coloration in the presence of Cu2+ ion with an intense band centered at 500 nm. The selective complexation can be attributed to the presence of a suitable ring containing N atoms to coordinate the copper ions.

N O R

NO2

Scheme 8. Mechanism of spiropyran dimerization via radical formation induced by Cu2+ ion.

played a shift in the major band from 517 nm to 478 nm. The receptor 26 with Cu2+ ion under visible light also displayed a hypsochromic shift in the absorption band from 514 nm to 480 nm. However, the shift or recovery of the intense band has not been observed entirely, which suggested the partial release of Cu2+ ions from the receptor solution. Interestingly, other metal ions like Zn2+, Ni2+, Fe2+, Co2+, Mn2+, Cd2+, Mg2+, Ca2+, Na+, K+ failed to produce any significant change.

NO 2

N O N O

OH

29: R =

O 2N H O

N O R

27: R = CH3

NO2

NO2

O 2N

NO2

25

OH

24

26

O EtO

O

N

HOOC

N

O

27 O 2N

Natali et al. have synthesized spiropyran based receptors 23–26 and evaluated them for affinity towards transition metal ions. The receptors (23–26) exhibited a bathochromic shift in the presence of Cu2+ ions only in acetonitrile, which indicated their selectivity [133]. The addition of one equivalent of copper ions to the solution of receptor 23 altered the coloration of the receptor from pink to deep orange with a broad band centered at 496 nm. The receptor 24 in the presence of Cu2+ ions produced a band at 515 nm with a shoulder at 493 nm and displayed deep orange color. Similarly, the receptor 25 displayed an intense band at 517 nm upon addition of Cu2+ ions in CH3CN. The receptor 26 on interaction with Cu2+ ions exhibited an intense band at 514 nm with an orange color visible to the naked eye. The receptor 23 displayed a shift in the absorption band from 496 nm to 452 nm and decrease in the absorbance intensity with one equivalent of Cu2+ ions, on irradiation with visible light. Similarly, the receptor 24 upon addition of Cu2+ ions followed by visible light irradiation displayed a hypsochromic shift from 504 nm to 472 nm. Similarly, the receptor 25 in the presence of Cu2+ ion under visible light exposure dis-

NO2 O

N

N 28

HO

O

OH

O2 N

NO2 O

N

N

O

29 O

O

O

O

O2 N

NO2 O

N

N

O

30 HO

O

O

OH

27

P.R. Sahoo et al. / Coordination Chemistry Reviews 357 (2018) 18–49

[a]

O N

OH

Fig. 7. [a] Chemical Structure of receptor 35 [b] absorption spectra of receptor 35 in the presence of UV light [c] five alternate cycles of switching ability of receptor 35 [138]. Reproduced from [138] with permission of The Royal Society of Chemistry.

any change in the absorption spectra, which indicated the selectivity of Cu2+ ions. Similar changes were also observed with receptors 29 and 30. Similar to receptors 23–26, the spiropyran dimers 27–30 on exposure to visible light in the presence of Cu2+ ions produced a hypsochromic shift in the absorption spectra without complete recovery, which implied partial release of Cu2+ ions in CH3CN [133].

Natali et al. also interestingly observed dimerization of spiropyran motifs facilitated by Cu2+ ion through a cross-coupling catalyzed channel using MALDI-TOF mass spectrometry [133]. MALDI-TOF MS experiment of receptor 23 with one equivalent of Cu2+ ions exhibited a mass value at m/z 643, which suggested the formation of [27]+ dimer. The mass profile of the receptor 24 with one equivalent of Cu2+ ions displayed a peak at m/z 702 corresponding to [28]+ dimer. Similarly, a peak at m/z 843 has been observed from the mass spectrum of receptor 25 with one equivalent of Cu2+ ions, which corresponded to [29]+ dimer. Similarly, the mass spectrum of receptor 26 with one equivalent of Cu2+ ions produced a peak at m/z 787, which matched with the [30]+ dimer. To understand the detailed mechanism for the formation of spiropyran dimers, the authors synthesized the respective dimers utilizing spiropyran monomer units with one equivalent of Cu2+ ion in large scale. The synthetic products were characterized by two-dimensional spectroscopic techniques such as COSY NMR, which confirmed the formation of spiropyran dimer coupled through the CAC single bond in a head-to-head fashion (Scheme 8).

H 3C n O

O

31

Shiraishi et al. have synthesized a spiropyran receptor 31 to serve as a colorimetric chemodosimeter for Cu2+ ions [134]. A selective bathochromic shift was reported with Cu2+ ions in an aqueous acetonitrile solution. The spectroscopic investigation through Job’s plot indicated a 1:2 (guest to host), Cu2+–receptor complex stoichiometry.

N

1-n

O

O

33 N

HN

32 O S O O

NH HO

CH3 H 2C C

H2 C C O

N O

NO2

O N

N O

O

O

NH

The UV–Visible studies of spiropyran dimer 27 indicated two distinct bands at 275 nm and 303 nm, which corresponded to the closed form. On exposure to short wavelength UV light (254 nm), a new band at 579 nm region with a blue color appeared, which disappeared completely in the dark, which implied SP–MC reversible isomerization. Upon addition of Cu2+ ions solution to the receptor 27, a new band at 475 nm was noticed with significant change in the titration curve, which implied Cu2+ ion sensing behavior. Similarly, the receptor 28 responded to UV light with an appearance of a significant band at 586 nm, which returned to its initial spiro form at 275–308 nm region under dark conditions. The addition of one equivalent of Cu2+ ions to the solution of receptor 27 produced a distinct absorption band at 515 nm due to the complexation with Cu2+ ions. However, no other metal ions produce

N N N

NO2

34

Suzuki et al. have also synthesized a spiropyran-based sulfobetaine copolymer (32), which exhibited a reversible response in water as well as in saline solution [135]. The copolymer 32 was able to show selective and reversible response towards Cu2+ ions in saline solution and selective Cu2+ ions adsorption in water. Scarmagnani et al. have also developed polymeric beads utilizing spiropyran motifs (33) [136]. The spiropyran polystyrene beads exhibited photo-reversible phenomena using LEDs (light-emitting diodes). The SP to MC isomerization of polystyrene beads was envisaged under irradiation with UV LED of 375 nm. The spiropyran-based beads were observed to show coloration in contact with Cu2+ ions and subsequently released Cu2+ ion when microbeads were exposed to the white LED (430–760 nm). Kim et al. have synthesized 1,2,3-triazole substituted spiropyran 34,

28

P.R. Sahoo et al. / Coordination Chemistry Reviews 357 (2018) 18–49

Cl OHg+

Hg2+

N O

N O

Cl UV

N O

N

O

O

N

43

N Vis

Scheme 10. Coordination of spiropyran receptor 43 with the Hg2+ ion.

35

Byrne et al. have synthesized a spiropyran receptor 37 and modified it for further development of polymers 38–40 by using diamino alkyl linkers with variable chain length (n = 4, 6, 8 methylene units) over a PMMA matrix support [143]. The UV light (380 nm) irradiation of a solution of 37 in CH3CN produced the open merocyanine form of the receptor with absorption maxima at 560 nm. When the solution of 37 was exposed to UV light in the presence of Co2+ ions a maximum at 460 nm was produced, which indicated that the complex formation between the merocyanine form of 37 with cobalt ion. The merocyanine form of 37 complexed to cobalt ion reverted to spiro form on exposure to visible light (525 nm) and released the bound metal ion [143]. A 2:1 merocyanine–Co2+ ion binding stoichiometry for the complex was indicated by the Job’s plot method. In order to explore further the effect of polymer surface on the efficiency of photoswitching and transition metal ion complexation tendency, the authors have utilized polymer units 38–40 and their thin films for Co2+ ion sensing. On increasing the chain length from n = 4 to n = 8, the absorbance increased at 570 nm, which confirmed that the light-induced isomerization process enhanced significantly on increasing chain length due to extensive conjugation. The UV–Visible absorption spectra of the synthesized thin films did not produce any bathochromic or hypsochromic shift, however, only a hyperchromic effect (increase in absorbance only) was observed. A small change in the absorbance value was noticed with the thin films of 38, 39, whereas thin films containing 40 produced a profound increase in absorbance value at 570 nm. On addition of Co2+ ions to the spiropyran films, significantly increased in the absorbance value at 430 nm as well as a large decrease in absorbance value at 570 nm was noticed in the case of 40, whereas other films such as 38–39 produced very little change in the absorbance value. The thin films coated with Co2+ ions on exposure to white light released Co2+ ion, which indicated the metal complexation–decomplexation tendency of the spiropyran film 40. The curiosity to unfold responsive surfaces with multi functional properties attracts everybody’s attention for a superior surface response. In an attempt to improve the performance of photochromic receptors, Samanta and Locklin have developed a photochromic polymer brush 41 by employing spiropyran [144]. The importance of polymer brush lies in its three-dimensional arrangement with extended chains due to which a number of functional groups available at a particular surface greatly increased.

