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

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.

Solvent Effect on Excited-State Intramolecular Proton-Coupled Charge Transfer Reaction in Two Seven-Membered Ring Pyrrole-Indole Hydrogen Bond Systems

“…So far, tremendous efforts have been invested to investigate O–H-type ESIPT molecules with a strong intramolecular hydrogen bond, ,,,,− while limited N–H-type ESIPT molecules have been reported owing to their relatively weak intramolecular hydrogen bond. ,− As one of the most fundamental and significant biological and chemical reactions, the N–H-type ESIPT reaction deserves more in-depth investigations. In our previous work, we synthesized a series of seven-membered ring pyrrole-indole intramolecular hydrogen bond systems, including 2-(5-(1 H -pyrrol-2-yl)-3 H -pyrrolizin-6-yl)-3,3-dimethyl-3 H -benzo­[ g ]­indole ( g -PPDBI ) and 2-(5-(1 H -pyrrol-2-yl)­3 H -pyrrolizin-6-yl)-1,1-dimethyl-1 H -benzo­[ e ]­indole ( e -PPDBI ).…”

Progress in Tuning Emission of the Excited-State Intramolecular Proton Transfer (ESIPT)-Based Fluorescent Probes

“…In a collaborated effort, study from Guo et al further revealed the details of the photophysical process, showing the fluorescence lifetime of 1.75 ns for 8 in CH 3 CN. 14 These findings consistently indicate that the ESIPT is a rapid process. Although this class of new fluorescent dyes have strong donor–acceptor interaction in the excited state, as illustrated in 6b , they exhibit reasonably high fluorescence quantum yields (φ fl ≈ 0.20–0.35), which opens the possibility for various applications.…”

Mechanistic Insights into the Photoisomerization of N,N′‐Disubstituted Indigos

“…The TA spectra at different time delays are presented exemplarily for toluene in Figure 2 and for other solvents in the Supporting Information (Figure S2). The lifetimes of TA signals were extracted by employing global analysis using a sequential model (Figure S3) [15] with two decay components τ 1 and τ 2 (E*) (Table 3). The obtained TA spectra for all three compounds in toluene, CHCl 3 , MeCN, and DMSO are similar to each other and do not show any significant spectral shape changes depending on substituent or solvent parameters.…”