Achieving large thermal hysteresis in an anthracene-based manganese(II) complex via photo-induced electron transfer

Crystal-Defect-Induced Longer Lifetime of Excited States in a Metal-Organic Framework Photocatalyst to Enhance Visible-Light-Mediated CO2 Reduction

We report the structural defects in Zr-metal-organic framework (MOFs) for achieving highly efficient CO2 reduction under visible light irradiation. A series of defective Zr-MOF-X (X = 160, 240, 320, or 400) are synthesized by acid-regulated defect engineering. Compared to pristine defect-free Zr-MOF (NNU-28), N2 uptake increases for Zr-MOF-X synthesized with the HAc modulator, producing a larger pore space and Brunauer-Emmett-Teller surface area. The pore size distribution demonstrates that defective Zr-MOF-X exhibits mesoporous structures. Electrochemistry tests show that defective Zr-MOF-X possesses a more negative reduction potential and a higher photocurrent responsive signal than that of pristine NNU-28. Consequently, the defective samples exhibit a significantly higher efficiency in the photoreduction of CO2 to formate. Transient absorption spectroscopies manifest that structural defects modulate the excited-state behivior of Zr-MOF-X and improve the photogenerated charge separation of Zr-MOF-X. Furthermore, electron paramagnetic resonance and in-suit X-ray photoelectron spectroscopy provide additional evidence of the high photocatalytic performance exhibited by defective Zr-MOF-X. Results demonstrate that structural defects in Zr-MOF-X also improve the charge transfer, producing abundant Zr(III) catalytically active sites, exhibiting a slower decay process than defect-free Zr-MOF. The long-lifetime Zr(III) species in defective Zr-MOF-X are fully exposed to a high-concentration CO2 atmosphere, thereby enhancing the photocatalytic efficiency of CO2 reduction.

Reversible light-induced spin state switching in a dinuclear Fe(II) spin crossover complex

A dinuclear Fe(II) spin crossover (SCO) complex with the formula [Fe2L5(NCS)4]·2DMF·2H2O (1) was synthesised from 1-naphthylimino-1,2,4-triazole (L). Complex 1 exhibits an incomplete thermally induced spin transition with a transition temperature T1/2 of 95 K and a thermally trapped metastable high-spin state at low temperatures. Furthermore, it undergoes a reversible light-induced spin crossover by alternate irradiation with 532 and 808 nm lasers.

Photo-Thermal Mediated Li-ion Transport for Solid-State Lithium Metal Batteries

The development of lithium-based solid-state batteries (SSBs) has to date been hindered by the limited ionic conductivity of solid polymer electrolytes (SPEs), where nonsolvated Li-ions are difficult to migrate in a polymer framework at room temperature. Despite the improved cationic migration by traditional heating systems, they are far from practical applications of SSBs. Here, an innovative strategy of light-mediated energy conversion is reported to build photothermal-based SPEs (PT-SPEs). The results suggest that the nanostructured photothermal materials acting as a powerful light-to-heat converter enable heating within a submicron space, leading to a decreased Li+ migration barrier and a stronger solid electrolyte interface. Via in situ X-ray diffraction analysis and molecular dynamics simulation, it is shown that the generated heating effectively triggers the structural transition of SPEs from a highly crystalline to an amorphous state, that helps mediate lithium-ion transport. Using the assembled SSBs for exemplification, PT-SPEs function as efficient ion-transport media, providing outstanding capacity retention (96% after 150 cycles) and a stable charge/discharge capacity (140 mA g-1 at 1.0 C). Overall, the work provides a comprehensive picture of the Li-ion transport in solid polymer electrolytes and suggests that free volume may be critical to achieving high-performance solid-state batteries.

