Orange luminescence of α-Al2O3 related to clusters consisting of F centers and divalent cations.

The orange luminescence of α-Al2O3 under UV excitation is characterized by a 2.07-eV orange broadband emission that has not yet been elucidated. This emission is present in natural and synthetic crystals and powders, as well as in Be-treated samples. All orange-luminescent materials have low Fe concentration (mostly <1000 ppm) with traces of divalent cations, mostly Mg, or Be in Be-diffused material (dozens of ppm). Mg2+, Mn2+, and Be2+ cations substitute for trivalent Al. To accommodate the charge deficit, several defects are created, including oxygen vacancies also called F centers. Indeed, our excitation spectra revealed the presence of several different F centers (F, F+, and clustered F2, F2 +, F2 2+) in those samples. However, the thermal stability and the measured luminescence lifetimes do not match with previously reported characteristics of isolated F centers. Based on our experiments, we suggest that a complex aggregate of two F centers (F2 2+) trapped at divalent cations is a major cause of this uncommon microsecond lifetime emission, even if a variety of other defects, including Cr3+, V3+, or interstitial Al3+, are present.

Mechanoresponsive luminescence triggered by phase transition of a supercooled copper(I) complex.

Under mechanical stimulation, a copper(I) complex in its supercooled liquid state transforms into a crystalline phase, showing a dramatic emission color change from red to green that is accompanied by a 20-fold increase in the photoluminescence quantum yield up to 87%. This reversible phase transition relies on the intriguing ability of this copper complex to form a supercooled metastable state.

Role of Hydrogen Bonding on the Design of New Hybrid Perovskites Unraveled by Machine Learning.

Selecting a set of reactants to accurately design a new low dimensional hybrid perovskite could greatly accelerate the discovery of materials with great potential in photovoltaics, or solid-state lighting. However, this design is challenging as most hybrid metal halides are not perovskites and no feature is clearly associated to the structural characteristics of the inorganic metal halide network. This work first demonstrates that the organic molecules are key parameters to determine the structure type of the inorganic network (i.e., perovskite versus non-perovskite). Then, machine learning (ML) algorithms are used to identify the key features of the organic cations leading to the perovskite structure type. Using a large dataset of hybrid metal halides, this work extracts the organic molecules of all hybrid lead halide compounds, calculates 2756 molecular descriptors and fingerprints for each of these molecules, and are able to predict through ML techniques if a specific organic amine will lead to the perovskite type with an accuracy up to 88.65%. Descriptors related to hydrogen bonding are identified as important features. Thus, a simple but reliable design principle could be demonstrated: the presence of primary ammonium cation is the primary condition to prepare hybrid lead halide perovskites regardless of their dimensionalities.

Cubane Dimerization: Cu4 vs Cu8 Copper Iodide Clusters.

Copper(I) halides are well-known for their structural diversity and rich photoluminescence properties, showing great potential for the development of solid-state lighting technology. A series of four molecular copper iodide clusters based on the [Cu4I4] cubane geometry is reported. Among them, [Cu8I8] octanuclear clusters of rare geometry resulting from dimerization of the tetranuclear counterparts were also synthesized. Two different phosphine ligands were studied, bearing either a styrene or an ethyl group. Therefore, the effect of the dimerization and of the ligand nature on the photophysical properties of the resulting clusters is investigated. The structural differences were analyzed by single-crystal X-ray diffraction (SCXRD), solid-state nuclear magnetic resonance (NMR), infrared, and Raman analyses. Compared to the ethyl group, the styrene function appears to greatly impact the photophysical properties of the clusters. The luminescence thermochromic properties of the ethyl derivatives and the intriguing photophysical properties of the clusters with styrene function were rationalized by density functional theory (DFT) calculations. Thus, the styrene group significantly lowers in energy the vacant orbitals and consequently affects the global energetic layout of the clusters. From this study, it was found that the nuclearity of copper iodide clusters eventually has less influence on the photophysical properties than the nature of the ligand. The design of proper ligands should therefore be considered when developing materials for specific lighting applications.

Tris-benzo[cd]indole Cyanine Enables the NIR-photosensitized Radical and Thiol-ene Polymerizations at 940†nm.

