Halogen Atom and Position Dependent Strong Enhancement of Solid‐State Fluorescence and Stimuli Responsive Reversible Fluorescence Switching

Abstract Halogen (F, Cl and Br) substituted triphenylamine (TPA)‐phenylacetonitrile donor‐acceptor aggregation enhanced emissive (AEE) compounds have been synthesized and investigated halogen atom and substitution position dependent solid‐state fluorescence. Interestingly, para position substituted compounds exhibited stronger fluorescence compared to meta position substituted compounds. Further, strong enhancement of fluorescence efficiency was observed with increase of atomic size from H to Br ( Φ f =11.81 to 33.05%). Solid state structural studies revealed formation of halogen‐H and halogen‐π interactions in the crystal lattice that rigidified the fluorophores and enhanced the fluorescence. Computational studies have been performed to get insight on the fluorescence efficiency modulation by halogen. Integration of propeller shaped TPA lead to external stimuli induced reversible/self‐reversible fluorescence switching. PXRD studies confirmed self‐recovering of crystalline state after crushing. Fluorescent PMMA polymer thin films of halogen derivatives showed off‐on fluorescence switching upon exposure of acid and base/heating. Thus, the present studies indicate that halogen could be employed for strongly enhancing fluorescence efficiency of organic compounds that might be of potential interest for optoelectronic device application.