Ultrafast UV photon echo peak shift and fluorescence up conversion studies of non-polar solvation dynamics

We present photon echo peak shift and femtosecond fluorescence up-conversion studies of non-polar solvation dynamics of a simple non-polar dye p-terphenyl in ethanol and cyclohexane, using excitation in the UV range at 290 nm. The UV fluorescence up-conversion experiments were combined with a polychromatic detection and the results highlight the high sensitivity of this approach to fully characterize the excited state dynamics of the dye. We also demonstrate the feasibility of UV photon echo and transient grating and its sensitivity for the detection of non-polar solvation dynamics by measuring the frequency correlation function of the dye in the ground state. While solvation dynamics in the picosecond regime is observed in ethanol, electronic coherence dephasing occurs on timescales faster than 100 fs in ethanol as well as in the non-polar solvent cyclohexane.     

Ultrafast dynamics of room temperature ionic liquids after ultraviolet femtosecond excitation

The photochemistry and relaxation dynamics of four room-temperature ionic liquids (RTILs) after ultraviolet (UV) photolysis were investigated by femtosecond pump-probe absorption spectroscopy. A pulse duration-limited rise of the induced absorption in halide-containing RTILs at various probe wavelengths was attributed to the generation of solvated electrons. With continuous irradiation (static conditions), di- and trihalide ion formation became apparent especially below 1000 nm. The formation of trihalide ions was further confirmed by steady-state UV absorption spectroscopy. All RTILs showed a rich photochemistry after UV photolysis leading to the build-up of various long-lived intermediate products as evidenced from the observation that ionic liquids turn yellow upon continuous irradiation. On the other hand, exposing RTILs to the excitation pulse for a short time (rapid-scan method) significantly suppressed the formation of halides. The results suggest that the development of flow-cell systems for highly viscous ionic liquids is urgently needed to quantitatively investigate their ultrafast dynamics.     

Ultrafast solvation dynamics of coumarin 153 in imidazolium-based ionic liquids

The dynamic Stokes shift of coumarin 153 has been measured in two room-temperature ionic liquids, 1-(3-cyanopropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-propyl-3-methylimidazolium tetrafluoroborate, using the fluorescence up-conversion technique with a 230 fs instrumental response function. A component of about 10-15% of the total solvation shift is found to take place on an ultrafast time scale < 10 ps. The amplitude of this component is substantially less than assumed previously by other authors. The origin of the difference in findings could be partly due to chromophore-internal conformational changes on the ultrafast time scale, superimposed to solvation-relaxation, or due to conformational changes of the chromophore ground state in polar and apolar environments. First three-pulse photon-echo peak-shift experiments on indocyanine green in room-temperature ionic liquids and in ethanol indicate a difference in the inertial component of the early solvent relaxation of <100 fs.     

Heterogeneous solute dynamics in room temperature ionic liquids

The excitation wavelength dependence of the emission kinetics of several solutes is used to demonstrate the presence of dynamic heterogeneity in two representative room temperature ionic liquids, dimethyl-isopropyl-propyl-ammonium bis(trifluoromethylsulfonyl)imide [N(ip311)(+)][Tf(2)N(-)] and N-propyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [Pr(31)(+)][Tf(2)N(-)]. The solute kinetics examined here include rotation and solvation of coumarin 153, isomerization of two malononitriles, and intramolecular charge transfer in crystal violet lactone. The rates of most of these processes vary significantly with excitation wavelength, especially for excitation on the red edges of the solute absorption bands, indicating that energetically selected subpopulations relax at distinct rates. The results presented here suggest more generally that dynamical processes taking place on the subnanosecond time scale in typical ionic liquids near room temperature are likely to be heterogeneous in character.     

Measuring electronic coupling in the reaction center of purple photosynthetic bacteria by two-color, three-pulse photon echo peak shift spectroscopy

One- and two-color, three-pulse photon echo peak shift spectroscopy (1C and 2C3PEPS) was used to estimate the electronic coupling between the accessory bacteriochlorophyll (B) and the bacteriopheophytin (H) in the reaction center of the purple photosynthetic bacterium Rhodobacter sphaeroides as approximately 170 +/- 30 cm-1. This is the first direct experimental determination of this parameter; it is within the range of values found in previously published calculations. The 1C3PEPS signal of the Qy band of the bacteriochlorophyll B shows that it is weakly coupled to nuclear motions of the bath, whereas the 1C3PEPS signal of the Qy band of the bacteriopheophytin, H, shows that it is more strongly coupled to the bath, but has minimal inhomogeneous broadening. Our simulations capture the major features of the data with the theoretical framework developed in our group to separately calculate the response functions and population dynamics.     

