Synthesis and Fluorescence Properties of Pyrimidine-Based Diboron Complexes with Donor-π-Acceptor Structures

Pyrimidine-based diboron complexes bearing β-iminoenolate ligands and phenyl groups as bulky substituents on the boron atoms were synthesized as novel fluorescent dyes, and their fluorescence properties were investigated in solution and in the solid state. The diboron complexes with donor-π-acceptor structures showed positive solvatochromism in the fluorescence spectra. The cyano derivative exhibited the most dramatic redshift of the fluorescence maximum Fmax with increasing solvent polarity (from 551 nm in n-hexane to 710 nm in acetonitrile). The diboron complexes showed solid-state fluorescence in the range of 578-706 nm with fluorescence quantum yields of 0.06-0.28. Additionally, the trifluoromethyl derivative exhibited solvent-inclusion solid-state fluorescence. The trifluoromethyl derivative formed toluene-inclusion and ethyl acetate-inclusion crystals. The toluene-inclusion crystal (Fmax = 668 nm, Φf = 0.16) showed a blueshifted Fmax and higher Φf value compared to the original trifluoromethyl derivative (Fmax = 694 nm, Φf = 0.08) in the solid state. On the other hand, the Fmax (709 nm) and Φf (0.04) values of the ethyl acetate-inclusion crystal were redshifted and lower, respectively.