Relation between s-polarized and p-polarized internal reflection spectra: application for the spectral resolution of perpendicular vibrational modes

The orientation of chemical bonds in thin films is commonly probed with polarized internal reflection Fourier transform infrared spectroscopy. Here we demonstrate how the internal reflection spectra obtained using s-polarized light are related with the corresponding p-polarized spectra. The relation between s- and p-polarized internal reflection spectra is applied to resolve the absorption bands of vibrational modes with components oriented perpendicular to the substrate. This is successfully demonstrated using Langmuir-Blodgett films containing smectite clay minerals and a surfactant deposited on ZnSe and Ge internal reflection elements.     

Measurement of ordered associate of protonated tetraphenylporphine formed at toluene/aqueous H2SO4 interface by attenuated total internal reflection spectroscopy with polarized light

Attenuated total internal reflection (ATR) spectroscopy with an s- or p-polarized visible light was examined for some species of protonated 5,10,15,20-tetraphenylporphine (tpp) at toluene/aqueous H₂SO₄ (3-6 mol dm⁻³) interface. Tpp initially dissolved in the toluene phase was diprotonated at the interface to form monomeric H₂tpp²⁺, the absorption peak of which was 438 nm. At the same time, a long H₂tpp²⁺ oligomer was formed, the absorption peak of which was 448 nm. The two interfacial species were transient. Just after their disappearance, a rod-shaped H₂tpp²⁺ associate was formed at the interface, the absorption peak of which was 417 and 478 nm. The former and latter wavelengths corresponded to H- and J-bands of the associate, respectively. Theoretical calculation of the strength of electric field of light at the interface allowed one to estimate the interfacial concentration of the three species with measured reflection absorbance (A_R). The monomeric H₂tpp²⁺ and its oligomer were at sub-monolayer levels, whereas the associate was at a multilayer level. Reflection absorption anisotropy (K_R), which was calculated from A_R with the s- and p-polarized lights, was adopted for the evaluation of out-of-plane orientation of the interfacial species for the first time. The K_R value suggested that the rod-shaped associate lay at the interface.     

Adsorption Behavior of a Water-soluble Porphyrin at the Glass-water Interface as Studied by Synchronous Total Internal Reflection Fluorescence Spectroscopy

Total imernal reflection fluorescence spectroscopy (TIRF) and synchronous scanning technique were combined to study the adsorption behavior of the meso-tetrakis (4-sulfonatophenyl)porphyrin (TPPS) at the glass-water interface without any surfactant. The pH dependence of synchronous fluorescence signal at the interface was analyzed. Both unprotonated (TPPS^4-) and diprotonated (H2TPPS^2-) forms of TPPS were observed at the interface. But the interface favored the adsorption of. The apparent estimated pKa2 value shifted from 5.00 in the bulk solution to 2.7 at the interface. STIRF provides a good technique to study multi-component systems at the interface.     

IR reflection spectra of monolayer films sandwiched between two high refractive index materials

Monolayer films adsorbed on substrates with high refractive indices such as metals or semiconductors yield strongly enhanced infrared reflection spectra when they are contacted with a transparent, high refractive index ambient medium and are probed with p-polarized light at high incidence angles. The sensitivity increase arises from the enhancement of the perpendicular electric field within a thin, low refractive index layer sandwiched between two high refractive index materials and gives rise to signal intensity gains up to 2 orders of magnitude in combination with an essentially exclusive detection of only perpendicular surface vibrations. Experimental spectra of ordered monolayer films of octadecanethiol on gold and of octadecylsiloxane on silicon in this sandwich configuration yield enhancement factors between 15 (on Si) and 30 (on gold) compared to conventional grazing incidence external reflection spectra and are governed by a common, simple surface selection rule, which allows immediate quantitative evaluation and comparison of the film structures on different substrates.     

Infrared studies of pyroelectric Langmuir–Blodgett films of arachidic acid and 1,2-bis(dodecyloxy)-4,5-diaminobenzene

Fourier Transform Infrared (FTIR), p-polarized grazing angle (GAIR) and Horizontal Attenuated Total Reflectance (HATR) spectra have been recorded of arachidic acid (AA)/1,2-bis(dodecyloxy)-4,5-diaminobenzene (DADB) Y-type alternate LB films deposited on an aluminium plate with 31 layers. It is well known that the frequencies of CH₂ stretching bands of a hydrocarbon chain are sensitive to the conformational ordering of the chain. Changes in frequency and intensity can be used to characterize film ordering and preferential molecular packing. The observed peak frequencies and intensities of these bands indicate that the alkyl chains are present in a mostly trans conformation and tilted from the normal direction with respect to the substrate in LB films. The FTIR–GAIR and HATR spectra of 31 layers alternate film show significant changes in the region 1700–1400 cm⁻¹ due to the partial proton transfer between acid and amine head groups. According to the HATR spectrum, the peak at 1731 cm⁻¹ is observed due to a proportion of the carboxylic acid groups forming sideways dimers indicating that if the carboxylic acid groups form sideways dimers, they are less likely to undergo proton transfer with the amino groups.     

