Structurally Defined High-LUMO-Level 66π-[70]Fullerene Derivatives: Synthesis and Application in Organic Photovoltaic Cells

Two new reactions for the synthesis of structurally defined 66 pi-electron [70]fullerene derivatives are reported. The first provides synthetic access to tetra-phenyl or [3 + 1] hybrid tetra-aryl C-70 adducts via oxidation of a fullerene copper complex [Ar3C70-Cu-Ar'](-) (Ar = Ph, 4-(BuC6H4)-Bu-n; Ar' = Ph, 4-MeOC6H4). The second provides access to alkyl fullerene ethers, C70Ar3(2-EH) via AgClO4-mediated coupling of a [70]fullerene bromide C70Ar3Br with 2-ethylhexanol (2-EH). The first reaction afforded two types of regioisomers including a 3,10,22,25-adduct (denoted type I) and a 7,10,22,25-adduct (type II). The haptotropic migration of the copper on a cuprio fullerene intermediate was suggested to be responsible for the generation of the two regioisomers. The second reaction gave only one regioisomer (type II). The eight new 66 pi-electron [70]fullerene derivatives synthesized are electrochemically and thermally stable, and their photoabsorption and electrochemical properties are closely related to the addition pattern. For example, the type II regioisomers have higher LUMO levels than the type I isomers. Through modification of the addends, the LUMO levels of these [70]fullerene derivatives can be raised by as much as 220 meV, that is, from -3.80 to -3.58 eV. Solution-processed p-n junction organic photovoltaic devices using five soluble compounds 5, 9, 10, 13, and 15 as the n-type semiconducting materials were fabricated. The device using compound 15 (LUMO = -3.58 eV) showed the highest open circuit voltage (V-oc = 0.90 V) and a respectable PCE value of 3.33%. For J(sc) and FF, type II compounds 10, 13, and 15 showed much higher values than did type I compounds 5 and 9, suggesting that the type II addition pattern on C-70 may be superior to the type I pattern for efficient electron transport, likely because of better molecular packing in crystals as suggested by crystallographic data.