Study of the excited-state proton-transfer reaction of 5-cyano-2-naphthol in sub- and supercritical water.

The excited-state proton-transfer (ESPT) reaction of 5-cyano-2-naphthol (5CN2) has been investigated in sub- and supercritical water using time-resolved fluorescence measurements. Under ambient conditions, a very fast decay of the fluorescence from the excited state of normal 5CN2 (ROH*) and a simultaneous increase of the fluorescence from the excited state of the anion species (RO(-)*) were observed, as reported previously. The very high ESPT rate was evaluated as 0.12 ps(-1). With increasing temperature at a constant pressure of 39.0 MPa, the proton transfer became slow. At 615 K and 39.0 MPa, another fluorescence from a new unknown chemical species appeared, which was assigned to the contact ion pair (CIP) of RO(-)* and the hydronium ion. With decreasing pressure at 664 K, the fluorescence from RO(-)* disappeared, and the fluorescence from ROH* and CIP was observed. At the very low density of supercritical water, only the fluorescence decay of ROH* was detected. The reaction dynamics was analyzed with the help of singular value decomposition and spectral decomposition using model functions. The ESPT rate was correlated with the solvent dielectric constant and/or the hydrogen-bonding ability.

Raman spectroscopic study on the solvation of p-aminobenzonitrile in supercritical water and methanol.

Raman spectra of the C[triple bond]N stretching vibration of p-aminobenzonitrile (ABN) have been investigated in water, methanol, and cyclohexane under sub- and supercritical conditions, and in acetonitrile under subcritical condition. In all solvent fluids covering the supercritical region, the vibrational frequency of the C[triple bond]N stretching mode decreased with increasing solvent density from the gaseous region to the medium density region rho(r) approximately = 2, where rho(r) is the reduced density by the critical density of the solvent. However, from the medium density region to the higher density region, the vibrational frequency turned to increase with the solvent density. The temperature-induced low frequency shift of the C[triple bond]N stretching Raman band was also ascertained by the measurement of the temperature dependence of Raman spectrum of ABN vapor above 543 K. The electronic absorption spectra in the UV region of ABN were also measured under the same experimental conditions. The absorption peak energies decreased with an increase of the solvent density, except in water above rho(r) = 2.8. The vibrational frequency shift in cyclohexane was explained by a sum of contributions of the repulsive interaction, the mean field attractive interaction, and the pure temperature effect probably due to the hot-band contribution. The residual frequency shift after the subtraction of the repulsive and temperature effects in water and methanol showed the low frequency shift with increasing solvent density from rho(r) congruent with 0 to 2.8. However, above rho(r) congruent with 2.8 in water, the residual shift showed a high frequency shift with increasing solvent density. The electronic state calculations based on the PCM model using the density functional theory (DFT) indicated that the solvent polarity change caused the low frequency shift of the C[triple bond]N stretching mode, which was also correlated with the shift of the electronic absorption spectrum. The results of the DFT calculations on the cluster of ABN with water molecules and the molecular dynamics simulations indicated that the high frequency shift of the C[triple bond]N stretching mode in water above rho(r) congruent with 2.8 could be due to the hydrogen bonding between water and ABN.

Study of the translational diffusion of the benzophenone ketyl radical in comparison with stable molecules in room temperature ionic liquids by transient grating spectroscopy.

Transient grating (TG) spectroscopy has been applied to the photoinduced hydrogen-abstraction reaction of benzophenone (BP) in various kinds of room temperature ionic liquids (RTILs). After the photoexcitation of BP in RTILs, the formation of a benzophenone ketyl radical (BPK) was confirmed by the transient absorption method, and the TG signal was analyzed to determine the diffusion coefficients of BPK and BP. For comparison, diffusion coefficients of carbon monoxide (CO), diphenylacetylene (DPA), and diphenylcyclopropenone (DPCP) in various RTILs were determined by the TG method using the photodissociation reaction of DPCP. While the diffusion coefficients of the stable molecules BP, DPA, and DPCP were always larger than those predicted by the Stokes-Einstein (SE) relation in RTILs, that of BPK was much smaller than those of the stable molecules and relatively close to that predicted by the SE relation in all solvents. For the smallest molecule CO, the deviation from the SE relation was evident. The diffusion coefficients of stable molecules are better represented by a power law of the inverse of the viscosity when the exponent was less than unity. The ratios of the diffusion coefficient of BP to that of BPK were larger in RTILs (2.7-4.0) than those (1.4-2.3) in conventional organic solvents. The slow diffusion of BPK in RTILs was discussed in terms of the fluctuation of the local electric field produced by the surrounding solvent ions.

