Large Room‐Temperature Magnetoresistance in a High‐Spin Donor–Acceptor Conjugated Polymer

Open‐shell conjugated polymers (CPs) offer new opportunities for the development of emerging technologies that utilize the spin degree of freedom. Their light‐element composition, weak spin‐orbit coupling, synthetic modularity, high chemical stability, and solution‐processability offer attributes that are unavailable from other semiconducting materials. However, developing an understanding of how electronic structure correlates with emerging transport phenomena remains central to their application. Here, the first connections between molecular, electronic, and solid‐state transport in a high‐spin donor–acceptor CP, poly(4‐(4‐(3,5‐didodecylbenzylidene)‐4H‐cyclopenta[2,1‐b:3,4‐b’]dithiophen‐2‐yl)‐6,7‐dimethyl‐[1,2,5]‐thiadiazolo[3,4‐g]quinoxaline), are provided. At low temperatures (T < 180 K), a giant negative magnetoresistance (MR) is achieved in a thin‐film device with a value of −98% at 10 K, which surpasses the performance of all other organic materials. The thermal depopulation of the high‐spin manifold and negative MR decrease as temperature increases and at T > 180 K, the MR becomes positive with a relatively large MR of 13.5% at room temperature. Variable temperature electron paramagnetic resonance spectroscopy and magnetic susceptibility measurements demonstrate that modulation of both the sign and magnitude of the MR correlates with the electronic and spin structure of the CP. These results indicate that donor–acceptor CPs with open‐shell and high‐spin ground states offer new opportunities for emerging spin‐based applications.


Introduction
Organic semiconductors (OSCs) are playing a defining role in the advancement of numerous (opto)electronic technologies, with DOI: 10.1002/adma.202306389significant breakthroughs leading to commercialized systems (e.g., organic light-emitting displays).Organic materials are also promising for emerging low-power electronic devices [1] that utilize the spin-degree of freedom (i.e., spintronic devices) due to their weak spin-orbit coupling emanating from their light element composition, [2,3] longer-spin relaxation times, [4,5] and longer spin diffusion lengths (i.e., ≈100 nm), [6] when compared with their inorganic counterparts.3][4][5][6] In a prototypical organic spintronic device, such as an organic spin valve, the organic material is sandwiched between two ferromagnetic electrodes, and a spinpolarized current is generated by a ferromagnetic spin-injector.Impressive values of magnetoresistance (MR) of ≈20-30% have been realized using organic materials systems, which is comparable to various inorganic and emerging semiconductor materials. [7,8]espite these technological developments, there is still debate on whether the origin of the MR response arises due to spinconserved transport in the organic layer or from the anisotropic MR of the ferromagnetic electrodes. [9,10]Moreover, the strict requirements associated with the fabrication of vertical spin-valve devices require careful engineering of the interfaces between the ferromagnetic electrodes and organic materials, features which have demonstrated profound effects on the observed MR. [10] One of the main goals of modern spintronics is to maintain spin polarization with sufficiently long lifetimes to perform manipulations at room temperature.Many efforts in inorganic semiconductor spintronics (e.g., InGaAs) have focused on defect-engineered paramagnetic centers to promote spin-filtering (i.e., enabling the transport of only one type of spin); however, issues with processability, attaining long spin diffusion lengths, and room temperature operation remain largely unresolved. [11]Moreover, microelectronic device fabrication with inorganic semiconductors requires complex thin film deposition steps, making it difficult to utilize these materials in commercial, and emerging spin-based device applications.Thus, there remains widespread interest in developing organic paramagnetic spin-filtering materials that do not require ferromagnetic electrodes to achieve high MR, that can be easily solution-processed, implemented into simple device architectures, and operate at room temperature.
