Electrical functions of photochromic molecules

Electrical functions of photochromic molecules

Journal of Photochemistry and Photobiology C: Photochemistry Reviews 11 (2010) 1–14 Contents lists available at ScienceDirect Journal of Photochemis...

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Journal of Photochemistry and Photobiology C: Photochemistry Reviews 11 (2010) 1–14

Contents lists available at ScienceDirect

Journal of Photochemistry and Photobiology C: Photochemistry Reviews journal homepage: www.elsevier.com/locate/jphotochemrev

Review

Electrical functions of photochromic molecules Tsuyoshi Tsujioka a,∗ , Masahiro Irie b a b

Department of Arts and Sciences, Faculty of Education, Osaka Kyoiku University, Asahigaoka 4-698-1, Kashiwara, Osaka 582-8582, Japan Department of Chemistry and Research Center for Smart Molecules, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan

a r t i c l e

i n f o

Article history: Accepted 3 November 2009 Available online 6 February 2010 Keywords: Photochromism Diarylethene Electrical function Current switch Isomerization

a b s t r a c t Recent progress in electrical properties of photochromic molecules is reviewed. A typical application of the properties is current switching based on the changes in ionization potential or carrier mobility induced by the photoisomerization of photochromic molecules. Carrier injection-type molecular memories have also attracted wide interest because they are a promising candidate for organic semiconductor devices in the field of organic electronics. Various new applications are proposed using photo-induced electrical, as well as optical, property changes of photochromic molecules. © 2010 Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photonics, electronics, and photochromism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Changes in electrical properties by photoisomerization (frame 2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Evaporated amorphous/dye-doped polymer films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Conductivity photoswitching at the single-molecule level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Conducting polymers with photochromic units in the main chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electric property changes by electrically induced isomerization (frame 3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Optical property changes by electrically induced isomerization (frame 4) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Tsuyoshi Tsujioka received his MS degrees from Osaka City University in 1985 and his PhD degree from Kyushu University in 1997 on high density optical memory using photochromic diarylethenes. He commenced research and development (high density memory and organic light emitting devices) in SANYO electric, Co. Ltd. in 1985. In 2002, he was appointed Associate Professor at Osaka Kyoiku University, and Professor in 2003. His interest is in photochromism and its application to electronics and photonics.

∗ Corresponding author. Tel.: +81 729 78 3633; fax: +81 729 78 3633. E-mail address: [email protected] (T. Tsujioka). 1389-5567/$20.00 © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jphotochemrev.2010.02.001

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Masahiro Irie received his BS and MS degrees from Kyoto University and his PhD Degree from Osaka University. He joined Hokkaido University as a research associate in 1968 and started his research on photochemistry. In 1973 he moved to Osaka University and developed various types of photoresponsive polymers. In 1988 he was appointed Professor at Kyushu University. In 2007, he moved to Tokyo and is now professor at Rikkyo University. He has been conducting research on photochromic molecular systems for the last 30 years. In the middle of the 80’s he invented a new class of photochromic molecules, diarylethenes, which undergo thermally irreversible and fatigue resistant photochromic reactions and are widely used as key elements of optical switches. He is currently interested in developing single-crystalline photochromism of the diarylethene derivatives.

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1. Introduction Photochromism is defined as a reversible change in color of a chemical species upon irradiation with the appropriate wavelength of light. The color change is caused by a reversible isomerization of the molecule [1,2]. In addition to such a color change, various other properties including refractive index [3,4], fluorescence [5–7], magnetic property [8,9], conductivity [10,11], crystal shape and surface morphology [12–15], shape of elastomer [16], hydrophobicity [17], and/or metal-deposition properties [18–21] change as a result of the photoisomerization. In addition to optical data storage [22–26], interest in photoswitching of conductivity or current photoswitching has been growing in recent years as a result of the dramatic growth in research and development in the field of organic electronics. The new application of photochromism was initiated by Yokoyama and Homma. They, for the first time, demonstrated that photoisomerization of diarylethene derivatives can switch the electrical current in solid film devices [10,11]. Here, we survey recent developments related to this aspect of photochromism.

