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The manipulation of spin-state switching (SSS) under ambient conditions is of significant importance for the construction of molecular switches. Herein, we demonstrate that reversible SSS can be mediated by the aggregation state of a near-infrared (NIR)-sensitive ferrous complex. The ferrous complex was J-aggregated in a DMF suspension and with a low-spin (LS) state; however, with the addition of water, it changed to H-aggregation and reached a high-spin (HS) state, owing to the enhanced intramolecular charge transfer and metal-to-ligand charge transfer. Interestingly, the following NIR irradiation can restore the J-aggregation and LS states owing to the enhanced ligand-to-ligand charge transfer. More interestingly, the ferrous complex can be further incorporated into a hygroscopic sponge that was capable of capturing humidity effectively for all weather conditions, which displayed reversible SSS via alternating atmospheric humidity capture and NIR irradiation under ambient conditions in the sponge state. This study thus opens up a new avenue for the development of novel smart molecular switches at the device level.

Numerous emerging applications require precise monitoring of humidity that is not only sensitive and miniaturized but also durable and portable. However, except for the existing sensing mechanisms, such as resistive, capacitive, impedance, and voltage-based devices, little attention has been paid to exploiting dual readout modes with novel properties that can meet the requirements of simplicity and convenience. In this work, an example of color/fluorescence dual-switching by using an aggregator building block of naphthalenediimide with a porous hygroscopic polymer matrix is proposed. With an increase in the water fraction, there are obvious variations in fluorescence, driven by H-aggregation of the aggregator, and in color, driven by intramolecular-charge transfer, which facilitate the monitoring of external humidity. More importantly, the prepared humidity-sensitive switch demonstrated excellent cycling stability and reversibility. This work may shed light on the precise fabrication of aggregation-activated materials for potential applications in humidity-sensitive systems and switching devices.

Three new pharmaceutical co-crystals: 1 PZA-MA (malonic acid), 2 PZA-SA (succinic acid, a new polymorph of a reported one), and 3 PZA-GA (glutaric acid) have been prepared and characterized by differential scanning calorimetry (DSC), thermogravimetric analyses (TGA), and single-crystal X-ray diffraction. Wherein, PZA formed 1:1 co-crystals with MA and GA by acid–amide and acid–py heterosynthon, while it formed 2:1 co-crystal with SA by amide–amide homosynthon in addition to acid–amide and acid–py heterosynthon. Their melting points follow the order, PZA-GA < PZA-MA < PZA-SA, which are lower than the melting points of the individual components. Hirshfeld surface analysis revealed that N–H···O hydrogen bonding and π···π interactions for PZA in them follow the order: PZA-MA > PZA-SA > PZA-GA, while H–H and O–O interactions follow the order: PZA-MA < PZA-SA < PZA-GA. We also compared the Hirshfeld surfaces of the present co-crystals with the nine reported PZA co-crystals, which obtained important results. The studies of the solubility and dissolution showed a semiempirical inverse relationship with the melting point: the solubility follows the order, PZA-SA < PZA-GA < PZA-MA and dissolution rate follows the order, PZA-SA < PZA-MA < PZA-GA.

Combining the fascinating advantages of atmospheric humidity with the phenomenon of spin-state switching (SSS) materials represents an attractive way of triggering the reversible SSS under ambient conditions. In this work, the occurrence of the reversible SSS triggered by the humidity capture/release cycle without the addition of external energy was achieved by mixing composite gels with the two-dimensional ferrous complex. A linear relationship between humidity absorption amounts and the color of the composite gel was established, which facilitates the manufacture of a novel SSS device. This work has provided a new design strategy to motivate SSS performance and may open the way for practical applications of SSS.

