Intramolecular Vibrational Modes Research Articles

50-fs Photoinduced Intramolecular Charge Separation in Triphenylmethane Lactones

Upon excitation of their structural subunits, phenolphthalein and malachite green lactone undergo ultrafast intramolecular charge separation with formation of a long-lived charge-transfer state (intramolecular “radical ion pair”). The kinetics of the primary charge separation in these molecules in aprotic solvents has been studied using femtosecond pump−probe spectroscopy. The phenol radical cation is formed by electron transfer within 50 fs after excitation of phenolphthalein to the S1 state. In malachite green lactone the products of charge separation are observed 150 fs after excitation to the S2 state. The results demonstrate that electron transfer in these molecules occurs faster than the time scale of inertial solvation dynamics. An intramolecular vibrational mode to promote charge separation is indicated.

Rovibrational and dynamical properties of the hydrogen bonded complex (CH2)2S-HF: a combined free jet, cell, and neon matrix-Fourier transform infrared study.

Fourier transform infrared spectra of the nu(s) (HF stretching) band of the (CH(2))(2)S-HF complex have been recorded at 0.1-0.5 cm(-1) resolution in a cooled cell, in a supersonic jet expansion seeded with argon and in a neon matrix at 4.5 K. The combination of controlled temperature effects over a range of 40-250 K and a sophisticated band contour simulation program allows the separation of homogeneous and inhomogeneous contributions and reveals significant anharmonic couplings between intramolecular and intermolecular vibrational modes similar to our previous work on (CH(2))(2)S-DF. The sign of the coupling constants is consistent with the expected strengthening of the hydrogen bond upon vibrational excitation of HF which also explains the observed small variations of the geometrical parameters in the excited state. The analysis of sum and difference combination bands involving nu(s) provides accurate values of intermolecular harmonic frequencies and anharmonicities and a good estimate of the dissociation energy of the complex. Frequencies and coupling parameters derived from gas phase spectra compare well with results from neon matrix experiments. The effective linewidth provides a lower bound for the predissociation lifetime of 10 ps. The comparison between effective linewidths and vibrational densities of states for (CH(2))(2)S-HF and -DF complexes highlights the important role of intramolecular vibrational redistribution in the vibrational dynamics of medium strength hydrogen bonds.

Femtosecond primary events in bacteriorhodopsin and its retinal modified analogs: revision of commonly accepted interpretation of electronic spectra of transient intermediates in the bacteriorhodopsin photocycle.

Femtosecond primary events in bacteriorhodopsin (BR) and its retinal modified analogs are discussed. Ultrafast time resolved electronic spectra of the primary intermediates induced in the BR photocycle are discussed along with spectral and kinetic inconsistencies of the previous models proposed in the literature. The theoretical model proposed in this paper based on vibrational coupling between the electronic transition of the chromophore and intramolecular vibrational modes allows us to calculate the equilibrium electronic absorption band shape and the hole burning profiles. The model is able to rationalize the complex pattern of behavior for the primary events in BR and explain the origin of the apparent inconsistencies between the experiment and the previous theoretical models. The model presented in the paper is based on the anharmonic coupling assumption in the adiabatic approximation using the canonical transformation method for diagonalization of the vibrational Hamiltonian instead of the commonly used perturbation theory. The electronic transition occurs between the Born-Oppenheimer potential energy surfaces with the electron involved in the transition being coupled to the intramolecular vibrational modes of the molecule (chromophore). The relaxation of the excited state occurs by indirect damping (dephasing) mechanisms. The indirect dephasing is governed by the time evolution of the anharmonic coupling constant driven by the resonance energy exchange between the intramolecular vibrational mode and the bath. The coupling with the intramolecular vibrational modes results in the Franck-Condon progression of bands that are broadened due to the vibrational dephasing mechanisms. The electronic absorption line shape has been calculated based on the linear response theory whereas the third order nonlinear response functions have been used to analyze the hole burning profiles obtained from the pump-probe time-resolved measurements. The theoretical treatment proposed in this paper provides a basis for a substantial revision of the commonly accepted interpretation of the primary events in the BR photocycle that exists in the literature.

Analysis of the vibronic fine structure in circularly polarized emission spectra from chiral molecular aggregates.

