Optical Activity In Solution Research Articles

Defect-Free Synthesis of a Fully π-Conjugated Helical Ladder Polymer and Resolution into a Pair of Enantiomeric Helical Ladders.

Fully π-conjugated ladder polymers with a spiral geometry represent a new class of helical polymers with great potential for organic nanodevices, but there is no precedent for an optically-active helical ladder polymer totally composed of achiral units. We now report the defect-free synthesis and resolution of a fully π-conjugated helical ladder polymer with a rigid helical cavity, which has been achieved by quantitative and chemoselective acid-promoted alkyne benzannulations of a rationally designed, random-coil achiral polymer followed by chromatographic enantioseparation. Because of a sufficiently high helix-inversion barrier, the isolated excess one-handed helical ladder polymer with the degree of polymerization of more than 15 showed a strong circular dichroism with the dissymmetry factor of up to 1.7 × 10-2 and is thermally stable, maintaining its optical activity in solution even at 100 °C, as well supported by molecular dynamics simulation.

Defect‐Free Synthesis of a Fully π‐Conjugated Helical Ladder Polymer and Resolution into a Pair of Enantiomeric Helical Ladders**

AbstractFully π‐conjugated ladder polymers with a spiral geometry represent a new class of helical polymers with great potential for organic nanodevices, but there is no precedent for an optically active helical ladder polymer totally composed of achiral units. We now report the defect‐free synthesis and resolution of a fully π‐conjugated helical ladder polymer with a rigid helical cavity, which has been achieved by quantitative and chemoselective acid‐promoted alkyne benzannulations of a rationally designed, random‐coil achiral polymer followed by chromatographic enantioseparation. Because of a sufficiently high helix‐inversion barrier, the isolated excess one‐handed helical ladder polymer with a degree of polymerization of more than 15 showed a strong circular dichroism with a dissymmetry factor of up to 1.7×10−2 and is thermally stable, maintaining its optical activity in solution even at 100 °C, as well‐supported by molecular dynamics simulation.

Self-Assembly of Chiral-at-End Diketopyrrolopyrroles: Symmetry Dependent Solution and Film Optical Activity and Photovoltaic Performance.

Chiral thiophene-diketopyrrolopyrrole derivatives have been synthesised to investigate the potential of stereochemistry and symmetry as a means of modulating properties by influencing self-assembly of these purely organic materials. In particular, derivatives of diketopyrrolopyrrole were employed because of their proven interest as dyes, especially for organic solar cells. The natural product myrtenal was used as the source of stereochemistry, introduced through a Kröhnke reaction of a thiophene-bearing pyridinium salt and diketopyrrolopyrroles were prepared through Suzuki coupling with this chiral moiety at one end only as well as at both ends. Absorption spectroscopy and electrochemistry confirmed the potential suitability of the compounds for photovoltaic devices. The nanostructures formed by the compounds have been probed with circular dichroism spectroscopy in solution and in films. It is shown that a chiral C2 symmetric molecule assembles in solution giving a strong circular dichroic signal while as a film this optical activity is nulled, whereas an asymmetric homologue is most optically active as a thin film. The X-ray crystal structure of the asymmetric compound shows a polar order of the molecules that might explain this observation. The lack of optical activity in solution is very likely a result of the high solubility of the compound. The results reaffirm the sensitivity of circular dichroism spectroscopy to inter-chromophore organisation, whereas absorption spectroscopy in the visible region reveals only slight changes to the bands. The differing order in the compounds also affects their performance in bulk heterojunction photovoltaic devices. Atomic force microscopy of the blended thin films with the fullerene derivative usually employed (PC61 BM) showed that smooth and well mixed films were achieved, with the conditions required during spin coating depending greatly on the derivative, because of their differing solubility. The apparently better performance of the symmetrical compound (although with very low efficiency) is probably a result of the alignment of the molecules inferred by the circular dichroism experiments, whereas the asymmetric compound presumably adopts a twisted supramolecular organisation.

