Abstract
The design of two-photon absorbing azobenzene (AB) derivatives has received much attention; however, the two-photon absorption (2PA) properties of bis-conjugated azobenzene systems are relatively less explored. Here, we present the synthesis of six azobenzene derivatives and three bis-azobenzenes substituted (or not) at para position(s) with one or two amino group(s). Their linear and nonlinear absorption properties are studied experimentally and theoretically. The switching behavior and thermal stability of the Z-isomer are studied for unsubstituted mono- (1a, 2a) and bis-azobenzene (3a) compounds, showing that when the length of the π system increases, the half-life of the Z-isomer decreases. Moreover, along with the increase of π-conjugation, the photochromic characteristics are impaired and the photostationary state (PSS) related to E-Z photoisomerization is composed of 89% of the Z-isomer for 2a and 26% of the Z-isomer for 3a. Importantly, the 2PA cross-section increases almost five-fold on extending the π-conjugation (2a vs 3a) and by about one order of magnitude when comparing two systems: the unsubstituted π-electron one (2a, 3a) with D-π-D (2c, 3c). This work clarifies the contribution of π-conjugation and substituent effects to the linear and nonlinear optical properties of mono- and bis-azobenzene compounds based on the experimental and theoretical approaches.