Conjugated Coordination Polymers Research Articles

Self-Assembled Conjugated Coordination Polymer Nanorings: Role of Morphology and Redox Sites for the Alkaline Electrocatalytic Oxygen Evolution Reaction.

Electrocatalytic water splitting provides a sustainable method for storing intermittent energies, such as solar energy and wind, in the form of hydrogen fuel. However, the oxygen evolution reaction (OER), constituting the other half-cell reaction, is often considered the bottleneck in overall water splitting due to its slow kinetics. Therefore, it is crucial to develop efficient, cost-effective, and robust OER catalysts to enhance the water-splitting process. Transition-metal-based coordination polymers (CPs) serve as promising electrocatalysts due to their diverse chemical architectures paired with redox-active metal centers. Despite their potential, the rational use of CPs has faced obstacles including a lack of insights into their catalytic mechanisms, low conductivity, and morphology issues. Consequently, achieving success in this field requires the rational design of ligands and topological networks with the desired electronic structure. This study delves into the design and synthesis of three novel conjugated coordination polymers (CCPs) by leveraging the full conjugation of terpyridine-attached flexible tetraphenylethylene units as electron-rich linkers with various redox-active metal centers [Co(II), Ni(II), and Zn(II)]. The self-assembly process is tuned for each CCP, resulting in two distinct morphologies: nanosheets and nanorings. The electrocatalytic OER performance efficiency is then correlated with factors such as the nanostructure morphology and redox-active metal centers in alkaline electrolytes. Notably, among the three morphologies studied, nanorings for each CCP exhibit a superior OER activity. Co(II)-integrated CCPs demonstrate a higher activity between the redox-active metal centers. Specifically, the Co(II) nanoring morphology displays exceptional catalytic activity for OER, with a lower overpotential of 347 mV at a current density of 10 mA cm-2 and small Tafel slopes of 115 mV dec-1. The long-term durability is demonstrated for at least 24 h at 1.57 V vs RHE during water splitting. This is presumably the first proof that links the importance of nanostructure morphologies to redox-active metal centers in improving the OER activity, and it may have implications for other transdisciplinary energy-related applications.

Control of the Hydroquinone/Benzoquinone Redox State in High‐Mobility Semiconducting Conjugated Coordination Polymers

Conjugated coordination polymers (c‐CPs) are unique organic‐inorganic hybrid semiconductors with intrinsically high electrical conductivity and excellent charge carrier mobility. However, it remains a challenge in tailoring electronic structures, due to the lack of clear guidelines. Here, we develop a strategy wherein controlling the redox state of hydroquinone/benzoquinone (HQ/BQ) ligands allows for the modulation of the electronic structure of c‐CPs while maintaining the structural topology. The redox‐state control is achieved by reacting the ligand TTHQ (TTHQ = 1,2,4,5‐tetrathiolhydroquinone) with silver acetate and silver nitrate, yielding Ag4TTHQ and Ag4TTBQ (TTBQ = 1,2,4,5‐tetrathiolbenzoquinone), respectively. In spite of sharing the same topology consisting of two‐dimensional Ag‐S network and HQ/BQ layer, they exhibit different band gaps (1.5 eV for Ag4TTHQ and 0.5 eV for Ag4TTBQ) and conductivities (0.4 S/cm for Ag4TTHQ and 10 S/cm for Ag4TTBQ). DFT calculations reveal that these differences arise from the ligand oxidation state inhibiting energy band formation near the Fermi level in Ag4TTHQ. Consequently, Ag4TTHQ displays a high Seebeck coefficient of 330 μV/K and a power factor of 10 μW/m·K2, surpassing Ag4TTBQ and the other reported silver‐based c‐CPs. Furthermore, terahertz spectroscopy demonstrates high charge mobilities exceeding 130 cm2/V·s in both Ag4TTHQ and Ag4TTBQ.

Control of the Hydroquinone/Benzoquinone Redox State in High-Mobility Semiconducting Conjugated Coordination Polymers.

