The Green Box: Selenoviologen-Based Tetracationic Cyclophane for Electrochromism, Host-Guest Interactions, and Visible-Light Photocatalysis

One-Pot 'Click' Synthesis of Ring-in-Rings Complexes with Customizable π-Stacked Dyads

We report an efficient one-pot aqueous synthesis of ring-in-rings complexes featuring customizable π-stacked dyads. Conventional methods for such complexes often suffer from poor solubility and low yields due to irreversible kinetically controlled reactions. To overcome these limitations, we developed a strategy combining noncovalent preassembly with efficient dynamic covalent bonding to secure a ring-in-rings complex as the exclusive thermodynamic product. Through complexation with cucurbit[8]uril (CB[8]), a folded conformation was induced in an aldehyde-functionalized bis(phenylpyridinium) derivative, predisposing the reactive aldehyde groups to promote acylhydrazone condensation with aromatic dihydrazides. The method achieves high conversion and purity, enabling direct single-crystal growth without the need for purification. We successfully synthesized π-stacked dyads across diverse aromatic moieties, including five distinct single-crystal structures demonstrating dimeric cofacial stacking. Kinetic analysis reveals that CB[8] complexation increases the ceiling temperature of the condensation reaction, rendering the process both thermodynamically and kinetically favorable. The modular nature of this strategy allows for precise tuning of photophysical properties by simply altering the linker lengths and the central aromatic cores, providing a facile platform for exploring structure-function relationships in ring-in-rings complexes and beyond.

Shape-Persistent Anthracene-Based Macrocycles Prepared by Reversible Boronic Ester Formation: Crystallization and Structural Analysis

Shape-persistent macrocycles with confined inner spaces have gained significant interest due to their unique properties and potential applications in gas/molecular recognition, nanoscale templates, and nanoelectronics. In this study, we present an efficient synthesis of macrocycles containing anthracene units through reversible boronic ester formation between 1,2-diols and boronic acids. These template-free macrocycles exhibited diverse internal cavities ranging from 11 Å to 20 Å and readily crystallized in solution and on solid substrates. Powder X-ray diffraction analysis revealed that the crystallinity remained after solvent removal. Single crystal X-ray analysis provided detailed insights into the molecular geometry and packing structure. Notably, a macrocycle with phenyl linkers resembles a pseudo-nanocapsule, as the bulky substituents on both sides of the macrocycles prevented the cavity filling by neighbouring molecules. Consequently, the crystalline powders of the macrocycle with phenyl linkers maintained its crystallinity even after annealing, likely resulting in the highest N2 gas adsorption properties among synthesized macrocycles. This work highlights a robust synthesis strategy for macrocycles, broadening their potential for advanced applications and enabling self-assembled nanoarchitectures.

Electric-Field Regulation of Adhesion/De-Adhesion/Release Capacity of Transparent and Electrochromic Adhesive

Removing adhesive nondestructively and intact from the adhered surface is a difficult challenge for advanced adhesive materials. Compared with the commonly used thermal or chemical release, the controlled adhesive release via electric-field offers practical application advantages. However, a noninvasive release mode such as this has not been available for the de-bonding of supramolecular adhesives that originate from small organic molecules. Herein, a conductive hydrogel with surface adhesion and electric field-triggered de-adhesion and release is fabricated from thioctic acid (TA) and L-arginine (LA). The non-covalent intermolecular attractions of poly[TA-LA], especially its electrostatic interactions, not only endow it with useful bulk-state properties and strong adhesion (up to 363.3 kPa) but also generate electric responsiveness for on-demand de-adhesion and release. The poly[TA-LA] adhesive layer can be easily released within a short time (<60 s) under a mild voltage (5≈10 V). After a combined experimental and theoretical investigation, It is concluded that the adhesive-layer morphological and mechanical changes, activated by a weak current (1.1≈3.2 mA), are responsible for the adhesion failure, which takes place primarily at the anode. Importantly, rapid electric release of poly[TA-LA] is applicable at low temperatures (5 V, 60 s, -40 °C) or underwater (5 V, 60 s, 25 °C).

