Precise synthesis and assembly of π-conjugated polymers enabled by metal–organic frameworks

Abstract

The π-conjugated polymers hold considerable promise as key materials in various devices because of their advantageous optical, electronic, and magnetic properties; however, their strong interchain π–π interactions render them insoluble without the introduction of solubilizing substituents, making their synthesis highly challenging. Confined synthesis of π-conjugated polymers inside metal–organic frameworks (MOFs) offer a solution to this challenge by isolating the individual polymer chains, effectively addressing solubility issues and regulating polymerization reactions to yield novel π-conjugated polymers that are otherwise inaccessible. This account reviews recent advances in the synthesis and assembly control of π-conjugated polymers within MOFs to enhance physicochemical properties. Additionally, we explore the nanohybridization of π-conjugated polymers with MOFs, leading to sophisticated architectures with intriguing functionalities.

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1. Introduction

The π-conjugated polymers play crucial roles in our daily lives and advanced technology owing to their versatile optoelectronic functions.^1–4 Their properties vary depending on the chemical structures, which has driven decades of research toward their precise synthesis.⁵ Nevertheless, synthesizing these polymers in solution is inherently challenging.⁶ Strong intermolecular π–π interactions often result in insolubility in common solvents, leading to precipitation during polymerization reactions. Designing polymer structures with solubilizing substituents is essential but may alter their intrinsic properties. Although alternative methods, such as on-surface synthesis, have been explored, there remains a need for precise and scalable synthetic techniques for practical applications.^7–9

Template-confined synthesis is a promising solution, providing π-conjugated polymers that cannot be obtained using conventional methods.^10–14 Confined growth of the polymers can overcome solubility limits by isolating individual polymer chains. Controlling monomer organization enables regulation of reaction sites. Furthermore, this approach offers precise control of polymer assemblies within the templates, facilitating the creation of unprecedented nanostructures with unique optoelectronic properties.^15,16

Recently, metal–organic frameworks (MOFs) have attracted considerable attentions owing to their applications in various fields, such as gas storage, catalysts, sensing, and drug delivery.^17–21 The key advantage of MOFs lies in their tunable channel structures, in which the pore size, shape, and dimensionality can be systematically tuned by selecting appropriate components, rendering them ideal for polymerization of encapsulated monomers and tailoring polymer structures.^22–28 MOFs can be readily digested under mild conditions because of their noncovalent frameworks, enabling the isolation of products without triggering undesired side reactions. In addition, the MOF template method is amenable to large-scale operations. These features make MOFs an attractive platform for the precise and scalable synthesis of novel π-conjugated polymers (Fig. 1).²⁹ The present review highlights our latest achievements, focusing on the synthesis, assembly, and nanohybridization of π-conjugated polymers using MOFs.

2. Precise and scalable synthesis of graphene nanoribbons (GNRs)

GNRs—narrow strips of graphene—represent a highly promising class of carbon-based π-conjugated polymers.^30–32 Their unique electronic and magnetic characteristics are fundamentally influenced by their width and edge geometry, rendering their precise fabrication of GNRs a key focus for materials scientists.^33,34 Initially, GNRs were produced using “top–down” techniques such as unzipping carbon nanotubes (CNTs) or lithographic cutting of graphene.^35–37 However, these methods lack control of the ribbon structures. In contrast, “bottom–up” methods can produce GNRs with atomic precision. Although the annulation of precursor polymers can yield precise GNRs in solution, this method necessitates solubilizing side groups, which can alter their intrinsic properties.^38,39 Surface-mediated chemical reactions exhibited greater efficacy in precisely synthesizing bare GNRs; however, the production is often limited to small amounts owing to the restricted reaction areas.^40–42 Templated growth of GNRs using the inner spaces of CNTs enables control of their chemical structures.^43,44 However, the high thermal stability of the CNTs poses significant challenges for their removal, making the isolation and analysis of the GNRs difficult.

