Changes in excited-state character of [M(L-1)(L-2)(CO)(2)(alpha-diimine)] (M = Ru, Os) induced by variation of L-1 and L-2

The character of the lowest excited state of the d(6)-complexes [M(L-1)(L-2 )(CO)(2)(alpha-diimine] (M = Ru, Os) varies with L-1 and L-2. In the case o f [Ru(Cl)(Me)(CO)(2)(alpha-diimine)] it has predominant metal-to-ligand cha rge transfer (MLCT) character. For [M(SnPh3)(2)CO)(2)(alpha-diimine)] (M = Ru, Os), having two axial Sn Ph, ligands, it has sigma(Sn-M-Sn)pi*(alpha-di imine) (Sigma-Bond-to-Ligand Charge Transfer, SBLCT) character. In a glass at 90 R the emission lifetimes of the [Ru(SnPh3)(2)(CO)(2)(alpha-diimine)] complexes are nearly a thousand times longer than those of the correspondin g [Ru(Cl)(Me)(CO)(2)(alpha-diimine)] compounds, although their emission ene rgies are very similar. In fact, these lifetimes are extremely long (e.g. t au = 1070 mu s for [Ru(SnPh3)(2)(CO)(2)(4,4'-dimethyl-2,2'-bipyridine)]) fo r a CT state. Although replacement of Ru by Os decreases both the emission energy and lifetime of complexes such as [M(bpy)(3)](2+) (M = Ru, Os), only the emission lifetime decreases in the case of the [M(SnPh3)(2)CO)(2)(alph a-diimine)] compounds. The latter effect is due to the increase of spin-orb it coupling (SOC), the influence of which could thus be studied without tak ing into account any energy-gap-law effect. Preliminary photophysical data are presented for [Pt(SnPh3)(2)(Me)(2)(i-Pr-DAB)], which has very similar e xcited state properties as the corresponding Ru- and Os-complexes. Replacem ent of a SnPh3 ligand by a methyl group to give [Ru(Me)(SnPh3)(CO)(2)(i-Pr- DAB)] increases the photoreactivity of the complex. The weaker Ru-Me bond i s broken homolytically and the formation of methyl radicals was studied in detail with FT-EPR spectroscopy. (C) 2000 Elsevier Science S.A. All rights reserved.