Photoexcitation is almost always a one-photon, one-electron process.202 As seen in reactions 5 and 6, water splitting is a multi-electron process. One significant challenge in the development of artificial photosystems has been addressing this mismatch. As seen in reactions 19 and 20, one-electron routes towards water splitting involve the formation of highly reactive intermediates and require considerably higher potentials202,203 than needed for the corresponding multi-electron reactions 5 at -0.414 V and -0.816 V, respectively. The redox potentials required to drive reactions 18 and 19 are not attainable by Ru-polypyridyl systems and are similarly out of the reach for many organic sensitizers. Furthermore, controlling and directing these highly reactive intermediates (e.g., OH, H, OOH) towards the desired products (H2 and O2) is an extremely challenging task which is best avoided if possible:
OH- ^ OH. + e- -2.33 V (+ 224.8 kJ/ mol, pH 7) (24)
It is clear that the electron stoichiometry strongly affects the reaction mechanism and therefore the potentials required. As demonstrated by the natural photosystems, properly designed co-catalysts can circumvent these high-energy one-electron path-
ways, however this is still a tremendous challenge for man-made systems. The first photoinduced charge separation event typically changes the physical properties of the acceptor and donor such that successive electron transfer events are not favora-
One of the first complexes capable of storing more than a single electron was the Run-IrnI-Run trimer 19. In this case, reductive quenching of the photoexcited complex with dimethylaniline led to the storage of two electrons, one in each of the n* orbitals of the two bridging polypyridyl ligands (highlighted in bold).206 More recently, the same group reported a trimeric Run-Rhm-Run complex 20 (see Fig. 10), structurally similar to 19, that undergoes photoreduction at the Rh(III) site to yield the Ru^Rh^Ru" complex 21. In this case, the photoproduct 21 has undergone a considerable structural rearrangement at the Rh site with loss of two chloride ligands. This doubly-reduced RhI center is an attractive catalytic site for heterolytic reactions such as oxidative addition or the formation of hydrides. In fact, the complex is reported to catalytically evolve hydrogen under photochemical conditions.207
MacDonnell and coworkers have shown that compounds 11 and 23 undergo two and four electron reductions to yield 22 and 24, respectively, under photochemical conditions in both MeCN and water (see Fig. 11).208-210 In this system, the photore-ductions are ligands based and are seen to occur as stepwise one-electron processes under basic conditions. At lower pH's (~6-8), protonation of the reduced cen-
tral bridging ligands becomes evident and the individual one-electron reductions merge such that only two electron reductions are observed at the relatively slow timescale of the photochemical reaction (minutes). The process is reversible and air oxidation reoxidizes the reduced complexes back to 11 and 23. Visible light irradiation of a acetonitrile solution of 23, Pd(bpy)Cl2, NH4PF6 and triethanolamine slowly produces H2 at a rate of ~ 3 turnovers (per 23) a day.211 As before when discussing the unusual bichromophore behavior of 11, the unusual acceptor capabilities of tatpp and tatpq bridging ligands seems to arise from the weak electronic coupling of the tetraazapentacene-like orbitals for tatpp (or tetraazapentacene-like and quinone-like orbitals for tatpq) and the bipyridine-like orbitals. As the bpy-like orbitals are the ones initially populated upon excitation, subsequent 'intramolecular electron-transfer' reduces the central portion of the ligand leaving the bpy-like portion open for another reductive cycle (after reductive quenching of the Ru(III) center with a sacrificial reductant).
As shown previously in Fig. 9, compound 16 is a promising addition to this family of complexes capable of photodriven multi-electron processes. Flash photolysis reveals a stepwise three electron oxidation at the MnnMnn center to yield the MnIIIMnIV complex 17.196,197 Unlike the preceding examples, multiple oxidizing equivalents or 'holes' are stored during the photochemical reaction making this system complimentary to those that collect multiple electrons.
Bocarsly, Pfennig and co-workers reported interesting multi-electron photoreac-tions for the trimeric Mn-PtIV-Mn complexes 25a-c.212-215 In this system, a single photon excitation into the intervalence charge transfer band results dissociation of the trimer into [Pt(NH3)4]2+ and two equivalents of a Mm. The initial photoexcited complex is though to dissociate first to a MIII complex and PtIII-MII intermediate. The latter dimer subsequently undergoes a thermal electron transfer reaction to yield the final products.
In a related fashion, Haga and coworkers showed that the tetranuclear [Ru4]8+ complex 26 is doubly-reduced by a combination of photo- and thermal-induced re-
duction.216 Initial photoexcitation of the Ru-diimine chromophores leads to reductive quenching of the photoexcited complex by N-benzyldihydronicotinomide, (BNA)2. The oxidized (BNA)2+ dimer then breaks into BNA+ and BNA; the latter radical species subsequently reduces the ruthenium tetramer to form a [Rm]6+ complex in which the electrons are stored in the n* orbitals of the bridging ligands. In this case, the sacrificial reductant (BNA)2 is 'non-innocent' in that the formation of a stable neutral radical leads to the second thermal reduction.
It is worth noting that similar processes could be in occurring with compounds 11, 19, 20 and 23 in which the oxidized sacrificial donor (e.g., TEA, DMA) deproto-nate to form a neutral radical species with good reducing power. While it is difficult to rule such a possibility out, MacDonnell and co-workers have shown that the singly-reduced version of compound 11, [(phen)2RuII(tatpp-)RuII(phen)2]3+ can be isolated and, when subject to photochemical reduction, cleanly undergoes the second reduction. Thus, while the overall reduction of 11 to 22 could include a thermal redox reaction, it does not require one.
Nocera and coworkers have shown that a two-electron catalytic manifold is accessible in the Rh°Rh0 dimer 27 which is formed in solution by photolabilization of the CO adduct.31,217,218 Earlier Gray and coworkers showed that certain RhIRhI dimers stoichiometrically form X-RhII-RhII-X and H2 when irradiated in the presence HX.219,220 Under photolytic conditions, the dimer 27 is thought to oxidatively add hydrohalides (HX) to form the mixed-valent dimer Rh0RhIIHX which then aggregates to form higher nuclearity species and H2. These aggregates absorb at visible wavelengths (~580 nm) and undergo a photochemical reaction to form Rh0RhIIX2 with the evolution of additional H2. In the presence of halogen atom traps, UV photolysis regenerates 27 and thus it is possibly to photocatalytically evolve H2.
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