Magnetic Ordering in a High‐Spin Donor–Acceptor Conjugated Polymer

Abstract

The development of open‐shell organic molecules that magnetically order at room temperature,which can be practically applied, remains a grand challenge in chemistry, physics, and materials science. Despite the exploration of vast chemical space, design paradigms for organic paramagnetic centers generally result in unpaired electron spins that are unstable or isotropic. Here, a high‐spin conjugated polymer is demonstrated, which is composed of alternating cyclopentadithiophene and benzo[1,2‐c;4,5‐c′]bis[1,2,5]thiadiazole heterocycles, in which macromolecular structure and topology coalesce to promote the spin center generation and intermolecular exchange coupling. Electron paramagnetic resonance (EPR) spectroscopy is consistent with spatially localized spins, while magnetic susceptibility measurements show clear anisotropic spin ordering and exchange interactions that persist at room temperature. The application of long‐range π‐correlations for spin center generation promotes remarkable stability. This work offers a fundamentally new approach to the implementation of this long‐sought‐after physical phenomenon within organic materials and the integration of manifold properties within emerging technologies.

Strong Acceptor Annulation Enables Control of Electronic Structure and Spin Configuration in Donor–Acceptor Conjugated Polymers

“…In contrast, P2 displays an onset of reduction at −3.73 eV with clear separation from the oxidation peak at −4.75 eV (Figure b). While the large change in the frontier molecular orbitals is anticipated from incorporating the highly electronegative acceptor ( M1 ), the broad spectral profile and very narrow bandgap are consistent with considerable mixing of the ground state and the emergence of diradical character within P1 . ,, …”

“…Strong configuration mixing between frontier MOs results in an orbital manifold in which α- and β-electrons occupy two degenerate or nearly degenerate singly occupied MOs (SOMOs). , The topological distribution of SOMOs within the π-conjugated backbone determines the degree of spin pairing (i.e., diradical character ( y )), energy gap between spin states, physicochemical properties, and (opto)­electronic functionality. , In this context, significant wave function overlap between near-degenerate SOMOs stabilizes singlet ground states with an antiparallel alignment of spins ( S = 0), whereas high-spin (triplet, S = 1) ground states arise from strong electron correlations that minimize SOMO overlap to stabilize a parallel alignment of spins. Alternating in chain donor and strong acceptor interactions and extensive π-delocalization endow high chemical stability, enabling the realization of new applications such as charge-neutral conductive polymers, wide-potential window supercapacitors, solution-processed infrared detectors, and other solid-state functionality not previously realizable using other organic materials systems. ,, Apart from offering new technological benefits, these materials are anticipated to give rise to new properties and physics, emanating from the influence of stronger correlation effects on charge transfer, spin, redox, and other processes, and from properties that reflect their light atom composition such as weaker spin–orbit coupling. ,, …”

“…High-spin DA CPs have an electronic structure in which long-range π-correlations emerge from narrow bandgaps, quinoidal bonding patterns, and rigid coplanar conformations. ,,,, This has been achieved by pairing strong, pro-quinoidal acceptors capable of lowering the lowest unoccupied MO (LUMO) with donors that reduce aromatic character and raise the highest occupied MO (HOMO) such as derivatives of cyclopenta­[2,1- b :3,4- b ′]­dithiophene (CPDT). , Prototypical acceptors include heteroannulated variants of benzo­[ c ]­[1,2,5]­thiadiazole (BT) such as [1,2,5]­thiadiazolo­[3,4- g ]­quinoxaline (TQ) and benzo­[1,2- c ;4,5- c ′]­bis­[1,2,5]­thiadiazole (BBT) (Figure ). While open-shell and high-spin DA CPs are an important emerging technology, controlling the electronic and spin structure of these structurally and energetically heterogeneous materials and establishing connections with emergent electronic phenomena remains a nascent challenge.…”

“…While open-shell and high-spin DA CPs are an important emerging technology, controlling the electronic and spin structure of these structurally and energetically heterogeneous materials and establishing connections with emergent electronic phenomena remains a nascent challenge. Furthermore, strategies to modulate important properties including exchange interactions, orbital topology, singlet–triplet splitting (Δ E ST ), and carrier polarity remain central to the rational design and implementation of these materials within emerging technologies. − , …”

Recent Progress of Organic Semiconductor Materials in Spintronics

“…The curie temperature of Pyrene-F4TCNQ [41] charge transfer composite crystal is 305 K, and the saturation magnetization is 15.9 memu/g. The saturation magnetization of the copolymer based on benzodithiadiazole and thiophene synthesized by Steelman et al [42] is close to 0.2 emu/g at 25 K. Organic charge transfer composites have attracted extensive attention in the field of semiconductors due to their good conductivity and excellent photoelectric conversion performance. [43] Although the saturation magnetization of the materials obtained at present is low, they still have a specific potential as organic spin injection materials.…”