Chiroptical Behavior Research Articles

Dispersive Optical Activity of (R)-Methylene Norbornene: Intrinsic Response and Solvation Effects.

The dispersive optical activity of a homoconjugated bicyclic diene, (R)-methylene norbornene (R-MNB), was interrogated under complementary vapor-phase and solution-phase conditions to elucidate the structural/electronic provenance of its unusual chiroptical signatures and to explore the marked influence of environmental perturbations. The intrinsic (isolated-molecule) values of specific rotation measured at 355 and 633 nm (1623.5 ± 5.5 and 390.4 ± 3.7 deg dm-1 (g/mL)-1) were found to be factors of 3.9 and 2.1 smaller in magnitude than analogous quantities obtained for the kindred enone, (R)-norbornenone (R-NBO), reflecting, in part, the loss of prominent magnetic-dipole contributions from the C═O moiety and the exclusion of electron delocalization from the oxygen lone pairs. The wavelength-resolved rotatory powers of R-MNB were enhanced dramatically (by ∼40% on average) upon dissolution in any of the four common solvents targeted by the present study (acetonitrile, di-n-butylether, cyclohexane, and chloroform), yet displayed only a slight dependence on the exact nature of the surrounding liquid (±2.7% variation from the mean at 589.3 nm). Quantum-chemical calculations built upon the linear-response frameworks of density-functional theory and coupled-cluster theory were enlisted to interpret experimental results, with the substantial effects incurred by nonspecific solvation phenomena being explored through use of polarizable continuum models and bulk property-response relationships. Aside from enumerating the varied quality of agreement attained between computational predictions and polarimetric measurements, these efforts have found that refractive-index correlations, akin to those embodied in the venerable (albeit often discounted) Lorentz local-field correction, afford a viable means of linking the chiroptical behavior of R-MNB across phases.

Spectroscopic studies of chiral perovskite nanocrystals

As an emerging type of optically active material, semiconductor nanocrystals (NCs) stabilized by chiral molecules have attracted much attention. Owing to the wide range of potential applications of chiral perovskite NCs, the development of these materials is of great importance, but there has been a lack of relevant studies. Here, we describe an investigation of the properties of chiral perovskite NCs obtained using post-synthetic ligand exchange (achiral ligand/chiral ligand). These are found to exhibit mirror-image circular dichroism spectra. It is the chirality of the ligand (enantiomeric 1,2-diaminocyclohexane, DACH) that is most likely responsible for the induction of chiroptical activity in these NCs. Furthermore, their chiroptical properties and the corresponding mechanisms are found to depend strongly on the amount of capping ligand. When excess DACH is used to cap the surface of the NCs, their chiroptical properties are induced mainly by aggregation of DACH on the surface in a chiral pattern. In contrast, when small amounts of DACH are used for the capping, it is mainly surface distortion (or defects) and electronic interaction mechanisms that contribute to the chiroptical behavior of the NCs. In both cases, the anisotropy factors of the NCs are of the order of 10−3, which is comparable to or larger than the values reported for other chiral semiconductor and metal NCs. This work opens the door toward further understanding of chiroptical perovskite NCs and their potential applications.

Optically Active Ultrafine Au–Ag Alloy Nanoparticles Used for Colorimetric Chiral Recognition and Circular Dichroism Sensing of Enantiomers

Despite a significant surge in the number of investigations into chirality at the nanoscale, especially thiolated chiral molecules capping gold clusters, only limited knowledge is currently available to elaborate the alloying effect on chiroptical behavior of bimetallic nanoparticles (NPs). Also, few successful cases as to the efforts toward the development of chirality-dependent applications on the optically active nanomaterial have been made. Herein, as a positive test case for chiral alloy nanoparticle synthesis, the stable and large chiroptical ultrafine Au-Ag alloy NPs were prepared by reduction of different molar fractions of HAuCl4 and AgNO3 with NaBH4 in the presence of d/l-penicillamine (d/l-Pen). Compared with those of monometallic Au and Ag counterparts with comparable size, the Au-Ag alloy NPs (Ag mole fraction, 70%) obviously displayed the largest optical activities with the maximum g-factors of ∼1.6 × 10-3. Impressively, the Pen-mediated synthesis of chiral Au-Ag alloy NPs possesses a colorimetric self-recognition function and can be used as an incisive circular dichroism (CD) probe toward d- and l-Pen enantiomers. The plasmonic CD signal amplification (ΔICD) shows good linearity with the amount of Pen over the range of 5.0-80.0 μM with a detection limit (3σ) of 1.7 μM for l-Pen and 1.5 μM for d-Pen, respectively. In addition, the sensing system exhibits good selectivity toward d- and l-Pen in the presence of other enantiomers; therefore, it is highly expected that the approach described here would open new opportunities for design of more novel enantioselective analyses of important species related to biological processes.

