INTRAMOLECULAR ELECTRON TRANSFER AND ENERGY TRANSFER IN BIMETALATED COMPOUNDS

Abstract

Intramolecular electron transfers (ET) within bi-chromophoric compounds have been examined by means of laser photolysis spectroscopy. The chromophores examined are Ru(II)L 2 (L = 2,2'-bipyridine (bpy) or 4,4'-dimethyl-2,2'-bipyridine (dmbpy)) and Rh(III)L' 2 (L' = 1,10-phenanthroline (phen)) which are linked by a tetradentate ligand, bis-2,2'-(2“-pyridyl)-bibenzimidazole (bpbimH 2 ). The Ru(II)-site in the MLCT state is a strong electron donor to the Rh(III)-site. The intervening ligand itself consists of two chromophores of 2-(2'-pyridyl)-benzimidazole (pbimH) which acts an electron acceptor in the diprotonated form (pbimH 3 2+ ). Quenching of the excited MLCT state of the Ru(II)-site are ascribed to ET to the Rh(III)-site in RuL 2 (bpbimH 2 )RhL' 2 5+ or to pbimH 3 2+ -moiety in the RuL 2 (pbimH-pbimH 3 ) on the basis of redox potentials of the sites. Rates of the Ru(II)-to-Rh(II) ET were measured at low temperatures (165-240 K) by monitorring the decay of excited state absorption (ESA). Rates of the Ru(II)-to-pbimH 3 2+ ET in Ru(dmbpy) 2 (pbimH-pbimH 3 ) 4+ were 2.6 × 10 10 s −1 at room temperature and 1 × 10 s −1 at 165 K. Analysis of the temperature dependence of the rate affords an activation energy of 0.2 eV for the ET process. Non-adiabaticity in the Ru(II)-to-Rh(III) ET is discussed in conjunction with the superexchange metal-metal interaction estimated from the spectroscopic and electrochemical data.