Exploring the effect of hydrogen bonding on protonation of 7,8-benzoquinoline with TFE: Water binary mixture

Abstract In this work the role of hydrogen bonding in the protonation of 7,8-Benzoquinoline has been investigated using 2,2,2,-trifluoroethanol (TFE): water binary mixture. In our earlier work, protonation of 7,8-BQ was explored in water, TFE and acidic media. It appeared that protonation is significantly promoted in strong hydrogen bonding TFE. However, to substantiate it, a systematic study consisting of TFE: water binary was considered necessary. In this work, we observe that on addition of TFE in water, the intensity of neutral emission gradually decreases while the intensity of emission corresponding to protonated species increases due to faster and enhanced protonation as compared to pure water. Excited state properties of 7,8-BQ upon addition of TFE in water have been discussed using steady state and time resolved fluorescence spectroscopy. Quantum yield and excited state protonation rates have been calculated and discussed. We would like to mention that the present study validates our claim that hydrogen bonding indeed promotes protonation in 7,8-BQ. Besides the importance and applications of excited state proton transfer process in chemistry and biology the work has prospects in studying the conformational changes in proteins.