A supramolecular polymeric heterojunction composed of an all-carbon conjugated polymer and fullerenes

Abstract

Herein, we design and synthesize a novel all-carbon supramolecular polymer host (SPh) containing conjugated macrocycles interconnected by a linear poly(para-phenylene) backbone. Applying the supramolecular host and fullerene C₆₀ as the guest, we successfully construct a supramolecular polymeric heterojunction (SPh⊃C₆₀). This carbon structure offers a means to explore the convex–concave π–π interactions between SPh and C₆₀. The produced SPh was characterized by gel permeation chromatography, mass spectrometry, FTIR, Raman spectroscopy, and other spectroscopies. The polymeric segment can be directly viewed using a scanning tunneling microscope. Femtosecond transient absorption and fluorescence up-conversion measurements revealed femtosecond (≪300 fs) electron transfer from photoexcited SPh to C₆₀, followed by nanosecond charge recombination to produce the C₆₀ triplet excited state. The potential applications of SPh⊃C₆₀ in electron- and hole-transport devices were also investigated, revealing that C₆₀ incorporation enhances the charge transport properties of SPh. These results expand the scope of the synthesis and application of supramolecular polymeric heterojunctions.

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Introduction

The design and fabrication of heterojunctions in varying dimensions have attracted much attention for many applications, such as transistors, solar cells, solid-state lasers, and diodes.^1–6 It is worth noting that organic heterojunctions played very important roles in organic thin-film solar cells.⁷ Various organic electron-donating molecules, such as thiophenes,⁸ porphyrins^9,10 and tetrathiafulvalene derivatives¹¹ can construct C₆₀-based donor–acceptor (D–A) heterojunction systems, which can effectively suppress aggregation of fullerene derivatives and facilitate energy conversion. Imahori and co-workers reported porphyrin/C₆₀ composite clusters and discovered photocurrent generation under light irradiation.^12,13 Inspired by these studies, crown-shaped small molecular heterojunctions were constructed using curved nanographene host molecules for photocurrent generation.¹⁴ Supramolecular polymers are regarded as an important source of intelligent materials and show broad applications.^15–19 These supramolecular structures can be formed by various different interactions, such as metal coordination,^20,21 π–π stacking,²² host–guest recognition,^23–25 and hydrogen bonding.^26,27 Würthner and co-workers achieved a co-assembled p–n-junction on a nanoscopic scale containing a p-type oligo(p-phenylene vinylene) donor and an n-type perylene bisimide acceptor, which are initially formed via hydrogen bonding and subsequently self-assemble into chiral stacks by π–π interactions.²⁸ Aida and co-workers reported an unusual supramolecular polymerization reaction upon heating as well as cooling using a metalloporphyrin-based tailored monomer to investigate its stimuli-responsive applications.²⁹ Recently, frustrated Lewis pairs were implemented into supramolecular polymers combined with π-conjugated O-bridged triphenylborane and triphenylamines by Meijer and co-workers.³⁰ The Yang group reported novel supramolecular polymeric nanoparticles comprised of chromophoric guest molecules and disulfidebridged bispillar[5]arene to construct a light-harvesting system.³¹ However, the construction of highly ordered, multi-stage, multilayered polymeric supramolecular systems is still an unsolved challenge.^32,33 The structural effects of charge mobilities in supramolecular polymeric heterojunction systems have been rarely explored.

Conjugated polymers (CPs) have attracted great interest for various optoelectronic applications due to their semiconductivity in pristine (neutral) or doped states.³⁴ CPs generally exhibit an efficient absorption or emission at the band edge.³⁵ For example, excitons with bounded electron–hole pairs are created to emit strong luminescence when photons are absorbed in poly(phenylenevinylene) materials.³⁶ The wide band-gap poly(p-phenylene) (PPP) materials have a rigid-rod character and contain very stable benzene rings.³⁷ Therefore, much attention has also been paid to their synthesis and electronic properties.^38–43 Linear or cyclic benzene rings in PPP and cyclo-para-phenylene (CPP) macrocycles can be considered as conjugated segments of graphene nanoribbons and single-walled carbon nanotubes (SWCNTs).^44–49 In addition, it has been found that these macrocycles show good supramolecular interactions with fullerenes such as C₆₀ (ref. 50 and 51) and C₇₀.⁵² Their electron transfer properties between a macrocycle and fullerenes have been recently investigated by von Delius and Guldi,^53,54 Martín,⁵⁵ and our group.^52,56,57 Thus, integrating the physical properties of macrocycle-based conjugated polymers and fullerenes to make supramolecular polymeric heterojunctions could have many interesting properties and applications in molecular electronics and materials.⁵⁸

