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Excited-state intramolecular proton transfer (ESIPT) is an important photophysical process which have wide applications in fluorescent probes, molecular switches, and organic light-emitting materials. The molecule with ESIPT is highly sensitive to its surroundings, such as solvents, and exhibits fruitful fluorescence properties. Theoretical study on the microscopic mechanism of proton transfer in regulating the fluorescence properties of organic molecules is very important. Recently, Yang et al. [Yang G, Li Y, He L, et al. 2024 Microchem. J. 198 110044] designed a fluorescent probe (FZ) based on ESIPT. They observed bimodal emission, strong long-wavelength emission and weak short-wavelength emission in low-polar, highly polar non-protic and highly polar protic solvents, respectively. To reveal the microscopic mechanism of these fluorescence properties, in this work, we theoretically investigate the proton transfer process of FZ molecule in various solvents including toluene, dichloromethane, ethanol, and dimethyl sulfoxide (DMSO) by using density functional theory and time-dependent density functional theory. Based on polarizable continuum model with the integral equation formalism variant (IEFPCM), the optimized structures are obtained and potential energy curves for proton transfer are scanned employing the CAM-B3LYP functional with Grimme’s D3 dispersion and 6-31+g(d,p)/6- 311+g(d,p) basis. Importantly, the excited-state dynamics behaviors of four intermolecular hydrogen-bonding systems in ethanol solvent are explored by using super-molecular model. The structures, hydrogen-bonding energies, and interaction region indicator (IRI) analysis show that the strength of the intramolecular hydrogen bond significantly enhances upon photo excitation. The potential energy curves indicate that FZ molecules tend to undergo the ESIPT process in all the solvents. The barriers of proton transfer decrease as the solvent polarity increases. As a result, a dual emission and a strong keto (K*) emission were observed in dichloromethane (low-polar) and DMSO (highly polar non-protic), respectively. In ethanol (highly polar protic), the excited-state behaviors of the four super-molecular systems (FZ-OH1, FZ-OH2, FZ-OH3, FZ-OH4) are quite different. In FZ-OH1, ESIPT cannot occur because enol (E*) is more stable than K*. As a result, FZ-OH1 can produce the E* emission. In contrast, ESIPT can take place almost barrierlessly in FZ-OH2, resulting in the K* emission. Interestingly, FZ-OH3 could undergo stepwise excitedstate double protons transfer (ESDPT) between FZ and ethanol molecules, resulting in a dark state of K*. Hole-electron analysis demonstrates that it is the twisted intramolecular charge transfer (TICT) that quenches the fluorescence of K*. Therefore, the observed weak short-wavelength emission in ethanol could ascribe to the E* emission of FZ-OH3. Our work is of great significance in understanding and predicting the photophysical properties of organic molecules in solvents and provides a useful theoretical basis for designing and developing ESIPT-based functional materials.
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Keywords:
- excited-state intramolecular proton transfer /
- excited-state double proton transfer /
- fluorescence /
- solvent
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