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

Abstract

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.

Time Evolution of Local pH Around a Photo‐Acid in Water and a Polymer Hydrogel: Time Resolved Fluorescence Spectroscopy of Pyranine

“…From the time‐resolved emission spectra we constructed the TRANES following literature methods . Figure displays TRANES spectra of HPTS in water and polymer hydrogel.…”

“…Presence of a clear iso‐emissive point indicates inter‐conversion (kinetically coupled) of HPTS and PTS − and rules out presence of any other step. Multiple intermediates generate more than one iso‐emissive points …”

“…Acid‐base reaction is one of the fundamental chemical reactions in nature and has implications in many natural and biological processes . The dissociation equilibrium of an acid HA into proton and its deprotonated species (A − ) is, …”

“…The primary prcocesses of excited state proton transfer (ESPT) of HPTS in a wide variety of media have been monitored by time evolution of the fluorescence from the anion (A − *) and the undissociated acid (HA*). This includes binary solvent mixtures, live biological cell, reverse micelles, micelle, nafione membrane and niosome . ESPT of HPTS has not been studied in polymer hydrogel.…”

A theoretical exploration and regulating about the excited state process for 2‐(4‐(diphenylamino)phenyl)‐3‐hydroxy‐4H‐chromen‐4‐one system

“…And this kind of process has been widely investigated in the past few years due to their significant characteristics in biological and chemical fields. In effect, these changes about properties have been extensively applied to fluorescent sensing technologies, fluorescence imaging, organic light‐emitting diodes (OLEDs), and so forth . However, some mechanisms of the ESPT reactions are complicated.…”