Herein, we report design and synthesis of a novel PPP-based supramolecular polymeric heterojunction containing conjugated macrocycles interconnected by a PPP backbone, which acts as a supramolecular host to form a host–guest supramolecular polymeric heterojunction with fullerene C₆₀ (SPh⊃C₆₀), representing the first polymeric segment of the supramolecular heterojunction of carbon nanopeapods (Fig. 1). In specific, the conjugated macrocycles mimic the curved polyphenylene-C₆₀ peapod segment of a CNT, and the linear PPP backbone is the linear polyphenylene part along the CNT. This polymer was fully characterized by different techniques. In addition, the supramolecular interactions between SPh and fullerene C₆₀ were investigated. Moreover, transient absorption measurements were performed to investigate the charge transport dynamics between SPh (electron donor) and C₆₀ (electron acceptor). The charge transport properties of the supramolecular polymeric heterojunction were examined using the space charge limited current (SCLC) method, revealing that C₆₀ incorporation enhances the charge transport properties of SPh.

Fig. 1 The design of a novel conjugated polymer (SPh) containing a conjugated macrocyclic host and a poly(para-phenylene) (PPP) backbone, representing the first supramolecular polymeric heterojunction as the segment of carbon nanopeapods.

Results and discussion

Molecular design of SPh

To obtain the target supramolecular polymeric heterojunction as the segment of carbon nanopeapods with desired structural features, the appropriate functional building unit for π-extension should be rationally selected. Thus, a bifunctional conjugated macrocyclic host with the reaction sites in the para position will be a good candidate for both linear polymeric π-extension and supramolecular interaction. A previous study showed that a cyclic precursor can react with the linear component containing three phenyl rings.⁵⁶ Then, a tetra-substituted phenylene unit with different reaction sites can connect with the cyclic molecule by the Suzuki–Miyaura coupling reaction to achieve the bifunctional monomer and subsequently to form a long π-conjugated PPP-based polymeric segment of CNTs. Once fullerene molecules are embedded into CPP moieties of the conjugated polymer, a polymeric segment of carbon nanopeapods can be achieved.

Synthesis of SPh

The synthesis procedure for SPh is summarized in Fig. 2. Compound 2′,5′-dibromo-4,4′′-dichloro-1,1′:4′,1′′-terphenyl (1) was prepared according to our previously reported procedure.⁵⁶ Then, two bromo groups in 1 were transformed into 4,4,5,5-tetramethyl-1,3,2-dioxaborolane groups by Pd-catalyzed borylation to produce compound 2 in a yield of ∼60%. Subsequently, linear compound 2 was embedded into the ring-shaped structure to generate the monomer for polymerization. Under the standard conditions for a Pd-catalyzed Suzuki coupling reaction, compound 2 reacted with the curved precursor 3 to synthesize the key macrocycle intermediate 4, which was successfully converted into the targeted macrocyclic monomer 5, a light yellow solid, by an aromatization reaction. All these small molecules were fully characterized by NMR, and MS (Fig. S16–S23†). Finally, a polymerization reaction using compound 5 was carried out in dry DMF by a nickel-mediated Yamamoto homocoupling reaction, and the polymer SPh was then obtained as an intense yellow solid in a yield of ∼72%.

Fig. 2 Synthesis procedure for compound 5 and the polymeric segment of carbon nanopeapods (SPh and SPh⊃C₆₀).