A Photoelectromagnetic 3D Metal-Organic Framework from Flexible Tetraarylethylene-Backboned Ligand and Dynamic Copper-Based Coordination Chemistry

Porous frameworks that display dynamic responsiveness are of interest in the fields of smart materials, information technology, etc. In this work, a novel copper-based dynamic metal-organic framework [Cu3TTBPE6(H2O)2] (H4TTBPE = 1,1,2,2-tetrakis(4″-(1H-tetrazol-5-yl)-[1,1″-biphenyl]-4-yl)ethane), denoted as HNU-1, is reported which exhibits modulable photoelectromagnetic properties. Due to the synergetic effect of flexible tetraarylethylene-backboned ligands and diverse copper-tetrazole coordination chemistries, a complex 3D tunneling network is established in this MOF by the layer-by-layer staggered assembly of triplicate monolayers, showing a porosity of 59%. These features further make it possible to achieve dynamic transitions, in which the aggregate-state MOF can be transferred to different structural states by changing the chemical environment or upon heating while displaying sensitive responsiveness in terms of light absorption, photoluminescence, and magnetic properties.

Guest-Induced Reversible Single-Crystal-to-Single-Crystal Transformation Involving Displacement of 2D Layers and Spin Crossover Behavior Change in a Hofmann-Type Coordination Polymer

A novel two-dimensional (2D) Hofmann-type coordination polymer, {FeII(PyHbim)2[Pd(CN)4]}·2CH3OH [1·2CH3OH, PyHbim = 2-(4-pyridyl)benzimidazole], has been synthesized, which can undergo a spontaneous guest exchange, transforming to 1·2H2O in a single-crystal-to-single-crystal (SCSC) manner, shifting from orthorhombic Cmmm to monoclinic C2/m involving the displacement of 2D layers. The solvent-induced SCSC transformation process was reversible and verified through powder X-ray diffraction (PXRD) and single-crystal X-ray crystallography analyses. Both 1·2CH3OH and 1·2H2O exhibit complete and abrupt spin crossover (SCO) behaviors in two steps, while their SCO temperature ranges drastically shift by ca.100 K, spanning room temperature, owing to different intermolecular interactions resulting from diverse interlayer packing manners and host-guest interactions. Besides, a structural phase transition is observed in 1·2CH3OH, contributing to the two-step spin transition.

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.

Superior Electrochemical Water Splitting and Energy-Storage Performances of In Situ Fabricated Charge-Separated Metal Organophosphonate Single Crystals

The design and exploration of advanced materials as a durable multifunctional electrocatalyst toward sustainable energy generation and storage development is the most perdurable challenge in the domain of renewable energy research. Herein, a facile in situ solvothermal approach has been adopted to prepare a methylviologen-regulated crystalline metal phosphonate compound, [C12H14N2][Ni(C11H11N2)(H2hedp)2]2•6H2O (NIT1), (H4hedp = 1-hydroxyethane 1,1-diphosphonic acid) and well characterized by several techniques. The as-prepared NIT1 displays excellent bifunctional electrocatalytic activity with dynamic stability toward oxygen evolution reaction (η10 = 288 mV) and hydrogen evolution reaction (η10 = 228 mV) in alkaline (1.0 M KOH) and acidic mediums (0.5 M H2SO4), respectively. Such a low overpotential and Tafel slope (68 mV/dec for OER; 56 mV/dec for HER) along with long-term durability up to 20 h of NIT1 make it superior to benchmark the electrocatalyst and various nonprecious metal-based catalysts under similar experimental condition. Further, the electrochemical supercapacitor measurements (in three-electrode system) reveal that the NIT1 electrode possesses much higher specific capacity of 187.6 C g-1 at a current density of 2 A g-1 (272 C g-1 at 5 mV s-1) with capacitance retention of 75.2% over 10,000 cycles at 14 A g-1 (Coulombic efficiency > 99%) in 6 M KOH electrolyte medium. Finally for a practical application, an asymmetric supercapacitor device (coin cell) is assembled by NIT1 material. The as-fabricated device delivers the maximum energy density of 39.4 Wh kg-1 at a power density of 450 W kg-1 and achieves a wide voltage window of 1.80 V. Notably, the device endures a remarkable cycle performance with cyclic retention of 92% (Coulombic efficiency > 99%) even after 14,000 charge/discharge cycles at 10 A g-1. Nevertheless, the extraordinary electrochemical activities toward OER and HER as well as the high-performance device fabrication for LED illumination of such a noble metal-free lower-dimensional charge-transfer compound are truly path breaking and would be promising for the development of advanced multifunctional materials.

