Chiroptical Changes Research Articles

Time-Resolved Circular Dichroism in Molecules: Experimental and Theoretical Advances.

Following changes in chirality can give access to relevant information on the function or reactivity of molecular systems. Time-resolved circular dichroism (TRCD) spectroscopy proves to be a valid tool to achieve this goal. Depending on the class of molecules, different temporal ranges, spanning from seconds to femtoseconds, need to be investigated to observe such chiroptical changes. Therefore, over the years, several approaches have been adopted to cover the timescale of interest, especially based on pump-probe schemes. Moreover, various theoretical approaches have been proposed to simulate and explain TRCD spectra, including linear and non-linear response methods as well as non-adiabatic molecular dynamics. In this review, an overview on both experimental and theoretical advances in the TRCD field is provided, together with selected applications. A discussion on future theoretical developments for TRCD is also given.

A pillar[5]arene-based planar chiral charge-transfer dye with enhanced circularly polarized luminescence and multiple responsive chiroptical changes.

The fabrication of circularly polarized luminescent (CPL) organic dyes based on macrocyclic architecture has become an importantly studied topic in recent years because it is of great importance to both chiral science and supramolecular chemistry, where pillar[n]arenes are emerging as a promising class of planar chiral macrocyclic hosts for CPL. We herein synthesized an unusual planar chiral charge-transfer dye (P5BB) by covalent coupling of triarylborane (Ar3B) as an electron acceptor to parent pillar[5]arene as an electron donor. The intramolecular charge transfer (ICT) nature of P5BB not only caused a thermally responsive emission but also boosted the luminescence dissymmetry factor (g lum). Interestingly, the specific binding of fluoride ions changed the photophysical properties of P5BB, including absorption, fluorescence, circular dichroism (CD), and CPL, which could be exploited as an optical probe for multi-channel detection of fluoride ions. Furthermore, the chiroptical changes were observed upon addition of 1,4-dibromobutane as an achiral guest.

Chiroptical Performances in Self-Assembled Hierarchical Nanosegregated Chiral Intermediate Phases Composed of Two Different Achiral Bent-Core Molecules

In this paper, chiral intermediate phases composed of two achiral molecules are fabricated by utilizing nanophase separation and molecular hierarchical self-organization. An achiral bent-core guest molecule, exhibiting a calamitic nematic and a dark conglomerate phase according to the temperature, is mixed with another achiral bent-core host molecule possessing a helical nanofilament to separate the phases between them. Two nanosegregated phases are identified, and considerable chiroptical changes, such as circular dichroism and circularly polarized luminescence, are detected at the transition temperatures between the different nanophase-separated states. The nanosegregated chiral phase-wherein the helical nanofilament and dark conglomerate phases are phase-separated-exhibits the highest chiroptical intensities. The luminescence dissymmetry factor, |glum|, in this phase is amplified by an order of magnitude compared with that of another nanosegregated phase, wherein the helical nanofilament and nematic phases are phase-separated.

Ground- and excited-state dynamic control of an anion receptor by hydrostatic pressure.

Stimulus-responsive supramolecular architectures have become an attractive alternative to conventional ones for many applications in sensing, drug-delivery and switchable memory systems. Herein, we used an anion receptor (H: host) as a hydrostatic-pressure-manipulatable fluorescence foldamer and halide anions as chiral (binaphthylammonium) and achiral (tetrabutylammonium) ion pairs (SS or RR·X and TBA·X; X = Cl, Br), and then investigated their (chir)optical properties and molecular recognition behavior under hydrostatic pressures. The conformational changes and optical properties of H in various organic solvents were revealed by UV/vis absorption and fluorescence spectra and fluorescence lifetimes upon hydrostatic pressurization. The anion-recognition abilities of H upon interactions with SS or RR·X and TBA·X at different pressure ranges were determined by hydrostatic-pressure spectroscopy to quantitatively afford the binding constant (Kanion) and apparent reaction volume changes . The results obtained indicate that hydrostatic pressure as well as solvation plays significant roles in the dynamic control of the present supramolecular system in the ground and excited states. This work will provide a new guideline for further developing hydrostatic-pressure-responsive foldamers and supramolecular materials.

