Some novel cholesteric liquid crystals

A novel series of cholesteric liquid crystals (CNTⁿ). made by quaternization of the new mesogenic unit cholesteryl isonicotinate (CN) with n-alkyl salts, C_nH_2n + ₁ (T) (n = 2 to 8, T = 4-MeC₆H₄SO₃) has been prepared. Differential scanning calorime-try and polarizing optical microscopy studies reveal that all of these salts exhibit an enantiotropic cholesteric phase but for the cholesteryl isonicotinate unit the cholesteric phase is monotropic. For all members of the series the cholesteric-isotropic transitions are not reversible as these salts begin to decompose before their cholesteric-isotropic transition point is reached.     

Enhanced smectic polymorphism of a cholesteric side chain co-oligosiloxane by blending with small molecule liquid crystals

We have shown recently that a side chain chiral industrial co-oligosiloxane exhibits an original blue phase behaviour at the cholesteric-isotropic transition [1-3]. This same material has been studied dissolved in various polar small molecule nematic or cholesteric liquid crystals and it was shown that these associations are responsible for a large induced smectic domain. These systems allow the adjustment of the smectic A-cholesteric transition temperature from 180°C to room temperature, depending on the nature and concentration of the associated small molecule liquid crystal. Moreover the kinetics of this transition is strongly slowed down at high co-oligomer concentrations, suggesting, as confirmed by a preliminary X-ray study on the pure material, that the presumed glass transition allowing for the quenching of the Bragg optical properties at room temperature is in fact strongly associated with strong smectic correlations between the mesogenic parts.     

Thermophysical Properties of Cholesteryl Oleyl Carbonate Determined with Microscopic Ftir/Dsc System

A microscopic Fourier transform infrared spectrometer (Micro FTIR) equipped for differential scanning calorimetry (DSC) was used to measure simultaneously the chemical structural variation and the thermal response of phase transition of cholesteryl oleyl carbonate (COC). The differential scanning calorimeter served also to determine the phase transitions of COC during heating or cooling. Two endothermic features due to phase transition were found in the thermogram: 18. 3 °C for a smectic-cholesteric transition and 37.5 °C for a cholesteric-isotropic transition during heating; 35.1 °C for an isotropic-cholesteric transition and 15.8 °C for a cholesteric-smectic transition during cooling. The breadth of the feature indicated sluggish phase transitions. The three-dimensional plot indicated that the intensities of lines due to the C-H stretching and scissoring deformation modes and C=O stretching mode of COC decreased suddenly near the temperature of phase transition during heating but the intensity of the line dues to the C-O stretching mode of carbonate ester of COC increased. Lattice vibrations or coupling of vibrational modes might be responsible for the result. These intensities in the cooling process varied inversely to those in heating process.     

Influence of spacer lengths on the properties of chiral triplet liquid crystals based on estradiol

A series of 3-[ω-(4-cyanobiphenyl-4'-yloxy)alkyl] ethers of estradiol 17-[ω(4-cyanobiphenyl-4'-yloxy)alkanoates] with variable spacers has been prepared as chiral triplet liquid crystals. The compounds show very broad range cholesteric phases and are transformed into cholesteric glasses at room temperature. They exhibit odd-even effects for their cholesteric-isotropic transition temperatures and the associated entropy changes as a function of spacer length. The odd-even effects are observed upon changing the parity of both the ester and of the ether spacer. The best ordering is observed for compounds with an even number of methylene groups both in the ester and the ether spacer. In these cases the three mesogen units are oriented more or less parallel when the alkyl spacers are in the all-trans-conformations. Odd-even effects are also observed for the selective reflection wavelength of the planar cholesteric phase, depending on the parity of both spacers. For several compounds with short spacers the selective reflection wavelength increases strongly with temperature, whereas for other compounds this is almost temperature independent.     

Magnetic phase transition in a heteromolecular hydrogen-bonded complex of nitronylnitroxide radicals

An organic molecular acid-base complex has been synthesized from pyridine-substituted biradical 2 in a triplet (S = 1) ground state and a benzoic acid derivative of monoradical 3 with S = 1/2. The two constituent molecules are bound by an OH-N hydrogen bond in a crystalline solid state. The complex has been found to exhibit an antiferromagnetic phase transition at 5 K. The complex is the first example of a hydrogen-bonded heterospin, heteromolecular complex exhibiting a magnetic phase transition in purely organic molecule-based materials.     

