Direct observation of geminate recombination within photochemically produced ion pairs in polar solutions

Pulsed laser excitation of pyrene (P)/N, N-diethylaniline (DEA) solutions in methanol at low temperatures leads to the generation of the P^? and DEA⁺ radical ions. A fast initial decay of the ions (τ_{$\text{1}\text{2}$} ≈ 37 nsec at ?40°C) is observed and is attributed to geminate recombination within the initially formed (²P^?…²DEA⁺) ion pair. The quenching of ¹P^*) by DEA is also associated with substantial intersystem crossing, yielding ³P^*. The data rule out both gemiate and non-geminate (homogeneous) inonic recombinations as sources for the initial yield of ³P^*, in keeping with a prompt triplet formation mechanism.     

Internal reorganization energy in return electron transfer within geminate radical ion pairs

The internal reorganization energies λ_v for return electron transfer (ET) reactions within geminate radical ion pairs were studied using the extended Nelsen method. In the ET systems studied, the common acceptor was 9,10-dicyanoanthracene (DCA). The donors were methyl-substituted compounds of benzene, biphenyl, naphthalene and phenanthrene. The calculated results indicated that the λ_v values were associated mainly with the carbon atoms of the aromatic rings and the atoms linked directly to the aromatic rings. Systems with similar substituted conditions are expected to have similar internal reorganization energies. For systems in which the two aromatic rings of the donor can rotate relative to each other, the calculated λ_v values include a contribution from the change in torsional angle in the ET process. Compared with the system in which the donor is a fluorene molecule, the contributions of the torsional angles (low-frequency vibration) to λ_v were estimated.     

Investigation of geminate recombination of radical ion pairs generated by dissociation of exciplexes in moderately polar solvents using the photoconductivity technique

A new approach to determination of the recombination rate of radical ion pairs in moderately polar solvents is presented. It is based on an investigation of transient photocurrents caused by dissociation of exciplexes generated in photoinduced electron transfer reactions. It has been shown that the recombination rate of geminate ion pairs can be found from the photocurrent rise time. We have applied such an approach to transient photocurrents observed by Hirata et al. [Y. Hirata, Y. Kanda, N. Mataga, J. Phys. Chem. 87 (1983) 1659] for the pyrene/dicyanobenzene system in solvents of moderate polarity. The increase of the obtained recombination rate of photogenerated ions with increasing polarity of solvent testifies that ions recombine mainly by the backward electron transfer from the dicyanobenzene anions to solvent-separated cations of pyrene.     

Recombination yield of initially separated geminate radical pairs

We have derived a general analytical expression for the high field recombination yield of a geminate radical pair (RP) that diffuses freely and is separated initially. The dependence of the recombination yield on the initial separation is obtained by a simple extention of the previously published Green's function method. An explicit expression is derived for diffusion controlled recombination through the singlet channel and it incorporates Zeeman and hyperfine interactions, intraradical relaxation (both transversal and longitudinal), and homogeneous scavenging.     

Formation and decay of localized contact radical ion pairs in DNA hairpins

The dynamics of charge separation and charge recombination in synthetic DNA hairpins possessing diphenylacetylene-4,4'-dicarboxamide linkers have been investigated by means of femtosecond time-resolved transient absorption spectroscopy. The lowest excited singlet state of the linker is capable of oxidizing nearest neighbor adenine as well as guanine. A large wavelength shift in the transient absorption spectrum accompanies the conversion of the singlet linker to its anion radical, facilitating the investigation of electron-transfer dynamics. The rate constants for charge separation are dependent upon the oxidation potentials of the neighboring nucleobase donors but not upon the identity of nonnearest neighbors. Thus, the charge separation processes yield a contact radical ion pair in which the positive charge is localized on the neighboring nucleobase. Rate constants for charge recombination are dependent upon the identity of the first and second nearest-neighbor nucleobases but not more remote bases. This dependence is attributed to stabilization of the contact radical ion pair by interaction with its nearest neighbor. The absence of charge migration to form a base-pair separated radical ion pair is a consequence of Coulombic attraction in the contact radical ion pair and the low effective dielectric constant (epsilon < 7) experienced by the contact radical ion pair. Photoinduced charge injection to form a base-pair separated radical ion pair is necessary in order to observe charge migration.     

Transient dc photoconductivity caused by geminate ion pairs in non-polar liquids

The kinetics of dc conductivity induced by pulsed photo-generation of ion pairs is discussed on the basis of a numerical solution of the time-dependent Onsager problem. The results calculated are compared with experimental data to estimate the geminate ion contribution in transient photoconductivity.     

Hot recombination of photogenerated ion pairs

The recombination dynamics of ion pairs generated upon electron transfer quenching of perylene in the first singlet excited state by tetracyanoethylene in acetonitrile is quantitatively described by the extended unified theory of photoionization/recombination. The extension incorporates the hot recombination of the ion pair passing through the level-crossing point during its diffusive motion along the reaction coordinate down to the equilibrium state. The ultrafast hot recombination vastly reduces the yield of equilibrated ion pairs subjected to subsequent thermal charge recombination and separation into free ions. The relatively successful fit of the theory to the experimentally measured kinetics of ion accumulation/recombination and free ion yield represents a firm justification of hot recombination of about 90% of primary generated ion pairs.     

