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Porphyrin are a group of heterocyclic organic compounds which are commonly found in the living tissues such as hemoglobin (Heme), chlorophyll-a etc. Chlorophyll-a assists in the conversion of sunlight into electronic energy and plays a major role in the reactions. However the energy conversion efficiency of chlorophyll through natural photosynthesis is very low which opens up the idea of replacing chlorophyll with porphyrin compounds which are structurally similar. Functionalization and modification with suitable functional moieties, a higher photoelectric conversion efficiency can be achieved. In 2011, our laboratory synthesized YD2-o-C8 with a conversion efficiency of 11.9% which was published in “Science”. Later in 2013, another compound GY50 with conversion efficiency of 12.75% was synthesized and published in Angew. Chem. Because of the intricacies in the synthesis, we attempted a different system, YD7 consisting of a pair of ethynyl group, with lesser synthesis steps and more red-shift in the absorption spectrum. Through simple SN2 reaction, electron donating groups could be easily replaced with long carbon chain aniline. Longer the carbon chain better was the solubility of porphyrin. Firstly, YD17 connected with different carbon chain lengths of aniline was considered as a prototype. The YD17 with 8 carbons chain length exhibited a better solubility and an efficiency of more than 9%. Further, retaining the same electronic (donating/withdrawing) properties, the prototype was replaced with simple structures (23) and (28). Later, the number of alkoxy groups in porphyrin (33) was increased to further enhance the solubility. In addition, the steric hindrance in porphyrin was augmented in order to reduce the stacking there by effectively blocking the electronic interference of TiO2 in liquid electrolytes. Eventually, a porphyrin dye was synthesized consisting of two pairs of alkynyl groups in the meso positions which in turn enhanced the resonance leading to the red shift. The maximum absorption spectrum of Q band was found to appear around 700 nm. With a high photoelectric conversion efficiency and considerably low cost of making, the dye-sensitized solar cells have been the focus of many research groups around the world. We firmly believe that the DSSC will soon be substituting the semiconductor based solar cells in the near future.
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