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Resonance hyper-Raman spectra are reported for zincblende and wurtzite CdSe quantum dots (QDs), CdS quantum dots, and CdSe/CdS tetrapods in toluene solution using 808–824 nm excitation. All structu...

Resonance Raman spectra, including absolute scattering cross sections, depolarization ratios, and overtone to fundamental intensity ratios, have been measured for three sizes of ZnSe quantum dots between 3.8 and 4.9 nm diameter (first excitonic peak maximum at 402–417 nm) using excitation wavelengths between 400 and 425 nm. Spectra were obtained both in cyclohexane solution and in thin films in order to quantitate the exciton–phonon coupling strength. The Raman data and optical absorption spectra were simulated using a particle in a sphere effective mass model for the excitonic transitions similar to that previously employed for CdSe [Lin, C.; Gong, K.; Kelley, D. F.; Kelley, A. M. J. Phys. Chem. C 2015, 119, 7491]. The Huang–Rhys parameter of the longitudinal optical phonon in the lowest excitonic transition is in the range S = 0.3–0.5, about a factor of 2 larger than for CdSe quantum dots of similar size. Smaller ZnSe quantum dots (∼3.0 nm diameter, first excitonic maximum at 379 nm) measured in films s...

Resonance Raman excitation profiles, including absolute Raman cross sections, have been measured for the optical phonons of organic ligand capped spherical CdSe nanocrystals with average diameters ranging from 2.6 to 5.2 nm. The absorption spectra, fundamental excitation profiles, overtone to fundamental intensity ratios, and depolarization ratio dispersion profiles are simulated using a model for the exciton phonon coupling strengths in the various excitonic transitions that contribute to the resonance enhancement. The resonance Raman cross sections increase strongly with nanocrystal size as expected from the increased transition dipole moments (oscillator strengths) in the larger particles. However, the exciton phonon coupling strengths, particularly for the lowest energy excitonic transition (1Se 1S3/2), are relatively independent of nanocrystal size over this range, perhaps decreasing slightly with increasing size. This result is consistent with calculations of exciton phonon coupling using an atomistic model for the phonon modes and an effective mass approximation envelope function model for the excitonic states if the electron and hole effective masses are adjusted to fit the experimental excitonic transition energies.

The spectroscopic and photophysical similarities and differences between charge trapping by surface ligands on CdSe quantum dots and charge delocalization into the shell in excited CdSe core/shell nanocrystals are discussed. Optical absorption and resonance Raman spectroscopies are used to study small CdSe quantum dots coated with organic ligands that accept electrons (methyl viologen) or holes (phenothiazine, 4-methylbenzenethiol), as well as with semiconductor shells that delocalize electrons (CdS) or holes (CdTe). The organic ligands have only a small effect on the optical absorption spectrum and contribute negligibly to the resonance Raman spectra, indicating little participation of ligand orbitals in the initial excitation. The semiconductor shells more strongly red-shift the absorption spectrum by delocalizing the electron and/or hole into the shell, and vibrations of the shell appear in the resonance Raman spectrum, showing that the shell is involved in the vertical excitation. The qualitative differences between ligand and semiconductor shells are discussed in terms of the energetics and coupling strengths.

The biexciton Auger dynamics of alloyed Zn1–xCdxSe/ZnS and Zn1–xCdxSe/CdS core/shell nanoparticles are examined. A series of particles with increasing cadmium mole fractions are prepared by synthesizing ZnSe nanocrystals followed by cation exchange with cadmium oleate. This is followed by the low-temperature deposition of a CdS or ZnS shell to passivate surface traps. The percent composition of cadmium in the core particles is determined from the absorption spectrum upon cation exchange, and the excited-state dynamics are measured using power-dependent transient absorption spectroscopy. The biexciton lifetimes are controlled by Auger processes, and the results show that Auger recombination is faster for alloys than for either pure ZnSe or pure CdSe. The lifetimes go through a minimum at a composition that is approximately 10–40% cadmium. This behavior is assigned to partial hole localization at clusters of cadmium atoms in the alloy, which increases Auger rates through the same mechanism that is responsib...