A near-infrared-absorbing heptamethine (HM+ ) incorporating three bulky benzo[cd]indole heterocycles was designed to efficiently prevent self-aggregation of the dye, which results in a strong enhancement of its photoinitiating reactivity as compared to a parent bis-benzo[cd]indole heptamethine (HMCl+ ) used as a reference system. In this context, we highlight an efficient free-radical NIR-polymerization up to a 100 % acrylates C=C bonds conversion even under air conditions. Such an important initiating performance was obtained by incorporating our NIR-sensitizer into a three-component system leading to its self-regeneration. This original photoredox cycle was thoroughly investigated through the identification of each intermediary species using EPR spectroscopy.

Chemistry in the Molten State: Opportunities for Designing and Tuning the Emission Properties of Halide Perovskites.

A series of monolayered lead halide hybrid perovskites (HO2C(CH2)n-1NH3)2PbX4, named (Cn)2PbX4 (n = 4-6, X = Cl, Br), exhibiting a low congruent melting temperature (Tm) (Tm = 130 °C for (C4)2PbBr4), high stability in the molten state, and whitish type emission, are reported. From the synthesis in the molten state, rare solid solutions of mixed organic cations (Cn1-xCn'x)2PbX4 (n, n' = 4-6; X = Cl, Br; 0 ≤ x ≤1) as well as solid solutions of mixed halides (Cn)2Pb(X1-yX'y)4 (n = 4-6; X, X' = Cl, Br; 0 ≤ y ≤1) have been prepared and characterized (thermal behavior, powder X-ray diffraction (PXRD), photoluminescence properties). The impact of substitutions is significant on the thermal properties, lowering the Tm down to 100 °C for (C4)2Pb(Br0.25Cl0.75)4. The emission properties are slightly tuned in the case of mixed organic cation systems, whereas modifications are more dramatic in the case of mixed halide systems, leading to emission properties through the entire visible region. These results illustrate the great opportunities offered by the congruent melting properties of halide perovskites allowing syntheses in the molten state.

Tuning the 1D-2D dimensionality upon ligand exchange in silver thiolate coordination polymers with photoemission switching.

Silver nanoparticles are known and widely used for their antimicrobial activities. Nevertheless, once they are released into the natural or biological environments, they can become toxic with time, because of the dissolution of some Ag(I) ions that can then react with thiol-based molecules, such as glutathione and/or compete with copper proteins. These assumptions are based on the high affinity of the soft acid Ag(I) and the soft base thiolates and the exchange reactions that are involved in complex physiological media. Here we synthesized and fully characterized two new 2D silver thiolate coordination polymers (CPs) that exhibit a reversible 2D-to-1D structural transformation in the presence of an excess of thiol molecules. This dimensionality change induces also a switch of the yellow emission of the Ag-thiolate CP. This study highlights that these highly stable silver-thiolate CPs, in basic, acidic and oxidant media can undergo a complete dissolution-recrystallization mechanism upon thiol exchange reactions.

Photoactive CuI-Cross-Linked Polyurethane Materials.

Using multinuclear copper iodide complexes as cross-linking agents in a polyurethane matrix, original photoluminescent stimuli-responsive materials were synthesized. The intrinsic photoluminescence properties of the covalently incorporated copper iodide complexes are thus transferred to the materials while retaining the beneficial characteristics of the polymer host. The transparent materials exhibit room-temperature phosphorescence with emission switching properties by displaying luminescence thermochromism and solvatochromism. The luminescence thermochromism is characterized by a change in the wavelength and intensity of the emission with temperature, and the vapochromic effect presents a contrasted response of extinction or exaltation according to the nature of the solvent of exposure. By combining the luminescence characteristics of photoactive copper iodide complexes with the ease of polymer processing, the application of these luminescent materials as phosphors in LED (light-emitting diode) devices was also demonstrated. The present study shows that the use of copper iodide complexes as cross-linkers in polymeric materials is a relevant strategy to design materials with enhanced functionalities in addition to their low cost and sustainable characteristics.

Perovskite or Not Perovskite? A Deep-Learning Approach to Automatically Identify New Hybrid Perovskites from X-ray Diffraction Patterns.