Using a Frenkel-exciton model, the degree of circular polarization of the luminescence (g(lum)) from one-dimensional, helical aggregates of chromophoric molecules is investigated theoretically. The coupling between the electronic excitation and a local, intramolecular vibrational mode is taken into account. Analytical expressions for the fluorescence band shape and g(lum) are presented for the case of strong and weak electronic coupling between the chromophoric units. Results are compared to those from numerical calculations obtained using the three particle approximation. g(lum) for the 0-0 vibronic band is found to be independent of the relative strength of electronic coupling between chromophores and excitation-vibration coupling. It depends solely on the number of coherently coupled molecules. In contrast, for the higher vibronic transitions[g(lum)] decreases with decreasing strength of the electronic coupling. In the limit of strong electronic coupling, [g(lum)] is almost constant throughout the series of vibronic transitions but for weak coupling [g(lum)] becomes vanishingly small for all vibronic transitions except for the 0-0 transition. The results are interpreted in terms of dynamic localization of the excitation during the zero point vibrational motion in the excited state of the aggregate. It is concluded that circular polarization measurements provide an independent way to determine the coherence size and bandwidth of the lowest exciton state for chiral aggregates.

Characterization of the molecular parameters determining charge transport in anthradithiophene.

The molecular parameters that govern charge transport in anthradithiophene (ADT) are studied by a joint experimental/theoretical approach involving high-resolution gas-phase photoelectron spectroscopy and quantum-mechanical methods. The hole reorganization energy of ADT has been determined by an analysis of the vibrational structure of the lowest ionization band in the gas-phase photoelectron spectrum as well as by density-functional theory calculations. In addition, various dimers and clusters of ADT molecules have been considered in order to understand the effect of molecular packing on the hole and electron intermolecular transfer integrals. The results indicate that the intrinsic electronic structure, the relevant intramolecular vibrational modes, and the intermolecular interactions in ADT are very similar to those in pentacene.

Temperature dependent exciton emission from herringbone aggregates of conjugated oligomers.

In this work, the effect of temperature, exciton bandwidth, and size on the photoluminescence spectra of defect-free two-dimensional herringbone aggregates of pi-conjugated oligomers such as oligophenylene vinylene and oligothiophene is investigated theoretically. The model is based on exciton-phonon coupling in two-dimensional herringbone lattices with the exciton deriving from the lowest optical (1Ag-->1Bu) transition and the phonon from the most strongly coupled intramolecular vibrational mode with frequency omega0. Simple analytical expressions are obtained for the line strengths of the emission origin (0-0) and first replica (0-1) as a function of the number of molecules comprising the aggregate, N, the free exciton bandwidth, WD, and the temperature, T. At a given temperature, the 0-0 emission intensity initially scales as N/Nth, where Nth is the superradiant threshold number, but eventually converges to NT/Nth, where NT is the size independent thermal coherence number. NT is inversely proportional to temperature and proportional to the exciton band curvature (omegac) near the band bottom; NT=1+4piomegac/kbT. In striking contrast, the 0-1 line strength is relatively insensitive to temperature and size, but scales as the inverse square of WD+omega0. The insensitivity of the first replica to the exciton coherence number makes the ratio of the 0-0 to 0-1 line strengths a measure of the exciton coherence number. The ratio can be used to test for crystal purity. Comparison to experiments on thin films of quaterthiophene shows that the thermal coherence size is given by NT approximately 1+450/T (K) and that superradiance, which requires NT>Nth, can only be observed at temperatures less than 1 K.

Magnetic field dependent vibrational modes in κ-(ET)2Cu(SCN)2organic superconductor

The infrared reflectance of the 10.4 K organic superconductor κ-(ET) 2 Cu(SCN) 2 has been measured as a function of applied magnetic field at 4.2 K. We investigate changes in intramolecular vibrational modes between the superconducting (low field) and normal (high field) states, of interest for mechanistic reasons. It is shown that the v3 (Ag), ν 60 (B 3g ), and ν 21 (B 1g ) modes display field dependence. These results suggest that intramolecular vibrational modes are involved in the superconducting to normal state transition in κ-(ET) 2 Cu(SCN) 2 below T c .