Ligand exchange with phenyl dithiocarbamates is known to significantly red-shift the lowest excitonic transition of CdSe quantum dots. Here we show that the resonance Raman spectra of CdSe quantum dots treated with p-methoxyphenyldithiocarbamate are nearly identical to those of CdSe quantum dots to which thin CdS shells have been added by standard synthetic methods. We conclude that the dithiocarbamate ligands decompose and react with Cd2+ not fully removed following the synthesis to form CdS on the surface of the quantum dots. This red-shifts the absorption spectrum primarily by delocalizing the electron into the CdS shell. These results are consistent with the common use of aliphatic dithiocarbamates as single-source precursors for adding ZnS or CdS shells to CdSe quantum dots and with the recent demonstration through NMR that phenyldithiocarbamate ligands decompose during nanocrystal ligand exchange.

Resonance Raman spectra and absolute differential Raman cross-sections have been measured for CdSe nanocrystals in both the wurtzite and zincblende crystal forms at four excitation wavelengths from 457.9 to 514.5 nm. The frequency and bandshape of the longitudinal optical (LO) phonon fundamental is essentially identical for both crystal forms at each excitation wavelength. The LO phonon overtone to fundamental intensity ratio appears to be slightly higher for the wurtzite form, which may suggest slightly stronger exciton-phonon coupling from the Fröhlich mechanism in the wurtzite form. The LO fundamental Raman cross-sections are very similar for both crystal forms at each excitation wavelength. © 2012 Elsevier B.V. All rights reserved.

For linear electron-phonon coupling, the Huang-Rhys factor, S, gives the intensity ratio of the one-quantum vibronic transition (0 → 1) to the purely electronic origin transition (0 → 0) in a vibrationally resolved, zero-temperature absorption or emission spectrum. It is often assumed that the overtone to fundamental integrated intensity ratio in resonance Raman scattering of semiconductor nanocrystals is equal to or proportional to S, or that S may be determined from the overtone intensity in some other straightforward manner. In fact, this is not generally possible because of different excitation profiles for overtones and fundamentals, differential sensitivity of overtones and fundamentals to electronic dephasing, and interference effects from partially overlapping electronic transitions. Here we examine the relationship between the Huang-Rhys factor and the overtone to fundamental intensity ratio through spectroscopic simulations using parameters appropriate to II-VI semiconductor nanocrystals such as CdSe. A simple equation relating the overtone to fundamental Raman intensity ratio to the Huang-Rhys factor is obtained only in the case of a single resonant electronic state, excitation at the maximum of the inhomogeneously broadened absorption band, and a homogeneous line width small compared with the phonon frequency. © 2013 American Chemical Society.

Hyper-Raman spectra were obtained for zinc phthalocyanine in a dilute pyridine solution at excitation wavelengths that are two-photon resonant with the one-photon-allowed B band (360-380 nm) as well as with the two-photon absorption near 440 nm reported by Drobizhev et al. ( J. Chem. Phys. 2006, 124, 224701 ). In both regions, the hyper-Raman spectra were very different from the linear resonance Raman spectra at the corresponding excitation frequencies. While the resonance Raman spectra show only g symmetry modes, almost all of the hyper-Raman frequencies can be assigned as fundamentals of E u symmetry that also are observed in the infrared absorption spectrum or E u symmetry combination bands. These results contrast sharply with previous observations of highly noncentrosymmetric push-pull conjugated molecules and are consistent with a structure for phthalocyanine in solution that is centrosymmetric or nearly so. The hyper-Raman spectra show different intensity patterns in the two excitation regions, consistent with different Franck-Condon and/or vibronic coupling matrix elements for the different resonant states.

Linear absorption spectra, resonance Raman spectra and excitation profiles, and two-photon-resonant hyper-Rayleigh and hyper-Raman scattering hyperpolarizability profiles are reported for the push-pull chromophore N,N-dipropyl-p-nitroaniline in seven solvents spanning a wide range of polarities. The absorption spectral maximum red shifts by about 2700 cm(-1), and the symmetric -NO2 stretch shifts to lower frequencies by about 11 cm(-1) from hexane to acetonitrile, indicative of significant solvent effects on both the ground and excited electronic states. The intensity patterns in the resonance Raman and hyper-Raman spectra are similar and show only a small solvent dependence except in acetonitrile, where both the Raman and hyper-Raman intensities are considerably reduced. Quantitative modeling of all four spectroscopic observables in all seven solvents reveals that the origin of this effect is an increased solvent-induced homogeneous broadening in acetonitrile. The linear absorption oscillator strength is nearly solvent-independent, and the peak resonant hyperpolarizability, beta(-2omega;omega,omega), varies by only about 15% across the wide range of solvents examined. These results suggest that the resonant two-photon absorption cross sections in this chromophore should exhibit only a weak solvent dependence.

