Solvent structural relaxation dynamics in dipolar solvation studied by resonant pump polarizability response spectroscopy

Resonant pump polarizability response spectroscopy (RP-PORS) was used to study the isotropic and anisotropic solvent structural relaxation in solvation. RP-PORS is the optical heterodyne detected transient grating (OHD-TG) spectroscopy with an additional resonant pump pulse. A resonant pump excites the solute-solvent system and the subsequent relaxation of the solute-solvent system is monitored by the OHD-TG spectroscopy. This experimental method allows measuring the dispersive and absorptive parts of the signal as well as fully controlling the beam polarizations of incident pulses and signal. The experimental details of RP-PORS were described. By performing RP-PORS with Coumarin 153(C153) in CH(3)CN and CHCl(3), we have successfully measured the isotropic and anisotropic solvation polarizability spectra following electronic excitation of C153. The isotropic solvation polarizability responses result from the isotropic solvent structural relaxation of the solvent around the solute whereas the anisotropic solvation polarizability responses come from the anisotropic translational relaxation and orientational relaxation. The solvation polarizability responses were found to be solvent-specific. The intramolecular vibrations of CHCl(3) were also found to be coupled to the electronic excitation of C153.     

Study on the vibrational energy relaxation of p-nitroaniline, N,N-dimethyl-p-nitroaniline, and azulene by the transient grating method

The vibrational energy dissipation processes of the electronic ground states of p-nitroaniline and N,N-dimethyl-p-nitroaniline have been studied by transient grating spectroscopy with subpicosecond laser pulses. The rise time of the acoustic signal produced by the energy dissipation process of the hot ground state molecule was monitored. The acoustic signal was analyzed by an equation including the acoustic damping. The solvent temperature rise times in various solvents have been determined. The acoustic signals of azulene in previous papers [Y. Kimura et al., J. Chem. Phys. 123, 054512 (2005); 123, 054513 (2005)] were also reanalyzed using this equation. The temperature rise times in all cases are longer than the vibrational energy relaxation times of the solutes determined by the transient absorption measurements. The difference is discussed in terms of the energy transfer pathways from the solute to the solvent. We concluded that both the hydrogen bonding between the solute and the solvent and the lower frequency modes of the solutes play important roles in determining the energy transfer pathway from the solute to the solvent.     

Influence of temperature on thymine-to-solvent vibrational energy transfer

At the instant following the non-radiative deactivation of its ππ* electronic state, the vibrational modes of thymine possess a highly non-equilibrium distribution of excitation quanta (i.e., >4 eV in excess energy). Equilibrium is re-established through rapid (5 ps) vibrational energy transfer to the surrounding solvent. The mechanisms behind such vibrational cooling (VC) processes are examined here using femtosecond transient grating and two-dimensional photon echo spectroscopies conducted at 100 K and 300 K in a mixture of methanol and water. Remarkably, we find that this variation in temperature has essentially no impact on the VC kinetics. Together the experiments and a theoretical model suggest three possible mechanisms consistent with this behavior: (i) vibrational energy transfer from the solute to solvent initiates (directly) in intramolecular modes of the solute with frequencies >300 cm(-1); (ii) the relaxation induced increase in the temperature of the environment reduces the sensitivity of VC to the temperature of the equilibrium system; (iii) the time scale of solvent motion approaches 0.1 ps even at 100 K. Mechanism (i) deserves strong consideration because it is consistent with the conclusions drawn in earlier studies of isotope effects on VC in hydrogen bonding solvents. Our model calculations suggest that mechanism (ii) also plays a significant role under the present experimental conditions. Mechanism (iii) is ruled out on the basis of long-lived correlations evident in the photon echo line shapes at 100 K. These insights into photoinduced relaxation processes in thymine are made possible by our recent extension of interferometric transient grating and photon echo spectroscopies to the mid UV spectral region.     

