Kinetics of Photoinduced <i>E</i> to <i>Z</i> Isomerization of Azobenzene in Polystyrene Films:  Thickness, Molecular Weight and Temperature Effects

Abstract

E to Z (trans-->cis) photoisomerization of azobenzene (Az) chromophores tagged to polystyrene (PS) in thin films was studied as functions of thickness, PS molecular weight, and temperature, using the change in absorption at 336 nm arising from the Az E isomer remaining upon ultraviolet light irradiation at 350+/-5 nm. The photoisomerization in solid films exhibited fast and slow modes. The fractional amount of the fast mode (I1) started to increase with decreasing film thickness once the films were thinner than a threshold value. This was explained in terms of a surface layer in which the photoisomerization reaction proceeds quickly, the effect of which becomes more noticeable with decreasing thickness due to a larger surface to volume ratio. The thickness dependence of the I1 fraction was insensitive to the molecular weight of the PS used. The thickness of the surface layer, estimated through a layer model analysis, increased with rising temperature up to 298 K. Interestingly, the surface layer markedly thickened at temperatures at which the molecular motion of PS is on a relatively small scale, namely, at the gamma and beta relaxation temperatures.