Intense Cotton Effects Research Articles

Structure and Chiroptical Properties of Anthra[1,2-a]anthracene-1-yl Dimers as New Biaryls.

Dimers of anthra[1,2-a]anthracene-1-yl units and its mesityl derivative were synthesized by Ni(0)-mediated coupling of the corresponding chloro derivatives as new biaryls. The X-ray analysis and DFT calculations revealed that two polycyclic aromatic units with nonplanar deformations took a twisted conformation about the single bond as a chiral axis. Enantiomers of the nonsubstituted compound were resolved by chiral HPLC, and the enantiopure samples showed intense Cotton effects at 321 nm in the circular dichroism (CD) spectra and emission bands at 449 nm in the circularly polarized luminescence (CPL) spectra with dissymmetry factor of |glum| 3.6×10-3. The absolute stereochemistry of this biaryl was determined by the theoretical calculation of CD spectrum by the time-dependent DFT method. The barrier to enantiomerization was determined to be 108 kJ mol-1 at 298 K. The dynamic process proceeded via a stepwise mechanism involving the helical inversion of each aromatic unit and the rotation about the biaryl axis as analyzed by the DFT calculations.

Helicity control of a polyaromatic coordination capsule through stereoselective CH-π interactions.

Although square-planar ML4 units are essential building blocks for coordination cages and capsules, the non-covalent control of the chirality and helicity of the resultant nanostructures is quite difficult. Here we report the helicity control of an M2L4 polyaromatic capsule, formed from metal ions with square-planar coordination geometry and bent bispyridine ligands, through stereoselective CH-π interactions with monosaccharide derivatives. Thanks to host-guest CH-π multi-interactions, one molecule of various permethylated monosaccharides is quantitatively bound by the capsule in water (K a up to >108 M-1). In the polyaromatic cavity, among them, the selective binding of a β-glucose derivative (>80 : 20 ratio) is demonstrated from a mixture of the α/β-glucoses, through the equatorial-selective recognition of the anomeric (C1) group. A similar stereoselective binding is accomplished from an α/β-galactose mixture. Interestingly, single equatorial/axial configurations on the bound monosaccharides can regulate the helical conformation of the capsule in water, confirmed by CD, NMR, and theoretical analyses. An intense capsule-based Cotton effect is exclusively observed upon encapsulation of the permethylated α-glucose (>20-fold enhancement as compared to the β-glucose derivative), via the induction of a single-handed host helicity to a large extent. Inverse capsule helicity is induced by the binding of a β-galactose derivative under the same conditions.

Chiroptical Properties of Planar Benzobisthiazole-Bridged Squaraine Dimers.

Five chiral squaraine dimers were synthesized by fusing chiral indolenine semisquaraines with three different benzobisthiazole bridges. The thereby created squaraine dimers show a strong splitting of the lowest energy absorption bands caused by exciton coupling. The intensities of the two exciton transitions and the energetic splitting depend on the angle of the two squaraine moieties within the chromophore dimer. The electric circular dichroism spectra of the dimers show intense Cotton effects whose sign depends on the used squaraine chromophores. Sizable anisotropies gabs of up to 2.6 × 10-3 could be obtained. TD-DFT calculations were used to partition the rotational strength into the three Rosenfeld terms where the electric-magnetic coupling turned out to be the dominant contribution while the exciton chirality term is much smaller. This is because the chromophore dimers are essentially planar but the angle between the electric transition dipole moment of one squaraine and the magnetic transition dipole moment of the other squaraine strongly deviates from 90°, which makes the dot product between the two moment vectors and, thus, the rotational strength substantial.

Integration of Metal Catalysis and Organocatalysis in a Metal Nanocluster with Anchored Proline.

Chiral metal nanoclusters have recently been attracting great attention. It is challenging to realize asymmetric catalysis via atomically precise metal nanoclusters. Herein, we report the synthesis and total structure determination of chiral clusters [Au7Ag8(dppf)3(l-/d-proline)6](BF4)2 (l-/d-Au7Ag8). Superatomic clusters l-/d-Au7Ag8 display intense and mirror-image Cotton effects in their CD spectra. Density functional theory (DFT) calculations were carried out to understand the correlation between electronic structures and the optical activity of the enantiomeric pair. Surprisingly, the incorporation of proline in a metal nanocluster can significantly promote the catalytic efficiency in asymmetric Aldol reactions. The increase of catalytic activity of Au7Ag8 in comparison with organocatalysis by proline is attributed to the cooperative effect of the metal core and prolines, showing the advantages of the integration of metal catalysis and organocatalysis in a metal nanocluster.

