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) × 10 4 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.