Sound velocity dispersion in room temperature ionic liquids studied using the transient grating method.

Sound velocity is determined by the transient grating method in a range from 10(6) to 10(10) Hz in three room temperature ionic liquids, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-butyl-3-methylimidazolium hexafluorophosphate, and N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide. In all room temperature ionic liquids studied, the sound velocity increased with increasing frequency. The cause of this change is posited to be structural relaxation in the room temperature ionic liquids. Frequency dependence of the sound velocity is not reproduced by a simple Debye relaxation model. The sound velocity dispersion relation in 1-butyl-3-methylimidazolium hexafluorophosphate matches a Cole-Davidson function with parameters determined by a dielectric relaxation [C. Daguenet et al., J. Phys. Chem. B 110, 12682 (2006)], indicating that structural and reorientational relaxations are strongly coupled. Conversely, the sound velocity dispersions of the other two ionic liquids measured do not match those measured for dielectric relaxation, implying that structural relaxation is much faster than the reorientational relaxation. This difference is discussed in relation to the motilities of anions and cations.

Thermodynamical properties of reaction intermediates during apoplastocyanin folding in time domain.

Two intermediates observed for the folding process of apoplastocyanin (apoPC) were investigated by using a photoinduced triggering system combined with the transient grating and transient lens methods. The thermodynamic quantities, enthalpy, heat capacity, partial volume, and thermal expansion volume changes during the protein folding reaction were measured in time domain for the first time. An interesting observation is the positive enthalpy changes during the folding process. This positive enthalpy change must be compensated by positive entropy changes, which could be originated from the dehydration effect of hydrophobic residues and/or the translational entropy gain of bulk water molecules. Observed negative heat capacity change was explained by the dehydration effect of hydrophilic residues and/or motional confinement of amino acid side chains and water molecules in apoPC. The signs of the volume change and thermal expansion volume were different for two processes and these changes were interpreted in terms of the different relative contributions of the hydration and the dehydration of the hydrophilic residues. These results indicated two-step hydrophobic collapses in the early stage of the apoPC folding, but the nature of the dynamics was different.

Observation of pressure wave generated by focusing a femtosecond laser pulse inside a glass.

The pressure (or stress) wave generated by focusing a femtosecond laser pulse inside a glass has been considered one of the important factors in determining structures created in the laser focal region. In this paper, a method of the transient lens (TrL) analysis was proposed to characterize the pressure wave. Experimentally, the TrL signal exhibited damping oscillation within 2 ns. Simulations of the TrL signal showed that the shape of the oscillating signal depended on the width and amplitude of the pressure wave. Comparing the observed TrL signal with the simulated one, we estimated these properties of the pressure wave generated after femtosecond laser focusing inside a soda-lime glass.

Heating and rapid cooling of bulk glass after photoexcitation by a focused femtosecond laser pulse.

To investigate the energy dissipation process after focusing a femtosecond laser pulse inside a zinc borosilicate glass, the time-dependent lens effect in the laser focal region was observed by a transient lens (TrL) method. We found that the TrL signal after 100 ns can be explained clearly by thermal diffusion. By fitting the observed signal, we obtained the phase change due to temperature increase, the initial diameter of the heated volume and the thermal diffusivity. On the basis of the results, the temperature increase and the cooling rate were estimated to be about 1800 K and 1.7X10(8) Ks(-1), respectively. We have also observed the signal change on a 100 ns scale, which can not be explained by the thermal diffusion model. This change was attributed to the relaxation of the heated material.

