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 S0 state, and upon excitation, the two intramolecular hydrogen bonds were strengthened in the S1 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 S1 and S0 states can explain the proton-transfer process. In the S1 state, three types of proton-transfer processes exist as type 1 (single-proton transfer: H2 from O1 to O3), type 2 (single-proton transfer: H5 from O4 to O6), 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.