Owing to their tunability, diverse functionality, and scalability, MOFs can be regarded as a promising reaction medium for GNRs. We have recently developed a simple and feasible method for the synthesis of GNRs using an MOF, achieving both precision and scalability. The first demonstration of GNR synthesis involved the production of an armchair-type GNR with five carbon atoms in the cross section (AGNR) in the one-dimensional (1D) nanochannels of [ZrO(bpdc)]_n (bpdc = 4,4′-biphenyldicarboxylate, pore size = 6.9 × 6.9 Ų) via the polymerization of perylene (Fig. 2).⁴⁵ Although the polymerization reaction of perylene did not occur at 400 °C in the bulk phase, increasing the temperature to 600 °C led to graphitic or branched structures. Remarkably, the coupling reaction of perylene proceeded even at 400 °C within the MOF, probably owing to the uniaxial alignment of perylene molecules along the nanochannels and/or the catalytic effect of the Lewis acidity of the host, giving rise to AGNR via selective coupling at the 3-, 4-, 9-, and 10-positions of perylene. Bulk quantities of AGNR with atomic precision can be obtained by removing the framework, as confirmed by nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR), Raman, and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS) analyses.

Strategic choice of host and monomer combinations will enable the synthesis of long-sought-after GNRs. Recently, the narrowest zigzag-edged GNR called polyacene was synthesized for the first time.⁴⁶ The strategy involved regulated synthesis of precursor polymers inside [ZrO(bpdc)]_n and subsequent conversion into polyacene (Fig. 3a). Polymeric hydroacenes, which are partially saturated derivatives of acenes, can be regarded as precursors for synthesizing polyacene.⁴⁷ 2,6-Bis(bromomethyl) naphthalene (BBMN) and 2,6-bis(bromomethyl)anthracene (BBMA) can be employed as a monomer. However, heat treatment of the monomers in the bulk state yielded branched and graphitic carbons owing to the higher reactivity at zigzag positions than at the 3- and 7-positions. The spatial constraint of the nanochannels of [ZrO(bpdc)]_n enabled site-selective cyclization reactions, providing linearly extended precursor polymers, poly(naphthalene-2,3:6,7-tetrayl-6,7-dimethylene) (PNTD) and poly(anthracene-2,3:6,7-tetrayl-6,7-dimethylene) (PATD). After the precursors were isolated, dehydroaromatization reaction was conducted at a high temperature, yielding the unsubstituted polyacene. A series of the characterizations, including NMR, FT-IR, ultraviolet/visible/near-infrared absorption analyses, confirmed the generation of polyacene through the dehydroaromatization reaction (Fig. 3b). The mean numbers of benzene rings in polyacene from PNTD and PATD were determined to be 17.8 ± 3.3 and 18.6 ± 3.5, respectively, via the FT-IR analysis, which exceeded those of the acenes reported thus far (Fig. 3c).^48–50

Acenes longer than pentacene is known to be more susceptible to oxidation and dimerization owing to their inherent biradical character.^51,52 Remarkably, the signs of degradation processes, such as the carbonyl group and bridgehead sp³-carbon, were not observable in the solid-state ¹³C NMR spectra of polyacene. The unexpectedly high stability of bulk polyacene is likely due to the restricted oxygen access and limited chain mobility required for oxidation and dimerization reactions.^53,54 These findings have prompted us to investigate the optical, electronic, magnetic properties of polyacene that has remained elusive so far; these are currently under investigation in our laboratory. In several previous works on controlled polymerizations using MOFs, it has been reported that the degree of polymerization can be modulated by changing the size of 1D nanopores because the monomer mobility is affected by the geometrical constraint of the nanopores.⁵⁵ Given that the length of polyacene likely affects the physicochemical properties, a comprehensive study on the synthesis of polyacenes using MOFs with different pore structures (pore size, shape, and surface environment) will also be a focus of our future research.⁵⁶

In the aforementioned two examples, aromatic ring molecules were employed as a starting monomer for GNRs. The MOF template method enables the synthesize of GNRs using flexible vinyl polymers.

