Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Rhodium 227
3.2
Dyads
Heteronuclear bimetallic species containing rhodium polypyridine complexes
are more interesting, as the Rh(III) unit can be involved in intercomponent
processes, particularly of the electron transfer type. The thermodynamic
requirements for the participation of Rh(III) polypyridine complexes in electron
transfer processes are summarized in Fig. 4, where Rh(III), ∗ Rh(III), and
Rh(II) represent the ground state, the triplet LC excited state (see Sect. 2.1),
and the one-electron reduced form, respectively, and the values of excitedstate
energy [31] and reduction potential [78] refer to Rh(phen)3 3+ (1). From
these figures, it is apparent that Rh(III) polypyridine complexes can behave
as extremely powerful photochemical oxidants and relatively good electron
transfer quenchers. On the other hand, because of the high excited-state energy,
these complexes are also good potential energy donors.
Fig. 4 Typical redox energy level diagram for Rh(III) polypyridine complexes. Values
(reduction potential vs. SCE) appropriate for Rh(phen)3 3+ (1)
As a matter of fact, Rh(III) polypyridine complexes have been extensively
used in bimolecular electron transfer processes, either as photoexcited
molecule [79, 80] or as quencher [81–83], with motivations of both fundamental
(testing electron transfer-free energy relationships) [80] and applied
nature (photoinduced hydrogen evolution from water) [81, 82]. Here, on the
other hand, we focus our attention on photoinduced processes where the reactants
are pre-assembled in some kind of supramolecular system. The most
common photoinduced processes taking place in simple two-component systems
(often called “dyads”) involving a Rh(III) polypyridine unit are shown
in Eqs. 6–8:
∗ Rh(III) –Q → Rh(III) – ∗ Q (6)
∗ Rh(III) –Q → Rh(II) –Q + (7)
∗ P – Rh(III) → P + – Rh(II) . (8)