Characterization of the SPh and SPh⊃C₆₀

The successful synthesis of π-extended polymer SPh from compound 5 was confirmed by gel permeation chromatography (GPC), FTIR, mass spectrometry, and Raman spectroscopy. After the polymerization reaction, Soxhlet extractions with methanol and acetone were performed to remove lower molecular weight oligomers; the GPC molecular weight data for SPh are presented in Fig. 3a and S1.† The weight average molecular weight (M_W), relative number-average molecular weight (M_n), and polydispersity index (PDI) of SPh were measured, and the results showed that M_n = 10 768 g mol⁻¹ using polystyrene as the standard. This weight is approximately equal to 11 repeating units. The large PDI value at 2.66 suggested a broad molecular weight distribution. The mass spectra are shown in Fig. S2 and S7,† demonstrating different lengths of the polymeric molecules with roughly similar mass distribution results before and after the chromatographic purification of the polymer. This is probably due to the high molecular weight, low solubility, and some aggregation in SPh, which make purification of each oligomer very difficult. MALDI-TOF MS spectrometry data of SPh show a series of equidistant mass peaks in the high-molecular-weight fraction with up to 17 repeating units. The m/z and interval values (Δm/z = 910.36) are consistent with the calculated values, indicating the successful polymerization of compound 5.

Fig. 3 (a) GPC trace of SPh. (b) Raman spectra (excited at 785 nm) of SPh (wine) and 5 (dark cyan). (c and d) FTIR spectra of SPh (red) and compound 5 (blue). (e) A high resolution STM image showing the polymer segment SPh. The sample was prepared by the simple drop casting method. Imaging conditions: U = 1.3 V, I = 20 pA. (f) A smaller area STM image showing the existing morphology of the target polymer segment with the hoop-shaped structure.

Moreover, we compared the GPC traces of SPh⊃C₆₀ in different solvents (Fig. S10†). Only small oligomers are present, probably due to the poor solubility of SPh⊃C₆₀ in chloroform (CHCl₃) and N,N-dimethylformamide (DMF) (Fig. S10c†). It should be noted that the concentration for GPC was approximately 5 mg mL⁻¹ and substantial insolubilities in the concentrated solutions cannot be introduced into the mobile phase. Thus, the results only reflect smaller soluble oligomers dissolved in CHCl₃ and DMF. In addition, SPh⊃C₆₀ was further characterized by FTIR (Fig. S8†). C₆₀ contains the C–C bond and C=C bond, and four absorption peaks are found in the FTIR spectra of SPh⊃C₆₀ at 526, 576, 1161, and 1430 cm⁻¹, respectively. Compared with the FTIR spectrum of SPh, the results suggest the formation of a supramolecular polymeric heterojunction (SPh⊃C₆₀). Raman spectroscopy was also used to characterize SPh⊃C₆₀. In Fig. S9,† the Raman dominant peak of SPh⊃C₆₀ at 1469 cm⁻¹, indicating the stretching mode of cages of C₆₀, slightly shifted compared with that of C₆₀ (1468 cm⁻¹), which is similar to C₆₀-β-CDP reported by Diao and co-workers.^59,60 These results indicate that the formation of supramolecular complexes (SPh⊃C₆₀) did not change the nature of C₆₀. No appreciable Raman peaks of SPh can be recognized in SPh⊃C₆₀, probably due to the strong fluorescence background signal of SPh.

Raman spectroscopy was further used to characterize SPh. Fig. 3b, S3, and S4† show the Raman spectra of SPh and monomer 5. A strong Raman G-band appears at approximately 1600 cm⁻¹ for both SPh and compound 5, which probably arises from the symmetrical vibration caused by the collective C–C stretching modes of the benzenoid rings along the transverse tube axis. The D-band at ∼1200 cm⁻¹ mainly originates from the in-plane C–H bending modes, and those around 1270 cm⁻¹ correspond to ring breathing modes. In addition, the main peaks of [10]cyclo-para-phenylene were at 1206, 1265, and 1582 cm⁻¹,⁶¹ and the signals of linear PPP are located at 1215, 1285, and 1602 cm⁻¹.⁶² Some of these peaks are quite similar to signals of SPh, which further confirms the existence of cyclo-para-phenylene macrocycle and one-dimensional PPP structures. All these results suggest the polymerization of monomer 5 and the formation of SPh.

