Polar solvents geminate recombination

Many experimental data concerning the M.I.R. pertain to the back e.t. between geminate ions, prior to separation in polar solvents [53]. In some cases direct spectroscopic measurements can be made, although these depend on the correctness of the assignment of the absorption spectra. In principle the geminate ion recombination is expected to follow first order kinetics, whereas the diffusional recombination of free ions follows second order kinetics, so there is here another possibility to distinguish between the two processes [54], In some cases, the rate... 

Braden DA, Parrack EE, Tyler DR. Solvent cage effects. I. Effect of radical mass and size on radical cage pair recombination efficiency. II. Is geminate recombination of polar radicals sensitive to solvent polarity Coord Chem Rev 2001 211 279-94. 

The formation of electron donor-acceptor complexes from excited singlet states can lead to triplet formation. In highly polar solvents where radical-ion formation readily occurs, triplets may be produced by recombination (25) of solvent-separated radical ions and of geminate radical ions (Schulten et al., 1976) A- + Dt-> A + D. Such an electron-transfer reaction occurs in many of the electrochemiluminescent reactions discussed earlier. There is also evidence that, in some solvents of medium polarity, triplet production occurs via an exciplex (Orbach and Ottolenghi, 1975). The extent to which each of these processes contributes is obviously highly dependent upon the solvent. 

Detailed analyses of intramolecular structures are possible. Comparison of NMR and fluorescence data shows meso- and racemic diastereoisomers are found from 2,4-di(2-pyrenyl)pentane 24 jhe polarization of monomer and excimer of 4,9, disubstituted pyrenes have been measured in nematic liquid crystals 25 Quenching of pyrene fluorescence by alcohols in cyclodextrin inclusion complexes has also been studied in detail 26 Solvent effects on the photophysical properties of pyrene-3-carboxylic acid has been used to measure the pJJ, in different solvents 27 Geminate recombination in excited state proton transfer reactions has been studied with... 

Proton transfer processes are specially important excited state properties, and several detailed time resolved studies have been reported. Time resolved fluorescence studies of excited l-naphthol-3,6-disulphonate shows there is geminate recombination by proton transfer. Another detailed study is the examination of proton transfer and solvent polarization dynamics in 3-hydroxyflavone . The dynamics of proton transfer using a geminate dissociation and recombination model has also been investigated with 8-hydroxypyrene-l,3,6-trisulphonate 5 and also with... 

Two types of ion are involved in ion recombination. Geminate ions, which constitute 90 % of the total, recombine within a few nanoseconds since the positive and negative ions which are formed do not escape each others influence. The other 10 % of the ions do escape each others influence and are termed free or non-geminate. They recombine over microsecond time scales. The high percentage of geminate ions in hexane explains why non-polar solvents support high yields of excited states and low yields of radical ions. 

Initial reaction steps including photodissociation and formation of the geminate radical pair were investigated for P-NH2-QH4-S , by picosecond laser photolysis in polar and non-polar solvents [24]. In polar solvents, the solvation process isolates the radical very quickly. The recombination process is affected by the magnetic field [25]. 

The most interesting aspect of decay dynamics of polymer ion radicals is that slow geminate recombination is observed even in polar solvents. During our systematic studies of carbazole polymers quenched by DMTP in N,N-dimethylformamide, we found that the decay of the ion radicals produced had both of the fast and slow components. Although only normal second-order decay kinetics could be observed in the cases of small molecule donor-acceptor systems and of polymers with low degree of polymerization, an additional first-order decay was detected for PVCz with n-values of 400 and... 

The most optimal conditions for monitoring quantum beats are in non-polar solvents for the following reasons [7] (1) the separation of the radial pair is usually much less than the Onsager radius and the radical ion pairs recombine geminately (2) a strong luminescence signal can be observed due to the small solvation energy ... 

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