Scheme 9. Light-induced isomerization of spiropyran receptor 36.

which selectively produced dark pink coloration in the presence of Cu2+ ions [137]. The interaction of spiropyran with metal ions was investigated in acetonitrile. The copper ions formed a complex with spiropyran by coordination through triazole unit and phenolic ion producing colored open merocyanine form. An absorption band at 520 nm was obtained from UV–Visible experiment due to the formation of a complex between the open form and Cu2+ ions. The Job’s plot indicated a 3:2 copper ion to the receptor 34 stoichiometry for the complex. Recently, Kumar et al. have reported a light controlled salicylal dimine–naphthopyran 35 as a photoswitch for the colorimetric detection of Cu2+ ions in aqueous solution and also established the chemical motif by single crystal X-ray crystallography. The authors have covalently positioned the salicylaldehyde group over a photochromic unit so that the receptor 35 can directly bind the heavy metal ions [138]. It has been observed that the open form of the receptor binds copper ions more strongly in comparison to the closed form with a difference of 3.25  104 in binding affinity. This receptor has been proposed as a heavy metal detection tool for future sensing purposes [138] (Fig. 7). 4.3. Photoactive Co2+ ion sensors Cobalt is a vital trace element essential in nutrition, however, its deficiency leads to many abnormalities such as anemia, loss of appetite, vasodilatation etc. [139]. Exposure to Co2+ ions at working sites such as diamond polishing, chemical, and pharmaceutical industry etc. poses a grave threat to the living specimens, which causes cardiac and thyroid enlargement, heart disease, bone marrow malfunctions etc. [140]. Disorders like allergy, nausea, gastrointestinal disorder also engulf the human system due to cobalt ion poisoning [141]. Hence, there is a strong need to detect a low level of cobalt ions for environmental remedies. Zakharova et al. have examined a biphenyloxazole substituted spiropyran receptor 36 for affinity towards transition metal ions and observed a hypsochromic shift in the absorption spectra owing to swift complexation with Co2+ ions (Scheme 9) [142].

NO2

NO2

O

O N

N

n = 4, 38 n = 6, 39 n = 8, 40 HN

N O

NO2

HN

O

O n

n HN

O

O

37 HOOC

O Hg2+

OH

PMMA

NH

29

P.R. Sahoo et al. / Coordination Chemistry Reviews 357 (2018) 18–49

NO2

N O

NO2

hv1, Mn+ hv2

N

O

N Mn+

N 44 closed

open

Scheme 11. Metal complexation model of spiropyran receptor 44.

Fig. 8. UV–Visible absorption spectra of receptor 43 with metal ions in dichloromethane [151]. Reproduced from [151] with permission of The Royal Society of Chemistry.

Shiraishi et al. have reported the synthesis of another spiro pyran–dipicolylamine conjugate (42), which displayed selectivity towards Co2+ ions with an intense band at 472 nm under UV irradiation, while no change was observed with other metal ions [145]. The conjugate was observed to be colorless in the dark or during light irradiation. However, UV irradiation (280 nm) with Co2+ ion produced a yellow coloration in aqueous CH3CN. A detection limit of 1.0 mM was observed for Co2+ ions. Job’s plot analysis revealed 2:1 conjugate–Co2+ ion binding stoichiometry. 4.4. Photoactive Hg2+ ion sensors

[a] N N O Fig. 9. [a] Chemical structure of receptor 51. [b] Paper-based test strips of receptor 51 with Fe3+ under Daylight and UV light (365 nm); A = Free receptor 51, B = Fe3+– 51 complex [162]. Reproduced from Ref. [162] with the permission from Elsevier.

The surface with increased functionality could show superior stimuli–responsive properties owing to the high density of molecules per unit area. The polymer brush was utilized as a switchable color surface with wettable characteristics. The application of wetting expands from oil recovery to advanced printing due to the super hydrophobic character. Normally, wettability can be investigated using the contact angle measurement, where the smaller contact angle (90°) signifies high wettability and contact angle 90° implies low wettability. The spiropyran–merocyanine isomerization produced reversible contact angle change equivalent to 5– 15°. The contact angle can be tuned by the addition of metal ions due to complexation. Reversible contact angle variation of 35° was achieved by irradiating the polymer brush in the presence of Co2+ ion [144].

O O2 N

N C nH 2n O

4.5. Photoactive Fe3+ ion sensors

O n [Ru]

O O Si O O O O

[Ru] n

Si O O 2N 41

N

O O

O

C nH 2n N

Mercury, the dreaded poison passes the human body via biological membranes like skin and respiratory systems [146,147]. Mercury ion contamination affects human health creating brain damage, cognitive disorders etc. [148]. Long-term exposure results in vigorous internal disorders even cell death [149]. Hence, there is a tremendous need to detect low-level mercury ion in drinking water [150]. Han et al. have observed the formation of a colored complex, when Hg(OAc)2 solution was mixed with a hydroxy substituted spiropyran receptor 43 (Scheme 10, Fig. 8) [152]. The mercury ions played a prominent role through proximity effect in the activation of the CspiroAO bond present in spiropyran derivative (43). The study has paved the way for mimicking heavy metal recognition for the future generation, where the receptor (43) can detect highly contagious Hg2+ ion both in solution phase and solid phase such as a thin film by displaying pink coloration recognizable to the naked eye. The receptor (43) displayed high selectivity for Hg2+ ions over a number of other metal ions like Zn2+, Cd2+, Mn2+, Cu2+, Fe2+, Pb2+, Ag+, Zn2+, Ca2+, Mg2+, K+ and Li+ [151] (Fig. 9). More recently, Winkler et al. have observed 20-fold enhancement of emission intensity, when one equivalent of HgCl2 was mixed with a receptor 44 based on a spiropyran skeleton (44, Scheme 11) [115]. However, only 9-fold enhancement intensity was noticed with the addition of one equivalent Zn2+ ions into the receptor solution and only 3–4-fold enhanced emission intensity was observed in the case of Cd2+ ions. The variation can be attributed to the reduced binding ability of the nitro group substituted quinoline system. The results supported the selectivity of the receptor (44) towards mercury ions.

N O 42

NH

N N

Iron plays a crucial role in the realm of oxygen uptake, oxygen metabolism and electron transfer in human body cells [153]. Regardless of heavy usage of iron in living species, iron is cytotoxic in nature. Clogging of excess iron in the central nervous system has shown to evolve a number of complicated diseases notably Alzheimer’s disease, Parkinson’s disease and Huntington’s disease [154,155]. Side by side, deficiency of iron leads to a disastrous effect on the human body, for instance, permanent loss of motor skills [156]. Therefore, there has been a resurgence of interest to develop the selective Fe3+ ion sensor, which can be both sensitive to our naked eye and practically viable for smooth monitoring.

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P.R. Sahoo et al. / Coordination Chemistry Reviews 357 (2018) 18–49

O

NO2 N

l=365nm

Fe3+

O l ñ 520nm

2,2'bipyridine

O

O

UV (365 nm) + Fe3+ N O

NO2

NO2

45

N

Visible light + 2,2'-bipyridine

3+

Fe

Fe

Fe3+ O l=365nm

2+

Na2S2O3 O N O

NO2

Scheme 12. Interconversion of spiropyran receptor 45 with light stimuli and Fe3+ ion.

Guo et al. have observed the complex formation between merocyanine form of receptor 45 and Fe3+ ions (Scheme 12) [157]. The complex formation between the merocyanine form of 45 and Fe3+ ion produced a spectral shift to 420 nm. The MC–Fe3+ complex was stable to visible light and did not respond to Na2S2O3 at room temperature [157].

F

F

S

S

O

N

50

O

O

N

I

N O

O N

N

N

UV

O

Vis

O 2N

NO2

46

C 6 H13S

F

F

N

C 6 H13S

F F

48 F

S S

S S

F F

F F

F

S(H 2 C)12O 47

S N

N O

S

O

N

O

NO2

O O

Guo et al. have synthesized a spirooxazine based molecular switch 46 and investigated the opening–closing chemistry using FeCl3 and diethylamine [158]. Guo et al. have also designed and synthesized an interesting dyad 47 bearing spiropyran and tetrathiafulvalene (TTF) [159]. The dyad on UV light (356 nm) exposure produced a band at 580 nm, which indicated that the formation of an open merocyanine form in THF. Insertion of one equivalent of Fe3+ ions to the solution of 47 followed by UV irradiation resulted in a significant absorption band at 424 nm due to complexation with Fe3+ ions. However, when Fe2+ ions were added to a solution of 47 followed by UV light exposure, a very weak absorption band was noticed. In addition, when one equivalent of Fe3+ ions were added to the dyad solution 47, a broad band at 610 nm surfaced due to the formation of radical cation TTF+. The radical cation (TTF+) formed as a result of the oxidation product of the TTF unit facilitated by Fe3+ ions, which caused the reduction of Fe3+ ions to Fe2+ ions. The electrochemical experiments such as differential pulse voltammetric studies indicated thermodynamically favorable intermolecular electron transfer reaction between TTF+ and MCFe2+. The electrochemical measurements could be an impetus for exploring the photoswitching response of spiro pyran–tetrathiafulvalene dyad (47) in the presence of Fe3+ ions. The dyad displayed promising response towards the regulation of

NO2 O 2N Scheme 13. Light controlled isomerization of receptor 50.

electron transfer process that proceeds through light via redox mechanism. Guo et al. have synthesized 48 and evaluated the metal complexation behavior in THF [157]. The solution containing 48 displayed a bathochromic shift in the presence of Fe3+ ions.

O

N

O

O N O

O N O O O2 N

N

O

49

NO2

31

P.R. Sahoo et al. / Coordination Chemistry Reviews 357 (2018) 18–49

OH I

OH

N N O Pd(PPh 3 )2 Cl2 CuI, NMP 51

N

N O

O N

N Δ

hv OH

N

N

N

N O

O

Scheme 14. Photoswitchable ring opening of spirooxazine receptor 52.

R2

N R2 N O

N

UV VIS

R1

N

R1

O

M2+

55: R1=R2 = SO3Scheme 15. Photoinduced ring opening of receptor 55.

