4 INTERCOMPONENT PROCESSES IN A RU(II)-RH(III) POLYPYRIDINE DYAD - ELECTRON-TRANSFER FROM EXCITED DONOR, ELECTRON-TRANSFER TO EXCITED ACCEPTOR, CHARGE RECOMBINATION, AND ELECTRONIC-ENERGY TRANSFER

The binuclear complex )(2)-(Mebpy-CH2-CH2-Mebpy)-Rh-III(Me(2)pby)(2)(5 +) (Me(2)phen = 4,7-dimethyl-1,10- phenanthroline; Mebpy = 4-methyl-2, 2'-bipyridine; Me(2)bpy = 4,4'-dimethyl-2,2'-bipyridine), hereafter re presented by Ru(II)-Rh(III), was synthesized and studied. Selective ex citation of the two moieties of the dyad was achieved with visible (10 0% Ru(II)-Rh(III)) or ultraviolet light (e.g., at 298 nm, 70% Ru(II)- Rh(III)), In room-temperature fluid solutions, both local excited sta tes are quenched by electron transfer, leading to a common Ru(III)-Rh( II) state. The two forward electron-transfer processes, as well as the recombination process leading back to the ground state, can be resolv ed by transient laser spectroscopy, using various excitation wavelengt hs and pulse widths (532 nm, 30 ps; 427 nm, 0.5 ps; 298 nm, 0.5 ps). R ate constants in acetonitrile are as follows: Ru(II)-Rh(III) --> Ru(I II)-Rh(II), 1.7 X 10(8) s(-1); Ru(II)-Rh(III) --> Ru(III)-Rh(II), 3.3 x 10(10) s(-1); Ru(III)-Rh(II) --> Ru(II)-Rh(III), 7.1 x 10(9) s(-1). The rate constants can be rationalized in terms of standard electron- transfer theory, assuming that the driving force (Delta G degrees =-0. 10, -0.70, and -2.07 eV, respectively) is the main variable parameter. The two forward processes belong to the ''normal'', and the back reac tion belongs to the ''inverted'' free-energy regime. In room-temperatu re fluid solution, no Ru(II)-Rh(III) --> *Ru(II)-Rh(III) energy trans fer (Delta G degrees =-0.61 eV) is observed, presumably because of eff icient competition by the faster Ru(II)-Rh(III) --> Ru(III)-Rh(II) el ectron-transfer quenching. By contrast, this process becomes efficient in rigid media (room-temperature or 77 K), where both the Ru(II)-Rh( III) --> Ru(III)-Rh(II) and Ru(II)-Rh(III) --> Ru(III)-Rh(II) electro n-transfer processes are blocked as a consequence of restricted solven t repolarization. In 77 K ethanol glass, the energy-transfer rate cons tant is 1.9 X 10(6) s(-1).