OSCs with open-shell electronic structures offer opportunities to address these limitations on account of the presence of unpaired electron spins that can be manipulated by external magnetic, electronic, and optical stimuli. [12,13][27] In particular, DA CPs with narrow bandgaps and open-shell diradical character are emerging materials that offer new opportunities for spin transport, organic magnetism, [28,29] and various (opto)electronic, spin-based, and quantum functionalities not readily available from conventional closed-shell OSCs.In closed-shell materials, doping is required to obtain open-shell character (i.e., unpaired electron spins) and to increase electrical conductivity; however, doped polymers are usually unstable, and their ionized nature leads to large structural and energetic disorders.In contrast, strong configuration mixing between the frontier molecular orbitals (MOs) arises in DA CPs when the bandgap is narrowed, resulting in weakly paired or unpaired spins occupying nearly degenerate singly occupied MOs (SOMOs) delocalized within the -conjugated backbones.The narrow bandgaps, open-shell diradical character (y), and strong electronic correlations enable new functionality, such as electrical conductivity, without the use of (external) dopants.Members of our team previously reported open-shell DA CPs that demonstrate excellent charge transport [21] with electrical conductivities exceeding 8 S cm −1 at room temperature.However, research on open-shell and high-spin DA CPs has largely focused on understanding the evolution of the ground-state magnetic and electronic properties and establishing the role of spin-spin interactions; thus, magnetic-field effects on charge transport remain relatively unexplored. [17,21,29]Developing a fundamental understanding of how electronic and spin-structure correlate with transport phenomena remains central to the development of a new generation of organic paramagnetic materials that can be applied in spin-based device technologies.
Here, we provide the first examples of correlations between electronic and spin-structure and magnetic-field-dependent transport in a high-spin DA CP (Figure 1a The copolymer considered in this study was synthesized as previously reported. [22]Salient design features include a crossconjugated 4H-cyclopenta[2,1-b:3,4-b']dithiophene donor that raises the highest occupied MO (HOMO) and provides strategic positioning of ─C 12 H 25 substituents to balance considerations associated with solution-processability and promoting a highly planar backbone.When paired with a strong, proquinoidal thiadiazoloquinoxaline (TQ) acceptor capable of lowering the lowest unoccupied MO (LUMO), this donor-acceptor combination promotes strong configurational admixing (i.e., HOMO-LUMO mixing) resulting in an open-shell electronic structure.Strong electronic correlations arise from extensive delocalization of electron density along the -conjugated backbone promoted by internal charge transfer between intrachain electron-donor and electron-acceptor sites.19][20][21] These properties coalesce to enable a large negative MR at low temperatures and positive MR at room temperature without the use of ferromagnetic electrodes or complex device architectures.Quantum chemical calculations, variable temperature (VT) electron paramagnetic resonance (EPR) spectroscopy, and magnetic susceptibility measurements provide valuable insight into connections between electronic structure and spin-dependent transport.The thermal (de)population of the triplet state and the corresponding giant negative MR show a correlation between the spin of the conduction electrons and the triplet ground state of the polymer.This is indicative of effective exchange interactions between charge carriers and paramagnetic centers that promote spin-polarized transport. [8]This performance milestone demonstrates the viability of DA CPs for spin-based device applications.

Results and Discussion
To provide an interpretive baseline, the gas phase electronic structure of an oligomer of this material was first characterized using density functional theory (DFT).Figure 1b shows the spin density distributions of the singlet and triplet states of an oligomer with eight repeat units (n = 8) at the unrestricted (U)B3LYP/6-31G** level of theory.These spin density distributions are indicative of extensive spin polarization and delocalization over the quinoidal and linear CP backbone.For the triplet, a more predominant localization of spin density at the chain ends reduces Coulomb repulsion, stabilizing this electronic configuration as the ground state.To prevent strong aggregation of the rigid-rod CP and isolate single polymer chains, a dilute solution of ≈10 −5 m in anhydrous degassed xylenes was used for EPR measurements.This solution was then rapidly frozen using liquid nitrogen to study low-temperature EPR characteristics.EPR measurements at 50 K showed a broad single line (i.e., 10-15 G-wide) with a g factor of 2.005, indicating the presence of stable organic radicals (Figure 1c).Decreasing the temperature from 50 to 5 K results in an increase in signal intensity, indicating the population of a high-spin (triplet) ground state.The VT EPR data were fit to the Bleaney-Bowers equation (see Supporting Information), which revealed a small, nearly degenerate ΔE ST of 9.90 × 10 −3 kcal mol −1 (J = 1.73 cm −1 ), consistent with a previous report (Figure 1d). [22]he positive value of the J-coupling (ΔE ST = 2J) indicates the presence of weak intramolecular ferromagnetic coupling between spins, consistent with a high-spin ground state.This weak intramolecular ferromagnetic coupling potentially allows external magnetic field and temperature-dependent manipulations to promote spin-dependent transport in the polymer system.