Guo et al. have developed a hybrid receptor 49 by coupling spiropyran units with a perylenediimide fluorophore, which produced intense fluorescence emission in the presence of Fe3+ ions mediated by UV light in THF [160]. Choi et al. have synthesized an amalgamated receptor 50, which consisted of diarylethene and tetrathiafulvalene moieties linked to the spiropyran. The receptor 50 formed a complex with Fe3+ ions selectively (Scheme 13) [161]. An enhancement in absorption intensity was observed upon addition of ferric perchlorate to a solution of the spiro form followed by UV irradiation within the 360–410 nm region. A hypsochromic shift was observed in the presence of metal ion due to the formation of a complex. Recently, a selective and sensitive photochromic spironapthooxazine receptor 51 for the detection of Fe3+ ions in aqueous solution was reported. The receptor 51 produced a high stokes shift and 90-fold fluorescence intensity in the presence of Fe3+ ions with an improved detection limit in aqueous methanol solution [162]. 4.6. Photoactive Pd2+ ion sensors Palladium, a rare transition metal, creates eye and skin irritation when swallowed through contaminated food and water

[163]. Palladium ion also interacts with DNA, thiol rich amino acids, thereby derange several life processes at the cellular level [163]. Undeniably, palladium is an efficient catalyst for the construction of many industrially important synthetic molecules due to its high functional group compatible nature [164]. The release of palladium residues from the laboratory environment following reaction work up is a serious concern. Palladium also finds many frontal applications in cancer treatment, medical devices, digital electronics, fuel cells, aviation etc [165]. Due to an excessive utilization of palladium ions, both in laboratory and industry, screening of palladium ion is urgent for a safe environment and continuous life processes [166]. Simply revamping the indole moiety by an alkyne aromatic substituent, the property changed so peculiarly that the target molecule 52 gets ready to sense Pd (Palladium) catalyst. The research work of Kumar et al. in building a spirooxazine dimer that responded to palladium catalyst in a way equivalent to bindingcum-releasing fashion (Scheme 14) [87]. The spiro form on treatment with UV light yielded a colored merocyanine form with an absorption band in the 570–605 nm region and the open one transformed into closed form thermally or upon visible light irradiation obeying a first order kinetics. The thermal ring closing fits parallel to that of solvent polarity. The open merocyanine form has been found to form 1:1 stoichiometric complex formation with Pd (PPh3)2Cl2. The kinetic experiments revealed a remarkable binding affinity of merocyanine form with Pd catalyst in THF that resulted in a reduced thermal bleaching rate. The more interesting point is that the simple spirooxazine didn’t produce a reduced thermal bleaching, which indicated that the spirooxazine dimer has the capability for binding with the catalyst due to the cage effect. The result was further utilized in achieving a comprehensive and accelerated switching to boost the frontier areas of light controlled absorption and release [167]. Watkins et al. have synthesized nano micelles utilizing spiro-naphtho-oxazine dimer (SNOD) and poly (ethylene glycol) [167]. By encapsulating a guest species such as

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2+

O O

O

O

UV

O

O

O

Pb

O

O

O

O

O Pb2+

O

O

2+

O Pb O

O

O Vis

O

O

O

O

O

56

O O

O O

O Pb

O

O Scheme 16. Light-induced isomerization of naphthopyran receptor 56.

palladium catalyst at the micelle nucleus, a significant decrease in thermal bleaching rate has been noticed. Interestingly, with SNOD connected to a solid support such as polymer beads, strong affinity towards palladium catalyst has been observed.

CH 3 H 2 CH 3 H2 C C C C n 1-n O C C O O O

HO

O

H2 C O

O 53

O2 N

HC CH2 n f-n O O

CH(OH)

n O

NH

H 2C

54

m

HC H 2 C O

N

(CF2) 7 F3 C

54a

O N

N O

Connal et al. have also developed a poly(acrylic acid) functionalized spiropyran polymer 53 and used it for the construction of advanced ordered materials such as photo-responsive honeycomb structures [168]. The polymer film prepared using 53 successfully absorbed Pd2+ ions, which could function as advanced sensor materials. 4.7. Photoactive lead (Pb2+) ion sensors Lead is a harmful poison and a potent neurotoxin spreading its toxic tails towards memory loss, anemia, paralysis etc. [169,170]. Screening and accurate measurement of bioavailable lead ion is crucial in cellular and sub-cellular level inside the body cells [171]. Lead ion exposure creates havoc particularly among children [172]. Lead ion pollution is a common issue in the areas of mining and industrial sites [173]. Therefore, selective lead ion determination through colorimetric naked eye detection or fluorescent output has broadened the sensing mechanism.

Keeping in mind the toxicity of lead ions, Suzuki et al. have synthesized a poly-(spiropyran)–methacrylate (54) and employed it for reversible metal complexation studies [174]. The polymer solution in the presence of Pb2+ ion exhibited yellow color, which turned colorless in the presence of visible light. The authors have also investigated the photo-reversible isomerization of Pb2+ ion bound polymer using 1H NMR spectroscopy [174]. Binding of Pb2+ ions was achieved with an amalgamated spiropyran acrylate and N-isopropyl acrylamide 54a by Suzuki et al. Appearance of a new band at 433 nm upon addition of lead perchlorate [Pb (ClO4)2] provided clear indication of MC–Pb2+ complex formation [175]. The open form was stabilized by a polar solvent such as water. The molecule transformed into the closed form with the disappearance of the complex band at 433 nm after irradiation of visible light. The polymeric material was observed to absorb metal ions above lower critical solution temperature (LCST) and displayed a square wave voltammetric signal. Tamaki et al. have observed that the spirooxazine receptor 55 (Scheme 15) displayed hypsochromic shift selectively in the presence of Pb2+ ions followed by UV light (500 W Hg lamp) irradiation in aqueous acetone solution [176]. The observed shift was largest

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R

R

N N O 57

N

365nm

N

N

O

N

N 600 nm

N

Ni(NO3)2 Ni(NO3)2

N R

N

O

R

N Ni2+

N 3

365nm

N

N

O

N

N

600nm

Ni2+

3 Scheme 17. Complex formation of the receptor 57 with metal ions.

for lead ions followed by copper ions, and calcium ions. However, other metal ions like Cd2+, Zn2+, Mg2+, and Co2+ didn’t produce any significant change. The same order (Pb2+ > Cu2+ > Ca2+) was obtained in thermal stability of photoreversible chelator. Stauffer et al. revealed that the naphthopyran 56 with adjoint crown ether displayed an affinity towards lead ions under dark conditions [177]. Benzo 15-crown-5 was observed to assist the binding of lead ions to the naphthopyran unit with a voltammetric signal. The result being a Pb2+ ions reduction band shifted with decreased intensity in response to increased receptor (56) concentration. The binding was again monitored using either photo or electrochemical techniques with Pb2+ ions and naphthopyran molecule displayed a broad band at around ca. 600 mV (cathodic reduction). The observed gradual negative reduction potential was due to the addition of crown ether. The photo dissociation of 56– Pb2+ was achieved upon initiation of light of 300–400 nm wavelength, which was confirmed from reduction wave. The reversible cycle, i.e., association-cum-dissociation was beautifully controlled through light (Scheme 16). 4.8. Photoactive Ni2+ ion sensors Nickel, although is an essential element but is toxic when present in large amount. Additives of nickel ions in the form of nickel sulfide fumes and its dust particles are carcinogenic in nature [178]. Further, excess nickel ion depositions can dysfunction respiratory and internal immune system. Loss of nickel homeostasis is dangerous to both prokaryotic and eukaryotic organisms of interest [179,180]. Due to the toxicity associated with nickel ions, it is essential to develop sensitive receptors for the detection of this ion. In this context, Zhang et al. have reported the synthesis of a variety of receptors 57 with different substituents by modifying the alkyl group as (a) R = CH3, (b) R = benzo, (c) R = Br, (d) R = Cl, (e) R = H, (f) R = CH3O. Methoxy group substitution at 50 -position of indole nucleus provided a red-shifted band along with more stable open form stereoisomers [23]. The thermal equilibrium shifted towards merocyanine in polar solvents due to the accumulation of electron density over nitrogen atom through conjugation and leading to remarkable color and stabilized open form. The addition of Ni2+ ion to the bromo substituted compound in methanol in the absence of light shifted an absorption band to 500–650 nm region, which confirmed that the complexation is due to the merocyanine form rather than spiro one (Scheme 17). The open form complexed with Ni2+ ions displayed negative photochromism. The open form reversed to closed spiro isomer in the presence of visible light showing rapid photo bleaching.

N NAD+

NADH N

N O

N

H O

O

O

H N

N O

Gd3+ N N O

O

H

O

H

H 2O 2

O

N

O

Gd3+ N N

H O O

59

H O

N O

O O

O Scheme 18. Structural change of spirooxazine receptor 59–Gd3+ triggered by NADH and hydrogen peroxide.

N N O

58

H3 CO

Tamaki et al. have synthesized a methoxy substituted spirooxazine chelator 58. A bathochromic shift was noticed when the receptor 58 was treated with Ni2+ ions in acetone [176]. However, the receptor 58 didn’t recognize metal ion in highly polar solvents like dimethyl formamide or alcohols. Methoxy group at ortho position played a vital role in coordination with the Ni2+ ions. 4.9. Photoactive Gd3+ ion sensors Gd3+ ion chelates are often utilized as MRI contrast agents [181]. Therefore, exploration of Gd3+ ions based contrast agents have increased manifold due to its role as a diagnostic tool in detection at the soft tissue level (Scheme 18). Smart contrasting agents are fundamental to imaging. One of the chief advantages of gadolinium ions (Gd3+) based contrast agents are due to their unique magnetic properties in enhancing relaxation rate of water protons at the cellular level [182]. Gadolinium ions based chelates containing organic fluorophores have the potential in deep tissue penetration, whereas normal chelators failed to penetrate into deeper cavities [183].

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The light-responsive contrast agents have garnered huge interest in the advanced imaging system. Hence, a stable photochromic complex of spirooxazine 59 with Gd3+ ion was developed by Tu et al. The open form of spirooxazine has displayed good relaxivity after Gd3+ ion coordination [184].

