π-Stacks of radical-anionic naphthalenediimides in a metal-organic framework

Antibacterial Properties of the Peptide Conjugated Naphthalene Diimide Radical Anion in the Aggregated State

A peptide-linked naphthalene diimide (PNDI) forms a nanospherical architecture in water medium in its J-type self-assembled state. In this aggregated state, the system becomes highly redox-active and undergoes single-electron reduction under photoreactive conditions (λmax = 365 nm). The resulting photoreduced red fluorescent radical anions have significant stability. The stability of the photoreduced radical anion formation is highly dependent upon its self-assemblies as it is found that in the monomeric state, the PNDI molecule fails to generate the photoinduced radial anion where the assembled state shows good stability. A computational study supports the stabilization of the extra electron in the stacking state. This stabilization of the extra electron causes an increase in the intermolecular binding energy of PNDI molecules in their aggregated state. Furthermore, the formation of the radical anion under the photoirradiated condition is highly concentration-dependent. At a higher concentration, the formation of radical dimer is evident; however, at a lower concentration, no such dimerization is found. Interestingly, this stable and noncytotoxic radical anion is capable of generating the reactive oxygen species (ROS) in the aqueous medium, and this results in its high antibacterial efficacy against both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus bacteria, with very low minimum inhibitory concentrations (MICs) of 16 and 30 μg/mL. Mechanistic studies revealed that the photoirradiated PNDI causes the rupture of the bacterial cell outer and inner membrane, as observed with the 8-anilino-1-naphthalenesulfonic acid dye test. The presence of reactive oxygen species (ROS) is also confirmed using the 2,7-dichlorofluorescein diacetate (DCFDA) test.

Synergistic ionic modification strategy enhances the stability of naphthalene diimide zwitterions for cost-effective aqueous organic redox flow batteries

Aqueous organic redox flow batteries (AORFBs) hold significant promise for energy storage due to their unique advantages and characteristics. However, their development is hindered by the lack of decomposition resistance and cycle stability over long periods. In this study, we synthesized naphthalene diimide (NDI) derivatives with zwitterions in their side chains via the atmospheric pressure method, namely (CBu)2NDI and (SPr)2NDI. The electrostatic repulsion between (CBu)2NDI precisely regulates π-π stacking into a parallel-staggered pattern. The synergistic zwitterions strategy effectively mitigates the positive charge of N+ in (CBu)2NDI compared with (NPr)2NDI and dex-NDI; this not only enhances the aromaticity of the naphthalene diimide core but also inhibits the side chain decomposition caused by the SN2 nucleophilic attack of hydroxyl ions (OH-) on the C=O. The calculation of the single point energy proves that during the charging processes of (CBu)2NDI, the K+ will be close to the naphthalene core to form dimers or monomers with lower energy configurations under electrostatic attraction. (CBu)2NDI achieved a water solubility up to 1.49 M, which can be paired with K4Fe(CN)6 under two-electron transfer with total electrolyte costs as low as $6.58 Ah-1. The 0.1 M battery maintains full capacity after 5070 cycles. Furthermore, the battery delivers an impressive 100% capacity retention under 2 M e- during 220 cycles.

Insights into Decoupled Solar Energy Conversion and Charge Storage in a 2D Covalent Organic Framework for Solar Battery Function

Decoupling solar energy conversion and storage in a single material offers a great advantage for off-grid applications. Herein, we disclose a two-dimensional naphthalenediimide (NDI)-based covalent organic framework (COF) exhibiting remarkable solar battery performance when used as a photoanode. Light-induced radicals are stabilized within the framework for several hours, offering on-demand charge extraction for electrical energy production. Our study reveals mechanistic insights into the long-term charge stabilization using optical spectroscopy and (photo)electrochemical measurements, in conjunction with density functional theory (DFT) simulations. Among several solvents, water provides the best dielectric screening and energetically favorable proton exchange to stabilize photoinduced radicals for more than 48 h without the need for additional metal cations. This study provides fundamental insights into the optoionic charge storage mechanism in NDI-COF, while introducing a highly tunable, nanoporous material platform that surpasses related materials, such as carbon nitrides, metal-organic frameworks (MOFs), or metal oxides, in terms of charge storage capacity. This study opens new perspectives for the design of optoionic charge-storing materials and the direct storage of solar energy to overcome the intermittency of solar irradiation.

