Multiple Electron Transfer in Semiconductive Ternary D-D'-A Metal-Organic Framework for Enhanced X-Ray Detection and Imaging

Semiconductive metal-organic frameworks (MOFs) with donor-acceptor (D-A) characteristics have garnered attractive attention due to their capacity for separating and transferring photogenerated charges, making them promising candidates for high-performance X-ray detectors. However, the low charge transfer efficiency between the metal nodes and organic ligands limits the X-ray-to-electricity conversion efficiency of these materials. Herein, an additional photoactive donor (D') is introduced by incorporating a heavy atom-containing polyoxometalate (POM) [α-SiW12O40]4- into a binary {[Ni·bcbp·(H2O)2]·(H2O)4·Cl}n (Ni-bcbp, bcbp: H2bcbp·2Cl = 1,1'-bis(4-carboxyphenyl)(4,4'-bipyridinium) dichloride) MOF, resulting in a semiconductive ternary D-D'-A framework {[Ni2(bcbp)2·(H2O)4·(DMA)]·(SiW12O40)}n (SiW@Ni-bcbp, DMA: dimethylacetamide). The obtained material features an unprecedented porous 8-connected bcu-net structure that accommodates nanoscale [α-SiW12O40]4- counterions, displaying uncommon optoelectronic responses. In contrast to binary Ni-bcbp, the SiW@Ni-bcbp framework exhibits distinctive photochromism and robust X-ray responsiveness, which can be attributed to the synergistic effects of the electron reservoir and multiple photoinduced electron transfer originating from the POMs. As a result, the X-ray detector based on SiW@Ni-bcbp demonstrates a sensitivity of 5741.6 µC Gyair -1 cm-2 with a low detection limit of 0.49 µGyair s-1. Moreover, the devices demonstrated the capability of producing clearness X-ray images, providing a feasible and stable solution for constructing high-performance direct X-ray detectors.

Photochromism and single-component white light emission from a metalloviologen complex based on 1,5-naphthyridine

Metalloviologens, as emerging electron-transfer photochromic compounds, have shown intriguing properties such as radiochromism, photochromism and photoconductance. However, only a limited number of them have been reported so far. Exploration of new metalloviologens is strongly desired. Herein, we report a new solvothermally synthesized metalloviologen complex [CdCl2(ND)2]n (1, ND = 1,5-naphthalenes) that exhibits photochromic and intrinsic white light emission properties. Density functional theory calculation results reveal that the photochromism could be assigned to photoinduced electron transfer from chlorine atoms to ND molecules. The photoinduced charge-separated states are heat/air stable, attributed to the delocalization of ND and strong intermolecular π-π interactions. Besides, complex 1 consistently emits intrinsic white light when excited with 340-370 nm UV light, achieving high color rendering index (CRI) values (82.54-94.04). By adjusting the excitation wavelength, both "warm" and "cold" white light emission can be produced, making it suitable for the application of a white light emitting diode (WLED). Thus, this work demonstrates that the ND-based metalloviologen is not only helpful in producing photochromism, but also beneficial for creating white-light emission.

X-ray-triggered through-space charge transfer and photochromism in silver nanoclusters

Materials exhibiting X-ray-induced photochromism have consistently piqued the interest of researchers. Exploring the photochromic properties of such materials is valuable for understanding the structural changes and electron transfer processes that occur under high energy radiation, such as X-ray irradiation. Here, a crystalline silver(I) nanocluster synthesized from tert-butylacetylene silver was found to have the ability to exhibit color and photoluminescence changes upon exposure to X-ray radiation. The responsive behavior was observed across a wide temperature range of 100-300 K, with the ability to respond particularly well to soft X-rays (λ > 1 Å) and exhibit light responsiveness to hard X-rays (λ < 1 Å). By combining experimental findings including X-ray diffraction, X-ray photoelectron spectroscopy, electron spin resonance, etc. with theoretical calculations, we have proposed that X-ray irradiation induces electron transfer from chloride (Cl-) located in the center of the silver(I) nanocluster to the surrounding Ag14 in the skeleton. This represents the first documented example in which electron transfer induced by X-ray excitation has been observed, accompanied by a photochromism process, in silver nanoclusters. This study contributes to our understanding of X-ray-induced photochromism and the electron transfer process in silver cluster compounds. It also provides valuable insights and potential design strategies for applications such as photochromism, photoluminescence color change, and photoenergy conversion.

