Raman Excitation Profiles Research Articles

Fluorescence Backgroundless Ti: Sapphire Laser Using Acousto-Optical Tunable Filter for Raman Spectroscopic Measurements

The background noise inherent to tunable lasers, which emit broad band spontaneous fluorescence from the laser-active medium, is detrimental for sensitive Raman measurement. Using the diffraction effect in an acousto-optic device, we have developed a fluorescence backgroundless Ti: sapphire laser suited for near-infrared Raman spectroscopy. A Raman excitation profile consisting of series of Raman spectra of deoxygenated hemoglobin aqueous solutions was measured by changing excitation wavelengths, revealing the high potential of this laser as a spectroscopic light source.

The Role of Specific Amino Acid Residues in The Vibrational Properties of Plastocyanin

Resonance Raman spectra for zucchini, cucumber, bean, and lettuce plastocyanins, blue copper proteins involved in plant photosynthetic electron transport, have been measured at wavelengths throughout their intense 600-nm absorption band. A comparison of the resonance Raman spectra of these four plastocyanins, and two others previously quantified, demonstrates that they all exhibit vibrational bands with similar frequencies but significantly different relative intensities. Self-consistent analysis of the absorption band and resulting resonance Raman excitation profiles using a time-dependent wave packet formalism for these four plastocyanins demonstrates that many of the derived molecular parameters are similar to those determined previously for two other species of plastocyanin. However, significant differences are observed in the mode-specific displacements and reorganization energies, although the total reorganization energy is 0.16 ± 0.01 eV for all six plastocyanins. A detailed comparison of the structural and compositional differences among the six plastocyanins, using the previously reported structure of poplar a plastocyanin, suggests that these mode-specific differences arise from specific amino acid differences within 10−12 Å of the copper site. These results are interpreted as arising from a through-bond mode-mixing mechanism and a through-space electrostatic mechanism and suggest that the normal modes of the copper site are delocalized into the protein. In addition, lettuce plastocyanin has a significantly greater homogeneous line width, probably as a result of compositional differences adjacent to the copper active site. These results demonstrate that the protein environment is strongly coupled to or mixed in with the copper site vibrational dynamics. Further evidence for the sensitivity of the resonance Raman intensities to particular electron-transfer pathways within the protein is also discussed.

Transform method in resonance Raman scattering: effect of mode mixing

AbstractThe effect of mode mixing on resonance Raman scattering is examined on the basis of exact transform relations for the resonance Raman excitation profiles and absorption lineshape in the time domain. General transform relations are presented which hold for an arbitrary number of mixed modes. A detailed description of the excitation profiles of two mixed Raman modes is given, taking into account an arbitrary number of other modes. The dependence of the mixing parameters, change of vibrational frequencies and equilibrium positions in the excited state on the vibronic coupling parameters is found. It is shown that in the case of a substantial change in chemical bonds at the electronic transition, the mode mixing drastically changes the Raman cross‐sections/excitation profiles. Copyright © 2002 John Wiley & Sons, Ltd.

Excited-state dynamics of alizarin-sensitized TiO2 nanoparticles from resonance Raman spectroscopy

Resonance Raman spectra of alizarin-sensitized TiO2 nanoparticles have been obtained at excitation wavelengths throughout the 488-nm charge transfer absorption band. The resonance Raman spectrum of the alizarin-sensitized TiO2 nanoparticle is significantly different from the spectrum of free alizarin, consistent with a chemisorption-type interaction. This interaction is probably chelation of surface titanium ions by the hydroxy groups of alizarin, supported by the observed enhancement of bridging C–O modes at 1326 cm−1. In contrast to resonance Raman intensity analysis of homogeneous electron transfer where vibrations of both the donor and acceptor are observed, self-consistent analysis of the resulting resonance Raman excitation profiles and absorption spectrum using the time-dependent wave packet propagation formalism show mode-specific reorganization along alizarin vibrations exclusively; no resonance-enhanced vibrations attributable to the TiO2 moiety are observed. Therefore, the total resonance Raman-derived reorganization energy is only 0.04 eV, significantly smaller than the observed outer-sphere reorganization energy of 0.2 eV for this system and inner-sphere reorganization energies measured for other molecular systems. The discrepancy is ascribed to a significant environmental component to the outer-sphere reorganization energy arising from rapid dephasing of surface TiO2 units involved in adsorption by strongly coupled interior bath vibrations.

