Natural photosynthesis is the process where plants convert CO2 into carbohydrate and water is oxidized to molecular oxygen using solar energy.
Chlorophyll inside the plant cells absorbs energy from blue- and red-light waves, and reflects green-light waves, making the plant appear green.
In artificial photosynthesis photocatalyst or photocatalytic system is used for conversion of solar energy into hydrogen via water-splitting process without the requirement of any external bias.
Photocatalytic system can be divided into three main classes:
(i) suspended nanopowder photocatalysts,
(ii) photoelectrochemical cells (PECs), and
(iii) photovoltaic cell-driven electrolysers.
In natural photosynthesis, plants use light-harvesting complexes
to collect incident photons, move them over large distances and direct them to
a site where the process takes place. The light-harvesting complex is a highly
organized array of chlorophyll molecules. Many such arrays act cooperatively
both to shuttle photons to the right place and to avoid the problems of
overload at high light intensity. Nature has opted to transfer excitation
energy between weakly coupled chromophores via Förster mechanism, which involves
through-space interactions. However, in artificial systems, this route is
supplemented with electron exchange interactions occurring within more strongly
coupled chromophoric arrays. The first artificial photon collectors comprised
covalently linked porphyrin dimers that displayed highly efficient excitation
energy transfer over short distances. These were followed by detailed studies
of clusters, arrays, dendrimers, and polymers. More recently, extremely long,
porphyrin-based ribbons have been reported and attempts have been made to
develop porphyrin-based wheels and rings. In all cases, electronic energy
transfer is fast and highly efficient. There is, however, an additional
requirement in that energy produced following excitation must percolate through
the system to terminate preferably on a single chromophore. By ensuring that an
energy gradient is set up within a molecular system, this cascade effect is
readily achievable but does require careful design and choice of chromophores.
Light harvesting in natural
photosynthesis
Photosynthetic organisms universally
exploit antenna systems to absorb light and funnel the excitation energy to the
reaction center proteins, where the charge separation occurs. This process
converts light energy to chemical energy. The use of antenna allows a
multitude of pigment molecules to direct light excitation energy to each
reaction center proteins. This architecture increases the number of photons and
the range of photon energies that can be directed to a reaction center to
perform charge separation. By increasing the probability that a given reaction
center will produce a charge separation per unit time, antenna enable the rate
of reaction center proteins turnover under ambient sunlight to be matched to
the rate of downstream biochemical processes for more efficient biosynthetic
function. Several major types of photosynthetic antenna exist, including the
green sulfur bacterial chlorosome, cyanobacterial phycobilisomes,
dinoflagellate peridinin−chlorophyll−protein, the Pcb (Prochlorococcus chlorophyll a2/b binding)
protein, higher plant light-harvesting complex II (LHCII), and the purple
bacterial light-harvesting complexes 1, 2, and 3 (LH1−LH3). In addition to the wide array of
light-harvesting protein complexes found in nature, a wide array of pigments is
also used by different organisms.
Exciton
energy present in the antenna system is directed to an reaction center to
convert light energy into chemical energy. The light energy is used to drive
the primary charge-separation reaction. Reaction center proteins are
universally transmembrane proteins, vectorially oriented in a lipid bilayer
membrane that is exploited as a diffusion barrier to store chemical potential
as a proton gradient across that membrane. The goal of the reaction center is
to quickly produce a stable charge separation with minimal wasteful back
reactions. A high quantum efficiency can be obtained when all absorbed photons
result in a long-lived charge separation (P+QA-).
This is achieved through the coordination of energetics, electronic couplings,
and reorganization energies.
All
bRCs couple multi-electron catalysis to single electron photochemistry through
double reduction of a quinone to quinol. However, PSII is unique in that it is
the only RC able to harness four single electron charge separation events to
power the four-electron oxidation of two water molecules to O2 in
the OEC. PSII is the only enzyme known to perform this function and it is able
to do so using light energy from the visible spectrum. This requires very
precise redox positioning of multiple cofactors. The OEC consists of an
inorganic Mn4Ca cluster and the associated ligating protein. Each
charge separation event removes one electron from the OEC via a redox active
tyrosine YZ in a proton-coupled reaction.
Light
harvesting in artificial photosynthesis
In an artificial system, light
harvesting can be achieved by using a single “reaction center” chromophore or
by the excitation of a light−absorbing antenna array followed by the
energy−transfer sensitization of a reaction center. Among the light−harvesting
chromophores, porphyrins and phthalocyanines are closely related to chlorophyll
derivatives. Metal coordination compounds, which exhibit metal−to−ligand charge
transfer at relatively low energy, have been widely employed as
photosensitizers. Artificial systems can be designed with enhanced
light−harvesting capacity and efficiency. As model compounds
for Bchl c, zinc chlorins are very efficient in harvesting blue
and red light, but not the significant green region. The artificial
bichromophoric assembly absorbs green light and demonstrates efficient energy
transfer from the NBI dyes to the zinc chlorines. In an artificial
photosynthetic assembly, the light−harvesting unit and the RC must be coupled
to water−splitting catalysts or CO2−reduction catalysts to produce
chemical fuels. Transition−metal complexes are known for their capacity to
store multiple redox equivalents.
For an efficient artificial
architecture, a membrane is usually needed to spatially separate oxidative and
reductive species, as occurs in natural photosynthesis. The transfer of
electrons across the photosynthetic membranes drives chemical reactions and can
also produce electricity. Similarly, electricity can be generated from light by
transporting photoexcited electrons between the front and the rear contacts of
a solar cell. Photoelectrochemical cells utilize this principle to convert solar
energy into electricity.
References
Andrew
C. Benniston, Anthony Harriman, Artificial photosynthesis, Materials Today.
11(12), 2008: 26-34; https://doi.org/10.1016/S1369-7021(08)70250-5.
Hideki
Hashimoto, Yuko Sugai, Chiasa Uragami, Alastair T. Gardiner, Richard J.
Cogdell, Natural and artificial light-harvesting systems utilizing the
functions of carotenoids, Journal of Photochemistry and Photobiology C:
Photochemistry Reviews. 25, 2015: 46-70; https://doi.org/10.1016/j.jphotochemrev.2015.07.004.
Creatore
Celestino, Chin Alex W., Parker Michael A., Emmott Stephen. Emergent Models for
Artificial Light-Harvesting , Frontiers in Materials. 2, 2015 URL=https://www.frontiersin.org/article/10.3389/fmats.2015.00006