Light harvesting materials harvest solar energy that can then be converted into chemical energy through photochemical processes. Synthetic light harvesting materials are inspired by photosynthetic biological systems such as light harvesting complexes and pigments that are present in plants and some photosynthetic bacteria. The dynamic and efficient antenna complexes that are present in photosynthetic organisms has inspired the design of synthetic light harvesting materials that mimic light harvesting machinery in biological systems. Examples of synthetic light harvesting materials are dendrimers, porphyrin arrays and assemblies, organic gels, biosynthetic and synthetic peptides, organic-inorganic hybrid materials, and semiconductor materials (non-oxides, oxynitrides and oxysulfides). Synthetic and biosynthetic light harvesting materials have applications in photovoltaics, photocatalysis, and photopolymerization.
Photochemical Processes
Organic Photovoltaic Cells During photochemical processes employing donor and acceptor chromophores in organic solar cells, a photon is absorbed by the donor and an exciton is generated. The exciton diffuses to a donor/acceptor interface, or heterojunction, where an electron from the lowest unoccupied molecular orbital (LUMO) of the donor is transferred to the LUMO of the acceptor. This results in the formation of electron-hole pairs. When the photon is absorbed by the acceptor and the exciton reaches a heterojunction, an electron will then transfer from the HOMO of the donor to the HOMO of the acceptor. In order to make certain there is effective charge transfer, the continuous donor or acceptor domains must be smaller than the exciton diffusion length (< ~0.4 nm).
Light Harvesting Efficiency The light harvesting efficiency of energy transfer in light harvesting materials can be enhanced by either decreasing the distance between the donor and acceptor or designing a material that contains multiple antenna chromophores per acceptor (antenna effect). Förster Resonance Energy Transfer (FRET) Efficiency corresponds to the light harvesting efficiency and is determined by the spectroscopic properties of dyes/pigments or chromophores and the distances between the donor and acceptor; the limitations of FRET can be overcome by enhancing the antenna effect through modifying the stoichiometry of the electron donor, transmitter, and acceptor.
Photosynthetic biological systems
Photosynthetic biological systems utilize sunlight, an abundant and ubiquitous energy source, as metabolic fuel. The highest efficiency for the conversion of energy from the sun into biomass by plants is around 4.6% at 30 °C and 380 ppm of atmospheric CO2 for carbon fixation during photosynthesis. Natural light harvesting complexes have molecular machinery that make possible the conversion of sunlight into chemical energy with almost 100% quantum efficiency. The ability of living organisms to harvest solar energy and achieve quantum efficiency near unity is due to the culmination of ~3.5 billion years of evolution. This efficiency is achieved in plants with a series of energy transfer steps, that are carried out through pigment-protein complexes (e.g. Photosystem II). Pigment-protein complexes (PPC) contain chromophore molecules, specifically chlorophylls and carotenoids that are embedded in a protein matrix. PPC serve as antenna complexes that absorb sunlight and the harvested energy from the sunlight then travels hundreds of nanometers to the reaction center; this energy essentially powers the electron transfer chain essential to photosynthesis and the downstream photosynthesis of plants. In order for charge or energy transfer to occur in the multielectron redox processes of the electron transfer chain, charge separation must occur first, which is induced by light harvesting.
Purple bacteria complexes
Purple bacteria, a photosynthetic organism also contains a PPC that is structurally different to the photosystems in plants but similar in terms of function. Exciton-transporting proteins found in purple bacteria such as Rhodospirillum photometricum or Rhodoblastus acidophilus, are light harvesting complex 1 and light harvesting complex 2. Light harvesting complex 2 in the purple bacteria Rhodoblastus acidophilus is shown in Figure 2. The light harvesting complex in purple bacteria is multifunctional; at high light intensities, the light harvesting complex typically switches into a quenched state through a conformational change of the PPC, and at low light intensities, the light harvesting complex typically reverts to an unquenched state. These conformational changes occur in light harvesting complex 2 in order to manage the metabolic cost corresponding to protein synthesis in purple bacteria.
Complexes in green plants
Conformational changes of proteins in PPC of vascular plants or higher plants also occur on the basis of light intensity. When there are lower light intensities for example on an overcast day, any absorbed sunlight by higher plants is converted to electricity for photosynthesis. When conditions allow for direct sunlight the capacity of PPC in higher plants to absorb and transfer energy, exceeds the capacity of downstream metabolic or biochemical processes. During periods of high light intensity plants and algae will enter a stage of non-photochemical quenching.
