Magnitude Of Circular Dichroism Research Articles

The large-range strength-switchable circular dichroism in a mechanically reconfigurable chiral metasurface

The strength-switchable circular dichroism (CD) is crucial in the fields of biological detection, analytical chemistry, etc. In this paper, we achieved large-range strength-switchable CD by mechanically stretching polydimethylsiloxane (PDMS) substrate. With normal incidence, the traditional C4 rotation symmetric 卍-shaped chiral array could exhibit obvious optical activity but near-zero CD. However, we found that the giant CD could be gained by breaking the period symmetry in x- and y-directions in the 卍-shaped array. By mechanically stretching PDMS substrate in x-direction, the magnitude of CD is dynamically switched from 0.99 to 0 at 1314.5 nm. Essentially, the giant CD originates from the circular polarization selective excitations of magnetic dipole and electric quadrupole resonances, which is demonstrated by quantitative multipole decomposition and electric and magnetic field analysis. The giant and large-range strength-switchable CD has great potentials in the next generation optical devices.

Design of chiral plasmonic metamaterials based on interpretable deep learning

Chiral plasmonic metamaterials can amplify chiral signals, resulting in circular dichroism (CD) responses that are several orders of magnitude far beyond those of nature. However, the design process of chiral plasmonic metamaterials based on conventional methods is time-consuming. In recent years, the combination of deep learning (DL) and nanophotonics have accelerated the design of nanophotonic devices. Here, we construct the fully connected neural network model for the forward prediction and inverse design of chiral plasmonic metamaterials structures and introduce the permutation importance approach to optimize the model and increase its interpretability. Our experimental results show that using the peak magnitude of CD and the corresponding wavelength instead of the entire spectrum as the output in the forward prediction improves the accuracy of the peak magnitude of CD prediction, avoids the introduction of auxiliary networks, and simplifies the network structure; the permutation importance analysis shows that the gold length of the resonator is the most critical structural parameter affecting the CD response. In the inverse design, the permutation importance method helps us to make feature selections for the input of the network. By reducing 251 inputs (the whole CD spectrum) to 4 inputs (the peak magnitude of CD and the corresponding wavelength), the network can still maintain a good prediction performance and decrease the training time of the network. Our proposed method can be extended not only to other DL models to study the CD response of chiral metamaterials but also to other areas where DL is combined with metamaterials to accelerate the system optimization and design process of nanophotonic devices.

Giant and reversible circular dichroism based on phase change materials for near-field image display

Switchable circular dichroism (CD) is in high demand in wireless communication, real-time imaging, etc. At present, the active switching range of CD magnitudes in a nanostructure is relatively narrow. Herein, the giant and reversible CD with large-range modulation is realized by controlling the phase transition of GST (Ge2Sb2Te5) in a double Z-shape array. The demonstrated CD undergoes the large range from −0.85 to 0.85, which is a major advance. The giant CD arises from the polarization-dependent excitations of electric and magnetic dipole resonances. Utilizing the giant and reversible CD, the application for dynamic near-field image display is demonstrated. The reconfiguration capability of our metasurface for large-range switchable CD will promote the prosperity of active chiro-optical devices.

Enhanced circular dichroism in Ge2Sb2Te5-loaded metasurface.

Circular dichroism (CD) is originally obtained from three-dimensional spiral structures by simultaneously exciting electric and magnetic resonances. To simplify construction, multilayer stacked asymmetric structures and the symmetric structures relying on oblique incidence are proposed for enhancing CD. Herein, we achieved the enhancement of dual-waveband CD by adding a Ge2Sb2Te5 (GST) layer on the top of a Z-shape gold array in a normally incident system. Benefited from the polarization selective excitations of electric and magnetic dipole resonances, the CD in a simple planar structure is immensely enhanced from near zero to 0.73 at 1.58 µm. Furthermore, the CD strengths is dynamically tuned by controlling the phase of GST. With the GST phase transition from amorphous (a-GST) to crystalline state (c-GST), CD magnitudes are switched by about 0.73 and 0.27 at dual wavebands respectively. The enhancement of CD by adding a layer on a simple planar array offers a new method for designing planar metasurfaces with strong chirality.