The FTIR spectra also demonstrate the successful polymerization of compound 5 into SPh (Fig. 3c, d, S5 and S6†). Fig. 3c shows the C–H stretching of 5 and SPh in the high wavenumber region. Three obvious peaks (3026 cm⁻¹, 2921 cm⁻¹, and 2850 cm⁻¹) were observed for compound 5. In sharp contrast, only the peak located at 2913 cm⁻¹ appears for SPh. In the FTIR spectra of the low wavenumber region (Fig. 3d), significant differences were also observed for SPh (mainly four peaks at 1260 cm⁻¹, 1093 cm⁻¹, 1017 cm⁻¹, and 800 cm⁻¹) and compound 5 (two sharp peaks at 1467 cm⁻¹ and 813 cm⁻¹, and one broad band maximized at 1095 cm⁻¹). Similar peaks at 813 cm⁻¹ for 5 and 800 cm⁻¹ for SPh are associated with out-of-plane C–H deformation from the vibration of a para-disubstituted benzene ring. Moreover, the peak at 1467 cm⁻¹ for compound 5 is completely gone after polymerization. A new peak at ∼1260 cm⁻¹ appears in SPh, consistent with the FTIR spectrum of PPP polymers.⁶²

The polymeric chains of SPh composed of a conjugated hoop-shaped structure were further confirmed by the scanning tunneling microscope (STM) imaging technique at the submolecular level. Using a drop casting method, the high-resolution STM image with a singly dispersed carbon nanohoop structure is shown in Fig. 3e. Many hoop-shaped structures with a disordered arrangement were adsorbed onto the surface, as well as a lot of aggregated particles with bright spots. To clearly observe the morphology of the polymeric segment, an enlarged image is shown in Fig. 3f. To test the composition of these tiny spots, an energy-dispersive X-ray spectroscopy (EDX) experiment was conducted and the results showed that these tiny spots should be carbon-containing moieties (see Fig. S11†). The polymeric chain containing around seven conjugated macrocycles can be identified and the hoop-shaped structure was clearly recognized, confirming the contracture of SPh.

Photophysical properties of SPh

The photophysical properties of SPh and monomer 5 were investigated in solution by UV-vis absorption spectroscopy, steady-state fluorescence spectroscopy, and time-resolved fluorescence decay spectroscopy (Fig. 4). The absorption spectrum of SPh showed an intense absorption band maximized at ∼326 nm with a shoulder band in the range of 372–432 nm, which is slightly redshifted compared to 5 (maximized at ∼317 nm). Because of the same CPP skeletons, SPh and 5 show nearly the same fluorescence spectra with maximum peaks at ∼470 nm and 465 nm, respectively. The fluorescence quantum yields determined for compound 5 and SPh are Φ_F = 23% for SPh and Φ_F = 14% for 5 (using anthracene in ethanol as reference, Φ_F = 30%). Both values are much lower than that of [10]CPP (Φ_F = 65%),⁶³ probably because the functionalized benzene rings increase the proportion of nonradiative decay of the excited states. The fluorescence colors of SPh and 5 are shown in Fig. 2b, indicating their similarity on emitting blue fluorescence.

Fig. 4 (a) UV-vis absorption (solid line) and fluorescence (dot line) spectra of SPh (red) and compound 5 (blue) in CH₂Cl₂. (b) SPh and compound 5 in CH₂Cl₂ solution under UV irradiation at 365 nm. Emission lifetimes for compound 5 (c) and SPh (d) in CH₂Cl₂.

The luminescence lifetimes (τ_s) of SPh and 5 were further measured by the time-resolved photoluminescence (TRPL) technique using a nanosecond pulsed laser system (Fig. 4c and d). The fluorescence decay of 5 follows first-order kinetics with a lifetime τ_s = 2.7 ns at 469 nm when excited at ∼300 nm. SPh shows a similar single-exponential decay with a much longer fluorescence lifetime of τ_s = 18.7 ns. Based on the above results, the similarity of these optical properties for SPh and 5 in solution indicates that the motif of cyclo-para-phenylene plays an important role.