Conjugated coordination polymers (c-CPs) are unique organic-inorganic hybrid semiconductors with intrinsically high electrical conductivity and excellent charge carrier mobility. However, it remains a challenge in tailoring electronic structures, due to the lack of clear guidelines. Here, we develop a strategy wherein controlling the redox state of hydroquinone/benzoquinone (HQ/BQ) ligands allows for the modulation of the electronic structure of c-CPs while maintaining the structural topology. The redox-state control is achieved by reacting the ligand TTHQ (TTHQ=1,2,4,5-tetrathiolhydroquinone) with silver acetate and silver nitrate, yielding Ag4TTHQ and Ag4TTBQ (TTBQ=1,2,4,5-tetrathiolbenzoquinone), respectively. In spite of sharing the same topology consisting of a two-dimensional Ag-S network and HQ/BQ layer, they exhibit different band gaps (1.5 eV for Ag4TTHQ and 0.5 eV for Ag4TTBQ) and conductivities (0.4 S/cm for Ag4TTHQ and 10 S/cm for Ag4TTBQ). DFT calculations reveal that these differences arise from the ligand oxidation state inhibiting energy band formation near the Fermi level in Ag4TTHQ. Consequently, Ag4TTHQ displays a high Seebeck coefficient of 330 μV/K and a power factor of 10 μW/m ⋅ K2, surpassing Ag4TTBQ and the other reported silver-based c-CPs. Furthermore, terahertz spectroscopy demonstrates high charge mobilities exceeding 130 cm2/V ⋅ s in both Ag4TTHQ and Ag4TTBQ.

Controlling Film Formation and Host-Guest Interactions to Enhance the Thermoelectric Properties of Nickel-Nitrogen-Based 2D Conjugated Coordination Polymers.

2D conjugated coordination polymers (cCPs) based on square-planar transition metal-complexes (such as MO4, M(NH)4, and MS4, M = metal) are an emerging class of (semi)conducting materials that are of great interest for applications in supercapacitors, catalysis, and thermoelectrics. Finding synthetic approaches to high-performance nickel-nitrogen (Ni-N) based cCP films is a long-standing challenge. Here, a general, dynamically controlled on-surface synthesis that produces highly conductive Ni-N-based cCP films is developed and the thermoelectric properties as a function of the molecular structure and their dependence on interactions with ambient atmosphere are studied. Among the four studied cCPs with different ligand sizes hexaminobenzene- and hexaaminotriphenylene-based films exhibit record electrical conductivity (100-200 S cm-1) in this Ni-N based cCP family, which is one order of magnitude higher than previous reports, and the highest thermoelectric power factors up to 10 µW m-1 K-2 among reported 2D cCPs. The transport physics of these films is studied and it is shown that depending on the host-guest interaction with oxygen/water the majority carrier type and the value of the Seebeck coefficient can be largely regulated. The high conductivity is likely reflecting good interconnectivity between (small) ordered domains and grain boundaries supporting disordered metallic transport.

Cobalt-based conjugated coordination polymers with tunable dimensions for electrochemical sensing of p-nitrophenol

Using planar π-conjugated 2,5-diamino-1,4-benzenedithiol as organic ligand, Co-based conjugated coordination polymers (CoCCPs) with different morphology were prepared through controlling the injection rate of Co2+. When the injection rate decreases from 1.00 to 0.25 mL min−1, the obtained CoCCPs change from 2D nanosheets to quasi-1D nanorods. It is found that the different-shaped CoCCPs exhibit varying electrochemical sensing performance. The prepared CoCCPs-1 with quasi-1D nanowires and porous network structure possesses larger active area, faster electron transfer and higher accumulation ability. Moreover, the CoCCPs-1 is more active for the oxidation of p-nitrophenol (PNP), and greatly enhances its oxidation signal. Based on the morphology-tuned sensing performance of CoCCPs, a highly-sensitive electrochemical sensor has been developed for PNP, with detection limit of 0.00986 μM (9.86 nM). It was used in the analysis of wastewater samples, and the results is validated by other instrumental method.

