CD Exciton Research Articles

Bulky anion effect on the architecture of chiral dysprosium single-molecule magnets.

The interest for chiral tris(β-diketonato)lanthanide complexes in coordination chemistry is huge due to its Lewis acid character, optical activity, and the control of the final compound architecture. The reaction of equimolar quantities of [Dy((-)/(+)hfc)3 (H2 O)] (hfc- = 3-(heptafluoropropylhydroxymethylene)-(+/-)-camphorate) and L led to the formation of a pair of enantiomers for dinuclear complexes [Dy((-)/(+)hfc)3 (L)]2 ⋅C7 H16 ([(-)/(+)1]⋅C7 H16 ) (L = 4'-(4'''-pyridyl-N-oxide)-1,2':6'1''-bis-(pyrazolyl)pyridine]). Starting from the previous experimental protocol with the addition of bulky BArF anions, a partial dissociation of the chiral [Dy((-)/(+)hfc)3 (H2 O)] was observed leading to the isolation of a mono-dimensional cationic chiral polymer {[Dy((-)/(+)hfc)2 (L)][BarF]}n ⋅nCH3 NO2 ([(-)/(+)2]n ⋅nCH3 NO2 ). Natural circular dichroism (NCD) highlighted an exciton CD couplet for [(-)/(+)2]n but not for (-)/(+)1. The latter behaves as a single-molecule magnet (SMM) with a blocking temperature up to 4K, whereas [(-)/(+)2]n is a 1D assembly of field-induced SMMs with a magnetic relaxation occurring through a Raman process only.

The Impact of Chiral Citronellyl‐Functionalization on Indolenine and Anilino Squaraine Thin Films

AbstractThe impact on chiral aggregation in solution processed and thermally annealed thin films of two indolenine and one anilino squaraines with chiral (S)‐citronellyl functionalization at the nitrogen of the squaraine backbone is investigated. A pseudo polymorphic crystal structure is obtained for one of the indolenine squaraines, but thin films of both compounds are basically non‐aggregated and truly isotropic as evidenced by spectroscopic ellipsometry. The anilino squaraine coalesces and readily aggregates to circular dichroic but discontinuous thin films. The extent of circular dichroism correlates with the morphology of the samples, which is quantified by Mueller matrix polarimetry in combination with atomic force microscopy. The shape of the CD spectra reinforces the hybrid Frenkel and charge‐transfer excitonic nature of the characteristic double hump signatures within unpolarized absorbance spectra of previously investigated non‐chiral anilino squaraines with linear alkyl chain functionalization. Such excitonic CD offers an additional design parameter for next‐generation opto‐electronic devices.

Study of aryl triazoles for absolute configuration determination

A variety of chiral mono- and di-1,4-disubstituted 1,2,3-triazoles were synthesized by CuAAC ‘click chemistry’ as model compounds and their spectroscopic properties characterized. The UV and CD studies of these compounds showed that 4-substituted aryl triazoles give rise to moderate exciton CD curves, under either homo- or hetero-coupling. The direction of the electric transition moment of the non-symmetric chromophores was determined by conformational analysis and supported by NMR data. The signs of the Cotton effects of the CD spectra were in complete agreement with the determined directions of the electric transition moments of the chromophores. Therefore, 4-substituted aryl triazoles can be used for the determination of the absolute configuration of organic compounds. In addition, the 4-(4-bromophenyl)-1,2,3-triazole allows for red-shifted chromophores to be obtained via Suzuki reactions, thus avoiding overlap with the substrate absorptions.

Action of Daptomycin on Membranes

Daptomycin is a clinically important 13 amino acid lipopeptide antibiotic. Its N-terminus is acylated with n-decanol and its C-terminal 10 amino acids form a ring. It is structurally and size-wise different from other well-known membrane-active antimicrobials. Daptomycin is known to disrupt the cytoplasmic membrane function of Gram-positive bacteria by causing leakage of potassium (and potentially other) ions, leading to the loss of membrane potential and cell death. The critical factor affecting the function of daptomycin is its interaction with negatively charged lipids such as PG in a calcium (Ca++) dependent manner. Based on previous research on cell membranes, daptomycin has been assumed to insert and aggregate in the membrane, and then to alter the membrane curvature. However these details have not been demonstrated by biophysical studies. In our aspirated GUV experiments, we found that with a DOPG-containing GUV and a sufficient concentration of Ca++, daptomycin can extract lipid molecules, and form lipid-peptide aggregations. The lipid-peptide aggregates did not occur if cardiolipin replaced PG, or if other divalent ions, such as Mg++, Ba++ replaced Ca++. Similarly, daptomycin did not bind to a GUV when cardiolipin substituted for DOPG or in the absence of Ca++. Daptomycin with Ca++ did bind to a pure DOPC GUV, but had no other effects. Furthermore, with the presence of daptomycin, DOPG and Ca++, we found Ca++ permeates into the GUV, while a content dye, Texas red dextran, did not leak out. This result suggests that daptomycin and Ca++ do not form pores on the membrane of DOPG-contained GUV, but cause leakage of ions. Finally, daptomycin with DOPG and Ca++ produces a negative exciton CD couplet centered at the 225 nm absorption peak of Trptophan1 and Kynurenine13, whereas in all other conditions, the exciton CD couplet is positive.

