Anne Myers Kelley

Solvent effects on ground and excited electronic state structures of p-nitroaniline

Resonance Raman intensities of p-nitroaniline, a prototypical “push–pull” chromophore with a large first hyperpolarizability (β), 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 near-ultraviolet 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 five or six principal Raman-active vibrations as well as estimates of the solvent reorganization energies. The total vibrational reorganization energy decreases and the solvent reorganization energy increases with increasing solvent polarity in all solvents except methanol, where specific hydrogen-bonding interactions may be important. The dimensionless normal coordinate geometry changes obtained from the resonance Raman analysis are conve...

Symmetry breaking effects in NO3−: Raman spectra of nitrate salts and ab initio resonance Raman spectra of nitrate–water complexes

Ground-state structures and vibrational frequencies are calculated for complexes of the nitrate anion with one and two water molecules at the ab initio Hartree–Fock level with a basis set including diffuse and polarization functions. Two local minimum geometries are found for each complex. Calculations of the electronically excited states at the CIS level are then used to find the forces on each of the atoms upon vertical excitation to the two lowest-lying (near-degenerate) strongly allowed electronic transitions. These forces are converted to gradients of the excited-state potential surfaces along the ground-state normal modes and compared with the parameters obtained previously from empirical simulations of the experimental resonance Raman intensities of NO3− in dilute aqueous solution. The calculations on two-water clusters agree well with the experimental excited-state geometry changes along the totally symmetric N–O stretch. The calculations underestimate the frequency splitting of the antisymmetric ...

Why hyperpolarizabilities fall short of the fundamental quantum limits.

Quantum sum rules impose limits on the hyperpolarizability, beta. A survey of the largest second-order molecular susceptibilities finds what appears to be a universal gap between the experimental results and the fundamental limits. In this work, we use theory, linear spectroscopy, Raman spectroscopy, and measured values of beta (using hyper-Rayleigh scattering and Stark Spectroscopy) to show that this gap is due to an unfavorable arrangement of excited state energies. The question of whether this result is a universal property of a quantum system or a matter of present paradigms for making molecules is discussed.

Far-ultraviolet resonance Raman spectroscopy of nitrate ion in solution

Resonance Raman spectra are presented for the nitrate anion, NO3−, in water, ethylene glycol, methanol, and acetonitrile solution at six excitation wavelengths from 246 to 204 nm, on resonance with the lowest π→π* excitation. Absolute Raman cross sections for the CH stretches of ethylene glycol and methanol at these wavelengths are also reported. The nitrate spectra in all four solvents are dominated by fundamentals, overtones, and combination bands of the totally symmetric NO stretch (ν1) near 1043 cm−1 and the out-of-phase NO stretches (ν3) at 1340–1400 cm−1, consistent with substantial changes in NO bond length upon π-electron excitation. The intensity in ν3 and the ≈60 cm−1 splitting of this nominally degenerate vibration are indicative of pronounced breaking of the isolated molecules D3h symmetry by the local solvent environment. Intensity in the overtone of the out-of-plane mode (ν2) near 830 cm−1 suggests a change in the equilibrium geometry from planar to pyramidal upon electronic excitation. The absorption spectra and absolute Raman cross sections are simulated with a model that considers resonance with two orthogonally polarized electronic states whose degeneracy is broken by the locally asymmetric environment. Both solvent reorganization and geometry changes along the nitrate molecular vibrations make major contributions to the breadth of the absorption band. No differences between resonant and nonresonant linewidths are observed for the ν1 band.Resonance Raman spectra are presented for the nitrate anion, NO3−, in water, ethylene glycol, methanol, and acetonitrile solution at six excitation wavelengths from 246 to 204 nm, on resonance with the lowest π→π* excitation. Absolute Raman cross sections for the CH stretches of ethylene glycol and methanol at these wavelengths are also reported. The nitrate spectra in all four solvents are dominated by fundamentals, overtones, and combination bands of the totally symmetric NO stretch (ν1) near 1043 cm−1 and the out-of-phase NO stretches (ν3) at 1340–1400 cm−1, consistent with substantial changes in NO bond length upon π-electron excitation. The intensity in ν3 and the ≈60 cm−1 splitting of this nominally degenerate vibration are indicative of pronounced breaking of the isolated molecules D3h symmetry by the local solvent environment. Intensity in the overtone of the out-of-plane mode (ν2) near 830 cm−1 suggests a change in the equilibrium geometry from planar to pyramidal upon electronic excitation. The ...