The Recent Development of Two‐dimensional Conjugated Coordination Polymers

AbstractTwo‐dimensional conjugated coordination polymers (2D c‐CPs), an emerging family of 2D materials, have received great research interest due to their versatile framework topologies and unique physical properties, which found a wide variety of applications. In this review, we summarized the state‐of‐the‐art works of 2D c‐CPs in terms of synthesis strategies, topological structures, and their applications. Essential factors that affect the chemical structures and performance, such as the type of ligand, metal nodes, and their stoichiometric ratios have been highlighted. Besides, the perspectives and challenges are also provided considering the future development of 2D c‐CPs.

Single Crystals of a Highly Conductive Three-Dimensional Conjugated Coordination Polymer.

Conjugated coordination polymers (CCPs), which possess long-range planar π-d conjugation, are fascinating for various applications because they inherit the merits of both metal-organic frameworks (MOFs) and conducting polymers. However, only one-dimensional (1D) and two-dimensional (2D) CCPs have been reported so far. The synthesis of three-dimensional (3D) CCPs is challenging and even seems theoretically infeasible because conjugation implies 1D or 2D structure. Besides, the redox activity of the conjugated ligands and the π-d conjugation makes the synthesis of CCPs very complicated, and hence, single crystals of CCPs are rarely achieved. Herein, we reported the first 3D CCP and its single crystals with atomically precise structures. The synthesis process involves complicated in situ dimerization, deprotonation of ligands, oxidation/reduction of both ligands and metal ions, and precise coordination between them. The crystals contain in-plane 1D π-d conjugated chains and close π-π interactions between the adjacent chains that are bridged by another column of stacked chains, thus forming 3D CCP with high conductivity (400 S m-1 at room temperature and 3100 S m-1 at 423 K) and potential applications as cathodes in sodium-ion batteries with high capacity, rate capability, and cyclability.

Immobilizing Quinone-Fused Aza-Phenazine into π-d Conjugated Coordination Polymers with Multiple-Active Sites for Sodium-Ion Batteries.

The development of coordination polymers with π-d conjugation (CCPs) provides ide prospects for exploring the next generation of environmental-friendliness energy storage systems. Herein, the synthesis, experimental characterizations, and Na-ion storage mechanism of π-d CCPs with multiple-active sites are reported, which use quinone-fused aza-phenazine (AP) and aza-phenazin (AP) as the organic ligands coordinated with the metal center (Ni2+ ). Among them, NiQAP as the cathode material exhibits impressive electrochemical properties applied in sodium-ion batteries (SIBs), including the high initial/stable discharge specific capacities (180.0/225.6 mAh g-1 ) at 0.05 A g-1 , a long-term cycle stability up to 10,000 cycles at 1.0 A g-1 with a high reversible capacity of 100.1 mAh g-1 , and good rate capability of 99.6 mAh g-1 even at 5.0 A g-1 . Moreover, the Na-ion storage mechanism of NiQAP is also performed by the density functional theory (DFT) calculation, showing multiple-active sites of C≐O and C≐N (in the quinone and phenazine structure) and NiO4 (in the coordination unit) for Na-ion storage. These results highlight the importance of organic electrode material with the coordination units and provide a foundation for further studying the CCPs with multiple active sites for energy storage systems.

Molecular Design for Symmetric All-Organic Batteries.

