LIGAND PHOTOLYSIS AND RECOMBINATION OF (DIMETHYLSULFOXIDE)(2)FE(II)PORPHYRIN COMPLEXES

Steady-state optical absorption and nanosecond transient absorption sp ectroscopy have been employed to examine ligand photolysis and recombi nation of (dimethylsulfoxide)(2)Fe(Il)protoporphyrinIX ((DMSO)(2)Fe(II )PPIX), O)(2)tetrakis-4-(sulfonatophenyl)-Fe(II)porphyrin, ((DMSO)(2)F e(II)4SP) and (DMSO)(2)Fe(II)uroporphyrin ((DMSO)(2)Fe(II)URO) complex es in neat DMSO. The steady state optical absorption data indicate tha t all three complexes are six-coordinace and low-spin. The correspondi ng nanosecond transient absorption data demonstrate that photo excitat ion of the (DMSO)(2)Fe(II)PPIX and (DMSO)(2)Fe(II)URO complexes result s in formation of a transient species with an absorption maximum and m inimum at similar to 434 and similar to 424 nm, respectively. These tr ansient species decay exponentially with rate constants of similar to 2 X 10(6) s(-1). Excitation of the ((DMSO)(2)Fe(II)4SP, on the other h and, produces a transient species with a broad absorption maximum cent ered at similar to 438 nm with a minima at similar to 424 nm in the ki netic difference spectrum. The initially formed transient decays expon entially with rate constants of similar to 1.5 x 10(5) s(-1). Examinat ion of the temperature dependence of the rate constants for ligand rec ombination reveals that the activation enthalpy is higher for ligand r ecombination to the (DMSO)Fe(II)4SP complex than either the (DMSO)Fe(I I)PPIX or (DMSO)Fe(II)URO complexes (8.8, 6.1 and 6.8 kcal mol(-1) for Fe(II)4SP, Fe(II)PPIX and Fe(II)URO, respectively). These results all ow for the interpretation of the activation energy for ligand recombin ation in terms of electrostatic, steric and solvent interactions. (C) 1998 Elsevier Science S.A.