Changes In Band Shape Research Articles

Carrier Dynamics in TiO2 and Pt/TiO2 Powders Observed by Femtosecond Time-Resolved Near-Infrared Spectroscopy at a Spectral Region of 0.9−1.5 μm with the Direct Absorption Method

Femtosecond time-resolved near-infrared spectra of TiO2 and Pt/TiO2 powders are measured at the wavelength region of 0.9−1.5 μm with the direct absorption method. Broad absorption bands of charge carriers, mainly free and trapped electrons, are observed for TiO2 and Pt/TiO2. The absorption band shape changes with a time constant of 160 fs after the photoirradiation, which most probably reflects the trapping of the generated free electrons. Population decay curves of the carriers after 1 ps, obtained for three different pump light powers, are well-explained by the second-order decay kinetics with a common second-order rate constant, indicating nongeminate recombination of the electron−hole pairs. When Pt is loaded to TiO2, an additional decaying process of 2.3 ps is observed. This decay component represents the transfer of generated electrons from TiO2 to Pt, which is consistent with the known increase of overall catalytic activities by the Pt cocatalyst.

Application of the time-resolved spectroscopy on the photo-dissociation dynamics of disulfide compounds in supercritical fluids

Photo-dissociation dynamics of two disulfide compounds [bis( p-aminophenyl) disulfide (BADS) and bis( p-dimethylaminophenyl) disulfide (BDMADS)] in supercritical carbon dioxide and trifluoromethane were studied by transient absorption spectroscopy. For bis( p-aminophenyl) disulfide in carbon dioxide, we found that the absorption bandshape of the p-aminophenylthiyl (APT) radical generated by the photo-dissociation shows an asymmetrical change with a time constant of about 40 ps, as was reported in the case of nonpolar solvents (Y. Hirata, Y. Niga, S. Makita, T. Okada, J. Phys. Chem. A. 101 (1997) 561). For (bis( p-dimethylaminophenyl) disulfide in carbon dioxide, we did not observe an evident change of the absorption bandshape of the p-dimethylaminophenylthiyl radical. On the other hand, we found that the absorption peak position of the radical in trifluoromethane shows a red-shift in the initial 40 ps. The bandshape change and the peak shift were discussed in terms of the radical pair dynamics and the solvation dynamics.

Femtosecond electron transfer dynamics of 9,9′-bianthryl in acetonitrile as studied by time-resolved near-infrared absorption spectroscopy

Photoinduced intramolecular electron transfer reaction in 9,9′-bianthryl has been studied with femtosecond time-resolved near-infrared absorption spectroscopy. In acetonitrile, a strong locally excited (LE) band at 1020 nm and a broad charge transfer (CT) band at 1250 nm are observed simultaneously. The decay of the LE band and the rise of the CT band have time constants of (0.38 ± 0.01) and (0.3 ± 0.1) ps, respectively. There is no intermediate with a sub-picosecond lifetime observed between the LE and CT states. In heptane, the LE band shape changes with a time constant of 1.4 ps, which probably reflects a conformational change around the central C–C bond.

Synthesis and cofluorescence of Eu(Y) complexes with salicylic acid and o-phenanthroline

A series of dinuclear complexes of salicylic acid (HSal) and o-phenanthroline (Phen) with different molar ratios of Eu 3+ to Y 3+ have been synthesized. Their compositions are Eu x Y 1− x (Sal) 3(Phen) ( x = 0 ∼ 1). Their UV spectra, IR spectra, and fluorescence spectra were studied. The UV spectra of the complexes reflect essentially absorption of the ligands for the fact that no obvious change of wavelength and band shape is found between the spectra of the complexes and that of the ligands except slight red shift. The IR absorption spectra indicate that salicylic acid is coordinated to the rare earth ions and chemical bonds are formed between rare earth ions and nitrogen atoms of o-phenanthroline. The fluorescence spectra of the complexes indicate that the fluorescence emission intensity of europium ion was enhanced by the addition of Y 3+, which is referred to as cofluorescence. These facts show that not only the ligands but also the yttrium complex can transfer the absorbed energy to Eu 3+ ion in the complexes. Formation of polynuclear complexes appears to be responsible for cofluorescence.

