Fullerene isomers for Organic Photovoltaics
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The as-produced isomer mixture of the organic photovoltaic device acceptor material bisPC62BM has been purified into its constituents by peak-recycling HPLC, and those individual isomers were characterised by UV-Vis absorption spectroscopy and cyclic voltammetry. A total of 18 isomers were purified from the mixture to a standard exceeding 99.5% with respect to other isomers. The HOMOs, LUMOs, and HOMO-LUMO gaps of the purified isomers vary from (-5.673 to -5.402 eV), (-3.901 to -3.729 eV), and (1.664 to 1.883 eV), respectively. We also find a correlation between HPLC retention time and the relative positions of the addends; in that generally the closer the addends are to each other the longer the retention time of the isomer, and vice versa. The OPV acceptor molecule PC71BM was also purified into its constituent isomers to a standard of at least 99%. The total three purified isomers were each characterized by 13C NMR and UV-vis spectroscopy, and cyclic voltammetry. These characterizations were supported by HF/DFT ab-initio calculations. All three isomers are methano-fullerenes. The most abundant isomer (85% of the mixture) exists as a racemate involving the 8-25 bond of C70. The other two isomers both involved the 9-10 bond of C70, but are distinguished by opposing orientations of the addend with r and s pseudo-asymmetry about carbon atom 71. The r and s isomers comprised 9% and 6% of the as-produced of the mixture, respectively. In order of decreasing abundance, the LUMO levels of the isomers were –3.9316, –3.9194 and –3.9197 eV and the HOMO-LUMO gaps were 1,772, 1.754 and 1.748 eV.
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