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.

Photochromism and Photomagnetism in Two Ni(II) Complexes Based on a Photoactive 2,4,6-Tris-2-Pyridyl-1,3,5-Triazine Ligand

It is still challenging to construct novel photochromic and photomagnetic materials in the field of molecular materials. Herein, the 2,4,6-tris-2-pyridyl-1,3,5-triazine (TPTz) molecule was found to display photochromic properties under room temperature light irradiation. Two mononuclear structures, [Ni(H2O)(TPTz)(C2O4)]·2H2O (1; C2O42- = oxalate) [Ni(H2O)(TPTz)(C2O4)]·0.5H2O (2), and one chain compound [Ni(TPTz)(H2-HEDP)]·2H2O (3; HEDP = hydroxyethylidene diphosphonate) were obtained by assembling TPTz with polydentate O-ligands (oxalate and phosphonate) and the paramagnetic Ni2+ ions. The electron-transfer (ET)-dominated photochromism was observable in 1 and 2 after light irradiation with the photogeneration of relatively stable radicals, and the resultant photochromism was demonstrated via UV-vis, photoluminescence, X-ray photoelectron spectra, electron paramagnetic resonance spectra, and molecular orbital calculations. Due to the denser stacking interactions between the adjacent organic molecules, 2 exhibited a faster photochromic rate than 1. Compared with 1 and 2, compound 3 did not show photochromic behavior, which was deciphered by the theoretical calculations for all of the compounds. Importantly, the magnetic couplings appeared between photogenerated radicals and paramagnetic Ni2+ ions, resulting in a scarcely photomagnetic phenomenon of 1 and 2 in the Ni-based electron transfer photochromic materials. This work enriches the available kind of ligands for the design of ET photochromic materials, putting forward a method to tune the electron transfer photochromic efficiency in the molecular materials.

Anthraquinodimethane-Based Molecular Switches Tethered by Four-Arm Star-like Polymers

Molecular switches that reversibly change their structures and physical properties are important for applications such as sensing and information processing at molecular scales. In order to avoid the intermolecular aggregation that is often detrimental to the stimuli-responses of molecular switches, previous studies of molecular switches have been often conducted in dilute solutions which are difficult for applications in solid-state devices. Here we report molecular design and synthesis that integrates anthraquinodimethane as molecular switching units into polymers with amenable processibility in solid states. Optical and electron spin resonance characterizations indicate that the four-arm polymers of poly(ϵ-caprolactone) or poly(D,L-lactide) tethered from anthraquinodimethane slow down the dynamics of the conformational switching between the folded and the twisted conformations, enhance the photoluminescence in solid states and impart materials with a small energy gap from singlet ground state to thermally accessible triplet state.

Photoinduced large magnetic change at room temperature and radical-quenched spin glass in a cyanide-bridged MnII-FeIII compound

By the coordination assembly of a redox photoactive functional motif and a cyanide-bridged moiety, a cyanide-bridged MnII-FeIII compound with large photoinduced magnetic change at room-temperature due to photoinduced electron transfer was obtanied. This compound also shows unprecedented radical-quenched spin glass in molecule based magnets.

Piezochromism and Conductivity Modulations under High Pressure by Manipulating the Viologen Radical Concentration

Manipulating the radical concentration to modulate the properties in solid multifunctional materials is an attractive topic in various frontier fields. Viologens have the unique redox capability to generate radical states through reversible electron transfer (ET) under external stimuli. Herein, taking the viologens as the model, two kinds of crystalline compounds with different molecule-conjugated systems were designed and synthesized. By subjecting the specific model viologens to pressure, the cross-conjugated 2-X all exhibit much higher radical concentrations, along with more sensitive piezochromic behaviors, compared to the linear-conjugated 1-X. Unexpectedly, we find that the electrical resistance (R) of 1-NO3 decreased by three orders of magnitude with the increasing pressure, while that in high-radical-concentration 2-NO3 remained almost unchanged. To date, such unusual invariant conductivity has not been documented in molecular-based materials under high pressure, breaking the conventional wisdom that the generations of radicals are beneficial to improve conductivity. We highlight that adjusting the molecular conjugation modes can be used as an effective way to regulate the radical concentrations and thus modulate properties rationally.

