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. 2024 Apr 5;29(7):1629.
doi: 10.3390/molecules29071629.

Modulating the ESIPT Mechanism and Luminescence Characteristics of Two Reversible Fluorescent Probes by Solvent Polarity: A Novel Perspective

Yang Wang  1 , Hongyan Mu  1 , Yuhang Sun  1 , Jiaan Gao  1 , Xiaodong Zhu  1 , Hui Li  1
Affiliations

Modulating the ESIPT Mechanism and Luminescence Characteristics of Two Reversible Fluorescent Probes by Solvent Polarity: A Novel Perspective

Yang Wang et al. Molecules. .

Abstract

As reversible fluorescent probes, HTP-1 and HTP-2 have favourable applications for the detection of Zn2+ and H2S. Herein, the impact of solvent on the excited-state intramolecular proton transfer (ESIPT) of HTP-1 and HTP-2 was comprehensively investigated. The obtained geometric parameters and infrared (IR) vibrational analysis associated with the intramolecular hydrogen bond (IHB) indicated that the strength of IHB for HTP-1 was weakened in the excited state. Moreover, structural torsion and almost no ICT behaviour indicated that the ESIPT process did not occur in HTP-1. Nevertheless, when the 7-nitro-1,2,3-benzoxadiazole (NBD) group replaced the H atom, the IHB strength of HTP-2 was enhanced after photoexcitation, which inhibited the twisting of tetraphenylethylene, thereby opening the ESIPT channel. Notably, hole-electron analysis and frontier molecular orbitals revealed that the charge decoupling effect was the reason for the fluorescence quenching of HTP-2. Furthermore, the potential energy curves (PECs) revealed that HTP-2 was more inclined to the ESIPT process in polar solvents than in nonpolar solvents. With a decrease in solvent polarity, it was more conducive to the ESIPT process. Our study systematically presents the ESIPT process and different detection mechanisms of the two reversible probe molecules regulated by solvent polarity, providing new insights into the design and development of novel fluorescent probes.

Keywords: excited-state intramolecular proton transfer; intramolecular charge transfer; intramolecular hydrogen bond.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Scheme 1
Scheme 1
Configurations and detection mechanism of HTP-1 and HTP-2.
Figure 1
Figure 1
Optimized geometries of HTP-1 using the MN15/6-31G (d, p) method in S0 and S1 states; blue atom: C, orange atom: N, red atom: O, white atoms: H.
Figure 2
Figure 2
Optimized geometries of HTP-2 using the MN15/6-31G (d, p) method in S0, S1 and S1′ states; blue atom: C, orange atom: N, red atom: O, white atoms: H.
Figure 3
Figure 3
Calculated S0- and S1-stated potential energy curves (PECs) of HTP-1 in (a) ACN, (b) ACE, (c) DCM and (d) n-Hexane solvents.
Figure 4
Figure 4
Calculated S0- and S1-stated potential energy curves (PECs) of HTP-2 in (a) ACN, (b) ACE, (c) DCM and (d) n-Hexane solvents.
Figure 5
Figure 5
Infrared (IR) vibrational spectra of O1–H1 and N1-H1 bonds for HTP-1 in the S0 and S1 states.
Figure 6
Figure 6
Infrared (IR) vibrational spectra of O1–H1 and N1–H1 bonds for HTP-2 in S0, S1 and S1′ states.
Figure 7
Figure 7
Computational the frontier molecular orbitals (FMOs) and HOMO-LUMO gaps (Egap) in HTP-1 and HTP-2.
Figure 8
Figure 8
Simulated absorption and emission spectra of HTP-1 in ACN, ACE, DCM and n-Hexane solvents.
Figure 9
Figure 9
Simulated absorption and emission spectra of HTP-2 in ACN, ACE, DCM and n-Hexane solvents.
Figure 10
Figure 10
Hole-electron analysis (green and blue regions represent electrons and holes, respectively) in ACN. (a) S1 form of HTP-1, (b) S1 form of HTP-2 and (c) S1′ form of HTP-2.
Figure 11
Figure 11
Reduced density gradient (RDG) scatters and isosurfaces for ACN of HTP-1.
Figure 12
Figure 12
Reduced density gradient (RDG) scatters and isosurfaces for ACN of HTP-2.
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