Temperature and field-dependent superconducting quantum interference (SQUID) measurements of a powder sample are consistent with Curie paramagnetism and a high-spin ground state.Moreover, these measurements are useful to account for intermolecular interactions and solid-state electronic and structural ordering that play a large role in overall solid-state electronic device performance.The magnetic susceptibility () measurements show a clear decrease in  as the temperature increases (Figure 1e), consistent with thermal depopulation of the highspin ground state (Figure 1c).[31][32] The spin quantum number (S) was determined by measuring the magnetic moment as a function of the magnetic field at a fixed temperature and by fitting it to the paramagnetic Brillouin function in Equation (1) here H is the field in Oersted, T is the temperature, g is the gfactor, k B is the Boltzmann constant, M S is the saturation magnetization,  B is the Bohr magneton, and S is the spin quantum number fitting parameter.At 2.5 K, a fit to Equation (1) results in a value of S = 0.95, which is consistent with a high spin (S = 1) ground state (Figure 1e inset).A common feature of the field-dependent isotherms observed in open-shell DA CPs is the diamagnetic contribution at higher magnetic fields (Figure S1, Supporting Information) because of the saturation of the paramagnetic phase.Thus, the associated diamagnetic contributions were corrected with a linear fit to the high-field region.The rapid removal of the solvent in spin-coated thin films creates a uniform but largely amorphous polymer network in which the magnetic properties closely resemble powdered samples. [29]he ability of external magnetic fields to modulate the spin state within these materials was characterized using magnetic field-dependent electrical measurements.Devices were fabricated by spin-coating films from chloroform solutions onto pre-patterned gold electrodes (Figure 2a).The two-point probe measurements showed linear (I-V) characteristics with a room temperature conductivity ( ) of 0.29 S m −1 (Figure S2, Supporting Information).When the thin film was gated with a third electrode (i.e., to form a field-effect transistor), the output and the transfer curves did not show any off-state or effect of gate voltage, suggesting the presence of free carriers in the material.The field-effect transistor characteristics are consistent with p-type conduction (Figure S3, Supporting Information).We note that the applied source-gate voltage (V g ) had no significant effect on the charge transport so a V g = 0 was used for all magnetic-field-dependent measurements.To study the effect of a magnetic field on charge transport, a magnetic field was applied perpendicular to the direction of current flow.The MR response of the devices was quantified using Equation ( 2) here R(B) is the resistance under an applied magnetic field and R(0) is the resistance under a zero magnetic field.As the thermal population of the triplet state increases at low temperatures (Figure 1 and Figure S6, Supporting Information), the highest MR of −98% was observed at 10 K under an applied bias of 5 V.The curves start to saturate at fields above 0.3 T (Figure 2b), and as the temperature is increased, the MR response starts to decrease with an MR of −30% attained up to 140 K.The MR changes its sign from negative to positive as the temperature rises above 140 K with a high positive MR of 70% achieved at 180 K (Figure 2c).It is also important to note that the linewidth is ≈100 mT at 10 K, and it increases with temperatures, a behavior distinct from other organic magnetoresistive materials in which the linewidths are only a few mT. [33] significant challenge in modern spintronics is to achieve high room temperature MR so as to enable practical applications.However, the majority of organic materials systems that have been reported are used in conjunction with poly(styrene-sulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT: PSS) in multilayer devices that mask the intrinsic contribution of the organic active layer to the MR (Table S1, Supporting Information).Figure 2c shows that a further increase in temperature reduces the MR of the device; however, a relatively large intrinsic MR of 13.5% is achieved at room temperature, which is the highest value for organic materials systems (Table S1, Supporting Information).This large MR at both low and practical operating temperatures is indicative of strong exchange interactions between charge carriers and singlet or triplet states of the polymer.The value of ΔE ST indicates that ≈75% of spins (Boltzmann population ratio of 0.98, Figure S6, Supporting Information) occupy the lower-energy triplet manifold at room temperature which may account for the high MR.This contrasts with low-spin diradicals that require heating above room temperature in order to thermally populate the triplet state, and which show similar trends in the MR. [34]emperature-dependent and magnetic field-dependent charge transport measurements provide valuable insight into the transport mechanism.Figure 3a shows the effect of an external magnetic field from 0 to 2 T on the I-V