Alain Garcia

Mechanochromic and Thermochromic Luminescence of a Copper Iodide Cluster

The mechanochromic and thermochromic luminescence properties of a molecular copper(I) iodide cluster formulated [Cu(4)I(4)(PPh(2)(CH(2)CH=CH(2)))(4)] are reported. Upon mechanical grinding in a mortar, its solid-state emission properties are drastically modified as well as its thermochromic behavior. This reversible phenomenon has been attributed to distortions in the crystal packing leading to modifications of the intermolecular interactions and thus of the [Cu(4)I(4)] cluster core geometry. Notably, modification of the Cu-Cu interactions seems to be involved in this phenomenon directly affecting the emissive properties of the cluster.

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.

Luminescence of Eu2+ in strontium and barium thiogallates

Abstract Solid state investigation of the MSGa2S3 systems (M = Sr, Ba) gives evidence of the existence of the following thiogallates: SrGa2S4, Sr2Ga2S5, BaGa4S7, BaGa2S4, Ba2Ga2S5, Ba3Ga2S6, Ba4Ga2S7, and Ba5(GaS4)2. When substituted for the alkaline earth, Eu2+ shows an emission in most host lattices. The emission band shifts to longer wavelengths with increasing M Ga atomic ratio of the host lattice. For most thiogallates, despite low phonon energies and small Stokes shifts, quenching of the Eu2+ emission starts below 300 K. Reasons for such behaviors are discussed.

Investigations of the MSAl2S3 systems (M = Ca, Sr, Ba) and luminescence properties of europium-doped thioaluminates

Abstract Solid state-investigation of the MSAl 2 S 3 systems ( M = alkaline earth ) shows the existence of the following thioluminates CaAl 2 S 4 , SrAl 2 S 4 , Sr 2 Al 2 S 5 , BaAl 4 S 7 , BaAl 2 S 4 , Ba 2 Al 2 S 5 , Ba 4 Al 2 S 7 and Ba 5 Al 2 S 8 . All are isostructural with homologous thiogallates. When substituted for M, divalent europium shows an emission in all host lattices. Compared with thiogallates, wider bandgaps and higher energies of the 5d emitting level are characteristics favourable to the thermal stability of the emission; the increase in quenching temperature is particularly pronounced for M-rich hosts.

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.

Fluoride crystals and high lying excited states of rare earth ions

Abstract We present the advantage of fluoride crystal hosts for the study of high lying localised excited states of rare earth ions. Special attention is devoted to the study of broad band UV emissions for solid state tuneable UV lasers and to the illustration of photoionisation mechanisms involving indium ions.

Structure and White Luminescence of Eu-Activated (Ba,Sr)13−xAl22−2xSi10+2xO66 Materials

The optical properties of Eu-activated (Ba,Sr)(13-x)Al(22-2x)Si(10+2x)O66 materials have been determined after the structural reinvestigation of the hypothetical Ba 13Al 22Si 10O 66 material on the basis of the Geberts model. The white fluorescence and phosphorescence of the (Ba,Sr)(13-x)Al(22-2x)Si(10+2x)O66:Eu series result from the existence of two broad emission bands associated with (8)H-4f(6)5d(1)-->(8)S-4f(7) transitions peaking at 534 and 438 nm, the intensities of which may be tuned at room temperature via the control of the europium concentration and the substitution of Sr for Ba. This suggests the possibility to adjust the emission of the material to white LED requisites.

Long-lasting luminescent ZnGa2O4:Cr3+ transparent glass-ceramics

Highly transparent ZnGa2O4 glass-ceramic materials are elaborated via a simple heat treatment of a 55SiO2–5Na2O–17ZnO–23Ga2O3 parent glass composition, which presents nanoscale spinodal phase separation. This optimized glass-ceramic exhibits 50 wt% of ZnGa2O4 nanocrystals showing a homogeneous and tuneable size. To describe the crystallization process, the glass and glass-ceramic nanostructures are studied by high resolution scanning transmission electron microscopy analysis coupled with in situ high temperature X-ray diffraction and optical measurements. From these results, an original mechanism is proposed to explain the crystallization process occurring in a spinodal phase separated glass. Remarkably, red long-lasting luminescence arising from the entire sample volume is observed in the Cr3+ doped transparent glass-ceramics, opening the route to a wider range of performing applications for this famous zinc gallate persistent phosphor.

Charge transfer excitation of the Nd3+, Sm3+, Dy3+, Ho3+, Er3+ and Tm3+ emission in CaGa2S4

Abstract The excitation spectra of the Nd3+, Sm3+, Dy3+, Ho3+, Er3+ and Tm3+ emission in the sodium-compensated CaGa2S4 host lattice, a sulfide with wide band gap, contain an intense band below the absorption edge. Comparison of the energy of its maximum with thermodynamic data and correlations to Jorgensens refined spin-pairing theory predictions allow one to ascribe this band to a charge transfer transition ending onto 4f orbitals. The irregular variation within the rare earth series contrasts with the monotonic variation of the absorption edge in stoichiometric rare earth sulfides (e.g. NaLnS2), associated with interband transitions.