Thierry Buffeteau

Polarization Modulation FT-IR Spectroscopy of Surfaces and Ultra-Thin Films: Experimental Procedure and Quantitative Analysis:

Polarization modulation of the incident electromagnetic field is used to increase the sensitivity and the in situ experiment ability of the well-known method of reflexion absorption FT-IR spectroscopy, for the characterization of surfaces and ultra-thin films. The experimental procedure and signal processing of the detected intensity are described and illustrated with the use of results obtained with Langmuir-Blodgett monolayers. The quantitative analysis of the spectra is then developed, and a linear behavior of the band intensities is found for ultra-thin films exhibiting no strong absorptions. This result is checked with the use of organic and inorganic ultra-thin films of increasing thicknesses.

Investigations at the air/water interface using polarization modulation IR spectroscopy

The ability of polarization modulation IR reflection absorption spectroscopy (PM–IRRAS) to study the air/water interface is presented. A brief description of the set-up and of the experimental procedure is given. Theoretical simulations accounting for the uniaxial nature of the spread monolayer lead to optimum experimental conditions (71 ° for the angle of incidence) and to a specific surface selection rule. Application to the study of cadmium arachidate, dimyristolyl phosphatidylcholine (DMPC) and polypeptidic Langmuir films illustrates the potential uses of this method.

Polarization modulation FTIR spectroscopy at the air-water interface

Abstract Mid-infrared spectra of monolayers spread at the air-water interface have been obtained, completely devoid of strong water vapor absorptions, using polarization modulation infrared reflexion absorption spectroscopy (PM-IRRAS). On normalized difference (covered vs. uncovered water) PM-IRRAS spectra, the monolayer absorption bands appear upwards or downwards depending on the orientation of their transition moment with respect to the water surface. Vibrational modes of the water subphase contribute to these difference spectra as broad dips. Study of a monolayer of cadmium arachidate has allowed observation of the vibrational modes of the polar heads and provides some evidence of their symmetrical anchoring at the water surface. Under surface compression, a monolayer of deuterated arachidic acid undergoes a molecular reorganization leading to a better ordering of the deuterated chains.

Vibrational Circular Dichroism in General Anisotropic Thin Solid Films: Measurement and Theoretical Approach

In this study, the measurement of the true vibrational circular dichroism (VCD) spectrum is considered from an experimental and theoretical approach for any general anisotropic thin solid sample exhibiting linear as well as circular birefringence (LB, CB) and dichroism (LD, CD) properties. For this purpose, we have made use of a simple model α-helix polypeptide, namely, the poly(γ-benzyl-L-glutamate) or PBLG, reference sample possessing a well-known VCD spectrum and giving rise to slightly oriented films by deposition onto a solid substrate. Also, we have used a different Fourier transform infrared modulation of polarization (PM-FTIR) optical setup with two-channel electronic processing in order to record the PM-VLD and PM-VCD spectra for various sample orientations in its film plane. All the corresponding general relations of the expected intensities in these experiments and the related properly designed calibration measurements were established using the Stokes–Mueller formalism; in addition, the residual birefringence of the optical setup and the transmittance anisotropy of the detector were estimated. From a comparative study of the results obtained in solution and in the solid state, we then propose a simple new experimental procedure to extract the true VCD spectrum of an oriented PBLG thin film: its consists of calculating the half-sum of two spectra recorded at θ and at θ ± 90° sample orientations. Moreover, the complete linear and circular birefringence and dichroism properties of the ordered PBLG thin film are estimated in the amide I and amide II vibrational regions. This allows us to establish for any sample orientation various theoretical simulations of the VCD spectra that agree nicely with the observed experimental results; this confirms that the measurement of LD and LB is in this case a prerequisite in simulating the true VCD spectrum of a partly oriented anisotropic sample. This validates our combined experimental and theoretical approach and opens the route to promising future vibrational CD studies on other macroscopic anisotropic thin film samples.

