Photochromic Semiconductive Hydrogen-Bonded Organic Framework (HOF) with Broadband Absorption

Rapidly Generated, Ultra-Stable, and Switchable Photoinduced Radicals: A Solid-State Photochromic Paradigm for Reusable Paper Light-Writing

Although photochromic molecules have attracted widespread interest in various fields, solid-state photochromism remains a formidable challenge, owing to the substantial conformational constraints that hinder traditional molecular photoisomerization processes. Benefiting from the significant color change upon radical generation, chemical systems enabling a photoinduced radical (PIR) behavior through photoinduced electron transfer (PET) could be ideal candidates for solid-state photochromism within minimized need of conformational freedom. However, the transient nature of radicals causes a dilemma in this Scheme. Herein, we present a general crystal engineering strategy for rapidly generated (7-s irradiation to saturation) and ultra-stable (lasting 12 weeks) PIRs in the solid state, based on the anti-parallel alignment of para-hydroxyphenyl groups of persulfurated arenes to form a strong non-covalent network for efficient PET and radical stabilization. Using this strategy, a PIR platform was constructed, with a superior photochromic behavior remaining in different solid forms (even in the fully-ground sample) due to their transcendent crystallization ability. On this basis, our compounds can be further processed into reusable papers for light-writing, accompanied by water fumigation for modulating the reversible process. This work provides new insights into addressing solid-state photochromism and can inspire a wide range of optical material design from the switchable radical perspective.

Efficient photocatalytic hydrogen production via single-atom Pt anchored hydrogen-bonded organic frameworks

Constructing single-atom catalysts (SACs) using organic porous framework materials as supports presents a promising approach for developing highly efficient photocatalysts for hydrogen evolution. However, the fabrication of SACs that are both highly stable and active poses a significant challenge, particularly in the precise anchoring of metal single atoms. In this study, we utilized 1,3,6,8-tetra (p-methyl benzoate) pyrene as a ligand to synthesize pyrene-based hydrogen-bonded organic frameworks (denoted as PFC-1) through a self-assembly approach. Subsequently, a liquid-phase photoreduction process was employed to deposit noble metal platinum (Pt) onto PFC-1, resulting in the fabrication of SACs (PFC-1@Pt). Characterization results confirmed that Pt existed in a monatomic state, anchored through PtC and PtO coordination bonds with PFC-1. Serving as electron capture and separation centers, the Pt single atoms effectively suppressed electron-hole recombination, thereby prolonging carrier lifetimes. Consequently, the PFC-1@Pt SAC exhibited efficient hydrogen evolution performance with a rate of 2202.5 μmol g-1 h-1 and maintained photocatalytic activity for over 40 h. Our findings provide a systematic approach for developing efficient and stable SACs based on HOFs, expanding the potential applications of HOF materials in photocatalysis.

Ionic Hydrogen-Bonded Organic Frameworks with a Two-Photon Synergistic Color Change and Their Information Encryption Applications

Photochromic hydrogen-bonded organic frameworks (HOFs) can introduce different luminescent functional groups to achieve synergistic controlled multiple color change properties, which are in great demand for diverse information encryption applications. We report in this paper switchable photochromic and photoluminescent dual luminescent functional group HOFs constructed with synergistic effects by N,N'-bis(2-phenylalanine)-1,4,5,8-naphthalenediimine (H2PheNDI) and benzenecarboximidamide 4,4'-azobis(hydrochloride) (AZBH). The crystal powder of iHOF-41 is orange-red in color, which can be changed to black under the irradiation of a 365 nm ultraviolet (UV) light source for 15 min. The photoisomerization rate of the crystal solution under continuous UV irradiation for 5 h was close to 99%. The composite membranes can achieve the properties of photochromism and photoluminescence when they are discolored under 365 nm UV irradiation and, at the same time, excite red bright fluorescence. This work achieves the construction of HOFs based on switching biluminescent functional groups and explores the synergistic mechanism of the photoisomerization process and photochromism as well as its practical application in information encryption.

Ultrafast mechanosynthesis of hydrogen-bonded organic frameworks with UV and NIR photoswitching of photochromic/photothermal behavior

We present a facile and ultrafast mechanosynthesis of hydrogen-bonded organic frameworks |C10N2H10‖HC2O4|2 with UV and NIR bidirectional photoswitching of photochromic/photothermal behavior. The reaction time is reduced to mere seconds, and the method is both high-yield and scalable.

Construction of a stable radical hydrogen-bonded metal-organic framework with functionalized tetrathiafulvalene linkers

A stable two-dimensional radical hydrogen-bonded metal-organic framework, constructed using a modified tetrathiafulvalene-tetrabenzoate ((2-Me)-H4TTFTB) linker and Cd2+ ions, exhibits a high electrical conductivity of 4.1 × 10-4 S m-1 and excellent photothermal conversion with a temperature increase of 137 °C in 15 s under the irradiation of a 0.7 W cm-2 808 nm laser.