Chiral liquid-crystalline polymers. XI. Thermotropic behavior and chiroptical properties in solution of a new series of optically active poly (ester β-sulfide)s

The synthesis and characterization are described of a series of optically active semiflexible poly(ester β-sulfide)s 6*Sn with repeating units constituted by two p-oxybenzoyl diads spaced in an alternating fashion by a sulfide-containing polymethylene segment of variable length (n = 2-10) and a chiral (R)-3-methylhexanediyl segment of high enantiomeric purity. The polymers were prepared with good yields and relatively high molecular weights and narrow degrees of dispersity by a Michael-type polyaddition reaction of bis-thiols to the same chiral mesogenic bisacrylate monomer. Data are reported on the characterization of their chiroptical properties in dilute solution and thermal-optical properties in the bulk, with attention being devoted to the distinct odd-even alternations of the cholesteric-isotropic melt transition parameters. A comparison of the mesomorphic behavior and relevant thermodynamic quantities is presented between the present series of polymers and the achiral analogues of series 6Sn as well as the chiral, positional isomers of series mS6* . The dependence of the overall properties and their trends on very subtle structural changes is stressed, thus highlighting the critical interconnection between mesogenic cores and spacer segments in semiflexible liquid-crystalline polyesters.     

Luminescence of organic dyes in silica matrices

The luminescence spectra of the acridine dyes adsorbed on the dispersed silica surface and introduced into the silica film composition at the sol-to-gel transition stage have been studied. The formation of protonated forms of adsorbed dyes, the appearance of the anomalously long-time recombination afterglow, and the photoradicalization of dye at low temperature have been found. The effect of the transition metal dopant ions of Fe and Mn in a SiO₂ matrix on dye luminescence quenching has been investigated. The luminescence quenching due to the transition metal ions is explained on the basis of an exchange-resonant interaction with contribution of the chemical interaction - complex formation.     

Finding transition pathways using the string method with swarms of trajectories

An approach to find transition pathways in complex systems is presented. The method, which is related to the string method in collective variables of Maragliano et al. (J. Chem. Phys. 2006, 125, 024106), is conceptually simple and straightforward to implement. It consists of refining a putative transition path in the multidimensional space supported by a set of collective variables using the average dynamic drift of those variables. This drift is estimated on-the-fly via swarms of short unbiased trajectories started at different points along the path. Successive iterations of this algorithm, which can be naturally distributed over many computer nodes with negligible interprocessor communication, refine an initial trial path toward the most probable transition path (MPTP) between two stable basins. The method is first tested by determining the pathway for the C7eq to C7ax transition in an all-atom model of the alanine dipeptide in vacuum, which has been studied previously with the string method in collective variables. A transition path is found with a committor distribution peaked at 1/2 near the free energy maximum, in accord with previous results. Last, the method is applied to the allosteric conformational change in the nitrogen regulatory protein C (NtrC), represented here with a two-state elastic network model. Even though more than 550 collective variables are used to describe the conformational change, the path converges rapidly. Again, the committor distribution is found to be peaked around 1/2 near the free energy maximum between the two stable states, confirming that a genuine transition state has been localized in this complex multidimensional system.     

A complex of methylthioadenosine/S-adenosylhomocysteine nucleosidase, transition state analogue, and nucleophilic water identified by mass spectrometry

An enzyme-stabilized nucleophilic water molecule has been implicated at the transition state of Escherichia coli methylthioadenosine nucleosidase (EcMTAN) by transition state analysis and crystallography. We analyzed the EcMTAN mass in complex with a femtomolar transition state analogue to determine whether the inhibitor and nucleophilic water could be detected in the gas phase. EcMTAN-inhibitor and EcMTAN-inhibitor-nucleophilic water complexes were identified by high-resolution mass spectrometry under nondenaturing conditions. The enzyme-inhibitor-water complex is sufficiently stable to exist in the gas phase.     

The IRC method in chemical reactions: Reaction ergodography for the addition of LiH to acetylene

The ab initio calculation have been performed on the addition of LiH to acetylene at RHF/3-21G basis set. The geometries and energies of the isolated reactant, molecular complex, transition state and product have been determined on the singlet potential energy surface of the ground state. Our results indicate that there is a meta stable molecular complex near the isolated reactant in the reaction pathway. The process from isolated reactant to molecular complex is a non-bonding-exchanging reaction process, and the process from molecular complex to product is the rate-controlling step of the reaction. We also estimate the activated entropy and the frequency factor of the rate-controlling step by using the RRKM theory. The FMO analysis for the transition state reveals the HOMO of transition state to be formed from both HOMO-LUMO and HOMO-HOMO interactions.     

Functional microporous materials of metal carboxylate: Gas-occlusion properties and catalytic activities

Copper(II) terephthalate is the first transition metal complex found capable of adsorbing gases. This complex has opened the new field of adsorbent complex chemistry. It is recognized as the lead complex in the construction of microporous complexes. This specific system has been expanded to a systematic series of derivatives of other isomorphous transition metals, molybdenum(II), ruthenium(II, III), and rhodium(II). These complexes with open frameworks are widely recognized as very useful materials for applications to catalysis, separation at molecular level, and gas storage.     

