Alternating terpolymers based on tunable Bi-donors with manipulating energy levels and molecular geometry

Abstract

Three novel regular acceptor-donor1-acceptor-donor2(A-D1-A-D2) terpolymers were prepared via embedding a second donor(D2) unit into the traditional D-A backbone to manipulate the energy levels and molecular geometry with no complex synthesis or solubility loss. In these A-D1-A-D2 terpolymers, benzodithiophene(BDT, D1) and diketopyrrolopyrrole(DPP, A) were selected as the basic skeleton, and the dithienopyrrole(DTPy), carbazole(CZ) and fluorine(FL) units with different electron donating ability were chosen as the second donor unit(D2). The HOMO energy levels can be effectively modulated by only varying D2 unit because of the push-pull interaction between donor and acceptor units. Versus the D-A bipolymer PDPP-BDT, incorporation of the D2 unit into the copolymers can distinctly lower the highest occupied molecular orbital(HOMO) levels to –5.47 eV for PDDPP-BDT-DTPy, –5.38 eV for PDDPP-BDT-CZ and –5.23 eV for PDDPP-BDT-FL, which shows the strong dependence on electron-donating ability. Density functional theory(DFT) simulation and X-ray diffraction(XRD) measurements also reveal the effect of the D2 units on the molecular geometry of the terpolymers and their molecular packing. Notably, a PDDPP-BDT-DTPy combined with a thiophene ring and forked tail pendant away from the backbone had less back-bone torsion and more compact packing than the other two counterparts. These results demonstrate that embedding a second donor(D2) unit into the backbone to construct an A-D1-A-D2 structure can be an effective and direct strategy to manipulate the energy levels and molecular geometry and develop organic semiconducting materials.