Effects of driving force on the rates of intramolecular and bimolecular electron-transfer reactions

Abstract

Intramolecular electron transfer (ET) in [Ir 2(μ-pz*) 2(CO) 2(Ph 2PO(CH 2) 2A +) 2] [B(Ph) 4 −] 2 ([IrPOpy +] 2; pz*=3,5-dimethylpyrazolyl, A=pyridinium derivative, Ph=C 6H 5) exhibits a Gaussian free-energy dependence. The temperature dependences (200–280 K) of both excited-state ET (Ir 2*→A +) and highly exergonic recombination (A →Ir 2 ) reactions also agree remarkably well with semiclassical ET theory. Resonance Raman measurements on the N-ethyl-4-phenylpyridinium iodide charge-transfer complex reveal distortions along several vibrational modes upon pyridinium reduction. Neither semiclassical ET theory nor single-mode quantum theories can account quantitatively for all of the ET observations in [IrPOpy +] 2. Bimolecular ET reactions between the triplet excited state of [Ir(μ-pz)(COD)] 2 (pz=pyrazolyl, COD=1,5-cyclooctadiene) and a series of N-alkylpyridinium acceptors remain limited by diffusion at driving forces where the recombination reactions are inverted.