Photo Responsive Electron and Proton Conductivity within a Hydrogen-Bonded Organic Framework

Flexible hydrogen-bonded organic frameworks (HOFs): opportunities and challenges

Flexible behavior is one of the most fascinating features of hydrogen-bonded organic frameworks (HOFs), which represent an emerging class of porous materials that are self-assembled via H-bonding between organic building units. Due to their unique flexibility, HOFs can undergo structural changes or transformations in response to various stimuli (physical or chemical). Taking advantage of this unique structural feature, flexible HOFs show potential in multifunctional applications such as gas storage/separation, molecular recognition, sensing, proton conductivity, biomedicine, etc. While some other flexible porous materials have been extensively studied, the dynamic behavior of HOFs remains relatively less explored. This perspective highlights the inherent flexible properties of HOFs, discusses their different flexible behaviors, including pore size/shape changes, interpenetration/stacking manner, H-bond breaking/reconstruction, and local dynamic behavior, and highlights their potential applications. We believe that this perspective will not only contribute to HOF chemistry and materials science, but will also facilitate the ongoing extensive research on dynamic porous materials.

Photoactivated Circularly Polarized Luminescent Organic Radicals in Doped Amorphous Polymer

Luminescent organic radicals, especially those with photoactivated circularly polarized luminescence (CPL) features, hold great significance for cutting-edge optoelectronic applications, but their development still remains a challenge. In this study, we propose a novel strategy to achieve photoactivated CPL radicals by bonding two phosphine centers within an axial chiral system, yielding a compound of R/S-5,5-bis(diphenylphosphino)-4,4'-bibenzo[d][1,3]dioxole (R/S-BDP). The photoactivated R/S-BDP molecules in polymer matrix display a robust quantum yield of 19.8 % and a dissymmetry factor (glum) of 1.2×10-4, marking this work as the first example of photoactivated CPL radicals. Furthermore, the glum is improved to 1.0×10-2 by using a liquid crystal as host. Experimental and theoretical analyses reveal that R/S-BDP molecules, endowed with double phosphine cores in axial chirality, offer a direct way for intramolecular electron transfer upon photoirradiation. This leads to the generation of radical ionic pairs, which subsequently trigger the donor-acceptor arrangement through intermolecular electron transfer, thereby resulting in stable radical emission. The extended photoactivated BDP-F exhibits a remarkably high quantum efficiency of 57.8%. Ultimately, the distinctive photo-responsive CPL radical luminescence has been successfully used for information displays and anti-counterfeiting.

Vitrification-enabled enhancement of proton conductivity in hydrogen-bonded organic frameworks

Hydrogen-bonded organic frameworks (HOFs) are versatile materials with potential applications in proton conduction. Traditional approaches involve incorporating humidity control to address grain boundary challenges for proton conduction. This study finds vitrification as an alternative strategy to eliminate grain boundary effect in HOFs by rapidly melt quenching the kinetically stable HOF-SXU-8 to glassy state HOF-g. Notably, a remarkable enhancement in proton conductivity without humidity was achieved after vitrification, from 1.31 × 10-7 S cm-1 to 5.62× 10-2 S cm-1 at 100 °C. Long term stability test showed negligible performance degradation, and even at 30 °C, the proton conductivity remained at high level of 1.2 × 10-2 S cm-1. Molecule dynamics (MD) simulations and X-ray total scattering experiments reveal the HOF-g system is consisted of three kinds of clusters, i.e., 1,5-Naphthalenedisulfonic acid (1,5-NSA) anion clusters, N,N-dimethylformamide (DMF) molecule clusters, and H+-H2O clusters. In which, the H+ plays an important role to bridge these clusters and the high conductivity is mainly related to the H+ on H3O+. These findings provide valuable insights for optimizing HOFs, enabling efficient proton conduction, and advancing energy conversion and storage devices.

Boron-Nitrogen-Embedded Polycyclic Aromatic Hydrocarbon-Based Controllable Hierarchical Self-Assemblies through Synergistic Cation-π and C-H···π Interactions for Bifunctional Photo- and Electro-Catalysis

Boron-Nitrogen-embedded polycyclic aromatic hydrocarbons (BN-PAHs) as novel π-conjugated systems have attracted immense attention owing to their superior optoelectronic properties. However, constructing long-range ordered supramolecular assemblies based on BN-PAHs remains conspicuously scarce, primarily attributed to the constraints arising from coordinating multiple noncovalent interactions and the intrinsic characteristics of BN-PAHs, which hinder precise control over delicate self-assembly processes. Herein, we achieve the successful formation of BN-PAH-based controllable hierarchical assemblies through synergistically leveraged cation-π and C-H···π interactions. By carefully adjusting the solvent conditions in two progressive assembly hierarchies, the one-dimensional (1D) supramolecular assemblies with "rigid yet flexible" assembled units are first formed by cation-π interactions, and then they can be gradually fused into two-dimensional (2D) structures under specific C-H···π interactions, thus realizing the precise control of the transformation process from BN-PAH-based 1D primary structures to 2D higher-order assemblies. The resulting 2D-BNSA, characterized by enhanced electrical conductivity and ordered 2D layered structure, provides anchoring and dispersion sites for loading two appropriate nanocatalysts, thus facilitating the efficient photocatalytic CO2 reduction (with a remarkable CH4 evolution rate of 938.7 μmol g-1 h-1) and electrocatalytic acetylene semihydrogenation (reaching a Faradaic efficiency for ethylene up to 98.5%).