Diblock copolymer adsorption from the aqueous micellar phase to solid surfaces: real time monitoring by ATR spectroscopy in the mid‐infrared

AbstractAttenuated Total Reflectance (ATR) in the mid‐infrared is employed to monitor the adsorption of poly(tert‐butyl styrene ‐ b ‐ styrene sulfonate), PtBS‐PSS, copolymers with a small hydrophobic head as well as of an analogous poly(styrene sulfonate), PSS, homopolymer on germanium and Au‐plated germanium surfaces from aqueous NaCl solutions. The surface density of the adsorbed polyelectrolyte is monitored via the growth of characteristic infrared absorptions of the sulfonated ring at 1036‐ and 1008 cm−1 in the 2nd derivative mode. These probe bands do not exhibit shifts or changes in bandshape over very prolongued adsorption experiments. Pronounced differences in the kinetics of adsorption are observed between the PSS homopolymer and the PtBS‐PSS copolymers in agreement to previous investigations by phase‐modulated ellipsometry on similar systems. Adsorption of the diblocks above the critical micelle concentration is found to involve a sequence of diffusion, micellar relaxation and brush‐limited processes.

Overloading study of bases using polymeric RP-HPLC columns as an aid to rationalization of overloading on silica-ODS phases.

The separation of ionized bases by reversed-phase liquid chromatography with alkyl silica columns often leads to severely tailed bands that are highly detrimental. Band shape and its dependence on sample mass are notably different when mobile-phase pH is changed, and this behavior has not been previously explained. Ionized silanols present in the stationary phase have been credited with a role in determining peak shape. In the present study, separations on two different polymer columns were compared with those previously obtained on alkyl silica phases. Because silanols are absent from polymer columns, this comparison enabled us to assess the role of silanols in separations on alkyl silica phases and to offer an explanation of why band shape changes with sample size and mobile-phase pH for both polymer and silica-based phases.

Physical Chemistry of the Lowest Excited Singlet State of trans-Stilbene in Solution as Studied by Time-Resolved Raman Spectroscopy

Abstract Structure and dynamics of the lowest excited singlet state of trans-stilbene (the S1 state of tSB) in solution are discussed with an emphasis placed on the elucidation of the mechanism of the trans–cis photoisomerization. Structure of S1 tSB in solution is first discussed based on the time-resolved Raman spectral data and their isotope substitution effects. Original data for an asymmetrically deuteriated tSB, tSB-d5, are presented to confirm the ethylene-like structure of the central C–C linkage of S1 tSB. Next, the time-, temperature-, and solvent-dependent band shape changes of the olefinic C=C stretch band are discussed in terms of the solvent induced dynamic polarization. It is shown that the dynamic polarization model accounts very well, not only for the observed band shape changes, but also for their relevance to the isomerization mechanism. A new microscopic view on the photoisomerization of tSB is thus presented. Picosecond energy dissipation process of S1 tSB is then discussed in relation to the microscopic solvation structure in chloroform. Finally, the mechanism of a new bimolecular reaction between S1 tSB and carbon tetrachloride is discussed on the basis of the diffusion-controlled reaction model.

Analysis of the Solvent- and Temperature-Dependent Raman Spectral Changes of S1trans-Stilbene and the Mechanism of the trans to cis Isomerization: Dynamic Polarization Model of Vibrational Dephasing and the CC Double-Bond Rotation

Solvent- and temperature-dependent band shape changes of the olefinic CC stretch Raman band of S1 trans-stilbene have been analyzed on the basis of the dynamic polarization model. The analysis has shown that the solvent-induced dynamic polarization gives rise to the dephasing of the CC stretch vibration and, concomitantly, triggers and facilitates the rotation around the CC bond leading to the trans to perpendicular (and eventually to cis) isomerization. Picosecond time-resolved Raman spectra have been measured in the three alkane solvents, hexane, octane, and decane at a number of different temperatures ranging from 268 to 338 K and a total of 40 peak positions and the bandwidths have been determined for the CC stretch band. The correlation plot of the bandwidth against peak position shows a clear linear relationship that is predicted by the dynamic polarization model. Picosecond time-resolved fluorescence measurements have been performed in the same three alkane solvents at five different temperatures f...

Investigation of the Self-Organization of Lutein and Lutein Diacetate by Electronic Absorption, Circular Dichroism Spectroscopy, and Atomic Force Microscopy

Spontaneously forming H- and J-type chiral self-assemblies of lutein and lutein diacetate were studied by means of ultraviolet−visible and circular dichroism spectroscopy (CD). Spectroscopic properties of their thin films has been determined for the first time. Existence of chiral superstructures in both solution and solid phases with opposite and same handedness was demonstrated. The molecular exciton theory and the exciton chirality rule were applied to define both qualitatively and quantitatively the spectral shifts, band shape changes, and the highly intense but quite different excitonic CD activity in the two forms of aggregates. Atomic force microscopy images revealed definite threadlike morphology in lutein diacetate films referring to the formation of a nematic liquid crystal phase which probably exists in the aqueous solution either. On this basis, the vibronically coupled CD bands of lutein diacetate are attributed to an excitonic interaction arised from the twisted nematic layers.