Toward a general method for CCC N-heterocyclic carbene pincer synthesis: Metallation and transmetallation strategies for concurrent activation of three C-H bonds

A general solution to the preparation of pincer complexes that require the formation of two N-heterocyclic carbenes (NHC’s) and the activation of an aryl C-H bond is reported. The reaction of a phenylene-bridged bis(imidazolium) salt with Zr(NMe₂)₄ generated the requisite CCC-NHC Zr pincer complex, which has been characterized by X-ray crystallography. Subsequent manipulation of the Zr coordination sphere by reaction with MeI demonstrated the robustness of the ligand geometry at Zr and led to the isolation and structural characterization of a CCC-NHC Zr triiodide pincer complex. A variation of the methodology has been applied to a saturated NHC analog to produce the corresponding CCC-NHC Zr pincer complex. Importantly, it has been found that CCC-NHC Zr pincer complexes can be generated in situ and transmetallated with an appropriate Rh source to generate CCC-NHC Rh pincer structures. These two methodologies, metallation of CCC-NHC precursors with transition metal amido complexes combined with transmetallation, hold great promise for opening general synthetic pathways to a wide variety of transition metal CCC-NHC pincer complexes.     

Paramagnetic ruthenium(III) cyclometallated complex. Synthesis, spectroscopic studies and electron-transfer properties

The reaction of Ru^II(PPh₃)₃X₂ (X = Cl, Br) with o-(OH)C₆H₄C(H)=N-CH₂C₆H₅ (HL) under aerobic conditions affords Ru^II(L)₂(PPh₃)₂, 1, in which both the ligands (L) are bound to the metal center at the phenolic oxygen (deprotonated) and azomethine nitrogen and Ru^III(L¹)(L²)(PPh₃), 2, in which one L is in bidentate N,O form like in complex 1 and the other ligand is in tridentate C,N,O mode where cyclometallation takes place from the ortho carbon atom (deprotonated) of the benzyl amine fragment. The complex 1 is unstable in solution, and undergoes spontaneous oxidative internal transformation to complex 2. In solid state upon heating, 1 initially converts to 2 quantitatively and further heating causes the rearrangement of complex 2 to the stable RuL₃ complex. The presence of symmetry in the diamagnetic, electrically neutral complex 1 is confirmed by ¹H and ³¹P NMR spectroscopy. It exhibits an Ru^II → L, MLCT transition at 460 nm and a ligand based transition at 340 nm. The complex 1 undergoes quasi-reversible ruthenium(II)—ruthenium(III) oxidation at 1.27V vs. SCE. The one-electron paramagnetic cyclometallated ruthenium(III) complex 2 displays an L → Ru^III, LMCT transition at 658 nm. The ligand based transition is observed to take place at 343 nm. The complex 2 shows reversible ruthenium(III)—ruthenium(IV) oxidation at 0.875V and irreversible ruthenium(III)—ruthenium(II) reduction at −0.68V vs. SCE. It exhibits a rhombic EPR spectrum, that has been analysed to furnish values of axial (6560 cm⁻¹) and rhombic (5630 cm⁻¹) distortion parameters as well as the energies of the two expected ligand field transitions (3877 cm⁻¹ and 9540 cm⁻¹) within the t₂ shell. One of the transitions has been experimentally observed in the predicted region (9090 cm⁻¹). The first order rate constants at different temperatures and the activation parameter ΔH^#/ΔS^# values of the conversion process of 1 → 2 have been determined spectrophotometrically in chloroform solution.     

Highly Active Phosphine‐Free Bifunctional Iron Complex for Hydrogenation of Bicarbonate and Reductive Amination

Based on a “transition metal frustrated Lewis pair” approach, a cyclopentadienone iron tricarbonyl complex has been designed and applied in the reductive amination and hydrogenation of bicarbonate. This well‐defined phosphine‐free complex displays the best activities reported to date for an iron complex in the reduction of bicarbonate into formate and in reductive amination.     

Direct preparation of allylic zirconium reagents from zirconocene-olefin complexes and alkenes

A novel method for preparation of allylic zirconium reagents directly from 1-alkenes via zirconocene-olefin complex has been developed. Selective transfer of the hydride of zirconocene allyl hydride complex, a tautomer of zirconocene-olefin complex, to diisopropyl ketone generates the corresponding zirconocene alkoxide allyl. The allylic zirconium reagents formed effects stereoselective allylation of aldehyde at 25 degrees C and -78 degrees C to provide syn- and anti-homoallyl alcohols, respectively. The anti-isomer is formed via a six-membered chair transition state under kinetic control. The syn-selectivity can be rationalized by considering isomerization of the anti-adduct by a retroallylation process.     