Metal catalyzed, CVD-grown silicon nanowires decorated by chemical assembly of closely spaced Ag nanocrystals were modified with the well-known "silver mirror" reaction and investigated as substrates for surface-enhanced Raman (SERS) and hyper-Raman (SEHRS) spectroscopy. Four chromophores were examined: Rhodamine 6G, crystal violet, a cyanine dye, and a cationic donor-acceptor substituted stilbene. After soaking the substrates overnight in 10(-4) M aqueous chromophore solutions, all four chromophores gave good-quality SERS spectra in < or =60 s using

The depolarization ratios in resonance Raman, resonance hyper-Raman and resonance hyper-Rayleigh scattering have been measured for two octupolar conjugated organic chromophores that possess nominal threefold symmetry, crystal violet (CV) and 1,3,5-tris[4-[(1E)-2-(4-nitrophenyl)-ethenyl]phenyl]benzene (TPB(PV)1(NO2)). For comparison, the same quantities are measured for two dipolar donor–acceptor substituted conjugated molecules. The dipolar molecules both exhibit depolarization ratios of for resonance Raman and ρ = 1/5 for hyper-Rayleigh and hyper-Raman, consistent with the polarizability and the hyperpolarizability tensors having a single non-zero element. The resonance hyper-Rayleigh scattering from both of the octupolar molecules exhibits a depolarization ratio close to the value of 2/3 expected for D3h or D3 symmetry, but the resonance Raman and resonance hyper-Raman depolarization ratios deviate significantly from the values expected for those symmetries. In TPB(PV)1(NO2), the depolarization ratios i...

Surface-enhanced Raman spectra (SERS) are presented for two water-soluble, donor–donor- or donor–acceptor-substituted phenylene vinylene derivatives adsorbed on the surfaces of colloidal silver nanoparticles. The normally water-insoluble, substituted stilbene and distyrylbenzene chromophores have been modified by attaching trimethylammoniumalkyl chains to the amino donor group, creating both aqueous solubility and a net positive charge that facilitates binding to citrate- or borohydride-reduced silver colloids. A symmetrically donor-substituted distyrylbenzene exhibits strong electronically pre-resonant SERS spectra that are not highly dependent on excitation wavelength and are qualitatively similar to the solution-phase resonance Raman (RR) spectra. A donor–acceptor-substituted stilbene exhibits weaker SERS enhancement despite being fully electronically resonant. The spectra vary considerably with excitation wavelength and show new lines not present in the solution-phase spectra, which are attributed to a photoproduct formed on the surface. Two-photon resonant hyper-Raman spectra are also presented for the donor–acceptor stilbene, both with and without surface enhancement. Copyright © 2006 John Wiley & Sons, Ltd.

Absorption and resonance Raman spectra have been measured over a wide range of solvents for two merocyanine dyes containing the indoline ("Fischer" base) electron donor group with different accepting groups. One appears to be near the cyanine limit (equal contributions of the neutral and zwitterionic resonance forms to both ground- and excited-state structures) based on electrooptic absorption data showing a very small dipole moment change upon electronic excitation. The resonance Raman spectra of both molecules show significant frequency shifts and intensity redistributions that evolve monotonically with increasing solvent polarity and are consistent with increasing zwitterionic character of the ground-state structure. The vibrational reorganization energies of both molecules, obtained by simulating the absorption band shapes, are smaller in polar solvents than in nonpolar or weakly polar ones, consistent with a more cyanine-like structure at higher solvent polarities. However, the vibrational reorganization energies of both molecules exceed 700 cm(-1) in all solvents, larger than in many true cyanine dyes, and the optical absorption maxima do not correlate well with either solvent polarity or vibrational reorganization energy. This indicates some limitations to the structural conclusions that can be reached from the two-state model for pi-conjugated donor-acceptor systems.

