Anisotropic organization and microscopic manipulation of self-assembling synthetic porphyrin microrods that mimic chlorosomes: bacterial light-harvesting systems.

Being able to control in time and space the positioning, orientation, movement, and sense of rotation of nano- to microscale objects is currently an active research area in nanoscience, having diverse nanotechnological applications. In this paper, we demonstrate unprecedented control and maneuvering of rod-shaped or tubular nanostructures with high aspect ratios which are formed by self-assembling synthetic porphyrins. The self-assembly algorithm, encoded by appended chemical-recognition groups on the periphery of these porphyrins, is the same as the one operating for chlorosomal bacteriochlorophylls (BChl's). Chlorosomes, rod-shaped organelles with relatively long-range molecular order, are the most efficient naturally occurring light-harvesting systems. They are used by green photosynthetic bacteria to trap visible and infrared light of minute intensities even at great depths, e.g., 100 m below water surface or in volcanic vents in the absence of solar radiation. In contrast to most other natural light-harvesting systems, the chlorosomal antennae are devoid of a protein scaffold to orient the BChl's; thus, they are an attractive goal for mimicry by synthetic chemists, who are able to engineer more robust chromophores to self-assemble. Functional devices with environmentally friendly chromophores-which should be able to act as photosensitizers within hybrid solar cells, leading to high photon-to-current conversion efficiencies even under low illumination conditions-have yet to be fabricated. The orderly manner in which the BChl's and their synthetic counterparts self-assemble imparts strong diamagnetic and optical anisotropies and flow/shear characteristics to their nanostructured assemblies, allowing them to be manipulated by electrical, magnetic, or tribomechanical forces.

Self-assembled chromophores within mesoporous nanocrystalline TiO2: towards biomimetic solar cells.

Artificial light-harvesting antennas consisting of self-assembled chromophores that mimic the natural pigments of photosynthetic bacteria have been inserted into voids induced in porous titania (TiO2, anatase) in order to investigate their suitability for hybrid solar cells. Mesoporous nanocrystalline TiO2 with additional uniform macropores was treated with precursor solutions of the pigment which was then induced to self-assemble within the voids. The chromophores were tailored to combine the self-assembly characteristics of the natural bacteriochlorophylls with the robustness of artificial Zn-porphyrins being stable for prolonged periods even upon heating to over 200 degrees C. They assemble on the TiO2 surface to form nano- to micro-crystalline structures with lengths from tens of nm up to several microm and show a photosensitization effect which is supposed to be dependent on the assembly size. The natural examples of these antennas are found in green sulfur bacteria which are able to use photosynthesis in deep water regions with minute light intensities. The implementation of biomimetic antennas for light harvesting and a better photon management may lead to a rise in efficiency of dye-sensitized solar cells also under low light illumination conditions.