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 <i>ortho</i>-arylthiomethyl-substituted phenyl groups on the boron atom as a new family of stimuli-responsive boron-containing π-conjugated molecules. The two <i>ortho</i>-arylthiomethyl groups coordinate to the boron atoms by forming five-membered rings in the DBA scaffolds to produce the <i>cis</i> 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.