Photochromism of a radical diffusion-inhibited hexaarylbiimidazole derivative with intense coloration and fast decoloration performance

We report the synthesis and the photochromic behavior of a newly designed, photochromic, radical diffusion-inhibited hexaarylbiimidazole (HABI) derivative with markedly improved photochromic performance in coloration and decoloration rates as well as greater optical density in the colored state. The thermal bleaching rate (tau1/2 = 260 ms at 295 K) is the fastest among the reported ones for HABI derivatives.     

Production of free radicals and triplets from contact radical pairs and from photochemically generated radical ions

The quantum yields of triplets and free radicals (or radical ions) that escaped recombination in photochemically created primary radical pairs (or radical ion pairs) are calculated. As the products of monomolecular photodissociation, the neutral radicals appear at contact, while the ions are initially distributed over the space due to distant photoionization (bimolecular electron transfer) in the liquid solution. The diffusional dependence of the quantum yields is shown to be different when recombination starts from contact or from separated reactants. The experimental data for recombination of ionized perylene with aromatic amine counterions is well fitted with the noncontact initial distribution provided the recombination is also noncontact and even more distant than ionization.     

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.     

Experimental testing of molecular dynamic functions of pairs by the isotope selectivity of radical recombination

A method for experimental testing of the molecular dynamic functionf(t), which describes the generation of reencounters in a dynamic pair of molecular particles, has been developed. The method is based on the analysis of isotope separation in spin selective reactions of a radical pair. Thef(t) function derived from the solution of the diffusion equation with physically rigorous initial and boundary conditions describes experimental results much better than the well-known Noyes function. Conditions that make it possible to increase the efficiency of isotope separation in spin selective reactions are formulated.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1639–1645, September, 1995.This work was financially supported by the Russian Foundation for Basic Research (Project No. 93-03-05227).     

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.     

On the usefulness of crown ethers in freezing out the intramolecular cation migration in radical ion pairs

The additon of stoichiometric amounts of dibenzo-18-crown-6 ether to solutions of the sodium ion pairs of pyrazine and therephthalonitrile has been found to reduce dramatically the rate constant for the intramolecular migration of the alkali cation. The proton and nitrogen hyperfine splitting in the slow exchange region, could be easily measured under these experimental conditions.     

Activation of weak nucleophiles: polyfluorocarbamates from polyfluoroalcohols via a fast radical reaction

A new fast radical mechanism has been observed for the reaction of polyfluorinated alcohols and phenylisocyanate, very sensitive to the change of solvents and the concentration of reactants. The acidity of polyfluoroalcohols seems to be responsible for the observed new reactivity and evidences from kinetic studies, electron paramagnetic resonance, cyclic voltammetry, and photostimulation suggest that polyfluoroalkoxy radical is the key intermediate in the chain. To the best of our knowledge, it is the first time that a radical mechanism is described for the preparation of carbamates.     

Reactivity of triarylphosphine peroxyl radical cations generated through the reaction of triarylphosphine radical cations with oxygen

One-electron oxidation of triarylphosphines (Ar3P, Ar = phenyl and substituted phenyl) in benzonitrile (PhCN) has been studied using pulse radiolysis technique. One-electron oxidation of Ar3P occurred to yield the radical cation (Ar3P*+) which showed an intense absorption with a peak at 360-370 nm together with a broad band at 500-600 nm. The addition of molecular oxygen (O2) to the phosphorus atom of Ar3P*+ took place at the second-order rate constant of 10(7)-10(9) dm(3) mol(-1) s(-1) to yield the peroxyl triarylphosphinyl radical cation (Ar3P+OO*). It is found that the electron-releasing substituents on the para position of the phenyl ring of Ar3P influence the rate constants of the reaction of Ar3P*+ with O2 and that o-methyl substituents on the phenyl ring influence the reactivity of Ar3P+OO*.     

Magnetic field and magnetic isotope effects on the recombination kinetics of covalently-linked ketyl-phenoxyl triplet radical pairs

The recombination kinetics of three photogenerated covalently-linked ketyl-phenoxyl triplet radical pairs,³[PhC^.(OD)C₆H₄O(CH₂) _n OC₆H₄C₆H₄O^.] (n=3, 6, and 10), and of the corresponding deuterated derivatives were examined by the laser flash technique under an external magnetic field (up to 0.2 T) in a CDCl₃/CD₃OD (21) mixture. In zero magnetic field, radical pairs (RPs) with small exchange interactions (n=6 and 10) are characterized by high values of the magnetic isotope effect (MIE), which reach 3 for pairs withn=10. Under strong magnetic fields (up to 0.2 T), the values of MIE decrease to 1.2 to 1.1. The photochemical behavior of covalently-linked RPs is compared with that of similar unlinked RPs in micelles.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 442–446, March, 1995.The authors are grateful to V. F. Tarasov, A. I. Shushin (N. N. Semenov Institute of Chemical Physics, RAS) and V. A. Nadtochenko (Institute of Chemical Physics in Chernogolovka, RAS) for helpful discussions.     

Calculation of activation-controlled reaction rate constants from parameters of electronic absorption spectra of ion pairs

An equation relating the rate constants of a bimolecular activation-controlled redox reaction to the parameters directly determined from the electronic absorption spectra was obtained in the framework of the Marcus-Hush theory. Applicability of the equation for the prediction of redox reaction rate constants was shown. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No.6, pp. 1007–1010, June, 2000.     

Radio frequency magnetic field effects on a radical recombination reaction: a diagnostic test for the radical pair mechanism

The photoinduced electron-transfer reaction of chrysene with isomers of dicyanobenzene is used to demonstrate the sensitivity of a radical recombination reaction to the orientation and frequency (5-50 MHz) of a approximately 300 muT radio frequency magnetic field in the presence of a 0-4 mT static magnetic field. The recombination yield is detected via the fluorescence of the exciplex formed exclusively from the electronic singlet state of the radical ion pair Chr*+/DCB*-. Magnetic field effects are simulated using a modified version of the gamma-COMPUTE algorithm, devised for the simulation of magic angle spinning NMR spectra of powdered samples. The response of a chemical or biological system to simultaneously applied radio frequency and static or extremely low-frequency magnetic fields could form the basis for a diagnostic test for the operation of the radical pair mechanism that would not require prior knowledge of the nature and properties of the radical reaction.