Structure–property correlation of solid-emissive boron–fluorine derivatives

A series of solid-emissive BOPIM (boron 2-(2-pyridyl)imidazole complex) dyes, bearing electron-donating or -withdrawing substituents (methoxy, hydrogen, or nitro), are facilely synthesized. The compounds were characterized by 1H NMR, 13C NMR and MS. X-ray single crystal diffractions of BOPIM 1 and 2 indicate that nonplanar rigid structures were formed through intermolecular non-covalent interactions. Due to these non-covalent interactions, these dyes exhibit intense fluorescence in solution and also in solid state. According to 1H NMR analysis, electronic effect of the substitutes plays an important role in contribution to these BOPIMs’ electronic states. The photophysical measurements reveal that electron-donating groups (methoxy) lead to significant bathochromism of the absorption and emission. In contrast, electron-withdrawing moieties (nitro) play the reverse role. However, BOPIM 3 bearing nitro groups emits long wavelength fluorescence in solid state, probably due to the formation of intermolecular hydrogen bond. This work elucidates the spectroscopic structure–property relationship of solid-emissive BOPIMs, which would be valuable for design of solid-emissive dyes.