Selective Photochromic Response to Low-Dose X-ray Radiation Detection in One-Dimensional Cadmium-Viologen Complexes

Photochromic viologen-based materials have emerged as one of the most promising candidates for the development of X-ray light detection applications, including medical diagnosis and treatment, environmental radiation inspection, and industrial crack detection. However, the design and construction of low-dose X-ray-sensitive complexes remains an immense challenge, especially for the in-depth dissection of their response mechanisms. Herein, by using N,N'-4,4'-bipyridiniodipropionate (CV) as functional sensitive structural units and cadmium as heavy atoms, two cadmium-viologen complexes with one-dimensional chained structures, namely, [Cd₂Cl₄(CV)(H₂O)₂]_n (1) and [CdBr₂(CV)]_n (2), have been constructed, which exhibit a remarkable and selective photochromic response to low-dose X-ray radiation detection. Compound 1 is visually sensitive to both X-ray and UV light due to the more accessible photoinduced electron transfer (ET) pathways, while compound 2 only shows a slight color-changing process in response to UV light, in conformity with UV-vis absorbance analyses and kinetic studies. Surprisingly, compound 2 has longer ET pathways than 1, but not in response to high-energy X-ray light, seeming to contradict the previous phenomena. On further analysis, the key point in achieving X-ray-sensitive behavior should be a good balance among the electron donor-acceptor distance, intermolecular interaction, and X-ray absorbing capacity, as verified by density functional theory (DFT) and X-ray absorption strength calculations, X-ray photoelectron spectra, electron paramagnetic resonance measurements, and independent gradient model analysis. In particular, compound 1 is unprecedentedly sensitive to soft X-ray radiation, accompanied by an X-ray detection limit of as low as 2.91 Gy. These findings push forward the further development of low-dose X-ray sensing materials.

Stepwise Crystallization of Millimeter Scale Thorium Cluster Single Crystals as a Bifunctional Platform for X-ray Detection and Shielding

Monitoring and shielding of X-ray radiation are of paramount importance across diverse fields. However, they are frequently realized in separate protocols and a single material integrating both functions remained elusive. Herein, a hexanuclear cluster [Th₆ (µ₃ -OH)₄ (µ₃ -O)₄ (H₂ O)₆ ](pba)₆ (HCOO)₆ (Th-pba-0D) incorporating high-Z thorium cations and 3-(pyridin-4-yl)benzoate ligands that can function as a brand-new dual-module platform for visible detection and efficient shielding of ionizing radiation is demonstrated. Th-pba-0D exhibits rather unique reversible radiochromism upon alternating X-ray and UV irradiation. Moreover, the millimeter scale crystal size of Th-pba-0D renders the penetration depth of X-ray visible to naked eye and leads to the unearthing of its high X-ray attenuation efficiency. Indeed, the shielding efficacy of Th-pba-0D is comparable to that of lead glass containing 40% PbO, and a Th-pba-0D pellet with a thickness of merely 1.2 mm can shield 99.73% X-ray (16 keV). These studies portend the possible utilization of thorium-bearing materials as a bifunctional platform for radiation detection and shielding.

