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.

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.