Synthesis and Isolation of <i>cis</i>-2 Regiospecific Ethylene-Tethered Indene Dimer–[70]Fullerene Adduct for Polymer Solar Cell Applications

Although the utilization of [70]­fullerene bis-adducts can enhance the power conversion efficiencies of polymer solar cells (PSCs) owing to their strong absorption intensities and high-lying lowest unoccupied molecular orbital energy levels, this synthetic strategy typically yields a mixture of regioisomers that would mask the intrinsic device performances depending on the substituent pattern on the [70]­fullerene derivatives. In this study, a single <i>cis</i>-2 regioisomer of C₇₀ bis-adduct (<i>cis</i>-2-[70]­BIEC) has been prepared for the first time by the same strategy that had been applied to [60]­fullerene to obtain a regioisomerically pure C₆₀ bis-adduct (<i>cis</i>-2-[60]­BIEC). Diels–Alder reaction was conducted between a rationally designed ethylene-tethered indene dimer and [70]­fullerene, followed by isolation using high-performance liquid chromatography suitable for the separation of fullerene derivatives. A series of structural analysis techniques including NMR spectroscopies and X-ray crystallography were used to identify the absolute configuration of the bis-adduct. A systematic study on the optical, electrochemical, and photovoltaic properties of <i>cis</i>-2-[70]­BIEC as well as the corresponding regioisomer mixture (bis-[70]­BIEC) and the monoadduct (α-mono-[70]­BIEC) has been performed to examine the effect of the pure <i>cis</i>-2 regioisomer. More importantly, their properties are compared with those of <i>cis</i>-2-[60]­BIEC to address the effect of fullerene cage structures, that is, C₆₀ versus C₇₀. The PSC based on <i>cis</i>-2-[70]­BIEC and poly­(3-hexylthiophene) showed a remarkable power conversion efficiency of 4.2%, which is higher than those with bis-[70]­BIEC (2.2%), α-mono-[70]­BIEC (2.2%), <i>cis</i>-2-[60]­BIEC (2.8%), and even a prevalent high-performance C₇₀ monoadduct ([70]­PCBM, 3.8%). Our synthetic strategy will pave the way for further development on the rational design and isolation of single fullerene bis-adduct regioisomers exhibiting high device performances