Computational studies on density function theory for the bimolecular metathesis reaction CH3+HCl⇔︁CH4+Cl

Density function theory has been applied to alkyl radical reaction to get helpful data for its geometric parameters, energy, and vibrational frequency compared with results obtained by ab initio methods and experimental values. The geometry optimization of the transition state, the precursor complex and the successor complex were performed at the 6‐311G* basis set level. The transition state of the CH₄Cl system of the reaction computed was in agreement with the prediction of Benson. From analysis of the vibration frequency and the net charge on the atom of the precursor complex, transition state, successor complex and the isolated state, the reaction mechanism was derived which we complicated with the bond‐rupture electron‐transfer theory. The atom H in molecule HCl attacks the atom C, forming a transition state via the precursor complex and the electron transfer happens in the precursor complex. The reaction rate of the electron transfer determines the rate of the whole reaction to a certain extent, and active energy, electronic coupling matrix element, and reorganization energy were obtained. © 2001 Wiley Periodicals, Inc. Int J Quantum Chem, 2001     

Molecular organization of the lipid-bound EDTA—chromium(III) complex

Ethylenediaminetetraacetic acid bound to a lipid (dioctadecyldimethylammonium bromide) has been used as an immobilized ligand system for extracting chromium(III) salts from an aqueous solution. The morphology and nature of the aggregation of the metal-ion-bound lipid—ligand complex has been studied by transmission electron microscopy and differential scanning calorimetry. A strong dependence of the vesicle size on the gel—crystalline phase transition has been observed and the results are rationalized in terms of higher-ordered packing of the large vesicles below the phase transition temperatures.     

Acetylene cyclotrimerization by early second-row transition metals in the gas phase. A theoretical study

The acetylene cyclotrimerization reaction mediated by the left-hand-side bare transition metal atoms Y, Zr, Nb, and Mo has been studied theoretically, employing DFT in its B3LYP formulation. The complete reaction mechanism has been analyzed, identifying intermediates and transition states. Both the ground spin state and at least one low-lying excited state have been considered to establish whether possible spin crossings between surfaces of different multiplicity can occur. Our results show that the overall reaction is highly favorable from a thermodynamic point of view and ground state transition states lie always below the energy limit represented by ground state reactants. After the activation of two acetylene molecules and formation of a bis-ligated complex, the reaction proceeds to give a metallacycle intermediate, as the alternative formation of a cyclobutadiene complex is energetically disfavored. All the examined reaction paths involve formation of a metallacycloheptatriene intermediate that in turn generates a metal-benzene adduct from which finally benzene is released. Similarities and differences in the behaviors of the considered four metal atoms have been examined.     

On the excited state dynamics of vibronic transitions. High-resolution electronic spectra of acenaphthene and its argon van der Waals complex in the gas phase

Rotationally resolved fluorescence excitation spectroscopy has been used to study the dynamics, electronic distribution, and the relative orientation of the transition moment vector in several vibronic transitions of acenaphthene (ACN) and in its Ar van der Waals (vdW) complex. The 0(0)(0) band of the S(1) ← S(0) transition of ACN exhibits a transition moment orientation parallel to its a-inertial axis. However, some of the vibronic bands exhibit a transition moment orientation parallel to the b-inertial axis, suggesting a Herzberg-Teller coupling with the S(2) state. Additionally, some other vibronic bands exhibit anomalous intensity patterns in several of their rotational transitions. A Fermi resonance involving two near degenerate vibrations has been proposed to explain this behavior. The high-resolution electronic spectrum of the ACN-Ar vdW complex has also been obtained and fully analyzed. The results indicate that the weakly attached argon atom is located on top of the plane of the bare molecule at ~3.48 ? away from its center of mass in the S(0) electronic state.     

Three-Step Spin State Transition and Hysteretic Proton Transfer in the Crystal of an Iron(II) Hydrazone Complex

A proton–electron coupling system, exhibiting unique bistability or multistability of the protonated state, is an attractive target for developing new switchable materials based on proton dynamics. Herein, we present an iron(II) hydrazone crystalline compound, which displays the stepwise transition and bistability of proton transfer at the crystal level. These phenomena are realized through the coupling with spin transition. Although the multi-step transition with hysteresis has been observed in various systems, the corresponding behavior of proton transfer has not been reported in crystalline systems; thus, the described iron(II) complex is the first example. Furthermore, because proton transfer occurs only in one of the two ligands and π electrons redistribute in it, the dipole moment of the iron(II) complexes changes with the proton transfer, wherein the total dipole moment in the crystal was canceled out owing to the antiferroelectric-like arrangement.