Supramolecular chemistry and ultrafast excited-state dynamics

For a long time, fullerenes have been widely studied as guests in various organic host architectures owing to their unusual shape (approximately spherical) and chemical nature (nonpolar polyenes).⁵⁵ These organic host architectures have specific sizes for C₆₀ to construct various host–guest molecular systems via supramolecular interactions and weak supramolecular van der Waals interactions, such as supramolecular organic frameworks,⁶⁴ cyclodextrin,⁶⁵ calix[8]arene,⁶⁶ crown ether,^67,68 macrocyclic bis-exTTF hosts,⁵⁰ and nanorings.^51,69 Cyclo-para-phenylene with ten phenyl groups has an appropriate size to complex with C₆₀, as evidenced by single crystal X-ray diffraction analysis and NMR studies.^69,70 These results confirmed that such a conjugated macrocycle has the right size to selectively encapsulate C₆₀. Recently, π-extended crown-shaped CPPs⁵⁶ and porphyrin-embedded nanorings⁷¹ showed highly enhanced binding constants with fullerenes C₆₀. Their supramolecular structures with C₆₀ can also be directly observed by single-crystal X-ray diffraction. All these results make it possible for SPh to form a host–guest supramolecular polymeric heterojunction with fullerene C₆₀ (SPh⊃C₆₀).

Based on its structure, SPh contains a macrocyclic unit for C₆₀ to construct the designed carbon nanopeapod segment. We performed UV-vis experiments and fluorescence quenching experiments on the C₆₀ titration to confirm the existence of the supramolecular interactions between SPh and C₆₀. When C₆₀ was added into a SPh solution in toluene, the solution color obviously changed from yellow to brown. Upon C₆₀ titration into the SPh solution, the absorption spectra of SPh showed an obvious enhancement in the range of 280–430 nm (Fig. 5a). The absorption band maximum at ∼330 nm should be mainly due to the presence of C₆₀. Also, the fluorescence intensity was significantly reduced under a hand-held UV lamp excited at 365 nm; measurements of its emission intensity during titration (Fig. 5b) suggest that fullerene C₆₀ can efficiently quench the fluorescence. Monomer 5 was chosen to study the supramolecular interaction with C₆₀ by fluorescence spectroscopy. The titrating results showed that its K_a is ∼1.02 × 10⁵ M⁻¹ (Fig. S12†). The interaction between monomer 5 and C₆₀ was further observed by ¹H NMR spectroscopy when C₆₀ was added into monomer 5 in CDCl₃ (Fig. S13†). The benzene signals shifted downfield by ∼0.07 ppm. These results confirmed that monomer 5 could act as a supramolecular host with C₆₀ to form 5⊃C₆₀, suggesting that polymer SPh can also interact with C₆₀ to form SPh⊃C₆₀. Mass spectrometry clearly shows the formation of the monomer 5⊃C₆₀ complex (Fig. S14†).

Fig. 5 (a) UV-vis absorption spectra of SPh (1.67 × 10⁻³ mg mL⁻¹) in the presence of C₆₀. (b) Fluorescence spectra of SPh (1.67 × 10⁻³ mg mL⁻¹) in the presence of C₆₀. The concentrations of C₆₀ are 0.00–5.50 × 10⁻⁶ M. The solvent is toluene.

To understand the detailed quenching mechanisms between the photoexcited SPh host and C₆₀ guest, we performed femtosecond transient absorption (TA) and fluorescence upconversion (FU) measurements (see ESI† for experimental details). In these experiments, the samples were pumped at 350 nm, which can excite both SPh and C₆₀ species. Fig. 6a plots the TA spectra of SPh at the indicated pump–probe delays, displaying a negative feature within 400–450 nm and broad-band absorptive features extending from 500 nm to the near-infrared (NIR) region. We assign the negative feature to the stimulated emission (SE) of photoexcited SPh and the positive features to its excited state absorption (ESA). This assignment was established by comparing the TA kinetics probed for SE and ESA to the FU kinetics measured for SPh (Fig. 6a). The close agreement between these three kinetic traces suggests that they all track the decay process of photoexcited SPh.