Optical Activity Anisotropy of Benzil

Optical activity (OA) along the optic axis of crystalline benzil has been measured by many over the past 150 years. However, the OA anisotropy remains uncharacterized due to difficulties in sample preparation as well as competition with linear birefringence (LB). The challenges associated with measuring OA along low-symmetry directions in crystals have too often left scientists with only average values of nonresonant OA in solution, i.e., specific rotations, which continue to resist interpretation in terms of structure. Measuring OA anisotropy has been facilitated by recent advances in polarimetry and optical modeling and here we compare results from two distinct division-of-time polarimeters. The absolute structure of crystalline benzil was established for the first time. The optical rotation (OR) of (+)-crystalline benzil (space group P3121) perpendicular to the optic axis at the sodium D-line is −24.6 ± 1.1°/mm. A spectroscopic optical model in the transparent region of the crystal is provided. Electro...

Facile Synthesis and Enhanced Aggregation-Induced Circular Dichroism of Novel Chiral Polyamides.

A series of optically active polyamides, PAnLATs (n = 9–12), were prepared by polycondensation of l-tartrate-derived diacid with achiral aliphatic diamines in which the number of the methylene is from 9 to 12. The monomers could be obtained easily, and the synthesis of the polyamides is facile. The number-average molecular weight (M̅n) of PA9LAT, PA10LAT, PA11LAT, and PA12LAT is 35 500, 24 400, 35 800, and 20 900 g/mol, respectively, and the related polydispersity index is 3.4, 3.3, 3.4, and 3.0. These polyamides display intense optical activity in solution. Particularly, the stronger ellipticities and different circular dichroism (CD) images were presented when the polymers were evaluated in the solid state. The crystalline properties of the polymers were studied to illuminate the enhanced aggregation-induced CD. Moreover, PA9LAT and PA11LAT have similar crystal parameters, and PA10LAT is similar to PA12LAT, which revealed why the chirality of the polyamides displays an odd–even effect. On the other hand, the solubility of the polymers in organic solvents was studied, and thermogravimetric-differential thermal analyzer was utilized to characterize the thermal properties.

Accurate measurement of the optical activity of alanine crystals and the determination of their absolute chirality

Wavelength dependence measurements of the chiroptical properties in alanine crystals have so far been unsuccessful using conventional spectroscopic techniques. We describe our attempts to measure the wavelength dependence of the optical activity in L- and D-alanine crystals along each crystallographic axis, and to determine the absolute chirality of alanine crystals by correlating the absolute structure to the optical activity using an x-ray diffractometer and a generalized high accuracy universal polarimeter. We have succeeded in accurately measuring the optical rotatory dispersion in the <010> direction, which shows that the optical rotation of the d-alanine crystal is dextrorotatory and that of the l-alanine crystal is laevorotatory, thereby determining the absolute chirality. Furthermore, comparison with the optical activity in solution shows that the optical activity in alanine crystals is different not only in value, but also in the sign. These results have led us to conclude that the optical rotatory power in the crystalline state should not be simply the summation of molecular optical rotatory power values. We propose the necessity of a theory, which contains the contribution of molecular interactions within the crystal, in order to calculate the optical rotatory power of the crystalline state.

Optical activity of solutions and films of chitosan acetate

The optical activity of solutions, oligomers, and films of chitosan acetate in a molecular weight (М) range of 660—640 000 g mol–1 was studied by the optical rotation dispersion method. Monotonic dispersion curves lying in the range of negative values of specific optical rotation [α] are observed for all samples in a wavelength range of 300—700 nm. The absolute value of [α] decreases as the degree of polymerization decreases for solutions of chitosan acetate. Three regions are observed on the dependence [α] = f(logМ): constant [α] values for solutions of highmolecular-weight chitosan samples, their relatively weak change for low-molecular-weight samples, and a sharp change for oligomers. The transition to the low-molecular-weight analog of the monomeric chitosan unit, d-glucosamine, is characterized by the inversion of the rotation sign as high positive values of [α] are attained. The nontrivial for high-molecular-weight compounds dependence [α] = f(logМ) was established for chitosan acetate films: the module of [α] of the films increases with a decrease in the degree of polymerization. It is shown that the module of optical activity of the studied samples is determined by the mobility of structural units of oligomer, polymer, or film. It is found by X-ray diffractometry that differences in optical activity of the solutions and films are due to restriction of the mobility of macromolecular segments during the formation of high-molecular-weight chitosan films and incomplete relaxation processes of film systems to the state of energetically favorable conformations.

