PHOTOINDUCED ELECTRON-TRANSFER AND ENERGY-TRANSFER PROCESSES OCCURRING WITHIN PORPHYRIN-METAL-BISTERPYRIDYL CONJUGATES

Photophysical properties have been measured for zinc and free-base porphyrins covalently linked to ruthenium(II) or rhodium(III) bisterpyridyl complexes using ultrafast transient absorption spectroscopy. The appended metal complex quenches porphyrin fluorescence due to rapid intramolecular electron transfer. For directly coupled systems, the rate of photoinduced electron transfer (k approximate to 10(12) s(-1)) approaches the inverse of the solvent reorientation time in. solvents that relax rapidly but greatly exceeds the relaxation rate in ethanol at low temperature. These electron-transfer processes, which remain rapid in an ethanol glass at 77 K, are considered in terms of the model introduced by Sumi and Marcus (J. Chem. Phys. 1986, 84, 4894). Inserting a phenyl ring between the reactants decreases the extent of their mutual electronic coupling so that the rates of electron transfer decrease. Because of the large amount of energy that must be dissipated, charge recombination is relatively slow in these latter systems, and the observed kinetic data can be well described in terms of current nonadiabatic electron transfer theory. In particular, the phenyl-bridged, ruthenium(II) bisterpyridyl-based conjugate possesses properties that appear suitable for its use as a molecular bridge in multicomponent photosynthetic systems where it should facilitate rapid long-range, multistep electron transfer. This latter conjugate also demonstrates Dexter-type triplet energy transfer from metal complex to porphyrin.