X-Ray Triggered Coordination-Bond Breakage in Dysprosium-Organic Framework and its Impact on Magnetic Properties

Photosensitive lanthanide-based single-molecule magnets (Ln-SMM) are very attractive for their potential applications in information storage, switching, and sensors. However, the light-driven structural transformation in Ln-SMMs hardly changes the coordination number of the lanthanide ion. Herein, for the first time it is reported that X-ray (λ=0.71073 Å) irradiation can break the coordination bond of Dy-OH₂ in the three-dimensional (3D) metal-organic framework Dy₂ (amp₂ H₂ )₃ (H₂ O)₆  ⋅ 4H₂ O (MDAF-5), in which the {Dy₂ (OPO)₂ } dimers are cross-linked by dianthracene-phosphonate ligands. The structural transformation proceeds in a single-crystal-to-single-crystal (SC-SC) fashion, forming the new phase Dy₂ (amp₂ H₂ )₃ (H₂ O)₄  ⋅ 4H₂ O (MDAF-5-X). The phase transition is accompanied by a significant change in magnetic properties due to the alteration in coordination geometry of the Dy^III ion from a distorted pentagonal bipyramid in MDAF-5 to a distorted octahedron in MDAF-5-X.

A Three-Component Donor-Acceptor Hybrid Framework with Low-Power X-ray-Induced Photochromism

Donor-acceptor (D-A) hybrid frameworks with visual X-ray photochromism at room temperature are fascinating because of their promising applications as X-ray detectors. Herein, a 3-fold interpenetrated D-A hybrid framework, [Eu(bcbp)_1.5(DMF)(H₂O)₂][Co(CN)₆]·4H₂O·CH₃OH (1), has been obtained by incorporating electron-rich Co(CN)₆^3- into the electron-deficient europium viologen framework, which interestingly exhibits ultraviolet and low-power X-ray dual photochromism with a remarkable color change from brown to green. Experimental and theoretical studies revealed that the X-ray photochromic behavior of hybrid 1 could be attributed to its D-A hybrid structural feature increasing the extent of photoinduced electron transfer and thus photogenerated radical species upon X-ray irradiation. Meanwhile, due to the introduction of emissive lanthanide cations in the D-A system, hybrid 1 exhibits photomodulated luminescence properties.

A photochromic and scintillation Eu-MOF with visual X-ray detection in bright and dark environments

The detection of X-rays has always been a frontier of scientific research. An Eu-MOF with both X-ray-induced photochromic and scintillation properties has been synthesized through the combination of a photochromism-active viologen ligand and rare earth Eu element with high-efficiency absorption of X-rays. In a bright environment, Eu-MOF exhibits different color changes under high-energy X-rays and low-energy X-rays, which can effectively distinguish X-rays. Eu-MOF can also be used for X-ray detection by scintillation properties in dark environments. This work provides a new perspective on the design of multifunctional materials that can perform simple X-ray detection in different environments.

Unveiling the new function of uranyl molecular clusters as fluorometric sensors for UV and X-ray dosimetry

Simple synthetic modulation based on uranyl acetate and phenanthroline has resulted in two uranyl clusters (1 and 2) with different topologies and nuclearities. Notably, the dimeric complex exhibits distinct luminescence quenching upon UV and X-ray irradiation with detection limits of 4.30 × 10^-6 J and 0.32 Gy, respectively. To advance the practical application, 1 was further fabricated with polyvinylidene fluoride into a flexible strip as a UV and X-ray indicator.

An inclusion complex of cucurbit[7]uril with benzimidazolyl benzyl viologen exhibits fluorescence and photochromic properties

A self-assembled supramolecular inclusion complex of Q[7] with benzimidazolyl benzyl viologen exhibits interesting fluorescence emission and reversible photochromism.

Emergence of Thorium-Based Polyoxo Clusters as a Platform for Selective X-ray Dosimetry

A brand-new application of thorium-bearing clusters in the field of ionizing radiation detection is exemplified by two novel hexanuclear thorium clusters, Th-bppCOO-1 and Th-bppCOO-2, which incorporate carboxylate-functionalized 2,6-di(pyrazol-1-yl)pyridine ligands. Notably, Th-bppCOO-1 is composed of an unprecedented [Th₆(OH)₄O₄(H₂O)₅]¹²⁺ secondary building unit, the Th₆ core of which is decorated by five H₂O molecules. Furthermore, selective photoluminescence quenching responses of Th-bppCOO-1 and Th-bppCOO-2 toward X-ray over UV radiation have been demonstrated for the first time.