Chiral Perovskite Nanoplatelets Exhibiting Circularly Polarized Luminescence through Ligand Optimization

AbstractChiral halide perovskite nanocrystals have many applications in next‐generation optoelectronic devices due to their interaction with polarized light. Through careful selection of chiral organic surface ligands, control over the circular dichroism (CD) and circularly polarized luminescence (CPL) of these materials can be achieved. However, while recent developments of CD‐active perovskites have seen significant advances, effective CPL remains a challenge. Here, colloidal perovskite nanoplatelets are synthesized exhibiting room temperature CPL with dissymmetry factors up to glum = 4.3 × 10−3 and gabs = 8.4 × 10−3. Methylammonium lead bromide nanoplatelets are synthesized with a mixture of chiral dimethylbenzylammonium ligands and achiral octylammonium ligands, the precise ratio of which is shown to be critical to achieving high g‐factors. The competitive binding of these surface ligands is investigated using 1H NMR, and an equilibrium model is used to demonstrate the ligand affinity. The magnitude of CPL and CD is quantitatively shown to exhibit a linear correlation, such that glum = 0.40 × gabs. Last, by screening several amines with close structures, it is shown that subtle differences in ligand structure have significant impact on the resulting nanoplatelets CD signal. These findings provide new insights for the effective design of perovskites exhibiting CPL and facilitate the development of high‐performance devices based on circularly polarized luminescence.

Design and simulation of a GST-based metasurface with strong and switchable circular dichroism.

Circular dichroism (CD) is required in the applications of biological detection, analytical chemistry, etc. Here, we numerically demonstrated large-range switchable CD by controlling the phase change of Ge2Sb2Te5 (GST) in a zigzag array. At the amorphous state of GST (a-GST), the strong and dual-waveband CD effects are realized via the selective excitations of electric, magnetic, and toroidal resonances. With the transition from a-GST to crystalline state GST, CD strengths are tailored dynamically in large ranges. In detail, the CD magnitudes change by about 0.93 and the modulation depths exceed 94% at dual wavebands. The strong CD effects and large-range switch capability in the GST-based metasurfaces will boost the development of active chiroptical devices.

High-efficiency and tunable circular dichroism in chiral graphene metasurface

Circular dichroism (CD) response is extremely important for dynamic polarization control, chiral molecular sensing and imaging, etc. Here, we numerically demonstrated high-efficiency and tunable CD using a symmetry broken graphene-dielectric-metal composite microstructure. By introducing slot patterns in graphene ribbons, the metasurface exhibits giant polarization-selective absorption for circularly polarized (CP) wave excitations. The maximum CD reaches 0.87 at 2.78 THz, which originates from the localized surface plasmon resonances in patterned graphene. Besides, the operating frequency and magnitude of CD are dynamically manipulated by gating graphene’s Fermi energies. The proposed chiral graphene metasurface with high-efficiency and tunable capability paves a way to the design of active CD metasurfaces.

Circular dichroism induced by tunable symmetry breaking in vertical Q-shaped nanostructure

Circular dichroism (CD) is widely used in bio-sensing, pharmaceuticals, and molecular chemistry. Symmetry breaking is an important method of achieving CD signals. However, the relationship between the magnitude of CD and symmetry breaking remains unclear. In this study, we introduce nanorods into symmetric nanostructures comprising vertical Q-shaped nanostructure arrays (VQNAs) to prove the relativity of the symmetry breaking. The numerical simulations results show that the CD spectra of VQNAs present four modes in the visible spectrum. The coupling between nanorods differs under left- and right-handed circular polarized illuminations, leading to the production of the CD. The CD magnitude is not monotonic when the parameters of VQNAs change, and it is the largest for the structure at an appropriate scale. Besides, VQNAs have stable optical properties. Results can help to understand more deeply the CD mechanism produced by symmetry breaking, and design chiral plasmonic nanostructures.

Co-occurrence of circular dichroism and asymmetric transmission in twist nanoslit-nanorod Arrays.

