The effect of cyano-containing acceptor moieties on the photoisomerisation mechanisms and UV/vis spectra of azochromophores with switchable nonlinear optical activity

Abstract

© 2020 Informa UK Limited, trading as Taylor & Francis Group. The photoisomerisation pathways of substituted aminoazobenzenes with cyano-containing acceptor moieties, AAB-DCV and AAB-TCV, are revealed by TD-DFT at B3LYP/6-31G** level. Stationary and transition points on ground and excited state potential energy surfaces are localised; both inversion and rotation trans- and cis-isomerisation mechanisms are considered and the former is shown to be the preferable for both chromophores in the ground state. UV/vis spectra of isomeric forms of AAB-DCV and AAB-TCV are calculated by TD-DFT in chloroform at CAM-B3LYP/PCM//def2-TZVP level, the effect of acceptors on the spectrum shape and absorption maximum is revealed. It is found that S0→S2 excitation determines the most intensive π-π* band in the case of AAB-DCV trans-isomer, while in the case of AAB-TCV this band is due to S0→S1 excitation. λmax is bathochromically shifted when passing from AAB-DCV to AAB-TCV for trans-isomers. As for cis-isomers, the spectrum shape is determined by several excitations up to S0→S4, corresponding transitions having close intensities. Switching ability of the composite nonlinear-optical polymer material is demonstrated at both molecular and macroscopic levels: first hyperpolarisability of trans- and cis-forms differs three times in favour of the former; estimations of macroscopic quadratic electric susceptibility, χ(2), performed for model PMMA-based doped materials reveal the corresponding almost twofold difference.