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. 2023 Oct 3;28(19):6921.
doi: 10.3390/molecules28196921.

Theoretical Investigation on the "ON-OFF" Mechanism of a Fluorescent Probe for Thiophenols: Photoinduced Electron Transfer and Intramolecular Charge Transfer

Yuxi Wang  1   2 , Meng Zhang  1 , Wenzhi Li  1   2 , Yi Wang  1 , Panwang Zhou  2
Affiliations

Theoretical Investigation on the "ON-OFF" Mechanism of a Fluorescent Probe for Thiophenols: Photoinduced Electron Transfer and Intramolecular Charge Transfer

Yuxi Wang et al. Molecules. .

Abstract

In this study, the sensing mechanism of (2E,4E)-5-(4-(dimethylamino)phenyl)-1-(2-(2,4dinitrophenoxy)phenyl)penta-2,4-dien-1-one (DAPH-DNP) towards thiophenols was investigated by density functional theory (DFT) and time-dependent DFT (TD-DFT). The DNP group plays an important role in charge transfer excitation. Due to the typical donor-excited photo-induced electron transfer (d-PET) process, DAPH-DNP has fluorescence quenching behavior. After the thiolysis reaction between DAPH-DNP and thiophenol, the hydroxyl group is released, and DAPH is generated with the reaction showing strong fluorescence. The fluorescence enhancement of DAPH is not caused by an excited-state intramolecular proton transfer (ESIPT) process. The potential energy curves (PECs) show that DAPH-keto is less stable than DAPH-enol. The frontier molecular orbitals (FMOs) of DAPH show that the excitation process is accompanied by intramolecular charger transfer (ICT), and the corresponding character of DAPH was further confirmed by hole-electron and interfragment charge transfer (IFCT) analysis methods. Above all, the sensing mechanism of the turn-on type probe DAPH-DNP towards thiophenol is based on the PET mechanism.

Keywords: ESIPT; d-PET; frontier molecular orbital; thiophenol.

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Conflict of interest statement

The authors declare no conflict of interest. The funders had no role in the design of the study; collection, analyses, or interpretation of data; writing of the manuscript, or decision to publish the results.

Figures

Scheme 1
Scheme 1
Sensing mechanism of probe DAPH-DNP towards thiophenols.
Figure 1
Figure 1
The optimized structures of DAPH-DNP in the S0 (a), LE (S1) (b), and CT (S2) states (c). The labeling of atomic color: O: red; C: blue; H: white; N: yellow, the bond length in Å, the dihedral angle in degrees.
Figure 2
Figure 2
Excitation processes of DAPH-DNP. Molecular orbitals are given in blue and red iso-surfaces, holes and electrons are given in blue and green iso-surfaces, respectively (ωB97XD/TZVP/IEFPCM).
Figure 3
Figure 3
The IFCT analyzing the electron excitation process of DAPH-DNP molecular fragments. (a) The amount of electron transfer between fragments from S0 to S1 (b) and S2 states (c) (ωB97XD/TZVP/IEFPCM).
Figure 4
Figure 4
The ωB97XD/TZVP/IEFPCM calculated energies of DAPH-DNP showing the PET mechanism. The red bars: S2 state.
Figure 5
Figure 5
The optimized geometries of DAPH in the S0 and S1 states. The labeling of atomic color: O: red; C: blue; H: white; N: yellow, the bond length in Å, the angles are in degrees.
Figure 6
Figure 6
The calculated IR spectra of the DAPH in the spectral region of the O1–H2 stretching band calculated.
Figure 7
Figure 7
The PECs of the S0 and S1 states for DAPH along with the O1–H2 bond length (ωB97XD/TZVP/IEFPCM).
Figure 8
Figure 8
Excitation processes of DAPH. Molecular orbitals are given in blue and red iso-surfaces, holes and electrons are given in blue and green iso-surfaces, respectively (ωB97XD/TZVP/IEFPCM).
Figure 9
Figure 9
The DAPH molecular fragments electron excitation process analyzed by the IFCT (a) and the amount of inter-fragment electron transfer from the S0 to S1 States (b) (ωB97XD/TZVP/IEFPCM).
Figure 10
Figure 10
The cLR-ωB97XD/TZVP/IEFPCM calculated energies of DAPH showing the ICT mechanism.
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