Emergence of a thorium-organic framework as a radiation attenuator for selective X-ray dosimetry

By first applying a thorium-organic framework (Th-SINAP-2) as a radiation attenuator and by incorporating a terpyridine derivative (Htpbz) as a photo-responsive guest, selective photochromism in response to X-rays was achieved in the host-guest assembly of Htpbz@Th-SINAP-2. Such a combination endows the afforded material with the lowest detection limit of X-ray dose among all photochromic sensors and a brand-new function of X-ray dosimetry for thorium containing materials.

Radiochromic Hydrogen-Bonded Organic Frameworks for X-ray Detection

Porous materials have been investigated as efficient photochromic platforms for detecting hazardous radiation, while the utilization of hydrogen bonded organic frameworks (HOFs) in this field has remained intact. Herein, two HOFs were synthesized through self-assembly of tetratopic viologen ligand and formic acid (PFC-25, PFC-26), as a new class of "all-organic" radiochromic smart material, opening a gate for HOFs in this field. PFC-26 is active upon both X-ray and UV irradiation, while PFC-25 is only active upon X-ray irradiation. The same building block yet different radiochromic behaviors of PFC-25 and PFC-26 allow us to gain a deep mechanistic understanding of the factors that control the detection specificity. Theoretical and experimental studies reveal that the degree of π-conjugation of viologen ligand is highly related to the threshold energy of triggering a charge transfer, therefore being a vital factor for the particularity of radiochromic materials. Thanks to its convenient processibility, nanoparticle size, and UV silence, PFC-25 can be further fabricated into a portable naked-eye sensor for X-ray detection, which shows obvious color change with the merits of high transmittance contrast, good sensitivity (reproducible dose threshold of 3.5 Gy), and excellent stability. The work exhibits the promising practical potentials of HOF materials in photochromic technology.

UV-vis/X-ray/thermo-induced synthesis and UV-SWIR photoresponsive property of a mixed-valence viologen molybdate semiconductor

A new design strategy through the synergy of Mo(vi)-Mo(v) intervalence charge transfer and π(radical)-π(radical/cation) interactions is proposed to obtain semiconductors with photoresponsive ranges covering the whole UV-SWIR (ultraviolet-shortwave near-infrared; ca. 250-3000 nm) region. With this strategy, a viologen-based molybdate semiconductor with a UV-SWIR photoresponsive range was obtained through UV/X-ray irradiation or thermal annealing. The thermally annealed semiconductor has the highest conversion and the best photocurrent response in the range of 355-2400 nm.

Concomitant Photoresponsive Chiroptics and Magnetism in Metal-Organic Frameworks at Room Temperature

Stimulus-responsive metal-organic frameworks (MOFs) can be used for designing smart materials. Herein, we report a family of rationally designed MOFs which exhibit photoresponsive chiroptical and magnetic properties at room temperature. In this design, two specific nonphotochromic ligands are selected to construct enantiomeric MOFs, {Cu₂(L-mal)₂(bpy)₂(H₂O)·3H₂O}_n (1) and {Cu₂(D-mal)₂(bpy)₂(H₂O)·3H₂O}_n (2) (mal = malate, bpy = 4, 4′ − bipyridine), which can alter their color, magnetism, and chiroptics concurrently in response to light. Upon UV or visible light irradiation, long-lived bpy⁻ radicals are generated via photoinduced electron transfer (PET) from oxygen atoms of carboxylates and hydroxyl of malates to bpy ligands, giving rise to a 23.7% increase of magnetic susceptibility at room temperature. The participation of the chromophores (-OH and -COO⁻) bound with the chiral carbon during the electron transfer process results in a small dipolar transition; thus, the Cotton effects of the enantiomers are weakened along with a photoinduced color change. This work demonstrates that the simultaneous responses of chirality, optics, and magnetism can be achieved in a single compound at room temperature and may open up a new pathway for designing chiral stimuli-responsive materials.