Fig. 6 TA results for SPh. (a) TA spectra of SPh probed at indicated time delays following 350 nm excitation. The SE and ESA features of SPh are indicated. (b) Normalized FU kinetics (green triangles) and TA kinetics of SPh probed at SE (blue squares) and ESA (red circles) peaks. The FU kinetics of SPh⊃C₆₀ is also shown with wine triangles. (c) TA spectra of SPh⊃C₆₀ probed at indicated time delays following 350 nm excitation. The absorptions of SPh⁺, , and C₆₀⁻ are also indicated. (d) Normalized TA kinetics of SPh⊃C₆₀ probed at the (red squares), SPh⁺ (blue circles), and C₆₀⁻ (green triangles, the black line shows the fit).

The TA spectra of the SPh⊃C₆₀ complex in the presence of encapsulated C₆₀ (SPh⊃C₆₀) are presented in Fig. 6c. In this case, the negative SE feature of SPh disappeared, indicating immediate depletion of the SPh excited state by C₆₀. This ultrafast process was further confirmed by the FU experiment. As shown in Fig. 6b, when measured under the same conditions as pure SPh, SPh⊃C₆₀ displayed negligible photon counts even near time zero, suggesting that the excited SPh is depleted on a timescale much faster than the instrument response (≪300 fs). Based on the energy levels of SPh and C₆₀, we can identify this ultrafast interaction as an electron transfer from photoexcited SPh to C₆₀. Indeed, in the TA spectra in Fig. 6c, we observe the instantaneous formation of a positive shoulder peak at ∼1080 nm that is consistent with the anion radical peak of C₆₀ (C₆₀⁻).⁵³ Meanwhile, a positive peak at ∼650 nm and a broad positive band centered at ∼950 nm also formed instantaneously, which can be assigned to the cation radical of SPh (SPh⁺).⁷² Clearly, C₆₀⁻ and SPh⁺ are products of electron transfer from photoexcited SPh to C₆₀, and their nearly-instantaneous formation further confirmed that this electron transfer process occurred on a femtosecond timescale. This ultrafast electron transfer is likely enabled by a strong electronic coupling between SPh and C₆₀, which are in intimate contact due to supramolecular interactions.

The charge-separated state (SPh⁺–C₆₀⁻) is not very long-lived, as reflected by the decay of C₆₀⁻ and SPh⁺ features on the ns timescale. Interestingly, accompanying this decay is the gradual formation of a positive peak at ∼750 nm that coincides with the spectroscopic features of the C₆₀ triplet excited state .⁷³ Thus, the recombination of SPh⁺–C₆₀⁻ does not regenerate the ground state SPh–C₆₀ but rather produces . This observation is consistent with the spin–orbit charge-transfer intersystem crossing (SOCT-ISC) phenomenon that has been well documented in the literature for organic donor–acceptor systems.⁷⁴ Specifically, the orbital angular momentum change of the electron during charge recombination can induce spin-flip and populates a triplet state. A related process is also reported for semiconductor nanocrystal–organic molecule hybrid donor–acceptor systems.⁷⁵ By fitting the kinetics of SPh⁺ and C₆₀⁻ plotted in Fig. 6d, the charge recombination (triplet formation) time was determined as ∼1.1 ns. The resulting is long-lived, showing no decay within 8 ns.

The measurements for compound 5 showed similar results to SPh (Fig. S15†). Thus, these time-resolved studies generally reveal femtosecond electron transfer from photoexcited compound 5 and SPh to encapsulated C₆₀, followed by nanosecond charge recombination to produce . This offers a novel approach to sensitize that can be used to, for example, generate singlet oxygen for photocatalysis and photodynamic therapy. However, the triplet formation pathway might be detrimental to optoelectronic devices utilizing SPh donors and C₆₀ acceptors, similar to the issue reported in organic solar cells.^76,77 Nonetheless, if the rate of charge sweeping out exceeds that of charge recombination in operating devices, triplet formation can be effectively suppressed.⁷⁸