Quantitative SNIFTIRS studies of (bi)sulfate adsorption at the Pt(111) electrode surface

Subtractively normalized interfacial Fourier transform infrared reflection spectroscopy (SNIFTIRS) was applied to study (bi)sulfate adsorption on a Pt(111) surface in solutions of variable pH while maintaining a constant total bisulfate/sulfate ((bi)sulfate) concentration without the addition of an inert supporting electrolyte. The spectra were recorded for both the p- and s-polarizations of the IR radiation in order to differentiate between the IR bands of the (bi)sulfate species adsorbed on the electrode surface from those species located in the thin layer of electrolyte. The spectra recorded with p-polarized light consist of the IR bands from both the species adsorbed at the electrode surface and those present in the thin layer of electrolyte between the electrode surface and ZnSe window whereas the s-polarized spectra contain only the IR bands from the species located in the thin layer of electrolyte. A new procedure was developed to calculate the angle of incidence and thickness of the electrolyte between the Pt(111) electrode surface and the ZnSe IR transparent window. By combining these values with the knowledge of the optical constants for Pt, H(2)O and ZnSe, the mean square electric field strength (MSEFS) at the Pt(111) electrode surface and for thin layer of solution were accurately calculated. The spectra recorded using s-polarization were multiplied by the ratio of the average MSEFS for p- and s-polarizations and subtracted from the spectra recorded using p-polarization in order to remove the IR bands that arise from the species present within the thin layer cavity. In this manner, the resulting IR spectra contain only the IR bands for the anions adsorbed on the Pt(111) electrode surface. The spectra of adsorbed anions show little change with respect to the pH ranging from 1 to 5.6. This behavior indicates that the same species is predominantly adsorbed on the metal surface for this broad range of pH values and the results suggest that sulfate is the most likely candidate for this adsorbate.     

Raman microprobe spectrometer installed in a super-conducting magnet.

A Raman microprobe spectrometer that could be installed in the bore of a cryogen free super-conducting magnet (10 T) was designed and constructed for the investigation of the external magnetic field effect on the Raman spectra of molecular aggregates in solutions and at interfaces. The performance of the present instrument was demonstrated by measuring the magnetic field effect (0 - 10 T) on the resonance Raman spectra of diprotonated meso-tetra-(sulfonatophenyl)porphine aggregates in an acidic solution. The Raman shifts of the aggregates were not significantly influenced even in 10 T. However, the relative intensity of 1123 cm(-1) peak (nu(C(a)-N)) was interestingly enhanced about 20% under the magnetic fields higher than 2.5 T.     

Lowest electronic excited states of platinum(II) diimine complexes

Absorption and emission spectra of Pt(diimine)L2 complexes (diimine = 2,2'-bipyridine (bpy) or 4,4'-dimethyl-2,2'-bipyridine (dmbpy); L = pyrazolate (pz-), 3,5-dimethylpyrazolate (dmpz-), or 3,4,5-trimethylpyrazolate (tmpz-)) have been measured. Solvent-sensitive absorption bands (370-440 nm) are attributed to spin-allowed metal-to-ligand charge-transfer (1MLCT) transitions. As solids and in 77 K glassy solution, Pt(bpy)(pz)2 and Pt(dmbpy)(pz)2 exhibit highly structured emission systems (lambda max approximately 494 nm) similar to those of the diprotonated forms of these complexes. The highly structured bands (spacings 1000-1400 cm-1) indicate that the transition originates in a diimine-centered 3(pi-->pi*) (3LL) excited state. The intense solid-state and 77 K glassy solution emissions from 3MLCT[d(Pt)-->pi*(bpy)] excited states of complexes with dmpz- and tmpz- ligands occur at longer wavelengths (lambda max = 500-610 nm), with much broader vibronic structure. These findings are consistent with increasing electron donation of the pyrazolate ligands, leading to a distinct crossover from a lowest 3LL to a 3MLCT excited state.     