Supramolecular polynuclear clusters sustained cubic hydrogen bonded frameworks with octahedral cages for reversible photochromism

Developing supramolecular porous crystalline frameworks with tailor-made architectures from advanced secondary building units (SBUs) remains a pivotal challenge in reticular chemistry. Particularly for hydrogen-bonded organic frameworks (HOFs), construction of geometrical cavities through secondary units has been rarely achieved. Herein, a body-centered cubic HOF (TCA_NH4) with octahedral cages was constructed by a C3-symmetric building block and NH4+ node-assembled cluster (NH4)4(COOH)8(H2O)2 that served as supramolecular secondary building units (SSBUs), akin to the polynuclear SBUs in reticular chemistry. Specifically, the octahedral cages could encapsulate four homogenous haloforms including CHCl3, CHBr3, and CHI3 with truncated octahedron configuration. Crystallographic evidence revealed the cages served as spatially-confined nanoreactors, enabling fast, broadband photochromic effect associated with the reversible photo/thermal transformation between encapsulated CHI3 and I2. Overall, this work provides a strategy by shaping SSBUs to expand the framework topology of HOFs and a prototype of hydrogen-bonded nanoreactors to accommodate reversible photochromic reactions.

A solvent-controlled photoresponsive ionic hydrogen-bonded organic framework for encryption applications

Two novel ionic hydrogen-bonded organic frameworks (iHOF-17 and iHOF-18) were obtained by integrating organosulfonic acids with amidine salts. Among them, iHOF-18 exhibits fast, reversible, and high-contrast UV-induced photochromic properties, and this property is solvent-controlled. This work provides valuable insights for designing advanced anti-counterfeiting techniques and encryption applications.

Charge-Assisted Ionic Hydrogen-Bonded Organic Frameworks: Designable and Stabilized Multifunctional Materials

Hydrogen-bonded organic frameworks (HOFs) are a class of crystalline framework materials assembled by hydrogen bonds. HOFs have the advantages of high crystallinity, mild reaction conditions, good solution processability, and reproducibility. Coupled with the reversibility and flexibility of hydrogen bonds, HOFs can be assembled into a wide diversity of crystalline structures. Since the bonding energy of hydrogen bonds is lower than that of ligand and covalent bonds, the framework of HOFs is prone to collapse after desolventisation and the stability is not high, which limits the development and application of HOFs. In recent years, numerous stable and functional HOFs have been developed by π-π stacking, highly interpenetrated networks, charge-assisted, ligand-bond-assisted, molecular weaving, and covalent cross-linking. Charge-assisted ionic HOFs introduce electrostatic attraction into HOFs to improve stability while enriching structural diversity and functionality. In this paper, we review the development, the principles of rational design and assembly of charge-assisted ionic HOFs, and introduces the different building block construction modes of charge-assisted ionic HOFs. Highlight the applications of charge-assisted ionic HOFs in gas adsorption and separation, proton conduction, biological applications, etc., and prospects for the diverse design of charge-assisted ionic HOFs structures and multifunctional applications.

Reticular synthesis of 8-connected carboxyl hydrogen-bonded organic frameworks for white-light-emission

The rational design and construction of hydrogen-bonded organic frameworks (HOFs) are crucial for enabling their practical applications, but controlling their structure and preparation as intended remains challenging. Inspired by reticular chemistry, two novel blue-emitting NKM-HOF-1 and NKM-HOF-2 were successfully constructed based on two judiciously designed peripherally extended pentiptycene carboxylic acids, namely H8PEP-OBu and H8PEP-OMe, respectively. The large pores within these two HOFs can adsorb fluorescent molecules such as diketopyrrolopyrrole (DPP) and 9-anthraldehyde (AnC) to form HOFs ⊃ DPP/AnC composites, subsequently used in the fabrication of white-light-emitting devices (WLEDs). Specifically, two WLEDs were assembled by coating NKM-HOF-1 ⊃ DPP-0.13/AnC-3.5 and NKM-HOF-2 ⊃ DPP-0.12/AnC-3 on a 330 nm ultraviolet LED bulb, respectively. The corresponding CIE coordinates were (0.29, 0.33) and (0.32, 0.34), along with corresponding color temperatures of 7815 K and 6073 K. This work effectively demonstrates the feasibility of employing reticular chemistry strategies to predict and design HOFs with specific topologies for targeted applications.