characteristics at 50 K.The increase in slope by the application of an external field demonstrates that the injected spin direction is constrained and aligned with the applied field.In contrast, Figure S4, Supporting Information, shows a completely opposite effect at higher temperatures (>180 K) in which the external magnetic field increases the device resistance.This can be associated with the higher temperature diamagnetic features of the polymer becoming dominant and difficulty in attaining efficient spin-aligned transport.The non-linearity in the I-V curves at low temperatures can be attributed to a thermal activation barrier, suggesting thermally activated hopping as the dominant charge transport mechanism.A plot of conductivity versus 1000/T in Figure 3b shows that conductivity increases at higher temperatures and is temperaturedependent. Figure 3c shows the fit to the Arrhenius equation in two temperature ranges (100-300 K) and (10-40 K), which gives an activation energy (E a ) of 97.5 and 1.78 meV, respectively.A higher E a at elevated temperatures indicates that charge transport is not dominated by a single pathway (i.e., nearestneighbor transport) and that activation energy is not independent of temperature.Thus, the electrical conductivity can be defined by the variable range hopping (VRH) transport mechanism as described by Equation ( 3) here  0 , T 0 , and T are the room temperature conductivity, characteristic temperature, and absolute temperature, respectively, and n is the fitting parameter.The value of n can be used to predict the charge transport mechanism in the system or assess whether there are multiple mechanisms.Figure 3d shows the log R versus T −n fit to the temperature-dependent resistance (R) data. [35]he fit can be divided into a low-temperature region (LTR, 10-60 K) and a high-temperature region (HTR, 65-300 K).The HTR region is defined by n = ¼, which is related to Mott-VRH, [36] that considers a constant density of states (DOS) in the vicinity of the Fermi energy.In contrast, the LTR fits well for n = ½, which corresponds to a well-known Efros-Shklovskii-VRH (ES-VRH) hopping mechanism. [37]This crossover from Mott-VRH to ES-VRH suggests the presence of a Coulomb gap around the Fermi level.The value of T 0 extracted from the fitting is 2.16 × 10 7 for n = 1/4 and 8.89 × 10 2 for n = ½.Utilizing the characteristic temperature and activation energy relations (Equations (S11) and (S13), Supporting Information) an E 0 of 52.92 meV is obtained for Mott-VRH, and an E 0 of 203 meV is obtained for ES-VRH.These different energies and VRH mechanisms imply that multiple pathways facilitate charge transport in our system (see Supporting Information). [38]Moreover, the change in the sign of the MR can be described by these competing mechanisms which would provide insight on the impact of magnetic field on charge transport.
6][47][48][49][50] In the open-shell DA CP, both positive and negative MR can be explained by the VRH model.For positive MR, the external applied magnetic field manipulates the wavefunction of the carrier in a manner that reduces their overlap and increases the resistance.This effect is generally observable at higher temperatures and magnetic fields which can explain the positive MR in the DA CP.More recently, theories pertaining to negative MR in the VRH regime have also been put forward.][54][55][56][57][58] The materials denoted in red, blue, and green were tested in multilayer vertical devices with an ITO/PEDOT:PSS/Polymer/Ca architecture using an in-plane magnetic field, while the DA CP in this work was tested in planar device architecture and out of plane magnetic field.
dependent upon the sign distribution of the localized electron wavefunctions. [51]Moreover, exchange interactions between the high-spin polymer and charge carriers could also lead to negative magnetoresistance because of reduced electron scattering events.DFT calculations at the unrestricted (U)B3LYP/6-31G** level of the theory of neutral, cationic, and anionic states provide additional insight into spin-dependent phenomena within the DA CP.Unlike closed-shell systems, electrons in open-shell organic materials are non-bonding, unpaired, weakly interacting, and highly susceptible to spin manipulation from external stimuli.Figure S7, Supporting Information, shows the isodensity surfaces for the frontier molecular orbitals for the n = 8 oligomers in the neutral state, while Figures S8 and S9, Supporting Information, shows isodensity surface in the charged state, indicating a difference in electron density distribution and spin-orientation for different states.Under the presence of an external magnetic field, the spin density distribution in the charged states is expected to produce an inherently different spin distribution, coincident with the presence of the MR effect in this high-spin DA CP.While the control of spin orientation and flipping by an external field has extensive importance in this system, a detailed investigation along with the development of theoretical models for correlated open-shell species is important to fully understand detailed mechanistic considerations.