2. Photonics, electronics, and photochromism As mentioned, photochromism is defined as a reversible change in color in response to light illumination. This definition can be expanded to reversible changes in optical or electrical properties of bistable molecules by various stimuli. In this expanded definition, both optical and electrical stimuli can cause various optical, as well as electrical, property changes, as shown in Fig. 1. Conventional photochromism, that is, change in optical properties such as absorption spectra or refractive indices, is located in frame 1. This area is out of the scope of the current review. (Refer to references [1], and [22–26].) Frames 2, 3, and 4 are new properties and applications discussed in this review. In frame 2, photoisomerization is used to induce the change in electrical properties. For example, two isomers of photochromic molecules have different ionization potentials. The difference affects the electrical conductivity of the photochromic

layer. The property change can be used to photocontrol the electrical current switching. Photoswitching of organic light-emitting devices and a nondestructive readout based on the photocurrent detection using this principle will be described. Frame 3 involves isomerization of molecules by the action of an electrical current, for example, by injection of carriers. The concept of electrically induced isomerization reported here may contain some kinds of electrochromism but the chromism without isomerization is excluded. The isomerization induced by the electrical current causes changes in electrical properties. This effect is typically applied in organic semiconductor memory. Various applications based on the above effects are introduced in detail. Frame 4 involves the optical property changes caused by electrically induced isomerization. No potential application of this process has yet been reported.

3. Changes in electrical properties by photoisomerization (frame 2) 3.1. Evaporated amorphous/dye-doped polymer films Electrical current photoswitching based on photochromism has been most widely studied. Difference in ionization potentials between two isomers changes the conductivity [10,11]. Frame 2 in Fig. 1 illustrates the principle of the current switching based on this effect. A photochromic diarylethene layer is sandwiched between two electrodes, and an electric field is applied. As a result, electrical carriers are injected into the layer. The difference between the ionization potential of the photochromic layer and the work function of the metal electrode produces a potential barrier to the carrier injection. Most organic photochromic materials exhibit a better mobility for holes than for electrons. Therefore, the difference in the potential barrier for holes can switch the current with light. On–off control of a current based on the isomerization of photochromic diarylethene films has been investigated. Fig. 2 illustrates a device structure with a diarylethene 1-doped polystyrene layer and its voltage–current characteristics. The current does not flow in the layer in the uncolored state (1A), while it does flow in the colored state (1B). Clear on–off characteristics have been demonstrated.

Fig. 1. Four frames in optical/photonic and electrical properties and their applications.

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Fig. 2. Device structure and on–off characteristics in current flow based on photoisomerization of the diarylethene. 1A and 1B indicate uncolored and colored states, respectively. (Reproduced from Ref. [10].)

Molecular structure dependence of on–off control for several diarylethene derivatives is shown in Fig. 3 [27]. In the colored photostationary state obtained upon irradiation with 365-nm light, amorphous films of the diarylethenes (Fig. 3(A)) formed by vacuum evaporation have similar Ip values in the range of 5.7–5.8 eV, while in their uncolored (open-ring) states, Ip values are higher than 6.2 eV. To investigate the current injection/transport characteristics of these films, samples composed of a hole-injection/transport buffer layer (NPB), a photochromic diarylethene layer, and a cathode layer (Mg) were prepared on an indium–tin oxide (ITO) anode substrate by a vacuum evaporation method. Fig. 3(B) shows

the voltage dependence of the injected current for a cell containing a layer of diarylethene 2. In the colored state, the current increases with the applied voltage, whereas there is no current flow when 2 is in its uncolored state. The hole-transport characteristics of the diarylethene derivatives in their colored states are shown in Fig. 3(C). Diarylethenes 2, 3, and 4, which contain triphenylamine groups, conduct a current in the colored states, whereas, despite having similar Ip values, the films of 5 or 6, which lack such groups, do not conduct a current. The electrical conductivity characteristics are dependent on two factors: the ionization potential barrier for carrier injection into the layer, and the mobility of carriers. Since the

Fig. 3. (A) Photochromic diarylethenes investigated in Ref. [27]. (B) Current–voltage properties of a photochromic device with a diarylethene 2 layer in the colored and bleached states. (C) Current–voltage properties (hole-transport characteristics) of various diarylethenes in the colored states. (Reproduced from Ref. [27].)

