Photobehavior and Nonlinear Optical Properties of Push-Pull, Symmetrical, and Highly Fluorescent Benzothiadiazole Derivatives

Three quadrupolar D-pi-A-pi-D compounds, bearing alkoxy phenyls as mild electron donors and a benzothiadiazole (A), two benzothiadiazoles (B), or a benzothiadiazole linked to two thiophenes (C) as the acceptor units, are collectively the object of this study. They proved to be efficient yellow/orange/red fluorophores, respectively, with fluorescence being their preferred deactivation pathway. These systems exhibited positive fluorosolvatochromism and a noticeable decrease of the fluorescence quantum yield in the most polar solvent (with the quenching following the trend B > A > C). These findings point to an intramolecular charge transfer (ICT) nature of the emitting state, whose photoinduced dynamics was investigated by femtosecond-resolved transient absorption and fluorescence upconversion. Remarkable values of hyperpolarizability were estimated by the solvatochromic method. The significant two-photon absorption cross sections measured for A-C (whose trend nicely parallels that of ICT efficiency), coupled. with the intense emission, make them promising for applications as environment-sensitive probes in two-photon excited fluorescence imaging.