Photochemical reactions of rhodium(III) diimine complexes in solid glycerol matrices

At temperatures below 150 K in rigid glycerol glasses [Rh(CN)2(s-NN)2]PF6 complexes display a 3ππ* → gs phosphorescence characteristic of the s-NN ligand [s-NN = 1,10-phenanthroline (phen), 4-Mephen, 4,7-Me2phen, and 3,4,7,8-Me4phen]. 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 the 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)3](PF6)3 complexes. These results are interpreted in terms of the thermal redistribution of energy from an unreactive 3ππ* excited term to a close-lying chemically reactive 3dd 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.