Vibrational Spectroscopy 43 (2007) 249–253 www.elsevier.com/locate/vibspec
A new class of molecules with large, switchable vibrational non-linear optical responses: Photochromic diarylethene systems M. Del Zoppo *, A. Lucotti, C. Bertarelli, G. Zerbi Dip. di Chimica, Materiali e Ingegneria Chimica ‘‘G. Natta’’, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy Received 26 July 2006; received in revised form 29 August 2006; accepted 2 September 2006 Available online 12 October 2006
Abstract Photochromic diarylethene molecules are characterized by electronic properties significantly different in the two isomers. In this work, we are particularly concerned with the vibrational contribution to second order hyperpolarizabilities. We show that a systematic increase of the non-linear optical properties is observed every time the closure reaction is induced by a suitable irradiation. The large change of molecular non-linear optical coefficients can be exploited e.g., for the realization of an optical switch. Moreover, we show that also the intrinsic response of the ‘‘closed’’ (i.e., more conjugated) form is, in some case, quite large. The structural reasons of this phenomenon are discussed. # 2006 Elsevier B.V. All rights reserved. Keywords: Photochromism; Diarylethenes; Non-linear vibrational polarizabilities; Raman spectra
1. Introduction Organic materials have been extensively investigated in the recent past as promising systems for the realization of nonlinear optical devices. Among the various classes of materials presently under study photochromic systems may acquire a particular relevance. Photochromism is a reversible chemical transformation induced at least in one direction by an electromagnetic radiation between two molecular states A and B which possess different chemical structures, thus showing two different absorption spectra. Photonics, optics, optoelectronics, optical storage and information technology have recently turned their close attention to such class of materials. In this work, we propose to explore suitable photochromic systems in order to design a photoswitch based on the variation of the non-linear optical (n.l.o.) response of the two forms. The existence of two stable forms characterized by different electronic properties lays the conceptual necessary (but not sufficient) basis for the fabrication of n.l.o. switches triggered by light. We are particularly concerned with diarylethenes
* Corresponding author. Fax: +39 02 2399 3231. E-mail address: [email protected]
(M. Del Zoppo). 0924-2031/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2006.09.002
whose basic unit consists of ethylene 1,2-disubstituted with two thiophene rings; a large number of derivatives have been synthesized and studied especially by the group of Irie . Diarylethenes have been extensively studied in the last 10 years because of the thermal and optical stability of the two isomers, their high fatigue resistance, high sensitivity and the rapid response that make them good candidates for technological applications. In the recent past many photochromic diarylethenes have been synthesized and characterized in our laboratory [2–5]. Upon irradiation with UV light, diarylethenes undergo structural rearrangements which lead to a so-called ‘‘closed form’’ characterized by a more extended p conjugation (see Fig. 1a). Clear evidence of such larger conjugation is given by a remarkable red-shift of the absorption maximum in the electronic spectrum (see Fig. 1b). The larger p delocalization in the ‘‘closed’’ form is accompanied by a significant change in the electronic properties. In particular, it is reasonable to think that the non-linear optical properties of these molecules should change in going from one form to the other. This observation justifies the efforts made in order to measure the non-linear optical properties of diarylethenes at the molecular level [6,7]. In this work, we have investigated a series of diarylethenes, both low molecular weight molecules (compounds A–E in Fig. 2) and the so-called ‘‘backbone photochromic’’ polymers (F and G).
M. Del Zoppo et al. / Vibrational Spectroscopy 43 (2007) 249–253
ing the vibrational frequencies and the infrared and Raman intensities of the molecular systems of interest according to the following expressions: 1 X 1 @mi @a jk @m j @aik @mk @ai j þ þ bvijk ¼ 2 2 4p c n @Qn @Qn @Qn @Qn @Qn @Qn n2k (1) g vijkl ¼
Fig. 1. (a) Photochromic reaction of a generic 1,2-dithienylethene. (b) Absorption spectra for the open (solid line) and closed form (dashed line) of 1,2-bis-(2methyl-4-phenyl-3-thienyl)perfluorocyclopenetene (A) in chloroform solution.
