Excited-State Proton Transfer

The chemical and physical characteristics of organic molecules in their electronic excited states are usually quite different than those of their ground-state counterparts. This is not surprising, since excitation by ultraviolet or visible light involves perturbation of the electron cloud that determines the chemical bonding characteristics of organic molecules. Following excitation by a short impulse of light, organic molecules that were, in their ground state, in equilibrium with their surroundings, will often be in a metastable excited state with respect to their environment. Fluorescence decay measurements often provide an experimental tool for investigating the kinetics of the subsequent approach to the excited-state equilibrium. Excited-state proton transfer reactions represent a well studied and reasonably well understood group of excited-state reactions. The classic papers of Forsterl and Weller2–4 established that organic acids exhibit excited-state ionization constants that differ by several orders of magnitude from those found in the ground state. Several reviews 5–7 describe the use of steady-state fluorescence methods for estimating the rate constants for excited-state proton transfer. These may also be obtained theoretically from absorption and emission spectra with the aid of thermodynamic considerations5,8,9 (Forster cycle) which will be discussed below.