Solvation Dynamics of a Radical Ion Pair in Micro‐Heterogeneous Binary Solvents: A Semi‐Quantitative Study Utilizing MARY Line‐Broadening Experiments

Abstract

This work aims at elucidating the mechanism of solvation of a radical ion pair (RIP) in a micro-heterogeneous binary solvent mixture using magnetically affected reaction yield (MARY) spectroscopy. For the exciplex-forming 9,10-dimethylanthracene/N,N-dimethylaniline system a comparative, composition-dependent MARY line-broadening study is undertaken in a heterogeneous (toluene/dimethylsulfoxide) and a quasi-homogenous (propyl acetate/butyronitrile) solvent mixture. The half-saturation field extrapolated to zero-quencher concentration, B(1/2), and the self-exchange rate constants are analyzed in the light of solvent dynamical properties of the mixtures and a dielectric continuum solvation model. The dependence of B(1/2) on the solvent composition is explained by cluster formation giving rise to shortened RIP lifetimes. The results are in qualitative agreement with the continuum solvation model suggesting that it could serve as a theoretical basis for quantitative modeling.

Magnetic Field-Sensitive Radical Pair Dynamics in Polymethylene Ether-Bridged Donor–Acceptor Systems

“…We observe that for n = 8 and 10, the B 1/2 -values are markedly larger than the corresponding value for the free fluorophore–quencher-pair DMAnt/DMA in the limit of low donor concentration ( B 1/2 = 5.3 mT). 5,6 This can be attributed to a reduced lifetime of the radical pair and/or singlet–triplet dephasing. In fact, the lifetime effect has been suggested to lead to a broadening of the MARY lines in a study of the micro-heterogeneous solvation of the radical pair of DMAnt/DMA in DMSO/toluene mixtures, which likewise promote large MFEs by increasing the reencounter probability.…”

“…6 Singlet–triplet dephasing is the consequence of the randomization of the S/T-coherences by a time-varying J ( r ) as caused by the molecular motion. 5,6,47 Given that for n ≥ 8, no spectral fingerprints of an average exchange coupling are revealed in Figure 5, the picture of brief random encounters with large exchange coupling, interrupted by comparably long stretches of “free” evolution, emerges. We note again that the exchange interaction decays exponentially with the distance between two radicals.…”

“…By systematically studying the MFE of the exciplex emission, details of the dynamics of the spin-correlated RIP can be revealed indirectly. 5,6,9−11 The MFEs are theoretically accounted for in the framework of the so-called radical pair mechanism . 4,12,13,17−20 It relies on the quantum-coherent mixing, by the hyperfine interaction (HFI) and its interplay with the Zeeman interaction, of the singlet (S) and three triplet sublevels (T ± , T 0 ) describing the spin configurations of the two uncompensated electron spins on the radical ions (hyperfine mechanism).…”

Effects of Preferential Solvation Revealed by Time-Resolved Magnetic Field Effects

“…Eventually, the singlet RIP concentration can be detected through the emission of the exciplex, which is produced by singlet RIP cage recombination (see Scheme 1b). 19,20,29−33 …”

“…Indeed, using magnetic-field-affected-reaction-yield (MARY) line broadening measurements, the B 1/2 values were found to vary strongly with the solvent composition in microheterogeneous solvents. 19,20 Solvent mixtures composed of toluene (TO) and dimethyl sulfoxide (DMSO) have been studied in detail. 15,34 Studies of the magnetic isotope effect revealed no direct transfer of spin density on solvent molecules.…”