Supramolecular Self-assembly of Chlorins in an Aerosolized Droplet to Synthesize Biomimetic Antennas
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Supramolecular Self-assembly of Chlorins in an Aerosolized Droplet to Synthesize Biomimetic Antennas Vivek B. Shah and Pratim Biswas Aerosol and Air Quality Research Laboratory, Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, MO 63130, USA ABSTRACT Natural light harvesting organisms have evolved to harvest sunlight efficiently. Green sulfur bacteria, which contain chlorosomes, can survive in extremely low light conditions mainly because of efficient light absorption and transfer of energy, facilitated by the assembled dye molecules. Due to these reasons, chlorosomes have been used in dye sensitized solar cells to improve the light absorption and performance. The chlorosome absorption spectrum is fixed and their size is dependent on the organism. Various solution-based techniques have been developed for synthesizing mimics by supramolecular self-assembly. However, controlling the size of agglomerates and their subsequent deposition on surfaces to fabricate a device is difficult. In this work, a one-step aerosol-based self-assembly technique has been developed for the first time, to assemble and deposit chlorin (Bacteria chlorophyll mimics) agglomerates. A shift in absorption spectra of 79 nm which is comparable to the natural system was obtained. The analysis shows that kinetics of nucleation play an important role in assembly. INTRODUCTION Large scale utilization of solar energy to generate electricity is economically unattractive because of the high cost of silicon based solar cells. Dye-sensitized solar cells are deemed to be low cost solar cells, however their efficiency needs to be improved. Various sensitizers could be used in metal oxide based solar cells such as dyes and quantum dots. However due to low extinction coefficient, high surface area is required to absorb all the sunlight. In contrast, chlorosomes harvest solar energy by using a stack of self-assembled Bacteriochlorophyll c (BChl c) dyes. The assembled BChl c dyes result in high extinction coefficient and absorption of all the incident photons.1 Due to these reasons, chlorosomes sensitized solar cells result in improved light absorption and performance.2 The energy absorbed by the dyes in chlorosomes hops between multiple layers and is finally transferred to the baseplate in the form of excitons. Thus energy transfer between multiple layers is critical to the functioning of chlorosomes. Efficient energy transfer takes place in selfassembled molecules because they are neatly stacked which allows exciton transfer. However for application in artificial devices the properties such as size and absorption need to be tailored. Thus biomimetic antennas with artificial dyes are created. Various chlorins3-5 and porphyrins6 have been assembled in solution and the effects of molecular structure and substituents have been studied extensively. Self-assembled agglomerates of these chromophores are formed by hydrophobic interactions,7 interaction with nonpolar solvents8 or an evaporating solvent.9, 10 Th
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