N O

NO2 N

N O

Gd 3+ N N

O

O

O

H3 CO N O

O

61

H3 CO N O

OCH 3

62

The aza crown modified spiropyran 60, where the three carboxyl units coordinate with Gd3+ ions in merocyanine form. The complex reverted back to the ring closing stage after visible light initiation with an 18% decrease in relaxivity and the closing–opening equilibrium was controlled by light. This particular compound is used as MRI (magnetic resonance imaging) contrast agent [185].

H3C O R

N N CH3

N N

R

N

N Eu3+

Eu3+ O

57A

57B 3

O

H

N H N O CH3

UV Vis

N N CH3

65 -Eu3+ +Eu3+

H3C

OCH3 N O 63

N O 64

Balmond et al. have synthesized the photochromic receptors 61–64 and studied their influence towards the binding of Gd3+ ions [186]. The photochromic studies have been conducted using UV (8 W UV source) and visible light (150 W tungsten halogen lamp) in ethanol. The receptor 61 on light exposure produced a broadband centered at 440 nm, which disappeared in visible light. The Gd3+ ions solution has added to receptors 39–42 solutions and kept for 15 min under dark conditions for equilibration. The receptor 39 produced a new band at 425 nm after incubation with Gd3+ ions for 15 min. Similarly, the receptor 62 exhibited a new broad absorption band at 470 nm on incubation for 15 min with Gd3+ ions. The receptor 63 on incubation with Gd3+ ion produced a significant band at 470 nm with increased absorbance intensity. Similarly, the receptor 64 displayed a new band at 500 nm after treatment with Gd3+ ions. Job’s plot analysis confirmed 1:9 (host to guest) binding stoichiometry between Gd3+ ions and receptor 62, which corroborated the nine coordination tendency of Gd3+ ions during complex formation [186].

O 60

H 3 CO

H 3 CO

H

N

N

4.10. Photoactive Eu3+ ion sensor Europium(III) complexes exhibit long-lived luminescent properties due to which these are suitable for in vitro and in vivo medical applications [187]. Due to the exacerbated utilization of Eu3+ complexes in industries and medical sector, qualitative detection of Eu3+ ion is important. Zhang et al. have studied the complex formation of receptors 57 and 65 with Eu3+ ion [188]. In the receptor 57, a phenanthroline unit and a variety of substituents at the indoline unit appropriately fine-tuned the coordinating ability, while the receptor 65 has an unsubstituted indoline moiety along with a phenanthrene unit. Therefore, the receptor 57 formed two types of the complex with the Eu3+ ion as shown in Schemes 17 and 19 [188]. However, in the absence of phenanthroline unit, the receptor 65 formed a complex with Eu3+ ions as per scheme 19 [188]. The spiro form on treatment with UV light has been converted into a highly colored open merocyanine form. The merocyanine form of the receptor 57 displayed a hypsochromic shift (geometry 57A, 15–18 nm) or 150 nm (57B) depending on the complex geometry, while the merocyanine form of the receptor displayed a 140 nm hypsochromic shift upon addition of Eu3+ ions in methanol [188]. For the receptor 57, the formation of complex geometries can be monitored via UV–Visible spectroscopy. The complex geometry 57A displayed an absorption band at 590 nm, while the complex geometry 57B displayed an absorption band at 450 nm. The complex geometry 57B has been observed to be more stable than the geometry 57A. Therefore, the complex geometry 57A gets converted into 57B in dark [188]. The complex formed between the nickel ions and the receptors (57, 65) have been found to be unstable and the receptor thermally reverted to the closed spiro form releasing the Eu3+ ions [188].

Eu3+

4.11. Photoactive Tb3+ ion sensors

O 3

Scheme 19. Photochromic transformation and complex formation of receptor 65 with Eu3+ ion.

Terbium, a rare earth element used extensively as phosphorescent and luminescent materials owing to its green emission as well as optical inducing property [189]. Terbium doped phosphors are

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potential candidates in the field of X-ray imaging components, light emitting diodes, lasers, fluorescence lamps, X-ray tubes etc. owing to its excellent radiation stability [190,191]. Therefore, it is necessary to explore environment suitable for coordinating Tb3+ ions for sensing and other multifunctional applications [192,193].

hyper phosphatemia etc. [195–199]. Owing to the size equivalence with Ca2+, La3+ also interacts strongly with Ca2+ binding sites in the protein, thereby mimicking efficiently [198,200]. Due to the rich diversity of lanthanum complexes in the industry, determination of La3+ ion is necessary.

NO2

N

NO2

N N O

N

HN SO 2

NH 2 67

N O

N O N

Lanthanum complexes play a diverse role in a variety of cellular and biological functions such as ATPase activity, lipid peroxidation, phosphate ester hydrolysis, antitumor agents and diagnosis of

NO2

NO2

O N

O

N

N

O O

3+

La

O O

N

Visible light 100 equiv. K+ O

Δ

N

O

K+ O

N

O

O

O 2N

O

N

N O N

68a

4.12. Photoactive sensor for La3+ ions

O O

O

66

The binding investigation of Popov et al. have suggested that it was difficult to achieve the complexation (negligible UV–Visible spectral change obtained) of receptors 66 and 67 with Ba2+, Mg2+, La3+, Tb3+ metal ions in dark condition. However, the merocyanine isomer of receptors 60–67 formed a complex with metal ions under light irradiation [194]. The rates of thermal bleaching in the presence of different metal ions have been calculated and used to access the complexation of the metal ion with the receptor molecule. The calculation of rate constant for the thermal bleaching revealed the selective formation of a complex between Tb3+ ions and the receptors (66, 67). The merocyanine form of the receptor 66 produced initially a very small bathochromic shift, but after prolonged UV light exposure, an absorption band at 650 nm has been observed in a solution containing acetonitrile and Tb3+ ions [194]. Similar changes have not been observed with other metal ions used in the study. The merocyanine isomer of 67 produced a significant hypsochromic shift due to high selectivity for metal cations possessing higher charge density, particularly with Tb3+ ions [194]. It has been suggested that the nitrogen atom and oxygen atoms of the oxazine ring in the merocyanine form have been involved in the complex formation with the metal ion. In addition, the La3+ and Tb3+ ions have increased the aggregation tendency of the receptor 67, but decreased the photostability [194].

n=2

O

N O

N O n=2

NO2

68b

Kimura et al. have synthesized and studied the metal ion trapping tendencies of crown ether modified spiropyran 68 [76]. The spiropyran derivative 68 in the presence of various metal ions like Cd2+, Pb2+, La3+, Eu3+, Li+, Na+, K+, Mg2+, Ca2+ in CH3CN produced a significant band at 500–600 nm selectively with La3+ ions in the dark. The formation of the band in the 500–600 nm indicated the formation of merocyanine form of the spiropyran that can form a complex with the metal ion. The absorption band in the visible region indicated that perhaps the phenolic oxygen atom of the spiropyran coordinate the metal ion that induced the conversion of the spiro form into the merocyanine form. Due to the smaller size of the crown ether unit, the receptor 68a prefer binding to the smaller alkali and alkaline earth metal ions along with La3+ ion [76]. However, the receptor 68b due to the presence of two spiropyran moieties formed complex selectively with the La3+ ion. The mass spectrum of receptor 68b with La(NO3)3 indicated an intense peak at 565 nm owing to La3+ ion complexation [76]. The competitive mass spectrum investigation further confirmed the selectivity of the receptor 68b towards La3+ ions over other metal ions used in the study [76]. The 139La NMR spectrum also confirmed the strong binding interaction between phenolate ion of the receptor and La3+ ion. The mass spectroscopic study further indicated the difference in the affinity of the receptor 68a and 68b towards metal ions, which has been confirmed through the peak intensity of the receptor–metal complex [76]. It has been further observed that the metal complex between the La3+ ion and the receptor 68b can be controlled by visible light in acetonitrile (Scheme 20). The visible light irradiation (>500 nm) of the 68b–La3+ complex in the presence of 100 fold potassium ion produced a decrease in the absorption at 500 nm, which indicated the conversion of the merocyanine form into the spiro form [76]. The process has been further investigated by the authors using mass spectrometry, which indicated that the visible light irradiation of the 68b–La3+ complex in the presence of 100 fold potassium ions produced an increase in the 68b–K+ complex ion peak and a decrease in the 68b–La3+ complex ion peak. The 68b–K+ ion peak in the mass spectrum regenerated in dark. The mass spectrometry and UV–visible investigation both indicated the release of a La3+ ion from the receptor and binding of the potassium ion on exposure to the visible light (Scheme 20) [76].

O 2N

4.13. Photoactive sensors for multiple metal ions N

Scheme 20. The light controlled release of a La3+ ion from the receptor 68b.

Apart from the selective, reversible sensing of metal ions, several receptors have been reported in the literature, which can detect multiple metal ions, such sensors are exemplified in the following sections.

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Fig. 10. Crystal structure of the receptor 69–Zn complex (Ref. code# MACBID, CCDC No. 771869) [202]. Taken with the permission of the publisher (Royal Society of Chemistry) from [201].