Two-photon resonant hyper-Raman spectroscopy from large molecules in solution is discussed as a method for probing excited electronic states that are both one- and two-photon allowed. A number of electron donor–acceptor substituted conjugated organic molecules that have large first hyperpolarizabilities (β) have recently been shown to give strong resonance hyper-Raman (RHR) spectra in solution. The quantum mechanical formulation of RHR scattering is reviewed, and practical approaches to calculating RHR spectra and excitation profiles are proposed by reference to methods already developed for simulating linear resonance Raman spectra and profiles. The potential benefits of a close interplay between experiments, empirical simulations, and electronic structure calculations of ground-state and excited-state properties are emphasized. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Absorption spectra, resonance Raman spectra and depolarization ratios, fluorescence spectra and emission polarizations, and simulations of the resonance Raman excitation profiles and absorption spe...

Two-photon-resonant hyper-Raman spectra are reported for three "push-pull" conjugated organic chromophores bearing -NO(2) acceptor groups, two dipolar and one octupolar. The excitation source is an unamplified picosecond mode-locked Ti:sapphire laser tunable from 720 to 950 nm. The linear resonance Raman spectra of the same molecules are measured using excitation from the laser second harmonic. Excitation on resonance with the lowest-lying band in the linear absorption spectrum yields nearly identical resonance Raman and resonance hyper-Raman spectra. However, excitation into a region that appears to contain more than one electronic transition gives rise to different intensity patterns in the linear and nonlinear spectra, indicating that different transitions contribute differently to the one-photon and two-photon oscillator strength. The promise of the hyper-Raman technique for examining electronic transitions that are both one- and two-photon allowed is discussed.

Resonance Raman excitation profiles and absolute cross sections are presented for three closely related merocyanine dyes as monomers in dichloromethane solution and as H-dimers in dioxane solution. For both monomers and dimers the absorption spectra and the resonance Raman intensities of the 24 to 30 strongest vibrations are quantitatively simulated using time-dependent wave packet propagation methods to determine the geometry changes along each Franck−Condon active mode upon electronic excitation, as well as the homogeneous and inhomogeneous electronic line widths. In addition, an approach previously formulated to describe the vibronic structure of an excitonically coupled homodimer with multiple vibrational modes [Kelley, A. M. J. Chem. Phys. 2003, 119, 3320−3331] is applied to model the dimer spectra. Comparison of these two calculations demonstrates that when the intermonomer coupling is strong the theoretical absorption and resonance Raman spectra of the dimer can be very well approximated by treatin...

The empirical modeling of the absorption spectra and resonance Raman excitation profiles of a large molecule in solution requires adjustment of a minimum of dozens of parameters to fit several hundred data points. This is a difficult optimization problem because all of the observables depend on all of the parameters in a highly coupled and nonlinear manner. Standard nonlinear least-squares fitting methods are highly susceptible to becoming trapped in local minima in the error function unless very good initial guesses for the molecular parameters are made. Here, we demonstrate a method that employs a real-valued genetic algorithm to force a broad search through parameter space to determine the best-fit parameters. The multiobjective genetic algorithm is successful at inverting absorption spectra and Raman excitation profiles to determine molecular parameters. When vibronic structure is evident in the absorption profile, the algorithm returns nearly quantitative results. For broad, featureless profiles, the algorithm returns the correct slope of the excited state surface but cannot independently determine the excited-state frequency and the equilibrium geometry change. Compared with manual adjustment of parameters to obtain a best fit, the genetic algorithm is computationally less efficient but requires less human time.

Resonance Raman spectra and absolute cross sections have been measured for cresyl violet in aqueous solution and bound to the surface of SiO 2 colloidal nanoparticles. The absorption spectra of cresyl violet on SiO 2 havepreviously been interpreted to show formation of H-type dimers on the surface. The spectra of the monomers in solution are simulated to obtain the excited-state geometry change along each normal mode and the electronic spectral broadening parameters. The spectra of the dimers on SiO 2 are then simulated with a model that assumes transition dipole coupling between the vibronic transitions on the two monomers. The simulations require increases in both the electronic homogeneous line width and the inhomogeneous line width upon binding to the colloid. The general features of the resonance Raman spectra are reproduced fairly well, but significant differences in the relative intensities of certain Raman lines upon binding to the surface suggest specific vibrational or vibronic effects not considered in this simple model.