Dual-radical-based molecular anisotropy and synergy effect of semi-conductivity and valence tautomerization in a photoswitchable coordination polymer

Organic radicals are widely used as linkers or ligands to synthesize molecular magnetic materials. However, studies regarding the molecular anisotropies of radical-based magnetic materials and their multifunctionalities are rare. Herein, a photoisomerizable diarylethene ligand was used to form {[Co^III(3,5-DTSQ^·-)(3,5-DTCat^2-)]₂(6F-DAE-py₂)}·3CH₃CN·H₂O (o-1·3CH₃CN·H₂O, 6F-DAE-py₂ = 1,2-bis(2-methyl-5-(4-pyridyl)-3-thienyl)perfluorocyclopentene), a valence-tautomeric (VT) coordination polymer. We directly observed dual radicals for a single crystal using high-field/-frequency (∼13.3 T and ∼360 GHz) electron paramagnetic resonance (EPR) spectroscopy along the c-axis, which was further confirmed by angle-dependent Q-band EPR spectroscopy. Moreover, a conductive anomaly close to the VT transition temperature was observed only when probes were attached at the ab plane of the single crystal, indicative of synergy between valence tautomerism and conductivity. Structural anisotropy studies and density functional theory (DFT) calculations revealed that this synergy is due to electron transfer associated with valence tautomerism. This study presents the first example of dual-radical-based molecular anisotropy and charge-transfer-induced conductive anisotropy in a photoswitchable coordination polymer.

Long-Lived Multiple Charge Separation by Proton-Coupled Electron Transfer

Multiple charge separation has been successfully realized by a proton-coupled electron transfer reaction in an organic cocrystal. Benefiting from the adjustable electronic energy level of the electron donor and acceptor through thermal-induced proton migration, distinct optical absorption behaviors combined with color changes to blue or green are observed in these charge-separated states. It is of interest to note that such charge-separated states exhibit a longer lifetime of over a month as a result of the excellent coplanarity and π-π interaction of the electron acceptors. Moreover, the enhanced absorption toward longer wavelengths endows the charge-separated state with near-infrared (808 nm) photothermal conversion for imaging and bacterial inhibition, whereby the conversion performance can be controlled by the degree of proton migration.

Leveraging Crystalline and Amorphous States of a Metal-Organic Complex for Transformation of the Photosalient Effect and Positive-Negative Photochromism

Uncovering differences between crystalline and amorphous states in molecular solids would both promote the understanding of their structure-property relationships, as well as inform development of multi-functional materials based on the same compound. Herein, for the first time, we report an approach to leverage crystalline and amorphous states of a zero-dimensional metal-organic complex, which exhibited negative and positive photochromism, due to the competitive chemical routes between photocycloaddition and photogenerated radicals. Furthermore, different polymorphs lead to the on/off toggling of photo-burst movement (photosalient effect), indicating the controllable light-mechanical conversion. Three demos were further constructed to support their application in information encryption and anti-counterfeiting. This work provides the proof-of-concept of a state- and polymorph-dependent photochemical route, paving an effective way for the design of new dynamically responsive systems.

Gradual Size Enlargement of Aluminum-Oxo Clusters and the Photochromic Properties

Metallic clusters, assembled by functional motifs, possess the attribute of regulating the properties by changing inorganic and organic components. In this work, a series of aluminum-oxo clusters, [Al₆O(dmp)₄(Hdmp)₂]·2ⁱPrOH [Al₆-1, H₃dmp = 2,2-bis(hydroxymethyl)propionic acid], [Al₆(H₂thmmg)₆]·2DMF·2H₂O [Al₆-2, H₅thmmg = N-tris(hydroxymethyl)methylglycine], [Al₈(OH)₄(NAP-OH)₁₂(MeO)₇(MeOH)]Cl·7MeCN·3MeOH (Al₈, HNAP-OH = 3-hydroxy-2-naphthoic acid), and [Al₁₀(NA)₁₀(MeO)₂₀] (Al₁₀, HNA = nicotinic acid), were obtained based on different carboxylic acids, realizing metallic ring size enlargement from 5.91 to 9.32 Å. They all exhibit good chemical stability. Importantly, the Al₈ cluster displays obvious photochromic behavior from pale yellow to orange yellow, originating from the generation of photoinduced radicals in the metal-assisted ligand-ligand electron transfer process of 3-hydroxy-2-naphthoic acid (HNAP-OH). This work enriches the metal ring cluster chemistry and reports the example of the aluminum-oxo cluster-based photochromic material, developing a novel system of photochromic materials.