Photochromic Metal-Organic Frameworks Based on Host-Guest Strategy and Different Viologen Derivatives for Organic Amines Sensing and Information Anticounterfeiting

The encapsulation of viologen derivatives in metal-organic frameworks (MOFs) to construct host-guest materials has been widely discussed owing to their distinctive spatial arrangement and physical/chemical properties. Herein, three new photochromic MOFs (NWM-1-3) have been successfully synthesized by 1,1,2,2-Tetra(4-carboxylphenyl)ethylene (H4TCPE) ligand as well as three different viologen derivatives based on host-guest strategy. Remarkably, NWM-1-3 exhibit a notable reversible photochromism change from yellow to green under 365 nm UV irradiation. The distance between the electron-deficient N atom in the viologens and the electron-rich carboxylate oxygens satisfies the electron transfer (ET) pathway, and thus ET occurs upon irradiation, producing intermolecular viologen radicals. NWM-1 is able to produce colored responses to different volatile amines by ET and can be recognizable to the naked eye. Differential pulse voltammetry (DPV) analysis and comparative experiments have demonstrated that the host-guest strategy significantly enhances the electron-accepting ability of viologens, thereby achieving superior amine sensing performance. NWM-2 and 3 have been realized in various applications, such as security code, fingerprint, and QR codes for anticounterfeiting. This work provides new host-guest strategy for designing highly sensitive photochromic materials and color-tunable luminescent materials, advancing the development of assembled photochromic materials closer to commercialization.

Electrochemical Gating of Host-Guest Charge-Transfer Interactions Enabled by Viologen-like Functionality Engineered Metal-Organic Frameworks

Using electrochemically responsive metal-organic frameworks (MOFs) as host matrices to afford gating properties for functional guests is rather attractive but remains unexplored. Herein, a series of functionalized Zr-MOFs with viologen-like skeletons were created by engineering 2,2'-bipyridinium bay substitution with different alkyl chains. Of the series, benefiting from the enhanced rigidity, the one bearing N,N'-ethylene bridge, UiO-67-EE, exhibited the strongest electron deficiency due to the lowest LUMO level, thereby leading to efficient electron transfer and favorable redox activity, which further endowed it with outstanding electrochromic properties. More importantly, the highly electron-deficient framework of UiO-67-EE could allow the accommodation of electron-rich guest molecules through host-guest charge transfer (CT) interactions. By leveraging the electroresponsiveness of the viologen-like functionality, UiO-67-EE served as an adaptable platform for controlled guest release and capture through efficient control of dynamic CT interactions upon stimuli of alternate potentials. This smart electrochemical gating behavior of the host-guest systems was also monitored in real time by distinguishable optical changes of the guests. Besides, it was exploited to develop high-performance sensing platforms by integrating a molecular gate constructed from the target-aptamer complex.

Hyperconjugation Engineering of π-Extended Azaphosphinines for Designing Tunable Thermally Activated Delayed Fluorescence Emitters

Implanting heteroatoms into organic π-conjugated molecules (OCMS) offered a great opportunity to fine-tune the chemical structures and optoelectronic properties. This work describes a new family of 1,4-azaphosphinines with extended σ-π hyperconjugations. The photophysical studies revealed that azaphosphinines exhibited narrow-band thermally activated delayed fluorescence (TADF) ( full width at half-maximum: 26-40 nm). According to the orbital localization analysis and natural bond orbital analysis, the effective σ*-π* hyperconjugation is believed to induce the multiple-resonance (MR) TADF, which is distinct from the p-π conjugation-induced MR-TADF in BN systems. Although having the large ΔES1-T1s (>3.0 ev), the study suggested that σ*-π hyperconjugation endowed the system with the structural vibration favorable for the spin-vibronic-assisted RISC. Having the tunable p-centers (lp, O, S, Se, and Me+), azaphosphinines showed a fine-tuned TADF. Generally, azaphosphinines with strong σ*-π* hyperconjugations showed small ΔES1-T1s, efficient RISCs, and high PLQYs. Leveraging on the efficient hyperconjugations, TADF emission of the system spanned from UV-blue to green. Particularly, extended azaphosphinines exhibited the high photoluminescence quantum yields (74% in toluene and 92% in the 10% doped PMMA). As a proof of concept, two azaphosphinines with a PO center were applied as the light-emitting materials in organic lighting-emitting diodes. The devices showed the narrow-band UV- and deep-blue emission with EQE as high as 10.3%. The current study offered us a new strategy, namely, σ-π hyperconjugation-induced MR-TADF, for designing OCMs with tunable light-emitting properties.

