Chiral selectivity noise in spontaneous mirror symmetry breaking.

Mirror symmetry breaking bifurcations, that occur in nonlinear chemical systems leading to final chiral states with very large enantiomeric excess, can be exploited as an efficient chiral signal selector for even the smallest chiral polarizations. This effect of the chiral polarization requires the system's capacity for overcoming thermal noise, which is manifested as fluctuating reaction rate constants. Therefore, we investigate the chiral selectivity across a range of tiny parity-violating energy differences (PVED) in the presence of inevitable non-equilibrium temperature fluctuations.

A Complex Reaction Network Model for Spontaneous Mirror Symmetry Breaking in Viedma Deracemizations.

Attrition-enhanced chiral symmetry breaking in crystals, known as Viedma deracemization, is a promising method for converting racemic solid phases into enantiomerically pure ones under non-equilibrium conditions. However, many aspects of this process remain unclear. In this study, we present a new investigation into Viedma deracemization using a comprehensive kinetic rate equation continuous model based on classical primary nucleation theory, crystal growth, and Ostwald ripening.

Photoswitchable assembly of long-lived azobenzenes in water using visible light.

Hypothesis: Switchable assemblies relevant for bio-applications may be accessed from water-soluble tetra-ortho-substituted azobenzenes that reversibly self-assemble and form complexes with β-cyclodextrin under visible light.

Experiments: Two azobenzenes bearing either four fluorines or two chlorines and two fluorines in the ortho positions were synthesised with short poly(ethylene oxide) tails for water solubility. Photophysical properties were determined by UV-vis and H NMR spectroscopies, complexation with β-cyclodextrin was assessed by H NMR spectroscopy, and self-assembly in water was investigated by static and dynamic light scattering.

Spontaneous Emergence of Transient Chirality in Closed, Reversible Frank-like Deterministic Models.

To explore abiotic theories related to the origin of biomolecular homochirality, we analyze two entirely reversible kinetic models composed of an enantioselective autocatalysis with limited stereoselectivity that is coupled to an enantiomeric mutual inhibition (Frank-like models). The two models differ in their autocatalytic steps in respect to the formation of monomer species in one model and of dimer species in the other. While fully reversible and running in a closed system, spontaneous mirror symmetry breaking (SMSB) gives rise to transient chiral excursions, even when starting from a strictly achiral situation.

Spontaneous Deracemizations.

Spontaneous deracemizations is a challenging, multidisciplinary subject in current chirality research. In the absence of any chiral inductors, an achiral substance or a racemic composition is driven into an enantioenriched or even homochiral state through a selective energy input, e.g.

A photochemical determination of luminescence efficiency of upconverting nanoparticles.

Upconverting nanoparticles are a rising class of non-linear luminescent probes burgeoning since the beginning of the 2000's, especially for their attractiveness in theranostics. However, the precise quantification of the light delivered remains a hot problem in order to estimate their impact on the biological medium. Sophisticated photophysical measurements under near infrared excitation have been developed only by few teams.

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NaClO is achiral in solution. If crystallization is performed under a static set-up, it is recognized that the stochastic nucleation probability results in a racemic mixture of the conglomerate. In this paper, we report a reexamination of the crystallization of NaClO from static solution in petri dishes that was conducted over a number of years and is based on the count and analysis of several thousand d- vs.

Terahertz Time-Domain Spectroscopy of Thermoresponsive Polymers in Aqueous Solution.

The behavior of highly concentrated aqueous solutions of two thermoresponsive polymers poly(N-isopropylacrylamide) (PNIPAm) and poly(N-vinylcaprolactam) (PVCL) have been investigated by terahertz time-domain spectroscopy (THz-TDS). Measurements have been performed for concentrations up to 20 wt %, over a frequency range from 0.3 to 1.

Proton catalysis in the redox responsivity of a mini-sized photochromic diarylethene.

A thermally irreversible dithienylethene (DTE) photochrom can be turned into a thermally reversible one in presence of Cu(II) triflate. A ring opening (DTEC closed→DTEO open) occurs through the formation of a copper-containing fast transient intermediate. Stopped-flow experiments monitored at 410 and 780 nm have allowed to show that the stoichiometry of this intermediate is DTE/Cu=1:1.

Amplification of enantiomeric excess, mirror-image symmetry breaking and kinetic proofreading in Soai reaction models with different oligomeric orders.

