Ligand photolysis and recombination of Fe(II) protoporphyrin IX complexes in dimethyl sulfoxide

Abstract

Steady-state and transient absorption spectroscopies have been employed to investigate ligand photolysis and recombination associated with (dimethyl sulfoxide) 2Fe(II) protoporphyrin IX (DMSO) 2Fe(II)PPIX), (imidazole) 2Fe(II) protoporphyrin IX ((Imid) 2Fe(II)PPIX), and (2-methylimidazole)(dimethyl sulfoxide)Fe(II) protoporphyrin IX ((2-MeIm)(DMSO)Fe(II)PPIX) complexes in neat dimethyl sulfoxide (DMSO). Steady-state optical absorption spectra of these complexes are characteristic of six-coordinate low-spin heme iron. Photo-excitation of the (DMSO) 2Fe(II)PPIX complexes results in the formation of a transient species consistent with a five-coordinate high-spin heme iron. The transient difference spectrum displays an absorption minimum at about 423 nm and an absorbance maximum at about 435 nm. This species decays with a first-order rate constant of (2.13 ± 0.04) × 10 6 s −1. In contrast, photolysis of the (2-MeIm)(DMSO)Fe(II)PPIX complex results in a species with a transient difference spectrum broadened and red-shifted ΔA min ∼ 423 nm, ΔA max ∼ 439 nm) relative to that of the (DMSO) 2Fe(II)PPIX complex. This transient species decays with biphasic kinetics. The fast phase of the decays was found to be dependent on the concentration of 2-MeIm exhibiting a second-order rate constant of (7.9 ± 0.5) × 10 6 M −1 s −1. The slow kinetic phase displays first-order kinetics with a rate constant of (4.6 ± 0.6) × 10 4 s −1. Photolysis of the (Imid) 2Fe(II)PPIX yielded kinetics that were faster than the detection limit of our instrument ( k > 1.0 × 10 8 s −10.