English

• Stretchable polymer semiconductors have sparked significant interest, which enable low-cost, large-area and high-density device fabrication for promising applications in health monitoring, artificial skins and implantable bioelectronics [1–7]. Molecular design on polymer backbone or side chains is a very important approach to achieve intrinsically stretchable polymer semiconductors without the aid of elastomers [8–18]. However, it remains a formidable and long-standing challenge to achieve high stretchability without compromising charge transport properties because that loose packing with low crystallinity is conducive to stretchability while dense packing with high crystallinity benefits charge transport [19–22].

  Dynamic chemical bonds such as hydrogen bonds and metal coordination have been incorporated into conjugated polymers to afford high stretchability, in which the strain energy can be dissipated effectively through reversible bond cleavage and formation (Fig. 1a) [14,17,23–28]. Oh et al. introduced a pyridine dicarboxamide units into diketopyrrolopyrrole (DPP) based polymers to construct N–H···O···H–N hydrogen bonding network, which showed a hole mobility greater than 0.10 cm² V⁻¹ s⁻¹ at 100% strain in fully stretchable transistors [14]. The effects of H-bonding interactions on the mechanical properties and electrical performance of polymer semiconductor were systematically investigated by Gu and coworkers [29]. Zhen et al. designed a high-performance stretchable polymer semiconductor by in situ continuous hydrogen bonded engineering, which exhibited mobility up to 1.08 cm² V⁻¹ s⁻¹ under 100% strain [26]. Compared with hydrogen bonds, few examples based on metal coordination showed high stretchability without degrading carrier mobilities [30,31]. Wu et al. achieved stretchable, high-performance polymer semiconductors by dynamically crosslinking a donor–acceptor polymer via Fe-coordination with ~61% of initial mobility at 100% strain [30]. Han and coworkers demonstrated that the carrier mobility of ZnCl₂-treated semiconducting polymer films moderately decreased by 35% under 100% strain, much slower than that of the pristine films [31].

  Figure 1

  

  Figure 1.  (a) Design strategy of intrinsically stretchable polymers by multiple dynamic bonds. (b) Synthetic route of PDVT-BPDCA containing urethane and bipyridine units for both hydrogen bonding and metal coordination.

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  Multiple dynamic bonds are formed by two or more functional units with a wide distribution of the binding strength for physical junctions, promoting the dynamic cross-linking for intrinsic stretchability via synergistic effects (Fig. 1a) [32–36]. Unfortunately, multiple dynamic bonds have not yet been investigated to develop stretchable polymer semiconductors. Herein, we choose [2, 2′-bipyridine]-5, 5′-diylbis(methylene) bis((2-(5-bromothiophen-2-yl)ethyl)carbamate) (BPDCA-Br₂) as conjugation breaker containing urethane and bipyridine units for incorporation into the DPP donor−acceptor conjugated polymer backbone (Fig. 1b), providing hydrogen bonds and Fe-coordination to form multiple dynamic interconnected polymer network after treatment with Fe ion, which simultaneously improved the stretchability and carrier mobility. Using Stille coupling reaction, we have synthesized a series of terpolymers with different ratios of BPDCA moieties (P1-P3), which can proceed varying degrees of Fe-coordination (P2-Fe and P3-Fe) to optimize the tensile property and charge transport characteristics. The terpolymer P2 with 5% BPDCA exhibited significantly enhanced carrier mobility up to 1.12 cm² V⁻¹ s⁻¹ after addition of 1 equiv. of Fe ion while the terpolymer P3 with 10% BPDCA showed the maximum carrier mobility of 0.32 cm² V⁻¹ s⁻¹ when treated with 0.25 equiv. of Fe ion. The stretchability was also improved substantially after Fe-coordination, as manifested by nearly twofold increased crack-onset strain for the above two polymers. Notably, the Fe-coordinated polymer containing 10% BPDCA moieties (P3-Fe) exhibited almost unchanged carrier mobilities parallel to the stretching direction, with 91% of initial values even under 150% strain, which is the unprecedented value for intrinsically stretchable semiconducting polymers without blending of elastomers. Therefore, the introduction of multiple dynamic bonds can be used as a powerful strategy for intrinsically stretchable and high-performance polymer semiconductor.