Orientational and translational dynamics in room temperature ionic liquids

The authors investigate the dynamics of a series of room temperature ionic liquids, based on the same 1-butyl-3-methylimidazolium cation with different anions, by means of broadband (10(-6)-10(9) Hz) dielectric spectroscopy and depolarized light scattering in the temperature range from 400 K down to 35 K. Typical ionic conductivity is observed above the glass transition temperature Tg. Below Tg the authors detect relaxation processes that exhibit characteristics of secondary relaxations, as typically observed in molecular glasses. At high temperatures, the characteristic times of cation reorientation, deduced from the light scattering data, are approximately equal to the electric modulus relaxation times related to ionic conductivity. In the supercooled regime and close to Tg, the authors observe decoupling of conductivity from structural relaxation. Overall, room temperature ionic liquids exhibit typical glass transition dynamics, apparently unaltered by Coulomb interactions.     

Dipolar solvation dynamics in room temperature ionic liquids: an effective medium calculation using dielectric relaxation data

Solvation dynamics in four imidazolium cation based room temperature ionic liquids (RTIL) have been calculated by using the recently measured dielectric relaxation data [ J. Phys. Chem. B 2008, 112, 4854 ] as an input in a molecular hydrodynamic theory developed earlier for studying solvation energy relaxation in polar solvents. Coumarin 153 (C153), 4-aminophthalimide (4-AP), and trans-4-dimethylamino-4'-cyanostilbene (DCS) have been used as probe molecules for this purpose. The medium response to a laser-excited probe molecule in an ionic liquid is approximated by that in an effective dipolar medium. The calculated decays of the solvent response function for these RTILs have been found to be biphasic and the decay time constants agree well with the available experimental and computer simulation results. Also, no probe dependence has been found for the average solvation times in these ionic liquids. In addition, dipolar solvation dynamics have been predicted for two other RTILs for which experimental results are not available yet. These predictions should be tested against experiments and/or simulation studies.     

Modelling room temperature ionic liquids

Room temperature ionic liquids (IL) composed of organic cations and inorganic anions are already being utilized for wide-ranging applications in chemistry. Complementary to experiments, computational modelling has provided reliable details into the nature of their interactions. The intra- and intermolecular structures, dynamic and transport behaviour and morphologies of these novel liquids have also been explored using simulations. The current status of molecular modelling studies is presented along with the prognosis for future work in this area.     

Solute rotation and solvation dynamics in an alcohol-functionalized room temperature ionic liquid

Steady-state and time-resolved fluorescence behaviors of two dipolar solutes, coumarin 153 and 4-aminophthalimide, have been studied in an alcohol-functionalized room-temperature ionic liquid, 1-(hydroxyethyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide. The steady-state fluorescence parameters have been exploited for the estimation of the polarity of this ionic liquid and to obtain information on the hydrogen bonding interaction between the ionic liquid and the probe molecules. The time-resolved measurements have been focused on the dynamics of solvation by studying the dynamic Stokes shift in the ps-ns time scale and solute rotation by measuring the time dependence of the fluorescence anisotropy. The time-resolved anisotropy studies reveal a significant slow down of the rotational motion of one of the probe molecules. The time-dependent fluorescence Stokes shift measurements suggest that the time-resolvable part of the dynamics is biphasic in nature, highly dependent on the probe molecule and the ultrafast component is comparatively less than that in other ionic liquids. The influence of the hydrogen bonding interaction between the probe molecules and the ionic liquids on the solute rotation and the various components of the solvation dynamics is carefully analyzed in an attempt to obtain further insight into the mechanism of solvation in these novel media.     

