Jeanne L. McHale

Vibrational resonance coupling and the noncoincidence effect of the isotropic and anisotropic Raman spectral components in orientationally anisometric molecular liquids

An expression for the splitting parameter associated with the noncoincidence of the peak positions of the polarized and depolarized Raman spectral components of a totally symmetric band in a molecular liquid has been derived. The derivation is based upon an angular dependent vibrational resonance coupling Hamiltonian, and has induced a more general interaction Hamiltonian than the induced dipole–induced dipole interaction. It has been shown that short‐range orientational order is not essential for the noncoincidence effect, although the presence of short‐range order can affect the magnitude of the splitting parameter. An example is given to show that the noncoincidence effect is not necessarily due to the induced dipole interaction mechanism. The present result shows that any angular dependent resonance coupling mechanism can give rise to the noncoincidence effect.

The influence of angular dependent intermolecular forces on vibrational spectra of solution phase molecules

The zeroth, first, and second moments of the solution phase infrared, isotropic Raman, and anisotropic Raman bands of a totally symmetric mode are calculated. The existence of a strong orientationally dependent intermolecular potential is shown to lead to concentration dependent peak frequencies and bandwidths, as well as possible deviations from Beer’s law. Assumining a dominant transition dipole–transition dipole interaction, expressions for the explicit concentration dependence of the infrared, isotropic Raman, and anisotropic Raman first moments are derived and applied to the interpretation of the observed Raman spectra of polar molecules in the solution phase.

Photoluminescence of dense nanocrystalline titanium dioxide thin films: effect of doping and thickness and relation to gas sensing.

The photoluminescence (PL) of dense nanocrystalline (anatase) TiO(2) thin films is reported as a function of calcination temperature, thickness, and tungsten and nickel doping. The dependence of the optical absorption, Raman spectra, and PL spectra on heat treatment and dopants reveals the role of oxygen vacancies, crystallinity, and phase transformation in the performance of TiO(2) films used as gas sensors. The broad visible PL from defect states of compact and undoped TiO(2) films is found to be much brighter and less sensitive to the presence of oxygen than that of mesoporous films. The dense nanocrystalline grains and the nanoparticles comprising the mesoporous film are comparable in size, demonstrating the importance of film morphology and carrier transport in determining the intensity of defect photoluminescence. At higher calcination temperatures, the transformation to rutile results in the appearance of a dominant near-infrared peak. This characteristic change in the shape of the PL spectra demonstrates efficient capture of conduction band electrons by the emerging rutile phase. The W-doped samples show diminished PL with quenching on the red side of the emission spectrum occurring at lower concentration and eventual disappearance of the PL at higher W concentration. The results are discussed within the context of the performance of the TiO(2) thin films as CO gas sensors and the chemical nature of luminescent defects.

Differing HOMO and LUMO mediated conduction in a porphyrin nanorod.

In this communication we provide the first UHV-STM images and STM-based current-voltage (I-V) and orbital mediated tunneling spectroscopy (OMTS) data on a self-assembled porphyrin nanostructure at the single structure level. We will show that transverse conductivity over distances less than 10 nm can occur by barrier type tunneling but that long distance conduction solely occurs through the LUMO band. These nanorods are very highly rectifying.

Aggregation of triphenylmethane dyes in aqueous solution : dimerization and trimerization of crystal violet and ethyl violet

Abstract The self-association of crystal violet and ethyl violet in aqueous solution has been studied using multiwavelength linear regression of the concentration dependent absorbance data. Dimerization ( K 1 ) and trimerization ( K 2 ) equilibrium constants were determined to be 680 M −1 and 730 M −1 for crystal violet, respectively, and 3970 M −1 and 1180 M −1 for ethyl violet. Although not imposed by the numerical analysis of the data, resolved spectra of the dimer and trimer were obtained having approximately the same integrated absorptivity per mole of dye as that of the monomer. A reasonable model of the dimer structure is presented and discussed in terms of the excitonic coupling of the transition moments for the two visible bands in the momomer absorbance spectrum.

Resonance Raman study of solvent dynamics in electron transfer. I. Betaine-30 in CH3CN and CD3CN

Raman excitation profiles for nine vibrational modes of the solvatochromic dye betaine-30 have been measured in CH3CN and CD3CN solution at wavelengths that span the S0→S1 charge transfer transition. Though the absorption spectra of the dye are the same in the protonated and deuterated forms of the solvent, Raman cross sections for all modes were found to be lower in CD3CN solution than in CH3CN. Wave packet theory has been applied to model the absorption and Raman profiles, using a solvent dephasing model which accounts for bimodal solvent dynamics. The solvent isotope effect on the Raman intensities is interpreted in terms of different amplitudes of the fast solvent response in the protonated and deuterated solvents, and possible coupling of low-frequency solute and solvent modes. The solvent reorganization energy greatly exceeds that due to internal vibrational modes of betaine-30, and most of the internal reorganization is due to the torsional modes of the solute.