Circular dichroism (CD) and asymmetric transmission (AT) are important in the field of negative refractive index media and perfect polarization converters. A large difference between T++ and T-- in the transmission matrix T leads to a large CD effect, whereas a large difference between T-+ and T+- leads to a large AT effect. To achieve large CD and AT effects simultaneously, we theoretically analyzed the transmission matrix T and proposed the chiral plasmonic nanostructure of twist nanoslit-nanorod arrays (TNNAs) in this study. Results calculated by the finite element method show that, at around resonant wavelengths, the spectra of T++ and T-- correspondingly present peaks and valleys leading to a large CD effect. Meanwhile one of the spectra for T-+ and T+- presents valleys and another presents peaks leading to a large AT effect. More importantly, the magnitude of CD is equivalent to that of AT. In addition, the CD and AT effects strongly depend on the geometric parameters of TNNAs. Overall, these results are useful for designing chiral plasmonic nanostructures with large CD and AT effects.

Chiroptical study on benzil crystal using G-HAUP

There has been a great interest in understanding chiroptical properties such as optical rotatory power (ORP) and circular dichroism (CD) in anisotropic materials because these properties provide valuable information on molecular/crystal structure. In particular, generation of chiroptical properties in crystals composed of achiral compounds is considered as one of the most intriguing topics in the field of crystal optics. Benzil (C6H5COCOC6H5), which consists of two phenyl rings and two carbonyl groups, is achiral in solution, but exhibits chirality in the crystalline state. In the crystalline state, the molecules are arranged in a helical form and are put along three-fold left-handed screw axis or right-handed one. The benzil crystal belongs to trigonal crystal system with non-centrosymmetric space group P3121 or P3221. Hence, the crystal symmetry allows the benzil crystal to exhibit ORP and CD and indeed it is often used as a chiral reference crystal in visible/ultraviolet region (S. Chandrasekhar, 1954; N. K. Chaudhuri & M. A. El-Sayed, 1967) as well as α-quartz and nickel sulphate hexahydrate crystals. Meanwhile, ORP and CD reflect spatial dispersion of a second-rank permittivity tensor that depends on not only frequency but also wave vector of an electromagnetic wave. Furthermore, the magnitude of ORP and CD usually vary with the direction of an incident light. It is therefore much important in chiral science to discuss the relationship between ORP and CD along the principal axes and the molecular arrangements along them, respectively. In the case of benzil crystal, it is expected to show different ORP and CD along the a and c axes because of its trigonal crystal system, D3. In this study, we attempt to measure the ORP and CD of benzil crystal in the direction of the c and a axes with the Generalized-High Accuracy Universal Polarimeter (G-HAUP)(M. Tanaka et al., 2012).

Optical Activity of Achiral Ligand SCH3 Adsorbed on Achiral Ag55 Clusters: Relationship between Adsorption Site and Circular Dichroism

The electronic circular dichroism (CD) spectra of a methyl-thiol adsorbed at different sites on an icosahedral silver nanoparticle is studied by using time-perturbed density functional theory. Despite that separately molecule and nanoparticle are achiral and consequently optically inactive, the Ag(55)-SCH(3) compound emerges with a new symmetry, which may be chiral or not depending on the adsorption site and orientation of the molecule. It is found that chirality is favored when the thiol is adsorbed between two atoms of different coordination number. Chiral compounds have characteristic CD spectra in the UV-visible region, where Ag(55) shows optical absorption but SCH(3) does not; revealing that highly degenerated molecular-like electronic states of Ag(55) are modified by the presence of the molecule inducing optical activity. It is concluded that CD line-shapes and magnitude strongly depend on the site where the adsorption takes place, while its intensity is modulated by the molecule orientation.

A photo-degradable helix: Synthesis, structure, and photolysis of optically active poly[2,7-bis(4-t -butylphenyl)-9-methylfluoren-9-yl acrylate

