An experimental evidence for the isotropic absorption by cis isomers of azo compounds in the n−π∗ band

Polymeric materials and molecular glasses containing azo moieties have been the subject of many investigations due to the light driven behavior of such materials. The primary photochemical process in these systems is the light-induced trans-cis isomerization of azo moieties. It is commonly accepted that the trans molecules of azo chromophores absorb light by dipole mechanism, i.e., proportionally to the square cosine of the angle between the transition dipole moment and the light polarization. However, there is no consensus regarding the anisotropy of the absorption cross section of cis molecules. In this study, p-azotoluene (AT) and 1-naphthyl-p-azo-methoxybenzene (NAMB) dissolved in ortho-terphenyl (OTP) are irradiated with linearly polarized light in the region of the $n-{\pi }^{\ast }$ absorption band in a wide temperature range including the glass transition temperature of OTP, $T_g$. Upon irradiation, the trans-cis isomerization and the reorientation of azo chromophores occur. Near $T_g$, a kink in temperature dependence of the photostationary absorbance is observed. This fact together with modeling the time profiles of absorbance and anisotropy lead us to conclude that cis molecules of both azo chromophores absorb light isotropically upon irradiation in the $n-{\pi }^{\ast }$ absorption band.