Vibrational circular dichroism spectroscopy of solid polymer films: effects of sample orientation

Vibrational circular dichroism (VCD) spectra of anisotropic thin solid samples are often superimposed with large contributions of linear birefringence and linear dichroism. In this study a theoretical approach is given on how to extract the true VCD spectrum out of such superimposed spectra. To verify this approach, the VCD spectra of achiral polymer films were examined. The polymers are supposed to give a zero line as VCD spectrum after eliminating the linear contributions. Applying our approach, in which four VCD spectra in different but selected sample orientations are recorded, and calculating their average, leads to the expected result, i.e., a zero line for achiral polymers. The advantage of this method for the elimination of artifacts from solid-state VCD spectra is that no further measurements are required (e.g., linear dichroism measurements or the determination of the orientation with the maximum anisotropy).     

Theoretical simulation of a polarization modulator based on mechanical rotation of a polarizing element

The properties of three alternative designs for a polarization modulator of potential use for the measurement of vibrational circular dichroism (VCD) are evaluated and compared by use of Mueller calculus. The analysis shows that the combination of a fixed polarizer plus either a photoelastic modulator or a rotating quarter-wave plate possesses nearly the same capability for generation of time-varying, circularly polarized light. However, a modulator composed of a rotating polarizer plus a fixed birefringent plate entails considerable theoretical and experimental difficulties for use in the measurement of VCD spectra. While VCD spectra obtained with the rotating devices can be calibrated in the same manner as spectra obtained with a photoelastic modulator, Mueller analysis shows that the form of the resultant calibration signal will have a different shape. The relevant expressions for VCD and linear dichroism as well as the calibration signals are presented, and consequences for practical realization of these experiments are discussed.     

Observation of Fourier transform near-infrared vibrational circular dichroism to 6150 cm-1

We report here the first measurement of near-infrared (NIR) vibrational circular dichroism (VCD) with Fourier transform (FT) instrumentation. The measurements were carried out using a commercial rapid-scan FT-IR VCD spectrometer modified for operation in the NIR region between 3750 and 6150 cm-1. Overtone and combination band VCD intensities, first reported by Keiderling and Stephens using dispersive VCD instrumentation in 1976, were reproduced with equivalent signal quality and spectral resolution using collection times of only a few minutes. These results offer substantial promise for the routine use of rapid-scan FT instrumentation for the measurement of overtone and combination band VCD spectra in the NIR region.     

Vibrational circular dichroism of matrix-assisted amino acid films in the mid-infrared region

Vibrational circular dichroism (VCD) spectra in the mid-infrared region of amino acid films are reported here for the first time. Amino acid films are formed from aqueous solutions with alpha-cyclodextrin (CD) serving as the matrix to facilitate the film formation. This film method eliminates the strong interfering water absorption seen in the solution study and makes it easier to measure the VCD in the 1800-1200 cm-1 region. VCD spectra for films of six amino acids, L-alanine, L-proline, L-methionine, Lhistidine, L-phenylalanine, and L-tryptophan, are obtained. For amino acids with low solubility (L-phenylalanine and L-tryptophan), VCD could not be measured in solution, so the film method is the only means of obtaining the VCD spectra for such amino acids. For amino acids with moderate solubility (L-alanine, L-proline, L-methionine, and L-histidine), VCD spectra are also obtained in the solution state and compared with their corresponding spectra in the film state. A good correlation is found between the film and solution spectra for both absorption and VCD. The VCD spectra of L-methionine, L-histidine, L-phenylalanine, and Ltryptophan are reported here in the mid-infrared region for the first time. The present study broadens the application range of the VCD technique and enhances its role in the detection and analysis of biologically important compounds.     

Extension of fourier transform vibrational circular dichroism into the near-infrared region: continuous spectral coverage from 800 to 10 000 cm(-1)

