C-H Direct Arylation: A Robust Tool to Tailor the Π-Conjugation Lengths of Non-Fullerene Acceptors.

Facile synthesis without involvement of toxic reagents is of great significance in the practical application of photovoltaic materials. In this work, four acceptor-donor-acceptor (A-D-A) type unfused-ring acceptors (UFRAs) with stepwise extension in pi-conjugation, i.e., CPFB-IC-n (n = 1 -4), involving cyclopentadithiophene (CPDT) and 1,4-difluorobenzene (DFB) as cores, are facilely synthesized by an atom-, step-economic and labor-saving method through direct arylation of C-H bond (DACH). Among them, CPFB-IC-4 has the longest conjugation lengths among the molecular UFRA ever reported. The dependence of optoelectronic properties and photovoltaic performances of CPFB-IC-n (n = 1 -4) on conjugation length were systematically investigated. CPFB-IC-2 with near zero highest occupied molecular orbital (HOMO) offsets (Delta E-HOMO = 0.06 eV) achieves the highest power conversion efficiency (PCE), due to the significantly enhanced open voltage (V-OC) and short current (J(SC)) caused by the balanced frontier molecular orbitals (FMOs) and complementary light absorption. Our work demonstrates that the optical properties and FMOs of UFRAs can be finely tuned by the stepwise elongation of conjugation lengths. Meanwhile, DACH coupling as a powerful tool here established will be a promising candidate for synthesizing high-performance oligomeric UFRAs.