Hydrogen-bonding dynamics of photoexcited coumarin 138 and 339 in protic methanol solution: Time-dependent density functional theory study

Abstract

The ground and electronically excited-state intermolecular hydrogen bonds between coumarin 138 (C138)/coumarin 339 (C339) and the protic methanol solvent are investigated by using time-dependent density functional theory method. The methanol solvent can act as a hydrogen-donating or hydrogen-accepting moiety at the proper site of organic chromophore. Our theoretical investigation explores the formation of one and multiple hydrogen bonds and demonstrates the strengthening of the intermolecular hydrogen bonds in the excited state. According to our calculation, upon photoexcitation the isolated coumarins and hydrogen-bonded clusters are initially excited to the first excited state and the intermolecular hydrogen bonds are strengthened in the excited state. The obvious red shifts, as large as 21nm, of the steady-state absorption spectra are observed. In addition, the infrared spectra in the ground and excited states are calculated to explore the hydrogen bonding dynamics. The calculated CO, NH, OH stretching modes are red shifted induced by the electronic excitation and intermolecular hydrogen bond interaction. The strengthening of the intermolecular hydrogen bonds is also confirmed by the geometric parameters in the ground and excited states with TDDFT method.