6 research outputs found
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(PMe<sub>3</sub>)<sub>2</sub>(CCC<sub>6</sub>H<sub>4</sub>R)<sub>2</sub>]‑Containing Materials (R = O(CH<sub>2</sub>)<sub><i>n</i></sub>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><sub><b>6</b></sub>; <i>n</i> = 12, <b>[Pt]C</b><sub><b>12</b></sub>), X-ray powder
diffraction (powder XRD), differential scanning calorimetry (DSC),
thermal gravimetric analysis (TGA, <b>[Pt]C</b><sub><b>12</b></sub>), and steady-state and time-resolved solid state UV–vis
and emission spectroscopy at 298 and 77 K. <b>[Pt]C</b><sub><b>6</b></sub> complex exhibits no phase change with the temperature.
Concurrently, <b>[Pt]C</b><sub><b>9</b></sub> (<i>n</i> = 9) and <b>[Pt]C</b><sub><b>12</b></sub> complexes
exhibit an irreversible <i>T</i><sub>endo</sub> values of
104 and 119 °C, respectively, associated with a thermal annealing.
Finally, <b>[Pt]C</b><sub><b>15</b></sub> (<i>n</i> = 15) complex exhibit a reversible thermal processes with a large
hysteresis (<i>T</i><sub>endo</sub> = 126 °C, <i>T</i><sub>exo</sub> = 140 °C) followed by a glass transition
(<i>T</i><sub>endo</sub> = 146 °C, <i>T</i><sub>exo</sub> = 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 [Cu<sub>4</sub>I<sub>4</sub>L<sub>4</sub>]
(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 = CH<sub>3</sub> (seven steps) or C<sub>17</sub>H<sub>35</sub> (10 steps)) with e.e. up to 96%. The X-ray structure
of [Cu<sub>4</sub>I<sub>4</sub>((<i>R</i>)-(CH<sub>3</sub>)(Ph)(<i>i</i>-Pr)P)<sub>4</sub>] 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 [Cu<sub>4</sub>I<sub>4</sub>((<i>S</i>)-(CH<sub>3</sub>)(Ph)(<i>i</i>-Pr)P)<sub>4</sub>] 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 C<sub>17</sub>H<sub>35</sub> 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 = CH<sub>3</sub> and C<sub>17</sub>H<sub>35</sub>. 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 [Cu<sub>4</sub>I<sub>4</sub>L<sub>4</sub>]
(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 = CH<sub>3</sub> (seven steps) or C<sub>17</sub>H<sub>35</sub> (10 steps)) with e.e. up to 96%. The X-ray structure
of [Cu<sub>4</sub>I<sub>4</sub>((<i>R</i>)-(CH<sub>3</sub>)(Ph)(<i>i</i>-Pr)P)<sub>4</sub>] 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 [Cu<sub>4</sub>I<sub>4</sub>((<i>S</i>)-(CH<sub>3</sub>)(Ph)(<i>i</i>-Pr)P)<sub>4</sub>] 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 C<sub>17</sub>H<sub>35</sub> 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 = CH<sub>3</sub> and C<sub>17</sub>H<sub>35</sub>. 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