Photoinduced Electron Transfer and Intramolecular Folding in a Tricarbonylrhenium (Bi)pyridine Based Donor/Crown/Acceptor Assembly: Dependence on Solvent
Received May 3, 1995 Abstract: 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, Ttr, 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
Ttr) 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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