Photochemistry and Photophysics of Coordination Compounds

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into chemical energy. A strategy to overcome this problem, largely inspired
by the architecture of natural photosynthetic reaction centers, is that of going
to more complex supramolecular systems, triads, etc., in which a sequence of
electron transfer steps is used to achieve long-range charge separation. The
simplest of such system is a triad, as schematically illustrated in Fig. 9 for two
possible reaction schemes. In Fig. 9a, the consecutive ET steps are from the
excited chromophore, P, to an acceptor molecular component, A, and from
a donor unit, D, to the oxidized chromophore. In Fig. 9b, the two consecutive
electron transfer steps are from the excited chromophore, P, to a primary
acceptor, A, and from the primary to a secondary acceptor unit, A ′ .These
strategies have been extensively implemented using organic [106, 107] and, to
a lesser extent, inorganic [108–110] molecular components.
Fig. 9 Two types of triads for photoinduced charge separation. Molecular components:
P (chromophore), D (donor), A (acceptor), A ′ (secondary acceptor). Electron transfer
processes: cs (primary PET), cr (primary charge recombination), cs ′ (secondary charge
separation), cr ′ (final charge recombination)
A number of systems involving Rh(III) molecular components that behave
in some respects as triads (or pseudo-triads) are discussed in this section.
The trimetallic species 23 has been synthesized and studied by Petersen
and coworkers [111] as a possible supramolecular system for photoinduced
multi-step charge separation. This system comprises a Fe(II) electron donor,
a Ru(II) photoexcitable chromophore, and a Rh(III) unit carrying a “monoquat”
acceptor as ligand. Two different types of bridging ligand are present
in 23, a bipyrimidine between Fe(II) and Ru(II) and a dipyridylpyrazine between
Ru(II) and Rh(III). All the other combinations of bridging ligands be-