PHOTOINDUCED ELECTRON-TRANSFER IN LINKED TRANSITION-METAL DONOR-ACCEPTOR COMPLEXES

Several complexes of the general type D-L-A have been synthesized in which D--L and A-L are well-characterized transition metal complexes, and the strength of the donor-acceptor electronic coupling has been inferred from measurements of: (a) the oscillator strengths of adjacent metal-to-metal charge transfer (MMCT) absorptions; (b) the shifts in half-wave potentials of Ru(NH3)53+,2+ couples; and (c) the back electron transfer (BET) behavior observed to result from irradiation of the MMCT absorption band. A special emphasis of this work has been complexes containing degenerate pairs of acceptors (or donors). In a typical complex, a ruthenium(II) center functions as the donor and the acceptor may be of any of several substitution inert metal complexes (Ru3+, Co3+, Rh3+ or Cr3+). When L=1,2-bis(2,2'-bipyridyl-4-yl)ethane, electronic coupling is weak and BET in the photogenerated Ru(III)-L-Co(II) intermediate is non-adiabatic and nearly identical to that of the outer-sphere Ru(bpy)32+/Co(bpy)3(3+) (bpy, 2,2'-bipyridine) couple, indicating no special role for the aliphatic linker. When L = CN-, the donor-acceptor electronic coupling is very strong and gives rise to intense MMCT absorption bands and to substantial shifts in the D/D and A/A electrochemical half-wave potentials. The electronic coupling inferred from the electrochemical shifts is consistently found to be much larger than that inferred from a simple perturbational interpretation of the MMCT absorption bands when the complex contains degenerate acceptors (or donors). Possible origins of this effect are discussed. Picosecond flash photolysis experiments indicate that the electron transfer intermediates (D-CN-A-) are much longer lived when A is a cobalt(III) complex than when A is a ruthenium(III) complex. The lifetime of the intermediate is further increased when the lowest energy electronic configuration of the cobalt(II) center has quartet spin multiplicity. The kinetic data imply that the electronic coupling matrix element (H(DA)kin) appropriate for the BET process in an order of magnitude smaller than the matrix element (H(DA)op) inferred from MMCT spectroscopy. It is proposed that this is a simple symmetry effect: the dpi-dsigma electronic transitions are ''x, y-allowed'', while A -to-D BET is a ''z-allowed'' process. Such considerations suggest that orbital symmetries play an important role in this strongly coupled limit.