2. Vibrational contribution to polarizabilities Since the early 1990s, we have proposed  to evaluate (both experimentally and theoretically) the vibrational contribution to molecular hyperpolarizabilities, simply by measur-
1 X 4p2 c2 n
½a2 injkl þ ½mbinjkl n2n
@mi @b jkl @m j @bikl @mk @bi jl @ml @bi jk þ þ þ @Qn @Qn @Qn @Qn @Qn @Qn @Qn @Qn
½a2 injkl ¼
@ai j @akl @aik @a jl @ail @a jk þ þ @Qn @Qn @Qn @Qn @Qn @Qn
where nk is the vibrational frequency of the kth normal mode Qk and the quantities @mn/@Qk, @anm/@Qk, and @bnmp/@Qk are the derivatives of the Cartesian components of the molecular dipole moment, polarizability, and hyperpolarizability with respect to normal coordinates. These parameters can be obtained from infrared intensities, Raman and hyperRaman cross-sections, respectively. It has been shown that in the case of conjugated molecules such contributions are large and in many cases related to the corresponding electronic quantities . In this work, we focus our attention on the second order hyperpolarizability g v which is usually ruled by Raman intensities. Since the very beginning of the conducting polymer age, Raman intensities are known to be particularly sensitive to the extent of p electron delocalization. In particular, Raman
Fig. 2. Molecular structures of the photochromic molecules studied in this work.
M. Del Zoppo et al. / Vibrational Spectroscopy 43 (2007) 249–253
spectra are dominated by very few lines which correspond to collective skeletal CC stretching modes whose intensities are enhanced by orders of magnitude when the delocalization increases . Since the state of conjugation is very different in the closed and open form of diarylethenes, we expect to observe in the two cases Raman spectra which exhibit large differences. Indeed, in Fig. 3 the Raman spectra of various diarylethenes in both forms are compared. The comparison clearly shows how the extended delocalization of the closed form strikingly modifies the intensity pattern. Using Eq. (2) it is immediately apparent that g v of the closed form must be much higher than that of the open form. This is true provided that the term [mb] is negligible with respect to the [a2] term. It has been proven that this is indeed the case for many classes of molecules (polyenes, eteroaromatic systems, condensed aromatic rings, etc.) . The estimate of this contribution can only come from a proper quantum chemical calculation since hyperRaman intensities are not yet experimentally available. Thus, for a proper treatment one should perform high-level quantum chemical calculations in order to prove that the [mb] term is negligible. However, some general considerations can be made which justify this hypothesis. It is generally accepted that hyperRaman intensities are negligibly small (this is indeed the reason why it is so difficult to obtain experimental values for this quantity); moreover, to give a significant contribution to g v the
hyperRaman activity must be multiplied by the infrared intensity of the same normal mode. It is apparent from the experimental data on infrared spectra of diarylethenes in the two forms (Fig. 4) that even if some intensity changes do occur they are not so large as to justify a large variation in the contribution to g v of the two forms. 3. Experimental Since, as discussed in the previous section, in most cases the [mb] contribution is taken as negligible or much smaller with respect to that of [a2] we can use the following expression for evaluating g v as a function of absolute Raman intensities : 1 1 X Ik (5) gv ¼ 15 4p2 c2 k n2k In order to obtain the absolute Raman intensities (Ik), and therefore the second order hyperpolarizability (g v ) by means of Eq. (5), we must know the concentration of the sample (cs). For this reason Raman spectra have been recorded from solutions of known concentration where the solvent (chloroform) is used as an internal standard. In the case of non-polarized scattered light Ik in Eq. (5) is given by: 2 7 (6) I in:p: ¼ 45 a2i þ h2i ¼ 45Ri 45 n:p: where ai and hi are the invariants of the Raman tensor and jRij2 is related to the scattering cross-section by: 2 r 4 v;s r e vr2 2 s vi L R ¼ xi v2 Rri (7) s i s n;r v2 v L e vs2 Knowing the concentration of the sample (cs), the concentration of the reference (cr), the area of the ith band of the sample (asi ) and the area of the reference band (ar) we can calculate the xi ratio: xi ¼
Fig. 3. Raman spectra of the photochromic compounds A and G both in their open and closed form.
