Halogen substituted tetraphenylethylene AIEgens: facile single-crystal-to-single-crystal transformation, polymorphism and mechanofluorochromism

Abstract

The molecular structure and supramolecular organization of fluorophores play an important role in generating solid-state fluorescent materials and understanding the mechanism of fluorescence switching/tuning. Molecules that retain the single crystalline nature after applying external stimuli can provide a deep insight into precise control over fluorescence modulation. Herein, we reported the synthesis of eight halogen-substituted tetraphenylethylene (TPE) aggregation-induced emissive (AIEgens) derivatives and investigated the impact of the halogens (F, Cl, Br, and I) on polymorphism and topochemical structural transformation. TPE derivatives with F showed the highest fluorescence (Φ_f = 58–91%) and heavy atom substitution (I) reduced the fluorescence (Φ_f = 14–16%). Solid-state structural studies revealed the formation of polymorphs in ClTPE and BrTPE derivatives whereas FTPE and ITPE did not show any polymorphs. The PXRD and DSC studies were further performed to substantiate polymorph formation, which revealed that the formation of stronger intermolecular interactions in FTPE prevented polymorph formation. Importantly, the ClTPE polymorph exhibited facile single-crystal-to-single-crystal structural transformation upon heating/solvent exposure. The formation of polymorphs, structural transformation and crystal growth characteristics from different solvents produced tuneable and switchable fluorescence. A twisted non-planar TPE unit was also prepared to demonstrate mechanical pressure-induced reversible fluorescence switching. The present work attempted to explore the structural design principles for generating polymorphs and facilitating topochemical transformations in AIEgens, which will enhance our comprehensive understanding of the mechanism underlying fluorescence switching/tuning.