The development of atmospheric water generators depends critically on the preparation of hygroscopic materials. In our systematic efforts toward producing porous frameworks for effective humidity reduction, a super hygroscopic material, which was composed of hetero-layered 2D (two-dimensional) metal–organic framework nanosheet composites, has been established. The effective hetero-layered interactions have endowed the material with the functionalities of simultaneous humidity capture and in situliquefaction, reaching an unprecedented value of water adsorption of over 658% (highest) of its own weight under ambient atmospheric and room-temperature conditions. Outdoor experiments have suggested that the fast water release is triggered by natural sunlight. Compared with classical hygroscopic salts that require plenty of energy input for water release, the present work has provided a promising strategy for developing next-generation hygroscopic materials for humidity-related management and advanced atmospheric water generator systems.

The coordination interactions between transition-metal ions (Cu2+, Ag+) and sulfur atoms on ultrathin two-dimensional (2D) nanosheets of spin-crossover (SCO) metal–organic frameworks {[Fe(1,3-bpp)2(NCS)2]2}n(1,3-bpp = 1,3-di(4-pyridyl)propane), which constructed the ultrathin 2D nanosheets into three-dimensional (3D) nanoparticles, have made a profound effect on the SCO performance. Compared with 2D nanosheets, both the intraligand π–π* transition band and the metal-to-ligand charge transition band from the d(Fe) + π(NCS) to π*(1,3-bpp), for the 3D nanoparticles, have shown dramatic blue-shifts; meanwhile, the d–d transition band for the high-spin (HS) state Fe(II) ions has been generated, suggesting significantly the influence of 3D assemble-caused dimensional changes on the solid-state SCO performance of ultrathin 2D nanosheets. More importantly, by loading on the ytterbium ion (Yb3+)-sensitized hexagonal phase upconverting nanoparticles in the aqueous colloidal suspension, the near infrared (NIR) light (980 nm) triggered HS (high spin) to LS (low spin) state transitions have been observed, demonstrating the achievement of challenging target of NIR light-triggered molecular conversion under environment conditions.

Two new isostructural iron(II) complexes 1and 2, with the formula of Fe(1,4-dcb)(N(CN)2)2(1,4-dcb = 1,4-dichlorobenzene), have been prepared and characterized. 1was obtained under the ambient condition, whereas 2was prepared by the application of an external magnetic field with a strength of 0.5 T. Single-crystal X-ray structural analysis and Hirshfeld calculations have revealed that the external magnetic fields have induced the disorder of 1,4-dcb molecules in 2, which strengthened the Cl···N halogen-bonding contacts between 1,4-dcb arrays and one-dimensional (1D) Fe(N(CN)2)2chains. As a consequence, coplane stacking of 1,4-dcb arrays and 1D Fe(N(CN)2)2chains has been observed in 2. Compared with the stepped stacking and paramagnetic character for 1, the coplane stacking in 2has increased the lattice cooperativity and ligand-field strength of N(CN)2, which thus provided partial spin-state transition. While under 1.0 T, the complex was broken and resulted in the formation of crystals Fe(ClO4)·6(H2O), suggesting significantly the different influence of magnetic field strength on paramagnetic ions and antimagnetic organic molecules. These results have provided an effective strategy for molecular design with desired physical–chemical properties.

The design and preparation of a porous high-valence metal–organic framework (MOF) featuring open coordination sites are of utmost importance for the development of adsorbent materials. Here in this work, the three-dimensional (3D) high-valence MOF [Er(dcbp)3/2(DMF)(H2O)2]·2H2O (HV-MOF-1; H2dcbp = 4,4′-dicarboxy-2,2′-bipyridine, DMF = N,N-dimethylformamide), which possesses permanent porosity and two open coordination sites, has been prepared and characterized. In the 3D framework, the dcbp molecules display two different bridging styles, resulting in ordered diamondlike pores with bared carboxyl oxygen and pyridine nitrogen atoms on dcbp exposed directly to the pores, generating hydrophilic characteristics and high water affinity. In addition, the open coordination sites act as arms to fix the adsorbed water molecules, providing high water adsorption capacity (5.95 mmol g–1) and selectivity. More importantly, the activated HV-MOF-1species shows an energy-saving step for recycling (operation under 120 °C), demonstrating promise as a candidate for an adsorbent material with considerable water adsorption–desorption cycles.

