INTRAMOLECULAR CHARGE SHIFT FOLLOWING BIMOLECULAR REDUCTIVE QUENCHING OF A RHODIUM(III) POLYPYRIDINE DIQUAT DYAD

The Rh(NN)3(3+)-DQ2+ dyad, which contains a Rh(III) polypyridine moiety (Rh(NN)3(3+)) and a N,N'-bridged diquaternarized 4,4'-dimethyl-2,2'-bipyridine (DQ2+) as covalently linked components, has been synthesized and used in the study of intramolecular electron transfer. The study, performed by laser flash photolysis, makes use of a relatively unconventional reaction scheme. First, bimolecular electron-transfer quenching of the Rh(NN)3(2+)-localized excited state of the dyad (using 1,2,3-trimeth-oxybenzene as external reductant) is used to generate the reduced dyad in the thermodynamically unfavored Rh(NN)3(2+)-DQ2+ form. Then, this species is observed to relax by a fast (k = (3 +/- 1) x 10(7) s-1) intercomponent charge-shift process to the stable (DELTA-G-degrees, ca. 0.2 eV) Rh(NN)3(3+)-DQ+ form. A slower bimolecular back-electron-transfer reaction with the radical cation of the external quencher (k = 3.3 x 10(9) M-1 s-1) finally brings back the dyad to its original oxidation state.