Chiroptical Spectra of a Series of Tetrakis((+)-3-heptafluorobutylyrylcamphorato)lanthanide(III) with an Encapsulated Alkali Metal Ion: Circularly Polarized Luminescence and Absolute Chiral Structures for the Eu(III) and Sm(III) Complexes

The luminescence and circularly polarized luminescence (CPL) spectra of M(I)[Eu((+)-hfbc)(4)] show a similar behavior to the exciton CD in the intraligand π-π* transitions when the alkali metal ions and solvents are manipulated. There is a difference in susceptibility in solvation toward the alkali metal ions but not toward the Eu(III) ion, as in the case of axially symmetric DOTA-type compounds. The remarkable CPL in the 4f-4f transitions provide much more information on the stereospecific formation of chiral Eu(III) complexes, since CPL spectroscopy is limited to luminescent species and reflects selectively toward helicity of the local structural environment around the lanthanide(III). While in comparison, exciton CD reveals the chiral structural information from the helical arrangement of the four bladed chelates. Of special importance, the observation of the highest CPL activities measured to date for lanthanide(III)-containing compounds (i.e., Eu and Sm) in solution supports the theory that the chirality of lanthanide(III) in the excited state corresponds to that in the ground state, which was derived from the exciton CD.

Circular Dichroism Spectroscopy Is a Sensitive Tool for Investigation of Bilirubin−Enzyme Interactions

Noncovalent complex formation of unconjugated bilirubin with various enzymes has been demonstrated by measuring induced circular dichroism (ICD) peaks associated with the pigment VIS absorption band. Preferential binding of the P- or M-helical conformer of bilirubin to dehydrogenases, catalase, alkaline phosphatase, and α-chymotrypsin is responsible for the characteristic exciton CD couplet that undergoes remarkable changes upon the addition of enzymatic cofactors (NADH, AMP) and an inhibitor (acridine). Alterations of the ICD spectra refer to a direct binding competition between bilirubin and NADH for a common binding site on alcohol dehydrogenase and catalase, suggesting a potential mechanism for the inhibitory effect of BR reported on NAD(P)H dependent enzymes. NADH and bilirubin form a ternary complex with glutamate dehydrogenase indicated by peculiar CD spectral changes that are proposed to be generated by allosteric mechanism. α-chymotrypsin binds bilirubin in its catalytic site, as indicated by CD displacement experiments performed with the competitive inhibitor acridine. Surprisingly, the closely related trypsin does not induce any CD signal with bilirubin. Taking into consideration the clinically relevant but controversial and poorly understood areas of bilirubin biochemistry, the fast and simple CD spectroscopic approach presented here may help to unfold diverse physiological and pathophysiological roles of BR on a molecular level.

Vibrational Circular Dichroism of Δ-SAPR-8-tetrakis[(+)-heptafluorobutyrylcamphorato]lanthanide(III) Complexes with an Encapsulated Alkali Metal Ion

The vibrational circular dichroism (VCD) spectra for Delta-M[Ln((+)-hfbc)(4)] (M(I) = Na, K, Rb, Cs; Ln(III) = La, Eu; abbreviated as M-Ln) and Delta-Cs[Yb((+)-hfbc)(4)] (Cs-Yb) were observed in CDCl(3) solutions. The VCD signs and/or intensities are mostly different from those of the Delta-tris(beta-diketonato) transition-metal complexes. The M-Ln complexes give the strong VCD peak(s) near 1550 cm(-1). These VCD patterns are different from each lanthanide(III) complex, but their intensities change with variation of the alkali metal ion. A positive-negative couplet VCD near 1550 cm(-1) for the Delta-M-La complexes is observed similarly for the Delta-tris(beta-diketonato) complexes, confirming the assignment to the Delta absolute configuration based on the exciton CD. The density functional theory calculated VCD for the two peaks at 1650 and 1550 cm(-1) reproduces the relative intensity and signs of the observed ones for the Delta-M-La(III) complexes.