Subtle integration of rotatable polar components into dielectric crystals can contribute significantly to adjustable switching temperatures (Ts) and dielectric relaxation behaviors. Currently, one of the biggest challenges lies in the design of optimal polar components with moderate motion resistance in a crystalline system. In this work, we demonstrate that under refrigerator conditions, rotatable hydrogen-bonded one-dimensional (1D) cationic chains, {[C2H6N5]+}n(C2H6N5= 3,5-diamino-1,2,4-triazolinium), and two-dimensional (2D) anionic layers, {[(H2O)2·SO4]2–}n, can be generated in an organic salt, 3([C2H6N5]2·[(H2O)2·SO4]). Compared with the nonhydrated precursor, 2([C2H7N5]·[SO4]), the rotation of these 1D and 2D ionic species triggers a reversible phase transition and dielectric switching in 3. In addition, the significantly sluggish rotation of the 1D cationic chains from parallel to unparallel stacking and the counter-clockwise rotation of the 2D anionic layers, compared with their reverse processes, induce a frequency-dependent dielectric response with a more highly adjustable heating Ts↑ than the cooling Ts↓. More importantly, 3possesses excellent self-recovery ability attributed to the highly dynamic character of the hydrogen-bonded ionic species. The strategy here can provide a fairly good model for designing dielectric crystals with desired rotatable polar components.

The protonation of different functional acidic and alkalic groups is of utmost importance for the crystal engineering field. By using hydrochloric acid, in this work, the amine group of 3-aminobenzoic acid (1, C7H7NO2) and the hydroxy group of (E)-4-hydroxy-3,5-dimethoxybenzaldehyde oxime (2, C9H11NO4) have been protonated, giving birth to the organic salts [C7H8NO2]+·[Cl]−([1H]+·[Cl]–) and [C9H12NO4]+·[Cl]−([2H]+·[Cl]–). Results revealed that both salts display similar 1D “stair-like” ribbon models via the formation of intermolecular hydrogen bonding contacts between chloride ion and the protonated groups into four-membered squares. Note that the protonated acidic groups have shown much closer and stronger intermolecular interactions than the alkalic groups. In addition, 3D Hirshfeld surfaces, 2D fingerprint plots, and crystal void calculations have been performed, which have suggested 21.4% and 17.5% interactions around the protonated acidic and alkalic groups, respectively. This work will provide a new insight into the rational design and control of molecular crystal with protonated functional groups.

Through a facile-operating ultrasonic force-assisted liquid exfoliation technology, the single-layered two-dimensional (2D) [Co(CNS)2(pyz)2]n(pyz = pyrazine) nanosheets, with a thickness of sub-1.0 nm, have been prepared from the bulk precursors. The atomically thickness and the presence of abundant sulfur atoms with high electronegativity arrayed on the double surfaces of the sheets are making this kind of 2D MOF (metal–organic framework) nanosheets highly sensitive to intermolecular interactions. As a result, it can be well dispersed in all kinds of solvents to give a stable colloidal suspension that can be maintained for at least one month, accompanied by significant solvatochromic behavior and various optical properties, which thus have shown the potential to be practically applicated as in situ visual test paper for solvent identification and solvent polarity measurements. More importantly, combined with a smartphone, this kind of 2D-MOF nanosheets can be developed into in situ visual test paper to identify isomers and determine the polarity of mixed solvents quantitatively and qualitatively, suggesting the promising application of a portable, economical, and in situ visual test strategy in real world.

