BELL-SHAPED ENERGY-GAP DEPENDENCE OF BACKWARD ELECTRON-TRANSFER OCCURRING WITHIN GEMINATE RADICAL PAIRS PRODUCED BY QUENCHING OF RUTHENIUM(II) POLYPYRIDINE COMPLEXES BY AROMATIC-AMINES

Laser flash spectroscopy has demonstrated that the bimolecular quenching of MLCT excited triplet states of RuL32+ (L = 2,2-bipyridine (bpy), 1,10-phenanthroline (phen), and 4,7-diphenyl-1,10-phenanthroline (dp-phen)) produces Ru(L•-)L2+ and a cation radical of the quencher (aromatic amine) in a mixed solvent of acetonitrile and water (1:1 by volume). Quantum yields of Ru(L•-)L2+ production in the bulk were spectroscopically determined by using the molar absorption coefficients of Ru(L•-)L2+ and 3RuL32+, which were obtained by use of energy transfer from 3Ru(phen)32+ and 3Ru(dp-phen)32+ to anthracene. Rate constants (kb) of the backward electron transfers that occur within the geminate radical pair formed in the quenching were estimated from the quantum yields of Ru(L•-)L2+. A decrease in kb of highly exergonic electron transfer with an increase in the exergonicity (-ΔG°) was observed, with the maximum in kb at ΔG° = -1.7 eV for the three compounds. The ΔG° dependence of kb is discussed in terms of nuclear rearrangement and nuclear tunneling. © 1990 American Chemical Society.