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.

Site-selective line-narrowed fluorescence spectra have been obtained for the S1↔S0 transitions of phenanthrene and perylene in polyethylene, polystyrene, and poly(methyl methacrylate) films at 1.5 K. The spectral distributions and intensities of the phonon sidebands on each vibronic transition are analyzed to examine the chromophore, matrix, and vibrational level dependence of the coupling of the chromophore’s electronic transition to matrix and/or intermolecular vibrations. The frequency distribution of the coupled phonons (spectral density) and the strength of the coupling vary considerably among polymers. Different vibronic transitions of the same chromophore in the same matrix exhibit similar spectral densities but slightly different coupling strengths. The coupling strengths appear to correlate with the extent to which the normal mode changes the molecule’s overall dimensions. The more redshifted absorbers within a given chromophore and matrix also exhibit slightly stronger coupling to matrix phonons...

Resonance Raman spectra have been obtained for two electron donor−acceptor substituted “push−pull” conjugated molecules possessing guaiazulene donor groups and thiobarbituric acid acceptor groups. ...

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.

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.

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.

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 ...

Resonance Raman spectra and absolute cross sections have been measured for the azide anion (N3-) in dilute aqueous solution at excitation wavelengths of 246, 228, 223, 218, and 208 nm, on resonance with the longest-wavelength UV absorption bands. The spectra are dominated by the fundamental of the 1343 cm-1 symmetric stretch, with much lower intensities in the first overtone of the symmetric stretch and the overtone of the bending mode at 1275 cm-1. The weak overtones and generally low resonance Raman cross sections suggest unusually small changes in the N−N bond lengths relative to those expected for valence transitions of small molecules, and/or particularly strong coupling of the electronic transitions to solvent degrees of freedom leading to rapid effective electronic dephasing. Ab initio calculations have been performed on complexes of N3- with three and four water molecules at the single CI level using the 6-311++g** basis with additional diffuse functions on the N atoms. These calculations predict ...

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...

A novel approach is presented for finding the vibrational frequencies, Franck–Condon factors, and vibronic linewidths that best reproduce typical, poorly resolved electronic absorption (or fluorescence) spectra of molecules in condensed phases. While calculation of the theoretical spectrum from the molecular parameters is straightforward within the harmonic oscillator approximation for the vibrations, “inversion” of an experimental spectrum to deduce these parameters is not. Standard nonlinear least-squares fitting methods such as Levenberg–Marquardt are highly susceptible to becoming trapped in local minima in the error function unless very good initial guesses for the molecular parameters are made. Here we employ a genetic algorithm to force a broad search through parameter space and couple it with the Levenberg–Marquardt method to speed convergence to each local minimum. In addition, a neural network trained on a large set of synthetic spectra is used to provide an initial guess for the fitting paramet...

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 ...

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.

Nonresonant Raman spectra and site-selective fluorescence spectra (polyethylene matrix, 1.5 K) are reported for 1,4-distyrylbenzene, its tert -butyl and paracyclophane substituted derivatives, and the paracyclophane derivative bearing two distyrylbenzene chains in a pseudo- para orientation, a simple model for the interchain interactions in poly(phenylene-vinylene) films. The first three compounds show pronounced fluorescence line-narrowing when excited near the red edge of their absorption spectra. The bichromophoric compound exhibits no narrowing although its Raman spectrum, room-temperature fluorescence spectrum, and lifetime are similar to those of the other three molecules. This may reflect increased low-frequency Franck–Condon activity in the electronically delocalized bichromophoric system.

A resonance Raman intensity analysis is presented for the carbazole/tetracyanoethylene donor−acceptor charge-transfer complex in dichloromethane solution. The intent is to determine the nuclear reorganization contributions to the rates of charge hopping in carbazole polymers used as hole-transport agents in the xerographic (electrophotographic) process. Resonance Raman cross sections have been measured at seven excitation wavelengths spanning the broad visible charge-transfer absorption known to consist of two strongly overlapping charge-transfer electronic transitions. Interference between the Raman amplitudes from these two transitions manifests itself in the Raman excitation profiles for a number of resonantly enhanced modes. Simultaneous modeling of the absorption spectrum and the Raman cross sections shows that explicit consideration of the signs as well as the magnitudes of the normal mode displacements is required to reproduce the experimental data. The signs of the mode displacements obtained from...