2,7-Bis(4-t-butylphenyl)-9-methylfluoren-9-yl acrylate (BBPMFA) was synthesized and polymerized using α,α′-azobisisobutyronitrile or n-Bu3B-air as a radical initiator and using the complex of 9-fluorenyllithium with (S)-(+)-1-(2-pyrrolidinylmethyl)pyrrolidine as an optically active anionic initiator. Although the radical polymerization led to rather low-molecular-weight products at low yields, the anionic polymerization afforded polymers with higher molecular weights in higher yields. The poly(BBPMFA) obtained by the anionic polymerization was slightly rich in isotacticity (meso diad 57%) and showed an intense circular dichroism (CD) spectrum and large dextrorotation. The intensity of the CD spectrum and magnitude of optical activity increased with an increase in Mn, suggesting that the polymer possesses a preferred-handed helical conformation. The CD spectrum disappeared within 1 s on irradiation to the polymer in a CHCl3 solution using a 500-W Hg-Xe lamp. This was ascribed to fast photolysis of the ester linkage leading to a loss of helical conformation of the entire chain. Photolysis products of poly(BBPMFA) were poly(acrylic acid) and 2,7-bis(4-t-butylphenyl)-9-methylenefluorene (2,7-bis(4-t-butylphenyl)dibenzofulvene). The photolysis reaction seemed to proceed through the “unzipping” mechanism. The rate constant of photolysis of poly(BBPMFA) under irradiation at monochromated 325 nm was around 0.01 s−1 independent of molecular weight. Photolysis at 325 nm was approximately 2400 times faster than that for chemical ester solvolysis under a neutral condition in the dark. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Ab Initio Modeling of the Electronic Circular Dichroism Induced in Porphyrin Chromophores

The optical activity in porphyrins can easily be induced by a chiral environment, but it is difficult to determine the underlying mechanisms purely on an experimental basis. Therefore, in this study, magnitudes of the perturbational, dipolar, and direct covalent contributions to the electronic circular dichroism (CD) are evaluated with the aid of quantum chemical computations. Electronic properties of model porphyrin chromophores are analyzed. Time-dependent density functional theory (TD DFT), particularly with the hybrid B3LYP functional, appeared suitable for estimation of the electronic excitation energies and spectral intensities. The transition dipole coupling (TDC) between chirally stacked porphyrins was determined as the most important mechanism contributing to their optical activity. This is in agreement with previous experimental observations, where chiral matrices often induce the stacking and large CD signals. About a 10 times smaller signal could be achieved by a chiral orientation of the phenyl or similar residues covalently attached to the porphyrin core. Also, this prediction is in agreement with known experiments. Perturbation models realized by a chirally arranged porphyrin and a point charge, or by a porphyrin and the methane molecule, provided the smallest CD signals. The electrically neutral methane induced similar CD magnitudes as those of the charge, but spectral shapes were different. For a complex of porphyrin and the alanine cation, a significant influence of the solvent on the resultant CD spectral shape was observed, while for the charge and methane perturbations, a negligible solvent effect was found. Detailed dependence of the induced optical activity on variations of geometrical parameters is discussed. The simulations of the induced porphyrin activity can thus bring important information about the structure and intermolecular interactions in chiral complexes.

CRP-DNA Complexes: Inducing the A-likeForm in the Binding Sites with an Extended Central Spacer

The consensus DNA sequence for binding of the Escherichia colicyclic AMP receptor protein (CRP) has two symmetrically related inverted recognition elements TGTGA:TCACA, separated by a variable spacer, normally 6 bp long. We have shown that the CRP-cAMP complex, when bound to synthetic binding sites with an extended 8 bp spacer segment, induces an increase in the DNA circular dichroism (CD). The CD change at λ>275 nm agrees with the shift of approximately one helical turn of DNA into A-like form. The B-conformation is preserved for CRP binding sites similar to that in the lacand uxaCApromoters with 6 bp spacers. Another effect accompanying DNA binding is a dramatic increase of the negative CD magnitude in the spectral region of the ligand cAMP, at λ<272 nm. This effect is observed when CRP binds to specific sites with 6 or 8 bp spacers as well as to non-specific DNA. We reason that the A-like form arises by compressing and unwinding the DNA in CRP-DNA complexes having 8 bp central spacers. This serves to maintain a fixed length and twisting angle and is controlled by the protein's relatively rigid frame. This model is consistent with the observation that some binding sites with 6 bp spacers may also show the CD increase inherent to the sites with the extended 8 bp spacers. These 6 bp spacers are characterized by an increased twisting angle that requires their unwinding to bind to CRP. We propose that a mutual adaptation between CRP and binding sites by local untwisting and a B→ A-like transition in the DNA is of general importance and may occur in other protein-DNA complexes, such as the complex of RNA polymerase with promoter DNA.

