New Insights into the Thermal Stability of the Smectic C Phase

Subtle differences in the molecular structure of mesogens can lead to very different experimental polymorphisms. The smectic C (SmC) phase can actually be exhibited by one isomer and not the other, or the range of temperature can be completely different. Unveiling the deep connection between atomic structure and the very existence of the SmC phase will lead to the design of new performing liquid crystalline materials for ferroelectric or nonlinear optical applications. Our approach is based on running molecular dynamics simulation from an initial SmC arrangement of molecules. When the temperature is increased, the molecules automatically adjust in a more favorable organization. Such modification in the imposed initial self-assembly is governed by values of the nonbonded energies. Thanks to the combined use of simulation and experimental phase diagrams, we have unveiled part of the deep connection between atomic structure and the very existence of the SmC phase. The actual display of the SmC mesophase stems from a subtle balance between short-range interactions, which reveal arrangement of molecules within a smectic layer, and long-range interactions, which disclose organization of layers

Thermodynamic Control in the Catalytic Insertion Polymerization of Norbornenes as Rationale for the Lack of Reactivity of Endo-Substituted Norbornenes

The catalytic insertion polymerization of substituted norbornenes (NBEs) leads to the formation of a family of polymers which combine extreme thermomechanical properties as well as unique optical and electronic properties. However, this reaction is marred by the lack of reactivity of endo substituted monomers. It has long been assumed that these monomers chelate the metallic catalyst, leading to species which are inactive in polymerization. Here we examine the polymerization of <i>cis</i>-5-norbornene-2,3-dicarboxylic anhydride (so-called carbic anhydride, CA) with a naked cationic Pd catalyst. Although <i>exo</i>-CA can be polymerized, the polymerization of <i>endo</i>-CA stops after a single insertion. Surprisingly, no chelate is formed between the catalyst and <i>endo</i>-CA. Using DFT calculation, it is shown that while the insertion of <i>exo</i>-NBEs is exergonic, the insertion of two <i>endo</i>-CA in a row is endergonic. In this latter case, the enthalpy gain corresponding to the insertion of a double bond is not sufficient to overcome the entropic penalty associated with ligand binding. Thus, the different reactivity between endo and exo NBEs is thermodynamic in nature, and it is not controlled by kinetic factors. Interestingly, thermodynamics is also the main factor controlling the stereochemistry of the chain. For CA polymerization, and even for unsubstituted NBE polymerization, the formation of <i>r</i> and <i>m</i> dyads is, respectively, exergonic and endergonic, resulting in a polymer which is essentially disyndiotactic. Thus, this study demonstrates that thermodynamics can control the chemo- and stereoselectivity of a catalytic polymerization

Solid–Solid Phase Transitions in [<i>trans</i>-Pt(PMe₃)₂(CCC₆H₄R)₂]‑Containing Materials (R = O(CH₂)_<i>n</i>H; <i>n</i> = 6, 9, 12, and 15)

The title complexes were prepared in Hagihara conditions and were investigated by single crystal X-ray crystallography (<i>n</i> = 6, <b>[Pt]­C</b>_<b>6</b>; <i>n</i> = 12, <b>[Pt]­C</b>_<b>12</b>), X-ray powder diffraction (powder XRD), differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA, <b>[Pt]­C</b>_<b>12</b>), and steady-state and time-resolved solid state UV–vis and emission spectroscopy at 298 and 77 K. <b>[Pt]­C</b>_<b>6</b> complex exhibits no phase change with the temperature. Concurrently, <b>[Pt]­C</b>_<b>9</b> (<i>n</i> = 9) and <b>[Pt]­C</b>_<b>12</b> complexes exhibit an irreversible <i>T</i>_endo values of 104 and 119 °C, respectively, associated with a thermal annealing. Finally, <b>[Pt]­C</b>_<b>15</b> (<i>n</i> = 15) complex exhibit a reversible thermal processes with a large hysteresis (<i>T</i>_endo = 126 °C, <i>T</i>_exo = 140 °C) followed by a glass transition (<i>T</i>_endo = 146 °C, <i>T</i>_exo = 69 °C) as depicted by DSC. These phase changes are accompanied by a decrease in triplet excited state lifetimes upon cycling the sample temperature over and under the transition temperatures. These various thermal processes induce a significant decrease in emission lifetimes, strongly suggesting the presence of a reorganization of the complexes in the solid state favoring more chain-chromophore contacts, thus promoting nonradiative “knocking” processes

