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 are of fundamental importance for understanding structure-property relationship and developing efficient materials for device applications. Herein, we have designed and synthesized blue emitting cyano phenyl ether attached triphenylamine based donor-acceptor derivatives (CPDB-MN, CPDB-ECA, CPDB-CA, CPDB-CAA and CPDB-MBA) and investigated the solid-state fluorescence and stimuli-responsive fluorescence switching. All five fluorophores exhibited tunable dual state (solution and solid-state) emission depend on the solvent polarity and acceptor functionality. CPDB derivatives showed dual fluorescence, shorter and longer wavelength, in solution, which might be attributed to the integration of blue emitting cyanophenyl with TPA donor-acceptor. In solid-state, the derivatives showed aggregation induced emission (AIE) predominantly from the TPA donor-acceptor unit. CPDB molecules exhibited tunable solid-state fluorescence between 532 and 615 nm (quantum yield (f) = 1.02 to 14.14%). Solid-state structural analysis revealed that the cyanophenyl and diphenylamine adopted different conformation depend on the acceptor. The weak intermolecular interactions formed in the crystal lattice produced network structure and increase the structural rigidity in the solid-state. Computational studies further supported the conformational change and acceptors influenced optical band gap and fluorescence tuning. CPDB-CA displayed mechanical stimuli-responsive reversible/self-reversible fluorescence switching due to reversible phase transition. Powder X-ray diffraction (PXRD) was performed to gain insight on the fluorescence switching and phase transition. Thus, the present work studied the effect of acceptor unit on the molecular conformation and solid-state fluorescence.