Mode-Selective Optical Kerr Effect Spectroscopy

We demonstrate that by using two pump pulses with independently controllable polarizations, intensity, and timing, different contributions to the optical Kerr effect (OKE) signal in liquids can be enhanced and suppressed. When both pump pulses have the same polarization, intramolecular vibrations can be enhanced or suppressed without affecting the reorientational diffusion contribution to the signal significantly. Similar control can be exerted over intramolecular vibrations when the pump pulses are perpendicularly polarized, and under these conditions it is also possible to suppress the reorientational diffusion component of the signal completely. When two intramolecular vibrational modes are present in the signal, it is possible to enhance one while completely suppressing the other if the pump polarizations and timing are chosen appropriately. This technique should be a useful means for enhancing contrast in OKE microscopy.

Far-infrared vibrational modes of polycrystalline saccharides

We have measured the temperature dependence of the far-infrared vibrational modes of polycrystalline saccharide molecules in the frequency range 0.1–3.0 THz using a time-resolved terahertz spectroscopy system. Coherent measurement of the transmitted terahertz electric fields showed distinct absorption features, in both monosaccharides and disaccharides, originating from inter- and intra-molecular vibrational modes. These results show that far-infrared absorption features are highly sensitive to the structure and spatial arrangement of molecules. Furthermore, we demonstrate a broad band terahertz spectroscopy system that can be used to probe molecular interactions up to higher frequencies (>6.0 THz).

Magnetic field dependent vibrational modes in isotopically decoratedκ−(ET)2Cu(SCN)2

We report the infrared reflectance of several isotopically decorated samples of the 10.4-K organic superconductor $\ensuremath{\kappa}\ensuremath{-}(\mathrm{ET}{)}_{2}\mathrm{Cu}(\mathrm{SCN}{)}_{2}$ as a function of applied magnetic field at 4.2 K, allowing us to explore changes in intramolecular vibrational modes between the superconducting (low-field) and normal (high-field) states. Comparison with previous variable temperature transport data in which the crystals have similar isotopic decoration patterns suggests that, while the ${\ensuremath{\nu}}_{3}$ ${(A}_{g}),$ ${\ensuremath{\nu}}_{60}$ ${(B}_{3g}),$ and ${\ensuremath{\nu}}_{21}$ ${(B}_{1g})$ are involved in the field-induced normal to superconducting transition below ${T}_{c},$ there is no direct correlation between the value of ${T}_{c}$ and the characteristics of these modes. Intramolecular vibrational modes are therefore not driving the superconducting to normal state transition in $\ensuremath{\kappa}\ensuremath{-}(\mathrm{ET}{)}_{2}\mathrm{Cu}(\mathrm{SCN}{)}_{2}.$ Some plausible mechanisms, which can account for the sensitivity of these intramolecular vibrational modes to the magnetic field, are briefly discussed.

The adsorption site of methoxy and ethoxy on Cu(1 0 0) as determined by vibrational spectroscopy and first principle calculations

In the present work we determine the adsorption site of two polyatomic molecules, methoxy and ethoxy, on Cu(1 0 0). This is accomplished by comparing experimental intramolecular vibrational modes to the corresponding modes calculated by first principle methods. We explore the three different high symmetry adsorption sites on top, bridge and hollow using several different metal clusters to represent the Cu(1 0 0) surface. The experimental results for both methoxy and ethoxy are best reproduced by the most realistic representation of the hollow position, a Cu 5 cluster.

Infrared investigation of the low-temperature structural and magnetic transitions in the spin-ladder candidate(DT−TTF)2Au(mnt)2

We report measurements of the variable temperature infrared response of the organic spin-ladder candidate dithiophentelrathiafulvalene gold maleonitrile dithiolate $[(\mathrm{DT}\ensuremath{-}\mathrm{TTF}{)}_{2}{\mathrm{Au}(\mathrm{mnt})}_{2}].$ The 220 K structural transition is driven by massive symmetry breaking along the rung direction, whereas the 70 K magnetic transition is associated with a change in symmetry of the vibronically activated ${A}_{g}$ modes in the rail direction. From molecular dynamics simulations, we assign the most important intramolecular vibrational modes involved in each transition. From an analysis of the charge-transfer behavior, we find that the localization of unpaired electrons in the interacting DT-TTF double chains is modified at these transition temperatures as well. To motivate future inelastic neutron-scattering studies, we have also calculated the spectrum of low-lying magnon excitations expected in this material, assuming an isotropic Heisenberg spin-ladder model. We estimate a gap of 0.6 meV for the one-magnon mode.