COOC6H13

Fe2+ N O

C6H13O

NO2

SP 70

O

O Flu+

UV

NO2

+e

2,2'-bipyridine visible light

Fe3+ Na2S2O3 N Fe2+ O

NO2

-e MC-Fe2+

COOC6H13

N C6H13O

O

O

Fe3+ O

MC-Fe3+

Flu0

ther confirmed using 1H NMR spectroscopy, which indicated the disappearance of signal due to the spiro form and appearance of the characteristic signal for the merocyanine form. The Job’s plot analysis revealed 2:1 metal–spiropyran binding ratio in the case of both Zn2+ and Cu2+ ions. The MALDI-TOF mass spectrum has also revealed intense signals due to (1:1) 69–Zn2+ (or 69–Cu2+) complex stoichiometry along with (1:2) 69–2Zn2+ (or 69–2Cu2+) complex stoichiometry. A detection limit of 3  107 M has been observed for Zn2+ ion using fluorescence studies. In addition, a variation in the binding constant value of the receptor for copper ions with different counter anions has also been reported (3.37  104 with Cu (ClO4)2) and 2.04  104 M1 with CuCl2) [201]. The merocyanine– Zn2+ complex structure was established through single crystal Xray crystallography, where Zn2+ ions attached to the oxygen atoms of phenolate and substituted carboxylic acid of spiropyran via a distorted square-pyramidal geometry (Fig. 10) [201]. The crystal packing diagram of the receptor–zinc complex revealed the formation of a 1-D polymer containing hydrophilic and hydrophobic regions [201].

4.13.2. Fe3+/Fe2+ ion sensitive photoactive sensors Guo et al. have developed an artificial communication process between two molecular switches such as fluorescein (Flu) and spiropyran 70 utilizing Fe3+/Fe2+ redox states via electron transfer (Scheme 21) [203]. Insertion of one equivalent of Fe3+ ions to the fluorescein enhanced fluorescence intensity at 480 nm owing to the formation of Flu+. However, the fluorescence intensity reduced on exposure to UV light for 5 min. Interestingly, the fluorescence regenerated completely after increasing relative molar ratio of spiropyran (SP 70) and fluorescein (Flu) followed by Fe3+ treatment in the presence of UV light. The communication process happened as per the following events: Insertion of Fe3+ ions to the solution containing spiropyran and fluorescein facilitated oxidation of Flu to Flu+ followed by reduction of Fe3+ to Fe2+ with an increase in fluorescence intensity. On exposure to UV light, the spiro form converted into merocyanine isomer (MC), which subsequently formed MCFe2+ complex [160]. The electron-transfer process from MCFe2+ complex to Flu+ has been observed to be thermodynamically feasible, which finally led to the reduction of Flu+ to its neu-

Scheme 21. Communication pattern between two molecular switches via Fe2+/Fe3+ ion.

4.13.1. Photoactive sensor for Zn2+ and Cu2+ ions

N O

NO2 O

O

69 OH

Natali et al. have synthesized a spiropyran receptor 69 for reversible recognition of Zn2+ and Cu2+ ions [201]. The spiro form of receptor 69 absorbed light with an absorption maxima at 358 nm in CH3CN [201]. Insertion of Zn2+ ions resulted in an intense band centered at 494 nm. Similar alternation has also been noticed when Cu2+ ions solution was added to receptor 69 in CH3CN. The absorption band reverted completely back to the initial form on exposure to visible light, which suggested the reversible release of both Zn2+ and Cu2+ ions [201]. The complex formation between the merocyanine form of the receptor 69 and zinc ion has been fur-

Fig. 11. The UV–visible absorption spectra of receptor 71 with Co2+ ions in CH3CN [204]. Reproduced from [204] with permission of The Royal Society of Chemistry (RSC) on behalf of the Centre National de la Recherche Scientifique (CNRS) and the RSC.

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for Co2+ ions [(log K11 = 8.0, log K21 = 8.8) and Ni2+ (log K11 = 8.1, log K21 = 8.0)] in comparison to the receptor 73 [(log K11 = 7.1, log K21 = 7.3) and Ni2+ (log K11 = 7.3, log K21 = 7.3)] [204]. The luminescence measurements have indicated an enhancement in the intensity of the receptor (72) emission upon addition of zinc ion. A strong emission band at 700 nm was recorded using 365 nm excitation wavelength for the receptors upon addition of zinc ions (Fig. 12) [204]. No change in the emission intensity of the receptor has been reported for Ni2+ or Co2+ ions. The fluorescence results provide an alternative method for the detection of zinc ion complexation with the receptor 72 [204]. 4.13.4. Photoactive sensor for Zn2+, Co2+, Cu2+ ions

N O

Fig. 12. The fluorescence spectra of equimolar solutions the receptor 72 with Zn2+ ions in CH3CN [204]. [72] = [Zn2+] = 1.0  105 M (a), 5  106 M (b), 2.5  106 M (c) 1.65  106 M. The fluorescence spectra have been recorded 2000 s after mixing the metal ion and receptor solutions at 296 K. Reproduced from [204] with permission of The Royal Society of Chemistry (RSC) on behalf of the Centre National de la Recherche Scientifique (CNRS) and the RSC.

tral species along with MCFe3+ complex formation. Treatment of 2,20 -bipyridine along with UV light exposure transformed MCFe3+ complex to its initial spiro form. 4.13.3. Photoactive sensor for Zn2+, Co2+ and Ni2+ ions

71: R 1=CH 3 72: R 1=C 3H 7 73: R 1=CH 2CH(CH3 )2

N O R1 N

Cl

S

Zakharova et al. have synthesized a series of receptors (71–73) by varying the alkyl chain [204]. The addition of Co2+, Ni2+, Zn2+ ions to the solutions of the receptors produced an absorption band in the 520–550 nm region (Fig. 11), which on comparison with the absorption spectra of the merocyanine form have indicated a significant hypsochromic shift. A shift in the absorption band of the merocyanine form has indicated the complexation of the metal cation. The Job’s plot indicated the formation of 1:1 and 2:1 merocyanine–metal complex stoichiometry with Co2+ ions in solution. However, Ni2+ and Zn2+ ions yielded 2:1 merocyanine–metal complex stoichiometry in solution [204]. The complexation process has been found to be reversible and can be accomplished with irradiation of white light to produce the closed spiropyran form. The formation of a closed form of the receptor released the bound metal ion. The rates of thermal bleaching for receptors have been monitored with time in the presence of different metal ions [204]. The rates of thermal bleaching have been used to obtain the strength of complex between the metal ion and the merocyanine form of the receptor. For example, the receptor 71 displayed higher affinity

74

NO2 N O

NO2

N 75

OH

Taylor et al. have developed spiropyran receptors (74–75) for the chelation of metal ions [89]. Good photochemistry has been reported for both the receptors (74 and 75) in organic solvents. Both the receptor produced a purple colored merocyanine form in organic solvents [89]. The receptor (74) formed a red colored complex with Zn2+, Co2+ and Cu2+ ions in an acetone–alcohol mixed solvent [89]. The formation complex has been reported to accelerate with the application of either heat or UV light. Similar results have not been observed in spiropyran without piperidinomethyl group [89]. The receptor 75, in turn, produced an orange colored complex with cobalt ions, while a wine red colored complex has been reported with zinc ions. Both the receptors 74 and 75–metal complex faded on keeping the solution at room temperature [89]. The complexes have been observed to regenerate with the application of heat, but no effect of light on the complexation process has been reported [89]. Therefore, it may be concluded that both receptors formed a complex with metal ions in their merocyanine forms. The change in color may be attributed to the interaction of the metal ions and the phenolic oxygen atom along with coordinating atom of the side chain. The rate of complex formation has been correlated to a variety of factors such as the structure of the receptor, temperature, light and solvent polarity [89]. 4.13.5. Photoactive sensor for Co2+, Cu2+, Ni2+ ions Liu et al. have synthesized the spiropyran receptors 76 and 77, which have been observed to bind Cu2+, Co2+ and Ni2+ ions in alcoholic solution [132]. The closed form of the receptors has displayed no absorption band in the visible region in ethanol. However, the addition of metal ions to the solution of the receptors produced a new band in the visible region. The UV–visible experiments have indicated the metal ion induced conversion of the closed form of the receptor into the merocyanine form. The highly polar merocyanine form formed a strong complex with the metal ions used in the study. Zhou et al. have synthesized substituted spirooxazine receptor 78 and evaluated binding response towards Cu2+, Ni2+, Co2+ ions under UV light exposure in acetone [88]. The unstable complexes have been reported between Ni2+ or Co2+ ions and the receptor 78, which required the use of UV light to achieve the complex formation. The unstable complexes reverted to the colorless spiro form under dark conditions or in the presence of visible light.

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The Cu2+ ion formed a stable complex with the receptor (78) in the dark. The association constant for the unstable complexes of the receptor (78) have been calculated by using the Scheme 2 that yielded 5.8  105 M1 for Ni2+ (Ni(NO3)2) ions and 5.7  105 M1 for Co2+ (Co(NO3)2) ions [88]. The authors have reported an excellent correlation between the photochemistry of the photochromic receptor and the stability of the complex, which have used for calculation of the association constant values [88]. The stable complex has produced an intense emission signal, while the unstable complex has displayed no fluorescence signal. The counter anion present in the metal salt has also influenced the stability of the complex [88].

O

O

O

O

+M2+

O -M2+

O

O

M2+ O

84

O

UV

UV O

O

O

O

O O

O

O

O

O +M2+

O

O

O

-M2+

M2+ O

O

H 3C CH 3

N O

Scheme 22. Isomerization of naphthopyran receptor 84.

NO2

O O 76

N

HN

N

HN

H 3C CH3

N O

NO2

O O

O

77

O

NO2

N O

N

N

HN

N O H3 C

NH HN

CH3

78

O

MeOOC

form have been recorded in the presence of different metal ions [205]. A comparison of the thermal fading rate obtained in the presence of metal ion with the free merocyanine form indicated the formation of a complex between the receptors (80–81) and the metal ions (Cu2+, Fe2+, Al3+). The formation of complex stabilized the merocyanine form in polar solvents like ethanol due to the chelation effect [205]. The metal ions like Cr3+, Mn2+, Ni2+, Co2+, Ba2+ and Mg2+ have displayed no effect on the thermal bleaching rate of the merocyanine form. The EPR spectra of a solution of the receptor–Cu2+ complex at 77 K revealed a strong anisotropy owing to the Jahn-Teller deformation, which indicated a square planar geometry [205].