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Fig. 4. (A) Device structure with a diarylethene 3 layer. (B) Plots of the current against the square root of the voltage for the device with and without irradiation. The inset shows the time of UV irradiation. (C) Mechanism for the decrease in the potential barrier in the device on UV irradiation. (Reproduced from Ref. [30].)

ionization potentials of the colored states of diarylethene derivatives are similar (i.e., the potential barriers for carrier injection are similar), the difference in electrical conductivity is considered to be controlled by another factor, that is, the mobility of holes. As mentioned, materials with triphenylamine groups show good hole-transport characteristics [28,29]. Diarylethene derivatives containing triphenylamine groups exhibit high hole mobility.

It is worth noting that, even in the photostationary state, the proportion of colored isomers to uncolored ones is not high. The colored isomers are randomly distributed in low concentration in the uncolored isomers and act as carrier traps. Charge carriers injected into the photochromic layer are transported through the layer by a hopping mechanism, and may obey the Poole–Frenkel (P–F) mechanism [30]. Fig. 4(A) shows the structure of a device for

red Fig. 5. (A) Synthetic process of diarylethenes 7 to 11. (B) Plot of the reduction potentials (E1/2 , V vs Ag/AgCl) versus the calculated LUMO energy for diarylethenes. The dotted

line (a) denotes correlations for the first reduction potential in the −1 V region for both the closed and open isomers of compounds 8, 9, and 11. The dotted line (b) denotes the second reduction potential in the −0.7 to +0.4 V region for the closed isomers of the same molecules. (Reproduced from Ref. [31].)

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Table 1 Electrochemical data for open- and closed-form diarylethenes obtained before and after UV irradiation, respectively. Sample

Ered (E)a (V)

2o 2c 3o 3c 6o 6c

−1.07 (0.33) −0.82 (0.15), −0.61 (0.07), −0.4 (0.08) −1.03 (0.28) −0.75 (0.11), −0.49 (0.18), −0.37 (0.21) −1.03 (0.12) −0.84 (0.17), −0.64 (0.07), −0.36c

Eox (V) c

>1.5 >1.5 >1.5c >1.5 0.92 (0.12), 1.1d , >1.5c 0.89 (0.1), 1.1d , >1.5c

HOMO (eV)b

LUMO (eV)b

 (LUMO–HOMO) (eV)

−8.92 −8.72 −9.508 −9.12 −8.64 −8.58

−1.54 −2.22 −1.74 −2.65 −1.51 −2.18

7.38 6.5 7.77 6.47 7.13 6.4

(Reproduced from Ref. [31].) a Anodic-cathodic peak separation. b Calculated using the AMI semi-empirical method. Open and closed isomers are represented as o and c. formed before (dark) and after UV exposure, respectively. c Irreversible process. d Quasi-reversible process.

investigating the carrier-transport mechanism in a diarylethene 3 layer. Logarithmic plots of the current upon UV irradiation against the square root of the applied voltage are shown in Fig. 4(B). The parallel shift in the straight-line plot upon UV irradiation indicates that the device obeys the P–F mechanism. Fig. 4(c) illustrates the P–F hopping mechanism. The mobility of charge carriers in organic layers is not solely determined by the molecular structure. Various factors affect the carrier mobility. Electron affinity, molecular packing and orientation, ␲-conjugation and ionization potential contribute to the mobility. The current-conduction mechanism in photochromic dye-doped polymer films is complex. Therefore, in practical experiments, all effects are totally taken into account simply by measuring the change in mobility produced by the isomerization reactions. HOMO and LUMO levels of photochromic molecules directly affect their current–voltage (I–V) characteristics, and are therefore very important for the current switching. Diarylethenes with donor–acceptor structures have been synthesized, and the relation between their reduction potential (electron affinity), LUMO levels, and conductivities have been investigated [31]. Fig. 5(A) shows the synthetic procedure of such diarylethene derivatives. The reduction potentials of several diarylethenes 8, 9 and 11 were determined by cyclic voltammetry. The correlation