Design of planar chiral metamaterials for near-infrared regime

Planar chiral metamaterials (PCMs) comprising double-layer dielectric–metal–dielectric resonant structures in the shape of a windmill are presented for near-infrared regime. The circular dichroism is retrieved from transmission spectra. Effects of used materials on circular dichroism characteristics of PCM arrays are investigated for the first time. The dependence of spectral characteristics on the geometrical parameters of the PCMs is analyzed by the finite-difference time-domain method. The observations indicated that the circular dichroism characteristics of the proposed PCM arrays are strongly dependent on the type of metal and dielectric materials. Due to the enhanced chiroptical near-field response and tunable spectral behavior, proposed PCM arrays may have potential for biosensing applications of chiral biomolecules.

Controlled formation of chiral networks and their reversible chiroptical switching behaviour by UV/microwave irradiation.

A 3D coordination polymer {[Cd(L(1))(L(2))]2}n (1) spontaneously resolves into enantiomorphic crystals of 1P and 1M without any chiral source. Bulk quantities of 1P and 1M can be predictably and reliably obtained by using chiral cyclohexane-1,2-diamine. 1 can be transformed into {[Cd(L(1))(L(3))0.5]2}n (2) by highly selective [2+2] photodimerization of L(2) ligands. The reverse process can be achieved by microwave irradiation. The cycloreversion/cycloaddition cycles result in chiroptical switching behavior.

Stimuli-responsive polymers. 11. Highly adaptive poly(urea-amide)s that display solvent, light and heat modulated chiroptical behavior

Commercially available 4,4′-methylene bis(phenyl isocyanate), when copolymerized with the atropisomeric monomer S-(-)-1,1′-binaphthyl-2,2′-diamine, provides convenient access to urea-linked polymers that exhibit exceptional levels of solvent-dependent chiroptical behavior. The use of different solvent media affords a high degree of control over both the sign and magnitude of the optical rotations that are ultimately displayed by these conformationally dynamic systems. The polarity of the solvent environment appears to play a role in shaping polymer responses. A number of related poly(urea-amide) constructs fitted with azobenzene backbone segments are also sensitive to the application of light and heat, ultimately providing for highly adaptive material systems with chiroptical outputs that can be dynamically modulated in real-time by a combination of different input signals.

Radical Polymerization of a Novel Methacrylamide Derivative

The radical polymerization of a novel methacrylamide derivative, N-[o-(4-ethyl-4, 5-dihydro-1, 3-oxazol-2-yl) phenyl] methacrylamide ((S)-EtOPMAM), was carried out to obtained optically active polymers. The polymer yield and the chiroptical behavior of the resultant polymers have been examined in detail by using IR and 1H NMR spectroscopies in comparison with our previous observation. The polymers showed relatively high molecular weights (Mn=8000-16000) and largest specific rotations ([α]25D =+120.6o). Particularly, the largest specific optical rotation of the polymer is almost the six times of the monomer.

Stimuli-responsive polymers. 10. Photo-regulation of optical rotations in azobenzene modified poly(ester-amide)s containing highly structured, atropisomeric backbone geometries

The chiral atropisomeric monomer R-(+)-2-amino-1,1'-binaphthalen-2-ol provides convenient access to a number of azobenzene modified poly(ester-amide)s having highly-ordered main-chain arrangements of their constituent monomer groups. When evaluated in dilute solution environments, each of the polymers that contain the azobenzene stimuliphore undergo photo-induced oscillations in optical rotatory power when stimulated by multiple UV light–visible light illumination cycles. Polymer behavior is strongly influenced by the particular sequence of monomer units positioned along the chiral ester-amide backbone. Periodic variants that are fashioned from R-binaphthylene, azobenzene and para-phenylene segments exhibit particularly interesting response profiles. In certain cases, their light-modulated chiroptical outputs alternate between two discrete states, fluctuating between positive and negative specific rotations, or reversibly interconverting between optically active and optically inactive endpoints. This chiroptical switching behavior is greatly influenced by the choice of solvent, providing an added means for tuning the binary outputs afforded by these structurally well-defined materials.