Determining the crystal structure is a critical step in the discovery of new functional materials. This process is time consuming and requires extensive human expertise in crystallography. Here, a machine-learning-based approach is developed, which allows it to be determined automatically if an unknown material is of perovskite type from powder X-ray diffraction. After training a deep-learning model on a dataset of known compounds, the structure types of new unknown compounds can be predicted using their experimental powder X-ray diffraction patterns. This strategy is used to distinguish perovskite-type materials in a series of new hybrid lead halides. After validation, this approach is shown to accurately identify perovskites (accuracy of 92% with convolutional neural network). From the identification of the key features of the patterns used to discriminate perovskites versus nonperovskites, crystallographers can learn how to quickly identify low-dimensional perovskites from X-ray diffraction patterns.

A chiral 3D silver(I)-benzenedithiolate coordination polymer exhibiting photoemission and non-linear optical response.

A new tridimensional metal-organic chalcogenolate, made of a 1,3-benzenedithiolate bridging ligand and Ag(I), [Ag2(1,3-BDT)]n, is reported. This coordination polymer has good thermal stability in air and displays both photoluminescence properties and a second harmonic generation response.

Templating effect of trans-2,5-dimethylpiperazine (TDMP) on the structural dimensionality of hybrid metal halides.

Templating effects are commonly investigated by comparing different organic structure-directing agents in a specific inorganic system. Herein, a specific secondary diamine, the trans-2,5-dimethylpiperazine, has been selected for different metal halide anions with the aim to analyze its influence on different inorganic networks. Thus, five new trans-2,5-dimethylpiperazine-1,4-diium based compounds with [CuBr4]2-, [CdBr4]2-, [CuBr2]-, [AgCl2]-, and [AgBr2]- have been synthesized, structurally characterized and compared to eight previously reported compounds containing [ZnCl4]2-, [ZnBr4]2-, [CoCl4]2-, [PbCl4]2-, [PbBr4]2-, [SnBr4]2-, [CuCl4]2-, and [CdCl4]2-. Despite the different crystal structures (space-groups, inorganic frameworks, etc.), the dimensionalities (from 0D to 2D) of the inorganic networks of these 13 hybrid materials could be rationalized according to only two parameters: the oxidation state (+1 or +2) and the coordination sphere (tetrahedron or octahedron) of the metal ions. The luminescence properties of the new hybrid silver bromide have been analyzed and discussed by comparing the luminescence mechanisms of the other previously reported trans-2,5-dimethylpiperazine-1,4-diium metal halides.

Copper Iodide Clusters Coordinated by Emissive Cyanobiphenyl-Based Ligands.

Copper(I) halides are currently the subject of intensive research because of their rich photophysical properties combined with economic and eco-friendly advantages for practical applications. The molecular copper iodide cluster of the general formula [Cu4I4L4] (L = ligand) is a well-known photoluminescent compound, and the possibility to enlarge the panel of its photophysical properties is studied here, by exploring ligands bearing a distinct emitter. The comparative study of five copper iodide clusters coordinated by different phosphine ligands functionalized by the emissive cyanobiphenyl (CBP) group is thus described in this work. The emissive properties of the ligands have a great impact onto the photophysical properties of the cluster. Compared with classical [Cu4I4L4] copper iodide clusters, the origin of the emission bands is largely modified. The CBP moiety of electron acceptor character significantly lowers in energy the vacant orbitals and consequently affects the global energetic layout. These clusters present dual emission based on two different emissive centers which interplay through energy transfer. This study demonstrates that the design of original ligands is an effective approach to enrich the photophysical properties of the appealing family of copper halide complexes.

Machine Learning Guided Design of Single-Phase Hybrid Lead Halide White Phosphors.

Designing new single-phase white phosphors for solid-state lighting is a challenging trial-error process as it requires to navigate in a multidimensional space (composition of the host matrix/dopants, experimental conditions, etc.). Thus, no single-phase white phosphor has ever been reported to exhibit both a high color rendering index (CRI - degree to which objects appear natural under the white illumination) and a tunable correlated color temperature (CCT). In this article, a novel strategy consisting in iterating syntheses, characterizations, and machine learning (ML) models to design such white phosphors is demonstrated. With the guidance of ML models, a series of luminescent hybrid lead halides with ultra-high color rendering (above 92) mimicking the light of the sunrise/sunset (CCT = 3200 K), morning/afternoon (CCT = 4200 K), midday (CCT = 5500 K), full sun (CCT = 6500K), as well as an overcast sky (CCT = 7000 K) are precisely designed.