Infrared reflectance as a tool to reveal preferential molecular orientation of polymers adsorbed onto flat substrates

FTIR-Reflectance experiments have been made on thin ethylene-vinyl acetate (EVA) copolymer layers deposited on aluminum mirrors in order to determine orientation of polymer functional groups at the interface. This was accomplished by using various reflection angles under p polarization state of the incident IR beam. Film thicknesses were estimated by ellipsometric experiments. Kramers-Kronig analysis is first applied to the external infrared reflection spectrum from a single copolymer surface measured near the normal incidence angle. Absorption spectra, k(v), are then deduced and used to calculate specular reflectance intensities of the functional groups of interest as a function of incidence angle, polarization state and film thickness. The calculated values are compared to those observed. A layer model is developed, which allows the molecular orientation of both EVA carbonyl groups and the main chain axis at the interface to be determined. Only carbonyl groups involved in specific electron donor-electron acceptor interactions at the interface appear to be subject to specific orientation. A persistence thickness of the preferential orientation in the film is determined, also from which it is concluded that even in nanofilms, preferential molecular orientation induced by an hydroxylated substrate does not persist throughout the film thickness. It is localized at a near interfacial region, the thickness of which depends on the ability of the comonomer to undergo conformational changes.     

Excited states of porphyrin macrocycles

S1 --> S(n) spectra of porphyrin, diprotonated porphyrin, and tetraoxaporphyrin dication have been measured in the energy range 2-3 eV above S1 at room temperature in solution by means of transient absorption spectroscopy exciting with femtosecond pulses. Highly excited pi pi* states not active in the conventional S0 --> S(n) spectrum have been observed. The experimental data are discussed on the basis of the time dependent density functional theory taking advantage of large scale calculations of configuration interaction between singly excited configurations (DF/SCI). The DF/SCI calculation on porphyrin has allowed to assign g states active in the S1 --> S(n) spectrum. Applying the same calculation method to tetraoxaporphyrin dication the S0 --> S(n) spectrum is reproduced relatively to the Q and B (Soret) bands as well as to the weaker E(u) bands at higher energy. According to our calculation the S1 --> S(n) transient spectrum is related to states of g symmetry mainly arising from excitations between doubly degenerate pi and pi* orbitals such as 2e(g) --> 4e(g). In the case of diprotonated porphyrin it is shown that the complex of the macrocycle with two trifluoroacetate anions plays a significant role for absorption. Charge transfer excitations from the anions to the macrocycle contribute to absorption above the Soret band, justifying the intensity enhancement of the S0 --> S(n) spectrum with respect to the other two macrocyclic systems.     

Chiral recognition of 2-alkyl alcohols with porphyrin J-nanoaggregates at the liquid-liquid interface

The J-aggregate of diprotonated tetraphenylporphyrin (H(4)TPP(2+)) formed at the dodecane-water interface showed circular dichroism spectra corresponding to the chirality of 2-alkyl alcohols, longer than 2-butanol, added to the dodecane phase. The phenomenon suggested the preferential interaction between the nano-sized J-aggregates and the chiral alcohols at the interface, and provided a potential use of the J-nanoaggregate as a chiral recognition probe.     

Molecular interaction of polyimide films probed by using soft-pulse dynamic compression ATR time-resolved infrared and double Fourier-transform based 2D-IR spectroscopy

An improved time-resolved step-scan Fourier transform rheo-optical system, based on the soft-pulse dynamic compression attenuated total reflection (ATR) approach, has been introduced to analyze inter-molecular interactions of poly(p-phenylene bi-phenyltetracarboximide) (BPDA–PDA) and poly(N,N-diphenyl ether bi-phenyltetracarboximide) (BPDA–ODA) films. In order to extract the full range of frequency distributions obtained from soft-pulse dynamic compression time response (relaxation) spectra, the second-Fourier transformation was applied along the relaxation time axis in addition to the interferogram axis. In the case of the BPDA–PDA, distinct periodic higher overtone bands are observed especially at C=O stretching and C=C tangential stretching vibration bands in the FFT relaxation time contour map spectra. For the BPDA–ODA, intensities of higher overtone bands are relatively weak. We have further performed 2D-IR correlation analysis at given Fourier frequencies, which includes molecular interaction information. The results suggest that the existence of molecular interaction between C=O stretching and C=C tangential stretching vibration bands.     