between the potentials and LUMO levels and related electronic values including HOMO/LUMO levels are shown in Fig. 5(B) and Table 1. The plot indicates that the reduction potential is strongly dependent on the nature of the substituents around the diarylethene molecular backbone. The slopes of the plot for the first and second reduction potentials against the LUMO levels are almost similar. To test the current-switching capability of compound 11, a device shown in Fig. 6 was prepared and its current–voltage (I–V) property was studied. The applied voltage dependence of the electric current for cells containing a different concentration of diarylethene 11 is shown in Fig. 6(a), (b) and (c). The slope of the I–V curve significantly increased when the cell was irradiated with UV light. The current of the UV-irradiated cell at 2 V was three times larger than that of the bleached cell. This illustrates that ␲electron conjugation between the donor and the acceptor groups in the diarylethene is delocalized throughout the molecule in the closed-ring isomer, resulting in a higher current response than that in the open-ring isomer. The charge-transport mechanism in poly(2-methoxy-5(2 ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV), a typical semiconducting polymer, doped with the photochromic spirooxazine (1,3,3-trimethyl-1,3-dihydrospiro[indole-2,2 -phenanthro

Fig. 6. (Top) Schematic representation of a device incorporating diarylethene 11. (Bottom) The current–voltage plots for the device with films comprising polystyrene and diarylethene 11 in ratios of (a) 90:10 and (b) 70:30 at −2 to +2 V, and (c) 70:30 at −5 to +5 V. The dashed line corresponds to films kept in darkness, the solid line corresponds to films subjected to UV irradiation at 365 nm, and the dotted line corresponds to films subjected to subsequent irradiation with visible light. (Reproduced from Ref. [31].)

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Fig. 7. (A) Structures of spiropyran dyes 12, 13, silicon polymers and oligomers. (B) Dark current–voltage curves for an ITO/mMPSi-SP/Al sandwich sample before and after UV irradiation. (1) initial state; (2) after UV irradiation; (3) at 3 h, and (4) at 24 h after irradiation, with relaxation at room temperature. (Reproduced from Ref. [33].)

[9,10-b][1,4]oxazine] (PIII)) has been studied [32]. The charges are occasionally trapped by the photochromic molecules during migration and the current is switched upon photoirradiation.

Theoretical as well as experimental studies have also been carried out for spiropyran-doped ␴-conjugated silicon polymers [33–37]. The chemical structures of the polymers (pMPSi) and dopants (SP, 12 and 13) are shown in Fig. 7(A), and the I–V charac-

Fig. 8. Mechanism of photo-induced photon transfer in a three-state molecular switch based on polyaniline containing a photochromic dye. (Reproduced from Ref. [38].)