Luminescent P‑Chirogenic Copper Clusters

P-chirogenic clusters of the cubanes [Cu₄I₄L₄] (L = chiral phosphine) were prepared from (+)- and (−)-ephedrine with L = (<i>S</i>)- or (<i>R</i>)-(R)­(Ph)­(<i>i</i>-Pr)P (with R = CH₃ (seven steps) or C₁₇H₃₅ (10 steps)) with e.e. up to 96%. The X-ray structure of [Cu₄I₄((<i>R</i>)-(CH₃)­(Ph)­(<i>i</i>-Pr)­P)₄] confirmed the cubane structure with average Cu···Cu and Cu···I distances of 2.954 and 2.696 Å, respectively. The cubane structure of the corresponding [Cu₄I₄((<i>S</i>)-(CH₃)­(Ph)­(<i>i</i>-Pr)­P)₄] was established by the comparison of the X-ray powder diffraction patterns, and the opposite optical activity of the (<i>S</i>)- and (<i>R</i>)-ligand-containing clusters was confirmed by circular dichroism spectroscopy. Small-angle X-ray scattering patterns of one cluster bearing a C₁₇H₃₅ chain exhibit a weak signal at 2θ ∼ 2.8° (<i>d</i> ∼ 31.6 Å), indicating some molecular ordering in the liquid state. The emission spectra exhibit two emission bands, both associated with triplet excited states. These two bands are assigned as follows: the high energy emission is due to a halide-to-ligand charge transfer, XLCT, state mixed with LXCT (ligand-to-halide-charge-transfer). The low energy band is assigned to a cluster-centered excited state. Both emissions are found to be thermochromic with the relative intensity changing between 77 and 298 K for the clusters in methylcyclohexane solution. Several differences are observed in the photophysical parameters, emission quantum yields and lifetimes for R = CH₃ and C₁₇H₃₅. The measurements of the polarization along the emission indicate that the emission is depolarized, consistent with an approximate tetrahedral geometry of the chromophores

Luminescent P‑Chirogenic Copper Clusters

P-chirogenic clusters of the cubanes [Cu₄I₄L₄] (L = chiral phosphine) were prepared from (+)- and (−)-ephedrine with L = (<i>S</i>)- or (<i>R</i>)-(R)­(Ph)­(<i>i</i>-Pr)P (with R = CH₃ (seven steps) or C₁₇H₃₅ (10 steps)) with e.e. up to 96%. The X-ray structure of [Cu₄I₄((<i>R</i>)-(CH₃)­(Ph)­(<i>i</i>-Pr)­P)₄] confirmed the cubane structure with average Cu···Cu and Cu···I distances of 2.954 and 2.696 Å, respectively. The cubane structure of the corresponding [Cu₄I₄((<i>S</i>)-(CH₃)­(Ph)­(<i>i</i>-Pr)­P)₄] was established by the comparison of the X-ray powder diffraction patterns, and the opposite optical activity of the (<i>S</i>)- and (<i>R</i>)-ligand-containing clusters was confirmed by circular dichroism spectroscopy. Small-angle X-ray scattering patterns of one cluster bearing a C₁₇H₃₅ chain exhibit a weak signal at 2θ ∼ 2.8° (<i>d</i> ∼ 31.6 Å), indicating some molecular ordering in the liquid state. The emission spectra exhibit two emission bands, both associated with triplet excited states. These two bands are assigned as follows: the high energy emission is due to a halide-to-ligand charge transfer, XLCT, state mixed with LXCT (ligand-to-halide-charge-transfer). The low energy band is assigned to a cluster-centered excited state. Both emissions are found to be thermochromic with the relative intensity changing between 77 and 298 K for the clusters in methylcyclohexane solution. Several differences are observed in the photophysical parameters, emission quantum yields and lifetimes for R = CH₃ and C₁₇H₃₅. The measurements of the polarization along the emission indicate that the emission is depolarized, consistent with an approximate tetrahedral geometry of the chromophores

Regio- and Stereoselective Synthesis of Spiropyrrolizidines and Piperazines through Azomethine Ylide Cycloaddition Reaction

A series of original spiropyrrolizidine derivatives has been prepared by a one-pot three-component [3 + 2] cycloaddition reaction of (<i>E</i>)-3-arylidene-1-phenyl-pyrrolidine-2,5-diones, l-proline, and the cyclic ketones 1<i>H</i>-indole-2,3-dione (isatin), indenoquinoxaline-11-one and acenaphthenequinone. We disclose an unprecedented isomerization of some spiroadducts leading to a new family of spirooxindolepyrrolizidines. Furthermore, these cycloadducts underwent retro-1,3-dipolar cycloaddition yielding unexpected regioisomers. Upon treatment of the dipolarophiles with <i>in situ</i> generated azomethine ylides from l-proline or acenaphthenequinone, formation of spiroadducts and unusual polycyclic fused piperazines through a stepwise [3 + 3] cycloaddition pathway is observed. The stereochemistry of these N-heterocycles has been confirmed by several X-ray diffraction studies. Some of these compounds exhibit extensive hydrogen bonding in the crystalline state. To enlighten the observed regio- and stereoselectivity of the [3 + 2] cycloaddition, calculations using the DFT approach at the B3LYP/6-31G­(d,p) level were carried out. It was found that this reaction is under kinetic control