π-building blocks for organic electronics: Revaluation of "inductive" and "resonance" Effects of π-electron deficient units

Organic electronics has rapidly advanced in the last two decades, owing to the development of semiconducting materials and the innovation of device technologies. One of the critical issues in the materials development, for achieving high performances in the organic devices, is to precisely control their frontier orbitals, i.e., the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. These energy levels are largely dependent on the chemical structures, and thus to evaluate the electronic properties of representative π-building blocks and to understand their structure-property relationship are of particular importance. In this review, we focus on various π-building blocks for semiconducting polymers and oligomers, especially electron poor heterocycles (acceptor units), and revaluate their electronic structures focusing on the model compounds and the corresponding polymers. A clear difference in the electronic structure is found depending on the chemical structure, which can be explained in terms of "inductive effect" and "resonance effect". We hope that this Review will give new insight into the electronic structure of the semiconducting materials and be an important guideline for the materials design.