Monochromatically excited total emission spectra have been measured for the S1→S0 transition of azulene in polyethylene, polystyrene, and poly(methylmethacrylate) matrices over a temperature range from 1.4 to 100 K. The spectra in all three polymers exhibit strong zero-phonon lines (excitation of azulene vibrations only) accompanied by well-defined Stokes-shifted phonon sidebands at the lowest temperatures. As the temperature is raised the phonon bands broaden and gain relative intensity at the expense of the zero-phonon lines, and the spectra become qualitatively similar to the room-temperature liquid-phase spectra with sharp Raman lines on a broad fluorescence background. The near-origin-excited data are simulated by calculating the complete emission spectrum as a χ(3) process that assumes no artificial partitioning between “Raman” and “fluorescence.” The internal vibrations of azulene are modeled as simple undamped displaced harmonic oscillators while the intermolecular or matrix phonons are either mod...

Quantitative analysis of resonance Raman scattering cross sections, together with charge-transfer absorption and emission spectra, can provide detailed information about the changes in nuclear equilibrium geometry undergone by both the electron donor and acceptor and the surrounding solvent in photoinduced charge-transfer processes. The molecular parameters that determine absorption and fluorescence band shapes and resonance Raman cross sections are summarized, and methods for extracting those parameters through spectral modeling are reviewed with emphasis on charge-transfer systems. Applications to the determination of molecular and solvent reorganization parameters to several organic intra- and intermolecular charge-transfer transitions are then presented, and prospects for further development of the technique are discussed.

Resonance Raman spectra of two p-methoxybenzyltrialkylsilanes (alkyl = methyl and ethyl) have been obtained both as their neutral charge-transfer complexes with tetracyanoethylene in steady-state cw experiments and as their radical cations via two-color pump−probe transient measurements. The ground-state charge-transfer resonant spectra exhibit intensity predominantly in phenyl-localized modes, suggesting that vertical excitation to the contact ion-pair state involves little participation of the bond that is known to undergo subsequent nucleophile-assisted cleavage in the separated radical cation. Quantitative modeling of the absolute cross sections for the methyl compound is used to determine the mode-specific reorganization energies accompanying vertical electron transfer. Transient spectra of the relaxed radical cations show more than 20 resonance-enhanced modes, several of which have significant contributions from the C−Si stretching coordinate based on frequency shifts between the natural abundance a...

Surface enhanced Raman spectroscopy is used to characterize the interaction of a common poly(thiophene) based component of organic polymer photovoltaic cells, PEDOT:PSS, with plasmonically active metal nanoparticles that are reported to enhance solar conversion efficiency.

Resonance Raman (RR) and resonance hyper-Raman (RHR) spectra have been obtained for a nominally centrosymmetric dialkylammonium-substituted distyrylbenzene that was previously shown to exhibit a strong solvent polarity effect on its two-photon absorption strength. Spectra have been obtained in acetonitrile, methanol, dimethylsulfoxide, and water. In each solvent, the RR and RHR spectra show very similar intensity patterns in the strongest lines in both the C=C and C-C stretching regions, but the RHR peaks are shifted to higher energies by 5-10 cm(-1). These are interpreted as vibrations of similar mode character but different symmetries, with u symmetry modes observed in RHR and g symmetry modes in RR. In all solvents the nonzero chromophore contribution to the hyper-Rayleigh scattering indicates that the center of symmetry is broken to some degree, and this effect is more pronounced in water than in the other three solvents. The ratio of RR intensity to one-photon fluorescence intensity shows an approximate inverse relationship with the fluorescence quantum yield, consistent with the approximate solvent independence of the one-photon transition strength implied by the linear absorption spectra. The ratio of RHR intensity to two-photon fluorescence intensity is smaller than the corresponding RR to one-photon fluorescence ratio in all four solvents.

