Electroabsorption Studies of Metal-to-Ligand Charge Transfer in Ru(phenanthroline)(3)(2+): Evidence for Intrinsic Charge Localization in the Initially Formed Excited State

Electroabsorption studies of ruthenium(II) tris(phenanthroline) show that a substantial change in dipole moment (|Delta&mgr;| = 6.7 +/- 1 D) accompanies ground state to "singlet" metal-to-ligand charge transfer (MLCT) excited-state formation. The change is nearly identical to that reported for the 2,2'-bipyridine analogue (Oh; et al. J.Am. Chem. Soc. 1989, 111, 1130). Since both species lack ground-state dipole moments, the finite values for Delta&mgr; are diagnostic of intrinsic charge localization in the excited states. The nominally triplet transition for the phenanthroline complex also involves the formation of a charge localized state. The combined results are inconsistent, therefore, with an alternative "delocalized" charge transfer excited-state interpretation suggested by time-resolved resonance Raman studies of the tris(phenanthroline) complex.

Primitive Molecular Recognition Effects in Electron Transfer Processes: Modulation of ((Trimethylammonio)methyl)ferrocenium/ferrocene Self-Exchange Kinetics via Hydrophobic Encapsulation

1H NMR line broadening measurements show that the electron self-exchange rate constant for ((trimethylamino)methyl)ferrocenium/ferrocene (TMAFc(2+/+)) in D(2)O as solvent is decreased by ca. 20-50 fold in the presence of excess beta-cyclodextrin. The rate effect is associated with the selective hydrophobic encapsulation of the ferrocene form of the redox couple (i.e., the ferrocenium form is not significantly encapsulated). Selective encapsulation leads to a coupling of electron transfer to host (cyclodextrin) transfer. Optical intervalence absorption measurements for a closely related mixed-valence system strongly suggest that the coupling decreases the self-exchange rate by increasing the thermal activation barrier-an inference that is corroborated by activation parameter measurements. The barrier increase ultimately can be understood in terms of a redox asymmetry effect upon the isolated electron transfer event, where the overall exchange mechanism likely entails sequential electron and host transfer.