In silico simulation of the excited state proton transfer reaction of 2-(2-furyl)-3-hydroxychromone (FHC) in solution by empirical valence bond (EVB) method in conjunction with classical molecular dynamics

Abstract

2-(2-Furyl)-3-hydroxychromone (FHC) is a dual emitter fluorescent probe exhibiting excited state intramolecular proton transfer (ESIPT) reaction in solution and inserted in biological relevant macromolecules. Its two fluorescence bands are attributed to normal form (N*) for the short wavelength and the long-wavelength band to the tautomer form (T*), the product of the ESIPT reaction. To rationalise the mechanisms that lead to the occurrence of the ESIPT reaction and predict the fluorescence spectra, the excited and ground state dynamic of this compound was followed in acetonitrile, ethanol, methanol, N-methylformamide and formamide, by the classical molecular dynamic simulation in conjunction with the empirical valence bond (EVB) model to describe bond dissociation process. A set of transferable parameters were used to fit the EVB potential energy curve to TDDFT data. Using these parameters, the occurrence time of the ESIPT reaction was predicted to be 7, 84, 113, 98, 207ps, respectively in the considered solvents, in excellent agreement with the experimentally reported values. These values are comparatively higher than those determined for the parent flavone dye (less than 100 fs), giving FHC a higher potential for sensor development. By examining structural parameters such as the distance from the proton to acceptor, the intermolecular H-bond between FHC and the solvent molecules and proton trajectories, we found that ESIPT occured in acetonitrile and alcohols as a combination of low frequency high amplitude donor-acceptor bending motion and charge redistribution in the excited state, that were modulated by the solvent H-bond donor potency. Differently in amides ESIPT occured via tunnelling mechanism due to the close energy of N* and T*. Fluorescence spectra calculated from potential energies of the EVB model along the reaction coordinates were qualitatively in agreement with the recorded spectra.