Benzothiaoline Three-Coordinated Organoboron Compounds with a BN Bond: Dual Emission and Temperature-Dependent Excimer Fluorescence

A series
of 2,2-disubstituted benzothiazoline-BMes2 (Mes
= mesityl) compounds containing a BN bond have been prepared
and fully characterized. Their photophysical properties were investigated
by UV–vis and fluorescence spectroscopy, which revealed the
presence of solvent- and concentration-dependent dual emission. On
the basis of the spectroscopic data, the dual emission was assigned
to monomer and excimer fluorescence of the molecule, respectively.
Experimental and TD-DFT computational data indicated that the purple-blue
monomer emission of these compounds is mainly from an intramolecular
charge transfer (CT) transition between the benzo-sulfur moiety and
the boron center. The yellow-green excimer emission is attributed
to intermolecular interactions involving the benzo-sulfur unit. Furthermore,
the excimer emission maxima of all compounds were found to be sensitive
to temperature, shifting to lower energy with decreasing temperature,
which illustrates the potential for this class of compounds to be
used as luminescent thermometers.