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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2018-03, Vol.122 (9), p.2394-2400
• Main Authors: Hossen, Tousif, Sahu, Kalyanasis
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.7b12055

The H-bonded coumarin 102 (C102)–phenol complex has been a model system usually used to understand the influence of H-bonding on photophysical processes. Zhao and Han first showed that significant H-bond strengthening occurs in the excited state and proposed the possibility of fluorescence quenching in the complex via internal conversion from a locally excited (LE) state to a low-lying charge transfer (CT) state. Later, we experimentally confirmed fluorescence quenching of C102–phenol complex in a nonpolar solvent (cyclohexane). However, we also found that the existence of the low-lying CT state is ambiguous. Here, we proposed an alternative mechanism for the fluorescence quenching in the H-bonded complex. For this, we evaluate the excited state potential energy surface considering complete H atom-transfer from phenol to C102 along the H-bonding coordinate. Surprisingly, we observed two distinct minima separated by a low-energy barrier. One minimum corresponds to the complex with shortening of H-bond consistent with that of Zhao and Han. On the other hand, the second minimum, which has even lower energy than the first minimum, is likely to be arising from the proton-coupled electron transfer (PCET) process. The nature of the lowest excited state alters from LE to CT type at the second minimum, which may account for the fluorescence quenching phenomena in the system.