Synthesis, Characterization, and Photoluminescence Properties of Boron Tropolonate Complexes: From Fluorescence to Room Temperature Phosphorescence

Boron complexes based on tropolone, a seven-membered non-benzenoid aromatic compound, were synthesized. We revealed that the aromaticity of tropolone was enhanced by boron complexation. Importantly, all boron complexes exhibited photoluminescence, and room-temperature phosphorescence (RTP) was observed from the halogenated derivatives, BrTpB and ITpB in the crystalline states. Furthermore, a non-substituted complex, TpB, showed phosphorescence without any heavy atoms in the solution state at 77 K. DFT calculation data suggest that TpB should have large enough spin-orbit coupling (SOC) constants to accept the intersystem crossing. The RTP of halogenated complexes were achieved by heavy atom effects. Significantly, this is the first report of RTP from boron tropolonate complexes.