Photo-orientation of azobenzene side chain polymers parallel or perpendicular tothe polarization of red HeNe light

Abstract

The mechanism of the light-induced orientation process of azobenzene-containingpolymers caused by irradiation with linearly polarized red light is investigated.This process is surprising because there is almost no absorption at 633 nm.Depending on the photochemical pre-treatment and the exposure time,the azobenzene moieties can undergo two different orientation processesresulting in either a parallel or a perpendicular orientation with respect tothe electric field vector of the incident light. The fast orientation of thephotochromic groups with their long axis in the direction of the light polarizationrequires a photochemical pre-treatment in which non-polarized UV lightgenerates Z-isomers. Due to this procedure the film becomes ‘photochemicallyactivated’ for the subsequent polarized irradiation with red light. But oncontinued exposure a second, much slower reorientation process occurswhich establishes an orientation of the azobenzene groups perpendicular tothe electric field vector. The fast mechanism is probably caused by anangle-selective photo-isomerization of the Z-isomers to the E-isomers, whilethe subsequent slow reorientation process is caused by the well-knownconventional photo-orientation taking place via the accumulation of a number ofphotoselection steps and the rotational diffusion minimizing the absorbance of theE-isomer. This process occurs in the steady state but at this wavelengthwith a very small concentration of Z-isomers. The competing mechanismstake place in the same polymer film under almost identical irradiationconditions, differing only in the actual concentration of the Z-isomers.