Infrared reflection-absorption spectra of C2H4 and C2H6 on Cu: effect of surface roughness

Infrared reflection absorption spectroscopy (IRRAS) of the highly symmetric molecules C2H4 and C2H6 adsorbed as mono- and multilayers onto copper films is studied in relation to the type of metal-film roughness. Spectra of C2H4 show Raman lines on cold-deposited Cu films but not on Cu deposited at room temperature. For C2H6, the IR spectra from both types of metal films are similar; the surface infrared selection rule holds and no Raman bands are observed. The Raman lines that appear in the IR spectra already at low exposures are attributed to species adsorbed at special defect sites, identical to the so-called active sites in surface enhanced Raman scattering (SERS). The IR excitation mechanism by transient electron transfer to the adsorbate pi* state can deliver a discrete vibrational band of a Raman-active vibration only under certain circumstances, for example, for adsorbates at the "SERS-active sites". C2H6 at these sites cannot deliver Raman bands in IRRAS, because it has no pi* state. We also discuss IRRAS measurements on Cu(111) and Cu(110) single crystals, where Raman bands of C2H4 have been observed.     

Excitonic interactions in covalently-linked porphyrin dimers

We have studied the absorption spectra, emission spectra, and fluorescence excitation polarization spectra of a series of free base and diprotonated etioporphyrin-I dimers covalently linked through (CH₂)_n bridges, n = 0–8. The absorption spectra of the n = 0 and n = 1 dimer show red shifts, which are largest (≈15 mm) for the Soret band of the n = 0 dimer. The Soret bands of the diprotonated dimers n = 0–3 show splitting (≈500–1000 cm^?1) which can be interpreted by an exciton model assuming a reasonable geometry. The fluorescence spectra and quantum yields are similar to that of the monomer, except for the same red shift seen in absorption; however, the n = 0 diprotonated dimer shows an anomalo vibronic structure. The fluorescence excitation polarization spectra for the n = 0 and the n = 1 dimers differ substantially from the monomer; dimers n ? 3 have fluorescence excitation polarization spectra that suggest that some of the excitation stays localized in one moiety while the r hops to the dimer partner.     

Structures of palmitoyl-L and DL-lysine monolayers at the air-water interface--polarization modulation infrared reflection absorption spectroscopic study

Polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) was applied to measure the IR spectra of palmitoyl-DL-lysine (L-PL) and palmitoyl-DL-lysine (DL-PL) at the air-water interface. The spectra in the amide I and II regions were simulated by using the extinction coefficients of the amide I and II bands of L-PL and DL-PL determined by the analyses of the IR external reflection spectra of the Langmuir-Blodget (LB) films prepared on a Ge plate (Yasukawa et al. J. Mol. Struct. 2005, 735-736, 53), indicating the angle between the plane of the secondary amide group (the amide plane) and the surface normal in the L-PL monolayer to be about 20 degrees and the angle in the DL-PL monolayer to be about 37 degrees. Comparison of the tilt angles with the corresponding angles in the LB films (about 20 degrees for the LB film of L-PL; about 49 degrees for the LB film of DL-PL) indicated that, upon being transferred to the solid substrate from the air-water interface, the L-PL monolayer keeps the orientation of the amide plane virtually unchanged, while the DL-PL monolayer changes the orientation appreciably to a horizontal direction. The orientation change of the amide plane was interpreted as due to the accommodation of irregularly oriented palmitoyl groups into the LB films of DL-PL on the solid substrate.     

An approach to compatible multiple nonlinear vibrational spectroscopy measurements using a commercial sum frequency generation system

In this paper, we designed a compatible multiple nonlinear vibrational spectroscopy system that can be used for recording infrared-visible sum frequency generation vibrational spectra (SFG) and infrared-infrared-visible three-pump-field four-wave-mixing (IIV-TPF-FWM) spectra using a commercial EKSPLA SFG system. This is the first time IIV-TPF-FWM signals were obtained using picosecond laser pulses. We have applied this compatible system to study the surface and vibrational structures of riboflavin molecules (also known as vitamin B2). The SFG spectra of eight polarization combinations have non-vanishing signals. The signals with incoming s-polarized IR are relatively weaker than the signals with incoming p-polarized IR. Under the double resonant conditions, the SFG signals of the conjugated tricyclic ring are greatly enhanced. For the IIV-TPF-FWM spectra with incoming p-polarized IR, only the sspp and pppp polarization combinations have non-vanishing signals. The IIV-TPF-FWM spectra show a very strong peak at 1585 cm(-1) that is mainly dominated by the N(5)-C(4a) stretch. The method developed in this study will be helpful for researchers, either using a home-built or commercial (EKSPLA) SFG system, to obtain independent and complementary measurements for SFG spectroscopy and more detailed structural information of interfacial molecules.     