  The polymers were synthesized by Stille polymerization of 3, 6-bis(5-bromothiophen-2-yl)-2, 5-bis(2-decyltetradecyl)pyrrolo[3, 4-c]pyrrole-1, 4(2H, 5H)-dione (DPP39), (E)-1, 2-bis(5-(trimethylstannyl)thiophen-2-yl)ethene (TVT) and the brominated conjugation breakers [2, 2′-bipyridine]-5, 5′-diylbis(methylene) bis((2-(5-bromothiophen-2-yl)ethyl)carbamate) (BPDCA-Br₂) as outlined in Fig. 1b and Scheme S1 (Supporting information). In case of terpolymers P2 and P3, the molar ratios of DPP39 units versus BPDCA moieties are 20:1 and 10:1 respectively. For comparison, P1 without conjugation breakers units was also prepared similarly. Next, the treatment with different proportions of Fe(BF₄)₂·6H₂O allowed the formation of Fe-coordination bonds. The experimental details are described in Supporting information.

  The obtained polymers exhibited high solubility in chlorobenzene or chloroform. Thermogravimetric analysis (TGA) in Fig. S1 (Supporting information) indicates that all polymers P2, P2-Fe, P3 and P3-Fe began to decompose at temperatures exceeding 380 ℃, demonstrating excellent thermal stability. High-temperature gel permeation chromatography (GPC) in Fig. S2 (Supporting information) revealed molecular weight (M_n) of 41.1 kDa for P2 and 27.8 kDa for P3.

  The charge transport properties of P1-P3 thin films with or without Fe-coordination were investigated using bottom-gate/top-contact (BGTC) field-effect transistor (FET) devices (Fig. 2a, Figs. S3 and S4 in Supporting information). The average carrier mobility of reference polymer P1 is 0.73 cm² V⁻¹ s⁻¹ with an I_on/I_off up to 10⁵. As conjugation breaker BPDCA segment was introduced into the polymer backbone, P2 showed the average mobility of 0.71 cm² V⁻¹ s⁻¹ at the same order of magnitude as P1 while P3 exhibited decreased mobility of 0.19 cm² V⁻¹ s⁻¹ due to the partial disruption of conjugation by incorporating more BPDCA moieties. For P2 polymer, a noticeable enhancement was observed in carrier mobility up to 1.12 cm² V⁻¹ s⁻¹ after addition of 1 equiv. of Fe ion (Fig. 2b). The increase in the intensity of the (0-0) band relative to the (0-1) band and the slight red shift in the UV absorption spectrum indicate stronger chain-chain interactions as well as a higher degree of backbone planarity after Fe-coordination (Fig. S5 in Supporting information) [37], which is beneficial for charge transport. In contrast, P3 polymer showed the maximum mobility up to 0.32 cm² V⁻¹ s⁻¹ by treatment with 0.25 equiv. Fe ion (Fig. 2b). Higher degrees of coordination probably disrupt the stacking of polymer chains, consequently affecting charge transport performance as confirmed by their UV absorption spectra in Fig. S6 (Supporting information). Particularly for P3-Fe by treatment with 1 equiv. Fe ion, a number of aggregated particles were formed as revealed by optical microscopy (OM) (Fig. S7 in Supporting information). In view of the electrical properties, we selected fully coordinated P2-Fe polymer and partially coordinated (0.25 equiv.) P3-Fe polymer for following research.