Optical Properties of Dye Molecules Adsorbed on Single Gold and Silver Nanoparticles

Despite the well-known relationship between the resonance Raman excitation profile and the absorption line shape, there is scant experimental evidence for effects in absorption or fluorescence spectroscopy related to the observations of surface-enhanced Raman scattering (SERS). On the other hand, numerous Raman studies have been done on the SERS phenomenon, where large enhancement factors have been determined. In this work, the absorption properties of molecules adsorbed on single gold and silver nanoparticles (monomers) have been investigated, with particular emphasis on an examination of the effect on the spectrum of the adsorbate. A number of the adsorbates studied are similar to those reported in SERS studies. The adsorbates can be divided into two classes according to the nature of the interaction with the adsorbent. Class I shows little change in the absorption spectrum. Class II shows a large reduction in absorption. The only examples of an increase in absorption arise from solvatochromic effects. ...

The Raman Spectrum of the Squarate (C4O4-2 ) Anion: An Ab Initio Basis Set Dependence Study

The Raman excitation profile of the squarate anion, C4O4-2 , was calculated using ab initio methods at the Hartree-Fock using Linear Response Theory (LRT) for six excitation frequencies: 632.5, 514.5, 488.0, 457.9, 363.8 and 337.1 nm. Five basis set functions (6-31G*, 6-31+G*, cc-pVDZ, aug-cc-pVDZ and Sadlej's polarizability basis set) were investigated aiming to evaluate the performance of the 6-31G* set for numerical convergence and computational cost in relation to the larger basis sets. All basis sets reproduce the main spectroscopic features of the Raman spectrum of this anion for the excitation interval investigated. The 6-31G* basis set presented, on average, the same accuracy of numerical results as the larger sets but at a fraction of the computational cost showing that it is suitable for the theoretical investigation of the squarate dianion and its complexes and derivatives.

Time Dependent Coupled Cluster Approach to Resonance Raman Excitation Profiles from General Anharmonic Surfaces

A time dependent coupled cluster approach to the calculation of Resonance Raman excitation profiles on general anharmonic surfaces is presented. The vibrational wave functions on the ground electronic surface are obtained by the coupled cluster method (CCM). It is shown that the propagation of the vibrational ground state on the upper surface is equivalent to propagation of the vacuum state by an effective hamiltonian generated by the similarity transformation of the vibrational hamiltonian of that surface by the CCM wave operator of the lower surface up to a normalization constant. This time propagation is carried out by the time-dependent coupled cluster method in a time dependent frame. Numerical studies are presented to asses the validity of the approach.

Heme charge-transfer band III is vibronically coupled to the Soret band.

A complete resonance Raman excitation profile of the heme charge-transfer band known as band III is presented. The data obtained throughout the near-infrared region show preresonance with the Q-band, but the data also clearly show the enhancement of a number of modes in the spectral region of band III. Only nontotally symmetric modes are observed to have resonance enhancement in the band III region. The observed resonance enhancements in modes of B(1g) symmetry are compared with the enhancements of those same modes in the excitation profiles of the Q-band of deoxy myoglobin, also presented here for this first time. The Q-band data agree well with the theory of vibronic coupling in metalloporphyrins (Shelnutt, J. A. J. Chem. Phys. 1981, 74, 6644-6657). The strong vibronic coupling of the Q-band of the deoxy form of hemes is discussed in terms of the enhancement of modes with both B(1g) and A(2g) symmetry. The comparison between the Q-band and band III reveals that, consistent with the theory, only modes of B(1g) symmetry are enhanced in the vicinity of band III. These results show that band III is vibronically coupled to the Soret band. The coupling of band III to modes with strong rhombic distortion of the heme macrocycle calls into question the hypothesis that the axial iron out-of-plane displacement is primarily responsible for the structure-dynamics correlations observed in myoglobin.