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teristics in Fig. 7(B) [33]. The curve of the undoped sample (curve 1 in Fig. 7(B)) is ohmic up to 2 V, but shows a superlinear dependence at higher voltage. When the sample was irradiated with 340-nm light to isomerize the dye to its metastable polar merocyanine (MR) form, the current decreased by about one order of magnitude. This change is reversible under a vacuum at room temperature. The current again increased to the values measured before illumination in the dark, with a time constant approximately equal to that of the thermal back reaction. This result was interpreted by assuming a reversible creation of traps for the current carriers during illumination of the samples and subsequent annihilation of the traps during the storage of the samples in darkness. Spiropyran-doped polyanilines also exhibit reversible photoinduced change of electrical conductivity [38]. The conductivity of the polyanilines is dependent on protonation, and the protonation is controlled by changing the photoisomerization of the spiropyran dyes (Fig. 8). The merocyanine (ME) form of the dye, generated by UV irradiation, abstracts a proton from the polyaniline salt, to give a protonated merocyanine (MEH) form of the dye. Upon irradiation with visible light, MEH releases the proton, which is captured by the polyaniline to restore the degree of protonation of the polymer. The electrical conductivity changed from about 0.75 S cm−1 to about 0.31 S cm−1 after UV irradiation for 10 min, as shown in Fig. 9. Upon irradiation with visible light for 8 h, the electrical conductivity was restored to almost its initial value. Thus, UV and visible-light irradiation induces reversible change of the electrical conductivity. Electrical properties of azobenzene-substituted polythiophene derivatives have also been studied. The conductivity was found to change upon photoirradiation; the conductivity of 1.1 × 10−7 S cm−1 before UV irradiation increased to 8.3 × 10−7 S cm−1 after visible-light irradiation [39]. 3.2. Conductivity photoswitching at the single-molecule level One of the favorite properties of photochromic materials is that their optical as well as electrical properties can be switched upon photoirradiations even at the single-molecule level. Photoswitching of fluorescence of photochromic molecules can be detected

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Fig. 9. (A) Changes in the electrical conductivity of a thin film of polyaniline doped with the SP form of spiropyran dye on UV irradiation, followed by storage in darkness for 2 h and then visible-light irradiation for 8 h. (B) Changes in electrical output on UV irradiation (l 1) and visible-light irradiation (l 2) over three consecutive switching cycles. (Reproduced from Ref. [38].)

Fig. 10. (A) Photochromic molecular switch between two gold contacts. On exposure to light at 500–700 nm, the molecule switches from the closed state (a) to the open state (b). (B) (a) Current–voltage plot of the closed-ring molecule. (b) Resistance–time plot with visible irradiation. (c) Typical current–voltage plot after switching by irradiation at 546 nm. (Reproduced from Ref. [41].)

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Fig. 11. (A) Molecular bridges between the ends of SWNT electrodes. (B) Current-time plot. (C) Reversibility of current switching upon visible and UV irradiation. (Reproduced from Ref. [42].)

at the single-molecule level, and potentially applied to ultra-high density optical memory [7]. The conductivity can also be switched at the single-molecule level. The single-molecule photoswitch is the key element in molecular devices [40]. In this subsection, we introduce electrical current switching at the single-molecule level. The change in conductivity of a single diarylethene molecule upon irradiation has been studied experimentally and theoretically [41–49]. The electrical resistance of a single diarylethene molecule 14 has been measured by the break-junction method [41]. Fig. 10(A) schematically illustrates the single-molecule conductivity measuring system. Upon exposure to light of a specific wavelength,

the covalent bonds in the switching element rearrange, and ␲conjugation through the molecule is turned on or off. When the closed-ring isomer is connected to two gold electrodes, conduction through the molecule can occur via the ␲-conjugated system. Upon exposure to light of a wavelength in the range of 500–700 nm, the molecule switches from the closed form to the open form, in which there is no longer an alternation between the single and double bonds, and the molecule becomes an insulator. Fig. 10(B) shows the I–V characteristics of the device. The I–V plot for the device in its closed-ring state (a) did not show any sharp changes. When the device was illuminated with light of a wavelength of 546 nm, there

Fig. 12. Diarylethenes used in the experiment of the diarylethene–Au nanoparticle network. (Reproduced from Ref. [50].)