  Figure 2

  

  Figure 2.  (a) Configuration of bottom-gate/top-contact FET device. (b) Charge carrier mobilities and threshold voltages of FET devices prepared based on P1, P2 and P3 coordinated with metals in different ratios. (c) FT-IR spectra of P1, P2, P2-Fe, P3 and P3-Fe. (d) XPS Fe2p spectra of P2, P2-Fe, P3 and P3-Fe. (e) UV–vis/NIR absorption spectra of P2, P2-Fe, P3 and P3-Fe in thin film state.

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  We used IR spectroscopy to verify the presence of hydrogen bonds for our synthesized polymers (Fig. 2c). The peak around 3325-3332 cm⁻¹ can be assigned to the stretching vibration band of free N-H bonds in the urethane groups according to the literatures [38,39]. Hydrogen bonding peaks are usually observed at lower wavenumbers than free states owing to weakened C=O bonds. Thus, the peak in the range of 3230-3236 cm⁻¹ can be attributed to the stretching vibration band of hydrogen bonded N-H bond, indicating the formation of hydrogen bonds in the polymers P2, P2-Fe, P3 and P3-Fe. We further characterized Fe-coordination in the polymers of P2-Fe and P3-Fe using X-ray photoelectron spectroscopy (XPS) (Fig. 2d). Compared with P2 and P3, both P2-Fe and P3-Fe exhibited two strong peaks at 729.5 and 713.5 eV in the XPS spectra, which can be originated from the 2p_1/2 and 2p_3/2 signals of Fe(Ⅱ) respectively. At the same time, an obvious shift from 399.0 eV to 403.2 eV was observed for the nitrogen atom signal of the bipyridine group after coordination (Fig. S8 in Supporting information). These results suggested the successful introduction of metal coordination bonds into the polymers.

  The ultraviolet-visible/near-infrared (UV-vis/NIR) absorption spectra were measured to examine the optical and aggregation properties of P1-P3 thin films with or without Fe-coordination as shown in Fig. 2e. The lower energy 0-0 vibronic peak around 790 nm, which was typically attributed to interchain stacking [40], became blue-shifted with decreased relative intensity for P2, P2-Fe, P3 and P3-Fe with respect to the reference polymer P1, indicating the reduced degree of aggregation after induction of conjugation breaker BPDCA moieties [41], P2-Fe and P3-Fe showed increased I_0-0/I_0-1 than pristine films, suggesting stronger interchain stacking states that are beneficial to efficient charge carrier transport.

  In order to study the mechanical properties of terpolymers, the crack onset strains were measured in the following way. Thin films of corresponding polymers were spin-coated on octadecyltrichlorosilane (OTS) modified Si/SiO₂ substrates and then carefully transferred to polydimethylsiloxane (PDMS) substrates, which were stretched at different strains and fixed on Si/SiO₂ substrates to observe the film morphology and crystallinity. As measured by OM and atomic force microscopy (AFM) analysis in Figs. 3a and b, and Figs. S9-S14 (Supporting information), dense cracks appeared at only 50% strain for P1 and P2, and 75% strain for P3. In contrast, P2-Fe and P3-Fe exhibited crack onset strains of 100% and 150% respectively, demonstrating a significant improvement in stretchability after Fe-coordination.

  Figure 3

  

  Figure 3.  AFM images of P3 (a) and P3-Fe (b) under various strains. Dichroic ratio values of P2, P2-Fe (c) and P3, P3-Fe (d). (e) Relative degree of crystallinity (rDoC) values and coherence lengths of P1, P2, P2-Fe, P3 and P3-Fe. (f) Changes of the rDoC for P1, P3 and P3-Fe under strain parallel to strain direction. (g) Schematic diagrams illustrating mechanisms for energy dissipation during strain in the films with multiple dynamic bonds.

