Thiazolothiazole based donor-π-acceptor fluorophore: Protonation/deprotonation triggered molecular switch, sensing and bio-imaging applications.

Stimuli modulated photophysical properties of a new asymmetric multifunctional molecular probe, 2-(pyren-1-yl)-5-(pyridin-4-yl)thiazolo [5,4-d]thiazole (PYTZ-P) bearing electron-rich pyrene and electron-deficient pyridine units linked through a π-conjugated thiazolo [5,4-d]thiazole (TTz) bridge are reported. Its sensitivity towards protons (TFA) in solution is manifested in the form of bathochromically shifted emission that spreads all over the visible region, and is related to the increased acceptor strength of pyridine upon protonation and subsequent enhanced magnitude of intramolecular charge-transfer in the probe. Similar modulation of the luminescence behaviour of the probe was also observed in the solid state. These observations are well rationalised by the theoretical calculations. The applicability of the sensitivity of the probe towards TFA has been demonstrated by developing the ready-to-use portable paper strips. Furthermore, PYTZ-P showed different acid/base induced absorption/emission switching property. Interestingly, when the response of the probe was noted in the presence of several cations, modulation of the electronic properties was observed only in the presence of Hg ions, with the lowest detection limit of 8.43 × 10 M. Additionally, the sensitivity of the probe towards TFA and Hg ions in solution and solid states is convincingly mimicked in the fluorescence imaging of HeLa and A375 cancer cells.