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FEATURE ARTICLE Elementary Steps in Excited-State Proton Transfer

Elementary Steps in Excited-State Proton Transfer
Noam Agmon*
Department of Physical Chemistry and the Fritz Haber Research Center, The Hebrew UniVersity,
Jerusalem 91904, Israel
ReceiVed: June 11, 2004; In Final Form: September 25, 2004
The absorption of a photon by a hydroxy-aromatic photoacid triggers a cascade of events contributing to the
overall phenomenon of intermolecular excited-state proton transfer. The fundamental steps involved were
studied over the last 20 years using a combination of theoretical and experimental techniques. They are surveyed
in this sequel in sequential order, from fast to slow. The excitation triggers an intramolecular charge transfer
to the ring system, which is more prominent for the anionic base than the acid. The charge redistribution, in
turn, triggers changes in hydrogen-bond strengths that set the stage for the proton-transfer step itself. This
step is strongly influenced by the solvent, resulting in unusual dependence of the dissociation rate coefficient
on water content, temperature, and isotopic substitution. The photolyzed proton can diffuse in the aqueous
solution in a mechanism that involves collective changes in hydrogen-bonding. On longer times, it may
recombine adiabatically with the excited base or quench it. The theory for these diffusion-influenced geminate
reactions has been developed, showing nice agreement with experiment. Finally, the effect of inert salts,
bases, and acids on these reactions is analyzed.
I. Introduction
Certain aromatic dye molecules undergo a dramatic change


Source: Agmon, Noam - Institute of Chemistry, Hebrew University of Jerusalem


Collections: Chemistry