4.13.7. Photoactive sensor for Pb2+ and Ba2+, Mg2+ ions

4.13.6. Photoactive Cu2+, Fe2+, and Al3+ ion sensor

H3 C CH3 N

N

O

N

CH 2

N

O

N

R

R

O

HO 80: R = CH 3 81: R = C 4 H9

O

O

O

O

O

82c

O Cu2+

CH 2 N

H3 C CH3 H3 C CH3

CH 2

O

N CH 2

N O

O

O

O

82b

N

O

N N N

82a

O

O

R

R

O O

O

O Fe 2+

Al3+

The merocyanine form of the receptors (80–81) have been observed to form a complex with metal ions namely Cu2+, Fe2+, and Al3+ ions, respectively, particularly in the polar solvent. A hypsochromic shift in the absorption spectra of the merocyanine form has been observed by Minkovska et al., which indicated the formation of a stable and strong complex [205]. The highest hypsochromic shift has been noticed in a solution containing Fe2+ ions. The rates of thermal fading of the merocyanine form to spiro

Chebun’kova et al. have developed a substituted morpholine and aza-crown substituted naphthopyran receptors (82a–c). The receptors have been found to exhibit shifts in their absorption spectra in the presence of Mg2+, Ba2+ and Pb2+ ions in acetonitrile [206]. Strokach et al. have studied the metal (Mg2+, Ba2+, Pb2+) ions complexation properties of spirooxazine receptor 83 in acetonitrile using UV–visible and fluorescence spectroscopy [207,208]. The additional substituent on the spirooxazine ring increased the absorption maxima. A decrease in the rates of thermal bleaching of the merocyanine form in the presence of different metal ions has been observed. The rates of thermal bleaching have been correlated to the binding of the merocyanine form to the metal ions.

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P.R. Sahoo et al. / Coordination Chemistry Reviews 357 (2018) 18–49

H 3C CH3

HO

N N

N O CH 3 83

Another chromene receptor 84 have been synthesized by Paramonov et al., which has also been found to detect metal ions like Mg2+, Ba2+ and Pb2+ ions (Scheme 22) [209]. Ions with small size have been found to form 1:1 complex and the atoms with large size formed sandwiched type complexes with respect to two crown ethers. A small hypsochromic shift has observed upon addition of metal ions like Pb2+ along with Mg2+ and Ba2+ ions. 4.13.8. Photoactive sensor for Zn2+, Ni2+, Fe3+, Cu2+, Ce3+ ions

the merocyanine form. The 1H NMR study indicated that the metal ion induced the conversion of the spiro form of the receptor 86 into a colored merocyanine form. The metal ions can interact with donor atoms such as pyridinium nitrogen, phenolate oxygen and the oxygen from the substituted alkyl carboxylic group present in the binding site of the receptor 86. The 2D-NMR identified the TTC stereoisomer–Zn2+ complex in the solution, which has donor atoms present in the appropriate position to coordinate the metal ions and yielded a 2:1 complex stoichiometry. The Job’s plot also indicated the formation of 2:1 (86:Mn+) complex stoichiometry, which has also been observed through mass spectrometry. The stability constant values have been determined using the spectrophotometric data to reveal the formation of strong complex with transition metal ions in the order Cu2+ (log K11: 8.09, log K21 5.18), Zn2+ (log K11: 7.74, log K21 5.59), Ni2+ (log K11: 7.06, log K21 5.69), Co2+ (log K11: 6.87, log K21 5.12), Mn2+ (log K11: 6.63, log K21 5.49).

Cl N N O

N

86

O

HO 85

N

O M

CF3

A trifluoromethylquinoline substituted spiropyran 85 has been developed by Guo et al. [210]. The receptor 85 displayed higher stability of the merocyanine form in polar solvents. The expected higher merocyanine form thermal stability has been observed due to the presence of trifluoromethyl and nitrogen from quinolone moiety [210]. The quinolone moieties are known to coordinate with the metal ions and produce a luminescence signal. Therefore, the receptor 85 has been used for the detection of metal ions using fluorescence spectroscopy. The Co2+ ions produced a quenching response due to spin–orbit coupling, while other metal ion produced an enhancement in fluorescence intensity. The metal ion such as Ni2+, Fe3+, Cu2+, and Ce3+ also produced a fluorescence quenching response along with a large hypsochromic shift. The metal ions like Zn2+ that produced an enhancement in fluorescence intensity, but the presence of such metal ions did not produce any hypsochromic shift. 4.13.9. Photoactive sensor for Cu2+, Zn2+, Mn2+, Co2+ and Ni2+ ions Chernyshev et al. have synthesized a quinoline substituted spiropyran 86, which displayed selective complex formation with Zn2+ and Ni2+ ions [211]. The merocyanine form of the receptor 86 appeared at 560 nm and 600 nm in acetone. The addition of metal ions such as Zn2+, Ni2+, Mn2+, Cu2+ and Co2+ ions shifted the absorption band hypsochromically with an increase in the intensity. The 1H NMR spectroscopy revealed a downfield shift in the N–CH3 and Ar–H signals, which indicated the formation of

4.13.10. Photoactive sensor for Gd3+, Eu3+, Zn2+, Ni2+, Mn2+, Cd2+ ions Sakata et al. have reported a carboxylate derivative of spiropyran receptor 87 to form a complex with Gd3+, Zn2+, Ni2+, Mn2+, Cd2+, Eu3+ ions, which was confirmed through spectroscopic techniques (Scheme 23) [212]. The absorbance and fluorescence studies confirmed that the open form possesses strong chelation tendency towards metal ions. It has been observed that the transition of the receptor 87 from the spiro form to the merocyanine form takes place with low efficiency in the presence of UV light. However, the transition has been facilitated by the presence of metal ions that indicated the affinity of the receptor 87 for metal ions. The coordination of the metal ion to the merocyanine form led to an increase in the intensity of the absorption band of the merocyanine form. The receptor has shown to be optically switchable, where the metal-bound open form can be transformed to the free closed from using 543 nm light source. The reverse transformation from spiro form to open merocyanine form can be accomplished within microseconds using 365 or 720 nm light source. The receptor also proved to be in vitro and in vivo fluorescent active within living cells. The receptor displayed switching response between the merocyanine and the spiro form using 720 nm and 535 nm light sources during in vivo and in vitro studies.

N

N N

350nm N

O

N

N

N Δ

O

N

NO2 N O

NO2

UV Vis

87

-

M2+

N

O-

N

N CO2CO2-

88

O 2C

Scheme 23. The optically switchable spiropyran receptor 87.

-

N

N

M2+

N N

O 2C

O Scheme 24. Formation of the MC–Mn+ complex of the receptor 88.

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O

R1

R1

O N O R2

O

OH

N R2

HO

89

O

O O

O R2 N

HO

O

R1

M2+ O OH

R1

M2+ = Mg2+, Zn2+, Ni2+, Cu2+, Hg2+, Pb2+

N R2 O O

Scheme 25. Metal complexation mode of spiropyran–coumarin conjugate receptor (89).

4.13.11. Photoactive sensor for Zn2+, Fe2+, Co2+, Mn2+, Ni2+, Cu2+ ions Kopelman et al. have developed a photochromic receptor 88, which sensed multiple metal ions namely Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+ (Scheme 24). As far as the electronic stability is concerned, the respective metal ions possess a different number of electrons and hence back bonding issue played an important role. The metal complexation of different metal species was confirmed by thermal equilibrium shift and was observed both in spirooxazine and merocyanine form. Due to the complex formation with the first-row transition metal ions, the thermal equilibrium has been shifted dynamically towards the open merocyanine form, which resulted in a low thermal bleaching rate [213]. A matching evaluation of magnetic properties and electronic coupling between photochromic and di-cationic metal spin states strengthened the concept of spirooxazine–merocyanine equilibrium. In addition, magnetic properties of the merocyanine–metal complex have also been investigated. The Magnetic susceptibility studies have suggested the magnetically dilute nature of the sample, which indicated the presence of a distorted octahedron coordination sphere. The metal complexes have also displayed contributions from zero-field splitting. In addition, the merocyanine–Ni or Co complexes have displayed a higher degree of anisotropy, which indicated the desymmetrization of the ligand field.

O O 2N O O O O Si O O O O

Si

5

O

H 3C CH3

N O CH3

R N

91. R=Br 92. R=Cl

N C 2H4 O Br m

n

O

4.13.12. Photoactive sensor for Zn2+,Co2+, Hg2+,Ni2+,Cu2+ along with Pb2+ or Mg2+ ions In 2009, Nikolaeva et al. have reported the binding affinity of receptor 89 towards different metal ions (Scheme 25) [214]. A shift in the equilibrium towards the open form has been observed in the presence of metal acetates or magnesium perchlorate. The hydroxyl moiety along with phenolate oxygen has been involved in coordinating the metal ions. A set of binding studies have been performed by varying the two different R1 and R2 groups. It has been observed that the methyl group (in both R1 and R2) facilitated the stabilization of the open isomer in metal-free solution. The presence of a nitro group at R1 position (and R2 as methyl) decreased the lifetime of the merocyanine form to a large extent. The complex formation was monitored by UV–visible spectroscopy. The hypsochromic shift of the absorption maxima was found to be dependent on the nature of the metal ion. Fries et al. have developed a spiropyran polymer brush 90 using a spiropyran methacrylate analog and methyl methacrylate (Fig. 13) [215]. The isomerization of spiropyran embedded polymer brush produced its merocyanine counterpart, which is extremely polar (due to high-density molecules and hence the large change in dipole moment) and therefore influenced the surface free energy. The surface free energy induced reversible wettability of the surface. The polymer brush on treatment with metal ions facilitated the contact angle switching and finally surface wettability. The polymer embedded films have been subjected to UV light (365 nm) exposure and then treated with various metal ions to observe distinct color change visible to the naked eye (Fig. 13). Due to complexation with various metal ions, hypsochromic shifts of an absorption band with a decrease in the absorbance were noticed. The polymer films underwent metal decomplexation under visible light exposure (30 W quartz halogen lamp) in a non-polar solvent like toluene within 10 min.

