Theoretical Investigation of an Excited-State Intramolecular Proton-Transfer Mechanism for an Asymmetric Structure of 3,7-Dihydroxy-4-oxo-2-phenyl-4H-chromene-8-carbaldehyde: Single or Double?

3,7-Dihydroxy-4-oxo-2-phenyl-4H-chromene-8-carbaldehyde in methylcyclohexane solvent was chosen to investigate excited-state intramolecular proton-transfer mechanisms by using a time-dependent density functional theory method. The results show that the single- and double-proton-transfer mechanisms are related and exist simultaneously in the excited states, which differs from those reported in previous experiments ( Serdiuk , I. E. et al. RSC Adv. 2015 , 5 , 102191 - 102203 ). The analyses of bond distance, bond angle, the molecular electrostatic potential surface, and infrared vibrational spectra show that two intramolecular hydrogen bonds were formed in the S state, and upon excitation, the two intramolecular hydrogen bonds were strengthened in the S state, which can facilitate the proton-transfer process. The calculated absorption and fluorescence spectra agree well with the experimental results. The constructed potential energy surfaces on the S and S states can explain the proton-transfer process. In the S state, three types of proton-transfer processes exist as type 1 (single-proton transfer: H from O to O), type 2 (single-proton transfer: H from O to O), and type 3 (double-proton transfer). The relationship of the potential barrier is type 1 (1.02 kcal/mol) < type 2 (1.57 kcal/mol) < type 3 (2.29 kcal/mol), which indicates that type 1 is most susceptible to proton transfer.