Influence of UV irradiation on the blue and red light photoinduced processes in azobenzene polyesters

Birefringence induced in a series of liquid crystalline side-chain azobenzene polyesters with different substituent groups was investigated under irradiation with 488 and 633 nm linearly polarized lights. Two different initial conditions have been used: the effect of a previous irradiation with UV light that yields the films into the isotropic state at room temperature (RT) was compared with the quenching from temperatures above the isotropic transition temperature Ti. UV–visible spectra of the thermally quenched films show the presence of aggregates when measured at RT. We have found that UV light irradiation creates a high concentration of cis isomers and breaks the aggregates, but they are formed again after a few days in dark at RT. Orientation of the chromophores perpendicular to the polarization of the 488 nm light and parallel to the polarization of the 633 nm light was confirmed by dichroism measurements.