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  To confirm improved ductility of these films, dichroic ratios of the semiconductor films were subsequently measured under various strains via polarized UV–vis/NIR absorption spectroscopy (Figs. S15 and S16 in Supporting information). The dichroic ratio initially increased due to the strain-induced chain alignment and then decreased if the fracture propagation diminishes the alignment effect [42]. The dichroic ratio of P1 at 150% strain is only 1.33 and lower than that at 50% strain, which is attributed to the formation and growth of cracks (Fig. S17 in Supporting information). As shown in Figs. 3c and d, P2-Fe and P3-Fe thin films showed much larger dichroic ratios relative to the pristine films P2 and P3, indicating a higher degree of polymer chain alignment, which are in good agreement with the trend for crack formation under strains. We used polarization optical microscopy to further confirm the polymer chain arrangement (Figs. S18-S20 in Supporting information). The unstretched film exhibited isotropic behavior, appearing darkness in both polarization directions. The 150% stretched P3-Fe thin films showed pronounced anisotropic light transmission when rotated at 45° relative to the polarization direction, confirming significant strain-induced chain alignment. The tensile moduli of the polymers P1-P3, P2-Fe and P3-Fe were further measured by AFM nanomechanical mapping as shown in Fig. S21 (Supporting information). The elastic modulus decreases in the order of P1, P2, P2-Fe, P3 and P3-Fe, which corresponds well with the overall content of hydrogen bonds and metal coordination bonds in the polymers. The distinctly reduced tensile moduli after Fe-coordination are closely correlated with the enhanced crack-onset strain and increased dichroic ratio.

  We further investigated the thin film microstructures of all the polymers on Si/SiO₂ substrate using grazing incidence wide-angle X-ray scattering (GIWAXS) as shown in Fig. S22 (Supporting information). All polymers exhibited an edge-on orientation with out-of-plane (h00) lamella stacking peaks. The calculation of relative degree of crystallinity (rDoC) was performed by using the (200) intensity after normalizing thin film thickness, and the coherence lengths of crystalline ordered domains were estimated based on the Scherrer equation, as summarized in Fig. 3e (Table S1 in Supporting information) [43]. Compared with P1, P2 and P3 displayed decreased rDoCs and coherence lengths since the disrupted conjugation in the polymer backbone interfered with the long-range order packing. P2-Fe proceeded complete coordination with 1 equiv. of Fe ions, and the directional nature and strong bond energy of metal coordination bonds enable the polymer chains to form ordered arrangements in space, thereby enhancing crystallinity. We observed a distinct enhancement in coherence length for P2-Fe and P3-Fe in comparison with pristine polymers P2 and P3, suggesting the formation of higher ordered microstructures through metal coordination network, which may be related to the increase in carrier mobilities. We characterized the change of crystalline domains under strains by examining the lamellar stacking distances extracted from (100) peaks. The mismatch of d-spacing values along the parallel and perpendicular direction indicates the rupture of the lamellar structure of polymer chains because of crack formation [17]. Lamellar spacing distances along both parallel and perpendicular stretching directions became inconsistent at strain of 50% for P1, P2 and P3. Remarkably, P2-Fe and P3-Fe did not show a discernible difference in two directions until stretched at 150% strain, demonstrating superior stretchability (Fig. S23 in Supporting information). We observed that the rDoC values of P1, P2, P2-Fe and P3 decreased with strain from 0% to 50% and then plateaued or increased along the strain direction (Fig. 3f, Figs. S24 and S25 in Supporting information), suggesting the partial energy dissipation through breakage of the crystalline regions and further cracks formation at higher strain. In contrast, the rDoC of P3-Fe decreased steadily and slowly with increasing strain from 0% to 150%. These results are in line with the morphological and electrical characterizations under various strains.