Although the current experiments cannot conclusively determine the temperature dependence mechanism of MR in this system, they do provide compelling insight and rule out several possibilities.Weak localization (WL) effects cannot account for the negative MR in this system since WL effects would cause a negative MR at low fields (<0.1 T) that becomes positive at higher fields (>0.1 T).In contrast, the MR curves for this system saturate at large magnetic fields and no transition to positive MR is observed.Zeeman splitting-induced MR is also inconsistent with these results.Hu et al. [52] have pointed out that MR between positive and negative values can be tuned in OSCs due to Zeeman splitting, which makes the intersystem crossing (ISC) dependent on the external magnetic field.For this to occur, the magnetic splitting energy ΔE B of three triplet sublevels should be larger than ΔE ST .Using the relationship, ΔE B = m s g b B, where m s , g,  b , and B are the spin quantum number, g-factor, Bohr magneton, and external magnetic field, respectively, ΔE B is estimated as 230 μeV at 2 T.This is approximately half the ΔE ST calculated from EPR as 430 μeV, suggesting that the negative to positive MR transition is unrelated to Zeeman splitting.
][54][55][56][57][58] Figure 4b shows the room temperature MR of ≈13.5% for our high spin DA CP that surpasses all other organic materials.Moreover, repeated room temperature measurements of the MR show no discernable changes over a period of 30 days (Figure S12, Supporting Information).At 300 K, PEDOT:PSS shows an MR of 5%, PFO an MR of 11%, and MEH-PPV an MR of 1.5%.However, all these materials were tested in a vertical device structure, similar in nature to an OLED, with an in-plane magnetic field.In these devices, the MR depends upon spin-dependent recombination and additional interface effects come into play. [33,52,57]Since these device structures require different hole-transporting, electrontransporting, and emissive layers, the intrinsic contribution of the native organic material is difficult to determine.In contrast, our outstanding performance is achieved in devices that use a simple, transverse, and planar structure (Figure 2a).Such geometries benefit from ease of fabrication and are amenable to rapid, high-throughput, and cost-effective manufacturing technologies.Furthermore, this contrasts with conventional spin valves in which spin-polarized electrons are injected by ferromagnetic electrodes and the value of MR primarily depends on the efficiency of spin injection.Thus, this study reports the highest MR for organic materials and the first example using a high-spin DA CP, highlighting the potential of DA CPs in magnetic-field applications where intrinsic magnetic properties and diradical character can be manipulated by external magnetic fields.These advantages can be useful for fast spintronic operations and for the study of dynamical aspects of nanoscale magnetism and spin transport.Moreover, the change in the sign of the MR with temperature indicates different transport mechanisms occurring in the system and provides a path to investigate this class of polymers further for their magnetic field-dependent applications.

Conclusions
The significant spin-filtering effect in this high-spin CP has both a concrete fundamental meaning and offers important practical advantages.Fundamentally, the spin-polarized current in the polymer emanates from interactions between charge carriers and spin properties intrinsic within the polymer.This provides a current dominated by only one spin type, which is dependent upon the external magnetic field and temperature.Practically, this material can be easily solution-processed and is highly stable in ambient conditions.Thus, its unique magnetic characteristics open opportunities for very fast and flexible magnetic manipulations by driving external electric and magnetic fields.Importantly, these results demonstrate, for the first time, the intimate connection between magnetism, chemical and electronic structure, diradical character, and the corresponding magnetic fielddependent transport.The transition from a singlet spin state to a triplet state at low temperature causes more resistive charge transport which can be improved by the application of an external magnetic field.The observed positive and negative MR is a signature of tunable magnetic properties that can be utilized for a broad range of applications.Moreover, charge transport without doping in these systems is a result of narrow band gaps and open-shell electronic structures, and the temperature-dependent magneto-transport is highly correlated with the spin state of the system.These attributes provide a means for control considering recent demonstrations of modulating important properties, including exchange interactions, orbital topology, ∆E ST , and carrier polarity within these materials systems.Such efficient spinfiltering from a paramagnetic system provides a large room temperature MR, which paves the way for fast and low-power spincurrent manipulations based on extremely small magnetic fields and a new generation of organic electronic and spintronic devices.