Temperature dependence of optical activity of sugar solutions

The temperature dependence of optical activity of solutions of metabolic sugars is investigated and the activation energy of the reaction of association into supramolecular structures is determined. On the basis of the values of the activation energy and with consideration of the effect of time and place and orientation of the instrument on the optical activity of sugars, a suggestion is expressed concerning their key role in the mechanism of adaptation of living systems to environmental changes.

Gradual change of one- and two-photon absorption properties in solution—Protonation of 4- N, N-dimethylamino-4′-aminoazobenzene

Linear (one-photon) and nonlinear (two-photon) spectral properties of 4- N, N-dimethylamino-4′-aminoazobenzene have been investigated under protonation in DMSO/water solvent mixtures. Observed spectral changes have been discussed in terms of ammonium–azonium tautomerism. The corresponding tautomeric constants have been determined for the first time for such kind of tautomeric equilibria. The process of protonation leads to changes of the two-photon absorption properties, suggesting that such process could be used as a tool for change of third-order nonlinear optical activity in solution.

Poly(ethylene terephthalate) copolyesters derived from (2S,3S)‐2,3‐dimethoxy‐1,4‐butanediol

AbstractNovel poly(ethylene terephthalate) (PET) copolyesters, abbreviated PEDMBT, containing optically active (2S,3S)‐2,3‐dimethoxy‐1,4‐butanediol (DMBD) as the second comonomer were investigated. Copolymers with ethylene glycol to DMBD ratios between 95/5 and 50/50 as well as the two parent homopolymers, PET and PDMBT, were prepared by a two‐step melt polycondensation. The resulting copolymers were found to approximately have the composition of the polymerization reaction feed and a random microstructure. Polymer intrinsic viscosities varied from 0.4 to 0.6 dL g−1 with weight‐average molecular weights ranging from 16,000 to 44,000. PEDMBT copolyesters were distinguished in being much more soluble than PET and showing an increasing affinity for water with the content in dimethoxy groups. According to the asymmetric constitution of DMBD, they displayed optical activity in solution. Both melting and glass‐transition temperatures of the copolyesters were observed to steadily decrease with the content in DMBD. PEDMBTs were found to be crystalline for contents in DMBD up to 30 mol %. Both powder and fiber X‐ray diffraction revealed that the same crystalline structure is shared by PET and the crystalline copolyesters. The homopolyester PDMBT resulted in becoming a polymer with a crystallinity comparable to PET but with a significantly different crystalline structure. © 2001 John Wiley &amp; Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3250–3262, 2001

Methacrylic polymers bearing side-chain permanent dipole azobenzene chromophores spaced from the main chain by chiral moieties: synthesis and characterization

Novel optically active polymethacrylates bearing in the side chain a chiral group of one prevailing absolute configuration linked to the trans-azoaromatic chromophore, have been synthesized by radical homopolymerization of the corresponding optically active monomers, prepared in turn from optically active precursors by synthetic methods avoiding any racemization of the chiral center. These polymeric derivatives, namely poly[( S)-3-methacryloyloxy-1-(4′-X-4-azobenzene) pyrrolidine], with X=CN, CHO, CHC(SO 2CH 3)(CN), CHC(CN) 2, are characterized by the presence of the 3-hydroxy-pyrrolidinyl ring as the chiral moiety and the azoaromatic donor-acceptor conjugated system as the moiety with permanent dipole moment. The polymers display optical activity in solution much higher than that afforded by the corresponding low molecular weight models, representative of the monomeric repeating unit, purposely synthesized. Owing to the substantially stereoirregular structure of the main chain, this suggests that the overall optical activity is mainly due to conformational dissymmetry of the macromolecules. Spectroscopic evidence suggests the presence in the above mentioned polymeric derivatives of dipole–dipole interactions between the side-chain azoaromatic chromophores, occurring as a consequence of their anchorage to the polymer backbone, which favors their aggregation.