Influence of chirality on the aggregation of a new A-π-D-π-A small molecule with a benzodithiophene core: Spectroscopic and morphological investigations

In this paper, the synthesis and characterization of a new alkoxy benzodithiophene-based small molecule bearing two stereocenters and two end-capping acetyl groups (BDTCOR) are reported. The molecule has been obtained in both enantiopure form (S, S) and stereoisomeric mixture. The spectroscopic properties of BDTCOR in solution were compared to computational data revealing that the molecule is a candidate for optoelectronic applications thanks to a broad UV–Vis absorption spectrum and suitable fluorescence properties. The molecule was deposited as a thin film by physical vapor deposition on SiO2 and PMMA substrates revealing a quite different behavior of the enantiopure form vs. the stereoisomeric mixture. In particular, significant differences were found in film morphology suggesting that, contrary to what observed for the stereoisomeric mixture, enantiopure BDTCOR tends to generate supramolecular well-ordered structures. Moreover, while electronic circular dichroism spectra of enantiopure BDTCOR casted films do not show significant signals, vapor-deposited films show intense Cotton effect, suggesting a synergic role of chirality and of the deposition technique on the molecular order.

Oxidation of sodium cholate catalyzed by Au NPs and chiral selective binding of R- and S-binaphthyl derivatives

Au NPs generated in situ by photochemical reduction method were used as catalysts for the oxidation of sodium cholate. The circular dichroism of the oxidation product of sodium cholate exhibits a positive Cotton effect at 335 nm and an intense negative Cotton effect at 281 nm. Electrospray ionization (ESI) mass spectra and nuclear magnetic resonance (NMR) spectra were performed to characterize the chemical composition of the oxidation product of sodium cholate, revealing that two of the hydroxyl groups were selectively oxidized to oxo groups and the oxidation product is 12-hydroxy-3,7-diketo-cholanic acid (3,7-DKCA). The chiral recognition abilities of the aggregates of 3,7-DKCA were investigated by using planar chiral molecules R- and S-binaphthyl compounds. Compared with sodium cholate, the aggregates of 3,7-DKCA displayed higher chiral selective host–guest binding to the model molecules and the chiral recognition mechanism was clarified.

Enhanced circularly polarized luminescence from silica-coated self-assemblies of chiral tetraphenylethene-based amphiphiles

A pair of tetraphenylethene-based pyridinium salts with valine-containing attachments, TL and TD, were designed and synthesized. They emit faintly and give weak circular dichroism (CD) signals in dilute solutions, but show intense fluorescence and Cotton effects in aggregated state, displaying aggregation-induced emission (AIE) and CD (AICD) features. Their assemblies exhibit circular polarization luminescence (CPL) properties. Furthermore, the assemblies can be utilized as templates to fabricate twisted hybrid silica nanofibers through a supramolecular templating approach. Compared with the supramolecular assemblies, the obtained hybrid silicas show enhanced CPL activities and fluorescence efficiency due to the confinement effect of the silica frameworks.

Specific chiroptical sensing of cysteine via ultrasound-assisted formation of disulfide bonds in aqueous solution

Cysteine (Cys) can serve as a biomarker to indicate diseases or disorders, and its chiral sensing has attracted increasing attention. Herein, we established an ultrasound-facilitated chiral sensing method for Cys using 4-chloro-7-nitro-1,2,3-benzoxadiazole (NBD-Cl) and electronic circular dichroism (ECD) spectroscopy. The formation of chiral disulfide bonds induced degenerate exciton coupling between two NBD chromophores, resulting in intense Cotton effects (CEs) of the sensing product. The anisotropy factor (g) was linearly correlated with the enantiomeric excess of Cys across the visible region (400–500 nm), and other natural amino acids or biothiols did not interfere with the detection. This ultrasound-promoted efficient and specific chiral sensing method of Cys has potential for application in the diagnosis of related diseases.

Dynamic Induction of Optical Activity in Triarylmethanols and Their Carbocations.