Ligand exchange reactions on the chiral Au38 cluster: CD modulation caused by the modification of the ligand shell composition.

Ligand exchange reactions have become a highly versatile post-synthetic strategy to accurately engineer the ligand shell of atomically precise noble metal nanoclusters. Modifying the chemical structure of the exchanging ligand with chromophore substituents or adding chiral centers allow direct functionalization of the cluster with desired properties. As such, post-functionalized gold nanoclusters with unique physicochemical properties find applications in optoelectronics, catalysis and biomedicine. Herein, we successfully carried out ligand exchange reactions between the chiral Au38(2-PET)24 cluster (both racemic and enantiopure forms) and the helically chiral but configurationally labile 2-thio[4]helicene ligand (TH4). The reaction products with a composition of Au38(2-PET)24-x(TH4)x were analyzed using UV-vis spectroscopy and MALDI mass spectrometry. It was found that up to ten 2-PET ligands can be replaced with the helicene ligand on the cluster surface according to MALDI analysis. Consequently, the UV-vis and CD spectra of the cluster have been strongly affected by the ligand exchange reaction. The intensities of the CD signals of Au38(2-PET)24-x(TH4)x were drastically reduced and red shifted with respect to the reference Au38(2-PET)24 cluster. Moreover, the appearance of the other enantiomer in the HPLC chromatogram revealed the partial racemization of the cluster. DFT calculations were performed and they support the experimental observations and show that the observed chiroptical changes in UV-vis and CD spectra are exchange-site dependent. The calculations also demonstrate that charge transfer (CT) transitions occur between the Au38 cluster and the helicene ligand. Thus the ligand is directly involved in these transitions and contributes to the electronic states comprising those transitions.

Circular Dichroism Photoswitching with a Twist: Axially Chiral Hemiindigo.

Chiroptical properties play a crucial role not only for molecular structures and their functions but also for advanced applications such as molecular sensing, absolute asymmetric synthesis, or information processing and storage. Manipulating chiroptical characteristics in a predictable and reversible fashion by outside means is therefore a highly desirable option to enhance the functions and reporting abilities of a molecular system. Herein, we present axially chiral hemiindigo photoswitches showing unusual chiroptical changes upon visible-light irradiation. While absorption remains high throughout the spectrum, the corresponding ECD signals can be reversibly erased and reestablished in an ON/OFF manner upon photoswitching. Taken together with exceptionally high thermal bistabilities, leading to half-lives of the metastable states up to 3400 years at ambient temperature, and high photoswitching quantum yields these chiral hemiindigos offer unique possibilities for, e.g., smart molecule, photonic materials, or sensing applications.

Size-confined fixed-composition and composition-dependent engineered band gap alloying induces different internal structures in L-cysteine-capped alloyed quaternary CdZnTeS quantum dots.

The development of alloyed quantum dot (QD) nanocrystals with attractive optical properties for a wide array of chemical and biological applications is a growing research field. In this work, size-tunable engineered band gap composition-dependent alloying and fixed-composition alloying were employed to fabricate new L-cysteine-capped alloyed quaternary CdZnTeS QDs exhibiting different internal structures. Lattice parameters simulated based on powder X-ray diffraction (PXRD) revealed the internal structure of the composition-dependent alloyed CdxZnyTeS QDs to have a gradient nature, whereas the fixed-composition alloyed QDs exhibited a homogenous internal structure. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) analysis confirmed the size-confined nature and monodispersity of the alloyed nanocrystals. The zeta potential values were within the accepted range of colloidal stability. Circular dichroism (CD) analysis showed that the surface-capped L-cysteine ligand induced electronic and conformational chiroptical changes in the alloyed nanocrystals. The photoluminescence (PL) quantum yield (QY) values of the gradient alloyed QDs were 27–61%, whereas for the homogenous alloyed QDs, the PL QY values were spectacularly high (72–93%). Our work demonstrates that engineered fixed alloying produces homogenous QD nanocrystals with higher PL QY than composition-dependent alloying.

Chirality Transfer and Modulation in LB Films Derived From the Diacetylene/Melamine Hydrogen-Bonded Complex.