Br m O

5 O

n O O O C2 H 4

O 2N O

N

90

Fig. 13. The UV–Visible absorbance of spiropyran embedded polymer brush 90 in presence of various metal ions [215]. Reproduced from [215] with permission of The Royal Society of Chemistry.

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P.R. Sahoo et al. / Coordination Chemistry Reviews 357 (2018) 18–49

R2

R2 N O

H 3CO

N O





N

N

R1 R1

H 3CO

Ph

N

Ph

O

=

R1

O

+M2+ R2

N O N M 93

Ph

O

O N

M2+



+M2+

R2 = H

N

R2

95

N

O

O

R1

O

H 3CO

M

N

O

Scheme 26. Interaction of spiropyran receptor 93 with metal ions.

Voloshin et al. have synthesized substituted spiropyran receptors 91–92 as fluorescent chemosensors for multiple metal ions [216]. The spiropyran receptors under UV light irradiation (365 nm mercury lamp) produced merocyanine isomer with kmax at 598 nm and 561 nm for 91 and 92, respectively, along with weak fluorescence with a quantum yield of 0.001 (kmax at 625 nm) in acetone. Insertion of metal ions like Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Mg2+ to the spiropyran receptors produced significant absorption band at 533–573 nm. The receptors also produced remarkable fluorescence at 610 nm with Zn2+, Cd2+ and Mg2+ metal ions in acetone. The metal–ligand complexation stoichiometry has been reported to be 1:2 as evidenced by Job’s plot. Chernyshev et al. have reported a spiropyran based receptor 93, which has displayed two different types of complex stoichiometry with different metal ions (Scheme 26). The receptor (93) has displayed a 1:1 complex stoichiometry with Zn2+, Cd2+, Mn2+ ions, while 1:2 (host to guest) complex stoichiometry has been exhibited with Co2+, Ni2+, Cu2+ ions. In addition, the receptor (93) displayed a negative photochromism, when complexed to Cd2+ and Zn2+ ions [217]. The complex formation between the receptor (93) and the metal ions can be detected by using fluorescence and UV–Visible spectral changes. A bathochromic shift has been observed with the methoxysubstituted spiropyran. The heavy metal ions played a crucial role in the transformation of the spiro form to the merocyanine form and vice versa. The appearance of a hypsochromic shift suggested the formation of a complex between merocyanine and a metal ion. In the case of Cu2+ ions, the stability of 1:1 complex has been found to be higher (log K as 5.21 ± 0.28) and for Cd2+ ions, the lowest value (log K; 3.97 ± 0.12) has been observed. The receptor was highly sensitive to the presence of Cd2+ and Zn2+ ions and produced an optical signal (luminescence) with maxima in the 620–660 nm region. The complexes have been observed to be very sensitive to the presence of visible light. Fabrication of polymer units with the dipole enriched merocyanine architectures or dyes has inculcated interesting properties in the material, which can be exploited for the development of a worthier environment for metal ion monitoring. Similarly, Fries et al. have assembled a copolymer supported photoreversible sensor 94, which displayed complexation tendency towards metal ions such as Cu2+, Fe2+, Zn2+, Co2+, and Ni2+ [218].

O N

Ph

M2+ Scheme 27. Binding sketch of spiropyran receptor 95 in presence of metal ions.

Fig. 14. (1) The absorption spectra of the receptor 97 (30 mM) and upon addition of (2) Eu3+ ions (80 mM) (3) Tb3+ ions (80 mM) in acetonitrile. Reproduced from the Ref. [222], Hindawi Publishing Corporation.

Fig. 15. The fluorescence spectra of (1) receptor 97 (30 mM) + Eu3+ (80 mM) in DMSO (2) receptor 97 (30 mM) + Eu3+ (80 mM) in acetonitrile using 380 nm excitation wavelength. Reproduced from the Ref. [222], Hindawi Publishing Corporation.

n

O2 N

Br m O O O

O C 2H 4 O N

94

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P.R. Sahoo et al. / Coordination Chemistry Reviews 357 (2018) 18–49

N

O O

Mn+

N O

N N

O

N

Mn+

N

O

O

O

n

Mn2+: La3+, Eu3+, Tb3+, Pb2+

O

O O

Mn+

n n = 1, 2

O

Mn+

O N

N

N

O

O M

N O

Mn+: Mg2+, Ca2+, Ba2+

N

O

M

O N

n+

N

O N

O

n

Mn+

O

O

N

O

n+

n

Mn+ O

O

O

n

Mn+

Scheme 28. The complex formation between the receptor 98–99 and different metal ions [223].

Fig. 16. A partial 1H NMR spectra of the receptor 98 ([98] = 5.0  103 M) upon addition of one equivalent lead ions in acetonitrile-d3. Reproduced with the permission of the publisher (John Wiley and Sons) [223].

The spiropyran copolymer fabricated thin film on irradiation with UV light (365 nm) displayed a bathochromic shift in the absorption band from 400 nm to 584 nm, which indicated the possible formation of merocyanine isomer. When the copolymer bound thin films were subjected to metal ions like Zn2+, Co2+ and Ni2+ ions, a blue shift in the UV–Visible absorbance spectra has been noticed along with a reduction in the absorbance intensity owing to the loss of planarity. The copolymer thin films with Fe2+ metal ion as guest produced a broader absorption band at 520 nm with a shoulder at 437 nm, whereas the Cu2+ ions as guest produced absorption bands at 520 nm as well as 410 nm. The synthesized copolymer has produced a distinct colorimetric response with each metal ion.

Chernyshev et al. have observed the development of color, when metal ions like Mn2+, Co2+, Ni2+, Zn2+, Cu2+, and Cd2+ ions were mixed with a solution of spiropyran derivative 95 (Scheme 27). A shift in the wavelength at kmax of the receptor 95 has been observed due to the complex formation in the dark [217]. The color of the solution discharged after addition of EDTA. The spiropyran has been found to form 1:1 and 1:2 types (metal– ligand) complex in acetone, which has been monitored via electron absorption spectroscopy and electrospray ionization mass spectrometry. The closed form has been observed to display an intense absorption band at 303 nm and two weak long wavelength charge transfer bands at 383, 403 nm. The equilibrium shifted towards the merocyanine isomer through exposure to UV light at a temperature

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P.R. Sahoo et al. / Coordination Chemistry Reviews 357 (2018) 18–49

between the spiropyran (96) and metal ions proceeded through two coupled steps that involved ring opening of the spiropyran, followed by metal coordination. In general, a 1:1 (metal to ligand) complex geometry has been observed. The thermal bleaching process followed first-order decay kinetics. The authors have calculated the thermal bleaching rate constants from the decoloration process of the receptor–metal complex. The rate constant values for the receptor 96 with Ni2+ and Pb2+ ions were observed to be 3.0  107 dm3 mol1 s1 and 1.2  109 dm3 mol1 s1, respectively, which indicated strong interaction of metal ions with merocyanine form. The merocyanine–metal complexes have also displayed intense fluorescence properties. The comparison of the fluorescence of the free merocyanine form and merocyanine–metal complex indicated a 90 nm blue shift. Low quantum yields have been observed for Co2+ and Cd2+ ions complexes with 96, while significantly higher quantum yields have been reported for Zn2+, Cd2+ and Pb2+ ion complexes. 4.13.13. Photoactive sensor for multiple lanthanide ions

OH N O

CHO

97

Fig. 17. The SERRS spectra of 98 with Eu3+ (a) [Eu3+]/[98] = 1 (b) [Eu3+]/[98] = 10 (c) free 98 (d) [Eu3+]/[100] = 10; (e) free 100. Reproduced with the permission of the publisher (John Wiley and Sons) [223].

below 273 K, particularly at 258 K. The colored merocyanine has been confirmed by the presence of an absorption band at 600 nm. The open form gets transformed into closed spiropyran in the dark.

N O 96

NO2

MeO

Chibisov et al. have synthesized a substituted receptor 96 and evaluated the complexation properties with various transitions and rare earth metal ions in acetone using steady-state and timeresolved spectroscopy [219–221]. The complexation process

The receptor 97 has been synthesized and used in the complex formation with lanthanide metal ions like La3+, Eu3+, Sm3+, and Tb3+. A solution of the receptor 97 displayed absorption bands at 255 nm and 520 nm in acetonitrile (Fig. 14). The study revealed that the closed form of the receptor (97) displayed absorption band at 255 nm, while the absorption band at 520 nm indicated the presence of the open merocyanine form [222]. The intensity of the absorption bands can be altered through the use of light. The addition of metal ions to a solution of the closed spiro form of the receptor (97) produced the open merocyanine form of the receptor (97) that formed a complex with the metal ions [222]. The formation of a complex between the metal ion and merocyanine form of the receptor shifted the absorption band hypsochromically in acetonitrile. The changes in the absorption band have been used to calculate the association constant between the receptor and the lanthanide ions. The order of the association constant values been determined to be Tb3+ (58  103 M1) > Eu3+ (56  103 M1) > Sm3+ (55  103 M1) by Attia et al., which indicated moderate selectivity of the receptor 97 towards Tb3+ ions [222]. In addition, the fluorescence spectroscopy has been used to investigate the formation of a complex between the receptor and the lanthanide ions in acetonitrile or DMSO using 380 nm excitation wavelength (Fig. 15). Upon addition of metal ions to the receptor solution, an enhancement in the fluorescence intensity has been reported [222]. The fluorescence quantum yields have been

Fig. 18. Change in the color of the receptor 102 (5.0  105 M) upon addition of nitrate salt of different metal ions in acetonitrile. Reproduced from [224] with the permission from Elsevier.