  We hypothesize that the multiple dynamic interconnected polymer network synergistically constructed by hydrogen bonds and Fe-coordination are highly important to achieve excellent stretchability (Fig. 3g). Through a comparative analysis of the magnitudes of the bond energies associated with hydrogen bonds and metal coordination bonds, it becomes feasible to rationally anticipate the transformations of these bonds throughout the stretching process. For urethane, the bond energy of hydrogen bonding (O···H) is around 33.35 kJ/mol [44]. The bond length of Fe-N_bipyridyl between the bidentate ligand bipyridine and Fe²⁺ is around 1.97 Å [45]. Based on the bond valence–bond length correlation [46], we estimated that the bond energy of Fe-N_bipyridyl was 227.69 kJ/mol (Supporting information), much greater than that of hydrogen bonding. At the onset of stretching, the energy was dissipated by stretching coiled polymer chains, which were effectively aligned with increased dichroic ratios as revealed by polarized absorption spectroscopy. When applied strain increased, the energy was dissipated by the breakage of hydrogen bonds while stronger Fe-coordination bonds continue to maintain the integrity of the polymer. In the further stretching process, the metal coordination bonds begin to dissociate to dissipate the higher strain energy, thus showing notably enhanced stretchability [44].

  Finally, we fabricated fully stretchable transistors using respective polymers as the semiconductor to evaluate their electrical properties under mechanical strains. The device had a BGTC configuration using polystyrene-block-poly(ethyleneranbutylene)-block-polystyrene (SEBS 1062) as substrate, SEBS 1052 as a dielectric layer and conductive carbon nanotubes (CNTs) as the source, drain and gate electrodes (Figs. 4a and b). The detailed device fabrication procedures are described in Supporting information. The typical transfer curves for all the polymers at different strains are shown in Fig. 4c, Figs. S26 and S27 (Supporting information), and the corresponding FETs parameters are summarized in Tables S2-S4 (Supporting information). For P2 thin films, the average mobility parallel to the stretching direction decreased rapidly from 0.63 cm² V⁻¹ s⁻¹ to 0.16 cm² V⁻¹ s⁻¹ when the applied strain increased to 150%. A severe degradation was also seen in the case of P3 thin film. On the contrary, P2-Fe thin films exhibited a carrier mobility of 0.33 cm² V⁻¹ s⁻¹ under 150% strain, maintaining 60% of the initial value (Fig. 4d). Particularly, P3-Fe thin films demonstrated highly stable charge transport properties during stretching, retaining 91% of the initial mobility even at 150% strain (Fig. 4e and Fig. S28 in Supporting information), which is the highest value for intrinsically stretchable semiconducting polymers without aiding of elastomers (Fig. 4h) [13,14,47–53]. Although the mobilities of P2-Fe and P3-Fe perpendicular to the stretching direction decreased due to the strain-induced vertical chain alignment in the conduction channel as well as altered device geometry and dielectric capacitances (Table S5 in Supporting information), on-currents remained almost unchanged during stretching even at 150% strain. After released from stretching, both P2-Fe and P3-Fe recovered the carrier mobilities mostly, demonstrating the good mechanical robustness due to the formation of multiple dynamic interconnected polymer network. We further performed the stretching durability test for P3-Fe film at 50% strain. We observed that the field-effect mobility, on-current and off-current of our stretchable transistors did not suffer any noticeable degradation in performance even after 500 cycles in both parallel and perpendicular directions (Figs. 4f and g, Fig. S29 in Supporting information). These results demonstrated that hydrogen bonds and metal coordination cooperatively endowed the conjugated polymers with high stretchability without compromising charge transport characteristics.

  Figure 4

  

  Figure 4.  (a) Device structure of fully stretchable transistor. (b) Images of the fully stretchable transistor from 0% to 100% strain. (c) Transfer curves of P3-Fe film at different strains along the charge transport direction and perpendicular to charge transport direction. Field-effect mobilities as a function of various strains for P2 and P2-Fe (d), P3 and P3-Fe (e). (f) Transfer curves of the stretchable transistor under 50% strain after multiple stretch-release cycles along the charge transport direction and perpendicular to charge transport direction. (g) Field-effect mobilities, on-currents and off-currents as a function of stretching-releasing cycles along the strain direction. (h) Retention rate (μ/μ₀) of carrier mobility as a function of various strains for P3-Fe compared with those of intrinsically stretchable polymers reported in the literatures.