Introduction of hydrogen-bonding interaction into π-conjugated systems is a promising strategy, since the highly selective and directional hydrogen-bonding can increase the binding strength, provide enhanced stability to the assemblies, and position the π-conjugated molecules in a desired arrangement. The helical packing of the rigid melamine cores seems to play a dominating role in the subsequent formation of the peripheral helical PDA backbone. The polymerized Langmuir-Blodgett (LB) films exhibited reversible colorimetric and chiroptical changes during repeated heating-cooling cycles, which should be ascribed to the strong hydrogen-bonding interaction between the carboxylic acid and the melamine core. Further, the closely helical packing of the melamine cores could be destroyed upon exposure to HCl or NH(3) gas, whereas the peripheral helical polyaniline and polydiacetylene (PDA) backbone exhibited excellent stability. Although similar absorption changes could be observed for the films upon exposure to HCl or NH(3) gas, their distinct circular dichroism (CD) responses enabled us to distinguish the above two stimuli.

Thermochromism and supramolecular chirality of the coumarin-substituted polydiacetylene LB films

Herein, a novel single-chain coumarin-substituted diacetylene monomer (CODA) was synthesized. CODA LB films exhibited obvious chirality although CODA monomer was achiral. Upon UV irradiation, CODA LB films could be polymerized into blue form and exhibit reversible thermochromism and chiroptical properties. Strong intermolecular interaction among the coumarin chromophores was assumed to be responsible for the complete reversibility of absorption and chiroptical changes for PCODA films upon thermal stimulus. And the helical packing of the coumarin chromophores seemed to play a dominating role in the subsequent formation of the helical PDA backbone. Moreover, the single-chain CODA monomer could be further photo-dimerized, and a novel gemini-type diacetylene monomer (DCODA) could be prepared. The effects of the photo-dimerization reaction of the coumarin moieties on the thermochromism and chiroptical properties of the resulting PDCODA films had been evaluated. The dimerization of head groups would enhance the thermal stability of PDCODA films but inhibit the chirality formation for PDA backbone.

Achiral guest-induced chiroptical changes of a planar-chiral pillar[5]arene containing one π-conjugated unit

A planar-chiral pillar[5]arene derivative containing one π-conjugated unit was prepared. Chiroptical changes were observed upon addition of the achiral guest 1,4-dicyanobutane.

Conformational Effects Induced by Guest Encapsulation in an Enantiopure Water-Soluble Cryptophane

A new water-soluble cryptophane 1 derivative (penta-hydroxyl cryptophane-A) has been synthesized from cryptophanol-A and the chiroptical properties of its two enantiomers MM-1 and PP-1 have been studied by polarimetry, electronic circular dichroism (ECD), and vibrational circular dichroism (VCD). Cryptophane 1 shows specific circular dichroism responses upon complexation that depend on the size of the guest and on the nature of the counterion (Li(+), Na(+), K(+), Cs(+)) present in the solution. In LiOH and NaOH solutions, chiroptical changes induced by the encapsulation of guests and by the presence of cations in the vicinity of hosts can be interpreted from molecular dynamics (MD) and ab initio calculations by subtle conformational changes of the bridges. In KOH solution, the exchange dynamics is dependent on the size of the guest molecules, whereas in CsOH solution no encapsulation effect is observed whatever the size of the guest molecule. This last behavior comes from the fact that host 1 exhibits a very high affinity for cesium cations.

Induced Chiroptical Changes of a Water‐Soluble Cryptophane by Encapsulation of Guest Molecules and Counterion Effects

We report the synthesis of the water-soluble cryptophanol derivative 1 and the study of the chiroptical properties of its two enantiomers (>99 % ee) by polarimetry, electronic circular dichroism (ECD), and vibrational circular dichroism (VCD). We show that cryptophanol 1 exhibits unusual chiroptical properties in water under basic conditions (pH>12). For instance, the shapes of the ECD and VCD spectra of 1 in water were strongly dependent on the nature of the alkali metal ions (Li(+), Na(+), K(+), Cs(+)) surrounding the cryptophane and whether or not a guest molecule is present inside the cavity of the host. To the best of our knowledge, this is the first example in which the nature of these counterions governs the chiroptical properties of a host molecule. Moreover, specific ECD spectra were obtained depending on the size of the guest molecules. This makes 1 a good sensor for small neutral molecules in aqueous solvent. Finally, VCD experiments associated with DFT calculations show that the chiroptical changes can be directly correlated to the presence of charges close to the aromatic rings and with a conformational change of the alkyl chains upon encapsulation.

