Molecular Structure and Conformational Design of Donor-Acceptor Conjugated Polymers to Enable Predictable Optoelectronic Property.

Tuning the optoelectronic properties of donor-acceptor conjugated polymers (D-A CPs) is of great importance in designing various organic optoelectronic devices ranging from photovoltaics, photodetectors, light emitting diodes, and photoresistors. However, there remains a critical challenge in precisely control of bandgap through synthetic approach, since the chain conformation also affects molecular orbital energy levels. Here, we explored several types of D-A CPs with different acceptor units that show a opposite trend in energy band gaps with the increasing length of oligothiophene donor units. By investigating their chain conformation and molecular orbital energy with experimental measurements and theoretical calculations, we found that the molecular orbital energy alignment between donor and acceptor units plays a crucial role in dictating the final optical bandgap of D-A CPs. For NDI polymers with donor and acceptor building blocks with staggered orbital energy alignment, the higher HOMO with increasing oligothiophene length leads to a narrowing of the optical bandgap despite decreased chain rigidity. On the other hand, for DPP polymers with donor and acceptor building blocks with sandwiched orbital energy alignment, the increased band gap with increasing oligothiophene length originates from the reduction of bandwidth due to more localized charge density distribution and more flexible chains with intrachain twisting. Thus, this work provides a molecular understanding on the role of backbone building blocks on the chain conformation and bandgaps of D-A CPs for organic optoelectronic devices through the molecular level conformation design and segment orbital energy alignment. This article is protected by copyright. All rights reserved.