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.

The extinction spectra of J-aggregated dyes on gold nanoparticles, which exhibit interferences between the plasmonic and dye resonances, are simulated by a quantum mechanical model that considers the dye transition to interact through transition-dipole coupling with a continuum of nanoparticle states. This alternative to the classical core-shell dielectric model provides the wavefunctions of the coupled molecule-nanoparticle system and qualitatively explains the enhancement of resonance Raman, fluorescence, and other light-driven processes of molecules adsorbed to nanoparticles.

The normally weak process of hyper-Raman scattering should be greatly enhanced when the excitation is both one- and two-photon resonant. This triply resonant process is explored both theoretically and experimentally for organic nonlinear chromophores.

Resonance Raman and resonance hyper-Raman spectra and excitation profiles have been measured for a "push-pull" donor-acceptor substituted conjugated polyene bearing a julolidine donor group and a nitrophenyl acceptor group, in acetone at excitation wavelengths from 485 to 356 nm (two-photon wavelengths for the nonlinear spectra). These wavelengths span the strong visible to near-UV linear absorption spectrum, which appears to involve at least three different electronic transitions. The relative intensities of different vibrational bands vary considerably across the excitation spectrum, with the hyper-Raman spectra showing greater variation than the linear Raman. A previously derived theory of resonance hyper-Raman intensities is modified to include contributions from purely vibrational levels of the ground electronic state as intermediate states in the two-photon absorption process. These contributions are found to have only a slight effect on the hyper-Rayleigh intensities and profiles, but they significantly influence some of the hyper-Raman profiles. The absorption spectrum and the Raman, hyper-Rayleigh, and hyper-Raman excitation profiles are quantitatively simulated under the assumption that three excited electronic states contribute to the one- and two-photon absorption in this region. The transition centered near 400 nm is largely localized on the nitrophenyl group, while the transitions near 475 and 355 nm are more delocalized.

Many conjugated organic chromophores with large first hyperpolarizabilities also exhibit strong two-photon-resonant hyper-Raman scattering. The hyper-Raman spectra, excitation profiles, and depolarization ratios provide structural and energetic information about overlapping electronic transitions in these molecules.

The effect of vibrational structure on the frequency dependence of the first molecular hyperpolarizability of two thiophene-based charge-transfer chromophores is investigated. A time domain formulation is used to express the polarizability. The new expression includes the solvent-induced inhomogeneous distribution of electronic transition frequencies as well as the effect of the motion of solvent molecules that modulates the vibrational and electronic transition frequencies of the nonlinear optical molecule on which the first molecular hyperpolarizability depends. Resonance Raman scattering and one-photon absorption spectra of the chromophores are measured. By simultaneously fitting the experimental one-photon absorption spectrum and Raman cross sections of vibrational lines derived from resonance Raman scattering to a theoretical model, important parameters needed for the calculation of the first molecular hyperpolarizability are obtained. The first molecular hyperpolarizability is calculated as a function of frequency covering both nonresonance and two-photon resonance regions. The calculated result is compared with the measured hyperpolarizability as a function of frequency of the excitation laser. The resonance Raman-based analysis is shown to account reasonably well for the dispersion of the hyperpolarizability of the two charge transfer chromophores.

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.

Resonance Raman spectra of the radical cations of phenylcyclopropane and trans-1-phenyl-2-methylcyclopropane are reported. A near-UV pump pulse excites a photosensitizer which oxidizes the species of interest, and a visible probe pulse delayed by 35 ns obtains the spectrum of the radical ion. The transient Raman spectra of the phenylcyclopropane radical cations show nine or ten enhanced modes for which assignments are suggested based on density functional theory (DFT) results, previously published calculations on the resonant excited state and comparison between the unsubstituted and methyl-substituted compounds. The transient spectra are consistent with the large effect of methyl substitution on the geometry of the radical cation predicted by DFT. The resonance Raman spectrum of the electron donor-acceptor complex between phenylcyclopropane and tetracyanoethylene is also obtained on resonance with the visible charge-transfer absorption band, but the spectra are very weak and only a few resonance enhanced lines are observed. These results are compared with previously published data on the p-methoxybenzyltrimethylsilane charge-transfer complex and radical cation.