A series of boron difluoride complexes of azinylcarbazoles: synthesis and structure-property relationships

A series of boron difluoride (BF2) complexes of azinylcarbazoles 1b-1h were synthesized, and the effects of the structure of azine moieties on the photophysical and electrochemical properties of the BF2 complexes were clarified. UV-vis analysis of 1b with quinoline, 1c with isoquinoline, and fully fused 1d revealed that fusion with a benzene ring to a pyridylcarbazole BF2 complex (1a) resulted in red shifts of longest-maximum absorption wavelengths (?(max)). UV-vis analysis of 1e and 1f with pyrimidine, 1g with pyridazine, and 1h with pyrazine revealed that substitution of a carbon atom to a nitrogen atom in 1a also resulted in red shifts of ?(max). The fluorescence quantum yields (f(f)) decreased from 1a to 1b-1h, and especially, the fluorescence of 1e, 1g, and 1h was quenched in solution. At 77 K, the emission intensities of 1b-1h were significantly increased compared with those at ambient temperature, and they also exhibited phosphorescence with relatively narrow energy gaps between the singlet and triplet excited states. These results on the emission at 77 K indicate that the quench of fluorescence from 1e, 1g, and 1h at ambient temperature originates from both internal conversions and intersystem crossing. In the solid state, all of the complexes including 1e, 1g, and 1h exhibited emission. Distinctive aggregation-induced emission properties were observed for 1e-1h. Electrochemical measurements revealed that the replacement of the pyridine moiety in 1a with azine moieties reduced electrochemical gaps mainly due to a decrease in the LUMO levels. The effects of azine moieties on electronic structures were also discussed based on theoretical calculations.

Automated summary

A series of boron difluoride complexes were synthesized, and the effects of different azine moieties on their photophysical and electrochemical properties were examined. The results showed that the addition of a benzene ring or replacement of a carbon atom with a nitrogen atom resulted in a red shift in the absorption wavelength. The fluorescence quantum yields decreased from one complex to another, and the fluorescence of some complexes was quenched in solution. At lower temperatures, the emission intensity significantly increased, indicating the involvement of internal conversions and intersystem crossing in fluorescence quenching. Aggregation-induced emission properties were observed in the solid state. Electrochemical measurements revealed a decrease in electrochemical gaps due to the substitution of pyridine moieties with different azine moieties.