Since their development in the 1960s, carbon fibers have been essential engineering materials owing to their exceptional physical properties, high chemical resistance, and low weight.⁵⁷ Polyacrylonitrile (PAN) is a commonly used precursor for carbon fibers. Among the various fabrication methods, thermal stabilization is crucial for producing high-quality carbon fibers. During this process, PAN undergoes intrachain cyclization and dehydration reactions to form a nitrogen doped GNR (NGNR).⁵⁸ However, the actual structure of NGNR is complex, comprising aliphatic carbon, carbonyl groups, and crosslinking (Fig. 4a). Controlling the ladderization reaction is essential, but both intrachain and interchain reactions are unavoidable because of chain entanglement in the bulk state.⁵⁹ The conversion process is exothermic with an sudden heat release, leading to various side reactions, such as chain scission. Confining polymer chains in porous materials can effectively control polymer assembly by preventing entanglement and conformational disorder. Extensive research has explored the effect of nanoconfinement within nanoporous materials, such as MCM-41 and a porous polymer, on the reactivity of PAN.^10,12 We investigated the conversion of single PAN chains inside two MOFs with different channel sizes: [Al(OH)(bpdc)]ₙ (pore size = 11.1 × 11.1 Ų) and [Al(OH)(ndc)]ₙ (ndc = 2,6-naphthalenedicarboxylate, pore size = 8.5 × 8.5 Ų) to assess the effect of the pore size on the transformation process (Fig. 4b).⁶⁰ Solid-state ¹³C NMR analysis confirmed that NGNR synthesized within the MOFs exhibited a more extended conjugated backbone and fewer aliphatic carbons compared with bulk conditions (Fig. 4c). The confinement of single polymer chains within the nanochannels mitigated the abrupt heat release during the transformation process, as indicated by the reduced heat generation (Fig. 4d). MOFs typically exhibit low thermal conductivity owing to the heterogeneous nature of bond stiffness and atomic masses.⁶¹ Encapsulating PAN chains individually within the nanochannels limits the heat transfer between chains, suppressing rapid heat release. Further heating of NGNR can potentially improve the physical properties of the resulting carbon fibers.

3. Controlled assembly of π-conjugated polymers

Molecule-based nanodevices will play a pivotal role in the future because of the increasing emphasis on miniaturization and energy efficiency of devices.⁶² Thorough elucidation of the electronic properties of π-conjugated polymers, which are utilized as molecular wires and emitting elements in the nanodevices, is required for future advancements. Interchain interactions, including charge/energy migration and exciton coupling, significantly affect the performance of π-conjugated polymers.⁶³ Therefore, molecular-level control of chain assembly is crucial for revealing the fundamental properties of conjugated polymers and enhancing their functions in devices. The designed nanochannels of MOFs enable precise assembly of polymer chains, providing a unique opportunity to explore the intrinsic optoelectrical properties of π-conjugated polymers and endow them with new functions.⁶⁴

Unsubstituted polythiophene (PTh) has a longer effective conjugation length than its substituted counterparts, owing to the preservation of backbone planarity unperturbed by bulky side groups. However, strong interchain interactions render PTh nonmelting and insoluble, preventing control of its packing structure, a critical factor for exploring new functionalities. The physicochemical properties of a few PTh chains have been reported.⁶⁵ PTh was prepared in [La(btb)]_n (btb = 1,3,5-benzenetrisbenzoate, pore size = 10.7 × 10.7 Ų) via the oxidative polymerization of terthiophene, in which the polymer bundle size was modulated by changing the monomer feed amount. Despite variations in bundle size, the PTh chains remained planar because of the host/guest interactions, as evidenced by the Raman spectroscopy measurements. Encapsulation of PTh in [La(btb)]_n caused a blueshift in emission bands relative to bulk PTh, with the blueshift increasing as the PTh loading decreased. Aggregated PTh, where excitons migrate between neighboring chains to reach lower-energy sites, exhibit a red-shifted emission peak compared with isolated chains.⁶⁶ Molecular dynamics (MD) simulations suggested that reducing polymer loading led to the formation of polymer assemblies with fewer chains (Fig. 5a); therefore, the largest blue shifted emission of the composites with the smallest loading amount is attributed to the increased likelihood of encountering a discrete polymer chain in the nanochannels. Time-resolved microwave conductivity (TRMC) measurements enabled the assessment of the carrier mobility of the PTh bundles, revealing that the sizes of the polymer assemblies affected the carrier mobility of the PTh via the formation of the π–π stacked structure (Fig. 5b).

