Steric hindrance effect on the excited-state proton transfer process: TDDFT study on the fluorescent sensing mechanism of a fluoride sensor

• Published in: Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy Vol. 271; p. 120872
• Main Authors: Liu, Dong, Li, Peng-Yuan, Wang, Si-Jia, Gong, Bo, Lu, Ting, Li, Guang-Yue
• Format: Journal Article
• Language: English
• Published: England Elsevier B.V 15.04.2022
• Subjects: Density Functional Theory; ESPT; Excited state; Fluorescent Dyes - chemistry; Fluorescent sensor; Fluorides - analysis; Protons; Spectrometry, Fluorescence; Steric hindrance; TDDFT; Steric hindrance; Fluorescent sensor; ESPT; TDDFT; Excited state
• ISSN: 1386-1425; 1873-3557
• DOI: 10.1016/j.saa.2022.120872

[Display omitted] •The fluoride-sensing mechanism of a fluorescent sensor is theoretically confirmed.•Calculated VTEs are in good agreements with experimental spectra.•Fluoride-sensor complex undergoes an ESPT process with a relatively high potential barrier.•ESPT alleviates the steric hindrance in the sensor and enlarges the conjugated system.•ESPT process leads to the red-shifted fluorescence upon the addition of fluoride. An understanding of the excited-state process and the sensing mechanism for specific anions can be helpful for the design and synthesis of fluorescent sensors in analytical chemistry and biotechnology. Here, we theoretically investigated the fluorescent response mechanism of a reported acylhydrazone-based fluorescent sensor (Soft Matter, 2019, 15, 6690) for fluoride recognition using the time-dependent density functional theory approach. At the M06/TZVP/SCM level, the vertical excitation energies, which were calculated based on the ground state and first singlet-state geometries of the sensor molecule, agreed well with the experimental ultraviolet–visible and fluorescence spectra. Therefore, the time-dependent density functional theory method was considered reasonable and effective. According to the frontier orbital analysis and an excited-state potential energy scan, we proposed an excited-state proton transfer mechanism for the sensor-fluorine complex, where the steric hindrance leads to a high potential barrier. The excited-state proton transfer process facilitates sensor molecule deprotonation, alleviates its steric hindrance effect and expands its conjugated system. As a result, the fluorescence emission band of the sensor molecule was red-shifted significantly with the addition of fluoride anion. Based on this fluorescence difference, the sensor could be used for fluoride anion identification. This work provides a strategy to study sensor–analyte interactions in the excited state and offers an approach to tune the fluorescence emission wavelength of sensor molecules in anionic environments.