Alternating Copoly(ester amide)s derived from Amino Alcohols andL-Tartaric and Succinic Acids

A set of copoly(ester amide)s (s-PEASTn) containing equal amounts of amide and ester groups has been synthesized from succinic and tartaric acids, and aliphatic linear amino alcohols with 2, 3 and 6 carbon atoms. In these mixed poly(ester amide)s the amino alcohol unit adopts a syndioregic orientation and the succinic and tartaric units alternate regularly along the polymer chain. These compounds were characterized by elemental analysis, SEC and FT-IR and 1 H/ 13 C NMR spectroscopy. s-PEASTn are stereoregular polymers which display optical activity in solution and are crystalline in the solid state with melting points between 90 and 160° C. They all are non-water soluble at room temperature but take up water in amounts up to 30% of their weights when exposed to a humid atmosphere.

Reporting and evaluating absolute-structure and absolute-configuration determinations

Detailed practical and numerical information is provided for undertaking and evaluating absolute-structure and absolute-configuration determinations. The interpretation of numerical values ofx, the Flack [Acta Cryst. (1983), A39, 876–881] parameter, and its standard uncertaintyuare explained in terms of the inversion-distinguishing power. Moreover, the conditions to obtain reliable values ofx(u) are detailed. Further explanatory material is provided on the use of right-handed axes, valid intensity data, the application to macromolecular structures, the dangers of polar-dispersion errors, Euclidean normalizers of space groups, the detection and reporting of molecular symmetry, enantiopurity and optical activity in solution. New CIF data names are introduced.

Poly(ester amide)s Derived from l-Tartaric Acid and Amino Alcohols. 1. Regic Polymers

Stereoregular poly(ester amide)s (PEATn) containing equal amounts of ester and amide groups have been obtained by polycondensation in solution from di-O-methyl-l-tartaric acid and aliphatic amino alcohols H2N(CH2)n-1CH2OH. Suitable synthetic routes were designed to prepare either syndioregic (s-PEATn, n = 5,6) or isoregic (i-PEATn, n = 5, 6, 11) polymers. All the newly synthesized poly(tartarester amide)s were characterized by elemental analysis, size exclusion chromatography, and IR and NMR spectroscopies. They all display optical activity in solution and, except for s-PEAT5, are highly crystalline with Tm's within the range 100−150 °C and Tg's oscillating between 10 and 30 °C. Thermal decomposition of PEATn onsets above 200 °C with Td values depending upon both the regicity of the chain and the length of the polymethylene segment. For comparative purposes, two poly(esteramide)s, one syndioregic (s-PEAS6) and another isoregic (i-PEAS6), were prepared from succinic acid and 6-amino-1-hexanol, and their properties were evaluated in parallel. Major differences between PEAS6's and PEAT6's concerned solubility and hygroscopicity whereas crystallinity and thermal transitions were found to be scarcely affected by the presence of the methoxy side groups.

Stereoregular polyamides derived from methylene‐L‐tartaric acid and aliphatic diamines

AbstractStereoregular polyamides containing two chiral backbone carbons in the repeating unit were prepared by polycondensation of bis(pentachlorophenyl) 2,3‐O‐methylene‐L‐tartrate with 1,9 and 1,12‐alcanediamines activated as N,N′‐bis(trimethylsilyl) derivatives. The polymers were characterized by elemental analysis, IR, and 1H‐NMR spectroscopy, and differential scanning calorimetry. Both viscosimetry and GPC were used to estimate the molecular weights which ranged between 6000 and 44000. These polytartaramides were readily soluble in chloroform, displayed moderate optical activity in solution, and formed highly crystalline films.

New Properties of Cyclodextrin Complexes in Solution and in Solid State

Abstract Chiroptical property studies of conformational enantiomers are difficult. However, in association with cyclodextrin, one chiral conformer is favoured by complexation and leads to optical activity in solution. Therefore crystallographic studies of the cyclodextrin complex can provide absolute configuration of the guest. Further results are discussed. We show that all β cyclodextrin clathrates with head to head dimers can be classified in three classes with the same dimer sheets. This fact may be usefull for future crystallographic studies A remarkable peculiarity may be pointed out: all these structures display a very similar partial water network.

