Roland Billing

Photoinduced electron transfer in ion pairs

Electrostatic attraction by oppositely charged ions leads to ion pairing. The influence of solvent polarity, the size and the charge of the ions as well as the electron-pair donor and acceptor strength on the equilibrium constant of ion-pair formation is discussed.

Luminescence quenching of *[UO2F4]2− in aqueous solutions by anions

Abstract The luminescence lifetime of uranyl ions is strongly increased by complex formation with fluoride ions in neutral aqueous solution. A monoexponential luminescence decay with a lifetime of 160 ± 2 μs is observed in the presence of 0.5 M NaF. The * [UO 2 F 4 ] 2− species oxidizes carboxylate anions by an outer-sphere mechanism. For anions with oxidation potentials higher than 2 V, the logarithms of the quenching rate constants (log k q ) are linearly dependent on the oxidation potentials of the anions ( E ox ). This allows for the estimation of such potentials from kinetic data. The oxidation potentials of carboxylates RCOO − are only slightly dependent on R. The difference between these observations and earlier results may be interpreted in terms of two different pathways for the one-electron oxidation of carboxylates, depending on the balance between the driving force of the electron transfer reaction and the reorganization energies of the redox couples involved.

Optical and photoinduced electron transfer in ion pairs of coordination compounds

Abstract Electrostatic attraction between charged coordination compounds and oppositely charged counter ions in solution leads to ion pairing. Although ion pairs are loosely bound species, their spectroscopic and photochemical properties may markedly differ from those of the individual components. Upon favourable mutual energetic positions of the redox orbitals additional ion pair charge-transfer (IPCT) optical transitions may be observed. The various contributions to the energy of the IPCT band absorption maximum and an increment system for the prediction of the position of these maxima are discussed. Depending on the choice of the components and the irradiation wavelength, ion pairs may undergo energy transfer, photoinduced electron transfer and optical electron transfer respectively. East subsequent thermal reactions have to compete with back electron transfer in order to achieve high yields of permanent photo-redox products. Mechanistic aspects of the product formation are discussed with respect to ion pairs of, for example, cobalt compounds may find use as photoinitiators for microimaging and curing applications, as well as photocatalysts for selective organic synthesis.

Ion pair charge transfer as a particular effect of second-sphere coordination

Abstract The interest in dealing with the classical phenomenon of second-sphere coordination has developed again during the past few years. This is due to some unusual chemical and physical properties caused by outer-sphere effects. Ion pair associates consisting of ionic Werner-type complexes and appropriate counter-ions are diverse representatives of second-sphere coordination. Such ion-pair compounds are of particular interest which are distinguished by spectroscopic ion pair charge transfer (IPCT) transitions. The spectroscopic and photochemical behavior of IPCT compounds, as well as a novel increment system for the prediction of the energy of optical IPCT transitions, are discussed in this Comment.

Optical and photoinduced electron transfer in tris(ethylenediamine)cobalt(III)-cyanometallate ion pairs

Abstract The ion pairs {[Co(en) 3 ] 3+ ; [M(CN) x ] 4− }, where MFe, Ru, Os ( x =6) and MMo, W ( x =8), and {[Co(en) 3 ] 3+ ; [Mn(CN) 5 NO] −3 }, in aqueous solution show up ion pair charge-transfer (IPCT) transitions in the visible and near-ultraviolet spectral regions. On excitation of the IPCT transitions ( λ irr = 405 nm), trinuclear complexes trans -{(en) 2 Co[NCM(CN) ( x −1 )] 2 } 5− are formed subsequent to an optical electron-transfer step with quantum yields close to unity. Excitation of the longest-wavelength ligand-field transition ( λ irr = 510 nm) of ion paired [Co(en) 3 ] 3+ results in a photoinduced electron-transfer reaction leading to the formation of binuclear pentacoordinate [(en) 2 CoNCM(CN) 5 ] − (MFe, Ru, Os) and trinuclear hexacoordinate trans -{(en) 2 Co[NCM(CN) 7 ] 2 } 5− (MMo, W) complexes. The redox-reactive excited state of [Co(en) 3 ] 3+ has a lifetime shorter than 3 ns and is tentatively assigned to the 5 T 2g ligand-field excited state.