Surface orientation of polypropylene. I. Theoretical considerations for the application of internal reflection spectroscopy

A theoretical consideration of the orientation of the transverse magnetic and the transverse electric polarization vectors of the evanescent field in attenuated total reflection spectroscopy was made. It was demonstrated that near the critical angle of total reflection the transverse magnetic polarization vector approaches a purely normal orientation to the interface between the sample and internal reflection element surfaces, while the transverse electric vector is parallel to this interface. With an appropriate experimental design a set of three orthogonal electric vectors can be obtained for the analysis of surface orientations. A method to obtain high-quality infrared spectra of polypropylene surfaces at a fraction of a degree above the critical angle was developed using a KRS-5 internal reflection element specially cut with a 40° entrance aperture. The surface orientation of uniaxially extended polypropylene films was studied by using the proposed experimental design. It was shown that the orientation of the helix axis of the macromolecular chains of polypropylene could be qualitatively detected. A method employing the 841- and 809-cm^?1 bands of polypropylene was used to show that the ratio of absorbances of these bands reflects the surface orientation of the uniaxially extended polypropylene films for a set of films extended 0–500%.     

Primary intermediates of oxygen photoevolution reaction on TiO2 (Rutile) particles, revealed by in situ FTIR absorption and photoluminescence measurements

Primary intermediates of oxygen photoevolution (water photooxidation) reaction at the TiO2 (rutile)/aqueous solution interface were investigated by in situ multiple internal reflection infrared (MIRIR) absorption and photoluminescence (PL) measurements. UV irradiation of TiO2 in the presence of 10 mM Fe3+ in the solution caused the appearance of a new peak at 838 cm(-1) and a shoulder at 812 cm(-1). Detailed investigations of the effects of solution pH, the presence of methanol as a hole scavenger, and isotope exchange in water (H2(16)O-->H2(18)O) on the spectra have shown that the 838- and 812-cm(-1) bands can be assigned to the O-O stretching mode of surface TiOOH and TiOOTi, respectively, produced as primary intermediates of the oxygen photoevolution reaction. The results give strong support to our previously proposed mechanism that the oxygen photoevolution is initiated by a nucleophilic attack of a H2O molecule on a photogenerated hole at a surface lattice O site, not by oxidation of surface OH group by the hole. The conclusion is supported by PL measurements. A plausible reaction scheme is proposed for the oxygen photoevolution on TiO2 (rutile) in aqueous solutions of pH less than about 12.     

Tuning delocalization in the radical cations of 1,4-bis[4-(diarylamino)styryl]benzenes, 2,5-bis[4-(diarylamino)styryl]thiophenes, and 2,5-bis[4-(diarylamino)styryl]pyrroles through substituent effects

Radical cations have been generated for 10 bis[4-(diarylamino)styryl]arenes and heteroarenes to investigate the effect of the electron-richness of the terminal groups and of the bridging (hetero)arene on delocalization. The intervalence charge-transfer bands of these radical cations vary from weak broad Gaussians, indicative of localized class-II mixed-valence species, to strong relatively narrow asymmetric bands, characteristic of delocalized class-III bis(diarylamino) species, to narrow symmetric bands in cases where the bridge contribution to the singly occupied molecular orbital is largest. Hush analysis of these bands yields estimates of the electronic coupling varying from 480 cm(-1) (electron-poor bridge, most electron-rich terminal aryl groups) to 1000 cm(-1) (electron-rich bridge, least electron-rich termini) if the diabatic electron-transfer distance, R(ab), is equated to the N-N separation. Computational and electron spin resonance (ESR) evidence for displacement of the diabatic states into the bridge (reduced R(ab)) suggests that these values are underestimates and that even more variation is to be expected through the series. Several dications have also been studied. The vis-NIR absorption of the dication of (E,E)-1,4-bis{4-[bis(4-n-butoxyphenyl)amino]styryl}-2,5-dicyanobenzene is seen at an energy similar to that of the strongest band in the spectrum of the corresponding weakly coupled monocation, with approximately twice the absorptivity, and its ESR spectrum suggests essentially noninteracting radical centers. In contrast, the electronic spectra of class-III monocations show no clear relationship to those of the corresponding dications, which ESR reveals to be singlet species.     

Study of π-adsorption by the electroreflection method

The electroreflection spectra for lead and indium electrodes in polarized light have been obtained in a solution containing aniline, benzene and 5-bromo-2-acetylthiophene. In the presence of the above-mentioned additions a new minimum appears at 2.5-2.8 eV in the spectra obtained in p-polarized light. It corresponds to the charge-transfer band in the absorption spectra for adsorbate/electrode complexes. The electrode potential has no influence on the position of this minimum.