Temperature‐Induced Chiroptical Changes in a Helical Poly(phenylacetylene) Bearing N,N‐Diisopropylaminomethyl Groups with Chiral Acids in Water

A stereoregular poly(phenylacetylene) bearing an N,N-diisopropylaminomethyl group as the pendant (poly-1) changed its structure into the prevailing one-handed helical conformation upon complexation with optically active acids in water. The complexes exhibited induced circular dichroism (ICD) in the UV/Vis region of the polymer backbone. Poly-1 is highly sensitive to the chirality of chiral acids and can detect a small enantiomeric imbalance in these acids, in particular, phenyl lactic acid in water. For example, a 0.005% enantiomeric excess of phenyl lactic acid can be detected by CD spectroscopy. The observed ICD intensity and pattern of poly-1 were dependent on the temperature and concentration of poly-1, probably due to aggregations of the polymer at high temperature as revealed by dynamic light scattering and AFM. On the basis of the temperature-dependent ICD changes, the preferred chiral helical sense of poly-1 was found to be controlled by noncovalent bonding interactions by using structurally different enantiomeric acids.

Electrochiroptical systems based on biphenyl-2,2′-diyl-type dicationic dyes: strong chiroptical signals through the transmission of point chirality to axial chirality

Huge CD amplitude ( A = +538) was attained by diastereomeric preference (50% de) for the title dication with two bis[4-( R)- sec-butoxyphenyl]methylium units in benzene at 23 °C. Intramolecular π–π stacking is the origin for effective transmission of point chirality to axial chirality to attain the chiroptical enhancement of 400-times. Thanks to the strong CD signaling, chiroptical changes upon electrolysis could be readily detected, thus realizing a new class of electrochiroptical response systems. Chiroptical properties could also be modified by solvent polarity.

Solvent-Induced Chiroptical Changes in Supramolecular Assemblies of an Optically Active, Regioregular Polythiophene

The solvent-induced chiroptical properties of an optically active, regioregular (head-to-tail) polythiophene bearing chiral oxazoline residues, poly[3-{4-((R)-4-ethyl-2-oxazolin-2-yl)phenyl}thiophene] (poly-1a), were investigated in mixtures of a good solvent, chloroform, and a variety of poor solvents by means of UV−visible and CD spectroscopies. Poly-1a exhibited an induced circular dichroism (ICD) in chloroform upon the addition of poor solvents such as methanol and acetone with similar Cotton effect patterns, while some poor solvents such as acetonitrile and nitromethane dramatically changed the Cotton effect patterns of poly-1a. The changes in the conformation, morphology, and the size of the poly-1a aggregates induced by different solvents were also investigated with 1H NMR titrations, X-ray diffraction, filtration experiments using membrane filters, and atomic force microscopy (AFM) measurements. On the basis of these results, possible models for the chiral supramolecular aggregates are proposed.

A Magnesium-Selective Ionophore Containing Four Amide Carbonyl Ligands Derived from L-Tartaric Acid and Axial Furano Oxygen Binding Sites

Octahedrally converging hexadentate macrocyclic receptors 1 and 2, based on L-tartaric acid and furanmoieties, were synthesized andtheir complexation properties studied. In this paper,magnesium ion selectivitiesdetermined by ISE experiments, chiroptical changes observedby circular dichroism(CD) on complexation, and association constants measuredby NMR titration arediscussed.

Separation and characterization of the metal-thiolate-cluster domains of recombinant sea urchin metallothionein.