Thermal unfolding of ribonuclease A in phosphate at neutral pH: deviations from the two-state model.

The thermal denaturation of ribonuclease A (RNase A) in the presence of phosphate at neutral pH was studied by differential scanning calorimetry (DSC) and a combination of optical spectroscopic techniques to probe the existence of intermediate states. Fourier transform infrared (FTIR) spectra of the amide I' band and far-uv circular dichroism (CD) spectra were used to monitor changes in the secondary structure. Changes in the tertiary structure were monitored by near-uv CD. Spectral bandshape changes with change in temperature were analyzed using factor analysis. The global unfolding curves obtained from DSC confirmed that structural changes occur in the molecule before the main thermal denaturation transition. The analysis of the far-uv CD and FTIR spectra showed that these lower temperature-induced modifications occur in the secondary structure. No pretransition changes in the tertiary structure (near-uv CD) were observed. The initial changes observed in far-uv CD were attributed to the fraying of the helical segments, which would explain the loss of spectral intensity with almost no modification of spectral bandshape. Separate analyses of different regions of the FTIR amide I' band indicate that, in addition to alpha-helix, part of the pretransitional change also occurs in the beta-strands.

Role of Clay-Solvent Inter and Intraparticle Interactions on Swelling Characteristics of Montmorillonite Nano-Meso-Micro Scale Particulate Systems

AbstractThis paper deals with fundamental molecular issues related to macroscopic mechanical properties of nano-meso-microscale montmorillonite clay particulate systems. Molecular interaction of high aspect montmorillonite particles with solvents has large influence on resulting swelling characteristics in clays consisting of montmorillonite. A new controlled uniaxial swelling (CUS) cell is designed which allows a simultaneous measurement of swelling and swelling pressure in addition to ease of removal of undisturbed samples for electron microscopic and fourier transform infrared spectroscopic investigations. Our work shows that the swelling behavior of montmorillonite directly results from breakdown of large particles to smaller nano and meso scale particles with increased water incorporation in the interlayers. The Si-O vibration band regions of the montmorillonite spectra obtained at controlled amounts of swelling from 0-100% (where 0% swelling is defined as fully saturated samples under no volume change) show changes in band shape, intensity, and positions resulting from increased H-bonding in the interlayers and surface of montmorillonite particles. In addition, orientation dependant micro-attenuated total reflectance (ATR) spectroscopic investigations are also conducted on the controlled swelled samples. Our results indicate that the reduced particle size with increased swelling is related to increased misorientation of the montmorillonite platelets. The relationship between molecular interactions microstructure and macroscopic response (swelling and swelling pressure) is the key to development of novel nanocomposite systems based on montmorillonite with tailored properties.

Raman spectroscopy of secondary structure of elastinlike polymer poly(GVGVP).

Raman spectra of the elastinlike polypentapeptide poly(GVGVP) were measured in H(2)O and D(2)O as solutions and, after increasing the temperature, as suspensions and sediments. In addition, spectra of the polypentapeptide in the solutions of increasing concentration and in the solid state were also investigated by gradually evaporating the water. Significant changes in band frequencies, intensities, and shapes were found for selected Raman bands in the measured spectra, particularly for the C-H stretching, the glycine CH(2) wagging, and some amide vibrations. The C-H stretching vibrations are influenced predominantly by the presence of water, the glycine CH(2) wagging vibrations are associated with conformational transitions. Three possible types of poly(GVGVP)s in the presence of water were indicated: polymer chains in a relatively extended state in the solution, a beta-spiral structure in the suspension, and irregularly bent chains in the sediment.

Raman spectroscopy of secondary structure of elastinlike polymer poly(GVGVP)

Raman spectra of the elastinlike polypentapeptide poly(GVGVP) were measured in H2O and D2O as solutions and, after increasing the temperature, as suspensions and sediments. In addition, spectra of the polypentapeptide in the solutions of increasing concentration and in the solid state were also investigated by gradually evaporating the water. Significant changes in band frequencies, intensities, and shapes were found for selected Raman bands in the measured spectra, particularly for the CH stretching, the glycine CH2 wagging, and some amide vibrations. The CH stretching vibrations are influenced predominantly by the presence of water, the glycine CH2 wagging vibrations are associated with conformational transitions. Three possible types of poly(GVGVP)s in the presence of water were indicated: polymer chains in a relatively extended state in the solution, a β-spiral structure in the suspension, and irregularly bent chains in the sediment. © 2001 John Wiley & Sons, Inc. Biopolymers (Biospectroscopy) 62: 150–157, 2001