Revealing the chromophoric composition of multichromophoric polypyridyl complexes of Re(I) and Os(II): a resonance Raman study

AbstractSeveral binuclear Re(I) and Re(I)Os(II) complexes with polypyridyl bridging ligands, 6,7‐dimethyl‐2,3‐di(2‐pyridyl)quinoxaline (dpqMe2) and [2,3‐a:3′,2′‐c]dipyrido‐6,7‐dimethylphenazine (ppbMe2), were synthesized and studied. The visible region of the electronic absorption spectrum is dominated by a single metal‐to‐ligand charge‐transfer band (ca 600 nm) and a dpqMe2‐ or ppbMe2‐based ligand‐centred band (ca 400 nm). The resonance Raman spectra of these complexes were recorded at several excitation wavelengths. Analysis of the resonance Raman spectra identified a number of strongly enhanced chromophore‐specific signatures with excitation wavelengths coincident with the main absorption bands, and weakly enhanced chromophore‐specific signatures at intermediate excitation wavelengths. Resonance Raman excitation profiles (RREPs) are presented for selected marker bands. Analysis of the chromophore‐specific signatures and the RREPs allowed a more complete assignment of the features in the electronic absorption spectra to be made. Copyright © 2002 John Wiley & Sons, Ltd.

Image Potential Surface States Localized at Chemisorbed Dielectric−Metal Interfaces

The work reported herein involves an extensive examination of alkanethiol self-assembled monolayers (SAMs) on roughened Au and Ag surfaces by surface-enhanced electronic Raman scattering. The observation of a novel resonance Raman-like process in these systems at excitation energies between 1.7 and 2.0 eV is described in detail. At excitation energies between these limits, a series of intense bands appear superimposed upon the normal surface-enhanced Raman vibrational spectrum of the film. The experimental evidence for an electronic Raman scattering process which leads to these bands is presented. The energy level diagram derived from these observed electronic transitions has led to the development of a model based on electronic Raman scattering between image potential surface states (IPSs). While the experimental Raman data indicates that the IPS electron exists within the dielectric film, quantum mechanical modeling of these image potential states for chemisorbed alkanethiol SAMs on roughened metal surfaces strongly argues that the electron is located in close proximity to the headgroup region of the film. Additionally, Raman excitation profiles for CH3(CH2)9SH SAMs on Au have provided evidence for both a filled, lower electronic state within the metal band gap (from which the IPS levels are filled via optical pumping prior to the Raman scattering event) and an upper electronic state; the energetic separation between the two states is 3.70 ± 0.03 eV. Furthermore, an analysis of these excitation profiles places the upper edge of the lower electronic state and the lower edge of the upper electronic state at approximately 3.60 and 7.29 eV above the clean Au(100) Fermi level, respectively. Thus, these states can be located within the surface band diagram of Au(100), and such a diagram is presented for the CH3(CH2)9SH/Au(100) system. The energetic locations of these states within the metal band structure indicate that these states are not intrinsic to the metal, and are consistent with those previously observed by two-photon photoelectron spectroscopy for alkanethiol films on Cu(111).

Characterization of carbon nanotubes using Raman excitation profiles

Resonance Raman excitation profiles for several radial-breathing modes in carbon nanotubes have been measured using tunable lasers. It is shown that the line shapes of the excitation profiles are a powerful tool for the characterization of the nanotubes. In particular, profiles that follow theoretical predictions for a single one-dimensional singularity in the joint density of states can be assigned to armchair tubes. These assignments do not rely on the quantitative details of electronic structure calculations.

Aromatic versus polyenic linkers in push–pull chromophores: electron–phonon coupling effects

The linear absorption spectra and absolute resonance Raman excitation profiles of [(2 E,4 E,6 E,8 E)-9-[4-(dibutylamino)phenyl]-2,4,6,8-nonatetraenylidene]propanedinitrile, an electron donor–acceptor push–pull chromophore with a polyenic inker group, have been measured in cyclohexane and acetonitrile solution. Numerical simulation of the spectra yields solvent reorganization energies and the excited-state geometry changes along the strongly Raman-active vibrations. The ground-state vibrational frequencies and excited-state reorganization energies are considered within the context of two-state valence bond models. Comparison of the results for this chromophore with p-nitroaniline, which has a purely aromatic linker group, suggests that the two-state model is more applicable to molecules with polyenic rather than aromatic linkers.