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Fig. 13. (A) Schematic representations of a diarylethene–Au nanoparticle network. (B) (a) Changes in the current–voltage curves following UV (290–380 nm) and visible-light (>560 nm) irradiation. (b) Corresponding changes for a shorter UV irradiation time showing the reversibility of the process. (Reproduced from Ref. [50].)

was a sharp increase in resistance after a short time (b and c). The resistance after switching was in the G range, about three orders of magnitude larger than the initial value. The sharp jump in current can be attributed to the switching of the molecule. Reversible current switching using a diarylethene molecule (15, or 16) between single-walled carbon nanotubes has also been studied (Fig. 11) [42]. The thiophene-based device (X = S) could be switched from the insulating state (open form) to the conducting state (closed form) upon UV irradiation, but not back again, while the pyrrole-based (X = NMe) device cycled between the insulating and conducting states. Fig. 11C indicates current-switching cycles

upon UV irradiation and being left in the dark for 12 h. Although irradiation with visible light did not restore the initial insulating state, the device was recovered to the insulating state thermally by aging it at room temperature overnight. Switching of conductivity of diarylperfluorocyclopentene nanowire has also been theoretically investigated using Green’s function method combined with density functional theory [45,46]. The difference in conductivity of both isomerized states is in the order of magnitude of several hundreds times. The main reason for the difference lies with the differences in molecular geometry and in ␲-conjugation between open- and closed-ring forms. The result

Fig. 14. Experimental setup of an ordered metal–diarylethene–metal network. (a) Diarylethene molecule. (b) Schematic picture of two-dimensional nanoparticle network. (c) Schematic setup to measure light-induced conductance switching. (d) Repeated conductance switching. (Reproduced from Ref. [52].)

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Fig. 15. Chemical structure of the polymer; the bold lines show the extension of the ␲-conjugated connections. (Reproduced from Ref. [53].)

concludes that diarylethene molecules are a good candidate for a photoswitching unit in the molecular electronic devices. Photoswitching of the conductance of a diarylethene–Au nanoparticle network has been investigated using diarylethene derivatives with a thiophenol unit at both ends (Fig. 12) [50]. The two thiophenols bridge Au nanoparticles and create a conducting pathway between electrodes, as shown in Fig. 13(A). The Au–diarylethene nanoparticle network was prepared on an interdigitated Au electrode (5-␮m gap), and the conductance was measured upon alternating irradiation with UV and visible light (Fig. 13(B)). Upon irradiation with UV light, the conductance increased significantly, whereas it decreased upon irradiation with visible light. The on/off ratio of conductance was around 5. Reversible light-controlled conductance switching of the molecular device, in which gold nanoparticles are ordered in twodimensional lattices based on dithiolated diarylethenes, has been demonstrated [51]. The device has hexagonally ordered monolayers of octanemonothiol-covered gold nanoparticles prepared using self-assembly techniques, as shown in Fig. 14. Due to the small size of nanoparticles, each metal–molecule–metal junction incorporates one or at most a few molecular bridges. The sheet conductance decreased rapidly upon visible-light irradiation, and steep conductance increase was observed by UV application. Fig. 14(d) shows eight conductance switchings by repeated UV-visible irradiation. Decrease of on–off amplitude originates from decomposition by excessive UV illumination. Experiments in an ultra-high vacuum are expected to yield many more switching cycles.

colored form and after visible irradiation. In the colored state, where 70% of the diarylethene sites are in the closed-ring form, the room-temperature conductivity is about 1 × 10−15 S cm−1 . The conductivity increases upon increasing the temperature. Such thermally activated conductivity is a typical characteristic of neutral ␲-conjugated semiconductor polymers. Upon irradiation with visible light, the temperature dependence of the conductivity was measured again. The conductivity of the bleached state was onetwentieth that of the photostationary state at about 280 K. Upon UV irradiation of the bleached state, the conductivity increased to about half the original value at room temperature. 3.4. Applications Several applications based on photoswitching of electrical conductivity based on photochromism have been proposed, including optical memories [57,58], electrical circuits [59], and organic lightemitting devices [60,61].

