Elucidating the H-Bonding Environment of Coumarin 102 in a Phenol–Cyclohexane Mixture by Molecular Dynamics Simulation: Implications for H-Bond-Guided Photoinduced Electron Transfer

Abstract

Recently, we have experimentally demonstrated that the fluorescence intensity of coumarin 102 (C102) modulates anomalously upon hydrogen bonding to phenol in a nonpolar solvent: cyclohexane. The fluorescence intensity is first quenched gradually up to a particular mole fraction (X ≈ 0.013) but thereafter increases with further increases in the phenol mole fraction. These studies speculate about the importance of C102-phenol H-bonding to induce photoinduced electron transfer (PET) and propose a competition between the C102-phenol and phenol-phenol H-bonding to account for the anomalous fluorescence modulation. In this work, we investigate the exact H-bonding environment around the acceptor C102 at various compositions by molecular dynamics simulation and correlate the H-bonding environment to the observed fluorescence quenching. In addition to the 1:1 C102-phenol complex, 1:2 C102-(phenol) complexes with two different types of geometries were also found. Furthermore, density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were carried out to understand the H-bonding in these complexes in the ground state and in the excited state and their possible contribution to the observed fluorescence quenching.

New Insights on Hydrogen-Bond-Induced Fluorescence Quenching Mechanism of C102–Phenol Complex via Proton Coupled Electron Transfer

“…In this relaxed scan, we also included the solvent model (cyclohexane, IEFPCM). We previously found that inclusion of solvent model makes the S 1 state bright (contrary to the S 2 state in the gas phase) and thus could be accessed by direct excitation . Hence, we focused our attention only on the S 1 and S 0 states for consideration of H-bond dynamics.…”

“…We previously found that inclusion of solvent model makes the S 1 state bright (contrary to the S 2 state in the gas phase) and thus could be accessed by direct excitation. 17 Hence, we focused our attention only on the S 1 and S 0 states for consideration of H-bond dynamics. Zhao and Han had already applied relaxed scan for the H-bonded complex using GGA functional BP86, but was restricted to a shorter range and also in the gas phase.…”

“…However, when we applied TDDFT calculations, we found that the energy ordering of the LE and CT states depends critically on the choice of the functional and nature of the excited state. 17 The popular B3LYP functional undermines the CT energy and the LE state was found to be higher in energy than the CT state. However, the use of longrange corrected CAM-B3LYP hybrid functional which are usually recommended for excited states with charge transfer characteristics, leads to a dramatic switching of LE and CT energy ordering.…”

“…The fluorescence intensity of C102 is drastically quenched upon addition of phenol in a noninteracting solvent cyclohexane (nonpolar and aprotic). However, when we applied TDDFT calculations, we found that the energy ordering of the LE and CT states depends critically on the choice of the functional and nature of the excited state . The popular B3LYP functional undermines the CT energy and the LE state was found to be higher in energy than the CT state.…”