Enantiopure [La(btb)]_n can be synthesized using a lanthanum complex with homochiral phenylalanine.⁶⁷ The chirality of [La(btb)]_n originates from the propeller-like twisted conformation of the BTB ligand. Circular dichroism (CD) spectroscopy indicated that PTh, when incorporated into the chiral [La(btb)]_n, acquired chiral properties despite its inherent optical inactivity (Fig. 6a).⁶⁸ Control of the polymer assembly size led to significant changes in the chiroptical behavior, which were attributed to varying interchain exciton interactions.⁶⁹ The host MOF could be removed under mild conditions, leaving PTh with preserved long-range ordering, as confirmed by transmission electron microscopy and selected-area electron diffraction.^70,71 Remarkably, PTh maintained its chirality even in the absence of the chiral MOF and exhibited high thermal stability up to 250 °C (Fig. 6b). This approach was extendable to a wide variety of guest species, enabling the creation of various chiral nanomaterials with distinctive chiral functions, such as chirality-induced spin selectivity and circularly polarized luminescence.^72–75

The precisely engineered nanospaces of MOFs enable the manipulation of the relative orientation of adjacent guest molecules. The spatial control of donor/acceptor interfaces is crucial for probing key photophysical processes in optoelectronic devices.⁷⁶ In organic photovoltaic cells, PTh and fullerene derivatives are classic electron donors and acceptors, respectively, and they readily stack through π–π interactions.⁷⁷ When confined in the 1D pores of [La(btb)]_n, PTh and fullerene selectively aligned in an end-on configuration, significantly enhancing the charge separation lifetime (Fig. 7).⁷⁸ Theoretical analysis revealed the minimal electron coupling between PTh and fullerene in the end-on orientation, which reduced the degree of charge recombination and extended the charge-carrier lifetime.

4. MOF/π-conjugated polymer nanohybrids

Considerable research has focused on synthesizing MOFs with novel structures, and the nanohybridization of MOFs with functional guests has been explored to leverage the advantages of both components.^79,80 π-conjugated polymers have distinctive properties that are highly beneficial for hybridization with MOFs, resulting in nanohybrids with exceptional characteristics.⁸¹

Gas detection is becoming increasingly critical in various applications. Most MOFs are insulators and require specific metal ions and ligands to achieve conductivity. A promising approach for creating conductive and porous materials involves encapsulating conducting polymers within MOFs.⁸² Poly(3,4-ethylenedioxythiophene) (PEDOT), a conducting polymer whose Fermi level can be tuned via analyte oxidation, was synthesized within a three-dimensional (3D) porous MOF [Cr₃(bdc)₃OF(H₂O)₂]_n (bdc = 1,4-benzenedicarboxylate) (Fig. 8a).⁸³ This process yielded a PEDOT–MOF nanohybrid with high conductivity while preserving its high porosity. Owing to the enhanced accessibility of NO₂ to the polymer chains, the nanohybrid exhibited high sensitivity for NO₂ detection at 200 ppb (Fig. 8b). Recently, we have demonstrated that the application of external electric fields during drop casting can lead to the spatioselective deposition of composite particles between finger electrodes, increasing both the conductivity and sensitivity to moisture.⁸⁴

