Photoinduced Intramolecular Electron Transfer in Peptide-Bridged Molecules

Abstract

Two series of molecules have been prepared and characterized in which a polypyridyl Ru(II) complex is linked to p-dimethoxybenzene (DMB) and p-benzoquinone (Q) moieties by peptide bridges containing the amino acid L-proline (Pro). The photophysical properties of the metal-to-ligand charge transfer (MLCT) excited state of the Ru(II) chromophore have been examined for the complexes with 0, 1, 2, 3, and 4 intervening Pro residues. Steady state and time resolved luminescence experiments on the Pro-bridged DMB system show that the properties of the Ru MLCT excited state are only slightly modified from those of an unsubstituted model complex by the presence of covalently attached DMB peptides. Experiments on the Pro-bridged Ru-Q complexes show that the Q site quenches the yield and lifetime of the Ru MLCT emission. Furthermore, the quenching efficiency is diminished as the number of peptide spacers is increased. The quenching process is ascribed to long-range intramolecular Ru-to-Q electron transfer. This hypothesis is supported by time-resolved luminescence data which suggest that the average electron transfer rate falls sharply with an increase in the peptide bridge length. © 1988, American Chemical Society. All rights reserved.