A series of artificial triarylmethanols has been synthesized and studied toward the possibility of exhibiting an induced optical activity. The observed chiroptical response of these compounds resulted from the chiral conformation of a triarylmethyl core. The chirality induction from a permanent chirality element to the liable triarylmethyl core proceeds as a cooperative and cascade process. The OH···O(R) and/or (H)O···HorthoC hydrogen bond formation along with the C–H···π interactions seem to be the most important factors that control efficiency of the chirality induction. The position of chiral and methoxy electron-donating groups within a trityl skeleton affects the amplitude of observed Cotton effects and stability of the trityl carbocations. In the neutral environment, the most intense Cotton effects are observed for ortho-substituted derivatives, which undergo a rapid decomposition associated with the complete decay of ECD signals upon acidification. From all of the in situ generated stable carbocations, only two exhibit intense Cotton effects in the low energy region at around 450 nm. The formation of carbocations is reversible; after alkalization, the ions return to the original neutral forms. Unlike most triarylmethyl derivatives known so far, in the crystal, the triarylmethanol, para-substituted with the chiral moiety, shows a propensity for a solid-state sorting phenomenon.

Circularly polarized luminescence from axially chiral binaphthalene-bridged BODIPY

Two axially chiral BODIPY-binaphthalene dyads, (R)-/(S)-BDP1-2, were synthesized by tethering BODIPY to the 3- or 3,3′-position of chiral binaphthalene. (R)-/(S)-BDP1 show a very weak Cotton effect and circularly polarized luminescence (CPL), demonstrating the inefficient perturbation effect of binaphthalene on BODIPY in both ground and excited state. In contrast, (R)-/(S)-BDP2 show an intense Cotton effect and CPL. According to the theoretical results, this difference should be originated from the energy transfer between the two BODIPY units in (R)-/(S)-BDP2.

Complementary double-stranded helical oligomers bearing achiral bifunctional groups that catalyze asymmetric aldol reaction.

Two novel chiral dimer and trimer strands composed of m-terphenyl groups linked through p-diethynylbenzene units with the chiral amidine group and achiral piperazine group introduced at the terminus or center of the strands, respectively, and its complementary achiral carboxylic acid dimer and trimer were synthesized. The complementary chiral/achiral strands form an excess-handed double-helical structure as supported by intense split-type Cotton effects in the absorption regions of the conjugated backbones biased by the chiral amidinium-carboxylate salt bridges. The double-helical trimer was found to catalyze the direct aldol reaction of cyclohexanone with 4-nitrobenzaldehyde and produce the products with a moderate enantioselectivity despite the fact that the catalytically active bifunctional piperazine/carboxylic acid pair introduced in the middle is achiral, indicating the key role of the one-handed double-helical framework for supramolecular bifunctional organocatalysis.

Synthesis and characterization of a polyisocyanide with thioether pendant caused an oxidation-triggered helix-to-helix transition

AbstractStimuli responsive helical polymers have attracted wide attention and many polymers responsive to the external stimuli have been synthesized. But, only a few examples have been reported regarding the redox-induced helicity inversion helical polymers. The polyisocyanide including thioether pendant (poly-MBTIP) was synthesized by polymerization of methyl (R)-3-(benzylthio)-2-isocyanopropanoate (MBTIP) monomer using NiCl2·6H2O as catalytic agent in dry DCM. The chiroptical and oxidation properties of the poly-MBTIP are investigated. The poly-MBTIPs exhibit intense specific optical rotations and Cotton effects compared to the monomer, strongly suggesting a helical conformation of the polymer backbone. Additionally, the thioether pendant of poly-MBTIP backbone was oxidized to sulfoxide group by H2O2. Interestingly, the specific optical rotations and Cotton effects of poly-MBTIP oxidized are reversed, probably suggesting a helical conformation inversion.

Concentration-Independent Stereodynamic g-Probe for Chiroptical Enantiomeric Excess Determination.

Enantiomeric excess (ee) determination is crucial in many aspects of science, from synthesis to materials. Within this subject, coupling molecular sensors with chiroptical techniques is a straightforward approach to the stereochemical analysis of chiral molecules, especially in terms of process immediacy and labor. Stereodynamic probes typically consist of racemic mixtures of rapidly interconverting enantiomeric conformers able to recognize a chiral analyte and greatly amplify its chiroptical readout. A great number of sensors have been developed, but their activity is generally restricted to one or a few classes of chemicals, and the analysis outcome relies on precise knowledge of the probe and analyte concentrations. This aspect in particular limits the potential practical applications. Here we report an oxo-vanadium(V) aminotriphenolate complex that was found to act as a concentration-independent stereodynamic sensor for a wide range of compounds. The bare complex is CD-silent, but coordination of an enantioenriched substrate immediately gives rise to intense Cotton effects in the visible region. Furthermore, a geometry change during the substrate-complex interaction leads to a marked optical response, as witnessed by a strong red-shift of the probe absorption bands, thus allowing the generation of dichroic signals in an "interference-free" area of the spectrum. This peculiarity allows for a linear correlation at high wavelengths between the ee of the analyte and anisotropy g-factor. This parameter derives from the differential circularly polarized light absorption of the sample but is independent of concentration. The newly developed sensor based on a simple coordination process has an unprecedented general character in terms of substrate scope and employment.

