Maleimide-Substituted Polyacetylene: A New Highly Conductive n-Type Polymer

The development of highly conductive organic semiconductors has been a longstanding goal in materials science due to their chemical tunability, solution processability, mechanical flexibility, and other unique properties. Conjugated polymers are some of the most promising materials in this respect and have been widely employed in diverse device architectures such as organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), and organic photovoltaics (OPVs). These applications generally require both n-type and p-type semiconducting materials, but the development of p-type conducting polymers has greatly outstripped that of n-type and ambipolar systems. Specifically, n-type materials generally display lower conductivities and mobilities in their doped states and are less stable due to oxidation under ambient conditions.<br/><br/>This bottleneck is largely synthetic. Stability against oxidation requires decreasing LUMO levels well below -4.0 eV, yet there are relatively few building blocks that are sufficiently electron withdrawing to achieve this. Furthermore, there are limited design strategies for assembling these building blocks into polymers that retain high backbone planarity and electron delocalization, which are both critical to high mobility and conductivity. Thus, the development of new polymer scaffolds is critical to closing the performance gap between n-type and p-type conjugated polymers.<br/><br/>Polyacetylenes (PAs) are attractive as high conductivity polymers, holding the record for highest reported conductivity in an organic semiconductor. However, they are inherently p-type materials. Although the introduction of strongly withdrawing groups to the PA core is straightforward conceptually, there are several practical challenges. Many standard withdrawing groups are too sterically bulky to preserve backbone planarity, and those that remain are generally incompatible with traditional PA syntheses.<br/><br/>We address this issue by developing a new class of PAs consisting of an alternating sequence of maleimide and vinylene units, dubbed maleimide-polyacetylene or mPA. This structure achieves high backbone planarity while still introducing a high density of strongly withdrawing groups, furnishing a highly conductive polymer with a low LUMO of -4.2 eV. The maleimide unit is also a facile synthetic handle, which enables the introduction of diverse functionality without perturbing backbone conformation. These polymers were accessed via a new synthetic approach, where a nonconjugated precursor polymer was synthesized via ring opening metathesis polymerization (ROMP), followed by an efficient elimination to furnish the final polyacetylene. This strategy is scalable and efficient and enables the synthesis of low dispersity polymers with highly controllable molecular weights. Beyond mPA, it should be generalizable to a range of other electron-deficient polyacetylenes, opening the door to a new class of n-type organic semiconductors.