The excited-state proton transfer mechanism in water-bridged 4-hydroxybenzoate: spectroscopy and DFT/TDDFT studies

Abstract

The excited-state proton transfer (ESPT) behavior of 4-hydroxybenzoic acid is studied by means of steady-state spectroscopy and theoretical calculations to obtain insight of the excited state dynamics. The large Stokes shift for 4-hydroxybenzoate (4HB) at the pH value of the microenvironment of 6.74 indicates that the ESPT process took place. The proton transfer dynamics of the water-bridged complexes, 4HB·(H₂O)_x, with two different water chain lengths is investigated using density functional theory and time-dependent density functional theory. The constructed potential energy curves among the optimized 4HB·(H₂O)_x (enol form) and 4KB·(H₂O)_x (keto form) geometries at the ground and the first singlet excited state indicate that the ESPT indeed occurs as the barrier is less than 10 kcal mol⁻¹. In addition, the driving force is confirmed by NBO population analysis.