Donor-π-Acceptor Type Unsymmetrical Triarylborane-Based Fluorophores: Synthesis, Fluorescence Properties, and Photostability.

A two-step synthesis to prepare tricoordinate organoboron compounds bearing three different aryl groups has been developed. After the first aryl substitution to an aryl boronic ester took place, the intermediate species, that is, bis(diarylborinate) species, was isolated as an air- and moisture-stable solid, which allowed the second aryl substitution to carry out in a selective manner. Subsequently, a series of unsymmetrical triarylboranes possessing a sterically bulky aryl group, triarylamine moiety, and para-functionalized phenyl ring was synthesized. Not only did these triarylboranes exhibit remarkable solvent-dependent fluorescence as expected for donor-π-acceptor (D-π-A) systems, they were also accompanied by profound persistence against photoirradiation especially for that bearing a 1,3,5-tri- tert-butylphenyl ring. This survey exemplifies that sufficient electronic and steric modification is key to construct photostable D-π-A type triarylborane-based fluorophores.