Intramolecular optical metal to ligand charge transfer between remote redox centers

Abstract

Long-range electron transfer involving transition metal complexes has been investigated extensively during recent years [I]. In this context it is of interest if intramolecular electron transfer between remote redox centers can occur also by direct optical excitation. This process requires the occurrence of charge transfer (CT) absorptions which belong to electronic transitions from the electron donor to a distant acceptor. The CT interaction between donor and acceptor may be facilitated by throughbond interaction even if donor and acceptor are separated by more than one bond. While systematic studies of organic systems were carried out quite recently [2,3] much less is known about CT interaction between remote redox centers of inorganic molecules. The majority of intramolecular optical CT transitions of metal complexes involves donors and acceptors which are directly connected by chemical bonds [4]. These are metal to ligand CT (MLCT) and ligand to metal CT (LMCT) transitions [4]. In addition, optical CT transitions between redox centers are known which are separated by one intervening metal or ligand. Binuclear complexes of the type M red-L-Max with a bridging ligand L are characterized by metal to metal CT (MMCT) absorptions [4] while the electronic spectrum of mononuclear complexes LedM-L,, which contain a reducing and an oxidizing ligand display ligand to ligand CT (LLCT) bands [5]. There seem to be only two reports on intramolecular optical CT between redox centers which are separated by more than one ligand or metal. A MMCT band was identified in the absorption spectrum of a trinuclear complex of the type MredL-M-L-M,, [6]. For a complex of the general composition M,,d-L-M-L,, evidence for the appearance of a MLCT band was obtained [7]. In this case donor and acceptor are separated by a ligand and a metal. The present study adds further evidence to the occurrence of long-range optical CT transitions in metal complexes. It is anticipated that optical CT