Symmetric batteries by using the same material as both cathodes and anodes have several advantages. However, traditional inorganic materials are facing challenges as electrode materials in symmetric batteries. The designable organic electrode materials (OEMs) make it possible to fabricate symmetric all-organic batteries (SAOBs), which are still in their infancy stage. Herein, we summarize the requirements of the OEMs for SAOBs and classify SAOBs based on the types of OEMs (n-type and bipolar, including carbonyl materials, materials with C=N groups, conducting polymers, free radical compounds, conjugated coordination polymers, arylamine derivatives etc.). We review the recent progress on SAOBs and analyze the advantages and shortcomings of different types of SAOBs. The strategies for designing OEMs in SAOBs with high performance are discussed. Thus, we hope this minireview could stimulate more interest in SAOBs and pave the potential application of SAOBs with high performance.

Linear Conjugated Coordination Polymers for Electrocatalytic Oxygen Evolution Reaction.

Conjugated coordination polymers (CCPs) have attracted extensive attention for various applications related to energy storage and conversion in the past few years, despite that there are many CCPs with unclear chemical states and structures. Here, linear CCPs (LCCPs), with metal-O4 active sites grown on carbon paper (CP) for oxygen evolution reaction (OER), are presented. The LCCPs with high crystallinity and simple structures exhibit the order of electrocatalytic activity of Co-O4 >Ni-O4 >Fe-O4 in terms of the metal-O4 centers. The Co-based LCCP shows higher OER performance (263mV at 10mAcm-2 ) and better durability (90h at 30mAcm-2 ) than commercial IrO2 /CP. The structures and chemical states of LCCPs are carefully investigated, and density functional theory is used to reveal the mechanism of OER at the central metal site. This investigation into LCCPs provides new sights for a better understanding of CCPs and expands the applications of LCCPs with metal-O4 sites.

Highly selective oxygen reduction to H2O2 on π-d conjugated coordination polymers: The effect of coordination atoms

Single-atom catalysts (SACs) feature the excellent potential for H2O2 production via 2e− oxygen reduction reactions (ORR). The coordination atoms of SACs play a critical role in regulating the catalytic performance, while the underlying relationship between the coordination atoms and performance remains obscure. Here, two kinds of Ni-based π-d conjugated coordination polymers with atomically dispersed NiN4 and NiN2O2 sites are rationally constructed for ORR. The NiN2O2 catalyst exerts a high H2O2 production rate of 374 mmol gcat−1 h−1 with an excellent selectivity of 91 %, while the selectivity on the NiN4 one is only 60 %, demonstrating a coordination atoms-dependent performance. Experimental and theoretical investigations reveal that the energy barriers for the formation of the key *OOH intermediate and the subsequent hydrogenation could be optimized by introducing O atoms in the first coordination sphere, which contributes to the efficient regulation of the selectivity and activity of 2e− ORR process.

Semiconducting Conjugated Coordination Polymer with High Charge Mobility Enabled by "4 + 2" Phenyl Ligands.

Electrically conductive coordination polymers and metal-organic frameworks are attractive emerging electroactive materials for (opto-)electronics. However, developing semiconducting coordination polymers with high charge carrier mobility for devices remains a major challenge, urgently requiring the rational design of ligands and topological networks with desired electronic structures. Herein, we demonstrate a strategy for synthesizing high-mobility semiconducting conjugated coordination polymers (c-CPs) utilizing novel conjugated ligands with D2h symmetry, namely, "4 + 2" phenyl ligands. Compared with the conventional phenyl ligands with C6h symmetry, the reduced symmetry of the "4 + 2" ligands leads to anisotropic coordination in the formation of c-CPs. Consequently, we successfully achieve a single-crystalline three-dimensional (3D) c-CP Cu4DHTTB (DHTTB = 2,5-dihydroxy-1,3,4,6-tetrathiolbenzene), containing orthogonal ribbon-like π-d conjugated chains rather than 2D conjugated layers. DFT calculation suggests that the resulting Cu4DHTTB exhibits a small band gap (∼0.2 eV), strongly dispersive energy bands near the Fermi level with a low electron-hole reduced effective mass (∼0.2m0*). Furthermore, the four-probe method reveals a semiconducting behavior with a decent conductivity of 0.2 S/cm. Thermopower measurement suggests that it is a p-type semiconductor. Ultrafast terahertz photoconductivity measurements confirm Cu4DHTTB's semiconducting nature and demonstrate the Drude-type transport with high charge carrier mobilities up to 88 ± 15 cm2 V-1 s-1, outperforming the conductive 3D coordination polymers reported till date. This molecular design strategy for constructing high-mobility semiconducting c-CPs lays the foundation for achieving high-performance c-CP-based (opto-)electronics.

