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-C H2-Mebpy) (Me, methyl; bpy, 2,2'-bipyridine) bridging ligand. An examp le is provided by the binuclear complex [Ru(Me,phen)2(Mebpy-CH2-CH2-Me bpy)Rh(Me2bpy)2]5+ (phen, 1,10-phenanthroline) abbreviated as Ru(II)-R h(III)). In this system, photoinduced electron transfer processes orig inating 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 recombin ation (Ru(III)-Rh(II) --> Ru(II)-Rh(III)), have been resolved in the n anosecond and picosecond time domains using transient absorption and e mission measurements. An energy transfer process (Ru(II)-Rh(III) --> Ru(II)-Rh(III)) can also be observed in rigid media, where the compet ing electron transfer process is suppressed. The factors affecting the kinetics of the various electron transfer steps can be discussed in t erms of current electron transfer theories. Polychromophoric complexes suited for the study of intercomponent energy transfer can be designe d using Re(I) and Ru(II) polypyridine units as molecular components an d cyanide bridges as connectors. The energy flow in such systems is de termined by the relative energy ordering of the metal-to-ligand charge transfer (MLCT) excited states of the various units, which can be con trolled synthetically through the type of metal, the type of polypyrid ine ligand and the binding mode of the bridging cyanide(s). For exampl e, 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-(C OO)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-r esolved IR and resonance Raman. Compounds of this series behave in man y respects as supramolecular antenna systems. Applications of the ante nna effect to the spectral sensitization of semiconductors are discuss ed.