Advantages and disadvantages of photocatalysis induced by light-sensitive coordination compounds

Abstract Light-sensitive transition metal compounds have attracted considerable interest with respect to light-induced catalytic processes since electronic excitation of these compounds may lead to species distinguished by considerable catalytic reactivity. As such species may be regarded coordinatively unsaturated compounds, complexes with changed oxidation state, free ligands and ligand redox products. Depending on the life-time of the catalytically active species it is advantageous to distinguish between photoinduced catalytic and photoassisted reactions . Definitorial approaches to characterize these limiting cases of photocatalysis are discussed and examples illustrating both kinds of photocatalytic reactions are given.

Dreiparameterbeschreibung von Lösungsmitteleinflüssen auf Ionenpaar-Charge-Transfer-Banden

Solvent effects on the position of ion-pair charge-transfer absorption bands are described using a three parameter approach. The values of calculated coefficients are discussed considering specific and non-specific interactions.

Photochemical behaviour of ion pairs of the type {[Cr(en)3]3+; [M(CN)6]4−} (MFe, Ru, Os) as a first example of second-sphere photoeffects of chromium(III) amine complexes

Abstract Ion pairs of the constitution {[Cr(en) 3 ] 3+ ; [M(CN) 6 ] 4− } show, in an aqueous buffer solution, second-sphere charge transfer (SSCT) absorption bands at 342 nm (MFe), 290 nm (MRu) and 303 nm (MOs) additionally to the spectra of the individual components. Two possible mechanisms of the optical electron transfer process involving the initial formation of low spin (t 2g 4 ) and high spin (t 2g 3 e g 1 ) [Cr(en) 3 ] 2+ respectively the discussed in order to explain the rather high energies of the SSCT absorption maxima. The ion pairs under discussion undergo photosubstitution reactions upon ligand field excitation (λ irr =450 nm) of the chromium complex. Binuclear complexes [(H 2 O(en) 2 CrNCM(CN) 5 ] − (MFe, Ru, Os) are formed, which are distinguished by intense metal-to-metal charge transfer absorption bands in the UV spectral region. These binuclear complexes do not undergo further photoreactions. Photoredox reactions, indicated by the formation of [M(CN) 6 ] 3− (MRu, Os), are initiated upon excitation of the SSCT transition (λ irr =320 nm) within the ion pair compounds. Additionally, also photosubstitution is observed upon short-wavelength irradiation.

Photoinduced Chain Reactions of Alcohols in the Presence of Diphenyliodonium Ion Pairs with Cyanometallates—Steady State UV/Visible Spectroscopic and Pulse Radiolysis Studies

Diphenyliodonium ions (Ph2I+) form donor-acceptor ion pairs with suitable cyanometallates such as [Mo(CN)8]4-, [W(CN)8]4-, [Ru(CN)6]4- and [Os(CN)6]4-. Such ion pairs are characterized by new spectroscopic transitions due to second-sphere interactions between donor ([M(CN)x]4-, x=6, 8) and acceptor (Ph2I+) ions. Photochemical excitation of these ion-pair charge-transfer (IPCT) states leads to efficient electron transfer reactions that yield short-lived diphenyliodyl radicals (Ph2I*) and oxidized cyanometallates ([M(CN)x]3-). Diphenyliodyl radicals decay to iodobenzene and phenyl radicals. This very convenient source for generating phenyl radicals was applied to the photoinduced chain oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones, respectively. However, unexpected side reactions led to undesired chain-terminating reactions. Adduct formation of diphenyliodonium ions with alpha-hydroxyalkyl radicals was verified by pulse radiolysis studies. These relatively longlived adducts give rise to chain-terminating reactions because of interactions with [M(CN)x]4- complexes that lead to oxidized cyanometallates [M(CN)x]3- upon regeneration of the starting alcohols.