Achieving UV and X-ray Dual Photochromism in a Metal-Organic Hybrid via Structural Modulation

Rational design and synthesis of new photochromic sensors have been active research areas of inquiry, particularly on how to predict and tailor their properties and functionalities. Herein, two thulium 2,2':6',2''-terpyridine-4'-carboxylate (TPC)-functionalized metal-organic hybrids, Tm(TPC)₂(HCOO)(H₂O) (TmTPC-1) and Tm(TPC)(HCOO)₂ (TmTPC-2) with different photochromic response behaviors, have been successfully prepared, allowing for straightforward investigations of the structure-property correlation. Single-crystal X-ray diffraction and electron paramagnetic resonance analyses revealed that the incorporation of a unique dangling decorating TPC unit in TmTPC-1 offers a shorter and more accessible π-π interaction pathway between the adjacent TPC moieties than that in TmTPC-2. Such a structural feature leads to the production of radical species via a photoinduced intermolecular electron-transfer (IeMCT) process upon UV or X-ray irradiation, which ultimately endows TmTPC-1 with a rather unusual UV and X-ray dual photochromism. A linear relationship between the change of UV-vis absorbance intensity and X-ray dose was established, making TmTPC-1 a promising dosimeter for X-ray radiation with an extremely high energy threshold (30 kGy). To advance the development for real-world application, we have fabricated polyvinylidene fluoride (PVDF) membranes incorporating TmTPC-1 for functioning either as a UV imager or as an X-ray radiation indicator. Lastly, TmTPC-1 exhibits high thermal stability (up to 400 °C) and radioresistance (at least 900 kGy), and also excellent reversibility of photochromic transformation (at least 5 cycles).

Encapsulating organic guest cations in anionic MOFs that exhibit multi-responsive photochromism and photocontrolled luminescence

Two MOF-based host–guest compounds were prepared by incorporating viologen into an anionic framework, and exhibited photochromism upon irradiation with various light sources. Moreover, their luminescence properties can be switched by irradiation.

Rational assembly of metal-oxo clusters into molecular materials via a “wheel mounting” mode

Presented here is the self-assembly of metal-oxo clusters into molecular materials of different shapes and sizes via a “wheel mounting” mode, and molecular transformation was optical-driven.

A multifunctional colorimetric sensor originating from a cadmium naphthalene diimide-based metal–organic framework: photochromism, hydrochromism, and vapochromism

A new cadmium metal–organic framework with photocontrolled luminescence properties and multiple chromic properties was synthesized for used as a multifunctional colorimetric sensor and provide a new route to design functional MOFs.

Photosynergetic amplification of radiation input: from efficient UV induced cycloreversion to sensitive X-ray detection

Five photochromic terarylenes which show reversible photocyclisation and cycloreversion with relatively high quantum yields are presented. Some of these have been observed to undergo a highly efficient cycloreversion cascade process from their coloured, closed forms to their uncoloured open forms that leads to cycloreversion quantum yields significantly larger than unity. This cascade effect can been induced with both chemical and X-ray initiation; the limit of detection from X-ray initiation has been tested and is comparable to existing systems with detection observed at values as low as 0.3 mGy.

New photochromic terarylene displays extremely high photosensitivity with quantum yield as high as 3300% and radio-sensitivity to X-ray dose as weak as 0.3 mGy.

Sun, UV and X-ray triple photochromic properties of three coordination polymers based on 1,1′-bis(3-carboxylatobenzyl)-4,4′-bipyridinium ligand

Reversible color changes for three compounds can be observed in the air upon sunlight, UV-light and X-ray irradiation.