A comprehensive kinetic analysis of three prototypical autocatalytic cycle models based on the absolute asymmetric Soai reaction is presented. The three models, which can give rise to amplification of enantiomeric excess and mirror-image symmetry breaking, vary by their monomeric, dimeric or trimeric order of the assumed catalytic species. Our numerical approach considered the entire chiral combinatorics of the diastereomeric interactions in the models as well as the multiplicity of coupled reversible reactions without applying fast equilibration or quasi-steady state approximations.

Gated photochromism and acidity photomodulation of a diacid dithienylethene dye.

The present study quantitatively analyses the gated photochromism and the acidity photomodulation properties of a diacid dithienylethene compound. Photoisomerisation between the open and closed isomers was investigated by UV/visible and (1)H NMR spectroscopy. It was found that the photocyclisation quantum yield of the diacid form was remarkably high (around 90%).

Photochromism and metal complexation of a macrocyclic styryl naphthopyran.

A macrocyclic benzo-15-crown-5 ether unit tethered to a photochromic naphthopyran by a styryl spacer (MEN) is shown to form a 1:1 complex with magnesium(II). The structure and dynamics of the specific host-metal interactions were investigated by PFG-NMR analysis. A combination of UV/Vis and variable temperature multi-dimensional (1)H NMR photokinetic analysis of the crown-containing styryl naphthopyran and its metal complex was used to probe the effect of metal complexation on the photochromism.

Metal ions drive thermodynamics and photochemistry of the bis(styryl) macrocyclic tweezer.

UV/Vis and NMR spectroscopy were used for the structural elucidation and thermodynamic and photochemical studies of the metal-coordinated crown-containing macrocyclic tweezer (E,E)-1. The bis(styryl) tweezer (E,E)-1 formed two types of complexes with magnesium(II): a 1:1 intramolecular asymmetric sandwich complex [(E,E)-1].Mg(2+) and a 1:2 complex [(E,E)-1].

Quantitative investigations of cation complexation of photochromic 8-benzothiazole-substituted benzopyran: towards metal-ion sensors.

The photochromic, thermochromic and metallochromic behaviour of a series of three spiro[indoline-8-(benzothiazol-2-yl)-benzopyrans] has been investigated. The thermodynamic and kinetic parameters of their thermal equilibrium between the ring-closed (spiro) and ring-opened (merocyanine) isomeric forms have been determined using UV-Vis absorption and (1)H NMR spectroscopies. By adding Co(ii) and Ni(ii) ions in acetonitrile solution, 1 : 1 and 1 : 2 metal : merocyanine complexes are formed simultaneously.

Partition and water/oil adsorption of some surfactants.

Adsorption isotherms have been determined at the water/oil interface for five biphasic systems involving surfactants (non-ionic and ionic) present in both phases at partition equilibrium. The systems studied were polyoxyethylene(23)lauryl ether (Brij35) in water/hexane and four ionic surfactants, hexadecyltrimethylammonium bromide (CTAB), and a series of three tetraalkylammonium dodecylsulfate (TEADS, TPADS, and TBADS) in water/CH 2Cl 2. Interfacial tension measurements performed at the water/air and water/oil interfaces provided all the necessary information for the determination of the adsorption parameters by taking partition into account.

Wavelength-dependent reactivity of a quinolinone: toward a photochromic three-state system.

The photochemical reactivity of the quinolinone 3 was investigated using NMR by monitoring its reactions under appropriate irradiation wavelengths. Besides the irreversible formation of degradation products which were structurally identified, the reversible formation of the enol 4 and cyclobutenol 5 was also observed. The enol and cyclobutenol can be switched or reversed back to the quinolinone 3, resulting in a photochromic three-state system in which the relative ratio of the three components largely depends on the irradiation wavelength used.

Transient absorption studies of the photochromic behavior of 3H-naphtho[2,1-b]pyrans linked to thiophene oligomers via an acetylenic junction.

The photophysical and photochemical properties of four 3,3-diphenyl-3H-naphtho[2,1- b]pyrans substituted, via an acetylenic junction, to (thiophene) n oligomers (n = 0-3 units) were investigated by transient absorption in the femtosecond to microsecond time domain and by stationary absorption and fluorescence. The decay of the initially produced excited S1(pi pi*) state is found to occur via three competing processes: fluorescence, intersystem crossing, and a ring-opening reaction leading to a colored merocyanine product, with relative yields varying drastically with n. Whereas ultrafast (sub-picosecond) reaction dynamics and high product quantum yield are observed for n = 0 and 1, the reaction is considerably slowed down on going to the n = 2 (105 ps) compound and does not occur for n = 3.