Radical Polymerizations of Optically Active Methacrylamides Having Oxazoline Moieties

A novel type of chiral N-substituted methacrylamide derivatives bearing an oxazoline pendant (OPMAM) had been synthesized. Radical polymerization were carried out to produced corresponding polymers with high specific rotation in moderate yields. The polymerization characteristics and the chiroptical behavior of the resultant polymers have been examined in detail by using HPLC and CD and UV-vis spectra. The polymers showed relatively high molecular weights(Mn=5000-15000). The abolutely value of the specific rotation of poly(OPMAM)s basically increased with increasing the molecular weights.

Polymerization and Characterization of N-[o-(4-phenyl-4,5-dihydro-1,3-oxazol-2-yl)phenyl] Methacrylamide

A novel type of chiral methacrylamide derivative, N-[o-(4-phenyl-4,5-dihydro-1,3-oxazol-2-yl) phenyl]methacrylamide ((R)-PhOPMAM), was synthesized and radically polymerized to the corresponding polymers in moderate yields. The polymerization characteristics and the chiroptical behavior of the resultant polymers have been examined in detail by using IR and 1H NMR spectroscopies in comparison with our previous observation. The polymers showed relatively high molecular weights(Mn=5000-10000) and large specific rotations([α] 25D = -326.0°～-357.4°).

Polymerization and Chiroptical Behavior of Polymethacrylamides Bearing a Bulky Oxazoline Pendant

The radical polymerization of optically active methacrylamide derivatives, N-[o-(4-phenyl-4,5-dihydro-1,3-oxazol-2-yl) phenymethacrylamide ((R)-PhOPMAM) and N-[o-(4-isopropyl-4,5-dihydro-1,3-oxazol-2-yl) phenymethacrylamide ((S)-PriOPMAM), were carried out under various conditions. The effect of the Lewis acid, that is, rare earth metal trifluoromethanesulfonates on the free-radical polymerization were examined under different solvents. The Lewis acids are effective in enhancing the isotacticity in the polymerization of (S)-PriOPMAM. The same phenomenon were not happened in the polymerization of (R)-PhOPMAM. The polymerization characteristics and the chiroptical behavior of the resultant polymers have been examined by using circular dichroism (CD), UVVis, and NMR spectroscopies.

Polymerization and Chiroptical Behavior of N-[o-(4-isopropyl-4,5-dihydro-1,3-oxazol-2-yl) phenyl] Methacrylamide

The radical polymerization of optically active methacrylamide derivative, N-[o-(4-isopropyl-4,5-dihydro -1,3-oxazol-2-yl)phenyl] methacrylamide ((S)-MeOPMAM), was carried out under various conditions. The effect of Lewis acid, that is, rare earth metal trifluoromethanesulfonates on the free-radical polymerization were examined under different solvents. The polymerization in the presence of rare earth metal trifluoromethanesulfonates produced isotactic-rich polymers compared with those obtained without the Lewis acid. The polymerization characteristics and the chiroptical behavior of the resultant polymers have been examined by using circular dichroism (CD), UV–Vis, and NMR spectroscopies.

