The impact of the heteroatom in a five-membered ring on the photophysical properties of difluoroborates

Abstract

A series of novel BF2 complexes, bearing a five-membered heterocyclic ring (with X = NMe, O, and S), were synthesized and characterized with a focus on the influence of atom exchange on the photophysical properties of both unsubstituted and dimethylamino derivatives. The experimental results show that the optical spectra substantially differ in both sets of dyes. In particular, the dimethylamino series are more strongly affected by heteroatom substitution, i.e., the insertion of X = O or X = S in lieu of X = NMe causes substantial bathochromic shifts of the absorption and emission bands, as well as marked changes in their topologies. In contrast, the optical spectra of the unsubstituted compounds undergo only relatively small red-shifts, and no variation of band shapes is observed. Moreover, the measured absorption spectra of the unsubstituted compounds bearing X = NMe and X = O are almost identical. Interestingly, the fluorescence yields of the dimethylamino derivatives are much larger (up to one order of magnitude) than those of the corresponding unsubstituted compounds. The experimental analyses are supported by state-of-the-art quantum chemistry calculations, which satisfactorily reproduced the experimental trends and provided further insights into the observed optical signatures.