Donor–Acceptor Conjugated Macrocycles: Synthesis and Host–Guest Coassembly with Fullerene toward Photovoltaic Application

Electron-rich (donor) and electron-deficient (acceptor) units to construct donor–acceptor (D–A) conjugated macrocycles were investigated to elucidate their interactions with electron-deficient fullerene. Triphenylamine and 4,7-bisthienyl-2,1,3-benzothiadiazole were alternately linked through acetylene, as the donor and acceptor units, respectively, for pentagonal <b>3B2A</b> and hexagonal <b>4B2A</b> macrocycles. As detected by scanning tunneling microscopy, both D–A macrocycles were found to form an interesting concentration-controlled nanoporous monolayer on highly oriented pyrolytic graphite, which could effectively capture fullerene. Significantly, the fullerene filling was cavity-size-dependent with only one C₇₀ or PC₇₁BM molecule accommodated by <b>3B2A</b>, while two were accommodated by <b>4B2A</b>. Density functional theory calculations were also utilized to gain insight into the host–guest systems and indicted that the S···π contact is responsible for stabilizing these host–guest systems. Owing to the ellipsoidal shape of C₇₀, C₇₀ molecules are standing or lying in molecular cavities depending on the energy optimization. For the <b>3B2A</b>/PC₇₁BM blended film, PC₇₁BM was intercalated into the cavity formed by the macrocycle <b>3B2A</b> and provided excellent power conversion efficiency despite the broad band gap (2.1 eV) of <b>3B2A</b>. This study of D–A macrocycles incorporating fullerene provides insights into the interaction mechanism and electronic structure in the host–guest complexes. More importantly, this is a representative example using D–A macrocycles as a donor to match with the spherical fullerene acceptor for photovoltaic applications, which offer a good approach to achieve molecular scale p<i>–</i>n junctions for substantially enhanced efficiencies of organic solar cells through replacing linear polymer donors by cyclic conjugated oligomers