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  To be summarized, we have incorporated both hydrogen bonding groups and Fe-coordination units into semiconducting polymer backbones, synergistically facilitating the multiple dynamic cross-linking for high stretchability without compromising carrier mobilities. In addition to the nearly double increased crack-onset strain, metal coordination gave rise to stronger interchain stacking states, contributing to a significant improvement in carrier mobilities. Remarkably, the stretchable transistors based on the Fe-coordinated polymer demonstrated almost unchanged carrier mobilities parallel to the stretching direction and as well as highly stable on-currents perpendicular to the stretching direction even at 150% strain, without any noticeable degradation in performance even after 500 cycles in both directions. The incorporation of multiple dynamic chemical bonds into the conjugated polymers opens up a new possibility to develop high-performance intrinsically stretchable polymer semiconductors.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Gongxi Li: Writing – review & editing, Writing – original draft, Methodology. Jun Jin: Writing – review & editing, Writing – original draft, Methodology. Junxuan Tu: Formal analysis. Haoguo Yue: Methodology. Ying Wang: Methodology. Xiaohui Jia: Methodology. Weiyuan Yin: Formal analysis. Zhenglin Han: Formal analysis. Yuxuan Deng: Formal analysis. Chunfeng Shi: Methodology, Investigation. Yonggang Zhen: Writing – review & editing.

  Acknowledgments

  We acknowledge the 1W1A beamline of Beijing Synchrotron Radiation Facility and the Bl02U2 beamline of Shanghai Synchrotron Radiation Facility for GIWAXS measurements. We are grateful for the Fundamental Research Funds for the Central Universities (No. buctrc202103), the National Natural Science Foundation of China (Nos. 52373170, 22171019), Beijing Natural Science Foundation (No. 2252015), SINOPEC (No. 225057), Open Project Program of the State Key Laboratory of Fine Chemicals (No. KF2201, Dalian University of Technology).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111716.

  
  
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  Figure 1  (a) Design strategy of intrinsically stretchable polymers by multiple dynamic bonds. (b) Synthetic route of PDVT-BPDCA containing urethane and bipyridine units for both hydrogen bonding and metal coordination.

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  Figure 2  (a) Configuration of bottom-gate/top-contact FET device. (b) Charge carrier mobilities and threshold voltages of FET devices prepared based on P1, P2 and P3 coordinated with metals in different ratios. (c) FT-IR spectra of P1, P2, P2-Fe, P3 and P3-Fe. (d) XPS Fe2p spectra of P2, P2-Fe, P3 and P3-Fe. (e) UV–vis/NIR absorption spectra of P2, P2-Fe, P3 and P3-Fe in thin film state.

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  Figure 3  AFM images of P3 (a) and P3-Fe (b) under various strains. Dichroic ratio values of P2, P2-Fe (c) and P3, P3-Fe (d). (e) Relative degree of crystallinity (rDoC) values and coherence lengths of P1, P2, P2-Fe, P3 and P3-Fe. (f) Changes of the rDoC for P1, P3 and P3-Fe under strain parallel to strain direction. (g) Schematic diagrams illustrating mechanisms for energy dissipation during strain in the films with multiple dynamic bonds.

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  Figure 4  (a) Device structure of fully stretchable transistor. (b) Images of the fully stretchable transistor from 0% to 100% strain. (c) Transfer curves of P3-Fe film at different strains along the charge transport direction and perpendicular to charge transport direction. Field-effect mobilities as a function of various strains for P2 and P2-Fe (d), P3 and P3-Fe (e). (f) Transfer curves of the stretchable transistor under 50% strain after multiple stretch-release cycles along the charge transport direction and perpendicular to charge transport direction. (g) Field-effect mobilities, on-currents and off-currents as a function of stretching-releasing cycles along the strain direction. (h) Retention rate (μ/μ₀) of carrier mobility as a function of various strains for P3-Fe compared with those of intrinsically stretchable polymers reported in the literatures.

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