Structural investigations of the xZnCl2–(1−x)AlCl3 glass-forming system: a Raman spectroscopic study

We report on a temperature-dependence Raman scattering study of the binary glass-forming (halide) system xZnCl2–(1−x)AlCl3 for various compositions (x=1, 0.9, 0.8, 0.6, 0.4, 0.2, 0.1, and 0 mol fraction). The elucidation of the structure of these glasses and liquids is attempted in order to clarify the structural modifications that the addition of AlCl3 brings about to the tetrahedral ZnCl2 structure. The analysis has shown that the vibrational modes of the mixtures arise from a superposition of the vibrational modes of the pure component salts. Emphasis is given on both the temperature-induced changes of the intermolecular and intramolecular vibrational modes.

High-pressure stability, transformations, and vibrational dynamics of nitrosonium nitrate from synchrotron infrared and Raman spectroscopy

The properties of nitrosonium nitrate (NO+NO3−) were investigated following synthesis by laser heating of N2O and N2O4 under high pressures in a diamond anvil cell. Synchrotron infrared absorption spectra of NO+NO3− were measured at pressures up to 32 GPa at room temperature. Raman spectra were obtained at pressures up to 40 GPa at room temperature and up to 14 GPa at temperatures down to 80 K. For both lattice and intramolecular vibrational modes, a smooth evolution of spectral bands with pressure indicates that NO+NO3− forms a single phase over a broad range above 10 GPa, whereas marked changes, particularly evident in the Raman spectra at low temperature, indicate a phase transition occurs near 5 GPa. NO+NO3− could be recovered at atmospheric pressure and low temperature, persisting to 180 K. The Raman and IR spectroscopic data suggest that the NO+NO3− produced by laser heating of N2O followed by decompression may differ in structure or orientational order–disorder from that produced by autoionization of N2O4.

Magnetic field dependent vibrational modes inκ−(ET)2Cu(SCN)2

We report the infrared reflectance of the 10.4 K organic superconductor $\ensuremath{\kappa}\ensuremath{-}(\mathrm{ET}{)}_{2}\mathrm{Cu}(\mathrm{SCN}{)}_{2}$ as a function of applied magnetic field at 4.2 K, allowing us to explore changes in intramolecular vibrational modes between the superconducting (low field) and normal (high field) states. Several vibrational modes display field dependence, including ${\ensuremath{\nu}}_{3}{(A}_{g}),$ ${\ensuremath{\nu}}_{60}$ ${(B}_{3g}),$ and ${\ensuremath{\nu}}_{21}{(B}_{1g}).$ Changes in ${\ensuremath{\nu}}_{3},$ which involve the central C=C bond of the ET building block molecule, are especially complex, and include frequency, linewidth, and intensity modifications. These results show that intramolecular vibrational modes are involved in the superconducting to normal state transition in $\ensuremath{\kappa}\ensuremath{-}(\mathrm{ET}{)}_{2}\mathrm{Cu}(\mathrm{SCN}{)}_{2}$ below ${T}_{c}.$

Theoretical Study of Ultrafast Photoinduced Electron Transfer Processes in Mixed-Valence Systems

The recently proposed self-consistent hybrid method is applied to study photoinduced electron transfer (ET) reactions in mixed-valence compounds (NH3)5RuIIINCRuII(CN)5- and (NH3)5RuIIINCFeII(CN)5- in solution. To describe these ET processes in the condensed phase, both intramolecular modes (inner sphere) and the solvent environment (outer sphere) are taken into account using a model that is based on the analysis of experimental optical line shapes. The dynamics of the back ET process after photoexcitation is studied in detail. In particular we investigate the effects of intramolecular vibrational modes and solvent dynamics, as well as the influence of important physical parameters such as the electronic coupling and the temperature. In qualitative agreement with results of time-resolved optical experiments, the simulations predict an ultrafast decay of the population of the charge-transfer state. Oscillatory features are observed superimposed on the population decay, and their relations to electronic and ...