Influence of hydrogen bonding on excitonic coupling and hierarchal structure of a light-harvesting porphyrin aggregate

Helical porphyrin nanotubes of tetrakis(4-sulfonatophenyl)porphyrin (TSPP) were examined in DCl/D(2)O solution using resonance Raman and resonance light scattering spectroscopy to probe the influence of hydrogen bonding on the excitonic states. Atomic force microscopy reveals similar morphology for aggregates deposited from DCl/D(2)O and from HCl/H(2)O solution. Deuteration results in subtle changes to the aggregate absorption spectrum but large changes in the relative intensities of Raman modes in the J-band excited resonance Raman spectra, revealing relatively more reorganization along lower-frequency vibrational modes in the protiated aggregate. Depolarization ratio dispersion and changes in the relative Raman intensities for excitation wavelengths spanning the J-band demonstrate interference from overlapping excitonic transitions. Distinctly different Raman excitation profiles for the protiated and deuterated aggregates reveal that isotopic substitution influences the excitonic structure of the J-band. The deuterated aggregate exhibits a nearly two-fold increase in intensity of resonance light scattering as a result of an increase in the coherence number, attributed to decreased exciton-phonon scattering. We propose that strongly coupled cyclic N-mers, roughly independent of isotopic substitution, largely decide the optical absorption spectrum, while water-mediated hydrogen bonding influences the further coherent coupling among them when they are assembled into nanotubes. The results show that, similar to natural light-harvesting complexes such as chlorosomes, hydrogen bonding can have a critical influence on exciton dynamics.

Observation of Charge Transport in Single Titanium Dioxide Nanotubes by Micro-Photoluminescence Imaging and Spectroscopy

We present the first report of photoluminescence spectra and images of single TiO(2) (anatase) nanotubes. In previous work using ensembles of conventional TiO(2) nanoparticles, we interpreted the broad photoluminescence (PL) spectrum to be a superposition of hole trap emission, peaking in the green, and broad red PL arising from electron traps. PL spectra of individual nanotubes in inert environment show a similar broad emission, with peaks at around 560-610 nm. The PL from single nanotubes differs from the more blue-shifted PL of ordered nanotube films. The intensity of PL is found to be larger for single nanotubes than for ordered arrays, as a result of competition from transport in the contiguous samples and from introduction of additional trap states when the nanotubes are dispersed. PL images of single nanotubes show the emission to be concentrated in the area of excitation, but the peaks in the red and green components of the PL are not spatially coincident. Remote PL, occurring away from the excitation point, is observed in the green (∼510 nm), showing the possible contribution of charge transport to the observed PL. While the PL from ensembles of TiO(2) nanotubes is fairly insensitive to contacting media, exposure of single nanotubes to air and ethanol changes the shape and intensity of the PL spectrum. Our results point to a very different trap state distribution in TiO(2) nanotubes compared to that of conventional TiO(2) nanoparticles, which we attribute to differences in exposed crystal facets. In addition, separation of nanotubes introduces additional photoluminescent trap states and changes the character of the emission from excitonic in the array to trap-mediated in single nanotubes.

Resonance Raman study of solvent dynamics on the spectral broadening and intramolecular charge transfer of a hemicyanine dye in aqueous solution

The spectroscopic properties of 4-[2-(4-dimethylaminophenyl)ethenyl]-1-methyl-pyridinium iodide (HR) in different solvents reveal the important effects of solvent dynamics on the spectral broadening and the intramolecular charge transfer of HR. In this article, Raman excitation profiles for 18 vibrational modes of HR are reported in aqueous solution at wavelengths that span the S0→S1 charge transfer transition. The absorption spectra, fluorescence spectra and resonance Raman profiles of HR are modeled using time-dependent wave packet theory and the Brownian oscillator solvent dephasing model. The solvent reorganization energy in the absorption process is much greater than that due to internal vibrational modes, and the solvent reorganization energy for the emission process is considerably smaller than that for the absorption process. The fluorescence spectrum is mainly broadened by the inhomogeneous Gaussian distribution of the electronic energy, perhaps due to internal rotations in the molecule. The resu...

Response to ‘‘Comment on the influence of the dielectric constant upon the noncoincidence of the isotropic and anisotropic Raman frequencies’’

A reply to P. Mirones comments on the role of dielectric constant in the description of the noncoincidence of the isotropic and anisotropic Raman frequencies in polar liquids is presented. (AIP)