Ssi asi =cs ¼ Sr ar =cr
Inserting this value in Eq. (7) we can then calculate jRsi j2 and from Eq. (6) the unpolarized absolute Raman intensity (Iin.p.). For more details and a thorough definition of the symbols used see Ref. . The value of the second order hyperpolarizability g v is finally obtained by inserting the absolute Raman intensity of each Raman line in Eq. (5). The Raman spectra of all the diarylethenes were recorded both in the open and closed form. We recorded the Raman spectrum of the photochromic compound in its open form (non-colored form) in chloroform solution and then we exposed the sample to UV irradiation until the photostationary state was reached; then we recorded the Raman spectrum of the closed form. The conversion ratio
M. Del Zoppo et al. / Vibrational Spectroscopy 43 (2007) 249–253
Fig. 4. Infrared absorption spectra of photochromic compounds A and G both in their open and closed form.
between the open and the closed form was monitored using a UV–vis–NIR spectrophotometer JASCO V570. The UV irradiation has been obtained from a low-pressure mercury lamp using the 366 nm and the 254 nm lines. In some cases a visible light irradiation was necessary in order to completely open the photocromic compound. In these cases we used a 250 W 120 V halogen lamp; the emitted light was suitably filtered in order to remove the UV and IR components and then focused on the sample. Raman spectra were carried out using a Nicolet 910 FTRaman equipped with a Nd-YAG laser with a 1064 nm excitation line and a liquid nitrogen-cooled germanium detector. In order to obtain a non-negligible Raman signal we used a concentration of diarylethene compounds in the 101 M/102 M range. Materials were prepared according to the general procedure for the synthesis of diarylethenes, following Dixon reaction
between bromo-aryl derivative and octafluorocyclopentene in THF at 78 8C [13–15]. The hydrazone derivative D was obtained by reacting 1,2-bis(2-methyl-5-formyl-3-thienyl) perfluorocyclopentene with dimethylhydrazine in presence of acetic acid. The hydrazone derivative E was analogously obtained by reacting 1,2-bis(2-methyl-5-formyl-3-thienyl) pefluorocyclopentene with phenylhydrazine in presence of acetic acid . The synthesis of poly-dithienylethene-alt-dihexyloxyphenylenevinylene (G) has been already reported in Ref. . 4. Results and discussion In Table 1, we collect the g v values evaluated from Raman spectra for the diarylethenes shown in Fig. 2. As expected on the basis of the previous discussion g v values of the open form are systematically lower than those of the closed counterpart. In some cases the ratio between the closed and the open form is
M. Del Zoppo et al. / Vibrational Spectroscopy 43 (2007) 249–253 Table 1 Absorption maxima in the electronic spectrum and vibrational second hyperpolarizability for the photochromic compounds of Fig. 2 both in their open and closed forms Compound
A B C D E F G
g v (esu)
g vclosed =g vopen
285 296 321 318 358 326 420
585 592 606 622 652 620 674
1.6 1035 0.9 1035 2.0 1035 1 1035 2.2 1035 0.8 1035a 1.0 1034a
0.9 1034 1.4 1034 0.9 1034 3.0 1034 8.2 1034 1.6 1034a 1.8 1033a
absolute g v value of the closed form (Fig. 5), is large with respect to other organic molecules usually considered for possible nonlinear optical applications. For this reason these systems can be considered themselves promising as photonic materials. 5. Conclusions
5.6 15.6 4.5 30 37.3 20 18
Units are esu. a Referred to the monomeric unit.
rather large, thus indicating that the chemical nature of the substituent groups favors an increase of the p electron delcalization upon the closure reaction. We observe that this ratio is larger for diarylethenes bearing strong electron–donor substituents (D and E) which establish a push–pull–push system when the molecule is in the closed form since the central moiety has an electron withdrawing capability. The stronger the donor capability of the substituents the larger the ratio. On the contrary, when neutral or electron withdrawing substituents are used the ratio becomes increasingly smaller because the delocalization path is restricted. Also the backbone photochromic polymers F and G are characterized by large differences in g v values between the two isomers. It is interesting to notice that the g v values of the closed form increase when the extent of p conjugation increases. This observation is supported by the correlation with the absorption maxima (lmax) reported in Table 1. Indeed, larger lmax correspond to larger conjugation hence larger g v values. It must be noticed that not only large variations of the non-linear optical response are observed, but that, in many cases, also the intrinsic
Fig. 5. Comparison of the experimental values of the second order hyperpolarizability of some photochromic compounds (closed form) with the values obtained for other organic molecules known for their non-negligible non-linear optical responses. Capital letters refer to the photochromic compounds of Fig. 2. (^) rylenes, (&) polyenes and (~) phenylenes . The second order hyperpolarizability values are plotted as a function of the total number n of pz electrons. Units are (esu).