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P.R. Sahoo et al. / Coordination Chemistry Reviews 357 (2018) 18–49

N N N

N O

O

O

O

NO2 O 2N

O

O

O O

O

OH OH

NO2 O2 N

O

O

O

O

O

OH OH O

O

102

N N

O

O Eu3+

NO2 O 2N

O

O O

O

O OH OH

O

Scheme 29. Complex formation of calixarene–spiropyran conjugate receptor 102 with cations.

Fig. 19. Partial 1H NMR spectra of the receptor (102) recorded at 400 MHz in CD3CN (a) Free receptor 100 (b) 2 h after addition of Eu3+ ions in dark (c) 18 h after addition of the Eu3+ ions in dark. The peak marked with ‘m’ represents the merocyanine structure peaks. Reproduced from [224] with the permission from Elsevier.

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spirooxazine chelators (98–101) produced an open merocyanine form induced by Pb2+ and lanthanides such as La3+, Eu3+, Tb3+ ions, while the alkaline earth metal ions formed a complex with the closed form of the receptors (98–101) (Scheme 28). The merocyanine form of the receptors has been observed to produce two types of the complex with the metal ions (lead and lanthanide ions). In the presence of an equimolar concentration of metal ions, complexes with crown ether moiety have observed, while in the presence of excess metal ions both phenolic oxygen atom of the merocyanine form and crown ether moieties have been involved in the complex formation. The addition of the lead and lanthanide ions to the receptors 98–99 increased the stability of the merocyanine form and produced a bathochromic shifted absorption band of the merocyanine form. The closed spiro form resurfaced with the visible light exposure [223]. The 1H NMR (Fig. 16), UV–Visible and SERRS (Fig. 17) spectroscopic techniques have been used to evaluate the binding of the metal ions to the receptors. The 1H NMR spectra recorded for the receptor (100) revealed the formation of merocyanine form and disappearance of the closed form upon addition of lead ions [223]. Keeping a solution of the receptor (100) and lead ions produced a stable complex (Fig. 16). Similar patterns have also been reported for other receptors by the authors. The association constants have been calculated using UV–visible spectroscopy for the receptors (98, 100) and metal (lead, magnesium) ions. The receptor 98 produced 1:1 complex geometries [(For 98; log K11 = 0.6 (Mg2+); 2.8 (Pb2+)], while the receptor 100 produced both 1:1 and 1:2 geometries with lead ions [(For 100; log K11 = 2.5 (Mg2+), 4.0 (Pb2+) along with log K12 = 4.4 (Pb2+) where K12 refer to 100:2Pb2+ association stoichiometry] [223]. The complexation between Eu3+ and the receptor 98 has been investigated by SERRS technique by the authors (Fig. 15). The Raman bands due to the presence of the open merocyanine form of the receptor 98 that included indoline and naphthalene units have been observed to dominate the SERRS spectra [223]. However, the vibrations due to the azacrown ether have not been observed due to the absence of any resonance with the electronic transitions. The transoid forms [ttc (trans–trans–cis) and ctc (cis– trans–cis)] have been observed through the SERRS spectra, which have been characterized by the presence of vibration bands at 1540 cm1 and 1558 cm1 [223]. The addition of Eu3+ ions (M to L ratio varied from 1 to 10) to the receptor 98 has produced a

Fig. 20. The change in fluorescence intensity of the receptor 102 (20 mM) upon incremental addition of Eu(NO3)3 in an acetonitrile solution; [Eu(NO3)3] = 0–80 mM. Reproduced from [224] with the permission from Elsevier.

calculated for metal ions–receptor complex in DMSO that followed the order Tb (UL = 0.056) > Eu (UL = 0.052) > Sm (UL = 0.047) [222]. However, higher quantum yields in PMMA (Tb = 0.108 and Eu = 0.085) have observed, which indicated that the relaxation pathway like cis–trans isomerization was hindered in the rigid media [222].

N N O O

N

N 98. n = 1 99. n = 2

O

O

100 : R = H

N O

O

101 : R =

n

N

O

R

Fedorova et al. have synthesized aza-crown substituted spirooxazine chelators 98–101 and evaluated them for affinity towards the lead, lanthanide metal and alkaline earth (Mg2+, Ca2+, Ba2+) metal ions in acetonitrile [223]. The substituted

Cu2+ selective (31)

Fe3+ selective (45) N H HO

R2 =

R3 & R 4 = N

R1 = OCH3 R2 = NO2

R1

R1 = R2 = OCH3

Zn2+ selective

N O

(4)

R2 R3

Gd3+ selective (62)

R4 R4 = OH

N O R2 = 2+

Co

NH

Hg2+ selective (43)

N N

selective (42)

Fig. 21. Role of substituent over a spiropyran on the metal ion selectivity.

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Pb2+ selective (55) SO3Zn2+ selective (3) R3 = COOH SO3S R3 = 2+

Zn

selective (2)

N

R1

R2 =

N O R5

O R5 =

N

R3

O R1 = OCH3

R2

NH2 Tb3+ selective (66)

R4

N N

Ni2+ selective (57)

Eu3+ selective (65)

Fig. 22. The modification of the substituent over a spirooxazine to achieve the desired metal ion selectivity.

change in the relative intensities and positions of vibrations bands observed in the SERRS spectra. No changes in the SERRS band intensities or positions have been observed when the metal to ligand ratio was increased beyond 10. In addition, the increase in the metal ion concentration stabilized the ctc stereoisomer of the receptor 98 [223]. The Liu et al. [224] have synthesized a calixarene derivative conjugated at the lower rim to the two spiropyran units (102). The synthesized receptor has been evaluated for affinity towards lanthanides (La3+, Pr3+, Eu3+, Gd3+, Er3+), transition (Fe3+, Cu2+, Zn2+), alkali (Na+, K+) and alkaline earth (Mg2+, Ca2+) metal ions. The receptor (102) has been observed to interact with lanthanide metal ions such as La3+, Pr3+, Eu3+, Gd3+, Er3+ and produced a color change to yellow from purple observable to the naked eye (Fig. 18). The results indicated that the receptor produced an open merocyanine form in acetonitrile that binds the lanthanide metal ions (Scheme 29) [224]. The UV–Visible spectroscopic studies have indicated that the merocyanine form of the receptor 102 displayed an absorption band at 550 nm, which shifted hypsochromically upon addition of different metal ions. The magnitude of the UV–visible spectral shift was higher for lanthanide metal ions, while no spectral shift has been observed in case of alkali, alkaline earth or transition metal ions [224]. Addition of lanthanides to a solution of the receptor (102) has produced a 68–84 nm UV–visible spectral shift. The magnitude of spectral change have been observed to follow the following order Yb3+ > Er3+ > Gd3+ > Dy3+ > Eu3+ > Pr3+ > La3+ that indicated that the recognition has been subject to the size-fit effect [224] . The formation of a complex between the receptor (102) and the lanthanides has been further investigated using 1H NMR spectroscopy (Fig. 19). The addition of Eu3+ ions to a solution of the receptor (102) led the reduction in the signal intensity around d 6.0 ppm, which indicated the disappearance of the spiro form [224]. The downfield shift in the NCH3 signal has also indicated the presence of the merocyanine form in the complex [224]. The open merocyanine isomer has also responded to the presence of visible light (475 nm) and produced an intense fluorescence band at 555 nm (Fig. 20). The stereoisomers of the open

form of the receptor 102 interacted with lanthanide ions in the dark and produced a hypsochromic shift of 68–84 nm as an output with a profound intensity of absorption in the visible region of the spectrum and also produced a difference of 42 nm (with Er3+) in emission spectra [224]. Specifically, lanthanide metal ions containing complexes have also displayed vibrant fluorescence responses. An enhancement in intensity upon incremental addition of lanthanide ions has been observed (Fig. 18). However, complexes of alkali metal ions like Na+, K+, alkaline earth metal ions like Mg2+, Ca2+ and transition metal ions like Fe3+, Cu2+, Zn2+ did not respond to the presence of UV or visible light [224].

5. Role of substituents over the photochrome in metal ion selectivity As depicted in the discussion in different sections of this review, the development of a selective photoreversible receptor for metal ion require an appropriate substituent for good photochromic property and metal ion specificity [15]. The substituents play a crucial role in imparting the selectivity to the receptor towards a specific metal ion by creating an appropriate guest cavity with the help of donor atoms [86]. The substituents also play a crucial role in imparting the good photochromic behavior to the receptor (Fig. 20). The change in the electron donating or withdrawing ability of the substituents has a bearing on the photochromic properties of the receptor molecule [186]. Therefore, in order to develop a suitable photoreversible receptor for a particular metal ion, it is important to consider the suitability of the photochromic unit as well as the substituent at the suitable position to achieve desired selectivity [186]. The shape of the photochrome allows different ligands to append it to coordinate the metal ions. The alteration in the structure allows for the binding and release of the bound metal ions either thermally or using the light of appropriate wavelength [225]. Several receptors were cited in this review where a change in the substituent led to the change in the selectivity for the metal ion [186]. The side chain can be altered easily over the photochromic unit to achieve the desired change in the selectivity towards a metal ion (Figs. 21 and 22).

P.R. Sahoo et al. / Coordination Chemistry Reviews 357 (2018) 18–49

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