Extraordinary Circularly Polarized Luminescence Activity Exhibited by Cesium Tetrakis(3-heptafluoro-butylryl-(+)-camphorato) Eu(III) Complexes in EtOH and CHCl3 Solutions

The largest CPL activity ever measured was observed for cesium tetrakis(3-heptafluoro-butylryl-(+)-camphorato) Eu(III) complexes in EtOH and CHCl3 solutions substantiating the stereospecific formation of chiral Delta-SAPR-(C4) configuration with the aid of Cs+...FC (fluorocarbon) interactions more clearly than the exciton CD spectra.

Stimulated emission of free excitons in Cd 1− xMn xTe under nonresonant two-photon excitation

We report on free excitons coexisting with exciton magnetic polarons (EMPs) in bulk semimagnetic semiconductors of Cd 1− x Mn x Te for 0.04 ⩽ x ⩽ 0.36 at 2 K under nonresonant two-photon excitation. This two-photon excitation not only generates free excitons but also more efficiently creates EMPs compared with ordinary one-photon excitation. Stimulated emission from free excitons is demonstrated under strong two-photon excitation.

Crystal structure and chiroptical spectra of sodium tetrakis (+)-hfbc Pr(III) complex

Abstract The X-ray crystal structure of tetrakis (+)-hfbc Pr(III) complex, Na[Pr((+)-hfbc)4]·CH3CN, was compared with that of the corresponding Na–La complex with a pseudo-achiral dodecahedron, and the solution CD spectra were also discussed in the 4f–4f transition in consideration of the exciton CD couplet showing Δ-SAPR-8 configuration in CHCl3.

Novel Configurational Structures of Sodium Tetrakis(3-heptafluorobutylryl-(+)-camphorato) Ln(III) Complexes with a Trapped Na+ by Na+···FC Interactions in the Solid State and in Solution

For newly prepared sodium tetrakis(3-heptafluorobutylryl-(+)-camphorato) Ln(III) complexes, the X-ray crystal analysis revealed an unprecedented pseudo-achiral dodecahedron (DD-8 (D(2d))), whereas in CHCl3 solution, the exciton CD and 19F NMR spectra showed a novel chiral Delta-SAPR-8 (C4) configuration which is stereospecifically formed by a trapped Na+ ion with Na+...FC (fluorocarbon) interactions.

Interaction of Cu(II)-meso-tetrakis(n-N-methylpyridiniumyl)porphyrin (n = 2,3,4) with Native and Synthetic Polynucleotides Probed by Polarized Spectroscopy

The interactions of Cu(II)-meso-Tetrakis(n-N-methylpyridiniumyl)porphyrin (n = 2,3,4), respectively referred to as o-, m- and p-CuTMPyP, and DNA, poly<TEX>$[d(A-T)_2]$</TEX> and poly<TEX>$[d(G-C)_2]$</TEX> were investigated by circular and linear dichroism (CD and LD). In the o-CuTMPyP case, in which the rotation of the pyridinium ring is prevented, the shape of the CD spectrum when associated to DNA and poly<TEX>$[d(A-T)_2]$</TEX> resembles and is characterized by a positive band at a low drug to DNA concentration ratio (R ratio) and is bisignate at a high R ratio. The former CD spectrum shape has been attributed to porphyrin that is bound monomerically outside of DNA while the latter can be attributed to those that are stacked. When o-CuTMPyP is bound to poly<TEX>$[d(G-C)_2]$</TEX>, the excitonic CD appeared at a relatively high R ratio. In contrast, a characteristic negative CD band in the Soret region was apparent for both m- and p-CuTMPyP when bound to DNA and poly<TEX>$[d(G-C)_2]$</TEX> at the low R ratios, indicating that the porphyrin molecule intercalates. However, the DNA is bent near the intercalation site and the plane of the porphyrin molecule tilts relative to the DNA helix axis, as judged by the magnitude of the reduced LD. Various stacking patterns were identified by the shape of the CD spectrum for m- and p-CuTMPyP when bound to poly<TEX>$[d(A-T)_2]$</TEX>. Three species for the former complex and two for the latter complex were found which may reflect the extent of the stacking.