The measure of the vibrational contribution to the second order molecular hyperpolarizability of diarylethenes has demonstrated that their n.l.o. response depends on the isomeric state. The more conjugated closed form of diarylethenes show g v values larger than those of the open-ring counterparts and the change in g v between the two isomeric states depends strongly on the molecular structure and above all on the electronic effect of the substituents in 5,50 -positions of the thiophene ring. The large g v values of the closed form of the backbone photochromic polymer and of low molecular weight diarylethene bearing strong electron donor groups suggest to study also their bulk n.l.o. properties (x3) in order to obtain materials for applications in photonics. Acknowledgement This work has been partly supported by the Italian Scientific and Technological Research Ministry through PRIN04 and FIRB03 projects. References  M. Irie, Chem. Rev. 100 (2000) 1685, and references therein.  F. Stellacci, C. Bertarelli, F. Toscano, M.C. Gallazzi, G. Zotti, G. Zerbi, Adv. Mater. 11 (1999) 292.  C. Bertarelli, M.C. Gallazzi, F. Stellacci, G. Zerbi, S. Stagira, M. Nisoli, S. De Silvestri, Chem. Phys. Lett. 359 (2002) 278.  C. Bertarelli, A. Bianco, V. Boffa, M. Mirenda, M.C. Gallazzi, G. Zerbi, Adv. Funct. Mater. 14 (2004) 1129.  A. Lucotti, C. Bertarelli, G. Zerbi, Chem. Phys. Lett. 392 (2004) 549.  S.L. Gilat, S.H. Kawai, J.-M. Lehn, Chem. Eur. J. 1 (1995) 275.  C. Bertarelli, M.C. Gallazzi, A. Lucotti, G. Zerbi, Synth. Met. 139 (2003) 933.  C. Castiglioni, M. Gussoni, M. Del Zoppo, G. Zerbi, Solid State Commun. 82 (1992) 13.  M. Del Zoppo, C. Castiglioni, P. Zuliani, G. Zerbi, in: T. Skotheim (Ed.), Handbook of Conducting Polymers, II ed., Dekker, New York, 1998; C. Castiglioni, M. Tommasini, M. Del Zoppo, J. Mol. Struct. 521 (2000) 137.  M. Gussoni, C. Castiglioni, G. Zerbi, in: R.J.H. Clark, R.E. Hester (Eds.), Spectroscopy of Advanced Materials, Wiley, New York, 1991, p. 251; C. Castiglioni, M. Tommasini, G. Zerbi Phil, Trans. R. Soc. Lond. A 362 (2004) 2425; V. Hernandez, C. Castiglioni, M. Del Zoppo, G. Zerbi, Phys. Rev. B 50 (1994) 9815.  A. Bianco, M. Del Zoppo, G. Zerbi, Synth. Met. 125 (2002) 81.  C. Castiglioni, M. Tommasini, M. Del Zoppo, J. Mol. Struct. 521 (2000) 137.  M. Irie, in: J.C. Crano, R.J. Guglielmetti (Eds.), Organic Photochromic and Thermochromic Compounds, Plenum Press, New York, 1999 , p. 207 (Chapter 5).  S.H. Kawai, S.L. Gilat, R. Ponsinet, J.-M. Lehn, Chem. Eur. J. 1 (1995) 285.  M. Irie, K. Sakemura, M. Okinaka, K. Uchida, J. Org. Chem. 60 (1995) 8305.  C. Bertarelli, A. Bianco, F. D’Amore, M.C. Gallazzi, G. Zerbi, Adv. Funct. Mater. 14 (2004) 357.