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.

Exogenous Co-Reactant-Free Electrochemiluminescent Biosensor for Ratiometric Measurement of α-Glucosidase Based on a ZIF-67-Regulated Hydrogen-Bonded Organic Framework

The development of highly sensitive and selective analytical approaches for monitoring enzymatic activity is critical for disease diagnosis and biomedical research. Herein, we develop an exogenous co-reactant-free electrochemiluminescence (ECL) biosensor for the ratiometric measurement of α-glucosidase (α-Glu) based on a zeolitic imidazolate framework (ZIF-67)-regulated pyrene-based hydrogen-bonded organic framework (HOF-101). Target α-Glu can hydrolyze maltose to α-d-glucose, which can subsequently react with GOx to produce gluconic acid. The resultant gluconic acid can dissolve ZIF-67, leading to the recovery of the HOF-101 cathodic ECL signal and the decrease of the luminol anodic ECL signal. The long-range ordered structure of HOF-101 can speed up charge transfer, resulting in a stable and strong cathodic ECL signal. Moreover, ZIF-67 can not only efficiently quench the ECL signal of HOF-101 due to ECL resonance energy transfer between HOF-101 and ZIF-67 as well as the steric hindrance effect of ZIF-67 but also enhance the anodic ECL emission of luminol in dissolved O2 system because of its ordered and porous crystalline structure and the atomically dispersed Co2+. Notably, HOF-101 possesses a higher ECL efficiency (32.22%) compared with the Ru(bpy)32+ standard. Importantly, this ratiometric ECL biosensor shows high sensitivity (a detection limit of 0.19 U L-1) and a broad linear range (0.2-50 U L-1). This biosensor can efficiently eliminate systematic errors and enhance detection reliability without the involvement of exogenous co-reactants, and it displays good assay performance in human serum samples, holding great promise in biomedical research studies.

Multistate structures in a hydrogen-bonded polycatenation non-covalent organic framework with diverse resistive switching behaviors

The inherent structural flexibility and reversibility of non-covalent organic frameworks have enabled them to exhibit switchable multistate structures under external stimuli, providing great potential in the field of resistive switching (RS), but not well explored yet. Herein, we report the 0D+1D hydrogen-bonded polycatenation non-covalent organic framework (HOF-FJU-52), exhibiting diverse and reversible RS behaviors with the high performance. Triggered by the external stimulus of electrical field E at room temperature, HOF-FJU-52 has excellent resistive random-access memory (RRAM) behaviors, comparable to the state-of-the-art materials. When cooling down below 200 K, it was transferred to write-once-read-many-times memory (WORM) behaviors. The two memory behaviors exhibit reversibility on a single crystal device through the temperature changes. The RS mechanism of this non-covalent organic framework has been deciphered at the atomic level by the detailed single-crystal X-ray diffraction analyses, demonstrating that the structural dual-flexibility both in the asymmetric hydrogen bonded dimers within the 0D loops and in the infinite π-π stacking column between the loops and chains contribute to reversible structure transformations between multi-states and thus to its dual RS behaviors.

A novel diarylethene-based fluorescence sensor for Zn2+ detection and its application

A series of fluorometric sensors of Zn2+ have been synthesized due to the significant function of Zn2+ in the human body and environment. However, most of probes reported for detecting Zn2+ have high detection limit or low sensitivity. In this paper, an original Zn2+ sensor, namely 1o, was synthesized by diarylethene and 2-aminobenzamide. When Zn2+ was added, the fluorescence intensity of 1o increased by 11 times within 10 s, along with a fluorescence color change from dark to bright blue, and the detection limit (LOD) was calculated to be 0.329 μM. According to Job's plot curves, the binding mode of 1o and Zn2+ was measured as 1:1, which was further proved by 1H NMR spectra, HRMS and FT-IR spectra. The logic circuit was designed to take advantage of the fact that the fluorescence intensity of 1o can be controlled by Zn2+, EDTA, UV and Vis. In addition, Zn2+ in actual water samples were tested, in which the recovery rate of Zn2+ was between 96.5 % and 109 %. Furthermore, 1o was successfully made into a fluorescent test strip, which could be used to detect Zn2+ in the environment economically and conveniently.

Multi-Stimuli-Responsive Chromic Behaviors of an All-in-One Viologen-Based Cd(II) Complex

A one-dimensional Cd(II) chain coordination polymer constructed by an electron-deficient viologen-anchored carboxylate ligand was successfully synthesized. Owing to the favorable stimuli-chromic properties of viologen, the title compound shows reversible photochromism, thermochromism, electrochromism, and naked-eye-detectable differentiable vapochromic response to different volatile amines. The chromic behaviors of it are ascribed to the formation of viologen radicals triggered by external stimuli. And the differentiated response to volatile amines is attributed to the size effect of the amines as well as the steric hindrance effect of forming α/β Cv-H···Namines interactions of the viologen unit to further affect the occurrence of electron transfer. Such an all-in-one crystalline material might have more practical applications in photoelectric, erasable inkless printing, light printing, and volatile amine detection fields.

