Pub No:
Porphyrins. 38. Redox Potentials, Charge Transfer Transitions, and Emission of Copper, Silver, and Gold Complexes
Antipas, A., David Dolphin, Gouterman, M., and Johnson, E.C.
J. Am. Chem. Soc.
100, 7705-7709
While CuII porphyrins are known to luminescence, AgII complexes do not. AgIII octethylporphyrin has no emission while AuIII tetraphenylporphyrin has a moderabely intense phosphorescence with a nonexponential decay fit with 2 decay times of 63 and 184 μs. In contrast to CuII porphyrins, the Ag complexes have a metal redox potential, II to III, between that of ring oxidation and ring reduction suggesting that luminescence is quenched by low-energy charge transfer transitions AgII → ring or ring → AgIII. Near-IR (700-1100 nm) absorption spectra confirm the presence of weak absorption bands in AgII and AgIII complexes that are not observed in complexes of CuII and AuIII. The near-IR absorption of CuII(TPP) and the quenching of its unusually broad emission by pyridine suggest that a charge transfer state is close to the emitting level. Iterative extended Huckel calculations explain these facts by the energy of orbital b1g(dx2-y2), which rises along the series Cu < Ag < Au.

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