DESIGN OF AN OPTICALLY ACTIVE POLYSTYRENE BEARING IMINE PENDANTS AND ITS ACID/BASE-TRIGGERED CHIROPTICAL SWITCH PROPERTY

The chiroptical properties of N-(2-hydroxy-1-naphthyl)methylidene-(S)-α-phenylethylamine(s-1) were studied as a model of chiral molecular switches.The Schiff base exhibited a pH-responsive chiroptical inversion behavior as a result of breaking and forming of the intramolecular hydrogen bonding and accompanying C—C single bond rotation far away from the chiral center.DFT computation results showed that both two preferred conformers of s-1 and its deprotonated species are stereochemically quasi-enantiotropic.On the basis of these findings we designed a chiral styrene-type monomer(VNP) containing imine linkage and synthesized corresponding polymers through RAFT technique.It was found that the chiroptical properties of PVNPs are highly sensitive to acid/base stimulus.The addition of acid produced an intensive Cotton effect at 382 nm for the THF solution of polymer,which is probably derived from the dissymmetric spatial arrangement of interacting chromophores in adjacent pendent groups due to the protonation of imine moiety.Furthermore,the CD signal can be totally reversed with addition of base.Chiroptical signals remained consistent after five acid-base reaction cycles,proving that a robust polymer-based chiroptical switch was established.This promising new type of chiroptical switches is relatively unique as it does not involve any covalent bond formation/breakage or conformational transition of polymer main chains and thus possess distinct advantages,such as fast switching rate,high reversibility and fatigue resistance as well as the nondestructive readout.Using such a reversible and reproducible chiral switching system,we designed the IHIBIT molecular logic gate that might have potential uses in information processing at molecular level.

On–Off Switching of the Novel Thermochromic Chiroptical Behavior of Pseudoisocyanine Driven by Association/Dissociation

Abstract The compound 1,1′-diethyl-2,2′-cyanine iodide forms a chiral fluorescent self-aggregate in aqueous solution at high salt concentration. This compound shows reversible quenching and emitting behaviors and exhibits an interesting on–off switching of thermochromic circularly polarized luminescence behavior as a result of the association–dissociation process.

Investigation of the chiroptical behavior of optically active polyaniline synthesized from naturally occurring amino acids

The chiroptical properties of optically active polyaniline PANI(+)-HCSA emeraldine salt products in dimethylformamide (DMF) depend strongly on stereochemistry and steric crowding due to the presence of different moieties on the amino acids. Amino acids including L-arginine, L-proline, L-leucine, L-isoleucine, L-phenylalanine and L-lysine hydrochloride were used as precursors in emeraldine base/DMF solutions prior to acid doping with (S)-(+)-10-camphorsulfonic acid. Characterization of these materials was carried out using CD, UV-Vis and FTIR spectroscopic techniques. Our results indicate that the optical activity can be maintained for longer times in the solid and solution phases owing to the interactions and stabilization of polyaniline by hydrogen bond formation between the amino-acid molecules and the amine (NH) and imine (−N=) sites on the emeraldine base form, while preserving the stereo-selectivity of the polymer chain. This paper addresses the factors responsible for influencing the chiroptical properties in helical conducting polyaniline and the chiral discrimination of different amino acids. The main factors affecting the chirality and stability of PANI(+)-HCSA are moiety present, concentration, stereochemistry, chain length of the amino acids, time period for the doping, hydrogen bonding and so on. The corresponding CD spectra exhibited no measurable optical activity and consistent with complete de-doping of ES to EB. Further, on doping the solution, we have got the precipitate, which showed a remarkable optical activity. The CD spectra shows retention of the optical activity in solid phase obtained after de-doping of the solution.

AB Block Copoly(3-alkylthiophenes): Synthesis and Chiroptical Behavior

In a first part of this article, the synthesis of AB type block copoly(3-alkylthiophene)s initiated by Ni(dppp)Cl2, the most commonly used initiator for these polymers, is investigated. For this study the respective 1H NMR resonances of all possible end-groups are identified. This result confirms the hypothesis that the Ni(dppp) species can walk back to the beginning of the polymer chain and that propagation can occur at both chain ends. The next part of the article studies the chiroptical behavior of AB-type block copoly(3-alkylthiophene)s with one chiral block and compares the results with those of the corresponding random copolymers. In order to obtain exclusively AB-type block copolymers, the polymers were prepared from a modified Ni initiator. They all have the same degree of polymerization but vary in the length of the respective blocks. The chiroptical behavior was studied by changing the ratio solvent/nonsolvent, meanwhile monitoring the UV–vis and circular dichroism (CD) spectra. Three series were investigated: one in which both blocks aggregate simultaneously, one in which the achiral block stacks before the chiral block, and one in which the chiral block stacks first followed by the achiral block. It was found that when the blocks stack independently, the (chiral/achiral) stacking of the latter is significantly influenced by the former. If both blocks of the polymer chains aggregate simultaneously, Cotton effects which are significantly larger than those of the chiral homopolymer are found.

