Environment-Sensitive Azepane-Substituted β‑Diketones and Difluoroboron Complexes with Restricted C–C Bond Rotation

Luminescent β-diketones (bdks) and difluoroboron coordinated complexes (BF2bdks) exhibit many environment-sensitive properties, such as solvatochromism, viscochromism, aggregation-induced emission (AIE), and thermal and mechanochromic luminescence (ML). In a previous study, an azepane-substituted bdk ligand (L1) and boron dye (D1) showed noteworthy luminescence properties but low quantum yields (Φ, L1: 0.26; D1: 0.02) due to free intramolecular bond rotation and twisted intramolecular charge transfer state formation with associated nonradiative decay. Thus, in order to improve the quantum yields, an azepane-substituted bdk ligand (L2) and boron complex (D2) with restricted C–C bond rotation were synthesized, and various luminescence properties were investigated. Restricting bond rotation blue-shifted absorptions and emissions, increased lifetimes, and greatly improved quantum yields (Φ, L2: 0.47; D2: 0.83). Excited state density functional theory calculations displayed twisted geometries for L1 and D1 but more planar geometries for L2 and D2. All compounds showed red-shifted emissions in more polar solvents. For viscochromism, L1 and D1 exhibited higher emission intensity in more viscous media. However, L2 and D2 did not show dramatic viscochromism, substantiating the relation between viscosity sensitivity and intramolecular bond twisting. Additionally, while both ligands showed quenched emission upon aggregation, the dyes exhibited AIE regardless of bond restriction. Thermal and ML studies showed a more dramatic emission shift for L2 than L1 between thermally annealed and melt-quenched states. In summary, the quantum yields of the azepane-substituted bdk ligand and boron dye were successfully improved by restricting the intramolecular C–C bond rotation, making various luminescence properties more promising for environment-sensitive applications.