https://doi.org/10.1140/epjd/s10053-025-01113-8
Research - Atoms, Molecules, Ions, and Clusters
Theoretical investigation of fluorescence turn-on mechanism for cysteine detection by quinazolinone-based probe HTPQC
Henan Key Laboratory of Infrared Materials & Spectrum Measures and Applications, School of Physics, Henan Normal University, 453007, Xinxiang, People’s Republic of China
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Received:
18
October
2025
Accepted:
22
December
2025
Published online:
21
January
2026
The fluorescence turn-on mechanism of the quinazolinone-based probe HTPQC for selective cysteine (Cys) detection has been theoretically investigated. Electron–hole analysis indicates that HTPQC has a charge transfer distance (D-index) of 4.86 Å, suggesting significant intramolecular charge transfer (ICT). Additionally, the energy levels of the frontier molecular orbitals (FMOs) for both the donor and acceptor components do not support the occurrence of a photoinduced electron transfer (PET) process. These results illustrate that the weak fluorescence of the probe is mainly attributed to ICT, rather than the PET mechanism proposed in previous studies. Upon reaction with Cys, the product HTPQ shows a reduced D-index (4.59 Å) in its enol form, resulting in enhanced fluorescence at 534 nm. Subsequently, the enol form transforms into the keto form via excited-state intramolecular proton transfer (ESIPT), with a low energy barrier of 1.951 kcal mol−1. In the keto form, complete charge separation occurs (D-index = 7.17 Å), and FMO analysis shows that the highest occupied molecular orbital (HOMO) of the acceptor is higher in energy than that of the donor, indicating PET-induced fluorescence quenching of HTPQ. This study provides a novel theoretical explanation for the photophysical mechanism of cysteine detection via the HTPQC probe, offering insights that diverge from previously established explanations.
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Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.
