Synthesis of dual-state emissive twisted donor–acceptor fluorophores: tunable fluorescence and self-reversible mechanofluorochromism†

Abstract

Exploring new organic solid-state fluorescent molecules with tunable and switchable fluorescence is of fundamental importance for understanding the structure–property relationship and developing efficient materials for device applications. Herein, we have designed and synthesized blue-emitting cyanophenyl ether attached triphenylamine (TPA)-based donor–acceptor derivatives (CPDB-MN, CPDB-ECA, CPDB-CA, CPDB-CAA and CPDB-MBA) and investigated their solid-state fluorescence and stimuli-responsive fluorescence switching. All five fluorophores exhibited tunable dual-state (solution and solid-state) emission, depending on the solvent polarity and acceptor functionality. The CPDB derivatives showed dual fluorescence at shorter and longer wavelengths in solution, which might be attributed to the integration of blue-emitting cyanophenyl with the TPA donor–acceptor unit. In the solid-state, these derivatives showed aggregation-induced emission (AIE), predominantly from the TPA donor–acceptor system. The CPDB molecules exhibited tunable solid-state fluorescence between 532 and 615 nm (quantum yield (Φ_f) = 1.02% to 14.14%). Solid-state structural analysis revealed that cyanophenyl and diphenylamine adopted different conformations depending on the acceptor. Weak intermolecular interactions in the crystal lattice created a network structure and increased structural rigidity in the solid-state. Computational studies further supported the conformational changes, showing that acceptors influenced the optical bandgap and fluorescence tuning. Notably, CPDB-CA displayed mechanical stimuli-responsive reversible/self-reversible fluorescence switching due to a reversible phase transition. Powder X-ray diffraction (PXRD) was performed to gain insights into fluorescence switching and phase transitions. Thus, the present work studied the effect of the acceptor unit on the molecular conformation and solid-state fluorescence.