Conjugated Coordination Polymer as a New Platform for Efficient and Selective Electroreduction of Nitrate into Ammonia.

Electroreduction of nitrate into ammonia (NRA) provides a sustainable route to convert the widespread nitrate pollutants into high-value-added products under ambient conditions, which unfortunately suffers from unsatisfactory selectivity due to the competitive hydrogen evolution reaction (HER). Previous strategies of modifying the metal sites of catalysts often met a dilemma for simultaneously promoting activity and selectivity toward NRA. Here, a general strategy is reported to enable an efficient and selective NRA process through coordination modulation of single-atom catalysts to tailor the local proton concentration at the catalyst surface. By contrast, two analogous Ni-single-atom enriched conjugated coordination polymers (NiO4 -CCP and NiN4 -CCP) with different coordination motifs are investigated for the proof-of-concept study. The NiO4 -CCP catalyst exhibits an ammonia yield rate as high as 1.83mmol h-1 mg-1 with a Faradaic efficiency of 94.7% under a current density of 125mA cm-2 , outperforming the NiN4 -CCP catalyst. These experimental and theoretical studies both suggest that the strategy of coordination modulation can not only accelerate the NRAby adjusting the adsorption energies of NRA intermediates on the metal sites but also inhibit the HER through regulating the proton migration with contributions from the metal-hydrated cations adsorbed at the catalyst surface, thus achieving simultaneous enhancement of NRA selectivity and activity.

Nine‐Electron Transfer of Binder Synergistic π‐d Conjugated Coordination Polymers as High‐Performance Lithium Storage Materials

AbstractTo solve the problems such as the dissolution and the poor conductivity of organic small molecule electrode materials, we construct π‐d conjugated coordination polymer Ni‐DHBQ with multiple redox‐active centers as lithium storage materials. It exhibits an ultra‐high capacity of 9‐electron transfers, while the π‐d conjugation and the laminar structure inside the crystal ensure fast electron transport and lithium ion diffusion, resulting in excellent rate performance (505.6 mAh g−1 at 1 A g−1 after 300 cycles). The interaction of Ni‐DHBQ with the binder CMC synergistically inhibits its dissolution and anchors the Ni atoms, thus exhibiting excellent cycling stability (650.7 mAh g−1 at 0.1 A g−1 after 100 cycles). This work provides insight into the mechanism of lithium storage in π‐d conjugated coordination polymers and the synergistic effect of CMC, which will contribute to the molecular design and commercial application of organic electrode materials.

Nine-Electron Transfer of Binder Synergistic π-d Conjugated Coordination Polymers as High-Performance Lithium Storage Materials.

To solve the problems such as the dissolution and the poor conductivity of organic small molecule electrode materials, we construct π-d conjugated coordination polymer Ni-DHBQ with multiple redox-active centers as lithium storage materials. It exhibits an ultra-high capacity of 9-electron transfers, while the π-d conjugation and the laminar structure inside the crystal ensure fast electron transport and lithium ion diffusion, resulting in excellent rate performance (505.6 mAh g-1 at 1 A g-1 after 300 cycles). The interaction of Ni-DHBQ with the binder CMC synergistically inhibits its dissolution and anchors the Ni atoms, thus exhibiting excellent cycling stability (650.7 mAh g-1 at 0.1 A g-1 after 100 cycles). This work provides insight into the mechanism of lithium storage in π-d conjugated coordination polymers and the synergistic effect of CMC, which will contribute to the molecular design and commercial application of organic electrode materials.