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

Rational design of crystalline porous materials with coupled proton-electron transfer has not yet been reported to date. Herein, we report a donor-acceptor (D-A) π-π stacking hydrogen-bonded organic framework (HOF; HOF-FJU-36) with zwitterionic 1,1'-bis(3-carboxybenzyl)-4,4'-bipyridinium (H2 L2+ ) as acceptor and 2,7-naphthalene disulfonate (NDS2- ) as donor to form a two-dimensional (2D) layer. Three water molecules were situated in the channels to connect with acidic species through hydrogen bonding interactions to give a 3D framework. The continuous π-π interactions along the a axis and the smooth H-bonding chain along the b axis provide the electron and proton transfer pathways, respectively. After 405 nm light irradiation, the photogenerated radicals could simultaneously endow HOF-FJU-36 with photoswitchable electron and proton conductivity due to coupled electron-proton transfer. By single-crystal X-ray diffraction (SCXRD) analyses, X-ray photoelectron spectroscopy (XPS), transient absorption spectra and density functional theory (DFT) calculations, the mechanism of the switchable conductivity upon irradiation has been demonstrated.

Incorporating Photochromic Viologen Derivative to Unprecedentedly Boost UV Sensitivity in Photoelectrochromic Hydrogel

Developing ultraviolet (UV) radiation sensors featuring high sensitivity, ease of operation, and rapid readout is highly desired in diverse fields. However, the strategies to enhance sensitivity of UV detection remain limited particularly for photochromic materials, which show colorimetric response toward UV irradiation. Guided by our initial goal of facilitating easier handling, we formulated a viologen derivative ([H₂L]-SC) incorporating hydrogel-based UV sensor which not only inherits the photochromism of [H₂L]-SC but also engenders an unprecedented reversible photoelectrochromic response that is absent in either [H₂L]-SC or hydrogel alone. Judicious synergy between photochromic [H₂L]-SC and polyacrylamide (PAM) converts the colorimetric response of [H₂L]-SC into the electrical resistance change of [H₂L]-SC@PAM, which amplifies the UV sensitivity of [H₂L]-SC by 2 orders of magnitude. Explicitly, the limit of detection (LOD) for UV decreases from 296.3 mJ/cm² based on the UV-vis absorption spectra of [H₂L]-SC to 2.83 mJ/cm² derived from the resistance variation of [H₂L]-SC@PAM. Moreover, linear correlation between the resistance reduction rate of [H₂L]-SC@PAM and UV dose rate can be established, rendering it as a dual platform for quantifying both the accumulated UV dose and the instant dose rate. In addition, the proposed strategy based on constructing photoelectrochromic hybrids offers a new pathway to boost the UV sensitivity that could be universal for other photochromic materials.

Novel Stimuli-Responsive Spiropyran-Based Switch@HOFs Materials Enable Dynamic Anticounterfeiting

Developing smart fluorescent materials having very advanced levels, showing dynamic displays of encrypted messaging, remains a huge challenge. In this paper, we present a unique method based on combining a common photochromic molecule spiropyran (SP) with hydrogen-bonded organic frameworks (HOFs), which allows for reversible switching of SP in solid states and shows dynamic displays of encrypted information. With the irradiation time extended, the fluorescence emission undergo an evident transformation from yellow-green to orange to red, because of the fluorescence resonance energy transfer (FRET) process between the unique HOFs and merocyanine (MC) isomer. By doping with polydimethylsiloxane (PDMS), we obtained free-standing membranes with high flexibility and mechanical strength, which can be reversibly and repeatedly bent and folded at angles of >90°. Notably, the comparison of fatigue resistance between SP2/PDMS (can be used for no more than 5 times) and SP2 ⊂ HOF2/PDMS (can be used for more than 100 times) further proved the importance of HOFs. This composite system has many advantages: (1) it has diverse dynamic fluorescence emission and visible colors regulated by ultraviolet radiation with high contrast and can be reversibly converted; (2) these changes in behavior can be achieved by simple UV illumination; and (3) compared with previous work, this work not only shows the dynamic fluorescence emission, but also shows the dynamic information during the decryption.

Advances in the Structural Strategies of the Self-Assembly of Photoresponsive Supramolecular Systems

Photosensitive supramolecular systems have garnered attention due to their potential to catalyze highly specific tasks through structural changes triggered by a light stimulus. The tunability of their chemical structure and charge transfer properties provides opportunities for designing and developing smart materials for multidisciplinary applications. This review focuses on the approaches reported in the literature for tailoring properties of the photosensitive supramolecular systems, including MOFs, MOPs, and HOFs. We discuss relevant aspects regarding their chemical structure, action mechanisms, design principles, applications, and future perspectives.