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

Several complexes of the general type D-L-A have been synthesized in w hich D--L and A-L are well-characterized transition metal complexes, a nd the strength of the donor-acceptor electronic coupling has been inf erred from measurements of: (a) the oscillator strengths of adjacent m etal-to-metal charge transfer (MMCT) absorptions; (b) the shifts in ha lf-wave potentials of Ru(NH3)53+,2+ couples; and (c) the back electron transfer (BET) behavior observed to result from irradiation of the MM CT absorption band. A special emphasis of this work has been complexes containing degenerate pairs of acceptors (or donors). In a typical co mplex, 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, electroni c coupling is weak and BET in the photogenerated Ru(III)-L-Co(II) inte rmediate is non-adiabatic and nearly identical to that of the outer-sp here 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-acc eptor electronic coupling is very strong and gives rise to intense MMC T absorption bands and to substantial shifts in the D/D and A/A electr ochemical half-wave potentials. The electronic coupling inferred from the electrochemical shifts is consistently found to be much larger tha n that inferred from a simple perturbational interpretation of the MMC T absorption bands when the complex contains degenerate acceptors (or donors). Possible origins of this effect are discussed. Picosecond fla sh photolysis experiments indicate that the electron transfer intermed iates (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 intermedi ate is further increased when the lowest energy electronic configurati on of the cobalt(II) center has quartet spin multiplicity. The kinetic data imply that the electronic coupling matrix element (H(DA)kin) app ropriate for the BET process in an order of magnitude smaller than the matrix element (H(DA)op) inferred from MMCT spectroscopy. It is propo sed that this is a simple symmetry effect: the dpi-dsigma electronic t ransitions are ''x, y-allowed'', while A -to-D BET is a ''z-allowed'' process. Such considerations suggest that orbital symmetries play an i mportant role in this strongly coupled limit.