PHOTOINDUCED ENERGY AND ELECTRON-TRANSFER IN INORGANIC COVALENTLY-LINKED SYSTEMS

Photoinduced electron transfer and charge recombination can be studied in metal-polypyridine polynuclear complexes based on the (Mebpy-CH2-CH2-Mebpy) (Me, methyl; bpy, 2,2'-bipyridine) bridging ligand. An example is provided by the binuclear complex [Ru(Me,phen)2(Mebpy-CH2-CH2-Mebpy)Rh(Me2bpy)2]5+ (phen, 1,10-phenanthroline) abbreviated as Ru(II)-Rh(III)). In this system, photoinduced electron transfer processes originating from both local excited states (*Ru(II)-Rh(III) --> Ru(III)-Rh(II) and Ru(II)-*Rh(III) - Ru(III)-Rh(II)), as well as charge recombination (Ru(III)-Rh(II) --> Ru(II)-Rh(III)), have been resolved in the nanosecond and picosecond time domains using transient absorption and emission measurements. An energy transfer process (Ru(II)-*Rh(III) --> *Ru(II)-Rh(III)) can also be observed in rigid media, where the competing electron transfer process is suppressed. The factors affecting the kinetics of the various electron transfer steps can be discussed in terms of current electron transfer theories. Polychromophoric complexes suited for the study of intercomponent energy transfer can be designed using Re(I) and Ru(II) polypyridine units as molecular components and cyanide bridges as connectors. The energy flow in such systems is determined by the relative energy ordering of the metal-to-ligand charge transfer (MLCT) excited states of the various units, which can be controlled synthetically through the type of metal, the type of polypyridine ligand and the binding mode of the bridging cyanide(s). For example, in [NC-Ru(bpy)2-CN-Ru(bpy)2-CN]+, the flow is from the C-bonded to the N-bonded (to bridging cyanide) unit; in [(CO)3Re(phen)-NC-Ru(bpy)2-CN]+, the flow is from Re(I) to Ru(II); in [NC-Ru(bpy), -CN-Ru(bpy-(COO)2)2-NC-Ru(bpy)2-CN]2-, the flow is from the bpy-containing units to the units containing dicarboxy-bpy. With regard to the identification of the lowest energy state in such systems, valuable information can be obtained using transient vibrational spectroscopies, such as time-resolved IR and resonance Raman. Compounds of this series behave in many respects as supramolecular antenna systems. Applications of the antenna effect to the spectral sensitization of semiconductors are discussed.