Intramolecular Arylsulfide-Coordinated Diboraanthracenes: Effect of B–S Coordination on Ground-State and Excited-State Behavior

Controlling boron–heteroatom interactions in triarylborane scaffolds can lead to stimuli-responsive photophysical properties. A key molecular design to this end is the utilization of a labile coordination bond between the boron atom and a Lewis basic heteroatom. Herein, we report the synthesis of a series of 9,10-dihydro-9,10-diboraanthracenes (DBAs) bearing ortho-arylthiomethyl-substituted phenyl groups on the boron atom as a new family of stimuli-responsive boron-containing π-conjugated molecules. The two ortho-arylthiomethyl groups coordinate to the boron atoms by forming five-membered rings in the DBA scaffolds to produce the cis isomers predominantly, where the strength of the boron–sulfur bonds can be tuned by structural and electronic modifications of the aryl groups. In the ground state, the B–S bond is cleaved upon heating in solution. In the excited state, the B–S bond undergoes dissociation, resulting in emission from tricoordinate species. The aryl groups on the sulfur atom also play a role in forming an intramolecular charge- transfer state, whereby the emissions are bathochromically shifted with large apparent Stokes shifts. Moreover, the B–S bonds are sensitive to solvent polarity and temperature, resulting in multiple emission properties depending on the surrounding environment.