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

Intramolecular electron transfer (ET) in [Ir-2(mu-pz*)(2)(CO)(2)(Ph2PO(CH2) (2)A(+))(2)] [B(Ph)(4)(-)](2) ([IrPOpy(+)](2); pz* = 3,5-dimethylpyrazolyl, A = pyridinium derivative, Ph = C6H5) exhibits a Gaussian free-energy depe ndence. The temperature dependences (200-280 K) of both excited-state ET (I r-2*->A(+)) and highly exergonic recombination (A(.)-->Ir-2(.)) reactions a lso agree remarkably well with semiclassical ET theory. Resonance Raman mea surements on the N-ethyl-4-phenylpyridinium iodide charge-transfer complex reveal distortions along several vibrational modes upon pyridinium reductio n. Neither semiclassical ET theory nor single-mode quantum theories can acc ount quantitatively for all of the ET observations in [IrPOpy(+)](2). Bimol ecular ET reactions between the triplet excited state of [Ir(mu-pz)(COD)](2 ) (pz = pyrazolyl, COD = 1,5-cyclooctadiene) and a series of N-alkylpyridin ium acceptors remain limited by diffusion at driving forces where the recom bination reactions are inverted. (C) 2000 Elsevier Science S.A. All rights reserved.