Owing to their diverse topologies and linkers, MOFs offer a range of electronic and conducting properties. Consequently, integrating π-conjugated polymers with electroactive MOFs can facilitate the development of optimized systems featuring periodic networks, tailored to the inherent structures of host materials. We introduced a novel hybrid system using an electron–acceptor MOF [Ti₆O₅(mdip)]_n (mdip = methylenediisophthalate) with infinite (Ti₆O₉)_n chains arranged in a hexagonally packed formation along the 1D nanochannels (Fig. 9).⁸⁵ The carrier mobility of [Ti₆O₅(mdip)]_n was determined to be at least 4 × 10⁻⁴ cm² s^−¹V^−¹ via TRMC measurements, which is comparable to reported values for nanosized TiO₂.⁸⁶ The accommodation of donor PTh in the nanochannels creates a precisely alternating heterojunction at the molecular scale (Fig. 9). Remarkably, the resulting nanocomposites exhibited an exceptionally long life time of charge-carrier (τ_1/2 = 1 ms), which is one of the longest reported for dye-sensitized Ti–O systems.⁸⁷ This finding demonstrates that encapsulating donor polymers in acceptor MOFs provides an ideal structure for charge separation, advancing the development of efficient energy-conversion systems.

Fabricating hybrid thin films combining π-polymers and MOFs would be beneficial for energy conversion and optoelectronic applications. However, the limited solubility of MOFs in solvents restricts their processability. In this context, surface-mounted MOFs (SURMOFs) represent a promising platform for developing such hybrid films.⁸⁸ We performed the oxidative polymerization of terthiophene and EDOT within SURMOFs [Zn(bdc)]_n and [Cu(bpdc)]_n, which yielded the π-conjugated polymers and MOF nanocomposite films on the substrate.⁸⁹ The results indicated that functional nanohybrids are promising for future applications in optoelectronics.

5. Conclusion

The fabrication of π-conjugated polymers has been hindered by the difficulty of their precise synthesis and solubility issues. This account describes recent developments in the synthesis, assembly, and nanohybridization of π-conjugated polymers utilizing MOFs. We highlight the scalable and precise synthesis of GNRs, which are challenging to obtain using solid, solution, and surface chemistry, opening new avenues for the extensive applications of GNRs in both scientific and industrial fields, such as heterogeneous catalysts and gas sensors. The nanoconfinement of π-conjugated polymers in MOFs should shed light on their unique low-dimensional physicochemical nature, which remains elusive. Furthermore, our approach facilitates the integration of new functions into π-conjugated polymers through synergistic host–guest interactions, which can result in functional nanohybrids with promising applications, including photovoltaic cells and field-effect transistors.

Although considerable progress has been achieved in this research area, several challenges persist. On-surface polymerization can yield atomically precise GNRs; however, their physical properties might be significantly altered by metal substrates through orbital hybridizations, complicating the assessment of their intrinsic properties. The tailor-made pore characteristics of MOFs enable the creation of precise GNR assemblies within the nanochanenls, allowing for the access to their properties. This approach facilitates the tuning of intramolecular and intermolecular interactions of GNRs, endowing them with new functions otherwise inaccessible by conventional design concepts based on the ribbon structures. The proper selection of designed monomers and MOFs enables the synthesis of a diverse range of well-defined GNRs with large dimensions, facilitating precise engineering of their topological properties.^90,91 In this regard, controlled synthesis of two-dimensional (2D) and 3D GNR networks are future challenges that are currently being pursued by our group.

Recent advancements in the confined synthesis of π-conjugated polymers have led to significant breakthroughs in materials science. We anticipate a growing demand for novel π-conjugated polymers and expect the development of innovative templates to drive further exciting new discoveries.

Acknowledgments

The author would like to express his sincere gratitude to Professor Takashi Uemura (The University of Tokyo) for his continuous support and guidance. The author also acknowledges all collaborators and coworkers whose works are cited herein.

Funding

This work was supported by Japan Science and Technology Agency, Precursory Research for Embryonic Science and Technology program (JPMJPR21A7) and a Grant-in-Aid for Science Research (JP24K01276) from the Ministry of Education, Culture, Sports, Science and Technology, Government of Japan.

References

Takashi Kitao

Takashi Kitao received his PhD degree at the Department of Synthetic Chemistry and Biological Chemistry, Kyoto University in 2017. After working at Kyoto University as a postdoctoral research fellow, he was promoted to Assistant Professor at the University of Tokyo in 2018. Since 2021, he has also been a researcher of the PRESTO program in JST. His research focuses on the development of functional materials, such as conjugated polymers and nanocarbons, using coordination nanospaces.

Author notes