Tuning the Oxidation States of Dopants: A Strategy for the Modulation of Material Photoluminescence Properties.

Doped single-phase materials have been widely investigated owing to their easy to implement synthesis and the variety of their properties. This Minireview covers strategies for the co-stabilization and the ratio control of several oxidation states of dopants inserted in the same host. The tuning of the oxidation states of dopants opens up many possibilities for the optimization of specific properties and can be envisioned for various applications such as telecommunication, medicine, displays, lasers or lighting. Technics used for the quantification of each valence state of dopant are also emphasized, and the importance of high throughput methods for the discovery of efficient materials with dopants in multiple valence states is discussed.

Combining Theory and Experiment to Get Insight into the Amorphous Phase of Luminescent Mechanochromic Copper Iodide Clusters.

In the field of stimuli-responsive luminescent materials, mechanochromic compounds exhibiting reversible emission color changes activated by mechanical stimulation present appealing perspectives in sensor applications. The mechanochromic luminescence properties of the molecular cubane copper iodide cluster [Cu4I4[PPh2(C6H4-CH2OH)]4] (1) are reported in this study. This compound can form upon melting an amorphous phase, giving an unprecedented opportunity to investigate the mechanochromism phenomenon. Because the mechanically induced crystalline-to-amorphous transition is only partial, the completely amorphous phase represents the ultimate state of the mechanically altered phase. Furthermore, the studied compound could form two different crystalline polymorphs, namely, [Cu4I4[PPh2(C6H4-CH2OH)]4]·C2H3N (1·CH3CN) and [Cu4I4[PPh2(C6H4-CH2OH)]4]·3C4H8O (1·THF), allowing the establishment of straightforward structure-property relationships. Photophysical and structural characterizations of 1 in different states were performed, and the experimental data were supported by theoretical investigations. Solid-state NMR analysis permitted quantification of the amorphous part in the mechanically altered phase. IR and Raman analysis enabled identification of the spectroscopic signatures of each state. Density functional theory calculations led to assignment of both the NMR characteristics and the vibrational bands. Rationalization of the photoluminescence properties was also conducted, with simulation of the phosphorescence spectra allowing an accurate interpretation of the thermochromic luminescence properties of this family of compounds. The combined study of crystalline polymorphism and the amorphous state allowed us to get deeper into the mechanochromism mechanism that implies changes of the [Cu4I4] cluster core geometry. Through the combination of multistimuli-responsive properties, copper iodide clusters constitute an appealing class of compounds toward original functional materials.

Role of specific distorted metal complexes in exciton self-trapping for hybrid metal halides.

A methodology enabling the discovery of hybrid metal halide phosphors through the selection of structural networks, which exhibit a specific distorted environment of the metal ions associated with the self-trapping of excitons, is proposed. This approach is demonstrated with the synthesis of an efficient near-UV emitting hybrid cadmium halide phosphor.

Polymorphic Copper Iodide Anions: Luminescence Thermochromism and Mechanochromism of (PPh4)2[Cu2I4].

The photoluminescent stimuli-responsive properties of two crystalline polymorphs with the formula (PPh4)2[Cu2I4] are reported. Distinct luminescence properties are exhibited by these ionic copper iodide compounds with blue or yellow emission, and original luminescence thermochromism and mechanochromism are demonstrated. While one polymorph displays contrasted temperature-dependent emission properties, the other shows great modification of its emission upon mechanical solicitation. The establishment of structure-properties relationships, supported by a theoretical approach, permits us to get insights into the origin of the photoluminescence properties and the mechanisms at play. According to DFT calculations, the different emission bands originate either from the (PPh4)+ organic cation or from the [Cu2I4]2- anion. Activation of these two emissive centers appears to be dependent on the crystalline packing of the polymorph. The thermochromism displayed by one polymorph can be attributed to a variation in temperature of the relative intensities of two emission bands of two different excited states. The origin is different for the other polymorph, with emission bands coming from two independent emissive centers: namely, (PPh4)+ and [Cu2I4]2-. The luminescence mechanochromism is attributed to a polymorphic transition. The mechanical solicitation induces a partial transformation of one polymorph into the other within a disordered phase. The mechanochromic mechanism can be related to mechanical modifications of intermolecular interactions between the (PPh4)+ cations. By displaying luminescence properties that depend on crystalline structure, excitation wavelength, temperature, and mechanical solicitation, the studied copper iodides offer a great possibility of emissive color control and switching, a clear demonstration of the great potentialities of this family of compounds for the development of photoactive materials.