We report the first vibrational circular dichroism (VCD) spectra with continuous coverage from 800 cm(-1) in the mid-infrared (MIR) region to 10 000 cm(-1) in the near-infrared (NIR) region. This coverage is illustrated with MIR and NIR absorbance and VCD spectra of 2,2-dimethyl-dioxolane-4-methanol (DDM), alpha-pinene, and camphor that serve as calibration samples over this entire region. Commercially available, dual-source Fourier transform (FT) MIR and NIR VCD spectrometers were equipped with appropriate light sources, optics, and detectors, and were modified for dual-polarization-modulation (DPM) operation. The combination of liquid-nitrogen- and thermoelectric-cooled HgCdTe (MCT) detectors, as well as InGaAs and Germanium (Ge) detectors operating at room temperature, permitted collection of the desired absorbance and VCD spectra across the range of vibrational fundamental, combination band, and overtone frequencies. The spectra of DDM and alpha-pinene were measured as neat liquids and recorded for both enantiomers in the various spectral regions. Spectra for camphor were all measured in CCl(4) solution at a concentration of 0.6 M, except for the carbonyl-stretching region, where a more dilute concentration was used. The typical anisotropy ratios (g) of the three molecules were estimated with respect to their strongest VCD bands in each spectral region. It was found that for all three molecules in the spectral regions above 2000 cm(-1), anisotropy ratios are approximately the same order (10(-5)) of magnitude. However, in the MIR region, the typical anisotropy ratios are significantly different for the three molecules. This study demonstrates that with modern FT-VCD spectrometers modified for DPM operation, VCD spectra can be measured continuously across a wide spectral range from the MIR to nearly the visible region with an unsurpassed combination of signal-to-noise ratio and spectral resolution.     

Optical properties of hybrid titanium chevron sculptured thin films coated with a semiconducting polymer

Optical and structural properties of a hybrid metallic chevron sculptured thin film from titanium coated with the semiconducting polymer poly(3-dodecylthiophene) (P3DDT) are reported. The nanostructured thin film with two subsequent layers of oppositely slanted nanocolumns was fabricated by glancing angle deposition and coated with P3DDT by a spin-cast process. Spectroscopic generalized ellipsometry is employed to determine geometrical structure properties and the anisotropic optical constants of the coated and uncoated film in the spectral range from 400 to 1700 nm. The nanostructured thin films before and after hybridization show highly anisotropic optical properties. The complex refractive indices along major polarizability directions of the hybridized chevrons are increased in the entire investigated spectral range with respect to the as-deposited chevrons. Changes in birefringence and dichroism upon polymer infiltration are observed.     

Polarimetric investigation of materials with both linear and circular anisotropy

Abstract

We investigate light propagation through materials with both linear and circular anisotropy and find the relation of the amplitude and polarization transfer functions to the four anisotropic characteristics: linear circular birefringence, and linear and circular dichroism. We determine these four characteristics of anisotropic samples by measuring the output intensity and polarization corresponding to different input polarization azimuths and fitting the theoretical and experimental results. In our experiments we have used films of side-chain azobenzene polyesters in which optical anisotropy had been previously induced on illumination with elliptically polarized light.     

Fabrication of Langmuir-Blodgett films using amphiphilic peptides

To introduce self-organization ability of transmembrane proteins into Langmuir (L) and Langmuir-Blodgett (LB) techniques, we focused on "amphiphilic peptide" (AP) which is composed of two distinct hydrophilic and hydrophobic domains. Three types of APs of different average hydropathies were used to prepare the AP/lipid mixed L and LB films. According to the circular dichroism spectra, the secondary structures of APs were not uniform but were a mixture of alpha-helix, beta-strand and random coil. The fraction of alpha-helix was higher for lower hydropathy AP. The interaction between AP and lipid in the L film and the structure of the LB film were also depended on the APs used.     

金属卟啉LB膜吸收光谱研究

采用Langmuir-Blodgett(LB)技术,在23mN/m的膜压下制备了四苯基卟啉铜(CuTPP)、四苯基卟啉锌(ZnTPP)及四苯基卟啉镍(NiTPP)超薄膜.分析了CuTPP、ZnTPP及 NiTPP 的LB膜分别对乙酸乙酯气体和四氢呋喃气体的紫外光谱变化,并通过薄膜对紫外-可见吸收光谱的变化阐述了构建有机气体-金属卟啉色谱探测阵列的可行性.实验结果表明,无论针对乙酸乙酯气体还是四氢呋喃气体,ZnTPP 薄膜光谱变化最明显,CuTPP薄膜次之,而NiTPP薄膜几乎没有光谱变化.因此认为CuTPP和ZnTPP可作为色谱阵列单元识别乙酸乙酯气体和四氢呋喃气体(THF).     

