Light harvesting materials

[7] 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).

[12] Natural light harvesting complexes have molecular machinery that make possible the conversion of sunlight into chemical energy with almost 100% quantum efficiency.

[11][12] The ability of living organisms to harvest solar energy and achieve quantum efficiency near unity[12] is due to the culmination of ~3.5 billion years of evolution.

[11] These conformational changes occur in light harvesting complex 2 in order to manage the metabolic cost corresponding to protein synthesis in purple bacteria.

When there are lower light intensities for example on an overcast day, any absorbed sunlight by higher plants is converted to electricity for photosynthesis.

[11] 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[7][13][14] and carotenoids[14][15][16] that are embedded in protein-pigment complexes in nature.

[15] 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[13][17] which are the most extensively used compounds in artificial light harvesting applications.

[17] Supramolecular assemblies of synthetic porphyrin-based materials for light harvesting are commonly studied and utilized for electronic energy transfer.

[13][17] The supramolecular assemblies typically employ coordination and hydrogen bonding as an efficient means of tuning interactions and directionality between donor chromophores and acceptor fluorophores.

When finely arranged with chlorophylls in biological photosynthetic systems, carotenoids effectively promote photoinduced charge separation and electron transfer.

[7] 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.

[19] 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.

[23][24][25] 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.

[28] Sun et al. developed two polymorphic organometallic nanocrystals formed from platinum (II)-β-diketonate complexes demonstrated light harvesting and photoluminescent properties.

[36] MOFs can be designed to have solar light harvesting properties through different synthetic strategies such as using porphyrin containing struts or metalloporphyrins as the primary organic building blocks.

[39] Dye-sensitized solar cells frequently incorporate titanium dioxide as a principal component because it imparts sensitizer adsorption as well as charge separation and electron transport characteristics.

[40] The dye molecules present in dye-sensitized solar cells, upon light harvesting, transfer excited electrons to titanium dioxide which then separates the charge.

[41] The field of organic photovoltaics in particular, has developed rapidly since the late 1990s and small solar cells have demonstrated power conversion efficiencies up to 13%.

The dynamic and responsive molecular machinery present in photosynthetic organisms as well as the principles of self-assembly has influenced the design of “smart” photovoltaic devices.

[3] Photodynamic therapy is a medical treatment that employs photochemical processes, through the combination of light and a photosensitizer to generate a cytotoxic effect to cancerous or diseased tissue.

[47] Photosensitizers can be used for the formation of singlet oxygen upon photoinduction and this plays an important role in photodynamic therapy and this capability has been displayed by titanium dioxide nanoparticles.

Figure 1: (A) Light absorption and the formation of a singlet exciton with the opposite “up” and “down” spins of electrons, followed by energy transfer to an electron donor and acceptor of a type II heterojunction and then (B) electron or (C) hole transfer can occur at the heterojunction. [ 8 ]
Figure 2: Light Harvesting Complex 2 in Rhodoblastus acidophilus a type of purple bacteria. PDB 2fkw EBI
Molecular structures of Chlorophyll a, a light-harvesting pigment in green plants (left) and an artificial porphyrin photosensitizer system (right). [ 18 ]
Figure 3: Triad Molecules containing Porphyrin Capable of Light Harvesting and Charge Separation A) Carotenoid, Porphyrin, and Fullerene Triad B) Ferrocene, Porphyrin, and Fullerene [ 13 ] [ 15 ]