Photochemical reactions of rhodium(III) diimine complexes. in solid glycerol matrices - Ligand-field influences on activation parameters

At temperatures below 150 K in rigid glycerol glasses [Rh(CN)(2)(s-NN)(2)]PF6 complexes display a (3)pi pi* -> gs phosphorescence characteristic of the s-NN ligand [s-NN=1,10-phenanthroline(phen),4-Mephen, 4,7-Me(2)phen, and 3,4,7,8-Me(4)phen]. Above 150 K but below the melting point of glycerol a temperature-dependent first-order reaction occurs upon irradiation with UV light. The temperature dependences of these photochemical reactions conform to them Arrhenius equation with an activation energy range of 2000-2500 cm(-1), which is a range averaging 725 cm-1 lower than that previously reported for [Rh(s-NN)31(PF6)3 complexes. These results are interpreted in terms of the thermal redistribution of energy from an unreactive (3)pi pi* excited term to a close-lying chemically reactive 3 dd level that has been lowered in energy with respect to that of the corresponding [Rh(s-NN)3](PF6)3 complex. The relationships of the Arrhenius activation energies to the electronic structures of the parent complexes and to the natures of the products are discussed. (c) 2005 Elsevier B.V. All rights reserved.