Synthesis, Chiroptical Behavior, and Sensing of Carboxylic Acid Functionalized Poly(phenylene ethynylene-alt-bithiophene)s

Several poly(phenylene ethynylene-alt-bithiophene)s with (chiral) nonfunctionalized substituents were synthesized with a variable phenylene ethynylene (PE) spacer length (up to 4 repeating units). The chiroptical behavior was evaluated with UV−vis and circular dichroism (CD) spectroscopy, revealing a highly solvent-sensitive aggregate formation. Based on this high sensitivity, both chiral and achiral carboxylic acid functionalized analogues were prepared, of which the length of the spacer connecting the carboxylic acid to the polymer backbone was varied. A combination of UV−vis, CD, and emission spectroscopy showed a clear affinity of chiral amines toward the functionalized polymers both in solution and in film. However, a different supramolecular behavior of the polymers was observed depending on the length of the carboxylic acid functionalized side chain.

Correction: On the Chiroptical Behavior of Conjugated Multichromophoric Compounds of a New Pseudoaromatic Class: Bicolchicides and Biisocolchicides

Correction: On the Chiroptical Behavior of Conjugated Multichromophoric Compounds of a New Pseudoaromatic Class: Bicolchicides and Biisocolchicides

On the chiroptical behavior of conjugated multichromophoric compounds of a new pseudoaromatic class: bicolchicides and biisocolchicides.

BackgroundIt is well known that, stemming from the mutual interplay between chromophores, circular dichroism (CD) is a powerful technique to deal with structural problems for both the small organic molecule and the biopolymer. However, quantitative interpretations of the spectroscopic and structural terms that give rise to the exciton couplet are usually presented for ideal cases, or a few CD bands only are taken into account, overlooking the role of the solvent medium.Methodology/Principal FindingsCircular dichroism and UV absorption spectra were carried out for colchicide (3) and isocolchicide (6), as well as their coupling products, 10,10′-bicolchicide (2) and 9,9′-biisocolchicide (5), in both hydrogen bonding and non hydrogen bonding solvents, as well as MeCN/H2O mixtures. A dramatic control by the solvent emerged, as even tiny changes in the composition of solvent mixtures, at ca 1 water molar fraction, induced a dramatic modification of their CD bands. A mutarotation phenomenon - long known for isocolchicine (8) - was also observed for 5, and can be attributed to the interconversion between atropisomers (R a,7S),(R a,7′S)-5a and (R a,7S),(S a,7′S)-5b.Conclusions/SignificanceOur data show that with molecules built on two structurally identical moieties which embody both hydrophilic and hydrophobic groups, even tiny changes in the composition of solvent mixtures cause a dramatic modification of the CD bands. Their analysis arrives at a qualitative rationalization of the observed CD couplets from the coupling of high energy transitions, while attempts at a quantitative interpretation of these phenomena through time-dependent density functional theory allowed to reproduce satisfactorily the CD spectrum in the 300–450 nm region only. Failure with higher energies probably reflects currently inadequate specific theoretical treatments of the solvent medium.

Self-assembling films of chiral bipyridinium bisthiols

The identification of viable mechanisms to control the chiroptical properties of organic compounds with electrical stimulation can lead to the development of electroactive materials able to modulate the polarization of electromagnetic radiations under the influence of electrical inputs. Indeed, experimental protocols to regulate the circular birefringence and dichroism of chiral compounds are starting to be developed on the basis of redox processes. These studies, however, have focused their attention so far on the analysis of the chiroptical properties of metal complexes and organic molecules in solution. At this stage of their development, it is not entirely clear if and how these mechanisms can be extended to the realization of functional electrochiroptical materials. On the basis of these considerations, we designed two chiral and electroactive building blocks with anchoring groups able to adsorb on metallic electrodes. Specifically, these compounds have a 1,1′-bi-2-naphthol core with two 4,4′-bipyridinium appendages each terminated by a thiol group and differ in the nature of the spacer connecting the chiral core to the electroactive units. The 1,1′-bi-2-naphthol core imposes a chiroptical response on the 4,4′-bipyridinium appendages of one of the two compounds, but not on the other. The pair thiol groups ensure the adsorption one of the two compounds on gold and platinum electrodes in the form of electroactive multilayers and electrochromic monolayers respectively, while the other can only form monolayers on gold. Thus, the structure of the spacers separating the chiral element from the electroactive units dictates the adsorption and chiroptical behavior of these compounds.