Strongly Absorbing and Emissive Diazaborinine-Cored Dyes

The synthesis and photophysical properties of diazaborinine dyes (BDX, X = O, N^R, S; R = Me, Ph) are described. Multiple routes for modification of the boracycle core are reported. These dyes absorb in the near-UV to blue regime of the spectrum with large extinction coefficients of (2-8) × 10⁴ M^-1 cm^-1 and emit with high quantum yields (> 80%) and short lifetimes (1.5-2.1 ns), consistent with fluorescence. The absorption and emission energies of the BDX compounds are significantly affected by the identity of the heteroatoms, with transition energies decreasing in the trend O > N > S. Luminescence spectra for BDO were broad and unstructured when compared with the sharp lines of BDN^R and BDS. BDX compounds have small singlet-triplet gaps (0.28-0.46 eV). The phosphorescent intensity of these dyes is enhanced by bromination, which results in a hundredfold increase in the fluorescent non-radiative decay rate compared with the parent dyes. The BDX compounds are calculated to have C_S symmetry in the ground state. The BDN^R and BDS molecules are calculated to have C_S symmetry in the triplet excited state geometry, whereas BDO is found to undergo symmetry breaking, leading to a distorted asymmetric structure in the excited state.