Time-resolved magnetic field effect as a method of studying radical pairs with rapid evolution of spin state

Abstract

103As a rule, organic radical ions in solutions have rather short lifetimes, which makes them unsuitable for study by ESR, and at lifetimes shorter than 10 ns, even by a more sensitive method of optically detected ESR (OD ESR) of spin correlated radical ion pairs [1,2]. At the same time, the method of time resolved mag netic field effect (TR MFE) in recombination fluores cence of such pairs [2, 3] enables the detection of even shorter lived particles. This possibility is determined, first of all, by the time resolution of a setup, which we have managed to improve to ≈1 ns. An additional nec essary condition for identification of such short lived radicals is the existence of hyperfine coupling (HFC) constants in them sufficiently large (≈10 mT) to induce singlet-triplet transitions in the nanosecond time range. It is worth noting that the class of com pounds of interest includes only those with rather short fluorescence times.In this work, the potentialities of the TR MFE method have been demonstrated by the examples of radical anions of two fluoro substituted aromatic compounds-1,2,3,5 tetrafluorobenzene (TFB) and pentafluoroaniline (PFA)-generated by X ray irradi ation of solutions. This method is based on analysis of specific features of the ratio between the kinetic curves of recombination fluorescence in a magnetic field (I B (t) and in the absence of a field (I 0 (t)). In low polar ity solutions, the fluorescence intensity decay caused by recombination of radical ion pairs, in the limit of infinitely short fluorescence time and narrow instru mental function, is given bywhere F(t) is the pair recombination rate, θ is the frac tion of pairs generated in the singlet correlated state, and ρ ss (t) is the population of the singlet stage of such pairs [3]. In this approximation, the ratio(2) referred to as time resolved magnetic field effect, is independent of recombination kinetics F(t). However, at sufficiently high frequencies of ρ ss (t) oscillations caused by large HFC constants, the fluorescence kinetics must be described, not by Eq.(1), but by its integral convolution with the fluorescence probability of excited molecules, which changes exponentially with time, and the Gaussian instrumental function (see, e.g., [4]). Radical ions studied in this work have more than two different HFC constants. Therefore, in simulation of TR MFE curves, the singlet state population ρ ss (t) in strong and zero fields was calculated numerically. The spin relaxation times of the radical ions in the pair were considered in the framework of approximations described elsewhere [3,5].Studies of radical anions by the TR MFE method were carried out in 1-10 mM liquid solutions of TFB and PFA in 2,2,4 trimethylpentane (isooctane). In isooctane, the mobility of excess electrons is very high, and radical anions of electron acceptors form in the subnanosecond time domain. To reduce the contribu tion of the isooctane radical cation (the ESR spectrum width, ≈2 mT) to the spin evolution of the radical pair, a positive charge acceptor-hexameth...

Dimer Radical Anions of Polyfluoroarenes. Two More to a Small Family

“…The primary RCs of this alkane exhibit a significant EPR spectrum width, Δ H pp ≈ 4.2 mT . The addition of HMDS has no effect on excess electron migration and scavenging, and, by our estimates, the EPR spectrum width of HMDS RC is about 0.5–0.6 mT . Therefore, to reveal the quantum beat pattern determined by HFCs in 3 –• and 3 2 –• , HMDS was added to the solution to provide fast capture of solvent RCs and to reduce the effect of large HFCs in the RC of isooctane on the TR MFE curve.…”

Potential energy surface as a key to understanding the structure and properties of short‐living radical ions of cyclic organic molecules

“…Other cases when the EPR spectra observed cannot be explained with a single RA structure are represented by the recently detected RAs of 1,2,3,5‐tetrafluorobenzene and pentafluoroaniline . The experimental HFC constants obtained for 1,2,3,5‐F 4 C 6 , a F = 14.1(2F), 9.5(1F), and 1.5(1F) mT were reproduced satisfactorily by averaging over the four equivalent minimum energy structures that yield the values a F = 16(2F), 10(1F), and 1.6(1F) mT.…”

“…Experimental time‐resolved recombination fluorescence study on the decay of the pentafluoroaniline RA was performed in nonpolar alkane solutions . These data were interpreted in the frame of the PCM/B3LYP and PCM/CAMB3LYP calculations with cyclohexane as a model solvent.…”