Magnetic and Luminescent Dual Responses of Photochromic Hexaazamacrocyclic Lanthanide Complexes

Herein, hexaazamacrocyclic ligand L^N6 was employed to construct a series of photochromic rare-earth complexes, [Ln(L^N6)(NO₃)₂](BPh₄) [1-Ln, Ln = Dy, Tb, Eu, Gd, Y; L^N6 = (3E,5E,10E,12E)-3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane-3,5,10,12-tetraene]. The behavior of photogenerated radicals of hexaazamacrocyclic ligands was revealed for the first time. Upon 365 nm light irradiation, complexes 1-Ln exhibit photochromic behavior induced by photogenerated radicals according to EPR and UV-vis analyses. Static and dynamic magnetic studies of 1-Dy and irradiated product 1-Dy* indicate weak ferromagnetic interactions among Dy^III ions and photogenerated L^N6 radicals, as well as slow magnetization relaxation behavior under a 2 kOe applied field. Further fitting analyses show that the magnetization relaxation in 1-Dy* is markedly different from 1-Dy. Time-dependent fluorescence measurements reveal the characteristic luminescence quenching dynamics of lanthanide in the photochromic process. Especially for irradiated product 1-Eu*, the luminescence is almost completely quenched within 5 min with a quenching efficiency of 98.4%. The results reported here provide a prospect for the design of radical-induced photochromic lanthanide single-molecule magnets and will promote the further development of multiresponsive photomagnetic materials.

One-Dimensional Chain Viologen-Based Lanthanide Multistimulus-Responsive Materials with Photochromism, Photoluminescence, Photomagnetism, and Ammonia/Amine Vapor Sensing

In this work, a series of novel multistimulus-responsive lanthanide coordination polymers {[LnL(H₂O)₄]Cl₃·3H₂O}_n (Ln = Dy, Tb, Eu) constructed using a dicarboxylic acid viologen derivative L (L = N,N'-4,4'-bipyridiniodipropionate) and LnCl₃·6H₂O were prepared. All materials showed positive responses to UV light, and the photochromic phenomena accompanied by significant photoquenching of photoluminescence could be observed through a photoelectron transfer mechanism. Strikingly, the Dy analogue displayed photomagnetic behavior, as well as responded positively to small molecules of inorganic ammonia/organic amines. Furthermore, the good photoresponsive and ammonia/amine vapor-responsive properties of the Dy-based material were further fulfilled in dual-function papers involving erasable inkless printing and visual amine detection applications. This work aims to advance the development of multistimulus-responsive multifunctional materials incorporating viologen derivates and versatile lanthanide ions and further enriches the research in this field.

Two Stable Cd-MOFs as Dual-Functional Materials with Luminescent Sensing of Antibiotics and Proton Conduction

Construction and investigation of dual-functional metal-organic frameworks (MOFs) with luminescent sensing and proton conduction provide widespread applications in clean energy and environmental monitoring fields. By selecting a phosphonic acid ligand 4-pyridyl-CH₂N(CH₂PO₃H₂)₂ (H₄L) and coligand 2,2'-biimidazole (H₂biim), two cadmium-based MOFs [Cd_1.5(HL)(H₂biim)_0.5] (1) and (H₄biim)_0.5·[Cd₂(L)(H₂biim)Cl] (2) with different structures and properties have been hydrothermally synthesized by controlling reaction temperature. Based on the excellent thermal and chemical stabilities, and good luminescent stabilities in water solution, 1 and 2 can serve as luminescent sensors of chloramphenicol (CAP) with different quenching constant (K_SV) values and detection limits (LODs) in water, simulated environmental system, and real fish water system. Meanwhile, different sensing effects and possible sensing mechanisms are analyzed in detail. Moreover, 1 and 2 can also serve as good proton-conducting materials. The proton conductivities can reach up to 1.41 × 10^-4 S cm^-1 for 1 and 1.02 × 10^-3 S cm^-1 for 2 at 368 K and 95% relative humidity (RH). Among them, 2 shows better luminescent sensing and proton conduction performance than 1, which indicates that different crystal structures have a great impact on the properties of MOFs. Through the discussion of the relationship between structures and properties in detail, the possible reasons for the differences in properties are obtained, which can provide theoretical guidance for the rational design of this kind of dual-functional MOFs in the future.

