Geminate recombination reversible transfer

This is in fact the yield of fluorescence from only the initial geminate part of quenching. If the transfer is reversible, the stationary detection of fluorescence also includes, besides this part, the contribution from the bulk recombination of transfer products back to the excited state (see Sections VIII and XI). However,... 

When the ionization is irreversible, tpa = 1, the charge separation quantum yield is equal to cpp, which becomes the share of RIPs that escaped geminate recombination into the ground state. In more general case, when the reverse electron transfer into the excited state can not be neglected, this is the fraction of ions that escaped any recombination, in either the ground or excited states. To clarify this point, let us illustrate it by an example of contact electron transfer. 

More direct evidence for the inherent microscopic reversibility of an excited-state proton transfer reaction was found in ps-time-resolved measurements of a strongly reactive photoacid, namely HPTS (Fig. 12.2). With its conjugated-base, fourfold charged, the observation of the back (geminate) recombination of the pro-... 

Acids are in equilibrium with their conjugate bases in protic solvents, where the relative concentrations depend on the pK value. The observed dynamics of an electronically excited photoacid, typically interpreted as the proton transfer rate to the (protic) solvent [77, 78], is thus governed by the equilibration dynamics to the new configuration - as long as the photoacid and conjugate photobase remain in the electronically excited state - as dictated by the new excited state pJ a value. Depending on the pFI of the solvent one can observe the reversible time-depen-dent geminate recombination of the photobase with the released proton [79-83], or even the reaction of the photobase with other protons present in solution. 

Let us consider the pathways of PET in solution shown in Fig. 18. After formation of a geminate ion-pair, ion dissociation may take place in competition with reversible electron transfer, electron return, triplet recombination, and product-forming reactions. If we disregard product-forming steps and triplet recombination, the quantum yield, [Pg.52]

This latter low quantum yield indicates that virtually none of the radical cation (DPA4) escapes out of the cage of the geminate pair TCA /DPA4, owing to an exceptionally efficient reverse electron-transfer (which is unlikely), or that the reaction of (DPA+) with molecular oxygen is too slow to compete with the diffusion-controlled recombination of (TCA ) and (DPA+). 

The aim of this lecture is to provide a qualitative description of reversible proton transfer reactions in the excited-state, using the extended theory of diffusion influenced reactions. The complete equations and numerical procedures may be found in the literature [10-14]. Major results include (i) the asymptotic power-law decay and the evidence for diffusive kinetics [10] (ii) The salt effect [11] and the Naive Approximation for the screening function [17, 11] and (iii) an extension [18] of the theory for approximating the effect of competing geminate and homogeneous proton recombination expected atdow pH values. 

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