Alexandre Fargues

Thermochromic Luminescence of Copper Iodide Clusters: The Case of Phosphine Ligands

Three copper(I) iodide clusters coordinated by different phosphine ligands formulated [Cu(4)I(4)(PPh(3))(4)] (1), [Cu(4)I(4)(Pcpent(3))(4)] (2), and [Cu(4)I(4)(PPh(2)Pr)(4)] (3) (PPh(3) = triphenylphosphine, Pcpent(3) = tricyclopentylphosphine, and PPh(2)Pr = diphenylpropylphosphine) have been synthesized and characterized by (1)H and (31)P NMR, elemental analysis and single crystal X-ray diffraction analysis. They crystallize in different space groups, namely, monoclinic P21/c, cubic Pa ̅3, and tetragonal I ̅42m for 1, 2, and 3, respectively. The photoluminescence properties of clusters 1 and 3 show reversible luminescence thermochromism with two highly intense emission bands whose intensities are temperature dependent. In accordance to Density Functional Theory (DFT) calculations, these two emission bands have been attributed to two different transitions, a cluster centered (CC) one and a mixed XMCT/XLCT one. Cluster 2 does not exhibit luminescence variation in temperature because of the lack of the latter transition. The absorption spectra of the three clusters have been also rationalized by time dependent DFT (TDDFT) calculations. A simplified model is suggested to represent the luminescence thermochromism attributed to the two different excited states in thermal equilibrium. In contrast with the pyridine derivatives, similar excitation profiles and low activation energy for these phosphine-based clusters reflect high coupling of the two emissive states. The effect of the Cu-Cu interactions on the emission properties of these clusters is also discussed. Especially, cluster 3 with long Cu-Cu contacts exhibits a controlled thermochromic luminescence which is to our knowledge, unknown for this family of copper iodide clusters. These phosphine-based clusters appear particularly interesting for the synthesis of original emissive materials.

Polymorphic Copper Iodide Clusters: Insights into the Mechanochromic Luminescence Properties

An in-depth study of mechanochromic and thermochromic luminescent copper iodide clusters exhibiting structural polymorphism is reported and gives new insights into the origin of the mechanochromic luminescence properties. The two different crystalline polymorphs exhibit distinct luminescence properties with one being green emissive and the other one being yellow emissive. Upon mechanical grinding, only one of the polymorphs exhibits great modification of its emission from green to yellow. Interestingly, the photophysical properties of the resulting partially amorphous crushed compound are closed to those of the other yellow polymorph. Comparative structural and optical analyses of the different phases including a solution of clusters permit us to establish a correlation between the Cu-Cu bond distances and the luminescence properties. In addition, the local structure of the [Cu4I4P4] cluster cores has been probed by (31)P and (65)Cu solid-state NMR analysis, which readily indicates that the grinding process modifies the phosphorus and copper atoms environments. The mechanochromic phenomenon is thus explained by the disruption of the crystal packing within intermolecular interactions inducing shortening of the Cu-Cu bond distances in the [Cu4I4] cluster core and eventually modification of the emissive state. These results definitely establish the role of cuprophilic interactions in the mechanochromism of copper iodide clusters. More generally, this study constitutes a step further into the understanding of the mechanism involved in the mechanochromic luminescent properties of metal-based compounds.

Siloxanol-functionalized copper iodide cluster as a thermochromic luminescent building block.

A copper iodide cluster bearing reactive silanol groups exhibits thermochromic luminescence properties sensitive to its chemical environment and is thus a suitable building block for the synthesis of optically active materials.

Mechanochromic Luminescence of Copper Iodide Clusters

Luminescent mechanochromic materials are particularly appealing for the development of stimuli-responsive materials. Establishing the mechanism responsible for the mechanochromism is always an issue owing to the difficulty in characterizing the ground phase. Herein, the study of real crystalline polymorphs of a mechanochromic and thermochromic luminescent copper iodide cluster permits us to clearly establish the mechanism involved. The local disruption of the crystal packing induces changes in the cluster geometry and in particular the modification of the cuprophilic interactions, which consequently modify the emissive states. This study constitutes a step further toward the understanding of the mechanism involved in the mechanochromic luminescent properties of multimetallic coordination complexes.

Luminescence thermochromism of acrylic materials incorporating copper iodide clusters

By incorporating molecular copper iodide clusters of formula [Cu4I4L4] (L = PPh2(CH2)2CH3) into an UV-polymerizable acrylic resin, namely BEDA (Bisphenol-a-EthoxylateDiAcrylate), transparent and highly emitting photoluminescent composite materials have been synthesized. In these materials, the original luminescence properties of the copper iodide cluster and the transparency and processability of the acrylic matrix are combined. Study of the photopolymerization kinetics shows that the clusters incorporated in low concentration have limited influence on the polymerization reaction leading to a highly cross-linking polymeric matrix. These composite materials exhibit thermochromic luminescence properties with intense emissions varying with the temperature. A perfectly controlled luminescence thermochromism is observed due to a ‘protecting effect’ of the matrix preventing the non-radiative phenomenon of the cluster luminescence. The patterning of these UV-polymerizable photoluminescent films has been also realized by the UV-NIL technique to optimize the light-emitting properties of these materials. The surface patterning acts as a diffraction grating to extract the light which was previously guided inside the film. Moreover, the nanopatterning allows tuning of the emission color of the film as a function of the viewing angle. These composite materials present potential applications as photoactive systems with emission wavelength sensitive to the temperature and the surface nanostructuration.