3.3. Conducting polymers with photochromic units in the main chain When conducting polymers have diarylethene units in the main chains, the conduction can be switched upon photoirradiation [52–56]. A photochromic conducting polymer in which diarylethene chromophores are inserted between large twisted oligophenylene–fluorene units has been reported [52]. As a result of the introduction of the oligophenylene–fluorene units, the distance between the diarylethene chromophores is as long as ∼4 nm (Fig. 15). Fig. 16 shows the temperature dependence of the electrical conductivity of the polymer, measured both in its original

Fig. 16. Temperature dependence of the electrical conductivity of a film of the polymer; the solid circles denote the initial colored state of the polymer, and the open circles denote its state after photobleaching with visible-light irradiation. The inset shows the reversible change in electrical conductivity on alternating irradiation with UV and visible light. (Reproduced from Ref. [53].)

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Fig. 17. Operating principle of a nondestructive readout method for a memory medium with a photochromic data-storage layer and a photo-absorbing layer sandwiched by electrodes. (Reproduced from Ref. [57].)

On–off switching of the carrier transportation of a diarylethene can be used as a nondestructive readout method using photocurrent detection of photochromic memory [57]. Fig. 17 illustrates the operating principle of the nondestructive readout method. The memory medium has a photochromic data-storage layer (diarylethene) and a photo-absorbing layer (phthalocyanine). Readout light is only absorbed by the photo-absorbing layer. The photo-absorbing layer generates electric carriers (holes) that drift to the direction of the electric field. When the diarylethene layer is in the colored state, the generated holes are conducted through the diarylethene layer and an external photocurrent is detected. On the other hand, when the diarylethene layer is in the open-ring

state, the potential barrier for the holes becomes high and no external current is detected. Since the readout light is not absorbed by the photochromic layer, nondestructive readout is achieved. Over 106 readout times without any change of photocurrent have been demonstrated. The efficient on–off switching of an organic light-emitting diode (OLED) using a diarylethene layer has also been reported [61]. In order to obtain high on–off ratios in light emission based on photoisomerization, a high isomerization ratio is required. This is achieved by using the oxetane-functionalized diarylethene (XDTE, 23) molecule shown in Fig. 18. The XDTE molecules in the closedring state are closslinked by photoinitiated cationic ring-opening

Fig. 18. (Left) Photoisomerization of the oxetane-functionalized diarylethene 23. (Right) Switching property of the current density in photoprogrammable organic lightemitting diodes and corresponding absorption change. (Reproduced from Ref. [61].)

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polymerization of the oxetane units in the presence of photoacid, making a rigid film. As a result, antiparallel conformation is preserved and high isomerization efficiency is achieved. Fig. 18 also shows the photoswitching property of current for an optimized device with the XDTE layer. The maximum on–off ratio of about 103 and light-controlled emission switching have been achieved. Various light emission patterning using patterned photoisomerization with masked photoirradiation has been demonstrated. 4. Electric property changes by electrically induced isomerization (frame 3) In general, photoisomerization of molecules takes place when the molecules absorb light and are excited to higher energy levels. Similar excited states can also be produced by injecting electrical carriers into the molecules. This principle of electrically induced isomerization suggests that photochromic molecules could be used not only in optical memory, but also in organic semiconductor memory in which data are written electronically [62–64]. Fig. 19 illustrates the principle of semiconductor memory using a diarylethene derivative [62]. Energy-level diagrams for the “writing” and “reading” processes are shown in Schemes 1 and 2, respectively. For writing, electrons injected from the cathode and holes from the anode at a relatively high voltage are transported through carrier-transport layers to the photochromic molecular layer where they excite the molecules, which are originally in the closed-ring state. The molecules are thereby transformed into the open-ring state and their ionization potential increases (Scheme 1). For reading, only holes flow in the organic layers because of the relatively low applied voltage and the unbalanced potential barrier height between the electrode and the organic layer. The magnitude of the current varies according to the two isomer states of the photochromic layer (Scheme 2). Erasure can be achieved by UV irradiation. To produce such a memory device, a bipolar diarylethene derivative is required. A nonsymmetrical bipolar diarylethene 24 containing a triphenylamine group as an electron donor and an oxadiazole group as an electron acceptor has been proposed (Fig. 20(A)). Fig. 20(B) shows the experimental results for the reversible “writing” process shown in Scheme 1. A constant voltage is applied to the sample in its initial colored state generated by UV irradiation and the current falls as a result of carrier injection. The decreased-current state corresponding to a recorded state is recovered to the initial (erased) state upon UV irradiation. Because of the stability of the diarylethene molecules, both the recorded state and the initial state are stable in the dark at room temperature (this is shown as “keeping” in Fig. 20(B)). The above writing technique requires an encounter between an electron and a hole at a molecule to produce an excited state.

