Strengthening of hydrogen bonded coumarin 102 in ethanol solvent upon photoexcitation

Abstract

Excited state hydrogen-bonding dynamics of coumarin in ethanol solvent has been studied using TDDFT/6-311++G(d,p) and a conductor-like polarizable continuum model. The geometry, hydrogen bond binding energy and frequency analysis indicate that the intermolecular hydrogen bond between C102 and ethanol is strengthened in the excited state. The binding energy is increased from 27.81 kJ mol⁻¹ in the ground state to 32.36 kJ mol⁻¹ in the S₁ state. The CO and O–H stretching bands in C102–EtOH are strongly red-shifted due to the formation of the intermolecular hydrogen bond. In the excited state, redshift occurs for the CO and O–H in the C102–EtOH complex. The excitation energy and frontier molecular orbital analysis indicate that S₁ of the C102–EtOH complex is the locally excited state. S₁ corresponds to the orbital transition from the HOMO to the LUMO with the ππ* character. Furthermore, the internal conversion from the excited to ground state is enhanced by hydrogen bond strengthening.