Polymorphism and benzene solvent controlled stimuli responsive reversible fluorescence switching in triphenylphosphoniumfluorenylide crystals

Triphenylphosphoniumfluorenylide (TPPFY), a fluorescent fluorene attached molecule, showed polymorphism and benzene solvent induced aggregation enhanced emission (AEE) in the solid state. Crystallization from CH₃CN produced non-fluorescent crystals of TPPFY (TPPFY-1), whereas intense yellow fluorescent crystals (plates and blocks) were obtained from benzene (TPPFY-2, λ_max = 538 nm, Φ_f = 38%). Structural analysis indicates that TPPFY-1 exhibits strong π⋯π interactions (3.371-3.399 Å) between fluorene units in the crystal lattice that quenched the solid state fluorescence. In contrast, the inclusion of a benzene molecule in TPPFY-2 prevents the close packing of fluorophores and shows intense yellow fluorescence in the solid state. TPPFY-2 showed only weak C-H⋯π interactions between fluorene and the phenyl group of triphenylphosphoniumylide. Hirshfeld surface analysis further supported the differences in the intermolecular interactions and molecular packing between TPPFY-1 and TPPFY-2. Interestingly, heating/strong crushing irreversibly converts fluorescent TPPFY-2 crystals to non-fluorescent TPPFY-1 crystals. However, recrystallization of TPPFY-1 from benzene produced fluorescent TPPFY-2 crystals. Thus TPPFY displays reversible off-on fluorescence switching heating/crushing and recrystallization. PXRD studies confirmed the polymorphism as well as the phase conversion of TPPFY-2 to TPPFY-1 by external stimuli. This study indicates the role of the benzene molecule in controlling the molecular packing and functional properties of TPPFY.