Molecular Conformational Effect on Optical Properties and Fluoride Induced Color Changes in Triarylborane–Vinylbithiophene–BODIPY Conjugates

Three new triads [bis(mesityl)boryl (Mes 2 B)–vinylbithiophene–BODIPY] bearing zero ( 1 ), two ( 2 ), and four ( 3 ) methyl groups on the BODIPY core are synthesized, and their optical properties are reported. The vinyl linker between the thiophene rings in the spacer moiety improved the electronic communication between the boryl and BODIPY units. It displayed a bathochromic shift in the absorption and emission spectra compared to the Mes 2 B–bithiophene–BODIPY triad reported elsewhere. These compounds exhibit intriguing multiple-emission features due to an incomplete energy transfer from donor borane to the acceptor BODIPY unit. These compounds’ photoluminescence color can be conveniently fine-tuned by fluoride binding at the coordinatively unsaturated tricoordinate boron center. Triad 3, with a rigid molecular structure, showed a sharp emission band, whereas triads 1 and 2, with flexible molecular structures, displayed broad emission bands with a robust bathochromic shift, ascribed to their excited state structural reorganizations. These triads selectively bind fluoride ions and show colorimetric responses, which can be seen with an unaided eye. DFT computational studies were performed to rationalize the optical signatures of these compounds.