Light harvesting materials

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.

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