Sensitied Photoreduction of Bis(acetylacetonato)nicket(II) by Triplet State Aromatic Ketones

Article abstract of DOI:10.1021/ja00284a027

Bis(acetylacetonato)nickel(II), Ni(acac)2, quenched triplet state aromatic ketones with high efficiently but only certain aromatic ketones with high triplet energy levels successfully sensitized the photoreduction of Ni(acac)2 to give the acetylacetone radical and Ni(I) complexes.A short-lived Ni(I) species was detected by ESR and UV spectroscopy, and the acetylacetonyl radical was trapped by 2-nitroso-2-methylpropane or phenyl-tert-butylnitrone as a stable nitroxide and by H atom donors as acetylacetone.The transient Ni(I) complexes decayed by either first order or second-order kinetics, depending on the concentrations, to give Ni(0) complexes or ferromagnetic colloidical nickel.The photoreduction of Ni(acac)2 occurs only if the acetylacetone radical was trapped.Various evidence suggested that the quenching mechanism was not likely to occur by energy transfer and was best interpreted to involve a partial charge transfer within an excited-state complex.The acetylacetonyl radical in the complex undergoes hydrogen abstractions from solvent in competition with reverse electron transfer.The quantum effeiciency of the photoreduction is attributed to the extent of charge-transfer contribution and the hydrogen atom donating ability of solvents.It is also confirmed that the ketyl radical derived from triplet state ketones is not responsible for the reduction of Ni(acac)2.