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.

Excitation wavelength dependence of the Raman-Stokes shift of N,N-dimethyl-p-nitroaniline.

Raman spectra of N,N-dimethly-p-nitroaniline have been measured in various solvents. The Raman-Stokes shift of the band assigned to the NO2 stretching mode excited at 488 nm was found to be linearly dependent on the pi-pi* absorption band center. Furthermore, it is found that the Raman-Stokes shift of the NO2 stretching mode is dependent upon the excitation wavelength. The extent of the shift when excited at 355 versus 488 nm is almost linearly dependent on the vibrational bandwidth of the NO2 mode. The phenomenon is interpreted as the result of the solvation state selective excitation of the vibrational mode as in the case of phenol blue [Yamaguchi et al., J. Chem. Phys. 109, 9075 (1998); 109, 9084 (1998)].

Vibrational energy relaxation of azulene studied by the transient grating method. I. Supercritical fluids.

The vibrational energy dissipation process of the ground-state azulene in supercritical xenon, carbon dioxide, and ethane has been studied by the transient grating spectroscopy. In this method, azulene in these fluids was photoexcited by two counterpropagating subpicosecond laser pulses at 570 nm, which created a sinusoidal pattern of vibrationally hot ground-state azulene inside the fluids. The photoacoustic signal produced by the temperature rise of the solvent due to the vibrational energy relaxation of azulene was monitored by the diffraction of a probe pulse. The temperature-rise time constants of the solvents were determined at 383 and 298 K from 0.7 to 2.4 in rho(r), where rho(r) is the reduced density by the critical density of the fluids, by the fitting of the acoustic signal based on a theoretical model equation. In xenon, the temperature-rise time constant was almost similar to the vibrational energy-relaxation time constant of the photoexcited solute determined by the transient absorption measurement [D. Schwarzer, J. Troe, M. Votsmeier, and M. Zerezke, J. Chem. Phys. 105, 3121 (1996)] at the same reduced density irrespective of the solvent temperature. On the other hand, the temperature-rise time constants in ethane were larger than the vibrational energy-relaxation time constants by a factor of about 2. In carbon dioxide, the difference was small. From these results, the larger time constants of the solvent temperature rise than those of the vibrational energy relaxation in ethane and carbon dioxide were interpreted in terms of the vibrational-vibrational (V-V) energy transfer between azulene and solvent molecules and the vibrational-translational (V-T) energy transfer between solvent molecules. The contribution of the V-V energy transfer process against the V-T energy transfer process has been discussed.

Vibrational energy relaxation of azulene studied by the transient grating method. II. Liquid solvents.

The vibrational energy dissipation process of the ground-state azulene in various liquids has been studied by the transient grating spectroscopy. The acoustic signal produced by the temperature rise of the solvent due to the vibrational energy relaxation of azulene was monitored. The temperature rise-time constant of the solvent has been determined both by the fitting of the acoustic signal to a theoretical model equation and by the analysis of the acoustic peak shift. We found that the temperature rise-time constants determined by the transient grating method in various solvents are larger than the vibrational energy relaxation time constants determined by the transient absorption measurement [D. Schwarzer, J. Troe, M. Votsmeier, and M. Zerezke, J. Chem. Phys. 105, 3121 (1996)]. The difference is explained by different energy dissipation pathways from azulene to solvent; vibrational-vibrational (V-V) energy transfer and vibrational-translational (V-T) energy transfer. The contribution of the V-V energy transfer is estimated in various liquid solvents from the difference between the temperature rise time and vibrational energy relaxation time, and the solvent V-T relaxation time.

Vibrational energy relaxation of naphthalene in the S(1) state in various gases.