Photoactive Hybrid Gelators Based on a Luminescent Inorganic [Cu4I4] Cluster Core

The formation of supramolecular self-assemblies of lowmolecular-weight gelators (LMWGs) is an attractive and elegant way to organize, through spontaneous aggregation, small molecules at the microand nanoscale level into stimuli-responsive soft materials. The discovery of new organogels that are sensitive to external stimuli is very appealing for tailored applications in biosciences, molecular electronics, sensing, or as confined reaction media and templates for well-defined inorganic materials. Metal-based gelators (metallogelators) have also emerged recently, affording smart gels in which the metal plays a central role and leads to new functional materials. Introduction of metal centers provides new properties such as magnetism, catalytic activity , and luminescence, properties that can be triggered by external stimuli such as the temperature. Despite the increasing number of gelation motifs being discovered, it remains a challenge to reliably predict the gelation capability of a compound, especially with nonconventional linear or disc-shaped gelators. New gelation motifs are, however, highly desirable from both fundamental and practical standpoints. Few polymetallic coordination complexes, such as the linear Pt2LL’2, [7] cylindrical Pd2L2 [8] and Ag3L3, [9] triangular M3L3 [10] (M =Au, Cu, Ag), or Zn4L4 grid [11] compounds (L= organic ligand), have been incorporated as cores in LMWGs. Recently, polyoxometallate-containing supramolecular gels have also been reported but hybrid gelators based on coordination clusters remain rare. Tetracopper(I) clusters formulated [Cu4X4L4] (X=Cl, Br, I; L= organic ligand) are well known for their rich photophysical properties. The molecular structure of these cubane-type clusters is formed by four copper and four halide atoms alternatively occupying the corners of a cube (Figure 1). Their remarkable photoluminescence properties,

Isotropic octupolar second harmonic generation response in LaBGeO5 glass-ceramic with spherulitic precipitation

A spherulitic crystallization of the crystalline phase LaBGeO5 is generated in the 25La2O3-25B2O3-50 GeO2 glass system. Linear and nonlinear optical properties of lanthanum borogermanate glass-ceramic have been investigated at both macroscopic and microscopic scales. Polarized μ-Raman analysis has evidenced a radial distribution of the crystallites along the c-axis inside spherulites, whereas polarized μ-Second Harmonic Generation (SHG) analysis revealed intensity maxima perpendicularly to the c-axis crystallites orientation. Polarized SHG mapping of a spherulite indicate that no dipolar response along the c-axis oriented crystallites occurs despite the individual dipolar symmetry C3 of the crystallites. At a larger mm scale, the isotropic scattering of spherulites recorded from macroscopic SHG experiment in the forward direction is consistent with an average coherent octupolar response per spherulite. These SHG analyses at different scale are both in accordance with radial antiferroelectric orientation along the c-axis of crystallites inside each spherulite.

Two-photon excited fluorescence in the LYB:Eu monoclinic crystal: towards a new scheme of single-beam dual-voxel direct laser writing in crystals

We report on two-photon excited fluorescence in the oriented Eu(3+)doped LYB monoclinic crystal under femtosecond laser tight focusing. Due to spatial walk-off, the two polarization modes of the incident femtosecond beam simultaneously provide the independent excitation of two distinct focuses, leading to a single-beam dual-voxel nonlinear excitation of fluorescence below material modification threshold. These observations emphasize on the anisotropy of both two-photon absorption as well as fluorescence emission. They demonstrate the localized control of the nonlinear energy deposit, thanks to the adjustment of both the input power and polarization, by properly balancing the injected energy in each voxel. Such approach should be considered for future direct laser writing of waveguides in propagation directions out of the dielectric axes, so as to optimally cope with the highly probable anisotropy of laser-induced material modification thresholds in these crystals. These results open new ways for further potential developments in direct laser writing as the simultaneous inscription of double-line structures for original waveguides processes.

Potential of the Eu:LYB crystal as a laser material for DPSS lasers emitting at 613 nm

We show the spectroscopic properties of a new potential laser crystal in the visible range. The Eu-doped borate compound has the structure Eu3+:Li6Y(BO3)3 with a doping concentration of about 25%. We studied the absorption and emission spectra of the crystal and calculated the gain cross sections. The compound shows a strong emission peak at 613 nm and absorption at 394 nm, which is compatible with pumping by new blue laser diodes. These features make the Eu:LYB crystal a very interesting candidate for the development of DPSS lasers emitting in the visible range.

Luminescence Mechanochromism Induced by Cluster Isomerization

Luminescent mechanochromic materials exhibiting reversible changes of their emissive properties in response to external mechanical forces are currently emerging as an important class of stimuli-responsive materials because of promising technological applications. Here, we report on the luminescence mechanochromic properties of a [Cu4I4(PPh3)4] copper iodide cluster presenting a chair geometry, being an isomer of the most common cubane form. This molecular cluster formulated [Cu4I4(PPh3)4]·2CHCl3 (1) exhibits a highly contrasted emission response to manual grinding, and, interestingly, the optical properties of the ground phase present striking similarities with those of the cubane isomer. In order to understand the underlying mechanism, a comparison with two related compounds has been conducted. The first one is a pseudopolymorph of 1 formulated as [Cu4I4(PPh3)4]·CH2Cl2 (2), which exhibits luminescent mechanochromic properties as well. The other one is also a chair compound but with a slightly different phosphine ligand, namely, [Cu4I4(PPh2C6H4CO2H)4] (3), lacking mechanochromic properties. Structural and optical characterizations of the clusters have been analyzed in light of previous electronic structure calculations. The results suggest an unpreceded mechanochromism phenomenon based on a solid-state chair → cubane isomer conversion. This study shows that polynuclear copper iodide compounds are particularly relevant for the development of luminescent mechanochromic materials.