Spectroscopic studies on complex formation and non-covalent interactions in ternary palladium(II) systems involving spermidine and 2,3-diaminoproprionate or 2,4-diaminobutyrate

NMR and electronic absorption spectral studies have been carried out on complex formation and non-covalent interactions in ternary palladium(II) systems involving spermidine (Spd) and 2,3-diaminopropionate (Dpa) or 2,4-diaminobutyrate (Dba). 13C NMR spectra indicated that Pd(Spd) and Pd(Spd) 2 with the N3 and N4 coordinations and Pd(AA) 2 with the N4 coordination were predominant species at neutral pH in the binary Pd(II)-Spd and Pd(II)-AA systems (AA = Dpa or Dba), respectively. Pd(Spd) 2 and Pd(AA) 2 were also found to be predominant in the 1:1:1 Pd(II)-AA-Spd systems, and in the l:2: x Pd(II)-AA-Spd systems ( x = 1–10), Pd(AA) 2 was the only complex species present with entire Spd uncomplexed. Circular dichroism (CD) spectral magnitudes as well as the NMR spectral shifts observed for the ternary systems and the CD magnitude dependence on solvent polarity indicated that the side chain carboxylate group of AA is involved in hydrogen bonds or electrostatic interactions with the protonated amino groups of Spd. The apparent stability constants, log K, for the 1:1 adduct Pd(AA) 2· Spd at pH 7.5 (25 °C and I = variable) have been calculated from the CD magnitudes to be 2.4 and 2.6 for AA = Dpa and Dba, respectively.

Structural effects on the circular dichroism of ethidium ion-nucleic acid complexes.

Binding of the frameshift mutagen ethidium bromide to eight dinucleoside phosphates of different complementary base sequences, to trinucleoside diphosphates with mismatched bases capable of forming bulged structures (CpUpG and GpUpG + CpC), and to calf thymus DNA at different counterion concentrations was studied by optical methods, notably UV-visible spectroscopy, circular dichroism (CD), and fluorescence-detected circular dichroism (FDCD). The ethidium ion intercalated between the base pairs of the minihelix formed by the complementary dimers and displaced the mismatched bases to form bulged structures with the trimers. The stoichiometry of the complexes was 2:1 oligonucleotide:dye (or 1:1:1 for non-self-complementary species). Equilibrium constants for complex formation showed a general preference for dye binding to complementary sequences. Above 300 nm, where only the dye absorbs light, the induced CD spectra of the complexes had bands at 510, 375, 330, and 307 nm, with the magnitude of the 307-nm band, per bound dye, being the largest and sequence dependent. The CD spectra down to 220 nm of some complexes were obtained by FDCD measurements; between 220 and 300 nm, both the positions and the magnitudes of the bands were sequence dependent. At very low ionic strength, complexes of calf thymus DNA plus ethidium ion inmore » different salt concentrations showed that the 307-nm CD magnitude increased with decreasing counterion concentration. Implications of the results for theories of the induced CD spectrum of the ethidium ion are discussed.« less

Ternary α-Amino Acid-Palladium(II) Complexes with Ligand-Ligand Hydrogen Bonding

Abstract Histidinate(His)-containing ternary α-amino acid-palladium(II) complexes have been studied by synthetic and spectroscopic methods. The following complexes with imidazolate (im), l- and d-His, l-asparaginate (l-Asn), and l-glutaminate (l-Gln) have been isolated as crystals: [Pd(im)2]·0.5H2O; [Pd(l-His) (d-His)]·H2O; [Pd(l-Asn)-(l-His)]·2H2O; [Pd(l-Gln) (l-His)]·2H2O. The ternary systems involving palladium (II), l-His, and an amino acid l-AA with an OH or a CONH2 group (AA=Asn, Gln, serinate, threoninate, or homoserinate) exhibit absorption maxima at 300–311 nm with ε 310–440 and negative and positive circular dichroism (CD) peaks at 307–328 nm and ≈300 nm, respectively. 13C and 1H NMR spectra gave the amounts of the cis and trans isomers of Pd(His)-(AA) in solution and confirmed that Pd(His)(AA) has an N3O donor set in the coordination plane. Whereas the CD magnitude additivity holds for the ternary systems Pd (l-His)(B) with B=glycinate, l-alaninate, or l-valinate and the systems with l-histidine methyl ester (l-HisOMe) and l-AA, Pd(l-HisOMe)(l-AA), the magnitudes for Pd (l-His) (l-AA) deviate significantly from those estimated on the basis of the additivity. The CD magnitude anomaly as well as the geometrical and rotational isomer populations calculated from the 1H NMR spectra establishes the existence of the ligand-ligand interaction between the carboxylato group of His and the hydroxyl or amido group of AA in solution.