Charge transport properties of SPh and SPh⊃C₆₀

Owing to the interesting photophysical properties of the polymeric structure of SPh and its host–guest interaction with C₆₀, we further investigated the potential applications of SPh and SPh⊃C₆₀ as the electron- and hole-transport layers for optoelectronic devices.^79,80 The electron and hole mobilities (μ_e and μ_h) were measured based on the SCLC method (Fig. 7a).⁸¹ When using only SPh as the electron transport layer, an electron-only device was fabricated as ITO/ZnO/SPh/Ca/Al. The result demonstrates that the μe value of SPh is ∼4.87 × 10⁻⁵ cm² V⁻¹ s⁻¹ using the Mott–Gurney equation, a value which is ∼2.5 times higher than that of PS1 which we reported previously as an electron-transport layer.⁴⁴ By incorporating fullerene C₆₀ into the SPh polymer, an electron-only ITO/ZnO/SPh⊃C₆₀/Ca/Al device was fabricated and the electron mobility was enhanced to ∼8.06 × 10⁻⁵ cm² V⁻¹ s⁻¹, which is higher than those of both SPh layer and pure C₆₀ (∼4.87 × 10⁻⁵ cm² V⁻¹ s⁻¹ for SPh and ∼6.13 × 10⁻⁵ cm² V⁻¹ s⁻¹ for C₆₀).

Fig. 7 J ^1/2–V plots and their corresponding fitting curves of the (a) electron-only ITO/ZnO/active layer/Ca/Al device and (b) hole-only ITO/PEDOT:PSS/active layer/MoO₃/Ag device.

To investigate the influence of C₆₀ incorporation on the hole mobility of SPh, we also fabricated hole-only devices with the structure of ITO/PEDOT:PSS/active layer/MoO₃/Ag, where the active layer is SPh, SPh⊃C₆₀ or C₆₀, and PEDOT:PSS is poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (Fig. 7b) The hole mobility of SPh was measured to be ∼2.86 × 10⁻⁵ cm² V⁻¹ s⁻¹ in the SCLC region. Interestingly, upon incorporating C₆₀ into the SPh polymer, the hole mobility is dramatically enhanced by more than ten times to ∼3.64 × 10⁻⁴ cm² V⁻¹ s⁻¹, which is even higher than that of pure C₆₀ (∼9.05 × 10⁻⁵ cm² V⁻¹ s⁻¹). Based on these results, we conclude that C₆₀ incorporation enhances te charge transport properties of SPh, promising for applications of SPh and SPh⊃C₆₀ in optoelectronic devices.

Conclusions

In conclusion, we successfully synthesized the first supramolecular polymeric heterojunction as the segment of carbon nanopeapods by incorporating the C₆₀ guest into the segment of carbon nanotubes (SPh) containing conjugated macrocycles as the host interconnected by a PPP backbone. In the presence of C₆₀, the fluorescence intensity of SPh was significantly quenched. Transient absorption measurements demonstrated that, upon photoirradiation, C₆₀⁻ and SPh⁺ are generated due to femtosecond (≪300 fs) electron transfer from the photoexcited SPh donor to the C₆₀ acceptor, followed by nanosecond charge recombination to produce the C₆₀ triplet excited state. Furthermore, compared to the host SPh and the guest C₆₀, the SPh⊃C₆₀ showed enhanced electron- and hole-mobilities, promising for applications of SPh and SPh⊃C₆₀ in optoelectronic devices. This study opens a door for bottom-up synthesis of carbon nanopeapod segments.

Data availability

All analytical data is provided in the ESI. All details about synthetic procedures, STM measurements, physical and photophysical characterizations are available in the ESI.

Author contributions

P. D. conceived and designed this research. S. W. synthesized the SPh material, conducted all characterizations and photophysical studies. J. W. and K. W. did ultrafast excited-state dynamics measurements. S. Y and X. L. performed SCLC measurements. S. W., X. L., X. Z., P. H., P. F., J. W., K. W., S. Y. and P. D. co-wrote the paper, and all the authors commented on it.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Key Research and Development Program of China (2017YFA0402800), the National Natural Science Foundation of China (21971229, 51772285, U1932214), the Fundamental Research Funds for the Central Universities and the Strategic Pilot Science and Technology Project of the Chinese Academy of Sciences (XDB17010100).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1sc03410c

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