An artificial photosynthesis system, which mimics the natural photosynthesis system, comprises of three units. The first is a light-harvesting unit. The second is a charge separation unit (reaction center). The third is a catalysis unit where fuel-forming multi-electron reactions take place. This article discusses the light-harvesting system.

The chromophore (photosynthesizer)

Light harvesting is the initial step in artificial photosynthesis. Light harvesting can be done by a chromophore, which is a light-absorbing component analogous to the pigments in natural photosynthesis. Chromophores act as reaction centers.

The chromophore absorbs and converts the incoming solar energy into an excited state so that an electron can be transferred to an acceptor. This creates a charge-separated state, therefore producing the needed thermodynamic driving force for the chemical reactions needed.

Electron transfer can be induced by both semiconductor and photoactive molecular dye materials. These materials can be used as light-harvesting chromophores for artificial photosynthetic devices for water splitting.

Phthalocyanines and porphyrins are among the light-harvesting chromophores and are closely related to chlorophyll derivatives. Also, metal coordination compounds that show metal-to-ligand charge transfer (MLCT) at relatively low energy have been utilized as chromophores.

Early research on the design of molecular model systems concentrated on the absorption of light and excited-state electron transfer. These systems involved the MLCT of [Ru(bpy)3]2+ (bpy = 2,20-bipyridine) and porphyrins p/p* transitions.

Light-harvesting systems in natural photosynthesis are well-developed and highly complex that use the full solar spectrum. It is vital to design artificial systems with improved capacity and efficiency for light harvesting.

An early study prepared a bichromophoric assembly comprising blue naphthalene bisimide (NBI) dyes at the periphery of aggregated zinc chlorines. Zinc chlorins, which are model compounds for Bchl c, are highly efficient in harvesting red and blue light. Zinc chlorins are not efficient in the significant green regions.

The absorption of green light is achieved by the artificial bichromophoric assembly. This assembly displays the efficient transfer of energy to the zinc chlorides from the NBI chlorines. An early study developed a perylene-based green chromophore. The redox and photophysical properties of the chromophore are analogous to the natural chlorophyll a.

However, the chromophore can easily be functionalized and incorporated into a broad variety of biomimetic electron donor-acceptor systems unlike chlorophyll a. Supramolecular light-harvesting arrays can be built via self-assembling chromophore building blocks on surfaces and in solutions.

In Germany, Evonik and Siemens jointly established the Rheticus II project to commercialize artificial photosynthesis. The project combines artificial photosynthesis with fermentation to create high-value fuels. The system uses electricity from sunlight to break down water and carbon dioxide to produce hydrogen and carbon.

In the U.K., researchers at the Imperial College London developed a catalyst from an organic material called hyper-crosslinked polymers. This catalyst was used to convert carbon dioxide into carbon monoxide. The polymers used to make the catalyst deliver more sustainability and are inexpensive.

Light-harvesting antenna systems

Light-harvesting can also be achieved by an exciting light-absorbing antenna array accompanied by an energy-transfer sensitization of a reaction center. Antenna systems can collect light and transfer energy efficiently and directly.

Coupling a photosynthetic reaction center to an antenna array allows the antenna chromophores encircling the reaction center to absorb incident photons. The electronic energy is then transferred from the excited states to the reaction center before undergoing nonradiative or radiative deactivation.

Studies have broadly employed highly-branched tree-like dendrimers as antenna systems. In dendrimers, large numbers of chromophores can be closely assembled in close proximity due to the divergent and/or convergent synthesis of dendrimers. This allows different functional groups to easily interact with each other.

Many known artificial antenna systems are built using porphyrin, multi-porphyrin arrays, or organic molecules and metal complexes-based dendrimers. Importantly, integrating antenna chromophores in an artificial system may be unbeneficial if the reaction center and the antenna array are improperly coupled in space, energy, and time dimensions. In reality, studies have shown that reducing the size of the antenna can enhance the conversion efficiency of photosynthetic solar energy.

A few studies have been conducted to boost the efficiency of light-harvesting antennas. Researchers at the Hubert Girault lab at ‎the Swiss Federal Institute of Technology in Lausanne‎ (EPFL) developed a new approach to artificial photosynthesis. They used a simple organic molecule, tetrathiafulvalene (TTF), assembled into microrods, to act as light-capturing antennas. The antennas capture the four electrons required to oxidize a molecule of water into oxygen.

Researchers at the Lawrence Berkeley National Laboratory at the U.S. Department of Energy (DoE) and the University of California (UC) Berkeley developed a computational model to simulate the light-harvesting activities of antenna proteins. The model simulates light-harvesting across several hundred nanometers of a membrane within a chloroplast which holds the photosystem II (an antennae complex). The results point toward the development of artificial photosynthesis technologies.

Dye-sensitized solar cells (DSSC)

Light-harvesting in a DSSC involves the transition of the MLCT of the dye molecules that are attached to a mesoscopic titanium dioxide (TiO2) thin film. The dye molecules are usually polypyridyl Ru complexes. A polypyridyl Ru complexes-sensitized titanium dioxide thin film can absorb visible light with a maximum absorbance of about 550 nm.

Recent studies have demonstrated that the structure of dye can be tuned to enhance the light-harvesting capacity and molar extinction coefficient. The porous, nanostructured titanium dioxide film has small particle sizes (about 20 nm) and very high surface roughness. These properties allow for efficient light harvesting.

Dye molecules monolayer on a flat titanium dioxide surface can only absorb a very small percentage of incident light. This is due to the occupation of one dye molecule in an area that is larger than its optical cross-section for light capture. When an incident light penetrates the sensitized mesoporous titanium dioxide film in a DSSC, it crosses hundreds of adsorbed dye molecules.

This improved light absorption is akin to what happens in green leaves where light-harvesting by chlorophyll is improved in stacked thylakoid vesicles. Further, titanium dioxide particles of sizes 200-400 nm are usually integrated into the titanium dioxide film to improve light harvesting in the near-infrared region.

Future research directions

Solar energy can be used for fuel generation through water-splitting to produce chemical fuels like hydrogen. Although several studies have reported the use of heterogeneous photocatalysts to drive visible light water-splitting.

The advantages of photoelectrochemical synthesis cells include the efficient separation of redox equivalents for the production of solar fuels. The development of light-harvesting systems for artificial photosynthesis can be beneficial for the design of photoelectrochemical cells.

Further studies on light-harvesting systems including the development of molecular model systems other than rubidium-based systems would improve the design of artificial photosynthetic systems. Also, developing more efficient light-harvesting antennas should be the focus of future research.

The knowledge gained from light-harvesting systems research can greatly impact the development of highly efficient devices leading to producing energy-rich and affordable fuels from solar energy.