Fig. 19. Writing (Scheme 1) and reading (Scheme 2) mechanisms for an organic semiconductor memory device with a diarylethene layer. (Reproduced from Ref. [62].)

After producing the excited state, both carriers disappear. This means the isomerization efficiency, defined as the ratio of reacted molecules to injected carriers, is below unity and, therefore, it is difficult to achieve a high sensitivity in information writing. Many diarylethene molecules have recently been reported to undergo isomerization via their radical ion states [65–73]. A device based on this mechanism is proposed using a dithienylcyclopentene derivative containing two triphenylamine groups (Fig. 21(A)) [74]. The device structure is shown in Fig. 21(B). The derivative 3 is used as the memory layer (ML), and 4,4 ,4 -tris[3methylphenyl(phenyl)amino]triphenylamine (MTDATA) and N,N -di(1-naphthyl)-N,N -diphenylbiphenyl-4,4 -diamine (NPB) are used as the hole-injection layer (HIL) and the hole-transport layer (HTL), respectively. An additional layer of NPB is also optionally used as an electron-blocking layer (EBL). The initial state of the memory layer is a colored state produced upon UV irradiation

Fig. 20. (A) Structure of a nonsymmetrical bipolar diarylethene 24 for use in organic memory devices. (B) Reversible current decrease based on isomerization by electrical carrier injection into the diarylethene memory device. (Reproduced from Ref. [62].)

T. Tsujioka, M. Irie / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 11 (2010) 1–14

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Fig. 21. (A) Molecular structure of a diarylethene derivative 3 containing two triphenylamine groups. (B) Structure of a solid-state device incorporating a diarylethene-based memory layer and an electron-blocking layer (EBL). (C) Current decrease at a constant voltage of 8 V for devices with electron-blocking layers (EBLs) of various thicknesses. (D) Applied voltage dependence of the current decrease for the EBL-100 device. (Reproduced from Ref. [74].)

with 365-nm light. The decrease in current is caused by the change in the ionization potential as a result of isomerization from the closed-ring form to the open-ring form. Fig. 21(C) shows the thickness dependence of EBL in current decrease. The thicker EBL device requires fewer injected carriers, indicating hole injection is a more efficient method than both-carrier injection in the writing process. In this system, the isomerization efficiency depends on the lifetime of the cationic state of the diarylethene molecule. Therefore, a longer lifetime of the cationic state, as a result of a slower movement of the holes (achieved by lowering the voltage), would increase the probability of the isomerization. The result shown in Fig. 21(D) confirms that the efficiency increases upon lowering the voltage: the efficiency for a voltage of 6 V is 60 times that at 10 V. This is an important result for the creation of thermally stable organic semiconductor memory with very low power consumption. 5. Optical property changes by electrically induced isomerization (frame 4) This frame corresponds to a change in optical properties caused by electrically induced isomerization. Although no potential applications have yet been reported, several new applications can be proposed. Generation of a photocurrent from a photochromic molecule have been reported [75–78]. The generation of an external photocurrent means that carriers are separated from the photoexcited photochromic molecule. This suggests that sensitivity of photoisomerization is controlled by the carrier separation. Photosensitivity control based on this principle is one of the possible applications. 6. Conclusions Photochromic molecules and systems change not only optical properties but also various electrical properties, such as ionization potential, electron affinity, and the hole transport property. These property changes can be applied to switching electric current upon photoirradiation. Various applications of such electric properties

have been described from fundamental as well as application points of view.

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