Photodissociation Spectroscopy of Zn+−Methanol

We report on studies of the photodissociation spectroscopy of Zn+−methanol complexes in the near-UV region. We observe two absorption bands assigned to Zn+-based 4p ← 4s transitions in a Cs Zn+−OHCH3 association complex. The 22A‘(4pπ) ← 12A‘(4sσ) band shows a significant vibronic resonance structure that can be assigned to a combination of both inter- and intramolecular vibrational modes of the complex. The experimental vibrational frequencies are in excellent agreement with the theoretical model predictions. The higher-energy 22A‘ ‘(4pπ) ← 12A‘(4sσ) band is strongly coupled to a lower lying charge-transfer state and appears as a structureless continuum. We observe very little reactive branching; rather, charge transfer to CH3OH+ + Zn products dominates the dissociation. These results are compared with previous studies of other main-group- and transition-metal-ion−methanol complexes.

Intramolecular vibrational energy redistribution, mode specificity, and nonexponential unimolecular decay dynamics of vibrationally highly excited states of DCO (X̃ 2A′)

The unimolecular dynamics of vibrationally highly excited states of DCO (X̃ 2A′) in the energy region up to Evib⩽9500 cm−1, beyond the D–CO (X̃) dissociation threshold, has been investigated using an effective polyad Hamiltonian obtained by fitting to the term energies from the measured B̃ 2A′←X̃ 2A′ stimulated emission pumping (SEP) spectra of the molecule [Stöck et al., J. Chem. Phys. 106, 5333 (1997); Temps and Tröllsch, Z. Phys. Chem. 215, 207 (2001)]. An added absorbing negative imaginary potential allowed for the unimolecular dissociation of the highly excited DCO via distinctive open reaction channels of the DC stretching vibration. The ensuing dynamics was explored using a wave packet propagation approach. Time profiles describing the intramolecular vibrational energy redistribution (IVR) and unimolecular decay kinetics were computed for the CO stretching zero-order basis states up to 6 quanta of excitation and the DCO bending zero-order basis states up to 12 quanta of excitation. The computed decay curves for the CO stretching zero-order basis states compare nicely with those of the respective coherent superposition states constructed directly from the measured SEP spectra (assuming the CO stretching mode as the Franck–Condon active bright zero-order mode that determines the observed transitions). A comparison of the decay curves with those of the almost isoenergetic DCO bending zero-order basis states in the respective polyads reveals large differences in the couplings of the two vibrational modes among each other and with the open dissociation channels. The obtained unimolecular decay profiles exhibit pronounced non-exponential kinetics. Comparison with statistically calculated decay rates shows a substantial degree of mode specificity of the dynamics, which can be attributed to a bottleneck in the IVR from the CO stretching vibration to the reaction coordinate. The model calculations explain the two-to-three orders of magnitude large difference between the measured eigenstate specific DCO (X̃) decay constants [Stöck et al.] and predictions by microcanonical statistical rate theories.

Phase transitions of solid CF4 at high pressures

The pressure-induced phase transformation of solid CF4 at roomtemperature was studied with x-ray and Raman scatteringexperiments. The space group of phase III was determined asP 21 /c.The pressure dependences of the intramolecular vibrational modes suggest phasetransitions from phase III to IV at 8.9 GPa and from phase IV to V at 13.6 GPa.

Nuclear dynamics in electronic ground and excited states probed by spectrally resolved four wave mixing

Time-resolved ground-state bleach and excited-state stimulated emission spectra have been measured for indocyanine green dissolved in methanol by employing spectrally resolved four wave mixing (SRFWM). The separation of the SRFWM signals into the ground-state bleach and excited-state stimulated emission contributions allows observation of intramolecular vibrational wave packet motions and intermolecular solvation dynamics upon impulsive excitation, while the molecule resides either in the ground or in the excited state. Frequencies of the indocyanine green intramolecular vibrational modes in the ground and excited states are practically the same. Vibrational dephasing times in the excited state range from a few hundred fs to ∼2 ps, and they are consistently shorter than those in the ground state. When excitation frequency is centered near the 0-0 transition, center frequencies of the stimulated emission redshift due to solvation of the excited state in nonequilibrium solvent configuration, whereas those of the ground-state bleach blueshift due to equilibrium fluctuation of the solvent molecules around the chromophore in the ground state. At early times, the solvation function obtained from the time-resolved ground-state bleach spectra is slower than the solvation function obtained from the time-resolved excited-state stimulated emission spectra.