ON THE ROTATIONAL DIFFUSION-CONTROLLED PHOTO-REACTION AND REACTION-INDUCED BIREFRINGENCE

We have treated photo-induced intramolecular reactions where the react ant molecule in liquids or in solids has a photo-active group which is assumed to react with another group whose rate is proportional to the product of the strength of the electric field of an external light an d an induced dipole moment on the former group, which is equivalent to the interaction energy between the field and the dipole. When the rea ctant is subject to a linearly polarized light, the reaction rate beco mes dependent upon the orientation of the former group relative to the direction of the light and the system becomes optically birefringent. Since the reaction redistributes the orientation of the reactant inho mogeneously, the diffusion tries to average out the distribution, The dynamics of the reaction is considered theoretically taking into accou nt these two opposing factors with the reference to the dichroism usin g a model for the rotational diffusion-controlled reaction based on th e Smoluchowski equation modified with a sink term arising from the rea ction. We treated two typical cases of the slow and fast reactions, an d obtained the following quantities exactly in terms of continued frac tions as a function of the rotational diffusion constant D and the rat e constant with application of the natural light: the relaxation time for the time-dependent dichroism and the stationary dichroism for the former case, and the reaction constant r for the reactant for the latt er case which is found to be r proportional to D-1/4 for intermediate values of D. Also, it is shown that theoretical results agree satisfac torily with those of experiments.