Design and characterization of synthetic materials
Materials based on Porphyrins, Chlorophyll, and Carotenoids Artificial light harvesting materials that serve as antenna are based on non-covalent supramolecular assemblies that contain motifs that are inspired by the pigment molecules chlorophyll and carotenoids that are embedded in protein-pigment complexes in nature. The class of pigments that are most commonly found in nature are chlorophylls and bacteriochlorophylls, the synthetic analogs of these biological chromophore molecules are porphyrins which are the most extensively used compounds in artificial light harvesting applications. The porphyrin moieties present in biological light harvesting complexes play a critical role in the efficient absorption of visible light, the harvested energy from the porphyrin-based molecules is then collected in the reaction center through the excitation energy transfer relay. The light-driven charge separation process occurs at the reaction center due to the cooperation of two porphyrin derivatives.
Porphyrin and chlorophyll bioinspired materials Supramolecular assemblies of synthetic porphyrin-based materials for light harvesting are commonly studied and utilized for electronic energy transfer. The supramolecular assemblies typically employ coordination and hydrogen bonding as an efficient means of tuning interactions and directionality between donor chromophores and acceptor fluorophores. Zinc porphyrin is frequently coupled to free-base porphyrin in synthetic electronic energy transfer systems due to the separated absorption features of both of these molecules. The zinc porphyrin serves as the donor and the free-base porphyrin serves as the acceptor, since the fluorescence of the zinc porphyrin overlaps with the absorption of the free-base porphyrin. Porphyrin arrays and oligomers have been combined with charge-separation molecules in order to emulate charge-separation functions that are present in photosynthetic proteins, in addition to the light harvesting properties of biological light harvesting complexes. The charge-separation molecules that are usually combined with donor chromophore zinc metallated porphyrins are ferrocene which serves as an electron donor and fullerene which serves as an electron acceptor.
Carotenoid bioinspired materials Carotenoids are another class of pigment/dye molecules found in retinal photoreceptors and biological light harvesting systems (e.g. Photosystem I, Photosystem II, and Light Harvesting Complex II). When finely arranged with chlorophylls in biological photosynthetic systems, carotenoids effectively promote photoinduced charge separation and electron transfer. Carotenoid is highly conjugated and is structurally very similar to polyacetylene oligomers. Naturally derived carotenoids have been combined with fullerene derivatives for photovoltaic applications. In photovoltaic devices, carotenoid molecules exhibited p-type semiconductor behavior since the molecular structure is very similar to polyacetylene. Artificial dyad and triad systems in which carotenoids are covalently bound have been able to mimic the charge separation and light harvesting mechanisms present in phototrophic organisms. Carotenoid that is covalently bound to porphyrin is a typical example of a dyad containing carotenoid, the dyad can then be covalently bound to a fullerene to form a triad (Figure 3). The triad systems display electron transport that results in long lasting charge separated states.
Biomaterials Natural light harvesting complexes contain proteins that combine through self-assembly with effective donor chromophores in order to promote light harvesting and energy transfer during photosynthesis; synthetic peptides can be designed to have optoelectronic properties that mimic this phenomenon in natural light harvesting complexes. Proteins in PPCs not only serve as a support for the arrangement of chromophores during light harvesting but also actively play a role in the photophysical dynamics of photosynthesis. Some biomimetic artificial light harvesting complexes have been designed to have proteins and peptides that self-assemble in such a way that chromophores in the complex are arranged for optimized light harvesting efficiency. Peptide self-assemblies and polypeptides modified with porphyrins have also been designed to have the dual function of charge separation and light harvesting. Other examples of peptide donor and acceptor chromophore conjugates utilize the self-assembly of amyloid fibrils into a beta sheet that allows the chromophores to become arranged in such a way that is fine tuned for efficient light harvesting. Synthetic peptides and proteins are one example of the biological materials that are utilized in artificial light harvesting systems, virus templated assemblies and DNA origami have also been employed for light harvesting applications.
Organic gels and nanocrystals Reversible molecular organic gel networks are held together by noncovalent interactions (e.g. hydrogen bonding, π-stacking, van der Waals interactions and donor–acceptor interactions). The gelator molecules can self-organize in one-dimensional arrays due to the directional nature of intermolecular interactions, producing elongated fibrous structures that can serve as antenna molecules. The organic gels assemble in such a way that there is proper arrangement of donor and acceptor chromophores which is the principle requirement for efficient energy transfer. π-conjugated molecules are commonly used in organic gels since these molecules are impacted by the orientation of chromophores in self assemblies. Some examples of π-conjugated molecules that are employed in organic gels are oligo-p-phenylenevinylene, anthracene, pyrene and porphyrin derivatives.