Polarization‐resolved coherent anti‐Stokes Raman scattering (CARS) spectroscopy: a new probe of molecular symmetry through accurate determination of the Raman depolarization ratio

A broad-band multiplex CARS spectroscopic system was constructed for precise polarization-controlled measurements. The directions of polarization of the two incident laser beams and that of the CARS signal beam are controlled precisely by two polarizers and an analyzer set on high-precision prism mounts. Polarized CARS spectra were recorded with high signal-to-noise ratios for a number of different analyzer settings, typically at intervals of 1° of the analyzer axis. The observed changes in band shapes are interpreted by a global simulation analysis in which all the band shapes at different analyzer settings are reproduced using a small number of intrinsic bandshape parameters. The analysis leads to very accurate determination of depolarization ratios, accurate within an error of ±0.002 in the most favorable case. The method may well be called polarization-resolved CARS spectroscopy. Depolarization ratios of two typical organic liquids, cyclohexane and 1,2-dichloroethane, were determined. The two non-totally symmetric bands of cyclohexane gave a value of 0.75 within an experimental uncertainty of 0.002. The two totally symmetric bands of 1,2-dichloroethane, which have been reported to be depolarized despite their symmetry, showed depolarization ratios very close to but meaningfully smaller than 0.75. Copyright © 2000 John Wiley & Sons, Ltd.

Polarization-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy: a new probe of molecular symmetry through accurate determination of the Raman depolarization ratio

A broad-band multiplex CARS spectroscopic system was constructed for precise polarization-controlled measurements. The directions of polarization of the two incident laser beams and that of the CARS signal beam are controlled precisely by two polarizers and an analyzer set on high-precision prism mounts. Polarized CARS spectra were recorded with high signal-to-noise ratios for a number of different analyzer settings, typically at intervals of 1° of the analyzer axis. The observed changes in band shapes are interpreted by a global simulation analysis in which all the band shapes at different analyzer settings are reproduced using a small number of intrinsic bandshape parameters. The analysis leads to very accurate determination of depolarization ratios, accurate within an error of ±0.002 in the most favorable case. The method may well be called polarization-resolved CARS spectroscopy. Depolarization ratios of two typical organic liquids, cyclohexane and 1,2-dichloroethane, were determined. The two non-totally symmetric bands of cyclohexane gave a value of 0.75 within an experimental uncertainty of 0.002. The two totally symmetric bands of 1,2-dichloroethane, which have been reported to be depolarized despite their symmetry, showed depolarization ratios very close to but meaningfully smaller than 0.75. Copyright © 2000 John Wiley & Sons, Ltd.

Conformational change of proline residues observed in swollen and shrunken phases: ?-Ray synthesis and variable temperature circular dichroism spectra of a thermo-responding polymer, poly(acryloyl-Pro-OMe)

Poly(acryloyl-L-proline-methyl ester) (1) has optically active side chain, and constitutes thermoresponsive hydrogels upon crosslinking. In this study, we have prepared uncrosslinked polymer of 1 with a 10-kGy irradiation dose of γ-ray. For this polymer, 1, variable temperature circular dichroism (CD) and 1 H NMR spectra have been studied in the range of 0-30 °C. The intense CD spectrum at 0 °C suggests that the side chains in 1 have an ordered orientation. The CD intensity decreases gradually with increasing temperature. The decreased intensity of CD spectra indicates that the disordering occurs for the side-chain orientation. The CD band shape changes discontinuously at 20 °C. In the 1 H NMR spectra, signals disappear above 20 °C. These spectral change at 20 °C indicate that the phase transition occurs at around 20 °C from swollen to shrunken phase. Even after the phase transition, the CD spectra are still changing with isochromic point at 212 nm. It appears that the side-chain conformation is still changing from one state to the other state in the shrunken phase polymers.

Optically Induced Band Shifts in Infrared Spectra of Mixed Self-assembled Monolayers of Biphenyl Thiols

Optically induced band shifts in reflection absorption infrared spectra (RAIR) of mixed self-assembled monolayers (SAMs) are reported. SAMs of 4-trifluoromethyl-4-mercaptobiphenyl dispersed in 4‘-methyl-4-mercaptobiphenyl on gold exhibit significant shifts of the spectral envelope of the symmetric CF3 stretch and the b1u skeletal mode. A linear relationship is observed between the band positions, the band intensity, and the surface CF3 group concentration. To rationalize the observed changes in band positions and shapes, a classical electromagnetic theory approach is used, providing an excellent agreement between theory and experiments. The simplified RAIR signature of 4‘-trifluoromethyl-4−mercaptobiphenyl on gold is a unique example of the surface selection rule.

