A biomimetic approach for synthesizing artificial light-harvesting systems using self-assembly

Abstract

Photosynthesis is an extremely important process on Earth as it is the only natural source of food and fossil fuel, which fulfil our daily needs. After a certain period, the natural source of food and energy will decrease due to rapid consumption. Therefore, future generations will require alternative food and fuel sources. This represents a strong driving force to do research in construction of artificial light-harvesting (or antenna) systems. Synthetic antennas can be achieved either by covalent or non-covalent approaches by employing different strategies. This work throws light on the non-covalent approach i. e. a supramolecular approach in quest of artificial antenna systems wherein self-assembly and self-aggregation are at the focus. Furthermore this approach is biomimetic in nature as it is inspired by the antenna system which operates in green photosynthetic bacteria. Bacteriochlorophyll-c, d and e we are selected as models for the syntheses of artificial mimics. The supramolecular interactions which are, the ligation of the central Mg atom by the 3 1 -hydroxy group of another molecule; cooperative hydrogen bonding of the same OH group to the 13 1 -carbonyl group of a third BChl-c molecule; and π-π interactions between the macrocycles are responsible for self-assembly of the building blocks or tectons. Well-defined architectures of self-assembling porphyrins find applications in mimicking the functions of light-harvesting. Porphyrins that are equipped with the same functional groups that are responsible for the self-assembly of bacteriochlorophylls-c, d and e within the chlorosomal antenna of some green photosynthetic bacteria, have been selectively synthesized from easily available and cheap starting materials, 10,20-Bis(3,5-di-t-butylphenyl)porphinato copper. All the target compounds were obtained after four to eight synthetic steps in good yields by employing different synthetic procedures involving also novel reactions. However, the four synthetic steps viz, the introduction of dicarbonyl functionality in the form of acyl or formyl groups into the periphery of starting porphyrin material, demetallation of the dicarbonyl compounds under highly acidic condition to obtain free base dicarbonyl compounds, monoreduction of only one of the carbonyl functionality to give chiral or achiral compound that posses carbonyl and hydroxyl group and zinc metallation of the monoreduced compounds were the general synthetic steps those were applied to synthesize the target compounds. These fully synthetic novel chromophores self-assemble in a similar way to the natural case and without the need of a protein-pigment complex acting as mechanical scaffold. The architectures of the monomeric building blocks determine both the chirality and the optical properties of the supramoleular assembly achieving broad and bathochromically shifted absorption maxima. More importantly, due to highly ordered arrangements of the dye molecules, neither the self-assembly nor the chemisorption onto nanocrystalline titania (TiO 2 ) with different grain sizes quenches the fluorescence of the aggregates. If light harvesting is to be harnessed in artificial devices, a broad absorption with high extinction coefficients over the entire spectral range is beneficial and some of the supramolecular assemblies reported herein are promising candidates for artificial antenna systems as they fulfil these requirements and thus lend hope for being useful in hybrid solar cells.