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