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

The photoisomerisation pathways of substituted aminoazobenzenes with cyano-containing acceptor moieties,AAB-DCVandAAB-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 S-0 -> S(2)excitation determines the most intensive pi-pi* band in the case ofAAB-DCVtrans-isomer, while in the case ofAAB-TCVthis band is due to S-0 -> S(1)excitation. lambda(max)is bathochromically shifted when passing fromAAB-DCVtoAAB-TCVfor trans-isomers. As for cis-isomers, the spectrum shape is determined by several excitations up to S-0 -> S-4, 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, chi((2)), performed for model PMMA-based doped materials reveal the corresponding almost twofold difference.