Study of fluorescence probe transfer mechanism based on a new type of excited-state intramolecular proton transfer

• The proton transfer mechanism of 3-HF and its derivatives have been studied theoretically.
• Analysis of the absorption and fluorescence spectra explains its physical mechanism.
• The ESIPT processes have been justified to occur in the first excited state.

3-Hydroxyflavone (3-HF) is a typical representative of a new type of fluorescent molecular probe. The intramolecular proton transfer mechanisms of 3-HF and its derivatives have been studied theoretically based on detailed density functional theory. An optical physical cycle diagram of intramolecular proton transfer of 3-HF and its derivatives has been found based on the optimal configuration before and after proton transfer. An analysis of the absorption and fluorescence spectra of these probes explains their optical physical mechanism, which agrees well with experimental results. This correlation indicates that the adopted theory is reasonable and effective. The primary bond lengths, angles and infrared vibrational spectra indicate that the intramolecular hydrogen bonds were strengthened, which is an indication of the excited-state intramolecular proton transfer (ESIPT) processes. The constructed potential energy curves of the ground and first excited state based on these three chromophores provide the ESIPT mechanism, which demonstrates that potential barriers lower than the 6 kcal/mol and justifies the ESIPT processes occur in the first excited state. The fluorescence quenching phenomenon has been explained based on the ESIPT mechanism.

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