Molecular conformational twist-controlled wide fluorescence tuning and white light emission in a single fluorophore via halochromism

Abstract

pH-responsive fluorescence molecules are interesting candidates for achieving tunable as well as white light emission from a single molecule. Herein, we have synthesized pyridine-integrated carbazole and triphenylamine donor–acceptor isomers, (Z)-3-(4-(9H-carbazol-9-yl)phenyl)-2-(pyridinyl)acrylonitrile (1–3) and (Z)-3-(4-(diphenylamino)phenyl)-2-(pyridinyl)acrylonitrile (4–6), and studied the effects of the structure and solvent on acid-responsive fluorescence modulation in the solution state. Both carbazole (1–3) and triphenylamine (4–6) isomers produced twisted intramolecular charge transfer (TICT) states and showed tunable fluorescence depending on the solvent polarity. Highly blue shifted fluorescence was observed in non-polar toluene, while polar DMF showed large red-shifted fluorescence. Further, 1–3 isomers exhibited relatively strong fluorescence compared to 4–6. Interestingly, 1–6 exhibited organic acid-dependent (trifluoroacetic acid (TFA) and p-toluene sulphonic acid (PTSA)) and solvent-dependent fluorescence tuning. Carbazole isomers (1–3) showed wide fluorescence colour tuning from blue to red as well as white light emission in toluene, whereas green to red transition was observed in CHCl₃. Addition of TFA, a stronger acid, to 1 and 3 afforded blue, green and red emission along with white emission in toluene, whereas weak PTSA produced blue and red emission along with white emission. In contrast, addition of TFA or PTSA to the triphenylamine isomers (4–6) exhibited complete quenching of fluorescence in all solvents. The meta-isomers did not show significant fluorescence tuning with acids. The wide fluorescence tuning of 1 in toluene with TFA has also been successfully transformed into a PMMA polymer matrix, which also exhibited clear fluorescence tuning from blue to red. Density functional theoretical (DFT) studies indicated high molecular twisting in the excited state and clear separation of charge transfer from the donor carbazole to the acceptor unit compared to the propeller-shaped triphenylamine isomer; this may contribute to the wide fluorescence tuning in the carbazole isomers. The present studies could be useful for choosing appropriate molecular structures and solvent systems for achieving wide fluorescence tuning and white light emission from a single donor–acceptor molecule.