Stereochemistry of Spiro-Acetalized [60]Fullerenes: How the <i>Exo</i> and <i>Endo</i> Stereoisomers Influence Organic Solar Cell Performance

Abstract

Exploiting bis-addition products of fullerenes is a rational way to improve the efficiency of bulk heterojunction-type organic photovoltaic cells (OPV); however, this design inherently produces regio- and stereoisomers that may impair the ultimate performance and fabrication reproducibility. Here, we report unprecedented <i>exo</i> and <i>endo</i> stereoisomers of the spiro-acetalized [60]­fullerene monoadduct with methyl- or phenyl-substituted 1,3-dioxane (<b>SAF</b><sub><b>6</b></sub>). Although there is no chiral carbon in either the reagent or the fullerene, equatorial (<i>eq</i>) rather than axial (<i>ax</i>) isomers are selectively produced at an <i>exo-eq</i>:<i>endo</i>-<i>eq</i> ratio of approximately 1:1 and can be easily separated using silica gel column chromatography. Nuclear Overhauser effect measurements identified the conformations of the straight <i>exo</i> isomer and bent <i>endo</i> isomer. We discuss the origin of stereoselectivity, the anomeric effect, intermolecular ordering in the film state, and the performance of poly­(3-hexylthiophene):substituted <b>SAF</b><sub><b>6</b></sub> OPV devices. Despite their identical optical and electrochemical properties, their solubilities and space-charge limited current mobilities are largely influenced by the stereoisomers, which leads to variation in the OPV efficiency. This study emphasizes the importance of fullerene stereochemistry for understanding the relationship between stereochemical structures and device output

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