Density functional theoretical investigation of intramolecular proton transfer mechanisms in the derivatives of 3-hydroxychromone

In this work, the intramolecular proton transfer (IPT) mechanisms of 5-(3-hydroxy-4-oxo-4H-chromen-2-yl)thiophene-2-carbaldehyde (3-HTCA) and 2-((5-(3-hydroxy-4-oxo-4H-chromen-2-yl)thiophen-2-yl)methylene)malononitrile (3-HTC-DiCN) have been systematically investigated. The constructed potential energy curves (PECs) of 3-HTCA and 3-HTC-DiCN in the ground state (S0) and first-excited singlet electronic state (S1) indicate that the ground state intramolecular proton transfer process is difficult to take place due to the high potential barriers (13.72 kcal mol−1 and 13.25 kcal mol−1 respectively); in comparison, the intramolecular proton transfer reaction occurs more readily with the H atom removing from the O atom of the O–H moiety to the O atom of the CO moiety after photo-excitation. The relative size of the reaction barriers of 3-HTCA (4.24 kcal mol−1) and 3-HTC-DiCN (6.43 kcal mol−1) in the S1 state well explains the difference in their experimental fluorescence spectra. Based on the stable structures on PECs, the electronic spectra were simulated by the time-dependent density functional theory (TDDFT) method. The experimental absorption spectrum and fluorescence spectrum were well reproduced by calculating the vertical excitation energies of 3-HTCA and 3-HTC-DiCN. Moreover, the charge redistribution and charge-transfer character during photo-excitation were discussed in detail through the frontier molecular orbitals (FMOs) and natural bond orbital (NBO) analysis, which rationalizes the changes in the hydrogen bond strength and the ESIPT process of 3-HTCA and 3-HTC-DiCN.