Photoinduced decarbonylation of ruthenium(II) carbonyl octaethylporphyrin in acetonitrile: studies on the origin of the excitation wavelength dependence

Photodecarbonylation of ruthenium(II) carbonyl octaethylporphyrin, CORu{sup II}OEP, in acetonitrile has been investigated by steady-light and laser flash photolysis techniques. The quantum yields, {Phi}, for photodecarbonylation in degassed solutions are markedly dependent on excitation wavelengths, {lambda}, {Phi} = 0.01 {plus minus} 0.02 (550 > {lambda} > 350 nm), and {Phi} gradually increases up to 0.08 {plus minus} 0.01 on going from 350 to 260 nm at 298 K. With the use of 1,4-benzoquinone as a quencher, photodecarbonylation upon excitation of the Q band has been demonstrated to occur solely via the triplet mechanism, i.e., the triplet CORu{sup II}OEP is involved as a reactive excited state. In order to elucidate the wavelength dependence observed for the yields for photodecarbonylation, they have measured the quantum yields, phosphorescence intensities, and the decay rate constants of the triplet CORu{sup II}OEP in the temperature range 273-350 K. On the basis of these results, a marked increase in the quantum yields observed in the excitation wavelength region {lambda} < 350 nm is interpreted on the assumption that the excited state of CORu{sup II}OEP produced immediately upon excitation at {lambda} < 350 nm loses the excitation energy through elongation of the distance between CO and the central ruthenium atom to yield a loosely bound complex, CO{hor ellipsis}Ru{sup II}OEP, as an intermediate. The intermediate thermally dissociates to give Ru{sup II}OEP and CO. Photodecarbonylation via the triplet mechanism is also discussed in detail.