Mechanochromic Properties of Boron‐Difluoride Complexes Bearing π‐Expanded Pyridine Ligands: Effects of π‐Conjugated Skeletons and Halogen Atoms

Effects of halogen atom substitution on luminescent radicals: a case study on tris(2,4,6-trichlorophenyl)methyl radical-carbazole dyads

A series of halogen-substitute carbazole TTM radicals was synthesized. The effect of halogen substituents on radical luminescence was systematically evaluated. It was found that the well-known heavy atom effect does not work in the emission of radicals and that halogen substitution of the donor carbazole can change the HOMO and alter the absorption and emission wavelengths. In addition, the photostability was found to be improved with respect to TTM but not significantly different from that of closed-shell fluorescent molecules.

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 (λ_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 λ_max. The fluorescence quantum yields (Φ_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.

Aryl/Heteroaryl Substituted Boron-Difluoride Complexes Bearing 2-(Isoquinol-1-yl)pyrrole Ligands Exhibiting High Luminescence Efficiency with a Large Stokes Shift

A series of 2-(isoquinol-1-yl)pyrrole-boron complexes possessing (hetero)aryl substituents on the pyrrole and/or isoquinoline moiety were prepared. These compounds exhibited the fluorescence emission character in both solution and solid state. In most cases, the large Stokes shift and high fluorescence quantum yield in the solution were compatible. Furthermore, the structural diversity allowed the precise tuning of emitting colors from light blue to red with strong emission intensity. The present paper describes their comprehensive optical characteristics dependent on the type and position of the substituted aryl groups by the experimental and computational studies.