The effect of a magnetic field on the singlet/triplet ratio in geminate ion recombination

Excited state formation by ion recombination in solutions of fluorene in squalene has been studied by pulse radiolysis at the center of a large electromagnet. The products of the slower part of the ion recombination are affected by an applied magnetic field. The fluorescence yield increases by a factor of about 1.5 whereas the triplet yield decreases.     

Independent pairs and Monte-Carlo simulations of the geminate recombination of solvated electrons in liquid-to-supercritical water

Independent pairs (IP) and Monte Carlo (MC) simulations are employed to model experimental femtosecond time-resolved pump-probe spectroscopic data on the geminate recombination dynamics of solvated electrons in liquid-to-supercritical water. The hydrated electron was created by two-photon ionization of the neat fluid with a total ionization energy of 9.3 eV. In both numerical approaches, the ejection length, , (i.e. the distance from the ionization core, at which the electron is thermally and spatially localized) is used as the primary adjustable fitting parameter that can bring both model simulations into quantitative agreement with the ultrafast kinetic experiment. The influence of the thermodynamic conditions on the ejection length and on the recombination mechanism is discussed. Whereas in the compressed liquid associated with a high dielectric constant (ε ≥ 20), the electron recombines predominantly with the OH radical, the dissociative recombination via charge neutralization with the hydronium cation takes over at small dielectric constants (ε < 20). The importance of charge-dipole interactions for Monte-Carlo simulations of the recombination reactions of the hydrated electrons in the low-permittivity region is stressed.     

Yields of excited states from geminate recombination of hydrocarbon radical ions

A theory similar to the radical pair model for CIDNP, etc., is used to describe the rate of loss of spin correlation in geminate pairs of radical ions produced by radiolysis. The ratio of the yields of triplet and singlet excited states and the effects of magnetic fields are discussed.     

Double-channel contact recombination of radical pairs subjected to spin conversion via the Delta g mechanism

The contact recombination from both singlet and triplet states of a radical pair is studied assuming that the spin conversion is carried out by the fast transversal relaxation and Delta g mechanism. The alternative HFI mechanism is neglected as being much weaker in rather large magnetic fields. The magnetic-field-dependent quantum yields of the singlet and triplet recombination products, as well as of the free radical production, are calculated for any initial spin state and arbitrary separation of radicals in a pair. The magnetic field effect is traced and its diffusional (viscosity) dependence is specified.     

Dependence of the recombination kinetics of geminate electron-ion pairs on the mean free path of electrons

We have investigated how the kinetics of geminate recombination of electron-ion pairs is influenced by the mean free path of electrons. By use of the Monte Carlo method previously developed the decay of the pair survival probability was numerically calculated for a variety of values of the mean free path of electrons. When the mean free path is negligibly small compared with the Onsager length, our result is in good agreement with the Hong-Noolandi result based on the Smoluchowski equation. However, as the mean free path increases, it deviates from their result increasingly. This indicates that the treatment based on the Smoluchowski equation is inadequate when the mean free path of electrons is not negligible compared with the Onsager length. We have found that for mean free paths comparable with or larger than the Onsager length the decay kinetics of the pair survival probability approximately follows the first-order kinetics except at very short and very long times. The dependence of the escape probability on the mean free path of electrons is also briefly discussed.     

Monte Carlo calculation of diffusion-controlled ion recombination for single and multiple ion pairs in a nonpolar liquid

The Monte Carlo simulation of the diffusion and reaction of the ions in clusters of two and three pairs of oppositely charged ions in a nonpolar liquid has been performed. The survival probability as a function of time and the escape probability at infinite time have been computed and compared with the results obtained for single pairs.     

A study of the time parameters of radical geminate pairs by stroboscopic magnetic field techniques

The magnetosensitivity time for the pyrene excimer delayed fluorescence of the exciplex system pyrene-N,N-diethylaniline in methanol was studied by pulsed magnetic field techniques. The half-life of the magnetic effect was shown to be about 40 ns. The maximum magnetosensitivity time after photoexcitation appeared to be greater than 70 ns. The values are interpreted as geminate-pair lifetimes.     

A supramolecular "ship in bottle" strategy for enantiomeric selectivity in geminate radical pair recombination

[reaction: see text] Reactions in which zeolites are modified with chiral inductors to serve as media for chiral induction are often limited by the propensity of both substrate and inductor to occupy the same supercage. Herein, we report a "ship in bottle" strategy utilizing the thermal decomposition of dioxetanes obtained from oxazolidinone-substituted enecarbamates for the enantioselective generation of methyl desoxybenzoin (MDB). Photoexcitation of the supramolecular geminate molecular pair results in enrichment of the opposite enantiomer of MDB.