Time-resolved fluorescence spectra of naphthalene in the S(1) state have been measured in various gases below 10(2) kPa. The band shape of the fluorescence changed in an earlier time region after the photoexcitation when an excess energy (3300 cm(-1)) above the 0-0 transition energy was given. The excitation energy dependence of the fluorescence band shape of an isolated naphthalene molecule was measured separately, and the time dependence of the fluorescence band shape in gases was found to be due to the vibrational energy relaxation in the S(1) state. We have succeeded in determining the transient excess vibrational energy by comparing the time-resolved fluorescence band shape with the excitation energy dependence of the fluorescence band shape. The excess vibrational energy decayed almost exponentially. From the slope of the decay rate against the buffer gas pressure, we have determined the collisional decay rate of the excess vibrational energy in various gases. The dependence of the vibrational energy relaxation rate on the buffer gas species was similar to the case of azulene. The comparisons with the results in the low temperature argon and the energy relaxation rate in the S(0) state in nitrogen were also discussed.

Dynamics of structure and energy of horse carboxymyoglobin after photodissociation of carbon monoxide.

The energetics and structural volume changes after photodissociation of carboxymyoglobin are quantitatively investigated by laser-induced transient grating (TG) and photoacoustic calorimetric techniques. Various origins of the TG signal are distinguished: the phase grating signals due to temperature change, due to absorption spectrum change, and due to volume change. We found a new kinetics of approximately 700 ns (at room temperature), which was not observed by the flash photolysis technique. This kinetics should be attributed to the intermediate between the geminate pair and the fully dissociated state. The enthalpy of an intermediate species is determined to be 61 +/- 10 kJ/mol, which is smaller than the expected Fe-CO bond energy. The volume of MbCO slightly contracts (5 +/- 3 cm(3)/mol) during this process. CO is fully released from the protein by an exponential kinetics from 25 to -2 degrees C. During this escaping process, the volume expands by 14.7 +/- 2 cm(3)/mol at room temperature and 14 +/- 10 kJ/mol is released, which should represent the protein relaxation and the solvation of the CO (the enthalpy of this final state is 47 +/- 10 kJ/mol). A potential barrier between the intermediate and the fully dissociated state is DeltaH(*) = 41.3 kJ/mol and DeltaS(*) = 13.6 J mol(-1) K(-1). The TG experiment under a high wavenumber reveals that the volume expansion depends on the temperature from 25 to -2 degrees C. The volume changes and the energies of the intermediate species are discussed.

A spectrally silent transformation in the photolysis of octopus rhodopsin: a protein conformational change without any accompanying change of the chromophore's absorption.

A spectrally silent transformation in the photolysis of octopus rhodopsin was detected by the time-resolved transient grating method. Our results showed that at least two photointermediates, which share the same chromophore absorption spectrum, exist after the final absorption changes. Previously, mesorhodopsin was thought to decay to the final photoproduct, acid metarhodopsin with a lifetime of 38 micros at 15 degrees C, but the present results show that there is at least one intermediate species (called transient acid metarhodopsin) with a lifetime of 180 micros at 15 degrees C, before forming acid metarhodopsin. This indicates that the parts of the protein distant from the chromophore are still changing even after the changes in microenvironment around the chromophore are over. From the signal intensity detected by the transient grating method, the volume change of the spectrally silent transformation was found to be DeltaV = 13 ml/mol. The activation energy of the spectrally silent transformation is much lower than those of other transformations of octopus rhodopsin. Since stable acid metarhodopsin has not been shown to activate the G protein, this transient acid metarhodopsin may be responsible for G protein activation.

Is the translational diffusion of organic radicals different from that of closed-shell molecules?

The translational diffusion of photochemically created intermediate radicals is measured by the transient grating technique. The diffusional behavior of these intermediates is different from that of stable molecules, which have already been investigated extensively. The investigation of the diffusion of these species will provide an opportunity to reveal the unique intermolecular interaction between the intermediates and matrix. This information will be valuable for understanding photochemistry in solutions.

Ultrafast transient Kerr lens in solution detected by the dual-beam thermal-lens method.

Time profiles of a transient lens on a subpicosecond time scale are observed by use of a dual-beam thermal-lens configuration. On this time scale and under nonresonant conditions the transient lens is composed mainly of the Kerr response. The electronic and nuclear responses are clearly separated by the polarization selection.