Structurally Defined High-LUMO-Level
66π-[70]Fullerene
Derivatives: Synthesis and Application in Organic Photovoltaic Cells
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Abstract
Two new reactions for the synthesis of structurally defined
66π-electron
[70]fullerene derivatives are reported. The first provides synthetic
access to tetra-phenyl or [3 + 1] hybrid tetra-aryl C<sub>70</sub> adducts via oxidation of a fullerene copper complex [Ar<sub>3</sub>C<sub>70</sub>–Cu–Ar′]<sup>−</sup> (Ar
= Ph, 4-<sup>n</sup>BuC<sub>6</sub>H<sub>4</sub>; Ar′ = Ph,
4-MeOC<sub>6</sub>H<sub>4</sub>). The second provides access to alkyl
fullerene ethers, C<sub>70</sub>Ar<sub>3</sub>(2-EH) via AgClO<sub>4</sub>-mediated coupling of a [70]fullerene bromide C<sub>70</sub>Ar<sub>3</sub>Br 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 66π-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 <b>5</b>, <b>9</b>, <b>10</b>, <b>13</b>, and <b>15</b> as the n-type
semiconducting materials were fabricated. The device using compound <b>15</b> (LUMO = −3.58 eV) showed the highest open circuit
voltage (<i>V</i><sub>oc</sub> = 0.90 V) and a respectable
PCE value of 3.33%. For <i>J</i><sub>sc</sub> and FF, type
II compounds <b>10</b>, <b>13</b>, and <b>15</b> showed much higher values than did type I compounds <b>5</b> and <b>9</b>, suggesting that the type II addition pattern
on C<sub>70</sub> 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