Pyrylazo Dye: A Novel Azo Dye Structure with Photoinduced Proton Release and Highlighted Photophysical Properties in Biological Media

A straightforward method for synthesizing a stable, photoreactive, and fluorescent-probe azo dye molecule is presented, highlighting the influence of azo and pyrylium groups within the electronic structure of the novel dye. This compound, named the pyrylazo molecule, is synthesized through the chemical reaction between 2,4,6-trimethylpyrylium and a 4-methoxybenzenediazonium species. The methyl group at the para position of the pyrylium readily reacts with the diazonium molecule, forming a stable protonated pyrylium-azo dye (N-protonated pyrylazo). The pyrylazo structure can easily change into its N-deprotonated form upon introduction of a weak base, such as an amine, promoting significant spectral shifts in the visible absorption and fluorescence bands. Because of that and other photochemical properties, this novel dye has shown significant potential for applications in photoinduced processes and biological contexts, particularly in Coulombic interactions with micelles and animal cells. In contrast to other nonfluorescent azo dyes, the singlet excited state of pyrylazo is deactivated through a radiative process in organized media, as evidenced by its behavior during micelle media, cell membrane permeation, and fluorescence emission in the cytoplasm. Nanosecond-transient absorption spectroscopy reveals a reversible photoinduced proton release process occurring in the excited singlet state, suggesting that the excited states of pyrylazo may play roles in transport through ion channels, artificial photosynthesis, and photoinduced protein folding. These promising applications underscore the pyrylium-azo structure as a novel dye with remarkable photochemical and photophysical properties not observed in other azo dye molecules reported before.

Directional Electron Flow in a Selenoviologen-Based Tetracationic Cyclophane for Enhanced Visible-Light-Driven Hydrogen Evolution

Directional electron flow in the photocatalyst enables efficient charge separation, which is essential for efficient photocatalysis of H2 production. Here, we report a novel class of tetracationic cyclophanes (7) incorporating bipyridine Pt(II) and selenoviologen. X-ray single-crystal structures reveal that 7 not only fixes the distances and spatial positions between its individual units but also exhibits a box-like rigid electron-deficient cavity. Moreover, host-guest recognition phenomena are observed between 7 and ferrocene, forming host-guest complexes with a 1 : 1 stoichiometry. 7 exhibits good redox properties, narrow energy gaps, and strong absorption in the visible range (370-500 nm) due to containing two selenoviologen (SeV2+) units. Meanwhile, the femtosecond transient absorption (fs-TA) reveals that 7 has stabilized dicationic biradical, efficient charge separation, and facilitates directional electron flow to achieve efficient electron transfer due to the formation of rigid cyclophane and electronic architecture. Then, 7 is applied to visible-light-driven hydrogen evolution with high hydrogen production (132 μmol), generation rate (11 μmol/h), turnover number (221), and apparent quantum yield (1.7 %), which provides a simplified and efficient photocatalytic strategy for solar energy conversion.

Recent advances in porous organic cages for energy applications

In recent years, the energy and environmental crises have attracted more and more attention. It is very important to develop new materials and technologies for energy storage and conversion. In particular, it is crucial to develop carriers that store energy or promote mass and electron transport. Emerging porous organic cages (POCs) are very suitable for this purpose because they have inherent advantages including structural designability, porosity, multifunction and post-synthetic modification. POC-based materials, such as pristine POCs, POC composites and POC derivatives also exhibit excellent energy-related properties. This latest perspective provides an overview of the progress of POC-based materials in energy storage and conversion applications, including photocatalysis, electrocatalysis (CO2RR, NO3RR, ORR, HER and OER), separation (gas separation and liquid separation), batteries (lithium-sulfur, lithium-ion and perovskite solar batteries) and proton conductivity, highlighting the unique advantages of POC-based materials in various forms. Finally, we summarize the current advances, challenges and further perspectives of POC-based materials in energy applications. This perspective will promote the design and synthesis of next-generation POC-based materials for energy applications.