The effects of solute-solvent electrostatic interactions on solvation dynamics in supercritical CO2

We present here the results of molecular-dynamics simulation of solvation dynamics in supercritical CO(2) at a temperature of about 1.05T(c), where T(c) is the critical temperature, and at a series of densities ranging from 0.4 to 2.0 of the critical density rho(c). We focus on electrostatic solvation dynamics, representing the electronic excitation of the chromophore as a change in its charge distribution from a quadrupolar-symmetry ground state to a dipolar excited state. Two perturbations are considered, corresponding to different magnitudes of solute excited-state dipoles, denoted as d5 and d8. The d8 solute is more attractive, leading to a larger enhancement in CO(2) clustering upon solute electronic excitation. This has a large impact on solvation dynamics, especially at densities below rho(c). At these densities, solvation dynamics is much slower for the d8 than for the d5 solute. For both solutes, solvation dynamics becomes faster at densities above rho(c) at which solvent clustering diminishes. We show that the slowest solvation time scale is associated with solvent clustering and we relate it to solute-solvent mutual translational diffusion and the extent of change in effective local density resulting from solute electronic excitation.     

Quenched by ice: transient grating measurements of vibronic dynamics in bromine-doped ice

In both water and in ice, the absorption spectra of bromine are dramatically broadened and blueshifted, and all fluorescence is quenched. Time resolved, electronically resonant transient grating measurements are carried out to characterize the vibronic dynamics of the trapped molecule in its electronic B(3Pi0u) state in ice. Independent of the initial excitation energy, after the first half-period of motion, a vibrational packet is observed to oscillate near the bottom of the potential, near nu=1. The oscillations undergo a chirped decay to a terminal frequency of 169 cm(-1) on a time scale of taunu=1240 fs, to form the stationary nu=0 level. The electronic population in the B state decays in taue=1500 fs. Adiabatic following to the cage-compression coordinate is a plausible origin of the chirp. Analysis of the absorption spectrum is provided to recognize that solvent coordinates are directly excited in the process. The observed blueshift of the absorption is modeled by considering the Br2-OH2 complex. Two-dimensional simulations, that explicitly include the solvent coordinate, reproduce both the time data and the absorption spectrum. The observed sharp vibrational recursions can be explained by overdamped motion along the solvent coordinate, and wave packet focusing by fast dissipation during the first half-period of motion of the molecular coordinate.     

Probing excited-state dynamics and intramolecular proton transfer in 1-acylaminoanthraquinones via the intermolecular solvent response

The dynamics of a series of 1-acylaminoanthraquinones with varying degrees of excited-state intramolecular proton transfer are studied in acetonitrile and dichloromethane. Events are followed via changes in the third-order intermolecular Raman response as a function of time after resonant excitation of the chromophore. Compared to electronically resonant probes of the solute, measuring the ultrafast dynamics using the nonresonant solvent response offers a new and complementary perspective on the events that accompany excitation and proton transfer. Experimentally observed changes in the nuclear polarizability of the solvent follow dynamic changes in the solvent-solute interactions. Reorganization of the solvent in response to the significant changes in the intermolecular interactions upon proton transfer is found to play an important role in the reaction dynamics. With transfer of the proton taking place rapidly, the solvent controls the dynamics via the time-dependent evolution of the free energy surface, even on subpicosecond time scales. In addition, the solvent response probes the effects of intermolecular energy transfer as energy released during the reactive event is rapidly transferred to the local solvent environment and then dissipates to the bulk solvent on about a 10 ps time scale. A brief initial account of a portion of this work has appeared previously, J. Am. Chem. Soc. 2004, 126, 8620-8621.     

Measuring the change in the intermolecular Raman spectrum during dipolar solvation

We demonstrate a method to directly measure the change in the spectrum of intermolecular solvent fluctuations as a function of time after electronic excitation of a solute, and this method is applied to the dye Coumarin 102 (C102) in acetonitrile. The complete intermolecular response is captured following resonant excitation with time domain third-order Raman spectroscopy. In a previous report, we introduced this method and used it to probe one point in the intermolecular response as a function of time after solute excitation (Underwood, D. F., Blank, D. A. J. Phys. Chem. A 2003, 107 (7), 956). Here we extend this approach to recover the change in the entire intermolecular response as a function of time. To our knowledge the results provide the first direct measurement of the difference in the equilibrated intermolecular response after excitation of a solute and its evolution during a dipolar solvation event. Excitation of C102 results in a significant increase in the solvent-solute interaction due to a large increase in the dipole moment. The observed change in the intermolecular response is consistent with a rapid change in local solvent density, with intermolecular kinetic energy transfer changing the response on longer time scales. Evolution of the response exhibits a strong frequency dependence and suggests changes over longer distances at longer delay times. The measured change in the spectrum of solvent fluctuations represents a direct experimental confirmation of the breakdown of linear response and confirms predictions from molecular dynamics simulations.     