Effective Regulation of Organic Radicals within a Radical-Doped Metal-Organic Framework by Post-Treatment

This work reports a rare radical-doped metal-organic framework (MOF) crystal with one-dimensional open channels achieved by coordination-driven self-assembly. X-ray single-crystal diffraction, solid-state UV-vis-NIR diffuse reflectance spectroscopy, and electron paramagnetic resonance analysis were used to clarify its unique structure and why many organic radicals stably exist in the crystal lattice. The study shows that this crystal has strong intermolecular cofacial π-π interactions, a narrow optical bandgap (only 1.47 eV), a low LUMO, and a shallow HOMO. These features allow an inherent charge transfer, leading to the genetation of many organic radicals under ambient conditions. Interestingly, post-treatment can effectively adjust the organic radicals in this MOF. Mechanical grinding quickly reduces the number of organic radicals. Then, soaking the sample in an N,N-dimethylformamide solution with diphenylamine, para-methylaniline, or carbazole (as electron donors), or physically grinding with these amine organic molecules, can recover the reduced radicals to some degree.

Giant Enhancement of Optical Nonlinearity by Manipulating Guest Molecular Stacking Modes in Metal-Organic Frameworks

The influence of guest stacking interactions in host-guest (H-G) MOF composites on third-order nonlinear optical (NLO) performance remains largely unknown. Herein, we propose for the first time a noncovalent aggregate confinement strategy for synthesizing H-G MOF composites with different guest stacking modes. And [perylene2]n (α-Pe) and [perylene]n (β-Pe) were selected as guests and confined into a novel Ca-based MOF {[Ca(TBAPy)(DMA)2]·3DMA·[N(CH3)2]·H2O}n (Ca-MOF-pts). The NLO results showed that compared to β-Pe@Ca-MOF-pts, the saturable absorption (SA) and self-defocusing properties of α-Pe@Ca-MOF-pts were increased by 2.71-fold and 3.82-fold, respectively. Interestingly, α/β-Pe@Ca-MOF-pts can be transformed into α/β-Pe@Ca-MOF-flu (Ca-MOF-flu = {[Ca1.5(TBAPy)(H2O)2]·DMA·[N(CH3)2]·2H2O}n) through self-adaptive topological evolution, and the corresponding NLO absorption signal change from SA to reverse saturable absorption (RSA). As expected, compared to β-Pe@Ca-MOF-flu, the RSA and self-defocusing properties of α-Pe@Ca-MOF-flu are improved by 2.94-fold and 4.07-fold, respectively, demonstrating the importance of guest stacking modes. Theoretical calculation and transient absorption spectra indicated the enhancement of NLO performance was attributed to the large π-π overlap of α-Pe, which promoted the electron delocalization/transfer and optimized the cross-sectional of the ground state and excited state. This study provides a new strategy for developing H-G MOF composites with excellent NLO properties.

Mechanical Grinding-Induced Intermolecular Charge Transfer for Near-Infrared Photothermal Conversion

In this study, charge-transfer-type compounds comprising synthesized naphthalenediimide derivative (H4NDISA) or its Pb-based coordination polymer (Pb-NDISA) and suitable primary or secondary amine organic molecules were prepared by the solvent-free mechanical grinding method. The coloration phenomenon arising from charge transfer during grinding serves as a discriminative tool for distinguishing various organic guest molecules. The porous structure of Pb-NDISA crystals facilitates the infiltration of guest molecules and contributes to the preservation of the intermolecular charge transfer state. Moreover, the intermolecular charge transfer induced by grinding exhibits remarkable stability in an ambient atmosphere, underscoring the pivotal role of well-ordered molecules in the mechanical grinding procedure. This mechanochromic phenomenon holds promise for the detection and sensing of organic molecules, while the exceptional charge-transfer absorption characteristics offer the potential for efficient near-infrared photothermal conversion.