Surface-Grafted Single-Atomic Pt-Nx Complex with a Precisely Regulating Coordination Sphere for Efficient Electron Acceptor-Inducing Interfacial Electron Transfer

Based on the electron-withdrawing effect of the Pt(bpy)Cl2 molecule, a simple post-modification amide reaction was firstly used to graft it onto the surface of NH2-MIL-125, which performed as a highly efficient electron acceptor that induced the conversion of the photoinduced charge migration pathway from internal BDC→TiOx migration to external BDC→PtNx migration, significantly improving the efficiency of photoinduced electron transfer and separation. Furthermore, precisely regulating over the first coordination sphere of Pt single atoms was achieved using further post-modification with additional bipyridine to investigate the effect of Pt-Nx coordination numbers on reaction activity. The as-synthesized NML-PtN2 exhibited superior photocatalytic hydrogen evolution activity of 7.608 mmol g-1 h-1, a remarkable improvement of 225 and 2.26 times compared to pristine NH2-MIL-125 and NML-PtN4, respectively. In addition, the superior apparent quantum yield of 4.01 % (390 nm) and turnover frequency of 190.3 h-1 (0.78 wt % Pt SA; 129 times compared to Pt nanoparticles/NML) revealed the high solar utilization efficiency and hydrogen evolution activity of the material. And macroscopic color changes caused by the transition of carrier migration paths was first observed. It holds profound significance for the design of MOF-Molecule catalysts with efficient charge carrier separation and precise regulation of single-atom coordination sphere.

RETRACTED: Synthesis and Properties of Novel Alkyl-Substituted Hexaazacyclophanes and Their Diradical Dications

Radicals based on arylamine cyclophanes can be used as functional materials and show application potential in fields such as synthetic chemistry, molecular electronic components, organic light-emitting diodes, and catalytic chemistry. Using a Buchwald-Hartwig palladium-catalyzed aryl halide amination method, we synthesized a series of neutral hexaazacyclophane compounds 1-3 with different substituents in the meta-meta-meta positions of the phenyl rings. Three characteristic high-spin hexaazacyclophane diradical dications were obtained by two-electron oxidation using AgSbF6: 12·+•2[SbF6]-, 22·+•2[SbF6]-, and 32·+•2[SbF6]-. The electronic structures and physical properties of these compounds were then investigated by 1H and 13C nuclear magnetic resonance spectroscopy, cyclic voltammetry, electron paramagnetic resonance spectroscopy, superconducting quantum interferometry, ultraviolet-visible spectroscopy, and density functional theory calculations. The findings provide new ideas for designing radical species with novel physical properties and electronic structures. Importantly, the obtained radical species are not sensitive to air, making them valuable functional materials for practical applications.

Exciplex Emission and Förster Resonance Energy Transfer in Polycyclic Aromatic Hydrocarbon-Based Bischromophoric Cyclophanes and Homo[2]catenanes

Energy transfer and exciplex emission are not only crucial photophysical processes in many living organisms but also important for the development of smart photonic materials. We report, herein, the rationally designed synthesis and characterization of two highly charged bischromophoric homo[2]catenanes and one cyclophane incorporating a combination of polycyclic aromatic hydrocarbons, i.e., anthracene, pyrene, and perylene, which are intrinsically capable of supporting energy transfer and exciplex formation. The possible coconformations of the homo[2]catenanes, on account of their dynamic behavior, have been probed by Density Functional Theory calculations. The unique photophysical properties of these exotic molecules have been explored by steady-state and time-resolved absorption and fluorescence spectroscopies. The tetracationic pyrene-perylene cyclophane system exhibits emission emanating from a highly efficient Förster resonance energy transfer (FRET) mechanism which occurs in 48 ps, while the octacationic homo[2]catenane displays a weak exciplex photoluminescence following extremely fast (<0.3 ps) exciplex formation. The in-depth fundamental understanding of these photophysical processes involved in the fluorescence of bischromophoric cyclophanes and homo[2]catenanes paves the way for their use in future bioapplications and photonic devices.