Solvent Effects on Charge Transfer Bands of Nitrogen-Centered Intervalence Compounds

Abstract

Electron transfer parameters are extracted from the optical spectra of intervalence bis(hydrazine) radical cations. Compounds with 2-tert-butyl-3-phenyl-2,3-diazabicyclo[2.2.2]octyl-containing charge-bearing units that are doubly linked by 4-sigma-bond and by 6-sigma-bond saturated bridges are compared with ones having tert-butylisopropyl- and diphenyl-substituted charge bearing units and others having the aromatic units functioning as the bridge. Solvent effect studies show that the optical transition energy (E(op)) does not behave as dielectric continuum theory predicts but that solvent reorganization energy may be usefully separated from the vibrational reorganization energy by including linear terms in both the Pekar factor (gamma) and the Gutmann donor number (DN) in correlating the solvent effect. Solvation of the bridge for these compounds is too large to ignore, which makes dielectric continuum theory fail to properly predict solvent effects on either E(op) or the free energy for comproportionation.

Computational and spectroscopic studies of organic mixed-valence compounds: where is the charge?

“…The optical ET is associated with the so-called intervalence charge transfer (IV-CT) band which usually appears in the NIR. Among the various issues that have been investigated are the distance dependence of ET, the influence of local bridge states (electron rich vs. electron-deficient), 7,8 temperature, 20,21 solvent 22,23 and counter ion influences 24 etc. A major aspect in MV systems concerns the shape of the ground and excited state potentials.…”

Synthese und photophysikalische Eigenschaften einer neutralen organischen gemischtvalenten Verbindung

“…[10] Leider wird die Anwendung dieser Methode bei geladenen Verbindungen durch die im elektrischen Feld auftretende Ionenwanderung, die nur durch die Verwendung von Glas-Matrices verhindert werden kann, erschwert. [11][12][13] Weitere Nachteile geladener MV-Verbindungen sind ihre geringe Löslichkeit, besonders in unpolaren Solventien, [14][15][16] sowie das Auftreten von Ionenpaareffekten. [17,18] Neutrale MV-Verbindungen, eine Verbindungsklasse, die nach unserem Kenntnisstand [19] bisher noch nicht beschrieben wurde, weisen keinen dieser Nachteile auf.…”

Comparisons of Measured Rate Constants with Spectroscopically Determined Electron-Transfer Parameters

“…51 The values obtained in this manner are λ v ) 10 100 cm -1 (28.9 kcal/mol) and λ s (CH 3 CN) ) 4000 cm -1 (11.5 kcal/mol), that is, the present resonance Raman . Figure 1.…”

“…These calculations were done using the partitioning of the room temperature IV band maximum in acetonitrile, taken as λ, into λ v ) 10 110 cm -1 and λ s ) 4020 cm -1 that arose from how λ changes with solvent. 51 However, other calculations showed that the results were indistinguishable if λ was not broken into vibrational and solvent components but taken as a single 14 130 cm -1 quantity (see Supporting Information). This result is very different from that obtained using Bixon-Jortner theory to calculate rate constants, where S ) λ v /ω v is a parameter of fundamental importance, so the partitioning of λ is crucial.…”