Open‐Shell Donor–Acceptor Conjugated Polymers with High Electrical Conductivity

Abstract

Conductive polymers largely derive their electronic functionality from chemical doping, processes by which redox and charge‐transfer reactions form mobile carriers. While decades of research have demonstrated fundamentally new technologies that merge the unique functionality of these materials with the chemical versatility of macromolecules, doping and the resultant material properties are not ideal for many applications. Here, it is demonstrated that open‐shell conjugated polymers comprised of alternating cyclopentadithiophene and thiadiazoloquinoxaline units can achieve high electrical conductivities in their native “undoped” form. Spectroscopic, electrochemical, electron paramagnetic resonance, and magnetic susceptibility measurements demonstrate that this donor–acceptor architecture promotes very narrow bandgaps, strong electronic correlations, high‐spin ground states, and long‐range π‐delocalization. A comparative study of structural variants and processing methodologies demonstrates that the conductivity can be tuned up to 8.18 S cm−1. This exceeds other neutral narrow bandgap conjugated polymers, many doped polymers, radical conductors, and is comparable to commercial grades of poly(styrene‐sulfonate)‐doped poly(3,4‐ethylenedioxythiophene). X‐ray and morphological studies trace the high conductivity to rigid backbone conformations emanating from strong π‐interactions and long‐range ordered structures formed through self‐organization that lead to a network of delocalized open‐shell sites in electronic communication. The results offer a new platform for the transport of charge in molecular systems.

Quinoidal conjugated polymers with open-shell character

“…Besides, the incorporation of quinoidal units in doped conjugated polymers can be a viable strategy to achieve robust organic materials with metallic ground states. 77 Last but not least, advances in the understanding of the open-shell character have also enabled the use of quinoidal polymers in organic thermoelectricity, spintronics, nonlinear optics, etc.…”

“…Thiadiazole [3,4-g]quinoxaline is another strong proquinoidal acceptor unit and the derived polymers P7 and P8 were synthesized by Azoulay. 76,77 The open-shell diradical character of these proquinoidal building blocks leads to appealing conducting properties in their applications.…”

“…Similarly, a high spin concentration was also observed for the P8 series with the triplet ground state and weak ferromagnetic coupling. 77 Interestingly, the quinoidal-aromatic copolymer P10 was reported recently as a good singlet fission candidate. 95 The fulfillment of energetic requirements for singlet fission is primarily ascribed to the distinctive biradical character of the heterocyclic quinoidal core.…”

“…The high conductivity rendered by the hopping of quinoidal segment has been applied in organic conducting and thermoelectric devices. 77 For example, the electrical conductivity of up to 6 S cm −1 can be achieved on proquinoidal poly(thieno[3,2-b] thiophene) (PTT). 98 To further enhance the conductivity, Zhu's group developed 99 an in situ solution casting polymerization method to prepare doped PTT-Tos with tosylate dopant anions (Fig.…”

Broadband infrared photodetection using a narrow bandgap conjugated polymer

“…The rich behavior of these materials emanates from the collective properties of π-electrons along the conjugated backbone and diminished intramolecular electron pairing, which enables new applications ( 33 – 36 ). The recent development of high-spin DA CPs has produced polymers with electronic bandgaps in the SWIR-MWIR with absorption extending well into the LWIR ( 34 , 35 ). Here, we demonstrate the first example of a photoconductive detector that uses an open-shell DA CP active layer that senses SWIR, MWIR, and LWIR photons.…”