EXCITED-STATE TRANSIENT OF VANADYL UROPORPHYRIN-I DETECTED BY RESONANCE RAMAN-SPECTROSCOPY

Transient Raman spectroscopy has been used to investigate excited states of vanadyl uroporphyrin I (VOUroP) in both monomeric and dimeric forms. Uroporphyrins are water-soluble porphyrins with propionic and acetic acid groups substituted at the β-pyrrole carbon positions. In solution, the metal uroporphyrins form π-π dimers at high ionic strength. Monomeric VOUroP in aqueous solution is known to be six-coordinate with a ligand trans to the oxo ligand. Upon dimerization, the sixth ligand site is inaccessible, and a five-coordinate species is observed. At high laser fluence, an excited-state transient is formed in the monomeric species. Raman spectra of this species are most consistent with an 2A1u (a1u(π) → dxy) charge-transfer state. In contrast, dimeric VOUroP shows little evidence of an excited state in the transient Raman spectra during a 10-ns laser pulse. © 1990 American Chemical Society.