Wavelength selective light-induced magnetic effects in the binuclear spin crossover compound {[Fe(bt)(NCS)2]2(bpym)}

International audienceUsing Fourier transform infrared spectroscopy, x-ray diffraction, and magnetic susceptibility measurements under light irradiation, the selective light-induced excited spin state trapping (LIESST) and the reversible-LIESST effect have been evidenced and studied in depth in the binuclear spin crossover compound {[Fe(bt)(NCS)2]2bpym}. In this system, each magnetic site can switch from low spin (LS) to high spin (HS), so that three states exist, namely, the LS-LS, HS-LS, and HS-HS. All these techniques shine a new light on the high phototunability of this system. In addition to the direct photoswitching from the LS-LS to the HS-LS or to the HS-HS state, here we show that photoinduced switching between the excited photoinduced states can be triggered in a reversible way: from HS-LS to HS-HS (irradiation around 800 nm), or reverse from HS-HS to HS-LS (irradiation around 1300 nm). The nature of the intermediate HS-LS state during the thermal and light-induced spin state changes is also discussed by comparing the spectroscopic measurements and the structural analysis. The loss of inversion symmetry in the HS-LS molecular state, where the two magnetic Fe sites are no more equivalent, is not accompanied by any long-range ordering of the noncentrosymmetric molecules in the crystal. Therefore the continuous double-step spin conversion corresponds to a double crossover

Dispersion and alignment of individual Single Wall Carbon Nanotubes in a chromonic liquid crystal

Single Wall Carbon Nanotubes (SWNTs) display remarkable anisotropic features (mechanical, optical and conductivity properties). Exploiting them at a macroscopic scale requires both a good dispersion of individual tubes and a control of their orientational order at a large scale. The use of a liquid crystal as a structured solvent for aligning the tubes is attractive and several studies have already examined the dispersion of SWNTs in thermotropic or lyotropic liquid crystals. In this work, we used Disodium Chromoglycate (DSCG), a chromonic liquid crystal (LCLC) to disperse a large SWNT concentration (more than 0.1 %) in an aqueous nematic phase. The doped nematics and their orientation were studied by polarized microscopy, polarized Raman and photoluminescence spectroscopies (from individual semiconducting SWNT only). A quantitative approach [1-3] allowed us to determine accurately the order parameter of the tubes, which was found to be in the range 0.9-1

Multimodal imaging under supercontinuum illumination and electric field stimulation

National audienc

Dispersion and alignment of individual Single Wall Carbon Nanotubes in a chromonic liquid crystal

Single Wall Carbon Nanotubes (SWNTs) display remarkable anisotropic features (mechanical, optical and conductivity properties). Exploiting them at a macroscopic scale requires both a good dispersion of individual tubes and a control of their orientational order at a large scale. The use of a liquid crystal as a structured solvent for aligning the tubes is attractive and several studies have already examined the dispersion of SWNTs in thermotropic or lyotropic liquid crystals. In this work, we used Disodium Chromoglycate (DSCG), a chromonic liquid crystal (LCLC) to disperse a large SWNT concentration (more than 0.1 %) in an aqueous nematic phase. The doped nematics and their orientation were studied by polarized microscopy, polarized Raman and photoluminescence spectroscopies (from individual semiconducting SWNT only). A quantitative approach [1-3] allowed us to determine accurately the order parameter of the tubes, which was found to be in the range 0.9-1

Dispersion and alignment of individual Single Wall Carbon Nanotubes in a chromonic liquid crystal

Single Wall Carbon Nanotubes (SWNTs) display remarkable anisotropic features (mechanical, optical and conductivity properties). Exploiting them at a macroscopic scale requires both a good dispersion of individual tubes and a control of their orientational order at a large scale. The use of a liquid crystal as a structured solvent for aligning the tubes is attractive and several studies have already examined the dispersion of SWNTs in thermotropic or lyotropic liquid crystals. In this work, we used Disodium Chromoglycate (DSCG), a chromonic liquid crystal (LCLC) to disperse a large SWNT concentration (more than 0.1 %) in an aqueous nematic phase. The doped nematics and their orientation were studied by polarized microscopy, polarized Raman and photoluminescence spectroscopies (from individual semiconducting SWNT only). A quantitative approach [1-3] allowed us to determine accurately the order parameter of the tubes, which was found to be in the range 0.9-1