Partial metal depletion and 113Cd-NMR studies have suggested that the recombinant Cd-containing metallothionein of the sea urchin Strongylocentrotus purpuratus (Cd7-MTA) binds its metal ions in a four-metal (Cd4Cys11) and a three-metal (Cd3Cys9) cluster associated with the N-terminal and C-terminal halves of the protein, respectively [Wang, Y., Mackay, E.A., Zerbe, O., Hess, D., Hunziker, P.E., Vasak, M. & Kägi, J.H.R. (1995) Biochemistry 34,7460-7467]. This partitioning has now been confirmed by bisecting native Cd7-MTA with subtilisin into products bearing only a single metal-thiolate cluster. Their separation by reverse-phase HPLC and on-line electrospray mass spectrometry in combination with sequence analysis revealed selective cleavage of the protein into a set of N-terminal polypeptides containing 37-39 residues with four Cd ions and a set of C-terminal polypeptides containing 24 and 25 residues with three Cd ions. Thus, sea urchin MTA like its mammalian counterparts is made up of two separate cluster-harboring domains. The fragmentation pattern indicated that the sites of cleavage are located in the peptide loop interspaced between the first two metal-bound cysteine residues of the C-terminal domain. Accordingly, with cleavage, one of the putative nine thiolate ligands of the three-metal cluster was lost to the N-terminal fragment. The coordinational consequences of this repartition were reflected in massive chiroptical changes accompanying the cleavage process. While the liberated N-terminal domain retained the CD profile of the four-metal cluster in the parent protein and thereby indicated preservation of its structure, the CD features attributable to the intact three-metal cluster were largely lost on cleavage. The vanished features bear strong resemblance to the large biphasic ellipticity signal at 250 nm which dominates the CD spectrum of native Cd7-MTA, and allow us thus to attribute this signal to excitonic coupling interactions of Cd-thiolate chromophores in the three-metal cluster.

O.r.d.-c.d. and protonation of amide-containing polymers in strong acid media: 1. The case of poly(−)1,2-diaminopropane sebacamide, an optically active nylon-type polyamide

Optical rotatory dispersion (o.r.d.) and circular dichroism (c.d.) of poly(−)1,2-diaminopropane sebacamide I and of its model compound, N- N′-didecanoyl(−)1,2-diaminopropane II, are investigated in trifluoroethanol-formic acid, -trifluoroacetic, -methanesulphonic and -sulphuric acid mixtures, with emphasis on the two latter solvent systems. It is shown that the optical activity of both compounds depends on the nature and the concentration of the acid. The stronger the acid, the larger the chiroptical changes observed for a given value of the acid/amide molar ratio τ. The drastic changes caused by small amounts of methanesulphonic and sulphuric acids are assigned to protonation of amide chromophores solely.

A physico-chemical study of heparin: Evidence for a calcium-induced co-operative conformational transition

The conformations of heparin in aqueous solution in the presence of sodium, potassium, magnesium and calcium cations were studied using circular dichroism, optical rotation, nuclear magnetic resonance and equilibrium dialysis. Potassium and magnesium cations, when added to sodium heparinate solutions, cause small chiroptical changes. Binding of calcium ions gives rise to large changes in both optical rotation and circular dichroism. This is indicative of a major change in chain conformation, which is also manifest in 13C and 1H n.m.r. § § Abbreviations used: n.m.r., nuclear magnetic resonance; p.p.m., parts per million. Equilibrium dialysis suggests one mole of calcium bound per mole of tetrasaccharide, which n.m.r. indicates to be appropriately sulphated iduronateglucosamine-iduronate-glucosamine. The calcium is chelated by two iduronate carboxyl groups. Proton-proton coupling constants, determined by convolution difference spectroscopy and Carr-Purcell sequences, indicate that, over the temperature range 285 to 353 K, the iduronate ring is best described as 1C 4( l) and the glucosamine residue as 4C 1( d) for both sodium and calcium forms. The conformational change induced by calcium is ascribed to rotation around the glycosidic linkages. The binding process is co-operative and the binding constant of 10 3 to 10 4 m −1 is biologically significant. The findings are consistent with intramolecular binding. Hence, this study represents the first report of a polysaccharide undergoing a cation-induced intramolecular disorder-order process. The authors postulate that a function of the post-polymerization epimerization of d-glucuronate to l-iduronate is the attainment of the precise geometry required for co-operative calcium binding with consequent modulation of the flexibility of the tetrasaccharide units.