Photophysical Properties of a Terarylene Photoswitch with a Donor-Acceptor Conjugated Bridging Unit.

A terarylene photoswitch composed of an electron-donating thienothiophene unit which is connected to the central bridging imidazole was designed. The electronic and geometrical structures of the central π-conjugation unit was controlled by chemical modifications on the imidazole unit, in which the distribution of frontier molecular orbitals was modulated between the neutral state and its protonated or quaternized cationic forms. These electronic and geometrical changes modulated the mixing of the closely lying excited state potential energy surfaces, resulting in the excitation wavelength dependent photocyclization performance. The chemical modifications on the imidazole ring also had an effect on the fluorescence property in the open-ring forms, which is explained by the formation of different push-pull π-conjugation systems.