A facile method to synthesize [A '(D ' AD)(2)]-based push-pull small molecules for organic photovoltaics

Abstract

An efficient route to synthesize, for the first time, a series of small molecules based on the [A'(D'AD)(2)] architecture was developed using selective direct heteroarylation of the C-H bond with a Pd(AcO)(2)/Bu4NBr simple catalytic system. The C-H arylation of the unsymmetrical compound 4-(2,3dihydrothieno[3,4-b][1,4]dioxin-5-yl)-7-(thiophen-2-yl)benzo[c][1,2,5] thiadiazole (5) showed that the (C5) in the ethyldioxythiophene moiety is more reactive towards C-H arylation than its counterpart in the thiophene unit. The new small molecules bear two different acceptors and/or donors, which led to decrease the electron band gap and improve the strength of the push-pull system as well as the amount of intramolecular charge transfer from donor to acceptor units. Lactams and imide containing acceptors are used as second electron withdrawing units due to their well conjugated structures, strong p-p interactions and high electron affinity as well as their potential as electron withdrawing units in photovoltaic conjugated materials. All small molecules showed broad absorption spectra with optical band gaps, which were estimated to be in the range of 1.72-1.29 eV. From cyclic voltammetry, the highest occupied molecular orbital (HOMO) energy level could be tuned by changing the second acceptor, suggesting high open circuit voltage (V-oc). The EHID(EDBTT)(2):PC71BM-based solar cells reached a maximum PCE of 3.24%.