Effects of a Paracyclophane Linker on the Charge-Transfer Transition of 4-(Dimethylamino)-4‘-nitrostilbene

Resonance Raman intensities of the push-pull chromophore 4-(dimethylamino)-4′-nitrostilbene (DANS) have been measured in dichloromethane, acetonitrile, and methanol at excitation wavelengths spanning its strong visible charge-transfer absorption band. The effects of inserting a paracyclophane moiety between the donor and the acceptor are investigated by performing similar measurements on a second molecule, PCP-DANS, which consists of p-dihexylaminostilbene and p-nitrostilbene moieties attached through their unsubstituted rings by a paracyclophane linker. The Raman excitation profiles and absorption spectra are simulated using time-dependent wave packet propagation techniques to determine reorganization energies along the Ramanactive normal coordinates. Excitation to the nominal charge-transfer state generates a much greater change in geometry in the vicinity of the nitro acceptor group in DANS than in PCP-DANS, while the geometry changes around the dialkylamino donor end are similar. Comparison with other push-pull chromophores coupled with the results of ZINDO electronic structure calculations both suggest that the transition in DANS is well-described as an intramolecular charge-transfer transition, whereas the transition in PCP -DANS has less charge-transfer character.

Vibronic effects on solvent dependent linear and nonlinear optical properties of push-pull chromophores: Julolidinemalononitrile

The linear absorption spectra and absolute resonance Raman excitation profiles of the “push-pull” chromophore julolidinemalononitrile have been measured in cyclohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol solution at excitation wavelengths spanning the strong visible charge-transfer absorption band. Numerical simulation of the spectra using time-dependent wave-packet propagation methods yields the excited-state geometry changes along the ∼15 strongly Raman-active vibrations as well as the solvent reorganization energies. The distribution of the total vibrational reorganization energy among the various normal modes is solvent dependent, indicating solvent polarity effects on the electronic structure. These results are compared with those previously obtained for two other push-pull chromophores, p-nitroaniline and julolidinyl-n-N,N′-diethylthiobarbituric acid. The frequency dispersion of the molecular first hyperpolarizability, β, is also calculated in each solvent using a time-domain form of the standard Oudar–Chemla two-state model modified to incorporate solvent reorganization, inhomogeneous broadening, and the vibronic structure of the charge-transfer state. We show that accurate extrapolation of β measured at frequencies in the near-infrared to zero frequency requires a realistic description of the excited state as the measuring wavelength approaches a two-photon resonance. This is particularly relevant to the high chromophore concentrations needed for device applications, where intermolecular interactions can strongly perturb the electronic transitions.

Mode mixing via resonance Raman excitation profiles

In the case of linear+quadratic vibronic coupling with an arbitrary number of configurational coordinates, the exact equation for the resonance Raman amplitude has been derived via the absorption Fourier transform. There are some actual cases when this equation has simple solutions: change of vibrational frequency at the electronic transition without mode mixing, mixing of two modes, mixing of modes with close frequency. The corresponding transform relations between Raman amplitudes (→excitation profiles) and the absorption are given, which allow to determine the mixing parameters, vibrational frequencies as well as the linear vibronic coupling parameters.

Raman excitation profile of diphenylamine

Vibrational analyses have been done on the basis of Raman and FTIR spectra of diphenylamine (DPA) and contributions of the two rings to different normal modes of vibration have been discussed. From the analyses of Raman excitation profiles (REPs) of several vibrational modes, useful information about the geometries of the molecule in different excited states have been obtained. Also vibronic coupling has been found to be important in the case of some Raman bands. Besides an interesting Molecular Orbital (MO) calculation, exploring the structure of DPA in the allowed singlet electronic state ( 1 L a) , has been presented. Moreover, possible orientations of the two rings with respect to the amine nitrogen atom in the ground and in the 1 L a states have been discussed.

Resonant Raman scattering characterization of carbon nanotubes grown with different catalysts

Single-walled carbon nanotube samples produced in the presence of different combinations of metal catalysts have been studied by resonant Raman spectroscopy. The diameter distribution of different samples has been determined by analysis of the laser excitation energy dependence of the tangential modes associated with metallic nanotubes. These modes are resonantly enhanced over a narrow range of the exciting energies, which shifts for different samples. The Raman cross-section expression has been used to fit the experimental Raman excitation profiles. This procedure was used to determine the mean value and the width of the distribution of diameters within each sample.