A supramolecular naphthalenediimide radical anion through host-guest interactions for photooxidation of alkylarenes to carbonyls

A supramolecular naphthalenediimide radical anion was developed through host-guest interactions between NDI and cucurbit[7]uril (CB[7]), which can be greatly promoted in the presence of chloride ions to obtain Cl˙ and NDI-2CB[7]˙-. Under the synergistic action of Cl˙ as a hydrogen atom transfer (HAT) agent and NDI-2CB[7]˙- transferring electrons to O2 to produce O2˙-, the photocatalytic oxidation reactions of alkylarenes to carbonyls can be realized with universal applicability.

Commercializable Naphthalene Diimide Anolytes for Neutral Aqueous Organic Redox Flow Batteries

Neutral aqueous organic redox flow batteries (AORFBs) hold the potential to facilitate the transition of renewable energy sources from auxiliary to primary energy, the commercial production of anolyte materials still suffers from insufficient performance of high-concentration and the high cost of the preparation problem. To overcome these challenges, this study provides a hydrothermal synthesis methodology and introduces the charged functional groups into hydrophobic naphthalene diimide cores, and prepares a series of high-performance naphthalene diimide anolytes. Under the synergistic effect of π-π stacking and H-bonding networks, the naphthalene diimide exhibits excellent structural stability and the highest water solubility (1.85 M for dex-NDI) reported to date. By employing the hydrothermal method, low-cost naphthalene diimides are successfully synthesized on a hundred-gram scale of $0.16 g-1 ($2.43 Ah-1), which is also the lowest price reported to date. The constructed full battery achieves a high electron concentration of 2.4 M, a high capacity of 54.4 Ah L-1, and a power density of 318 mW cm-2 with no significant capacity decay observed during long-duration cycling. These findings provide crucial support for the commercialization of AORFBs and pave the way for revolutionary developments in neutral AORFBs.

Square-planar Tetranuclear Cluster-based Alkaline Earth Metal-organic Frameworks with Enhanced Proton Conductivity

Alkaline earth (AE) metal complexes have garnered significant interest in various functional fields due to their nontoxicity, low density, and low cost. However, there is a lack of systematic investigation into the structural characteristics and physical properties of AE-metal-organic frameworks (MOFs). In this research, we synthesized isostructural MOFs consisting of AE4(μ4-Cl) clusters bridged by benzo-(1,2;3,4;5,6)-tris(thiophene-2'-carboxylic acid) (BTTC3-) ligands. The resulting structure forms a truncated octahedral cage denoted as [AE4(m4-Cl)]6(BTTC)8, which further linked to a porous three-dimensional framework. Among the investigated AE ions (Ca, Sr, and Ba), the Ca4-MOF demonstrated good chemical stability in water compared to Sr4-MOF and Ba4-MOF. The N2 adsorption and solid-state UV-vis-NIR absorption behaviors were evaluated for all AE4-MOFs, showing similar trends among the different metal ions. Additionally, the proton conduction study revealed that the Ca4-MOF exhibited ultra-high proton conductivity, reaching 3.52×10-2 S cm-1 at 343 K and 98 % RH. Notably, the introduction of LiCl via guest exchange resulted in an improved proton conduction of up to 6.36×10-2 S cm-1 under similar conditions in the modified LiCl@Ca4-MOF. The findings shed light on the regulation of physical properties and proton conductivity of AE-MOFs, providing valuable insights for their potential applications in various fields.

