Carbazole fluorophore with an imidazole/thiazole unit: contrasting stimuli-induced fluorescence switching, water-sensing and deep-blue emission

Abstract

Carbazole-based, π-conjugated donor–acceptor fluorophores were synthesized by integrating imidazole/thiazole units. Then, we investigated the impact of subtle structural changes on fluorescence properties. Carbazole integrated with imidazole (Cz-I) and carbazole integrated with thiazole (Cz-T) showed strong fluorescence in solution (quantum yield (Φ_f) = 0.18 (Cz-I) and 0.14 (Cz-T) compared with the standard quinine sulfate) and solid-state (Φ_f = 8.0% (Cz-I) and 14.6% (Cz-T)). Cz-I showed relatively more blue-shifted emission in solution compared with the solid-state (λ_max = 417 nm (CH₃CN) and 460 nm (solid)). Cz-T exhibited deep-blue emission in the solid-state compared with solution (λ_max = 455 nm (CH₃CN) and 418 nm (solid)). Interestingly, Cz-T exhibited a drastic change in fluorescence in organic solvents (CH₃CN, THF, CH₃OH, DMSO) with a low percentage (1%) of water. Cz-I showed reversible fluorescence switching between two fluorescence states upon exposure to trifluoracetic acid (TFA)/ammonia (NH₃). In contrast, Cz-T displayed reversible/self-reversible off–on fluorescence switching upon exposure to TFA or NH₃. Mechanofluorochromic studies of Cz-I showed a slight reduction in fluorescence intensity upon crushing and reversal to the initial state upon heating. Cz-T exhibited off–on reversible/self-reversible fluorescence switching upon crushing/heating. Computational studies indicated that thiazole integration improved the electron-withdrawing characteristics compared with imidazole and contributed to contrasting fluorescence responses. Thus, a simple change of nitrogen with sulfur produced contrasting self-assembly in the solid-state that led to different functional properties and stimuli-induced fluorescence switching.