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In natural photosynthesis in photosynthetic bacteria,
a very effective charge separation occurs with a lifetime of several seconds
over a long distance, (about 100 Angström). Our studies of artificial
photosynthesis focus on catenanes and rotaxanes in which the constituent
elements, ring and wire, each have a different pendent porphyrin. Such
arrays are particularly well adapted to the study of electron transfer
in chromophores between which there is absolutely no connection.
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Several series of arrays have been prepared and studied: in the above
example the thread is terminated by two zinc porphyrin stoppers (electron
donors in the singlet excited state) which act as the blocking groups,
and the ring incorporates a gold porphyrin. This system involves perforce
very little friction between the ring and the axle. A rotational movement
of 180 degree is observed as a result of decomplexation of Cu(I) from
the central site. There is a notable difference between the rate of
electron transfer for the situation 'close' and 'distant'. In the free
rotaxane the transfer occurs over several tens of
picoseconds, while in the extended form several hundred picoseconds
are required. Charge recombination appears to be relatively slow (tens
of nanometres).
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The study of artifical photosynthesis concerns research
into systems in which vectorial electron transfer leads to species with
a long-lived charge separated state. In the molecular triad below, Ir(III)
bis-terpy is the central electroactive species which
acts as an electron relay between the primary electron donor, the free-base
porphyrin, and the secondary electron acceptor, a gold porphyrin. The
use of Ir(III) as the central metal circumvents difficulties encountered
with analogous Ru(II) complexes in which energy rather than electron transfer
was the dominant process.
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Using time-resolved emission measurements and transient absorption
spectroscopy, it is possible to establish the lifetimes of the various
electron transfer processes. The fully charge-separated state has a
lifetime of 3.5 ns in dichloromethane and is formed with a 50% efficiency
by two successive electron transfers. Its deactivation proceeds quantitatively
through the triplet state.
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