Ultrafast excited state intramolecular proton/charge transfers in novel NIR-emitting molecules

The unusual large bathochromic shift from a novel near-infrared (NIR)-emitting molecule, 2-[3-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methylstyr-yl]-3-ehtylbenzo[d]thiazol-3-ium iodide (named cyanine 1) with combination of intramolecular charge transfer (ICT) and intramolecular proton transfer (IPT) process in one molecular framework, is systematically investigated using ultrafast transient absorption (TA) spectroscopy and quantum chemical calculations. In order to understand the synergetic coupling effect of the excited state intramolecular proton/charge transfers (ESIPT/ESICT) for the intense near-infrared emission of cyanine 1, an analogue non-ESIPT molecule, 2-[5-(benzo[d]thiazol-2-yl)-2-hydroxystyryl]-3-ehtylbenzo[d]thiazol-3-ium iodide (named cyanine 2) has also been investigated as comparison. Steady-state spectra and theoretical calculations suggest that the large Stokes shift and high fluorescence quantum yield in cyanine 1 originate from the ultrafast ESIPT, which leads to the efficient extension of π-conjugation in the molecular backbone in its excited states. Femtosecond transient absorption spectra further confirm above-mentioned conclusion that an extremely fast ESIPT process occurs in cyanine 1 upon excitation, followed by a solvent reorganization process (ca. 1.5 ps). This solvation is obviously slower compared to cyanine 2 (ca. 0.8 ps), indicating the extent of ESICT concerned ESIPT in keto* form of cyanine 1 is slightly weaker than that of ESICT in cyanine 2, where the fast ESIPT plays an important role in extending the efficient π-conjugation in the molecular backbone by adjusting the electronic charge distribution in keto* form. Such an effect can reduce the radiationless transition due to weak solvation process in keto* form, and then promotes the quantum yield of the large red-shifted fluorescence in cyanine 1.