A R T I C L E S
Megiatto et al.
acceptors.5 Due to their low reduction potentials and small
reorganization energy, they simultaneously enhance dynamics
for charge separation (CS) while inhibiting charge recombination
(CR). As a result, a very large number of porphyrin-fullerene
systems have been synthesized and their electrochemical and
photophysical properties have been extensively investigated.6
dominant principle for construction of molecular motors,
shuttles, muscles, and information storage devices.10
Rotaxanes incorporating both porphyrin and C60 moieties have
been prepared and studied by our group11 and others.8k-p In
our first-generation systems, modeled on the Cu(I)-templated
purely porphyrinic systems elegantly studied by Sauvage and
co-workers,8a electronic excitation induces a series of long-range
energy and electron-transfer processes resulting in the generation
of long-lived charge separated radical pair (CSRP) states, i.e.,
porphyrin•+-C60•-. CSRP lifetimes between 0.59 and 1.17 µs
in dichloromethane (DCM) were observed for three first-
generation rotaxanes in which C60 is covalently linked to the
polyether macrocycle, while two ZnP groups serve as termini
on the thread.11a,d For rotaxanes in which the groups were
switched, i.e., ZnP covalently linked to the macrocycle and C60
moieties now serving as blocking groups on the threads, CSRP
lifetimes of 320 ns, 730 ns, and 29 µs were observed in DCM;
in THF, the lifetimes were even longer, 890 ns and 32 µs.11b,d
The inverse dependence of the rates of charge recombination
(CR) on solvent polarity for these interlocked DA materials is
evidence that CR is occurring in the Marcus inverted region,
Recently, supramolecular concepts have been introduced into
the synthesis of porphyrin-fullerene photosynthetic model
systems.7 Among the most promising of these are mechanically
interlocked electron donor-acceptor systems,8 in which the
noncovalent linkage of D and A moieties mimics the natural
system more closely than covalently linked D-A systems, which
have received the greatest attention to date.6
Catenanes are comprised of two or more interlocked rings,
while rotaxanes possess a ring threaded on a rod bearing terminal
bulky substituents to prevent dissociation. In such systems,
constituent photoactive components are located within a fixed
distance without any covalent linkage, allowing the components
to undergo submolecular motions by application of an external
stimulus, such as light, electrochemical, or chemical inputs.9
This molecular topology has been elegantly explored as the
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