Two excited state proton transfer mechanisms of 3-hydroxyisoquinoline (3HIQ) in cyclohexane and acetic acid (ACID) were investigated based on the time-dependent density functional theory (TDDFT), suggesting a different double-proton transfer mechanism from the one proposed previously (J. Phys. Chem. B, 1998, 102, 1053). Instead of the formation of keto–enol complexes for 3HIQ self-association in cyclohexane, our theoretical results predicted that 3HIQ self-association exists in two forms: the normal form (enol/enol) and the tautomer form (keto/keto) in cyclohexane. A high barrier (37.023 kcal mol⁻¹) between the 3HIQ enol monomer and 3HIQ keto monomer form indicated that the 3HIQ keto monomer in the ground state should not exist. In addition, the constructed potential energy surfaces of the ground state and excited state have been used to explain the proton transfer process. Upon optical excitation, the enol/enol form is excited to the first excited state, then transfers one proton, in turn, transition to the ground state to transfer another proton. A relatively low barrier (8.98 kcal mol⁻¹) demonstrates two stable structures in the ground state. In view of the acetic acid solvent effect, two protons of 3HIQ/ACID transfer along the dihydrogen bonds in the first excited state, which is a different transfer mechanism to 3HIQ self-association. In addition, the proton transfer process provides a possible explanation for the fluorescence quenching observed.

The excited state intramolecular proton transfer (ESIPT) mechanisms of 2-(2-hydroxyphenyl)benzoxazole (HBO), bis-2,5-(2-benzoxazolyl)-hydroquinone (BBHQ) and 2,5-bis(5′-tert-butyl-benzoxazol-2′-yl)hydroquinone (DHBO) have been investigated using time-dependent density functional theory (TDDFT). The calculated vertical excitation energies based on the TDDFT method reproduced the experimental absorption and emission spectra well. Three kinds of stable structures were found on the S₁ state potential energy surface (PES). A new ESIPT mechanism that differs from the one proposed previously (Mordzinski et al., Chem. Phys. Lett., 1983, 101, 291. and Lim et al., J. Am. Chem. Soc., 2006, 128, 14542.) is proposed. The new mechanism includes the possibility of simultaneous double proton transfer, or successive single transfers, in addition to the accepted single proton transfer mechanism. Hydrogen bond strengthening in the excited state was based on primary bond lengths, angles, IR vibrational spectra and hydrogen bond energy. Intramolecular charge transfer based on the frontier molecular orbitals (MOs) also supports the proposed mechanism of the ESIPT reaction. To further elucidate the proposed mechanism, reduced dimensionality PESs of the S₀ and S₁ states were constructed by keeping the O–H distance fixed at a series of values. The potential barrier heights among the local minima on the S₁ surface imply competitive single and double proton transfer branches in the mechanism. Based on the new ESIPT mechanism, the observed fluorescence quenching can be satisfactorily explained.

Inspired by the brilliant photochemical and photophysical properties of organic molecules containing chalcogenide substitutions that could be potentially applied across various disciplines, in this work, the effects of the atomic electronegativity of chalcogens (O, S, and Se) on hydrogen bond interactions and excited state proton transfer (ESPT) are mainly focused. We present characteristic oxygen-hydroxybenzazole-substituted 2,5-bis(4,5-diphenyl-1H-imidazol-2-yl)benzene-1,4-diol (BDIBD) derivatives that contain intramolecular double hydrogen bonds. The main objective of this study was to explore in detail the influence of the change of chalcogen atomic electronegativity on dual hydrogen bond interaction and ESPT behavior. By comparing the structural changes and infrared (IR) vibrational spectra of BDIBD derivative (BDIBD-O, BDIBD-S and BDIBD-Se) fluorophores in S₀ and S₁ states, combined with the preliminary detection of hydrogen bond interaction via the core-valence bifurcation (CVB) index and predicted hydrogen bonding energy (E_HB), we conclude that dual hydrogen bonds should be strengthened in the S₁ state, which is favorable for the occurrence of ESPT reactions. The charge recombination behavior of hydrogen bonds, induced by photoexcitation, further illustrates this point. By constructing potential energy surfaces (PESs) based on restrictive optimization and by searching the transition state (TS) structure, we finally elucidate stepwise excited-state double proton transfer (ESDPT). Specifically, we confirm that a change in atomic electronegativity has a regulatory effect on the ESDPT behavior in BDIBD derivatives, that is, lower atomic electronegativity is more conducive to stepwise ESDPT.