Highly efficient red emissive zwitterionic fluorophores across wide pH range: White light emission, mechanofluorochromism, sensing chlorinated solvent and bioimaging

The design and development of red/NIR emissive organic solid-state fluorescent materials have received significant interest because of their application potential in optoelectronics and bioimaging. Herein, we have synthesized strong red emitting triphenylamine-picoline based zwitterionic donor–acceptor molecules, (E)-3-(4-(4-(diphenylamino)styryl)pyridin-1-ium-1-yl)propane-1-sulfonate (TPA-P-PS) and (E)-4-(4-(4-(diphenylamino)styryl)pyridin-1-ium-1-yl)butane-1-sulfonate (TPA-P-BS) in the solid-state and structurally characterized. TPA-P-PS and TPA-P-BS showed strong red emission at 613 and 630 nm, respectively (quantum yield (Φf) = 29.6 and 16.2 %). Single crystal analysis of TPA-P-PS revealed twisted molecular conformation without any close π…π stacking. The strong intermolecular H-bonding formed between the sulphonate oxygen and picoline hydrogen/lattice water of TPA-P-PS rigidified the fluorophore and contributed to enhanced solid-state emission. Further, both compounds exhibited distinct red-shifted absorption and strong fluorescence in CHCl3, which was utilized for sensing CHCl3 in methanol. Across a wide pH range (3.0 to 11.0), TPA-P-PS and TPA-P-BS aqueous colloidal dispersion showed strong fluorescence without any significant change. The white emissive stable aqueous colloidal solution was developed by mixing red emissive TPA-P-PS/TPA-P-BS with polystyrene sulphonate (PSS). TPA-P-BS also displayed reversible mechanofluorochromism upon crushing and heating. Bioimaging studies revealed that increasing a carbon chain in TPA-P-BS improved the infiltration into the planktons and exhibited strong light-up of cells with low background noise.