Direct attachment of putative electron acceptors ((bi)pyridinium ions) to either Ru(bpy)3 2+ or Ru(1,10-phenanthroline)3 2+ (excellent photo-excited-state electron donors) leads to enhancement of excited-state lifetimes, rather than the diminution anticipated from electron transfer quenching. An evaluation of photo redox and emission energetics suggests that charged “quencher” attachment lengthens luminescence lifetimes by (a) diminishing the reductant strength of the photo excited state, (b) inhibiting intersystem crossing to a short-lived d−d state, and/or (c) diminishing Franck−Condon factors for nonradiative decay.

Among the emerging themes in chemical, biological, and photosynthetic studies of electron-transfer (ET) reactivity is the apparently essential role played by superexchange and related phenomena in facilitating long-range donor-acceptor coupling.1 Indeed, numerous recent model studies2•3 have emphasized the importance of the intervening medium-typically a covalently bound bridge-in providing energetically accessible virtual states or "conduction" pathways. Often, however, in naturally occurring ET processes the intervening medium (typically a collection of protein residues) is not directly covalently bound to either redox partner; one might question, therefore, whether modeling approaches which emphasize direct synthetic linkages can be fully successful in mimicking thoseparticular processes. An appealing alternative approach for biological redox systems involves the systematic replacement and alteration of key residues (i.e. virtual charge-transfer sites) via site-directed mutagenesis.4•5 While spectacular effects have indeed been seen, the inherent complexity of the systems themselves has sometimes precluded simple interpretations.4 We report here a new approach which, in a sense, blends elements of these two strategies. A comparatively simple family of linked donor-(chromophore-) acceptor complexes has been synthesized and photochemically characterized. The linkages themselves are saturated (and thus relatively ineffective as bridges). They are designed, however, to permit noncovalent encapsulation of real or virtual charge-transfer centers along the "line of fire" between primary donor and acceptor sites.

The title assembly diplays an emissive rhenium-to-pyridine charge-transfer state that is partially quenched by electron transfer to an attached acceptor (nitrobenzene). Quenching is preceded by intramolecular folding (J. Am. Chem. Soc. 1993, 115, 2048). Variable-temperature quenching measurements can be used to determine the characteristic temperature, T tr, above which unfolded photoexcited state conformations become favored over folded conformations. Similar information for the ground state can be obtained from variable-temperature NMR measurements. Studies in eight solvents show that excited state folding is (1) enthalpically favored but entropically disfavored (all solvents), (2) correlated (via T tr) with the inverse dielectric strength of the solvent, and (3) enhanced in comparison to folding in the electronic ground state (studies in one solvent). The combined evidence points to a folding reaction that is driven by optimization of localized Coulombic interactions. Optimization of solvent cohesive interactions, however, may possibly also play a role in the folding reaction.

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