Femtosecond evolution of the pyrrole molecule excited in the near part of its UV spectrum

The evolution of the isolated pyrrole molecule has been followed after excitation in the 265-217 nm range by using femtosecond time delayed ionization. The transients collected in the whole excitation range show the vanishing of the ionization signal in the femtosecond time scale, caused by the relaxation along a πσ(?) type state (3s a(1)←π 1a(2)), which is the lowest excited electronic state of the molecule. This surface is dissociative along the NH bond, yielding a 15 ± 3 fs lifetime that reflects the loss of the ionization cross-section induced by the ultrafast wavepacket motion. Although a weak πσ(?) absorption is detected, the state is mainly reached through internal conversion of the higher bright ππ(?) transitions, which occurs with a 19 ± 3 fs lifetime. In addition to its resonant excitation, the intense ππ(?) absorption extending in the 220-190 nm interval is also out-of-resonance populated at energies far to the red from its absorption onset. This coherent adiabatic excitation of the ππ(?) transition should follow the excitation pulse (coherent population return effect), but instead the system relaxes toward the lower πσ(?) surface through a conical intersection during the interaction time, leading to the population of πσ(?) state at wavelengths as long as 265 nm. According to the observed behavior, the time evolution of the system in the full excitation range studied is modeled by a coherent treatment that provides key insights on the photophysical properties of the molecule.     

Vibrational energy relaxation of a diatomic molecule in a room-temperature ionic liquid

Vibrational energy relaxation (VER) dynamics of a diatomic solute in ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI(+)PF(6) (-)) are studied via equilibrium and nonequilibrium molecular dynamics simulations. The time scale for VER is found to decrease markedly with the increasing solute dipole moment, consonant with many previous studies in polar solvents. A detailed analysis of nonequilibrium results shows that for a dipolar solute, dissipation of an excess solute vibrational energy occurs almost exclusively via the Lennard-Jones interactions between the solute and solvent, while an oscillatory energy exchange between the two is mainly controlled by their electrostatic interactions. Regardless of the anharmonicity of the solute vibrational potential, VER becomes accelerated as the initial vibrational energy increases. This is attributed primarily to the enhancement in variations of the solvent force on the solute bond, induced by large-amplitude solute vibrations. One interesting finding is that if a time variable scaled with the initial excitation energy is employed, dissipation dynamics of the excess vibrational energy of the dipolar solute tend to show a universal behavior irrespective of its initial vibrational state. Comparison with water and acetonitrile shows that overall characteristics of VER in EMI(+)PF(6) (-) are similar to those in acetonitrile, while relaxation in water is much faster than the two. It is also found that the Landau-Teller theory predictions for VER time scale obtained via equilibrium simulations of the solvent force autocorrelation function are in reasonable agreement with the nonequilibrium results.     

Separating Vibrational Coherences in Ground/Excited Electronic States of Solvent/Solute Following Non-Resonant/Resonant Impulsive Excitation

In a quest to track down the origin of coherent vibrational motions observed in femtosecond pump-probe transients, whether they arise from ground/excited electronic state of solute or are contributed by the solvent, we demonstrate a method for extricating vibrations under resonant and non-resonant impulsive excitations using a diatomic solute in condensed phase (iodine in carbon tetrachloride) with aid of spectral dispersion of the chirped broadband probe. Most importantly, we show how a sum over intensities for a select region of detection wavelengths and Fourier transform of data over select temporal window untwine contributions from vibrational modes of different origins. Thus, in a single pump-probe experiment, vibrational features specific to solute as well as solvent are disentangled that are otherwise spectrally overlapping and are non-separable in conventional (spontaneous/stimulated) Raman spectroscopy employing narrowband excitation. We envision wide-ranging applications of this method to unveil vibrational features in complex molecular systems.     