Microspectroscopie EPI-CARS multiplex ultrabroadband

International audienc

Evidence of a two-step process and pathway dependency in the thermodynamics of poly(diallyldimethylammonium chloride)/poly(sodium acrylate) complexation

Recent studies have pointed out the importance of polyelectrolyte assembly in the elaboration of innovative nanomaterials. Beyond their structures, many important questions on the thermodynamics of association remain unanswered. Here, we investigate the complexation between poly(diallyldimethylammonium chloride) (PDADMAC) and poly(sodium acrylate) (PANa) chains using a combination of three techniques: isothermal titration calorimetry (ITC), static and dynamic light scattering and electrophoresis. Upon addition of PDADMAC to PANa or vice-versa, the results obtained by the different techniques agree wellwith each other, and reveal a two-step process. The primary process is the formation of highly charged polyelectrolyte complexes of size 100 nm. The secondary process is the transition towards a coacervate phase made of rich and poor polymer droplets. The binding isotherms measured are accounted for using a phenomenological model that provides the thermodynamic parameters for each reaction. Small positive enthalpies and large positive entropies consistent with a counterion release scenario are found throughout this study. Furthermore, this work stresses the importance of the underestimated formulation pathway or mixing order in polyelectrolyte complexation

Fast epi-detected ultrabroadband multiplex CARS ans SHG imaging of mouse skull cells

International audienc

Symmetry breaking and light-induced spin-state trapping in a mononuclear Fe^II complex with the two-step thermal conversion

Crystallographic, magnetic, and Raman investigations of the mononuclear [FeII(Hpy-DAPP)](BF4)2 complex are presented. Its particular feature is a two-step thermal spin conversion in spite of a unique symmetry-independent iron site per unit cell. The plateau around 140 K is associated with a symmetry breaking visible by the appearance of weak (0k0) k odd Bragg peaks. Symmetries of the high-temperature high-spin state and of the low-temperature low-spin state are both monoclinic P21/c, so that the symmetry breaking on the plateau is associated with a reentrant phase transition. It is discussed in relation with Ising-type microscopic models. At the plateau level, the two symmetry-independent molecules differ both by their spin state and the conformation (chair versus twist-boat) of one metallocycle. At low-temperature photoinduced phenomena have been investigated: a partial phototransformation [light-induced excited spin-state trapping (LIESST) effect] is observed under visible red irradiation. Raman spectroscopy shows that the molecular photoinduced state is the high-spin one. Nevertheless, as no macroscopic symmetry breaking is observed, the unique average cationic [FeII(Hpy-DAPP)] state of the unit cell is intermediate between pure low-spin and high-spin states and presents a conformational disorder for one metallocycle. Reverse-LIESST has also been evidenced using near infrared excitation. Thus, the mononuclear [Fe(Hpy-DAPP)](BF4)2 compound offers the opportunity to discuss the interplay between spin conversion, molecular conformational change, and ordering processes

Spectroscopic and Magnetic Properties of the Metastable States in the Coordination Network [{Co(prm)(2)}(2){Co(H(2)O)(2)}{W(CN)(8)}(2)]*4H(2)O (prm = pyrimidine).

The study of the metastable states, obtained by thermal quenching or by light irradiation in the [{Co(prm)(2)}(2){Co(H(2)O)(2)}{W(CN)(8)}(2)]*4H(2)O complex, is reported using powder X-ray diffraction, Raman spectroscopy, optical reflectivity, and magnetic measurements. This compound is characterized by a electron-transfer (ET) phase transition occurring between a high-temperature phase (HT phase) formed by paramagnetic Co(II)-W(V) units and a low-temperature phase (LT phase) formed by diamagnetic Co(III)-W(IV) units. Metastable phases can be induced at low temperature either by thermal quenching rapidly cooling phase named RC or by irradiation photo-induced phase named PI similar to the well-known Light-Induced Excited Spin State Trapping effect. The relaxation dynamics of the metastable phases have been studied and revealed some differences between the RC and PI phases. The sigmoidal shape of the relaxation curves in the RC phase is in agreement with the cooperative nature of the process. Thermodynamic parameters that govern the relaxation have been determined and used to reproduce the experimental Thermal-Induced Excited Spin State Trapping curve