Porphyrins. 38. Redox Potentials, Charge Transfer Transitions, and Emission of Copper, Silver, and Gold Complexes

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.