Time scales and pathways of vibrational energy relaxation in liquid CHBr3 and CDBr3

Molecular dynamics simulations are used in conjunction with Landau-Teller, fluctuating Landau-Teller, and time-dependent perturbation theories to investigate energy flow out of various vibrational states of liquid CHBr3 and CDBr3. The CH stretch overtone is found to relax with a time scale of about 1 ps compared to the 50 ps rate for the fundamental. The relaxation pathways and rates for the CD stretch decay in CDBr3 are computed in order to understand the changes arising from deuteration. While the computed relaxation rate agrees well with experiments, the pathway is found to be more complex than anticipated. In addition to the above channels for CH(D) stretch relaxation that involve only the hindered translations and rotations of the solvent, routes involving off-resonant and resonant excitations of solvent vibrational modes are also examined. Finally, the decay of energy from low frequency states to near-lying solute states and solvent vibrations are studied.     

Polarizable solute in polarizable and flexible solvents: simulation study of electron transfer reaction systems

A polarizable solute model, based on the empirical valence bond approach, is developed and applied to electron transfer (ET) reactions in polarizable and flexible water solvents. The polarization effect is investigated in comparison with a nonpolarizable solute and solvent model. With free energy curves constructed by a molecular dynamics simulation, the activation energy barrier and the reorganization energy related to ET processes are investigated. The present simulation results show that the activation energy barrier becomes larger in the polarizable model than in the nonpolarizable model and that this makes the ET rate slower than that with the nonpolarizable model. It is shown that the effect of the electronic energy difference of solute molecule on free energy profiles is remarkable and that, corresponding to this effect, the reorganization energy is significantly modified. These results indicate that the process of solvent polarization by the polarized solute to enhance the solute-solvent interaction is a key factor and that treating the polarization of both solute and solvent at the same time is essential. Also, the polarization effect on the diffusive motion of the solute molecule in the polarization solvent is studied. The polarized solute molecule shows slower diffusive motion compared with that in the nonpolarizable model.     

Picosecond solvation of electronically excited solutes in alcoholic solvents: non-debye behaviour of the time-dependent fluorescence shift related to h

Time-dependent fluorescence shifts (TDFS) of polar compounds in a series of alcoholic solvents have been studied on a pico-second time scale. A non-exponential TDFS kinetic behaviour is observed, particularly at short times (0<t< 50 ps). It cannot be represented by a sum of decreasing exponentials. The effects of solute-solvent interactions on the kinetics of TDFS have been analysed. It is shown that the non-standard kinetics is due to hydrogen bonding in the solvent, particularly to the presence of alcoholic hydrogen-bonded aggregates. Following electronic excitation of the polar solute reorganization of solvent aggregates controls the TDFS time development. Our results show that a simple Debye model is inadequate to explain such a process.     

Coupling quantum Monte Carlo to a nonlinear polarizable continuum model for spherical solutes

Starting from the nonlinear dielectric response model of Sandberg and Edholm, we derive an analytical expression of the polarization contribution to the solvation free energy in terms of the electronic density of the solute and the dielectric properties of the solvent. The solvent inhomogeneity is taken into account with the use of a smooth switching function whose spacial variation is established on the basis of how the solvent is arranged around the solute. An explicit form of a local potential representing the solvent effect on the solute is thus obtained by functional analysis. This effective potential can be combined with density functional or quantum chemical methods for the quantum mechanical treatment of the solute. Here, we use quantum Monte Carlo techniques for the solute and apply the method to the hydration of atomic ions finding very good agreement with experimental data.     

On Raman optical activity sign-switching between the ground and excited states leading to an unusual resonance ROA induced chirality

Raman optical activity (ROA) spectra recorded for a chiral naphthalene diimide derivative (nBu-NDI–BINAM) dissolved in a series of solvents exhibit strong solute to solvent induced chirality with: (1) dominating bands of solvents, (2) nBu-NDI–BINAM resonance ROA (RROA) bands which are barely visible, (3) monosignate RROA Solvent spectra with an unexpected sign concordant with that of the ECD band of the resonant electronic state, (4) bisignate RROA bands for a few solvents, and (5) superposition of non-resonant and resonant ROA bands of the chiral solvents. The unusual ROA enhancement was explained in terms of resonance energy transfer with resonant Raman emission. The surprising RROA sign-switching was found to be due to specific conformational equilibria where one solute conformer dominates in the ground and the other in the first excited singlet state, and, the signs of the related ECD bands of these two conformers are opposite.

Unusual solute to solvent induced chirality in ROA comes from specific conformer equilibria in the ground and the excited states.     