Synthesis of novel optically active poly(thiophenyleneethynylenephenylene)s. Effects of chirality competition and cooperation at the side chains on higher order structures

Novel poly(thiophenyleneethynylenephenylene)s having optically inactive/active amide groups were synthesized by the Sonoghashira-Hagihara coupling polymerization of 3-substituted 2,5-dibromothiophene 1 with 1-substituted 3,5-diethynylbenzenes 2N, 2R, 2S derived from glycine and d-/ʟ-alanines. Poly(1–2S) exhibited an intense Cotton effect based on negative exciton chirality at 405 and 366 nm. On the other hand, poly(1–2N) exhibited a Cotton effect similar to that of poly(1–2S) but weaker, and poly(1–2R) exhibited a weak Cotton effect based on positive exciton chirality. The chiral secondary structures of the polymers were dependent on chirality cooperation and chirality competition of the side chains. The trend of the circular dichroism (CD) signs and intensities of the polymers can be explained by the concepts of “chiral cooperation” and “chiral competition” between the optically active thiophene unit and optically inactive/active phenylene unit. These polymers formed aggregate structures with increasing MeOH content of CHCl3/MeOH mixed solvents, with accompanying decrease of the UV–vis absorptions and photoluminescence (PL) intensities.

Optically active tyrosine‐containing poly(N‐propargylamides): Synthesis, helical conformation, and infrared emissivity study

ABSTRACTNovel optically active and racemic poly(N‐propargylamides) bearing tyrosine pendants (LPT, DPT, and RPT) were synthesized and characterized. The polymers possessed moderate molecular weights and were thoroughly soluble in common organic solvents. The secondary structure of these three polymers in chloroform was studied and the results indicated that LPT and DPT could adopt helical structures with predominantly one‐handed screw sense according to their intense Cotton effect and large specific rotation values, while RPT adopted random coiled polymer chain. The amide groups could form amide‐amide hydrogen bonds to engender the twist of dihedral angle and the strong steric repulsion of phenol groups might have the opposite effect on the helix. The helical structure of the optically active polymers were conclusively constructed and stabilized by the optimization of intra‐ and intermolecular hydrogen bonding and steric repulsion of the side chains. Moreover, the infrared emissivity values of LPT and DPT at 30 °C were 0.682 and 0.671, which were much lower than RPT. LPT and DPT also showed superior heat resistance due to their unique helical conformation. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44824.

Reversible Cis-Cisoid to Cis-Transoid Helical Structure Transition in Poly(3,5-disubstituted phenylacetylene)s

A series of novel 3,5-disubstituted phenylacetylenes, rM-1, sM-1 to sM-5, bearing an achiral methoxycarbonyl pendant group and various chiral N-alkylamide pendant groups, were synthesized. They were converted to the corresponding polymers, rP-1, sP-1 to sP-5, with high cis-structure contents under the catalysis of [Rh(nbd)Cl]2, aiming to understand how the environmental variation and the structure of pendant group influence the chiroptical properties of polymers. sP-1/rP-1 were soluble in CHCl3 and THF at the molecular level and exhibited much larger optical rotations with opposite signs to those of sM-1/rM-1 and displayed the intense Cotton effects centered at 360 nm in the circular dichroism (CD) spectra, ascribed to the one-handed, contracted cis-cisoid helical polyene backbone. The reversible conformation transition between the contracted cis-cisoid helix and the frustrated, extended cis-transoid helix was achieved by alternately adding trifluoroacetic acid (TFA) and triethylamine into the hydrogen bo...

A switchable self-assembling and disassembling chiral system based on a porphyrin-substituted phenylalanine-phenylalanine motif.

