Blue fluorescence from: N, O -coordinated BF₂ complexes having aromatic chromophores in solution and the solid state

We prepared amide-heterocycle (HC) compounds having various aromatic π-electron systems (Ar), such as phenyl, naphthyl, furyl, thienyl and phenanthryl moieties, and converted them as ligands to difluoroboronated complexes, Ar@HCs. Blue fluorescence from Ar@HCs was observed in solution and the solid state, and the fluorescence quantum yields (Φ_f) and lifetimes (τ_f) were determined. The Φ_f values in CHCl₃ were as small as 0.1 except for the phenanthrene derivatives (0.4-0.6). Observation of the triplet-triplet absorption upon laser flash photolysis of Ar@HCs in solution indicated that the fluorescence process competes with intersystem crossing to the triplet state. Blue fluorescence in the solid state was observed with the Φ_f values of 0.3-0.7. Based on the crystallographic data, the relationship between the crystal structures and emission features of Ar@HCs in the solid state is discussed.