Magnetic and natural circular dichroism spectra of cytochromes P-450 11 β and P-450 scc purified from bovine adrenal cortex

Magnetic (MCD) and natural circular dichroism (CD) spectra of various complexes of cytochrome P-450 11 β ( P-450 11 β ) and cytochrome P-450 scc ( P-450 scc) from bovine adrenal cortex were measured from 250 nm to 700 nm. MCD and CD spectral contours of cytochromes P-450 11 β and P-450 scc in the Soret and visible regions were, as a whole, analogous to those of cytochromes P-450 from rabbit liver microsomes and also from Pseudomonas putida in their high-spin ferric, high-spin ferrous and ferrous-CO complexes. MCD spectrum of the low-spin ferric P-450 scc free from the substrate, cholesterol, was very similar to that caused by addition of 20α-hydroxycholesterol, a reaction intermediate. However, it was distinct from those of the low-spin ferric P-450 11 β and P-450 scc complexes caused by addition of external nitrogenous ligands. The electronic states of the heme in the low-spin ferric P-450 free from substrates seemed to be subtly different from those of low-spin complexes coordinated with external nitrogenous ligands. Soret CD spectra of ferric low-spin complexes were not so different from each other. Upon reduction of high-spin ferric P-450 11 β or P-450 scc, the Soret CD magnitudes increased significantly in contrast with those of other P-450s, the Soret CD magnitudes of which decrease upon reduction. This may reflect an increased proximity of the neighbouring aromatic groups upon reduction of high-spin P-450 11 β or P-450 scc. High substrate specificity of adrenal P-450 s compared with liver P-450s can be explained in view of the above findings. The CD spectra in the near ultraviolet region (250–350 nm) were found to be quite sensitive to the spin change for ferric P-450 scc, while the MCD spectra in this region did not reflect substantially the spin state of the enzyme. MCD parameters of cytochrome P-450s were compared to those of other hemoproteins in diagrams describing selected MCD spectral values of hemoproteins so far available and were discussed in connection with the structures of the heme environment of P-450.

Circular Dichroism Spectra of Cobalt(III)–Ammine Complexes Containing Optically Active 1,3-Diphenyl-1,3-propanediamine or 1,2-Diphenyl-1,2-ethanediamine

Abstract A series of optically active cobalt(III) ammine complexes containing (S,S)-1,3-diphenyl-1,3-propanediamine (dppn) or (S,S)-1,2-diphenyl-1,2-ethanediamine (stien) were prepared. The sign of circular dichroism (CD) of [Co(NH3)4(S,S-dppn)]Br3 depends on solvents in the first absorption band region. The CD magnitude of trans-[Co(NH3)2(S,S-dppn)2]3+ varies significantly with solvent variations in the same energy region. The CD curve of the vicinal effect of (S,S)-dppn derived from the two diastereomers of cis-[Co(NH3)2(S,S-dppn)2]3+ is different from the CD spectra of both [Co(NH3)4(S,S-dppn)]3+ and trans-[Co(NH3)2(S,S-dppn)2]3+. The CD spectra of the (S,S) -stien complexes are insensitive to solvent variations and the vicinal contributions of the (S,S)-stien calculated from a variety of the (S,S)-stien complexes are similar to one another.

ChemInform Abstract: SPECTRAL PROPERTIES OF MIXED LIGAND COPPER(II) COMPLEXES CONTAINING L‐α‐AMINO ACIDS

AbstractDurch Reaktion von Cu(II)‐perchlorat‐hexahydrat mit den Dicarbonsäuren und den Hydrochloriden der Aminosäuren werden die Komplexe (I) ′dargestellt.