Density Functional Calculation of the Electronic Circular Dichroism Spectra of the Transition Metal Complexes [M(phen)3]2+(M = Fe, Ru, Os)

The circular dichroism spectra of the tris-bidentate metal complexes Lambda-[M(phen)3]2+, with M = Fe, Ru, Os and phen = 1,10-tris-phenanthroline, are investigated computationally, employing time-dependent density functional theory. Good agreement with experimental spectra is obtained for Ru and Os. The Lambda-[Os(phen)3]2+ spectrum is analyzed in detail. It is shown how relativistic effects red shift CD bands where the Os 5d-orbital participates to a large extent in the excitations. Further, the participation of the metal in the ligand pi --> pi exciton CD is determined to be of the order of 10%. Though solvent effects can have a noticeable effect on individual transitions and rotatory strengths, they are demonstrated to have only a very small overall effect on the resulting simulated CD spectra. For Lambda-[Fe(phen)3]2+, the results are shown to be rather sensitive to the choice of the applied hybrid and nonhybrid density functionals, and the optimized geometries based thereupon. In particular, the sign pattern of the lower-energy part (up to 33 x 10(3) cm(-1)) of the Lambda-[Fe(phen)3]2+ CD spectrum is difficult to reproduce. Some combinations of functionals and geometries yield good agreement with experiment, but no "best" approach can be devised based on the available results. Possible sources of errors in the spectrum of Lambda-[Fe(phen)3]2+ due to deficiencies in the functionals and the exchange-correlation kernels are investigated.

Exciton Chirality. (A) Origins of and (B) Applications from Strongly Fluorescent Dipyrrinone Chromophores

(A) The origin of exciton interaction and examples of its application to organic stereochemistry are presented. (B) N,N′-Carbonyl-bridged dipyrrinones constitute a new class of highly fluorescent chromophores suitable for investigations of stereochemistry and absolute configuration. N,N′-Carbonylxanthobilirubic acid esters are strongly fluorescent, with a fluorescence quantum yield (φF) ∼0.8, but produce only weak exciton CD from the trans-1,2-cyclohexanediol template. The ester of an analog with benzoic acid replacing propionic, N,N′-carbonyl-8-(4-carboxyphenyl)-3-ethyl-2,7,9-trimethyl-(10H)-dipyrrin-1-one, exhibits strong fluorescence (φF=0.68, λem=493 nm, λex=422 nm in CHCl3) and UV-Vis absorption (ɛ∼21000 at 424 nm) in organic solvents. Its diester with (1S,2S)-cyclohexanediol is fluorescent and exhibits exciton circular dichroism (Δɛ=+15 dm3·mol−1·cm−1, λ=432 nm; Δɛ=−4 dm3·mol−1 cm−1, λ=380 nm) that correlates with the Exciton Chirality Rule.

Influence of quantum dot density on excitonic transport and recombination in CdZnTe/ZnTe QD structures

We report on dynamics of excitons in Cd x Zn 1− x Te/ZnTe quantum dots (QDs) and present information of excitonic transport and recombination. Due to different growth methods, samples with different QD's densities were obtained. Time-resolved measurements reveal three decay mechanisms: (i) radiative recombination of excitons in the individual QDs; (ii) thermally activated escape of excitons and (iii) escape due to tunneling (hopping). In the high QD-density samples the hopping ( r HB=2700 ns −1) is two orders of magnitude more efficient than in the low QD-density samples ( r HB=33 ns −1). Radiative recombination rates are similar in both types of samples, r R=1–1.3 ns −1. Due to the good radiative to nonradiative recombination ratio, the low-density QDs can be a potential source of entangled photon pairs.

Localization of neutral and charged excitons in (Cd,Mn)Te quantum well: a microphotoluminescence study

Microphotoluminescence mapping measurements were performed on a (Cd,Mn)Te quantum well, iodine modulation-doped at about 10 1 0 cm - 2 . Intensity maps reveal dark lines with preferential directions [110] and [-110]. We attribute them to misfit dislocations propagating from substrate/buffer interface to the quantum well, as evidenced by a better spatial resolution observed at direct QW excitation compared to that at above-barrier energy. Photoluminescence spectra contain usual neutral (X) and negatively charged (X - ) exciton lines. Besides, a defect-related line (D) is observed at the low energy tail of the X - line. The presence of D line anticorrelates with that of X line, demonstrating an efficient trapping process of excitons at the defect centres. Lack of correlation between the X - and X intensities with direct excitation in the quantum well is interpreted as an evidence of electrostatic potential fluctuations.

Binding of meso-tetrakis(N-methylpyridium-4-yl)porphyrin to triplex oligonucleotides: evidence for the porphyrin stacking in the major groove.