Quantitative Orientation Measurements in Thin Lipid Films by Attenuated Total Reflection Infrared Spectroscopy

Quantitative orientation measurements by attenuated total reflectance (ATR) infrared spectroscopy require the accurate knowledge of the dichroic ratio and of the mean-square electric fields along the three axes of the ATR crystal. In this paper, polarized ATR spectra of single supported bilayers of the phospholipid dimyristoylphosphatidic acid covered by either air or water have been recorded and the dichroic ratio of the bands due to the methylene stretching vibrations has been calculated. The mean-square electric field amplitudes were calculated using three formalisms, namely the Harrick thin film approximation, the two-phase approximation, and the thickness- and absorption-dependent one. The results show that for dry bilayers, the acyl chain tilt angle varies with the formalism used, while no significant variations are observed for the hydrated bilayers. To test the validity of the different formalisms, s- and p-polarized ATR spectra of a 40-A lipid layer were simulated for different acyl chain tilt angles. The results show that the thickness- and absorption-dependent formalism using the mean values of the electric fields over the film thickness gives the most accurate values of acyl chain tilt angle in dry lipid films. However, for lipid monolayers or bilayers, the tilt angle can be determined with an acceptable accuracy using the Harrick thin film approximation. Finally, this study shows clearly that the uncertainty on the determination of the tilt angle comes mostly from the experimental error on the dichroic ratio and from the knowledge of the refractive index.

Enantiopure dendritic polyoxometalates: chirality transfer from dendritic wedges to a POM cluster for asymmetric sulfide oxidation.

The quest for new catalytic and highly enantioselective processes is currently one of the most intensively studied areas in chemical synthesis. In this context, chiral polyoxometalates (POMs) have become a topic of recent interest owing to their potential application in medicine and asymmetric catalysis. Among numerous applications of POMs, catalysis is by far the most studied, owing to the enormous versatility that POMs offer in the clean synthesis of fine chemicals. However, few chiral POM compounds for asymmetric catalysis are known. Of those reported, none are based on dendritic structures, although dendritic counter cations are known to play a critical role in determining the properties of anionic POMs. Two main approaches have been developed for the synthesis of chiral POM-based frameworks. The first involves the use of a chiral species (organic moieties or metal complexes) as a chirality transfer agent. The second strategy is based on spontaneous resolution upon crystallization in the absence of any chiral auxiliary, which yields conglomerates. For example, enantiopure hafnium-substituted POMs, which are obtained by spontaneous resolution upon crystallization in the absence of any chiral source have been reported. Recently, two enantiomerically pure 3D chiral POM-based architectures were developed from achiral moieties without any chiral auxiliaACHTUNGTRENNUNGry.[6b–c] However, despite their potential applications in catalysis and separation, enantiopure POM frameworks are still under represented. The search for suitable enantiomerically pure POM-based hybrid materials remains a challenging issue in synthetic chemistry and materials science. To date, chiral dendritic POM systems, as well as the study of chiroptical activity of enantiopure POM-based frameworks in asymmetric catalysis have, to the best of our knowledge, never been reported. Herein, we report the synthesis and characterization of four enantiopure polyoxometalate-cored dendrimers, which contain n-propyl [(R)-(+)-6 and (S)-( )-6] and epoxy [(R)(+)-7 and (S)-( )-7] groups, based on electrostatic interactions between enantiopure dendritic quaternary ammonium ions and an achiral trianionic POM. To the best of our knowledge, these compounds represent the first examples of optically active dendritic POM systems. Importantly, the POM cluster displays chiroptical effects, indicating chirality transfer from the enantiopure dendritic ammonium ions. The optical and chiroptical properties of the n-propyl-terminated POM-cored dendrimers (R)-(+)-6 and (S)-( )-6 have been demonstrated in solution by UV/Vis spectroscopy, circular dichroism (CD) and vibrational circular dichroism (VCD) spectroscopy, as well as fluorimetry. In addition, the use of 6 as a catalyst in the asymmetric oxidation of thioanisole with aqueous H2O2 provides the corresponding optically active sulfoxide with up to 14% enantiomeric excess (ee) as a result of a chirality transfer process from the dendritic wedge to the catalytically active POM unit. Despite the modest enantioselectivity, this proof-of-principle catalytic experiment demonstrates and confirms the transfer of chiroptical properties from organic moieties to a catalytically active POM unit. Interestingly, the catalyst was recovered and reused in three successive cycles without discernable [a] C. Jahier, Dr. M. Cantuel, Dr. N. D. McClenaghan, Dr. T. Buffeteau, Dr. D. Cavagnat, Dr. S. Nlate ISM, UMR CNRS No. 5255, Universit Bordeaux I 351 Cours de la Lib ration, 33405 Talence Cedex (France) Fax: (+33)5-4000-6994 E-mail : s.nlate@ism.u-bordeaux1.fr [b] Dr. F. Agbossou UCCS UMR CNRS N8 8181, Universit de Lille 59652 Villeneuve d’Asq Cedex (France) [c] Dr. M. Carraro, Prof. M. Bonchio Department of Chemical Sciences and ITM-CNR University of Padova, Via Marzolo, 1 35131 Padova (Italy) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200901512.