Figure 1 .
Figure 1.Electronic structure and magnetic characterization.a) Molecular structure of the donor-acceptor conjugated polymer and b) spin density distribution for the triplet ground state and thermally excited singlet state of the n = 8 oligomer at the UB3LYP/6-31G ** level of theory.The arrows highlight ferromagnetic spin coupling in the triplet and antiferromagnetic coupling in the singlet.c) Temperature-dependent EPR spectra from 5 to 50 K.d) Temperature-dependent fit to the Bleaney-Bowers equation with ΔE ST = 9.90 × 10 −3 kcal mol −1 .e) SQUID magnetometry of a solid sample.Main plot: Magnetic susceptibility,  versus T, from 2 to 300 K fit to the Curie-Weiss law (red line).Inset: Magnetic field (H) dependence of the magnetization at 2.5 K, with Brillouin functions (Equation (1)) for S = 1/2, S = 0.95, and 3/2.The fit with S = 0.95 confirms the high-spin ground state.

Figure 2 .
Figure 2. The device architecture with a magnetic field (B z ) applied perpendicular to the direction of current flow and temperature-dependent magnetoresistance.a) Schematic illustration of spin-dependent transport between two gold electrodes with active channel area of 60 μm × 1 mm and out of the plane magnetic field (B z ) denoted by a red arrow.b) MR curves for 10 K ≤ T ≤ 140 K for fixed V = 2 V.A fixed bias V = 2 V is applied for 50 K ≤ T ≤ 140 K and V = 5 V is applied at 10 and 30 K to account for an injection barrier.The largest MR of −98% is measured at 10 K, which decreases with an increase in temperature.c) MR curves between 180 K ≤ T ≤ 300 K.At T = 180 K, the MR becomes positive with a value of 70% and decreases to 13.5% at room temperature.

Figure 3 .
Figure 3. Temperature-dependent charge transport of the polymer device.a) Current-voltage characteristics at 50 K from 0 to 2 T. The red arrow indicates the change in slope of the curve upon application of the magnetic field.b) Temperature-dependent conductivity measurements (10-300 K) of the polymer films.The inset shows the I-V behavior from 4 to 300 K and ohmic conduction at high temperatures.c) ln  versus 1000/T with Arrhenius fit from 100-300 K and 10-40 K, respectively.Different activation energies in these two regimes indicate that activation energy is not independent of temperature.d) Plot of resistance versus inverse temperature log R versus T −n with 10-60 K shown as the low-temperature region (LTR, light blue) and 65-300 K shown as the high-temperature region (HTR, light red).The black line is fit to Equation (3) in these two different regions.The LTR is well-defined by an ES-VRH mechanism where n = ½.The HTR region fits well by the Mott-VRH mechanism with n = 1/4.

Figure 4 .
Figure 4. Overall change in magnetoresistance and comparison of the MR of the DA CP in this work (orange star) with other organic materials.a) The overall change in resistance versus temperature.The MR is negative below 180 K and positive at or above 180 K.The data points represent the average value measured for four different DA CP films and the error bars represent the standard deviation from the data points.The size of the data point is small to show the clear standard deviation.b) Comparison of the room temperature MR response of the DA CP against closed-shell and open-shell moleculesand macromolecules in devices using non-magnetic electrodes.[33,34,[52][53][54][55][56][57][58]The materials denoted in red, blue, and green were tested in multilayer vertical devices with an ITO/PEDOT:PSS/Polymer/Ca architecture using an in-plane magnetic field, while the DA CP in this work was tested in planar device architecture and out of plane magnetic field.