Abstract
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.
Highlights
INTRODUCTIONThe excited state intramolecular charge transfer (ESICT) is fused with Excited state intramolecular proton transfer (ESIPT) mechanism, which leads to the synthesis of an NIR-emitting 2-[3-(benzo[d]thiazol-2 yl) - 2 - hydroxy - 5 - methylstyr-yl] - 3 -ehtylbenzo[d]thiazol-3ium iodide (cyanine 1, see Scheme 1) for intracellular lysosome detection
Excited state intramolecular proton transfer (ESIPT) is a photo-induced process, during which a proton transfers to the neighboring heteroatom through intramolecular hydrogen bond, resulting in the respective keto form.1 Demchenko, Tang and Chou,2 As a typical example of ESIPT chromophore, 2-(2 -hydroxyphenyl)benzothiazole (HBT) gives two emission bands, attributing to its enol form and the lower energy keto form, respectively (Scheme 1).3 Due to ESIPT, fluorescence from the keto∗ form is characterized by abnormally large Stokes shift, in comparison with that from the enol∗.4 most of the common ESIPT molecules such as HBT or SCHEME 1
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
Summary
The excited state intramolecular charge transfer (ESICT) is fused with ESIPT mechanism, which leads to the synthesis of an NIR-emitting 2-[3-(benzo[d]thiazol-2 yl) - 2 - hydroxy - 5 - methylstyr-yl] - 3 -ehtylbenzo[d]thiazol-3ium iodide (cyanine 1, see Scheme 1) for intracellular lysosome detection.. An electron-withdrawing group (benzothiazolium iodide) is attached to HBT to enable both ICT and ESIPT in 1, producing intense red/NIR emission with 9176 cm-1 Stokes shift upon photoexcitation.. The study here shows extremely fast ESIPT process occurs in cyanine 1 upon photoexcitation, and the fast ESIPT plays an important role to adjust the electronic charge distribution, leading to the efficient extension of π-conjugation in the molecular backbone in its excited state, which leads to large red shifted emission. In order to aid the study, we synthesized 2-[5-(benzo[d]thiazol-2-yl)-2-hydroxystyryl]-3ehtylbenzo[d]thiazol-3-ium iodide (cyanine 2, Scheme 1) as comparison, in which ESIPT is no longer available and only the excited intramolecular charge transfer is expected. The study here shows extremely fast ESIPT process occurs in cyanine 1 upon photoexcitation, and the fast ESIPT plays an important role to adjust the electronic charge distribution, leading to the efficient extension of π-conjugation in the molecular backbone in its excited state, which leads to large red shifted emission.
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