105 K Wide Room Temperature Spin Transition Memory Due to a Supramolecular Latch Mechanism

Little is known about the mechanisms behind the bistability (memory) of molecular spin transition compounds over broad temperature ranges (>100 K). To address this point, we report on a new discrete Fe^II neutral complex [Fe^IIL₂]⁰ (1) based on a novel asymmetric tridentate ligand 2-(5-(3-methoxy-4H-1,2,4-triazol-3-yl)-6-(1H-pyrazol-1-yl))pyridine (L). Due to the asymmetric cone-shaped form, in the lattice, the formed complex molecules stack into a one-dimensional (1D) supramolecular chain. In the case of the rectangular supramolecular arrangement of chains in methanolates 1-A and 1-B (both orthorhombic, Pbcn) differing, respectively, by bent and extended spatial conformations of the 3-methoxy groups (3MeO), a moderate cooperativity is observed. In contrast, the hexagonal-like arrangement of supramolecular chains in polymorph 1-C (monoclinic, P2₁/c) results in steric coupling of the transforming complex species with the peripheral flipping 3MeO group. The group acts as a supramolecular latch, locking the huge geometric distortion of complex 1 and in turn the trigonal distortion of the central Fe^II ion in the high-spin state, thereby keeping it from the transition to the low-spin state over a large thermal range. Analysis of the crystal packing of 1-C reveals significantly changing patterns of close intermolecular interactions on going between the phases substantiated by the energy framework analysis. The detected supramolecular mechanism leads to a record-setting robust 105 K wide hysteresis spanning the room temperature region and an atypically large T_LIESST relaxation value of 104 K of the photoexcited high-spin state. This work highlights a viable pathway toward a new generation of cleverly designed molecular memory materials.

Photoactive Anthracene-9,10-dicarboxylic Acid for Tuning of Photochromism in the Cd/Zn Coordination Polymers

Electron transfer photochromic materials with photo-triggered radicals have received huge interest from chemists due to their potentialities in anticounterfeiting, displays, energy conversion, and information storage. However, utilizing the sole carboxylic acid to synthesize novel electron transfer photochromic species is still confronted with huge challenges. Herein, an acentric three-dimensional network Cd₂(ADC)₂(DMF)₂(H₂O) (1; ADC = anthracene-9,10-dicarboxylate; DMF = N,N-dimethylformamide) and a two-dimensional layer Zn(ADC)(H₂O)·DMA·H₂O (2; DMA = N,N-dimethylacetamide) were synthesized and characterized via a photoactive H₂ADC ligand. Both compounds exhibited electron transfer photochromism with the formation of radical photoproducts at the solid state, which was revealed by IR, UV-Vis absorption, photoluminescence and electron spin resonance spectra, and magnetic susceptibility measurements. Density functional theory calculations for 1 showed that the coloration process is a metal-assisted ligand-to-ligand electron transfer process between adjacent ADC molecules, and photogenerated stable radicals are delocalized over the ADC components. Compared with 1, the shorter distances between ADC components via coordination bonds promoted 2 to exhibit a higher coloration efficiency and larger quantity of photogenerated radicals. Furthermore, both compounds showed unexpected radical-actuated photochromism in aqueous solution. This work showed that the carboxylic acid ligands, without viologen acceptors, could construct the electron transfer photochromic complexes, showing a novel kind of ligand for the design of hybrid photochromic materials.