Framework Dimensional Control Boosting Charge Storage in Conjugated Coordination Polymers

Conjugated coordination polymers (CCPs) with extended π-d conjugation, which can effectively promote long-range delocalization of electrons and enhance conductivity, are superior to traditional metal-organic frameworks (MOFs) and attracted great attention for potential applications in chemical sensors, electronics, energy conversion/storage devices, etc. However, the precise construction of CCPs is still challenging due to the complex and uncontrollable reactions of CCPs. Herein, two different framework dimensions of CCPs are controllably realized by employing the same ligand (2,3,5,6-tetraaminobenzoquinone (TABQ)) and the same metal (copper) as center ions. The manipulation of reaction leads to different valences of ligands and metal ions, different coordination geometries, and thereby 1D-CuTABQ and 2D-CuTABQ frameworks, respectively. High performance of charge storage is hence achieved involving the storage of both cations and anions, and therein, 2D-CuTABQ shows a high reversible capacity of ≈305mAhg-1 , good rate capability and high capacity retention (≈170mAhg-1 after 2000 cycles at 5Ag-1 with 0.01% decay per cycle), which outperforms 1D-CuTABQ and almost all of the reported MOFs as cathodes for batteries. These results highlight the delicate structural control of CCPs for high-performance batteries and other various applications.

Tuning the Charge Transport Property and Photocatalytic Activity of Anthracene-Based 1D π-d Conjugated Coordination Polymers by Interlayer Stacking.

One-dimensional (1D) π-d-conjugated coordination polymers (CCPs) with charge delocalization have attracted significant attention due to their potential application in energy conversion and storage. However, the fundamental understanding of the correlation of their structural parameters with photophysical and photocatalytic properties remains underexplored. Herein, we report three novel Cu-node anthracene-based 1D π-d CCPs with systematic variation of steric groups (Ph > Me > H) at the 9 and 10 position of anthracene (denoted as AnPh, AnMe, and AnH), which is aimed at altering the stacking of the polymer chains and its impact on the inter-chain charge transport property. Using the combination of steady-state X-ray absorption spectroscopy, optical transient absorption spectroscopy, X-ray transient absorption spectroscopy, and electrochemical impedance spectroscopy, we show that the linear ligands (AnPh, AnMe, and AnH) with different degrees of steric groups (Ph > Me > H) introduced at the 9 and 10 position of anthracene can alter the stacking of the polymer chains and thus impact their crystallinity, charge separation, and charge transport property, which in turn impacts their photocatalytic performance for hydrogen evolution reaction.

Dimensionality Modulates Electrical Conductivity in Compositionally Constant One-, Two-, and Three-Dimensional Frameworks.

We reveal here the construction of Ni-based metal-organic frameworks (MOFs) and conjugated coordination polymers (CCPs) with different structural dimensionalities, including closely π-stacked 1D chains (Ni-1D), aggregated 2D layers (Ni-2D), and a 3D framework (Ni-3D), based on 2,3,5,6-tetraamino-1,4-hydroquinone (TAHQ) and its various oxidized forms. These materials have the same metal-ligand composition but exhibit distinct electronic properties caused by different dimensionalities and supramolecular interactions between SBUs, ligands, and structural motifs. The electrical conductivity of these materials spans nearly 8 orders of magnitude, approaching 0.3 S/cm.

Regulating the metal nodes of 1D conjugated coordination polymers for enhancing the performance of sodium-ion batteries

1D conjugated coordination polymers M-DHBQ (M = Mn, Zn, and Ni) were constructed for sodium-ion batteries. Ni-DHBQ delivered the highest performance. These results highlighted the importance of metal ions for high performance batteries.

Diradicals or Zwitterions: The Chemical States of m -Benzoquinone and Structural Variation after Storage of Li Ions

Diradicals or Zwitterions: The Chemical States of <i>m</i> -Benzoquinone and Structural Variation after Storage of Li Ions