To date, many high-mobility semiconducting polymers are based on the copolymerization of “large fused aromatics” and “small-size aromatics”. Thus, we collected potential polymer building blocks from recently published reviews22,29 and separate them into two groups: “large fused aromatics” and “small-size aromatics” (Fig. 2a-b). Based on the above assumptions, we first performed DFT calculations to obtain their ΔES-T values. Among all the building blocks, TDPP, BDOPV, TQ and BBT exhibits the smallest ΔES-T. Then, we performed DFT calculations to estimate the planarity of the polymer building block combinations by using a recently developed planarity indexes ⟨cos2φ⟩30. We found that TDPP can form planar polymer backbone with TQ and BBT with high torsional barriers and large ⟨cos2φ⟩ (Fig. 1c). Whereas when BDOPV was copolymerized with TQ and BBT, the resulting polymers show small ⟨cos2φ⟩ values, suggesting the polymers significantly deviate from planarity (Fig. S2). Backbone planarity is important for good conjugation and also crucial for efficient intra- and interchain charge transport. To have a systematically understanding of the relationship between molecular structure and open-shell property, we choose TDPP as the “large fused aromatic”, and BT, TQ and BBT with decreased ΔES-T values, as the “small-size aromatic” to construct polymers. Note that TDPP, BT, TQ and BBT are usually consider as “acceptors” in conjugated polymers. Unlike traditional donor-acceptor design27, this “acceptor-acceptor” design allows the polymers to have enough bandgap to show semiconducting rather than conducting property.
The three polymers, p(TDPP-BT), p(TDPP-TQ) and p(TDPP-BBT), were synthesized via Pd-catalyzed Stille polymerization reaction between the trimethyltin TDPP and the dibromo compounds of BT, TQ and BBT (Fig 3a). The synthesis and purification procedures are detailed in Supplementary Materials. The stability of those polymers was proved by thermogravimetric analysis, and all the polymers showed high decomposition temperatures over 350 ℃ (Fig. S4). Three polymers show gradually red-shifted absorption spectra as decreasing the ΔES-T of the “small-size aromatic” (Fig. 3b). The HOMO/LUMO energy levels of p(TDPP-BT), p(TDPP-TQ) and p(TDPP-BBT) obtained by cyclic voltammogram (CV) measurements are −5.34/−3.55, −5.23/−3.94, and −5.20/−4.17 eV, respectively (Fig. S3 in SI). The increase of the HOMO and the decrease of the LUMO level is probably due to the enhanced planarity, better conjugation, and more readily aromatic-to-quinoidal transformation (Fig. 3c and Fig. S2).
The high-spin characteristics of the polymers were investigated by electron paramagnetic resonance (EPR) and superconducting quantum interference device (SQUID) both in solution and in solid state. At room temperature, the solid-state EPR intensity increases dramatically in the order of p(TDPP-BT), p(TDPP-TQ) and p(TDPP-BBT) (Fig. 4a), which reflects an increasing of spin density. More importantly, the EPR intensities showed negligible change after storing the polymers in air for 90 days, suggesting the high air stability of these open-shell polymers (Fig. S5). The EPR intensity of p(TDPP-BT) is very weak and shows a closed shell feature, and thus we will not study this polymer in detail. To compare the spin dynamics of p(TDPP-TQ) and p(TDPP-BBT), temperature-dependent EPR measurements in solid state were performed (Fig. 4b, S5a). Their EPR intensities decrease with the increase of temperature, suggesting that they probably have a triplet ground state1. It is known that if there exists a triplet state ground state, the EPR signal of |Δms| = 2 forbidden transition can be observed, even with very weak intensity1. To determine the existence of triplet ground state in the polymers, the |Δms| = 2 forbidden transition is measured. For p(TDPP-BBT), a low intensity but clear signal at |Δms| = 2 is observed (Fig. 4c), suggesting the existence of triplet ground state. For p(TDPP-TQ), the forbidden transition could not be observed, which is largely due to its relatively weak EPR intensity compared to p(TDPP-BBT). Furthermore, the variable temperature EPR of both polymers in solution were also measured and showed a similar behavior compared to solid state EPR. The EPR intensities of p(TDPP-TQ) and p(TDPP-BBT) decrease with the increase of temperature (Fig. 4d, S5b). The EPR data of p(TDPP-BBT) from 4.9 K to 50 K were fitted by Bleaney-Bowers equation, which provides an energy gap between singlet and triplet (ΔES-T) of 4.92×10−3 kcal mol−1 (J = 0.86 K) (Fig. 4e).
