Spiropyrans modeling

G. Pepe, D. Siri, A. Samat, E. Pottier, and R. Guglielmetti, Modeling of spiropyran aggregates with the help of GenMol program, Mol. Cryst. Liq. Cyst., 246, 247-250 (1994). 

Figure 8.7. Changes in optical densities of vacuum-deposited thin solid films of compounds la (R = H), and la (R = Me), spiropyrans 19, 20, and a model compound 16 (photochemically inert) measured at the absorption maxima of the long-wavelength absorption band of the photocolored isomers as a function of time of irradiation (high-pressure mercury lamp DRSH-250, 365 nm). Relative colorabilities of the compounds, Arcl, were calculated to be equal to 19 (6-N02BIPS) = 1 1 (R = H) = 0.9 1 (R = Me) = 3.2 20 = 2.1.

Chapter 7 (Structural Studies by X-ray Diffraction). The geometric parameters of photochromic compounds have been determined by X-ray diffraction on single crystals and yield interesting correlations with photochromic behavior (colorability, thermal bleaching rates, or absorption spectrum of the colored species). More intensive studies have been carried out on members of the spiropyran series (indolinospiropyrans), examining both closed spiranic forms and permanent merocyanines. The latter served as models of open photomer-ocyanines, the transient species produced by UV irradiation of spiropyrans. 

Several models have been proposed to explain the properties and photochromic reaction mechanisms of spiropyrans and spirooxazines based on both frequency2,5 and time810 field spectroscopic studies. The photochromic reaction features the dissociation of the Cspiro-0 bond, producing a distribution of isomers3-15 (Scheme 2). 

E. Pottier, A. Samat, R. Guglielmetti, D. Siri, and G. Pepe, Modeling of photochromic spiropyrans and spirooxazines by molecular mechanics and comparison with experimental data, Bull. Soc. Chim. Belg. 101, 207-213 (1992). 

In view of the difficulties encountered in direct interpretation of the vibrational spectra of spiropyrans, Davin-Pretelli et al.w proposed a more rational approach. They investigated first the IR and Raman spectra of the two constituent moieties of the photochrome under study. Thus, from a preliminary IR and Raman study of benzothiazolines and 2/f-chromenes, which served as models, they were able to give a satisfactory assignment of the spectra of a series of benzothiazolinic spiropyrans.9 Moreover, on the basis of the vibrational data for 2//-chromenes, the set of bands at ca. 1250 cm-1 was unambiguously assigned to vibrations involving... 

In both cases (1 and 2) although the reason for the observed transitions on the basis of the photoisomerization of the spiropyran unit remains to be clarified, the spiropyran-modified polypeptides can be considered as macromolecular models resembling the behavior of naturally occurring photoreceptors. 

Table 2. Maximum values of volume obtained by computer-based modelling for spiropyranes and merocyanines (in A )...

The wavelength of the absorption maximum of the colored form of spiropyrane bound on poly(methyl methacrylate) differs from that of the maximum of a low-molecular weight model in the same solvent. Similarly to Irie et al.,39 we have observed that the absorption maximum of the merocyanine form bound to PMMA in benzene, dioxane, tetra-hydrofurane, ethyl acetate, and chloroform solutions is shifted to shorter wavelengths compared with the maximum of absorption of the low-... 

Fig. 16 Schematic coupling model of the photo-switchable interactions between cytochrome c and the mixed SAM of spiropyran/merocyanine-terminated and 4-pyridine thiol with (a) the reduction of O2 by COx and (c) the oxidation of lactate by lactate dehydrogenase (LDH). (b) When the electrode is in the cationic merocyanine state, repulsive interactions disallow the functioning of the bioelectrocatalytic processes [130].

Two types of cteviation from the first-orda kinetics are noted for photo-aialthamal isomerization reactions in polymer films. The first is the normal type, in which the reaction rate is the same as or smaller than that in solutions at the initial stage and then progressively becomes smaller. Typical examples are thermal decoloration of the photocolored merocyanine form of spirobenzopyran molecularly dispersed in or chemically bound to a polymer matrix > and photoisomerization of the transazobenzene residue incorporated in polymer main chains The first interpretation for the decoloration of the merocyanine form assumed the existence of different isomers, each of which fades independently following first-order kinetics On the other hand, Kryszewski et al. proposed the kinetic matrix effect, which means that the distribution of free volume may lead to the deviation from first-order kinetics. His idea was based on the finding that deviations from first-order kinetics can be observol even in simple molecules such as azobenzene which has only one trans or cis isomeric form. The effect of free volume distribution on reactivity was further demonstrated by studies of annealed polymer films The distribution function of free volume as well as the critical free volume v were estimated for the merocyanine form of spiropyran in poly(methyl methacrylate) derivatives of azobenzene in polystyrene and azobenzene in polycarbonate The deviation from first-order kinetics was also observed in cyclizing imidization of model poly(amic acid) in a polyamide matrix... 

Levitus, M., TaUiavini, M., Negri, R.M., Atvars, T.D.Z., and Aramendia, P.F., Novel kinetic model in amorphous polymers. Spiropyran-merocyanine system revisited, /. Phys. Chem. B., 101, 7680,1997. 

In addition to photomodulation of enzymatic activity, an azobenzene moiety was incorporated into a model peptide antigen Glu-azoAla-Gly-Gly that allows for photoregulation of the antigen-antibody interaction. More recently, incorporation of an azobenzene moiety was used to photomodulate peptide conformation.The reader is referred to a recent review for more examples of photoresponsive peptides produced using azobenzene or spiropyran moieties. 

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