Resonance Raman and resonance hyper-Raman intensities: structure and dynamics of molecular excited states in solution.

Resonance Raman scattering is discussed as a vibronic spectroscopy that can provide detailed information about the structure and dynamics of excited electronic states of molecules. The emphasis is on molecules in liquid solution. The theory of resonance Raman intensities and experimental and interpretive methods are discussed both in a historical context and in their present and future implementations. The related but much less developed technique of two-photon-resonant hyper-Raman scattering is also discussed in a similar context.

Electron–Phonon Coupling in CdSe Nanocrystals from an Atomistic Phonon Model

Phonon frequencies and eigenvectors, electron-phonon couplings, and the associated resonance Raman spectra have been calculated for approximately spherical, wurtzite form CdSe nanocrystals having radii of 1.4 to 2.3 nm and containing 318 to 1498 atoms. Calculations of the equilibrium geometries and phonon modes are carried out using an empirical force field, and the electron and hole wave functions are calculated as particle-in-a-sphere envelope functions multiplying the Bloch functions, with valence-band mixing included for the hole functions. The coupling of each phonon mode to the 1S(e)-1S(3/2) and 1S(e)-2S(3/2) excitations is evaluated directly from the change in Coulombic energy along the phonon coordinate. Ten to 50 different modes in each crystal have significant Huang-Rhys factors, clustered around two frequency regions: acoustic phonons at 20-40 cm(-1) depending on crystal size, and optical phonons at 185-200 cm(-1). The Huang-Rhys factors are larger for the acoustic modes than for the optical modes and decrease with increasing crystal size, and the Huang-Rhys factors for each group of modes are smaller for the 1S(e)-2S(3/2) than for the 1S(e)-1S(3/2) excitation. These results are compared with measurements of electron-phonon coupling in CdSe nanocrystals using different experimental techniques.

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 f...

A multimode vibronic treatment of absorption, resonance Raman, and hyper-Rayleigh scattering of excitonically coupled molecular dimers

The linear absorption spectra, resonance Raman excitation profiles and depolarization dispersion curves, and hyper-Rayleigh scattering profiles are calculated for excitonically coupled homodimers of a model electron donor–acceptor “push–pull” conjugated chromophore as a function of dimer geometry. The vibronic eigenstates of the dimer are calculated by diagonalizing the matrix of transition dipole couplings among the vibronic transitions of the constituent monomers. The absorption spectra show the usual red- or blueshifted transitions for J-type or H-type dimers, respectively. When the electronic coupling is large compared with the vibronic width of the monomer spectrum, the dimer absorption spectra exhibit simple Franck–Condon progressions having reduced vibronic intensities compared with the monomer, and the resonance Raman excitation profiles are shifted but otherwise only weakly perturbed. When the coupling is comparable to the vibronic width, the H-dimer absorption spectra exhibit irregular vibronic ...

Hyper-Raman Scattering by Molecular Vibrations

This article reviews the experimental and theoretical aspects of vibrational hyper-Raman scattering from molecules. Particular emphasis is placed on hyper-Raman scattering enhanced by nanostructured metal surfaces and by two-photon electronic resonance.

Excited state molecular dynamics simulations of nonlinear push–pull chromophores

Abstract Julolidinemalononitrile, p -nitroaniline, and julolidinyl- n - N , N ′ -diethylthiobarbituric acid are studied with ground and excited state molecular dynamics simulations in conjunction with the collective electronic oscillator formalism and Onsager’s cavity model. Ground and excited state geometries are calculated in the gas phase and four solvents. The results are interpreted in the context of a two-state valence bond model for charge-transfer transitions of conjugated organic molecules, and are compared to recent resonant Raman experimental results. The calculated geometries are qualitatively consistent with both the two-state model and experiment. In addition, calculated transition density matrices are presented to visualize the changes in charge distribution accompanying photoexcitation.