Dynamics of Intramolecular Electron Transfer in Dinitrodibenzodioxin Radical Anions

Abstract

The activation energy for intramolecular electron transfer in radical anions of 2,7-dinitrodibenzodioxin and 2,8-dinitrodibenzodioxin, obtained by simulation of their temperature-dependent EPR spectra, are well predicted by the values calculated by the two-state Marcus-Hush model from the optical charge-transfer bands using quartic-adjusted energy surfaces. The electronic coupling is higher in the 2,8-dinitrodibenzodioxin (H(ab) = 485 cm(-1)) than in the 2,7-dinitrodibenzodioxin radical anion (H(ab) = 250 cm(-1)), but for each solvent the reorganization energy, taken as the maximum of the optical band, is only slightly higher in the latter. These values are consistent with the fact that the reaction is faster in the 2,8-dinitrodibenzodioxin radical anion isomer, as determined by EPR spectroscopy. The pre-exponential factors obtained combining the EPR-derived rate constants and the activation energies calculated from the optical bands fit well the theoretical (modified) nonadiabatic values in the less viscous solvents. However, for the more viscous solvents, the trend of the pre-exponential values with solvent can only be explained if dynamical solvent effects increasingly influence their value. The influence of solvent dynamics in the 2,8-dinitrodibenzodioxin radical anion starts in the less viscous solvents DMF and DMSO, but in the 2,7-dinitrodibenzodioxin isomer this is only fully evident for the more viscous PhCN and HMPA. The influence of solvent dynamics is higher in the radical with the lowest activation barrier.

Dicyanoaromatic radical anions as mixed valence species

“…Dinitroaromatic radical anions have been the subject of our studies on MV chemistry for the last several years . A wide range of properties can be achieved by changing the nature of the aromatic bridge on these radicals.…”

“…Decreasing the conjugation (and hence H ab ) by the twisting around the central bond on substituted biphenyl bridges, as in 2,2′‐dimethyl‐4,4′‐dinitrobiphenyl radical anion, also leads to localization of charge . Dinitroaromatic radical anions with non‐Kekulé substitution patterns like 1,3‐dinitrobenzene, 2,7‐dinitronaphthalene, or 2,7‐dinitrodibenzodioxin radical anions have small enough H ab values to make them localized Class II radicals in all solvents studied …”

Mechanically induced intramolecular electron transfer in a mixed-valence molecular shuttle

“…Marcus-Hush theory is now widely applied to the study of electron transfer in a plethora of organic-based mixed-valence systems in order to understand (i) spatial interactions between redox centers (10)(11)(12); (ii) the use of different solvents and counterions (13)(14)(15); and (iii) how the existence of covalent bridges (16,17) affects the kinetics (k ET ) and thermodynamics (ΔG ‡ ) of electron transfer. In this article, we demonstrate how the switching mechanism present in a molecular shuttle-comprised of a bistable [2]rotaxane (R 6þ ) (Fig.…”

Computational study on intramolecular electron transfer in 1,3-dintrobenzene radical anion

“…According to the relative magnitudes of H ab and λ, MV systems are categorized in class I, class II, or class III in terms of the Robin and Day classification . Typical organic MV compounds studied so far include bis(hydrazine) radical cations, bis(triarylamine) radical cations, and dinitroaromatic radical anions . Among them, 1,3‐dinitrobenzene radical anion (hereafter abbreviated as DNB − ) is the simplest dinitroaromatic radical anion and has long been studied using spectroscopic techniques such as ESR and near‐IR spectroscopies in various solvents .…”

“…Typical organic MV compounds studied so far include bis(hydrazine) radical cations, bis(triarylamine) radical cations, and dinitroaromatic radical anions . Among them, 1,3‐dinitrobenzene radical anion (hereafter abbreviated as DNB − ) is the simplest dinitroaromatic radical anion and has long been studied using spectroscopic techniques such as ESR and near‐IR spectroscopies in various solvents . The optical spectrum of DNB − exhibits a broad Gaussian‐shaped inter‐valence charge‐transfer (IV‐CT) band in the near‐IR region, which is a characteristic of class II MV systems.…”