Elementary Steps in Excited-State Proton Transfer

Abstract

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.

Excited-State Dynamics of Nitroperylene in Solution: Solvent and Excitation Wavelength Dependence

“…The substantial isotopic effect, τ D /τ H ∼ 1.4, observed in methanol, strongly points to the occurrence of reversible intermolecular proton transfer. 48,49 Such a process agrees with the increase of electron density at the nitro group in the S 1 state. Moreover, the occurrence of the H-bonding interaction was already suspected to be at the origin of the mirror image symmetry between absorption and emission spectra in protic solvents.…”

Superior Photoprotective Motifs and Mechanisms in Eumelanins Uncovered

“…Such proton transfer often occurs along a pre-existing hydrogen bond and therefore becomes very fast, frequently on the ∼100 fs time scale or even faster. 36 ESPT to solvent has been studied for more than 60 years, 52,57,58 and many theoretical models have been proposed to describe various aspects of the transfer process, e.g. the dependence of transfer kinetics on solvent, temperature, pressure, deuterium substitution, complex formation, etc.…”

Excited-state dynamics of the fluorescent probe Lucifer Yellow in liquid solutions and in heterogeneous media

“…The strong increase of fluorescence lifetime when going from H 2 O to D 2 O strongly supports the occurrence of this process. ESPT is a well-known quenching process of many aromatic ketones 45, 46 and has been reported with other aminonaphthalimide derivatives. 25 The ESPT hypothesis is further supported by a TCSPC experiment with a methanol solution in which a drop of LYen in DMSO had been added, which yielded (Fig.…”