Solute-solvent intermolecular vibronic coupling as manifested by the molecular near-field effect in resonance hyper-Raman scattering

Vibronic coupling within the excited electronic manifold of the solute all-trans-β-carotene through the vibrational motions of the solvent cyclohexane is shown to manifest as the "molecular near-field effect," in which the solvent hyper-Raman bands are subject to marked intensity enhancements under the presence of all-trans-β-carotene. The resonance hyper-Raman excitation profiles of the enhanced solvent bands exhibit similar peaks to those of the solute bands in the wavenumber region of 21,700-25,000 cm(-1) (10,850-12,500 cm(-1) in the hyper-Raman exciting wavenumber), where the solute all-trans-β-carotene shows a strong absorption assigned to the 1A(g) → 1B(u) transition. This fact indicates that the solvent hyper-Raman bands gain their intensities through resonances with the electronic states of the solute. The observed excitation profiles are quantitatively analyzed and are successfully accounted for by an extended vibronic theory of resonance hyper-Raman scattering that incorporates the vibronic coupling within the excited electronic manifold of all-trans-β-carotene through the vibrational motions of cyclohexane. It is shown that the major resonance arises from the B-term (vibronic) coupling between the first excited vibrational level (v = 1) of the 1B(u) state and the ground vibrational level (v = 0) of a nearby A(g) state through ungerade vibrational modes of both the solute and the solvent molecules. The inversion symmetry of the solute all-trans-β-carotene is preserved, suggesting the weak perturbative nature of the solute-solvent interaction in the molecular near-field effect. The present study introduces a new concept, "intermolecular vibronic coupling," which may provide an experimentally accessible∕theoretically tractable model for understanding weak solute-solvent interactions in liquid.     

A Simple Approach Toward Solution of the Problems of Signal Instability and Interferences Observed with Spectrophotofluorometers

Abstract

Unexpected fluorescence signal changes observed by analysts are frequently due to lamp instability and interferences from other equipment near the fluorometer; The addition of a constant voltage transformer between the line and the power supply is an inexpensive and simple procedure to provide long term stability of the lamp, which leads to more reliable use of the fluorometer. Interference problems were significantly reduced by adding a filter system to the instrument.

The problem of long term stability and interferences from nearby electronic instrumentation are frequently observed by individuals using spectrophotofluorometers. Unexpected fluorescence intensity differences for samples which are believed to contain similar quantities of an analyte are an outward manifestation of these problems, which may lead to the incorrect report of the concentration of the compound of interest. We have observed such difficulties in a study of the effect of solvent composition on the luminescence of tyrosine and tryptophan (which will be reported separately) and would like to report our difficulties and the remedies to the problem, We feel that this situation is quite widespread, based on informal discussions with other individuals.

The observed fluorescence signal due to a given solution was found to vary quite drastically (up to ± 20%) as a cuvet containing an identical sample was placed in the sample chamber at various times during a day. In some cases, such drastic changes could be observed with measurements taken 15 minutes apart. The changes were not due to solute decomposition, solvent interaction with the solute, photochemical decomposition or thermal decomposition of the solute, as these changes were not reproducible, and appeared to be independent of the solute and the solvent composition.

A block of MgO was placed in the sample compartment and the fluorometer was set up for front surface emission measurement, so that the refleactance could be monitored over a period of time. The reflectance intensity therefore mirrored the lamp intensity, and any change in lamp intensity would appear as a change in the reflectance observed as a function of time. It was observed that random changes in intensity did take place, with such changes occurring over a very short time period. The reflectance intensity changes were quite random with respect to time, and individual shifts were often as much as ± 10%.

When we turned the Xe lamp off and placed a flashlight bulb powered by a 6V lantern battery in the sample chamber, the light measured by the photomultiplier did not exhibit any abrupt changes over a period of 16 hours. This confirmed that the changes in signal intensity observed with the various samples and the reflectance experiments were due to changes in the intensity of the xenon lamp. It was determined that variations in the observed reflectance could be correllated with variations in line voltage.

Our instrument is used in a large laboratory with a number of other instruments within 5 meters. We observe that noise from “external sources” was bothersome, especially on higher amplification steps of the photomultiplier amplifier. In general, the variation in signal was ± 3% full scale at the highest amplification.

We report below our solutions to the problems described above as we feel that our solutions are of interest to those using spectrophotofluorometers.