Origin of Luminescence in La2MoO6 and La2Mo2O9 and Their Bi-Doped Variants.

In this article, lanthanum molybdenum oxides (La2MoO6 and La2Mo2O9) and their Bi-doped derivatives were investigated as potential rare-earth-free phosphors. An X-ray diffraction analysis coupled with an EDX study confirmed the purity of the samples and the insertion of bismuth in a 1 molar % amount. Kubelka-Munk-transformed reflectance spectra clearly indicated that the insertion of Bi induces a shortening of the optical gap in La2MoO6 but has no impact on that of La2Mo2O9. Moreover, excitation and emission spectra evidenced a strong temperature quenching effect in all materials. Also, the CIEx,y parameters at 77 K are almost identical with or without Bi doping for the two host lattices. Clearly, it was shown, by combining experimental data, ab initio calculations, and the empirical positioning of absorption bands that the luminescence of the Bi-doped La2MoO6 sample is mainly related to the host lattice itself and distortions induced by La/Bi substitution. The role of the Bi3+ dopant is indirect, and the luminescence is mainly due to a Mo-O charge transfer rather than an on-site Bi3+ 3P1,0 → 1S0 transition. Concerning La2Mo2O9, there is no effect following the insertion of Bi, implying that the role of Bi is insignificant.

Hydrogen Bonding and Broad-Band Emission in Hybrid Zinc Halide Phosphors.

In solid-state lighting (SSL) applications, hybrid zinc halide phosphors are a promising family because they meet specific criteria such as high color rendering, low cost, and nontoxicity. However, contrary to hybrid lead halide phosphors, their quantum efficiencies are low and the origin of this luminescence remains unclear. To unravel this origin and provide new insights into enhancement of this emission, four hybrid zinc halides have been investigated. These four compounds exhibit similar crystal structures but different photoluminescence properties. We show that photoemission requires the formation of Vk centers, which can be promoted by specific hydrogen bonding. We anticipate that the selection of a specific environment for the zinc halide units could lead to a promising family of low-cost and environmentally friendly phosphors for SSL.

Enhanced bactericidal activity of brucite through partial copper substitution.

Brucite Mg(OH)2 belongs to a family of two-dimensional compounds with a CdI2-type structure built up from layers of edge-sharing octahedra delineating 2D galleries. In the current study, nanometer-sized platelets of copper substituted Mg(OH)2 were prepared by co-precipitation at room temperature in mixed alkaline (NaOH/Na2CO3) medium. Very weak substitution of a few hydroxyl ions by carbonate groups was highlighted at first by infrared spectroscopy and then quantified by thermogravimetric (TG) and mass spectrometric (MS) evolved gas analyses. The presence in a very low amount of water molecules in the galleries induces disorder in the stacking of layers of edge-sharing octahedra along the c-axis. The dehydration of the hydroxides taking place below 225 °C preserves the brucite-type structure of the samples while suppressing the stacking disorder. Copper substitution greatly enhances the bactericidal activity of nanometer-sized platelets of brucite against two bacteria frequently involved in healthcare-associated-infections. 10 mol% of cupric ions in Mg(OH)2 (a copper loading of 0.102 mg mL-1 in the suspension) were sufficient to induce, after 3 h in contact, 100% and 99.3% reductions in viability of Gram-negative E. coli and Gram-positive S. aureus, respectively (reductions as low as 23% and 48% are reported for the parent compound Mg(OH)2 in the same conditions). A good compromise between fast bactericidal kinetics and a high reduction in viability is reached by the 15 mol% copper-substituted Mg(OH)2 hydroxide. Its use gives the opportunity to five-fold reduce the copper loading of the bactericidal agent while being at least equally or even more efficient compared to the conventional CuO (a Cu loading of 0.799 mg mL-1 and 0.154 mg mL-1 in the suspension of CuO and 15 mol% copper substituted Mg(OH)2 particles, respectively).