In photosynthesis and also as elaborated further in Chapter 8, nature has developed a molecular-based system able to capture light and transiently store it as a reducing potential (Eq. 2) and as high energy molecules (Eq. 3).26 This energy is either quickly used or stored in the form of reduced CO2
products (e.g., glucose, see Eq. 4) which are more stable forms of stored energy. Water is the ultimate source of electrons for reactions 2 and 4 and dioxygen is the byproduct that is lost to the atmosphere. The development of artificial photosynthetic systems that would mimic the natural process, at least in basic function, is a challenging yet realistic chemical problem with obvious long-term benefits for mankind.
As seen in Eq. 1, the water-splitting reaction has an overall energy requirement of 4.92 eV per O2 molecule formed (or +474.7 kJ/mol O2 formed). The most abundant solar radiation to strike the earth's surface falls in the visible range (750-400 nm) and fortunately, these photons are energetic enough (1.65-3.1 eV)27 so that as little as two photons are required to drive this process thermodynamically. When broken down into redox half-reactions (5 and 6), the multi-electron nature of reaction 1 is readily apparent.
2 H2O ^ O2 + 4 H+ + 4 e- + 314.9 kJ/ mol O2, pH 7 (6)
These two reactions are often referred to as the hydrogen evolving reaction (HER) and oxygen evolving reaction (OER), respectively. The problem of driving these two half-reactions with light is two-fold. First, water does not absorb light in the visible and a chromophore is needed to capture and concentrate the solar energy. Second, the absorption of a photon by a chromophore is typically associated with the excitation of a single electron in the molecule. Even if this electron has sufficient reducing potential for reaction 5, the electron stoichiometry is not met. Similar arguments apply to the holes generated and their ability to drive reaction 6. Nature has addressed these problems by developing enzymes proficient at stepwise storage of multiple redox equivalents (electrons or holes) until the appropriate redox stoichi-ometry is met. For example, the oxygen-evolving center (OEC) in photosystem II is composed of a tetramanganese cofactor that does not evolve O2 until 4 electrons have been removed.28 In general, we observe that the transformation of many small molecule substrates into desirable products, such as H2O to O2 and H2, CO2 to C6H12O6, N2 to NH3, etc. are multi-electron processes and require not only the appropriate driving force but cofactors that enable the appropriate redox stoichiometry to be met. Artificial photosynthetic systems will similarly require entities/catalysts capable of driving multi-electron transfer (MET) reactions and proton-coupled electron transfer (PCET) reactions.29-31
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