Low Band Gap Polymers Design Approach Based on a Mix of Aromatic and Quinoid Structures

Topological Transition in Aromatic and Quinonoid π-Conjugated Polymers Induced by Static Strain

A topological quantum phase transition has been identified for the first time for 24 π-conjugated polymers as a function of external longitudinal strain due to a level crossing of the frontier orbitals at the topological phase transition. Topological phase is determined by the presence/absence of edge states. Out of the 24 polymers 15 are traditionally assigned an aromatic character, and 9 are traditionally assigned a quinonoid character. We find that all aromatic ones correspond to the trivial topological phase (Zak invariant, Z2 = 0), while all of the quinonoid ones to the nontrivial topological phase (Z2 = 1) replacing the intuitive characterization of aromatic/quinonoid with the physically well-defined Zak invariant. Unique topological phase transition as a function of tensile strain can be achieved for four of the quinonoid ones. Tensile strain in these cases leads to a reduction of the bandgap. We introduced a figure of merit for indicating the efficiency of achievable very small bandgap upon the application of external strain.

Carbon-based nanostructures as a versatile platform for tunableπ-magnetism

Emergence ofπ-magnetism in open-shell nanographenes has been theoretically predicted decades ago but their experimental characterization was elusive due to the strong chemical reactivity that makes their synthesis and stabilization difficult. In recent years, on-surface synthesis under vacuum conditions has provided unprecedented opportunities for atomically precise engineering of nanographenes, which in combination with scanning probe techniques have led to a substantial progress in our capabilities to realize localized electron spin states and to control electron spin interactions at the atomic scale. Here we review the essential concepts and the remarkable advances in the last few years, and outline the versatility of carbon-basedπ-magnetic materials as an interesting platform for applications in spintronics and quantum technologies.

Quinoidal conjugated polymers with open-shell character

Quinoidal π-Conjugated polymers with open shell character represent an intriguing class of macromolecules in terms of both fundamental research and practical applications.

Impact of π-Conjugated Linkers on the Effective Exciton Binding Energy of Diketopyrrolopyrrole-Dithienopyrrole Copolymers

The effect of the nature of the π-conjugated linker that is positioned between electron-deficient 2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP) and electron-rich dithieno[3,2-b:2',3'-d]pyrrole (DTP) units in alternating DPP-DTP copolymers on the optical and electrochemical band gaps and the effective exciton binding energy is investigated for six different aromatic linkers. The optical band gap is related to the electron-donating properties of DTP and the electron-withdrawing properties of DPP but likewise strongly affected by the nature of the linker and varies between 1.13 and 1.80 eV for the six different linkers. The lowest optical band gaps are found for linkers that either raise the highest occupied molecular orbital or lower the lowest unoccupied molecular orbital most, while the highest optical band gap is found for phenyl linkers that have neither strong donating nor strong accepting properties. Along with the optical band gap, the electrochemical band gap also changes, but to a lesser extent from 1.46 to 1.89 eV. The effective exciton binding energy (E b), defined as the difference between the electrochemical and optical band gaps, decreases with an increasing band gap and reaches a minimum of 0.09 eV for the copolymer with the highest band gap, that is, with phenyl linkers. The reduction in E b with an increasing band gap is tentatively explained by a reduced electronic interaction between the DTP and DPP units when the HOMO localizes on DTP and the LUMO localizes on DPP. Support for this explanation is found in the molar absorption coefficient of the copolymers, which shows an overall decreasing trend with decreasing E b.

Quinonoid vs. aromatic structures of heteroconjugated polymers from oligomer calculations

Conjugated polymers with quinonoid ground states can display low optical band gaps. The design of novel conjugated polymers with quinonoid ground states offers insights into the relative stabilities of aromatic vs. quinonoid structures. In this work, we present parameters such as the quinonoid (Q)/aromatic (A) energy difference, the band gap, and the C-C distances between the repeat units. This study reveals eight new polymers which exist in quinonoid ground state among twenty-nine polymers of varying structural composition that were subject to analysis. We expect that copolymerizing such quinonoid ground state monomers with aromatic ground state monomers will modulate the bandgap of the resulting polymers.

Development of a quantum chemical descriptor expressing aromatic/quinoidal character for designing narrow-bandgap π-conjugated polymers

Guidance for designing narrow-bandgap copolymers was established by developing a chemical descriptor (QSE) that considers only the monomer and linking sites.

Naphthalenediimides Fused with 2-(1,3-Dithiol-2-ylidene)acetonitrile: Strong Electron-Deficient Building Blocks for High-Performance n-Type Polymeric Semiconductors

Naphthalenediimides (NDI) fused with two 2-(1,3-dithiol-2-ylidene)acetonitrile moieties (NDI-DTYA2), a novel strong electron-deficient monomer, was designed and readily synthesized by aromatic nucleophilic substitution to develop n-type polymers. Two NDI-DTYA2 based donor-acceptor (D-A) polymers P(NDI-DTYA2-1T) and P(NDI-DTYA2-2T) have been prepared (1T = thiophene, 2T = 2,2'-bithiophene) and showed low-lying LUMO energies (<-4.2 eV) and near-infrared optical absorptions (optical band gap <1.3 eV). Although the film feature of these polymers is amorphous, pure electron transport with high mobility of up to 0.38 cm² V ^-1 s ^-1 could be achieved for their bottom gate organic thin film transistors, which was among the highest performances for unipolar n-type polymers. The results demonstrate that NDI-DTYA2 derivatives are promising building blocks for developing electron transport π-functional materials.