Difluoroboron Complexes Based on Benzimidazole-Phenolates as Blue Emitters

Novel four-coordinated boron complexes (1-5) were synthesized via a reaction between BF₃·CH₃OH and benzimidazole-phenolate ligands (L1-L5), which are N,O-donors. These complexes exhibit intense blue emission in the solution and solid states accompanied by notable fluorescence quantum yields (Φ_F). The study of the structure-property relation, through theoretical and experimental approaches, revealed a distinctive trend where compounds incorporating electron-donating substituents (methyl and ethoxy groups) in the phenolate moiety manifest shifts in emission wavelengths across the blue spectrum, concomitant with an increase in Φ_F. Furthermore, the incorporation of an aromatic ring into the benzimidazole moiety considerably intensifies the rate of radiative relaxation from excited states. Notably, in the solid phase, either as a crystalline powder or loaded into polymer films, these modified complexes maintain or even surpass Φ_F values observed in molecular solutions, ranging from 0.18 to 0.57, depending on the substitution. This characteristic makes these compounds attractive for applications in optoelectronics. All of the compounds were characterized using ¹H, ¹³C, ¹¹B, and ¹⁹F NMR, elemental analysis, and the molecular structures were corroborated through single-crystal X-ray diffraction analysis. Computational calculations via time-dependent density functional theory further elucidate the tunability of optical bandgaps through group substitution on ligands, aligning well with experimental observations.