Optically active polyamides, polymers from α‐, β‐, γ‐, and ϵ‐Methyl‐ϵ‐Caprolactam

AbstractOptically active polyamides were prepared from β‐, γ‐, and ϵ‐methyl‐ϵ‐caprolactam, and optically active α‐methyl‐ϵ‐caprolactam was racemized under polymerization conditions. The optically active polymers in solution and in the solid state were examined for the effect of an optically active carbon on the polymer structure and for changes occurring with the change in location of the optically active center on the polymer backbone. Significant differences were observed among these polyamides, in ease of formation, degree of crystallinity and melting points or softening ranges, infrared spectra, and optical activity in solution.

Closing remarks

In the 150 years of history of this subject, one can distinguish six peculiarly definitive periods or moments, spaced roughly 30 years apart. The first was the period around 1820, when Biot discovered optical activity, and Herschel noticed its relation to crystal morphology in quartz. The second was the period of 1850, when Pasteur, by his observations on the morphology of crystalline tartrates in relation to their optical activity in solution, narrowed the cause to one of chirality (handedness) in molecules. The third was the year 1874, when both van’t Hoff and Le Bel advanced the first specific model of molecular chirality, that of structures built from a tetrahedral carbon atom, the model that applies throughout organic chemistry, though other forms of chirality in octahedral inorganic complexes, and in helical secondary structures have since been recognized. Thus was the question of the physics of the light-matter interaction that leads in chiral matter to optical activity asked in the last century: it has been answered in this one. The fourth period was that around 1900, when Drude showed that the dispersion of rotation, like that of refraction, obeyed frequency terms, each with its own resonance denominator, leading in simplest principle to an infinity, and in practice, as we know, to an absorption band. So he forged the first link with spectroscopy, at a time when the electron itself was only in the process of being discovered, when there was no electronic theory of the atom, and no quantum theory, and when nobody had the least idea as to what an absorption band is in molecular terms.

Optical Properties of Molecular Aggregates. II. Classical Theory of the Refraction, Absorption, and Optical Activity of Solutions and Crystals

Classical theory is used to derive the optical parameters (refraction, extinction coefficient, reflectivity, rotation, and ellipticity) of a macroscopic sample containing aggregated monomer chemical units. The samples considered are dilute solutions of molecular aggregates or polymer molecules, and molecular crysstals. In this theory, each electronic transition of a monomer is represented by an electronic and a magnetic oscillator with complex polarizabilities which are evaluated empirically from the absorption and optical activity of unaggregated monomers. The complex refractive index for a refracted wave in the sample is obtained from Maxwell's equations and the electric and magnetic polarizations, taking into account Coulomb interactions between the oscillators. The optical parameters at any frequency are evaluated from the refractive indices for linearly or circularly polarized light. The results agree, in the same order of approximation, with those derived by several quantum theories. Sum rules are derived for the conservation of the monomer oscillator strengths and rotational strengths in the aggregates.

HYDROLYSIS AND POLYMERIZATION OF CYCLIC DIMETHYLETHYLENE OXALATES

Macrocrystalline cyclic oxalates (2,3-dimethyl-5-6-p-dioxanediones) were prepared by vacuum distillation of the polymeric mixture formed on heating ethyl oxalate, and 1, 2-dimethylethylene glycol with an ester-interchange catalyst. Monomeric dimethylethylene oxalate polymerizes less readily than propylene oxalate, and only at an elevated temperature in the presence of a catalyst. Dimethyl substitution therefore confers marked stability on the six-membered ring. Hydrolysis reaches 50% completion almost instantaneously, followed by a slow first order reaction resulting in complete cleavage of the monobasic acid in about one month at room temperature. Hydrolysis constants for the levo- and meso- oxalates are 0.069 and 0.076 days−1 respectively. The crystalline monomers do not possess sharp melting points and represent stereoisomeric mixtures which display optical activity in solution.