Analysis of structurally heterogeneous Langmuir-Blodgett films of folded/unfolded long-chain molecules by infrared multiple-angle incidence resolution spectroscopy

Infrared multiple-angle incidence resolution spectroscopy (IR MAIRS) has been employed for the structural analysis of a heterogeneous multilayer Langmuir-Blodgett (LB) film of ketomycolic acid (keto-MA). Keto-MA was chosen for the study because the molecule changes its conformation largely, which depends on the layer structure of the LB film: the long alkyl chain with a keto group remains folded when it has no counter layer for interdigitation. IR MAIRS is a surface analytical technique that was originally developed to analyze anisotropic structure in a thin film prepared on an IR transparent material. In the present study, however, a newly found characteristic of MAIRS has been used to discuss the molecular conformation and the interdigitation architecture between two adjacent monolayers. As expected theoretically, the molecular folding was clearly detected as "IR MAIRS shift". Subtraction of the IR MAIRS spectra of single- and triple-monolayer LB film of keto-MA yielded unambiguous results for the interdigitated double layer in the triple-monolayer LB film, and the layer sequence was also revealed.     

Rapid injection linear dichroism for studying the kinetics of biological processes

Linear dichroism is defined as the differential absorbance of linearly polarized light oriented in two orthogonal directions by an aligned sample. The measurement of a linear dichroism (LD) spectrum of a sample provides two key pieces of structural information. First, that the sample and the chromophores within the sample are able to align. Second, given knowledge of the transition polarization directions of the chromophores, the orientation of the chromophores within the aligned sample can be resolved. It has been shown that LD can provide unique information on the structure of some of the more challenging biomolecular complexes. This has included macromolecular protein and peptide fibers such as actin, tubulin, and amyloids as well as protein-membrane complexes and DNA-protein complexes. Much of this work has been enabled by the development of a low volume Couette flow cell that efficiently aligns long molecules in solution. However, the current Couette system is inherently complex to assemble for each experiment and hence not suited to measurement of rapid reactions. In this paper we detail the development of the first rapid injection LD cell. The system utilizes a conventional stopped-flow injection system coupled to a modified low volume Couette cell, where a narrow bore capillary replaces the normal solid central rod. The system is shown to have similar optical characteristics to the conventional LD Couette flow cell but with the added benefit of a much shorter dead time (0.60 s compared to ～60). The rapid injection Couette cell has been used to measure the degradation of DNA by DNA exonuclease I, providing data that would not be available using a conventional system.     

抗菌聚四氟乙烯双向拉伸薄膜的制备

以聚四氟乙烯(PTFE)为基料,添加3种不同种类的抗菌剂,制备了系列抗菌PTFE双向拉伸薄膜;并测定了薄膜对大肠杆菌、金黄色葡萄球菌、黑色芽孢变种以及白色念珠菌的抑菌性能;并测试了添加抗菌剂后对薄膜力学性能的影响。研究发现,添加编号为CAH-2的抗菌剂制备的薄膜抗菌效果均达到100%,具有广谱抗菌性。此外,添加液体抗菌剂会使薄膜的力学强度略有下降,添加固体粉末抗菌剂有助于增强薄膜的力学性能。     

Determination of surface selection rule of surface plasmon resonance near-infrared spectroscopy by using a Langmuir-Blodgett film

Near-infrared (NIR) absorption spectra of a cadmium arachidate Langmuir-Blodgett (LB) film were measured by surface plasmon resonance near-infrared spectroscopy (SPR-NIRS) based on the Kretschmann configuration with a 18.8-nm gold film. An NIR spectrum enhanced severalfold was obtained as a top ridge of the SPR-NIR spectra measured at different incident angles by using the principle of absorption-sensitive SPR. In order to determine the surface selection rule of SPR-NIRS, the enhanced NIR absorption spectrum of the LB film was compared to an unenhanced one without the gold film and to a normal incidence transmission spectrum. Moreover, a pair of out-of-plane (OP) and in-plane (IP) spectra were obtained by multiangle infrared spectroscopy analysis from a series of oblique incidence transmission measurements in the NIR region. It became obvious that the salient feature of the enhanced NIR absorption spectrum, i.e., the top ridge of the SPR-NIR spectra is almost equivalent to that of the OP spectrum. On the other hand, the unenhanced spectrum showed IP modes. These experimental results were well explained by calculation of the mean-square electric field based on the Fresnel formula.     