Ultrastable Anion Radicals in Ligand-Dimerized Frameworks for Self-Accumulated Electrochemiluminescence

Electrochemiluminescence (ECL) is a light-emitting process that occurs via an annihilation reaction among energetic radical intermediates, whose stabilities determine the ECL efficiency. In this study, a ligand-dimerized metal-organic framework (MOF) with ultrastable anion radical is designed as an efficient nanoemitter for self-accumulated ECL. Due to the nonplanar structure of perylene diimide (PDI) derivate, two PDI ligands in the framework form a J-dimer unit with a vertical distance of ∼5.74 Å. In cathodic scanning, the ligand-dimerized MOF demonstrates three-step ECL emissions with a gradual increase in ECL intensity. Unlike the decrease in the PDI ligand, the self-accumulated ECL of the MOF was observed with 16.8-fold enhancement due to the excellent stability of radical intermediates in frameworks. Electron paramagnetic resonance demonstrated the ultrastability of free radicals in the designed frameworks, with 82.2% remaining even after one month of storage. Density functional theory calculations supported that PDI dimerization was energetically favorable upon successive electron injection. Moreover, the ECL wavelength is 610 nm, corresponding to the emission of excited dimers. The radical-stabilized reticular nanoemitters open up a new platform for decoding the fundamentals of self-accumulated ECL systems.

Radical-Driven Crystal-Amorphous-Crystal Transition of a Metal-Organic Framework

Self-assembly-based structural transition has been explored for various applications, including molecular machines, sensors, and drug delivery. In this study, we developed new redox-active metal-organic frameworks (MOFs) called DGIST-10 series that comprise π-acidic 1,4,5,8-naphthalenediimide (NDI)-based ligands and Ni2+ ions, aiming to boost ligand-self-assembly-driven structural transition and study the involved mechanism. Notably, during the synthesis of the MOFs, a single-crystal-amorphous-single-crystal structural transition occurred within the MOFs upon radical formation, which was ascribed to the fact that radicals prefer spin-pairing or through-space electron delocalization by π-orbital overlap. The radical-formation-induced structural transitions were further confirmed by the postsynthetic solvothermal treatment of isolated nonradical MOF crystals. Notably, the transient amorphous phase without morphological disintegration was clearly observed, contributing to the seminal structural change of the MOF. We believe that this unprecedented structural transition triggered by the ligand self-assembly magnifies the structural flexibility and diversity of MOFs, which is one of the pivotal aspects of MOFs.

A Supramolecular Naphthalene Diimide Radical Anion with Efficient NIR-II Photothermal Conversion for E. coli-Responsive Photothermal Therapy

We report a supramolecular naphthalene diimide (NDI) radical anion with efficient NIR-II photothermal conversion for E. coli-responsive photothermal therapy. The supramolecular radical anion (NDI-2CB[7])⋅- , which is obtained from the E. coli-induced in situ reduction of NDI-2CB[7] neutral complex, formed by the host-guest interaction between an NDI derivative and cucurbit[7]uril (CB[7]), exhibits unexpectedly strong NIR-II absorption and remarkable photothermal conversion capacity in aqueous solution. The NIR-II absorption is caused by the self-assembly of NDI radical anions to form supramolecular dimer radicals in aqueous solution, which is supported by theoretically predicted spectra. The (NDI-2CB[7])⋅- demonstrates excellent NIR-II photothermal antimicrobial activity (>99 %). This work provides a new approach for constructing NIR-II photothermal agents and non-contact treatments for bacterial infections.

Transformation of a copper-based metal-organic polyhedron into a mixed linker MOF for CO2 capture

A new mixed linker metal-organic framework (MOF) has been synthesized from a copper-based metal-organic polyhedron (MOP-1) and 2,2'-bipyridine (2,2'-bipy). The CuMOF-Bipy with a formula of [Cu₂(2,2'-bpy)₂(m-BDC)₂]_n is comprised of a binuclear Cu(II) node coordinated to 2,2'-bipy, and isophthalic acid (m-BDC), which bridges to neighboring nodes. The crystal structure of CuMOF-Bipy consists of a stacked two-dimensional framework with the sql topology. CuMOF-Bipy was characterized by single-crystal X-ray diffraction (SC-XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and CO₂ sorption. CuMOF-Bipy was shown to have one-dimensional sinusoidal channels that allow diffusion of CO₂ but not N₂.