Association, partition, and surface activity in biphasic systems displaying relaxation oscillations.

Several biphasic systems giving rise to periodical Marangoni instability have been analyzed from the point of view of the physicochemical properties of the involved compounds. In each case, the compound at the origin of the oscillatory behavior has been identified: the reactant cetyltrimethylammonium bromide (CTAB) for the CTAB/picric acid (PH) system and the product of reaction dodecyl sulfate tetraalkylammonium (TAADS) for the sodium dodecyl sulfate/tetraalkylammonium bromide (SDS/TAAB) system. The properties of the latter system have been varied progressively by increasing the chain length of the tetraalkylammonium ion.

Enantioselectivity reversal by achiral additives in the Soai reaction: a kinetic understanding.

Numerical kinetic simulations have been employed to understand the origin of enantioselectivity reversal of chiral beta-amino alcohol catalysts occurring in the presence of their achiral analogs in the autocatalytic addition of diisopropylzinc to pyrimidine carbaldehydes (Soai reaction). In a preceding work (Rivera Islas, J.; Lavabre, D.

Spectroscopic and theoretical evidence for the elusive intermediate of the photoinitiated and thermal rearrangements of photochromic spiropyrans.

Spectral properties and photochromic behavior of a series of novel 1',3',3'-trimethyl-1,2-tetramethylenespiro[7H-furo(3,2-f)-(2H-1)-benzopyran-7,2'-indolines] 1-4 have been studied. The mechanism of the photoinitiated ring-opening reaction involves the formation of an acoplanar cis-cisoid intermediate, the lifetime of which in the case of 6-(tert-butyl) derivative 4 is long enough to observe its absorption and fluorescence spectra under conditions of continuous irradiation. The occurrence of the intermediate on the reaction paths of the thermal and photochemical ring-opening processes has been also shown by the DFT and CIS calculations.

Periodic marangoni instability in surfactant (CTAB) liquid/liquid mass transfer.

Periodic Marangoni convective instability has been observed in a biphasic system during the mass transfer of cetyltrimethylammonium bromide (CTAB) from an aqueous to a dichloromethane organic phase. Visualization of the convective fluxes was possible thanks to the CTAB crystals that are formed in the aqueous phase at a temperature below the Krafft point. Surface tension and electrical potential oscillations have been shown to be correlated with the fluid motion.

Determining the association constant and adsorption properties of ion pairs in water by fitting surface tension data.

Association constants and adsorption parameters of tetraalkylammoniumdodecyl sulfate (TAADS) ion pairs in water were determined. We have analyzed water/air surface tension measurements obtained for mixtures of sodium dodecyl sulfate (SDS) and tetraalkylammonium bromide of increasing chain lengths (TMAB, TEAB, TPAB, and TBAB). To reproduce the experimental isotherms, we coupled the association equilibrium of the ion pairs to the equations proposed by Fainerman and co-workers to model the adsorption of binary mixtures of surfactants (SDS and TAADS) with different molar areas at a nonideal surface layer.

Mirror-symmetry breaking in the Soai reaction: a kinetic understanding.

Kinetic modeling using nonlinear differential equations is proposed to analyze the spontaneous generation of enantiomeric excess in the autocatalytic addition of diisopropylzinc to prochiral pyrimidine carbaldehydes (Soai reaction). Our approach reproduces experimentally observed giant chiral amplification from an initial enantiomeric excess of <10(-6)% to >60%, high sensitivity and positive response to the presence of minute amounts of chiral initiator at concentrations <10(-14) M, and spontaneous absolute asymmetric synthesis from achiral starting conditions. From our numerical simulations using kinetic schemes derived from the Frank model, including stereospecific autocatalysis and mutual inhibition, we have shown that it is possible to reproduce the mirror-symmetry-breaking behavior of the Soai reaction under batch conditions leading to a bimodal enantiomeric product distribution.

Unexpected halogen substituent effects on the complex thermal relaxation of naphthopyrans after UV irradiation.

The kinetics of the thermal fading of four halogenated naphthopyrans (NP) have been analyzed using NMR spectroscopy in CD(3)CN. Two photomerocyanines (TT and TC) were detected after UV irradiation. The main relaxation process TC --> NP was coupled with TT/TC isomerization.