Resonance Raman and crystallographic studies on the complex [Fe 2(bbpnol) 2]·2DMF (bbpnol= N, N′-bis(2-hydroxybenzyl)-2-ol-1,3-propanediamine)

The ligand N, N′-bis(2-hydroxybenzyl)-2-ol-1,3-propanediamine (H 3bbpnol) reacts with iron(III) perchlorate forming two dinuclear bis-μ-alkoxo complexes, a ‘ cis– trans’ isomer (complex 1) previously reported and a ‘ cis– cis’ isomer (complex 2) characterized in this work. The main differences found in complex 2 structure are, (a) the four phenolic oxygens are trans to the alkoxo bridges; (b) each ligand is shared by the two Fe(III) ions occupying two coordination positions at each center. The Fe(III) centers in the resulting centrosymmetric structure in complex 2 have a distorted-octahedral geometry with the equatorial plane occupied by the phenolic and alcoholic oxygen atoms and the apical positions are filled by the aminic nitrogen atoms. The resonance Raman (RR) spectra of these two isomeric complexes are somewhat different with the intensity of some low-frequency modes increasing in the less symmetric core. The electronic spectra of both complexes are similar, but the molar absorptivities are substantially increased in complex 2, indicating the presence of an electronic coupling between the phenolate moieties trans in relation to the alkoxo bridge, and that phenolates coordinated cis to the alkoxo bridge do not seem to contribute to LMCT oscillator strength. This is directly reflected in the Raman excitation profiles (REP) of the chromophore modes, with the vibrational modes of the ‘ cis– cis’ isomer showing a greater intensification compared with the ‘ cis– trans’ isomer.

Electrostatic and conformational effects on the electronic structures of distortional isomers of a mixed-valence binuclear Cu complex.

The electronic structure of the binuclear copper complex [Cu(2)(L)](3+) [L = N(CH(2)CH(2)N(H)CH(2)CH(2)N(H)CH(2)CH(2))(3)N] has been investigated by resonance Raman and electroabsorption spectroscopy. Crystallographic Cu(2) distances of 2.364(1) and 2.415(1) A determined for the nitrate and acetate salts, respectively, are consistent with a substantial metal-metal interaction. The Cu-Cu bonding interaction in the binuclear complex is modulated both in the solid state and in solution by the ligand environment through coupling to ligand torsional modes that are, in turn, stabilized by hydrogen bonding. Electroabsorption data on the three major visible and near-infrared electronic transitions of Cu(2)L, lambda(max) (epsilon(max)) = 1000 nm ( approximately 1200 M(-1) cm(-1)), 748 nm (5600 M(-1) cm(-1)), and 622 nm (3350 M(-1) cm(-1)), reveal a difference dipole moment between the ground and excited states (Deltamu(A)) because of symmetry breaking. The difference polarizability for all three of the transitions is negative, indicating that the ground state is more polarizable than the excited state. A general model to explain this behavior in terms of the proximity of accessible transitions involving copper d electrons is proposed to explain the larger polarizability of the ground state. Raman excitation profiles (REPs) provide evidence for multiple conformational states of [Cu(2)(L)](3+). Separate REPs were obtained for each of the components of the two major Raman bands for nu(1) (a Cu-Cu stretching mode) and nu(2) (a Cu-Cu-N(eq) bending mode). The Raman data along with quantum chemical ZINDO/S CI calculations provide evidence for isomeric forms of Cu(2)L with strong coupling between the conformation of L and the Cu-Cu bond length.

Solvent Effects on Ground and Excited Electronic State Structures of the Push-Pull Chromophore Julolidinyl-n-N,N‘-diethylthiobarbituric Acid

Resonance Raman spectra and cross sections of a “push−pull” chromophore containing a julolidine donor and a thiobarbituric acid acceptor have been measured in dilute solution in five solvents having a wide range of polarities (cyclohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol) at excitation wavelengths spanning the strong visible charge-transfer absorption band. The absolute Raman excitation profiles and absorption spectra are simulated using time-dependent wave packet propagation techniques to determine the excited-state geometry changes along the ∼30 Raman-active vibrations as well as the solvent reorganization energies. Several vibrational modes undergo significant (5−15 cm-1) frequency changes as the solvent is varied, signaling solvent polarity effects on the ground-state electronic structure. The excited-state geometry changes are solvent dependent for some vibrational modes but not for others. The total vibrational reorganization energy decreases, and the solvent reorganization ...