Imidazolium-based ionic liquids: quantitative aspects in the far-infrared region.

The optical constants of some imidazolium-based ionic liquids (ILs) are determined in the mid- and far-infrared regions by combining polarized attenuated total reflection (ATR) and transmittance spectra. The internal vibrations of the cations and anions and the interionic vibrations can thus be quantitatively evaluated. A comparison of the far-IR spectral response of several imidazolium derivatives associated with the (CF(3)SO(2))(2)N(-) anion shows that methylation of the more acidic C((2))H imidazolium group does not change the far-IR intensity and hence that the CH...anion hydrogen bonds play a negligible role compared with electrostatic interactions. The calculated spectra of ion-pair dimers reproduce the far-IR density of states better than those of simple ion pairs.

Enantioselective Complexation of Chiral Propylene Oxide by an Enantiopure Water-Soluble Cryptophane

ECD and NMR experiments show that the complexation of propylene oxide (PrO) within the cavity of an enantiopure water-soluble cryptophane 1 in NaOH solution is enantioselective and that the (R)-PrO@PP-1 diastereomer is more stable than the (S)-PrO@PP-1 diastereomer with a free energy difference of 1.7 kJ/mol. This result has been confirmed by molecular dynamics (MD) and ab initio calculations. The enantioselectivity is preserved in LiOH and KOH solutions even though the binding constants decrease, whereas PrO is not complexed in CsOH solution.

C3-symmetrical self-assembled structures investigated by vibrational circular dichroism.

We demonstrate by using vibrational circular dichroism (VCD) spectroscopy that it is possible to investigate the chirality of a supramolecular polymeric system in relatively dilute solutions. Chiral C(3)-symmetrical discotic molecules, based on a trialkylbenzene-1,3,5-carboxamide, form supramolecular columnar stacks with a right-handed helical structure in solution due to intermolecular hydrogen bonds. The handedness of the supramolecular chirality is determined using electronic spectroscopy measurements. Under dilute conditions (at 10(-3) M concentrations), it was also possible to probe the hydrogen bonding moieties with IR and VCD spectroscopy on these self-assembled structures. In combination with density functional theory (DFT) calculations, we could verify the preference for a right-handed chirality in the helical stacks and the nonplanar orientation of the carbonyl groups present in the molecule. This chiral arrangement is in agreement with the structure determined for a related benzene-1,3,5-tricarboxamide by X-ray diffraction. Chirality, 2008. (c) 2008 Wiley-Liss, Inc.

Thin-Film Optical Constants Determined from Infrared Reflectance and Transmittance Measurements

A general method based upon reflectance and transmittance measurements in the infrared region has been developed for the determination of the optical constants n(v) and k(v) of thin films deposited on any substrate (transparent or not). The corresponding computer program, written in FORTRAN 77, involves three main parts: (1) a matrix formalism to compute reflection and transmission coefficients of multilayered systems; (2) an iterative Newton-Raphson method to estimate the optical constants by comparison of the calculated and experimental values; and (3) a fast Kramers-Krönig transform to improve the accuracy of calculating the refractive index. The first part of this program can be used independently to simulate reflection and transmission spectra of any multilayered system using various experimental conditions. Two practical examples are given for illustration. Simulation of reflection spectra at grazing incidence for thin films deposited on a metal surface and determination of the optical constants for thin CaF2 layers deposited on a silicon substrate are presented.