A Ratio-Analysis Method for the Dynamics of Excited State Proton Transfer: Pyranine in Water and Micelles

The emission spectrum of a fluorophore undergoing excited state proton transfer (ESPT) often exhibits two distinct bands each representing emissions from protonated and deprotonated forms. The relative contribution of the two bands, best represented by an emission intensity ratio (R) (intensity maximum of the protonated band/intensity maximum of the deprotonated band), is an important parameter which usually denotes feasibility or promptness of the ESPT process. However, the use of a ratio is only limited to the interpretation of steady-state fluorescence spectra. Here, for the first time, we exploit the time dependence of the ratio (R(t)), calculated from time-resolved emission spectra (TRES) at different times, to analyze ESPT dynamics. TRES at different times were fitted with a sum of two log-normal functions representing each peak, and then, the peak intensity ratio, R(t), was calculated and further fitted with an analytical function. Recently, a time-resolved area-normalized emission spectra (TRANES)-based analysis was presented where the decay of protonated emission or the rise of deprotonated emission intensity conveniently accounts for the ESPT dynamics. We show that these two methods are equivalent but the new method provides more insights on the nature of the ESPT process.