Bimolecular electron transfer at high driving forces. Photoreactions of gold(I) and iridium(I) complexes with pyridinium derivatives

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Abstract

The rates of electron transfer from electronically excited [Au2(1,2-bis(dicyclohexylphosphino)ethane)3]2+ (AU22+*) and [Ir(1,5-cyclooctadiene)(μ-pyrazolyl)]2 (Ire2*) to N-alkylpyridinium (py+) acceptors have been measured by Stern-Volmer quenching of Au22+* and Ir2* in acetonitrile solution at 25 °C. The quenching rates increase with reaction driving force, reaching the diffusion limit at −ΔGo=λ and remaining at this limit for higher driving forces. The recombination reactions of pyridinyl radicals with Ir2+ (measured by transient absorption), however, exhibit inverted behavior at −Go values greater than λ: rates decrease with increasing driving force. Formation of electronically excited products (py/sd*/Ir2+*) is suggested by the absence of inverted behavior for the Ir2*/py+ reactions.

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