Naphthylacetonitrile isomerism and donor conformational flexibility controlled molecular self-assembly and solid-state fluorescence properties

Organic fluorophores with propellor/partially planar donor unit and naphthylacetonitrile positional isomer were synthesized and explored their impact on the solid-state structural assembly, fluorescence properties and stimuli-induced fluorescence switching. Partially planar phenyl carbazole (Cz) and propeller shaped triphenylamine (TPA) donor units integrated with 1-naphthylacetonitrile/2-naphthylacetonitrile acceptors (Cz-1-Naph, Cz-2-Naph and TPA-2-Naph) by simple condensation reaction. All three compounds showed weak and tunable fluorescence in solution (λmax = 452–520 nm, Φf = 0.24 compared to quinine sulphate standard). However, Cz-Naph isomers showed relatively strong solid-state fluorescence compared to TPA-Naph (Φf = 5.9% (Cz-1-Naph), 13.8% (Cz-2-Naph) and 1.2% (TPA-2-Naph)). Further, Cz-Naph isomers showed relatively blue shifted emission (λmax = 517 (Cz-1-Naph) and 498 nm (Cz-2-Naph)) compared to TPA-2-Naph isomer (λmax 543 nm). Solid-state structural analysis revealed varied molecular conformation and arrangement depending on the naphthyl isomerism and donor units. The modulation of molecular conformation and packing contributed for fluorescence tuning. Computational studies indicated a clear intramolecular electron transfer from TPA/Cz donor to naphthylacetonitrile acceptor unit. Interestingly, Cz-2-Naph exhibited reversible mechanical crushing and heating induced fluorescence switching that was attributed to the reversible phase transformation between crystalline and amorphous phase and vice versa.