Investigating the structure–fluorescence properties of tetraphenylethylene fused imidazole AIEgens: reversible mechanofluorochromism and polymer matrix controlled fluorescence tuning

Abstract

A series of stimuli-responsive AIEgens of tetraphenylethylene (TPE) fused imidazole derivatives (1–7) were synthesized, and their substituent controlled fluorescence properties in the solid state were explored. The structure of the synthesised compounds was characterized by NMR, mass and single crystal X-ray diffraction spectroscopy techniques. 1–7 exhibited a typical aggregation induced emissive (AIE) behaviour in the THF : water mixture. These compounds exhibited substituent dependent strong solid state fluorescence (Φ_f = 16.6–55.7%) with a tunable λ_max between 458 and 496 nm. Chlorine substituted 6 showed the highest fluorescence efficiency (Φ_f = 55.7%), whereas acetyl substituted 3 exhibited the lowest fluorescence efficiency (Φ_f = 16.6%). All the seven compounds showed a mechanical pressure induced reversible fluorescence switching. Crushing 1–7 resulted in red shifted fluorescence, which was reversed to its initial state by heating/solvent vapour exposure. Interestingly, integration of 1–7 into a hydrophobic PMMA polymer resulted in blue shifted fluorescence at 470 nm, whereas that into hydrophilic PVA polymer resulted in red shifted fluorescence at 491 nm. Thus highly solid state emissive TPE fused imidazole compounds were prepared, and their stimuli-induced reversible fluorescence switching and polymer controlled tunable fluorescence were demonstrated.