LIGAND PHOTOLYSIS AND RECOMBINATION OF FE(II) PROTOPORPHYRIN IX COMPLEXES IN DIMETHYL SULFOXIDE

摘要

Steady-state and transient absorption spectroscopies have been employed to investigate ligand photolysis and recombination associated with (dimethyl sulfoxide)(2)Fe(II) protoporphyrin IX ((DMSO)(2))Fe(II)PPIX), (imidazole)(2)Fe(II) protoporphyrin IX ((Imid)(2)Fe(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)(2)Fe(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) x 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 Delta A(min) similar to 423 nm, Delta A(max) similar to 439 nm) relative to that of the (DMSO)(2)Fe(II)PPIX complex . This transient species decays with biphasic kinetics. The fast phase of the decay was found to be dependent on the concentration of 2-MeIm exhibiting a second-order rate constant of (7.9 +/- 0.5) x 10(6) M(-1) s(-1). The slow kinetic phase displays first-order kinetics with a rate constant of (4.6 +/- 0.6) x 10(4) s(-1). Photolysis of the (Imid)(2)Fe(II)PPIX yielded kinetics that were faster than the detection limit of our instrument (k > 1.0 x 10(8) s(-1)). [References: 27]