Bidirectional photoswitching of molecular materials under ambient condition is of significant importance. Herein, we present for the first time that a core–shell UCNP–SCO nanosphere (UCNP = upconversion nanophosphor, SCO = spin crossover), which was composed of a UCNP core (NaYF4: 20 mol % Yb3+, 1 mol % Er3+) and an SCO iron(II) shell ([Fe(H2Bpz)2(bipy-COOH)], H2Bpz = dihydrobis(1-pyrazolyl)borate, bipy-COOH = 4,4′-dicarboxy-2,2′-bipyridine), can be reversibly photoswitched between the high-spin and low-spin states at room temperature in the solid state, via alternating irradiation with near-infrared (λ = 980 nm) and ultraviolet (λ = 310 nm) light. What’s more, this reversible spin-state switching was accompanied by a variation of fluorescent spectrum and dielectric constants. The strategy here, that is, integrating the SCO iron(II) complex into a UCNP–SCO nanosphere for molecular photoswitching, may open a new area in the development of photocontrolled molecular devices.

The organometallic cation 1(Fe(bipy-NH2)32+, bipy-NH2= 4,4′-diamino-2,2′-bipyridine), which was constructed in situ in solution, can bind CO2from air effectively with a stoichiometric ratio of 1:4 (1/CO2), through the formation of “H-bonded CO2” species: [CO2–OH–CO2]−and [CO2–CO2–OH]−. These two species, along with the captured individual CO2molecules, connected 1into a novel 3D (three-dimensional) architecture, that was crystal 1·2(OH–)·4(CO2). The adsorption isotherms, recycling investigations, and the heat capacity of 1have been investigated; the results revealed that the organometallic cation 1can be recycled at least 10 times for the real-world CO2capture applications. The strategies presented here may provide new hints for the development of new alkanolamine-related absorbents or technologies for CO2capture and sequestration.

Magnetism of a complex [Fe(H2Bpz2)2(bipy-NH2)] (H2Bpz2= dihydrobis(1-pyrazolyl)borate, bipy-NH2= 4,4′-diamino-2,2′-bipyridine) has been altered from paramagnetic to spin-crossover (SCO) behavior, through protonation of one amino group of bipy-NH2with CF3SO3H. Complete SCO transition, both in solid state and in solution, occurs at ambient temperature.

Organic–inorganic hybrid perovskites have attracted tremendous interest due to their unique properties and promising applications for solar cells and other optoelectronics. Two-dimensional organic–inorganic halide perovskite materials are of essential importance in luminescence fields. The existing methods of regulating optical properties mainly focus on changing the chemical composition and ion doping. Herein, by adopting top-down and dimensional-change strategies, a series of atomically thin organic–inorganic halide perovskites (X-PMA)2PbBr4(R = 2,3,4 substituted, PMA = benzylamine, X = Cl, Br) have been synthesized and characterized. The material characterizations show that they are structurally similar but slightly different. The optical measurements illustrate that the prepared compounds exhibit not only a broad band but also strong fluorescence emission within 400–800 nm. This feature can be explained by their unique structure, with both substitution of halogen and low dimension. This study enables their promising application for next-generation light-emitting materials and devices and provides an effective strategy for the design of novel optical materials.

The controlled preparation of two-dimensional (2D) nanosheets of metal-organic frameworks (MOFs), with tailorable methodologies, properties and applications, is of significant importance. Here in this work, by subtle control of the ultrasonic duration and solvents polarity, the iron(II)-based 2D MOF Fe(pyz)2Cl2(pyz = pyrazine) have been elegantly tailored into 2D nanosheets (lateral size ≥ 500 nm in aqueous, with ultrasonic duration of 30 min) and one-dimensional (1D) nanoribbons (lateral size ≤ 100 nm in ethanol solution, with ultrasonic duration of 90 min). The aqueous suspension of 2D nanosheets was featured with thermal-induced spin-state transition at around room temperature, and can be used as effective Fenton catalysts for degradation of water-soluble organic dyes. While the ethanol suspension of 1D nanoribbons can act as versatile nano-platform for trans to cis isomerization of 4, 4’-azopyridine. These results may provide a novel strategy for the controlled preparation of layered nanomaterials.