Effects of Rapid Intramolecular Electron Transfer on Vibrational Spectra

Abstract

Single-electron reductions of linked triruthenium clusters of the general type Ru,-pyrazine-Ru, produced mixed valence systems showing spectroscopic characteristics of rapid intramolecular electron transfer. Reflectance infrared spectroelectrochemistry was used to characterize the vibrational spectra of mixed valence systems that contained one carbon monoxide ligand on each Ru, cluster. Infrared spectra in the CO stretching region showed two resolved, partially coalesced, and coalesced v(C0) bands for clusters with rate constants for intramolecular electron transfer k, increasing from = 1 x 1 O9 s-' up to 5 x 10" and 9 x 10'' s-', respectively. These data provide a strong correlation between rates of intramolecular electron transfer and infrared spectral bandshape.Single-electron transfers are the simplest chemical reactions and are f~tndamentally important in biology (photosynthesis and respiration) and technology (photography, electrophotography, and solar energy conversion) (1-3). The fastest electron transfers involve the transfer of charge frorn an electron-donor site to an electron-acceptor site within the same molecule. The Creutz-Taube ion, [(NH,) jRu-pyrazine-R~(NH3) j]5+, is a now classical example of an intramolecular chargetransfer complex (4-6). Rather than being viewed as a localized valence (Ru"1Ru"') state, it is generally accepted that charge is delocalized over the complex (5, 6). The semiclassical expressio~l for the rate constant for intramolecular electron transfer ke in a symmetric charge-transfer complex with no net free energy change (lGO = 0) is given by k, = KV, X exp [-(AG,,' " H.q, + HA4,"4AG,*)/RT]where K is the adiabaticity factor (unity for adiabatic reactions), v.. is the nuclear frequency factor, which ir;'cludes both the solvent dielectric response frequency and bond-length/bond-angle reorganizations required by charge transfer between the localized valence states, AG,'%s the reorganizational energy barrier, and HAB is the electronic coupling between the metal centers (6-8). Thus, the theoretical maximum rate show that the rates of intramolecular electron transfer in a series of pyrazine-bridged assemblies of ruthenium clusters can be varied over orders of magnitude by moderating the electronic coupling HAB through ancillary ligand variation. In the rnost rapidly exchanging systems, electron transfer dynamical effects were observed in the infrared (IR) vibrational spectra.The pyraiine-bridged ruthenium clusters R y i P3-O)( P-CH~CO:)~(CO) Qi ~z -p z ) Ru3( p,-0) ( k-CH,CO2),(CO) (L) [pi = pyraiine; L = 4-dirnethylaminopyridine (dmap, I ) , pyridine (py, 2 ) , and 4-cyanopyridine

Electron transfer at the class II/III borderline of mixed valency: dependence of rates on solvent dynamics and observation of a localized-to-delocalized transition in freezing solvents

“…In the singly reduced mixed-valence state, these complexes are highly electronically coupled and undergo groundstate ET on the picosecond time scale. IR spectroscopy in the n(CO) region has proven to be a useful method to determine rate constants in the range of 10 11 -10 13 s K1 by n(CO) band coalescence (Ito et al 1997). This range of measurable rates is also relevant to the time scale of solvent dipolar reorientation, which we will show here plays an important role in these ET reactions.…”

Propriedades e aplicações de clusters trinucleares de carboxilatos de rutênio

“…Trabalhos recentes de Ito et al 52 tem focalizado os efeitos da transferência eletrônica entre duas unidades de clusters unidas por ponte de pirazina ou de 1,4-di(isociano)-benzeno. A presença de diferentes ligantes, como CO e piridinas substituídas tem permitido a monitoração dos desdobramentos provocados pelas interações intervalência, por espectroscopia vibracional e técnicas eletroquímicas 48,89 .…”

Probing the localized-to-delocalized transition

“…IR line broadening and coalescence occur if there is chemical site exchange during the lifetime of the vibrational excited state. The use of IR band broadening to measure electron transfer chemical site exchange has been explored extensively by Kubiak and co-workers in measurements on ligand-bridged m-oxo ruthenium clusters (Ito et al 1997;Londergan & Kubiak 2003;Salsman et al 2005).…”