Induced CD spectra of meso-tetrakis(N-methylpyridium-4-yl)porphyrin (TMPyP) complexed with d(A)12.d(T)12, d(G)12.d(C)12 duplex and d(A)12.[d(T)12]2, d(G)12.d(C)12.d(C)12+ triplex in the Soret band were compared in this study. When TMPyP is complexed with the duplex, a monomeric CD spectrum at a low [TMPyP]/[oligomer] ratio was apparent, while at a high mixing ratio, the excitonic CD was dominant. In contrast, when TMPyP was complexed with the triplex, the excitonic CD disappears at a relatively high mixing ratio, indicating the TMPyP exciton formation is inhibited by the third strand, which is located in the major groove. This observation indicates that the exciton is formed at the major groove of both AT- and GC-rich DNA, while the monomeric TMPyP binds at (or near) the minor groove of the AT site.

Anomalous excitonic CD of meso-tetrakis(3-N-methylpyridiniumyl)porphyrin bound to poly[d(A-T)(2)

When meso-tetrakis(3-N-methylpyridiniumyl)porphyrin (m-TMPyP) formed a complex with poly[d(A-T)(2)], an intense bisignate excitonic CD in the Soret absorption region was observed. The excitonic CD of the m-TMPyP-poly[d(A-T)(2)] complex is unique in that no other combination of the related porphyrin, namely, meso-tetrakis(n-N-methylpyridiniumyl)porphyrin (where n = 2, 4), and polynucleotide including calf thymus DNA, poly[d(G-C)(2)], poly[d(I-C)(2)], and poly(dA).poly(dT), exhibits a comparable CD spectrum. From the [drug]/[DNA] ratio-dependence of the intensity and the shape of the CD spectrum, this porphyrin species is assigned to an extensively aggregated form. The extensively aggregated porphyrin disperses in 1 h after mixing to form moderately stacked porphyrin at a low mixing ratio. The magnitude of linear dichroism of the extensively aggregated porphyrin was small and the sign was negative in the Soret band, which indicated that the molecular plane of porphyrin in the complex is strongly tilted. On the other hand, the molecular plane of porphyrin is almost parallel to the DNA base plane (perpendicular to the DNA helix axis) in the moderately stacked form.

Rotation of Periphery Methylpyridine of meso-Tetrakis(n-N-methylpyridiniumyl)porphyrin (n = 2, 3, 4) and Its Selective Bindingto Native and Synthetic DNAs

By utilizing circular and linear dichroism, the binding mode of meso-tetrakis(n-N-methylpyridiniumyl)porphyrin (n=2, 3, 4) to various DNAs was studied in this work. 2-N-(methylpyridiniumyl)porphyrin(o-TMPyP), in which rotation of the periphery pyridinium ring is prevented, exhibits similar spectral properties when bound to DNA, poly[d(G-C)2] and poly[d(A-T)2], suggesting a similar binding mode. Close analysis of the spectral properties led us to conclude that o-TMPyP sits in the major groove. However, both 3-N- and 4-N-(methylpyridiniumyl)porphyrin (m- and p-TMPyP), of which the periphery pyridinium ring is free to rotate, intercalate between the basepairs of DNA and poly[d(G-C)2]. In the presence of poly[d(A-T)2], m-TMPyP exhibits a typical bisignate excitonic CD spectrum in the Soret band, while p-TMPyP shows two positive CD bands. The excitonic CD spectrum of the m-TMPyP-poly[d(A-T)2] complex and the positive CD band of the o-TMPyP-poly[d(A-T)2] complex were not affected by the presence of the minor groove binding drug, 4′,6-diamidino-2-phenylindole (DAPI), indicating that this porphyrin is bound in the major groove. In contrast, two positive CD bands of the p-TMPyP-poly[d(A-T)2] complex altered in the presence of DAPI. From the changes in CD spectrum and other spectral properties, a few possible binding modes for p-TMPyP to poly[d(A-T)2] are suggested.

Radiative Lifetimes of Excitons in CdMgTe/CdTe Tilted Superlattices Grown on Vicinal Surfaces

The radiative decay of one-dimensional (1D) excitons in Cd 0.74 Mg 0.26 Te/CdTe tilted superlattices (TSLs) grown on vicinal substrates is studied. The temperature dependence of the radiative lifetime of 1D excitons in the TSLs is measured using time-resolved photoluminescence spectroscopy, and is compared with that of two-dimensional (2D) excitons in Cd 0.74 Mg 0.26 Te/CdTe quantum wells (QWs). Radiative lifetimes of 1D excitons in the TSLs and 2D excitons in the QWs increase with T but show clearly different behaviours. The observed radiative lifetimes in the TSLs and QWs are expressed as 100T 1/2 psK -1/2 and 11T psK -1 , respectively, from which the intrinsic radiative lifetime of the k ∼ 0 excitons is found to be 72 ps (1D) and 5 ps (2D). The decrease of the exciton coherence length imposed by the lateral confinement in the TSLs causes the large intrinsic radiative lifetime in the TSLs.