Understanding the torsion effects on optical properties of azobenzene derivatives

Abstract

Most azobenzene derivatives require the use of ultraviolet light for photoisomerization, this limits the utilization of them in biological systems, energy-storing material fields and so on. It’s useful to develop azo-based photo-switches triggered with visible light. Three synthesized azobenzene derivatives (tetra-o-methoxyl-4,4′-diacetamidoazobenzene, 5,6-dihydrodibenzo[c,g][1,2]diazocine and 3,5-dimethyl-bisazo) who can be driven by visible light were studied by DFT calculations in this article. We found that they all have the same structural features of the twisted azobenzene trans geometries. The planarity of azobenzene, which can be reflected by ϕCNNC (the dihedral angle connecting the azo moiety and two phenyl rings) and θCCNN (the dihedral angle connecting the azo moiety and one of the phenyl rings). To better understand the relationship between structure and optical properties, absorption spectra of azobenzene in vacuum as the function of ϕCNNC and θCCNN rotation were investigated by using DFT and TDDFT methods. Calculations demonstrated that distorting ϕCNNC and θCCNN can both result in the red shift of the absorption bands to the visible light wavelength, and ϕCNNC has a greater impact than θCCNN. Furthermore, the influence of twisted structure on the absorption band of 4-(4-aminophenylazo) nitrobenzene (a D-π-A type azobenzene derivative) in DMSO solution was also studied. Our results indicated that both ϕCNNC and θCCNN(NH2) rotation decreased the HOMO–LUMO gap, causing a red shift of the S0→S1 absorption band. However, the θCCNN(NO2) rotation increased the HOMO–LUMO gap slightly, resulting in a little blue shift of the whole bands. This work may provide useful theoretical foundation for fabricating of new visible light driven azobenzene-based photo-switches.