Photochemical Reactions of Rhodium(III) Diimine Complexes in Solid Glycerol Matrixes. Thermal Activation of a Triplet dd Level from a Triplet ππ* Manifold

At temperatures below 150 K in rigid glycerol glasses, [Rh(III)(s-NN) 3 ](PF 6 ) 3 complexes display a 3 ππ* 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. The temperature dependences of these photochemical reactions conform to the Arrhenius equation with an activation energy range of 2500−3800 cm -1 . These results are interpreted in terms of the thermal redistribution of energy from the unreactive 3 ππ* excited term to a close-lying chemically active 3 dd level. The relationship of the Arrhenius activation energies to the electronic structures and the spectra of the parent complexes is discussed. At 77 K all the photoproducts emit 3 ππ* → gs phosphorescence. The spectral characteristics of the photogenerated species indicate that a single product has been produced in every case. Moreover, the energy of the phosphorescence of each photoproduct and its decay time at 77 K indicate that the newly generated species contains a monodentate s-NN ligand. We conjecture that one octahedral site on the metal is occupied by a glyceroxide moiety that is bonded directly to the rhodium ion through the oxygen atom.