To further investigate the static magnetic properties of the polymers, temperature--dependent magnetic susceptibility was measured by SQUID from 2 K to 300 K under DC field (Fig. 4f). The values of the product of magnetic susceptibility and temperature (χMT) increase linearly with temperature. This feature usually suggests strong antiferromagnet coupling or temperature-independent paramagnetism (TIP). Due to the relatively low calculated spin density and weak spin-orbit coupling in organic polymers, this phenomenon is usually attributed to TIP31,32. TIP comes from Curie and Pauli paramagnetism (χtotalT = C + χPauliT), where C is Curie paramagnetism and T is temperature. For organic molecule, Curie paramagnetism is negligible. Thus, we can calculate the χPauli of p(TDPP-TQ) and p(TDPP-BBT) to be 8.2×10−4 and 3.5×10−3 cm3/mol, respectively. From the relation χPauli = μB2N(EF), where μB is the Bohr magneton, the densities of states at the Fermi energy N(EF) for those p(TDPP-TQ) and p(TDPP-BBT) were calculated to be 1.5× 1022 and 6.5 × 1022 eV−1 cm−3. The Pauli paramagnetism and large value of N(EF) reveal that the spins in these polymers are highly delocalized.33,34 In particular, for p(TDPP-BBT), the χMT and T are slightly deviated from the linear relationship at low temperature (red circle in Fig. 4f). This behavior could be due to the weak antiferromagnetic coupling between polymer chains, which we will prove later by studying the aggregation behavior of the polymers. To determinate the spin ground state of those polymers, field-dependent magnetization measurements (M-H) were performed. For p(TDPP- TQ), it shows a close to S = 1 triplet state (Fig. 5a, S7), consistent with EPR results; while for p(TDPP-BBT), it shows a close to S = 1/2 doublet state in the measurement (Fig. 5b, S8). It seems that this result conflict with the EPR measurement. However, we will show that this phenomenon can be well explained with the high spin density and strong interchain interactions in p(TDPP-BBT).
It has been reported that conjugated polymers with rigid backbones are strongly aggregated even in dilute solutions35,36. The strong intermolecular interaction will affect spin delocalization31. To understand the polymer chain interactions, we performed the temperature-dependent UV-vis absorption spectra of p(TDPP-TQ) and p(TDPP-BBT) (Fig. S9-S10). We and others have shown that in good solvent (dissolving polymers better), conjugated polymers can be more readily disaggregated at elevated temperature35,37. Therefore, three solvents, toluene, o-dichlorobenzene (o-DCB), and 1-chloronaphthalene (1-CN), with increased solubility for conjugated polymers were used. For both polymers, the maximum absorption peak decreases with increasing temperature and decrease more significantly in good solvent, such as 1-CN, suggesting that both polymers are strongly aggregated in solution. The strong aggregation of the polymer supports the observed large N(EF) values, which could explain the TIP phenomenon. Previous study has shown that strong spin-spin interactions in triplet small molecules could result in doublet state in magnetization measurements15. p(TDPP-TQ) has a low spin density and direct spin-spin interaction can hardly happen (Fig. 5c), thus exhibiting a triplet ground state in solid state. In p(TDPP-BBT), the high spin density makes the spin at the chain end can directly interact with each other, leading to an apparently doublet ground state, consistent with the field-dependent magnetization measurement. Therefore, both spin density and interchain interaction contribute to the observed different spin ground states.
To further understand the magnetic properties, density functional theory (DFT) calculations were performed on their oligomers (n = 6). The calculated ΔES-T of the oligomers were −19.50 kcal/mol for p(TDPP-BT), −3.69 kcal/mol for p(TDPP-TQ), and −0.14 kcal/mol for p(TDPP-BBT), which is consistent with the above observed trend that p(TDPP-BBT) could have very small ΔES-T value and a triplet ground state. The significant decrease of ΔES-T indicates the increased stability of the open-shell triplet ground state, which agrees well with the observed EPR results and magnetic property. The large absolute ΔES-T value of p(TDPP-BT) lead to the inaccessible of the triplet state even at room temperature. While for p(TDPP-TQ) and p(TDPP-BBT), the small ΔES-T value of their oligomers could make the polymers have triplet ground state if the polymers have high degree of polymerization and strong interchain interactions. The singlet diradical character index (y0), which is often used to estimate the diradical property, was also calculated38,39. The value of y0 ranges from 0 to 1; 0 means closed-shell, while y0 = 1 stands for pure diradical. The calculated y0 values of (TDPP-BT)6, (TDPP-TQ)6, and (TDPP-BBT)6 are 0.012, 0.350, and 0.996 (Table S2-S4). The difference of the spin triplet states for each polymer can be visualized by the spin density distribution in Fig. 4g. From (TDPP-BT)6 to (TDPP-BBT)6, the triplet spin density distribution changes from localized in the center, to uniformly distributed along the chain, and to mostly distributed at the end of the chain. This result agrees well with the increase of the y0 value (Fig. S13-S15). The Bond length alternation analysis (Fig. S16-S18) also shows that the bond length difference between open-shell singlet and triplet of (TDPP-BT)6 become obvious only in the middle of molecule. For (TDPP-TQ)6, it emerges throughout the entire oligomer. For (TDPP-BBT)6, the bond length difference is negligible, which is consistent to its small ΔES-T value.
