Effects of Solvent Viscosity and Polarity on the Isomerization of Azobenzene Francesca Serra* and Eugene M. Terentjev CaVendish Laboratory, UniVersity of Cambridge, J.J. Thomson AVenue, Cambridge CB3 0HE, UK ReceiVed September 8, 2007; ReVised Manuscript ReceiVed NoVember 19, 2007 ABSTRACT: Isomerization of azobenzene has been studied for many years, but some aspects are still unclear or controversial. This work provides further insight and clarifies the role that environmental parameters play in determining the reaction kinetics of azobenzene derivatives in solution. Detailed spectroscopic measurements of the photoisomerization and thermal isomerization kinetics in mixtures of various solvents and polystyrene have been carried out. The results indicate a strong dependence of both photoisomerization and thermal isomerization rates on the polarity, but not on the viscosity of the solvent. 1. Introduction Isomerization of azobenzene and its derivatives has been extensively studied for the past 50 years because this molecule presents many interesting features and its applications range from electronics to biomedicine. It has been used as a model molecule for all the biological processes that involve similar reactions, 1-3 like the isomerization of retinal in rhodopsin, or as a probe for measuring the free volume in polymers. 1,4 More recently, its characteristic response to the polarization of light made it a suitable molecule for surface patterning. 5,6 Finally, azobenzene-containing elastomers constitute an interesting area of inhomogeneous photomechanical effects and their applica- tions as photoactuators and artificial muscles are under study. 7-10 However, in spite of the large literature on the subject, many fundamental mechanisms and effects have not been clarified yet. It is known that the isomerization reaction is very sensitive to both electrical and mechanical characteristics of the environ- ment which surrounds the molecules, but identifying and separating these effects is a difficult and often ambiguous task. UV-vis spectroscopy constitutes an appropriate tool for the study of this system because during isomerization the rear- rangement of the molecular orbital energy levels induces a change in the molecular absorption spectrum. 3 When the azobenzene molecule is in the ground-state trans configuration, the transition with the highest quantum yield is that between the bonding and antibonding (π-π*) orbitals: the result is a big absorption peak around 320 nm. The absorption of this high- energy photon stimulates the molecular transition into the metastable cis configuration. In this state the highest quantum yield belongs to the nonbonding-antibonding (n-π*) transition, which is identified by a peak around 440 nm. 1,3 This cis-trans transition can occur spontaneously, when thermal fluctuations carry the molecule over the energy barrier; it can also be stimulated by absorption of a low-energy 440 nm photon. During isomerization, the relative intensity of these two peaks changes, and in this way one can follow the course of the reaction. Different chemical groups can be attached to the basic azobenzene molecule. These substitutions dramatically change the spectroscopic properties, affecting the relative position and intensity of the two absorption peaks, and the kinetics of isomerization. This allows fine-tuning of the optical response to many purposes; for this reason different azobenzene-based molecules have been used as probes in a wide range of wavelengths. In addition to molecular substitutions in the chromophore molecule, environmental parameters like solvent polarity and viscosity also play a role. Over 30 years ago, Whitten et al. 11 found an increase of several orders of magnitude in the thermal isomerization rate of an azobenzene derivative, with an aminic and a nitrate group as substituents, in solvents at increasing polarity. At the same time, only a little change was detected using a molecule with a simple aminic substitu- tion. 11 An increase of the speed of the reaction in some azobenzene molecules was also reported by Gille 12 and Asano. 13 Bortolus reports an increase in quantum yield of cis-trans photoisomerization in nonpolar environment for azobenzene molecules. 14 Sanchez and De Rossi studied in detail the effect of polarity on thermal cis-trans transition, and they found that hydroxide ion plays an important role in speeding up the cis- to-trans isomerization for azobenzene compounds with aminic substitutions. 15 The effect of solvent viscosity has been investigated too, but no universal conclusion has been reached. Gegiou et al. reported only a slight effect on the photostimulated cis-trans isomer- ization quantum yield, but they observed a viscosity-dependent quantum yield in the trans-cis isomerization. 16 Gille et al. reported a slowing of thermal relaxation (γ) in viscous environ- ment, 12 while Eisenbach found no effect at all. 17 Many groups have therefore studied the effect of a very viscous matrix, i.e., a molten or glassy polymer instead of a solution, and the results agree that the kinetics is the same as in solution if the polymer is in rubbery state, but below the glass transition temperature its characteristics change. 17-20 The models that were proposed for reaction kinetics are basically first-order, with the important exception of azobenzene in glassy polymers. In solution, the fraction of isomers in the cis state, c, varies in time as 21,22 where I is the irradiation intensity, k CT and k TC are the cis- trans and trans-cis constants of photoisomerization, respec- tively, and γ is the rate of spontaneous thermal cis-trans isomerization. Accordingly, (1 - c) represents the fraction of molecules in the ground-state trans configuration. There is an assumption that the rate of spontaneous thermal trans-cis isomerization is negligible at room temperature, supported by the estimates of the energy barrier much exceeding k B T. The dc dt ) Ik TC (1 - c) - Ik CT c - γc (1) 981 Macromolecules 2008, 41, 981-986 10.1021/ma702033e CCC: $40.75 © 2008 American Chemical Society Published on Web 01/04/2008