Determination of enantiomeric excess in samples of chiral molecules using fourier transform vibrational circular dichroism spectroscopy: simulation of real-time reaction monitoring

The first use of Fourier transform vibrational circular dichroism (FT-VCD) to follow changes in the percent enantiomeric excess (% EE) of chiral molecules in time using a flow cell sampling apparatus is reported. FT-VCD, as opposed to dispersive scanning VCD, eliminates the need to scan the VCD spectrum in time to monitor the % EE at more than one spectral location. The first use of partial least-squares chemometric analysis to determine % EE values from kinetic sets of VCD spectral data is also reported. These two advances have been used to monitor simultaneously changes in the fractional composition and the % EE of a mixture of two different chiral molecules. This simulates the progress of the chemical reaction from a chiral reactant to a chiral product where the % EE of both molecules can change with time. For the molecules studied, alpha-pinene, camphor, and borneol, the accuracy of following % EE changes for one species alone is approximately 1%, while for simultaneously following % EE changes in two species is approximately 2% for 10-20-min sampling periods at 4 cm(-)(1) spectral resolution. This accuracy can be increased for the same collection times or maintained for shorter periods of collection by lowering the spectral resolution. These findings demonstrate the potential for VCD to be used for real-time monitoring of the composition and % EE of chemical reactions involving the synthesis chiral molecules.     

Investigation of the monolayer LB film properties of a silsesquioxane hybrid containing phenyl groups using SPM

In this study, we used a silsesquioxane hybrid material between phenyl group containing carbosilane dendrimer and siloxane polymer (G1-3Ph). As expected, G1-3Ph has a networked structure. Generally, it is difficult to generate uniform monolayers as common materials. However, if the materials have a networked structure, we can easily obtain uniform monolayers because of the molecular interactions. Moreover, this networked structure should have a cavity in which ions could be placed.We have fabricated Langmuir–Blodgett (LB) films of the G1-3Ph in a thin film state. We investigated the monolayer behavior using the π-A isotherm at the air–water interface. The π-A curves indicated that the surface pressure of this LB film from the liquid to the solid state falls between 5 and 8 mN/m. The monolayer was deposited onto highly oriented pyrolytic graphite (HOPG) substrates via Y-type deposition at a surface pressure of 5.5 mN/m. We also studied the morphological properties of the silsesquioxane hybrid LB films using SPM and determined the electrical properties of the LB films using STM. The I–V and the morphological characteristics of the sample were investigated at room temperature. The experimental results correlate with the calculated I–V curves when we use the parameters indicated in the caption. This study will enable us to apply a molecular electronic device to measure morphological and electrical characteristics on the nano scale.     

Spectral and quantum chemical examination of the Si clusters nascent inside the SiO bulk

In the present communication the results of the synthesis, the quantum chemical (QC) and the spectral examination of the SiO thin film and powder are reported. The QC and Raman studies indicate the formation of the Si clusters inside the bulk of the SiO protoparticle at the very first stages of its gas-phase growth. Infrared, Raman, inelastic neutron scattering and electron spin resonance spectra for the synthesized SiO thin films and the commercial sample have been recorded and simulated in order to verify the theoretical model. The QC calculated radial distribution function and vibrational spectra reproduce well the experimental ones.     

Studies of optical anisotropy in opals by normal incidence ellipsometry

Anisotropy of thin opal films was studied by ellipsometric technique in a visible spectral range. At normal light incidence, the ellipsometric data were directly related to anisotropy parameters measured by polarization modulation technique. In the (111)-oriented thin films, the optical anisotropy was mainly caused by internal strain-induced birefringence with anisotropy axes oriented along [110] and [-112] directions. The deviation from 180°-symmetry, which has been observed for ellipsometric parameters in the in-plane sample rotation experiments at normal incidence, was enhanced at oblique incidence and assigned to particular properties of opal. Experimental data were discussed in the model of stacked anisotropic layers.     

Cartesian-structure analysis in cast films by advanced infrared multiple-angle incidence resolution spectroscopy

Multiple-angle incidence resolution spectroscopy (MAIRS) has been improved to be an advanced algorithm so that the Cartesian structure in organic thin films can be analyzed. The infrared MAIRS technique was originally proposed as a totally new spectroscopic technique to reveal structural anisotropy in thin films on an infrared-transparent substrate, which yields both in-plane- (IP; X and Y) and out-of-plane (OP; Z)-mode spectra from an identical sample. Since this technique employs an analytical concept based on a signal decomposition of light intensity (not absorbance spectra), the algorithm intrinsically has high potential for further development. In the present study, the theoretically deduced matrix that correlates the light intensity to the angle of incidence has been modified to further decompose the IP-mode spectrum into X and Y components. As a result, anisotropic measurements of infrared spectra of thin film have become possible for the X, Y, and Z directions (Cartesian coordinate) simultaneously. With this advanced algorithm, the Cartesian structural changes in a cast film prepared on a germanium substrate have readily been analyzed, and a change from the biaxial to the uniaxial film structure with aging has spectroscopically been revealed.