Based on the magnetic properties and DFT calculations, the following conclusion can be obtained: (1) p(TDPP-BT) has a singlet ground state because of its large ΔES-T value; (2) p(TDPP-TQ) and p(TDPP-BBT) all showed triplet ground state because of their oligomers’ intrinsically small ΔES-T values; (3) Because of the strong interchain interactions and separately distributed spin-density in p(TDPP-BBT), it shows doublet state in field-dependent magnetization measurement; whereas the low spin density and relatively uniformly distributed spin density in p(TDPP-TQ) make it exhibit triplet state.
The charge transport properties of the polymers were evaluated by field-effect transistors (FET) with a top-gate/bottom-contact (TGBC) configuration (Fig. 6e, Fig. S1&Fig. S19-S21). Unlike previously reported high-spin polymers with conducting properties27, our polymers show typical semiconducting properties with good ambipolar charge transport properties. The polymers also showed good on/off ratios if the VDS is small. p(TDPP-BT) exhibits good electron mobilities of up to 3.83 cm2 V−1 s−1 and hole mobilities of up to 2.77 cm2 V−1 s−1. p(TDPP-TQ) exhibits high electron mobilities of up to 7.76 cm2 V−1 s−1 and high hole mobilities of up to 6.16 cm2 V−1 s−1. These values are the highest reported to date in high-spin ground state polymers, and also among the highest in all reported organic semiconductors22. p(TDPP-BBT) shows relatively lower electron mobilities of 0.25 cm2 V−1 s−1 and hole mobilities of 0.37 cm2 V−1 s−1. To date, only a few reported open-shell small molecules have shown moderate charge carrier mobilities, usually on the order of 10−3 cm2 V−1 s−1 (Fig. 1 and Table S5)10,15,16,17,40. Therefore, our approach successfully addressed the challenge to design high-mobility and high-spin ground-state organic semiconductors.
Grazing incidence wide-angle X-ray scattering (GIWAXS) and atomic force microscopy (AFM) were employed to study the microstructures and morphology of the polymer films. (Fig. 6a-6c). All the polymers show typical edge-on dominated molecular packing in solid state. p(TDPP-BT) shows the narrower full width at half maximum (FWHM) (Fig S24 and Table S6) with a lamellar distance of 21.67 Å and a π-π stacking distance of 3.57 Å, suggesting its higher crystallinity. p(TDPP-TQ) shows wider FHWM with a lamellar distance of 20.94 Å and a π-π stacking distance of 3.70 Å. For p(TDPP-BBT), it shows a similar lamellar distance of 21.67 Å but larger π-π stacking distance of 3.88 Å. AFM height images show that the polymer films have very smooth surface with root-mean-square surface roughness < 1 nm (Figure S22). Clearly, the charge transport properties of the three polymers are not strongly correlated with their molecular packings. This phenomenon is common in conjugated polymers, because many studies have shown that crystallinity and π-π stacking distance of polymers are not the only parameters affecting their charge transport properties, and other parameters, such as interchain short contacts41, packing conformation42, and energetic disorders43, also strongly influences charge carrier mobilities.
Previous studies suggest that spin-spin interactions in materials could lead to enhanced thermopower in some thermoelectric materials44,45. Since organic thermoelectric materials are usually achieved under doped states, we explored the charge transport properties after doping. We found that p(TDPP-TQ) can be both effectively n-doped and p-doped. For n-doping, a commonly used n-dopant, N-DMBI, was used. After optimizing the doping concentration and annealing temperature, the n-type electrical conductivity of p(TDPP-TQ) polymer achieved 16.1 S/cm (Fig. 6e), which is among the highest in n-doped organic semiconducting polymers46,47. P-doping was performed by immersing p(TDPP-TQ) films in a 10 mM FeCl3 solution. The polymer can be easily p-doped as observed in the UV−vis−NIR absorption spectra (Fig. S23). By varying the immersion time, the polymer showed a p-type electrical conductivity of 348.3 S/cm (Fig. 6f), which is also among the highest in p-doped conjugated polymers. One polymer that can achieve both high n-type conductivity and p-type conductivity after doping is rare in literature48,49. Such unique doping behavior of p(TDPP-TQ) could be probably due to the ease of accepting and donating electrons of the triplet state. These results also suggest the great potential of using high-spin semiconducting polymers in the applications requiring heavily doping, e.g. organic thermoelectrics and organic bioelectronics48,50,51.