Narrow Bandgap Conjugated Polymers with Strong Correlations and Open-Shell Electronic Structures—Towards New Phenomena and Emergent Technologies

For over forty years, conjugated polymers (CPs) have been a source of enormous fundamental breakthroughs, enabling foundational insight into the nature of π-bonding and electron pairing, the creation of novel optoelectronic functionalities, and the development of commercially relevant technologies. Despite the achievement of significant technological milestones, the complex structural and energetic heterogeneities that define these materials preclude bandgap control at low energies, tailored interactions with infrared (IR) light, the study of fundamental physical phenomena, and the design and realization of new device functionalities. To address these modern challenges, we developed precision synthetic methods that enable control of the frontier orbital energetics, coplanarity of the conjugated backbone, intermolecular interactions, and many chemical, electronic, and structural features that affect electronic coherence within these π-conjugated macromolecules and enable unprecedented levels of bandgap control from 1 → 0.1 eV. We discovered that narrow bandgaps afforded through extended π-conjugation are intimately related to the coexistence of nearly degenerate electronic states. Through articulating novel mechanisms of spin alignment, topology control, and exchange, we have enabled the synthesis of neutral CPs with ground states that span the entire range from “conventional” closed-shell structures, to biradicaloids with varying degrees of open-shell character, to diradicals in both singlet (<i>S</i> = 0) and triplet (<i>S</i> = 1) spin-states. These materials exhibit weaker intramolecular electron-electron pairing and stronger electronic correlations than their closed-shell counterparts, which imparts novel phenomena not previously measured in soft-matter (polymer) systems.