Smart-Responsive HOF Heterostructures with Multiple Spatial-Resolved Emission Modes toward Photonic Security Platform

Luminescent hydrogen-bonded organic frameworks (HOFs) with the unique dynamics and versatile functional sites hold great potential application in information security, yet most of responsive HOFs focus on the single-component framework with restrained emission control, limiting further applications in advanced confidential information protection. Herein, the first smart-responsive HOF heterostructure with multiple spatial-resolved emission modes for covert photonic security platform is reported. The HOF heterostructures are prepared by integrating different HOFs into a single microwire based on a hydrogen-bond-assisted epitaxial growth method. The distinct responsive behaviors of HOFs permit the heterostructure to simultaneously display the thermochromism via the framework transformation and the acidichromism via the protonation effect, thus generating multiple emission modes. The dual stimuli-controlled spatial-resolved emission modes constitute the fingerprint of a heterostructure, and enable the establishment of the smart-responsive photonic barcode with multiple convert states, which further demonstrate the dynamic coding capability and enhanced security in anticounterfeiting label applications. These results offer a promising route to design function-oriented smart responsive HOF microdevices toward advanced anticounterfeiting applications.

A Versatile Strategy for the Generation of Air-stable Radical-functionalized Materials

The exploration of a facile approach to create structurally versatile substances carrying air-stable radicals is highly desired, but still a huge challenge in chemistry and materials science. Herein, a non-contact method to generate air-stable radicals by exposing pyridine/imidazole ring-bearing substances to volatile cyanuric chloride vapor, harnessed as a chemical fuel is reported. This remarkable feat is accomplished through a nucleophilic substitution reaction, wherein an intrinsic electron transfer event transpires spontaneously, originating from the chloride anion (Cl- ) to the cationic nitrogen (N+ ) atom, ultimately giving rise to pyridinium/imidazolium radicals. Impressively, the generated radicals exhibit noteworthy stability in the air over one month owing to the delocalization of the unpaired electron through the extended and highly fused π-conjugated pyridinium/imidazolium-triazine unit. Such an approach is universal to diverse substances, including organic molecules, metal-organic complexes, hydrogels, polymers, and organic cage materials. Capitalizing on this versatile technique, surface radical functionalization can be readily achieved across diverse substrates. Moreover, the generated radical species showcase a myriad of high-performance applications, including mimicking natural peroxidase to accelerate oxidation reactions and achieving high-efficiency near-infrared photothermal conversion and photothermal bacterial inhibition.

Bionic Luminescent Skin as Ultrasensitive Temperature-Acoustic Sensor for Underwater Information Perception and Transmission

Bioinspired artificial luminescent skin (L-skin) integrated with multiple sensing functions significantly promotes the development of smart devices. It is considerably challenging to realize underwater sensing technologies. Here, a sharkskin-inspired Eu@HOF-TJ-1@TA L-skin (1) is prepared for both temperature and sound sensing. 1 is an ultrathin and flexible temperature sensor, in 298.15-358.15 K, exhibiting ultrahigh maximum relative sensitivity (97.669% K-1 ) and low minimum uncertainty (0.000 952 K). The temperature response mechanism is analyzed deeply. As a waterproofing acoustic sensor, 1 can monitor sound in both air and water with the greatest sound response frequencies of 400 and 300 Hz in air and water, respectively. The maximum sensitivities of 1 in air and water are 6 593 765.2 and 1 346 124.5 cps Pa-1 , respectively. The response times of 1 in air and water are as fast as 20 and 10 ms. The sound response processes of 1 in air and water are simulated by finite element simulation. Moreover, by using sharkskin-inspired 1, the actual water temperature can be monitored, and a series of water sound information can be recognized by using an artificial neural network. This work proposes a sharkskin-inspired L-skin for temperature and acoustic sensing and promotes the development of underwater sensing technology with high performances.

Hydrogen-Bonding Assembly Meets Anion Coordination Chemistry: Framework Shaping and Polarity Tuning for Xenon/Krypton Separation

Hybrid hydrogen-bonded (H-bonded) frameworks built from charged components or metallotectons offer diverse guest-framework interactions for target-specific separations. We present here a study to systematically explore the coordination chemistry of monovalent halide anions, i.e., F- , Cl- , Br- , and I- , with the aim to develop hybrid H-bond synthons that enable the controllable construction of microporous H-bonded frameworks exhibiting fine-tunable surface polarity within the adaptive cavities for realistic xenon/krypton (Xe/Kr) separation. The spherical halide anions, especially Cl- , Br- , and I- , are found to readily participate in the charge-assisted H-bonding assembly with well-defined coordination behaviors, resulting in robust frameworks bearing open halide anions within the distinctive 1D pore channels. The activated frameworks show preferential binding towards Xe (IAST Xe/Kr selectivity ca. 10.5) because of the enhanced polarizability and the pore confinement effect. Specifically, dynamic column Xe/Kr separation with a record-high separation factor (SF=7.0) among H-bonded frameworks was achieved, facilitating an efficient Xe/Kr separation in dilute, CO2 -containing gas streams exactly mimicking the off-gas of spent nuclear fuel (SNF) reprocessing.