Photoinduced electron/energy transfer across molecular bridges in binuclear metal complexes.

Molecular bridges that efficiently move charge between remote donor and acceptor sites can be thought of as molecular wires. Insight into the properties of molecular wires can be obtained by studying photoinduced electron transfer in covalently linked donor--bridge--acceptor systems. This article summarizes some of the recent progress in the study of such systems involving transition metal complexes as donor and acceptor units. Specific classes of molecular bridges are considered, namely, polyphenylene, and polyquinoxaline bridges. Basic questions are discussed, such as the transfer mechanisms, the associated distance and bridge structure dependence, and the interplay between energy and electron transfer.

Solvent switching of intramolecular energy transfer in bichromophoric systems: photophysics of (2,2'-bipyridine)tetracyanoruthenate(II)/pyrenyl complexes.

The supramolecular systems [Ru(Pyr(n)bpy)(CN)(4)](2-) (n = 1, 2), where one and two pyrenyl units are linked via two-methylene bridges to the [Ru(bpy)(CN)(4)](2-) chromophore, have been synthesized. The photophysical properties of these systems, which contain a highly solvatochromic metal complex moiety, have been investigated in water, methanol, and acetonitrile. In all solvents, prompt and efficient singlet-singlet energy transfer takes places from the pyrene to the inorganic moiety. Energy transfer at the triplet level, on the other hand, is dramatically solvent dependent. In water, the metal-to-ligand charge transfer (MLCT) emission of the Ru-based chromophore is completely quenched, and rapid (200 ps for n = 1) irreversible triplet energy transfer to the pyrene units is detected in ultrafast spectroscopy. In acetonitrile, the MLCT emission is practically unaffected by the presence of the pyrenyl chromophore, implying the absence of any intercomponent triplet energy transfer. In methanol, triplet energy transfer leads to an equilibrium between the excited chromophores, with considerable elongation of the MLCT lifetime. The investigation of the [Ru(Pyr(n)bpy)(CN)(4)](2-) systems in methanol provided a very detailed and self-consistent picture: (i) The initially formed MLCT state relaxes toward equilibrium in 0.5-1.3 ns (n = 1, 2), as monitored both by ultrafast transient absorption and by time-correlated single photon counting. (ii) The two excited chromophores decay with a common lifetime of 260-450 ns (n = 1, 2), as determined from the decay of MLCT emission (slow component) and of the pyrene triplet absorption. (iii) These equilibrium lifetimes are fully consistent with the excited-state partition of 12-6% MLCT (n = 1-2), independently measured from preexponential factors of the emission decay. Altogether, the results demonstrate how site-specific solvent effects can be used to control the direction of intercomponent energy flow in bichromophoric systems.

Photophysics of pyridylporphyrin Ru(II) adducts: heavy-atom effects and intramolecular decay pathways.

Eight adducts between different pyridylporphyrins and ruthenium complexes, MPyP[RuCl(2)(DMSO)(2)(CO)], c-DPyP[RuCl(2)(DMSO)(2)(CO)](2), TrPyP[RuCl(2)(DMSO)(2)(CO)](3), TPyP[RuCl(2)(DMSO)(2)(CO)](4), (MPyP)(2)[RuCl(2)(DMSO)(2)], [c-DPyP[RuCl(2)(DMSO)(2)]](2), MPyP[RuCl(2)(CO)(3)], and [c-DPyP[RuCl(2)(CO)(2)]](2), have been investigated. The results show that in all the adducts the porphyrin singlet is quenched, to a greater or lesser extent, relative to the parent-free molecule. This study provides insight into the mechanisms of singlet quenching in the adducts. Two mechanisms for singlet quenching, both related to the "heavy-atom effect" of the ruthenium center and experimentally distinguishable by transient spectroscopy, are examined. Enhanced intersystem crossing within the porphyrin chromophore is demonstrated for the series of adducts MPyP[RuCl(2)(DMSO)(2)(CO)], c-DPyP[RuCl(2)(DMSO)(2)(CO)](2), TrPyP[RuCl(2)(DMSO)(2)(CO)](3), and TPyP[RuCl(2)(DMSO)(2)(CO)](4), where a nice correlation is observed between the magnitude of the effect and the number of ruthenium centers attached to the pyridylporphyrin chromophore. Singlet-triplet energy transfer from the pyridylporphyrin chromophore to the ruthenium center(s) is an additional efficient quenching channel for adducts containing ruthenium centers with weak field ligands and low triplet energies, such as (MPyP)(2)[RuCl(2)(DMSO)(2)] and [c-DPyP[RuCl(2)(DMSO)(2)]](2).