Artificial light-harvesting systems have until now not been able to self-assemble into structures with a large photon capture cross-section that upon a stimulus reversibly can switch into an inactive state. Here we describe a simple and robust FLFL-dipeptide construct to which a meso-tetraphenylporphyrin has been appended and which self-assembles to fibrils, platelets or nanospheres depending on the solvent composition. The fibrils, functioning as quenched antennas, give intense excitonic couplets in the electronic circular dichroism spectra which are mirror imaged if the unnatural FDFD-analogue is used. By slightly increasing the solvent polarity, these light-harvesting fibres disassemble to spherical structures with silent electronic circular dichroism spectra but which fluoresce. Upon further dilution with the nonpolar solvent, the intense Cotton effects are recovered, thus proving a reversible switching. A single crystal X-ray structure shows a head-to-head arrangement of porphyrins that explains both their excitonic coupling and quenched fluorescence.

Photoinduced formation of an azobenzene-based CD-active supramolecular cyclic dimer.

A series of new photo-responsive amino acid-derived azobenzenedicarboxylic acid derivatives (S)-1 a-e were synthesized. Compound (S)-1 a in the trans form exhibited no circular dichroism (CD) signal in DMF under ambient conditions, whereas intense Cotton effects were observed upon UV irradiation, indicating the formation of a chiral supramolecular structure in the cis form. The CD signals disappeared when trifluoroacetic acid (TFA) was added to the solution. The ester counterpart [(S)-1 a'] showed no CD signal. Hydrogen bonding between the carboxy groups seemed necessary for constructing the supramolecular structure. The kinetic studies of cis to trans isomerization of (S)-1 a demonstrated that the formation of a chiral supramolecule enhances the stability of the cis-azobenzene structure. The ESI mass spectrum of stilbenedicarboxylic acid (S)-4, an analogue of (S)-1 b, confirmed the formation of a dimer. A theoretical CD study revealed that (S)-1 a in the cis form should be present as a cyclic chiral dimer.

Synthesis, helical conformation, and infrared emissivity property study of optically active substituted polyacetylenes derived from serine

ABSTRACTSerine‐based monosubstituted acetylene monomers were synthesized and polymerized with rhodium zwitterion catalyst in THF to afford optically active polyacetylene derivatives (LPA and DPA) and corresponding racemic polyacetylenes (RPA) with moderate molecular weights in good yields. All of the substituted polyacetylenes (SPA) were characterized by FT‐IR, NMR, GPC, UV‐Vis spectroscopy, circular dichroism (CD) spectroscopy, and TGA. LPA and DPA were soluble in common organic solvents and possessed single‐handed helical conformation according to their intense Cotton effect and large specific rotations, while RPA presented random coiled polymer chain. The characterization results showed that helical structure of these SPAs was stabilized by intra‐ and intermolecular hydrogen bonding between the substituents which played a significant role in creating and maintaining the helix. Further, the infrared emissivity properties of them at wavelength of 8 to 14 μm were investigated at room temperature. Consequently, the LPA and DPA exhibited lower infrared emissivity values than RPA, which came down to 0.632 and 0.616. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41210.

Optically active amino acid-based polyacetylenes: Effect of tunable helical conformation on infrared emissivity property

Chiral amino acid-based monosubstituted acetylene monomers [N-tert-butoxy-carbonyl-l-phenylalanine-N′-propargylamide (LP), and N-tert-butoxycarbonyl-l-serine-N′-propargylamide (LS)] were (co)polymerized with rhodium zwitterion catalyst in THF to afford helical polyacetylenes (PAs) with moderate molecular weights (4400–14,800) in good yields. The optically active PA copolymers were soluble in common organic solvents and proven to adopt predominately single-handed helical conformations according to their intense Cotton effect and large specific rotations. Various contents of amino acid units in side chains facilitated controllable helical secondary structure while the helix was primarily stabilized by hydrogen bonding and steric repulsion between the substituents. Intramolecular hydrogen bonds constructed between hydroxyl and amide groups, in particular, played a significant role in adjusting the helicity and orderliness of the helix. The infrared emissivity values of the PAs at wavelength of 8–14μm were measured and the correlation between helical conformations and their effect on infrared emission were systematically investigated. The results showed that more well-ordered and compact screw-sense could enhance the performance of organic polymers in lowering their infrared emissivity. Among all the as-prepared PAs, poly(LP50-co-LS50) exhibited the lowest infrared emissivity value (ε25=0.632) and possessed excellent resistance against heat.