Luminescence spectra and kinetics of disordered solid solutions

We have studied both theoretically and experimentally the luminescence spectra and kinetics of crystalline, disordered solid solutions after pulsed excitation. First, we present the model calculations of the steady-state luminescence band shape caused by recombination of excitons localized in the wells of random potential induced by disorder. Classification of optically active tail states of the main exciton band into two groups is proposed. The majority of the states responsible for the optical absorption corresponds to the group of extended states belonging to the percolation cluster, whereas only a relatively small group of ``radiative'' states forms the steady-state luminescence band. The continuum percolation theory is applied to distinguish the ``radiative'' localized states, which are isolated in space and have no ways for nonradiative transitions along the tail states. It is found that the analysis of the exciton-phonon interaction gives the information about the character of the localization of excitons. We have shown that the model used describes quite well the experimental cw spectra of ${\mathrm{CdS}}_{(1\ensuremath{-}c)}{\mathrm{Se}}_{c}$ and ${\mathrm{ZnSe}}_{(1\ensuremath{-}c)}{\mathrm{Te}}_{c}$ solid solutions. Further, the experimental results are presented for the temporal evolution of the luminescence band. It is shown that the changes of band shape with time come from the interplay of population dynamics of extended states and spatially isolated ``radiative'' states. Finally, the measurements of the decay of the spectrally integrated luminescence intensity at long delay times are presented. It is shown that the observed temporal behavior can be described in terms of relaxation of separated pairs followed by subsequent exciton formation and radiative recombination. Electron tunneling processes are supposed to be responsible for the luminescence in the long-time limit at excitation below the exciton mobility edge. At excitation by photons with higher energies the diffusion of electrons can account for the observed behavior of the luminescence.

Surface-Enhanced Raman Scattering of Ultrathin Cadmium Chalcogenide Films on Gold Formed by Electrochemical Atomic-Layer Epitaxy: Thickness-Dependent Phonon Characteristics

The utility of surface-enhanced Raman scattering (SERS) for characterizing phonon and related interfacial vibrations within ultrathin semiconductor layers on gold substrates is demonstrated, specifically for cadmium chalcogenide (CdSe, CdS) films formed by electrochemical atomic layer epitaxy. The occurrence of quantum-confinement effects for CdSe films less than ca. 10 monolayers thick is clearly evident from the observed ca. 12 cm-1 phonon redshifts and bandshape changes. Milder phonon redshifts are seen for the ultrathin CdS films, suggesting the presence of offsetting blueshifts from lattice compression. The observed sensitivity of the phonon spectra to the order of initial Cd/Se or Cd/S monolayer electrodeposition, along with the retention of Au−Se and Au−S vibrational bands upon layer growth, signal the importance of the initial chemisorbate to the epitaxial film structure and vibronic properties.

Intramolecular excimer formation in short- and long-chainlength di(9-anthryl) bichromophoric compounds and relation to ground state properties

The ground state conformational, UV absorption and fluorescence spectral as well as decay time data for a series of polymethylene- and one polyoxyethylene-linked di(9-anthracenes) in solution are compared, for chainlengths varying from n=0 (directly linked system A0A, 9,9′-bianthryl) to n=∞ (A∞, reference system 9-decylanthracene). 1H NMR spectroscopy of A2A to A11A shows that there is no clearly preferred ground state conformation of the anthracene moieties. The crystal structure of A2A was solved and the anti conformation of the anthracene units was established. The UV spectra show a redshift interpreted within the Forster exciton model (predominance of extended conformations). Fluorescence spectra for (CH2)n-bridged systems show no clear excimer band for short-chain systems but redshifted excimer emission for A4A to A9A. Loss of structure and fluorescence band shape changes as well as biexponential decay times allow the conclusion that excimer formation occurs in all chained systems. For a related polyoxyethylene-chained system, the excimer band is substantial, probably a consequence of the increased flexibility of the chain and moderate photoreactivity. Solvent polarity changes are strong for A0A [formation of a twisted intramolecular charge transfer (TICT) state] but also significant in the systems A1A and A2A. This is rationalized by the possiblity for competitive formation of an excimer-type (parallel but tilted chromophore arrangement) and a TICT-type (near perpendicular) conformation. For AnA (n>2) the solvent polarity was found to have virtually no influence.