Ultrafast dynamics of pyrrolidinium cation ionic liquids

We have investigated the ultrafast molecular dynamics of five pyrrolidinium cation room temperature ionic liquids using femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The ionic liquids studied are N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide P14+/NTf2-), N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide P1EOE+/NTf2-), N-ethoxyethyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide P1EOE+/NTf2-), N-ethoxyethyl-N-methylpyrrolidinium bromide P1EOE+, and N-ethoxyethyl-N-methylpyrrolidinium dicyanoamide P1EOE+/DCA-). For comparing dynamics among the five ionic liquids, we categorize the ionic liquids into two groups. One group of liquids comprises the three pyrrolidinium cations P14+, P1EOM+, and P1EOE+ paired with the NTf2- anion. The other group of liquids consists of the P1EOE+ cation paired with each of the three anions NTf2-, Br-, and DCA-. The overdamped relaxation for time scales longer than 2 ps has been fit by a triexponential function for each of the five pyrrolidinium ionic liquids. The fast ( approximately 2 ps) and intermediate (approximately 20 ps) relaxation time constants vary little among these five ionic liquids. However, the slow relaxation time constant correlates with the viscosity. Thus, the Kerr spectra in the range from 0 to 750 cm(-1) are quite similar for the group of three pyrrolidinium ionic liquids paired with the NTf2- anion. The intermolecular vibrational line shapes between 0 and 150 cm(-1) are fit to a multimode Brownian oscillator model; adequate fits required at least three modes to be included in the line shape.     

Excited-state proton-transfer dynamics of 7-hydroxyquinoline in room temperature ionic liquids

Excited-state proton transfer (ESPT) reaction of 7-hydroxyquinoline (7-HQ) mediated by methanol molecules has been studied in two room temperature ionic liquids (RTILs) using steady-state and time-resolved fluorescence measurements. While no ESPT is observable in neat RTILs, characteristic tautomer fluorescence of 7-HQ could be observed in the presence of small quantity of methanol (0.5-4.1 M). The observation of a rise time (350 ps-1.4 ns) associated with the tautomer fluorescence suggests that proton transfer in 7-HQ is indeed an excited-state phenomenon that requires considerable solvent reorganization prior to the relay of proton from the hydroxyl group to the distant ring nitrogen atom through suitably organized dimeric chain of methanol molecules. The rise time of the tautomer fluorescence, which has been found to decrease with increasing methanol concentration, is attributed to the change of viscosity of the medium upon methanol addition. While the influence of viscosity on the ESPT kinetics is evident from the data, lack of any definite correlation between the bulk viscosity and the rise time has been interpreted in terms of the microheterogeneous nature of the media that does not allow assessment of the microviscosity around 7-HQ from the bulk viscosity.     

Raman spectroscopic study on solvation of diphenylcyclopropenone and phenol blue in room temperature ionic liquids

We investigated the solvation of several room temperature ionic liquids by Raman spectroscopy using diphenylcyclopropenone (DPCP) and phenol blue (PB) as probe molecules. We estimated acceptor numbers (AN) of room temperature ionic liquids by an empirical equation associated with the Raman band of DPCP assigned as a C=C stretching mode involving a significant C=O stretching character. According to the dependence of AN on cation and anion species, the Lewis acidity of ionic liquids is considered to come mainly from the cation charge. The frequencies and bandwidths of the C=O and C=N stretching modes of phenol blue are found to be close to those in conventional polar solvents such as methanol and dimethyl sulfoxide. The frequencies of these vibrational modes show similar dependence upon the electronic absorption band center as is observed in conventional liquid solvents. However, peculiar behavior was found in the Raman bandwidths and the excitation wavelength dependence of the C=N stretching mode in room temperature ionic liquids. Both the bandwidth of the C=N stretching mode and the extent of the excitation wavelength dependence of the Raman shift of the C=N stretching mode tend to decrease as the absorption band center decreases, in contrast to the case of conventional solvents. This anomaly is discussed in terms of the properties of room temperature ionic liquids.     

Peptide synthesis in room temperature ionic liquids

We demonstrate that chemical peptide coupling using modern coupling agents is efficient in rt ionic liquids. This new approach presents some advantages, especially in the case of hindered amino acids, which are not easy to couple under standard conditions, since high purities for the crude peptides were observed with respect to coupling in classical solvents.     

Nanoscale segregation in room temperature ionic liquids

Room-temperature ionic liquids (RTILs) are organic salts that are characterized by low melting points. They are considered to possess a homogeneous microscopic structure. We provide the first experimental evidence of the existence of nanoscale heterogeneities in neat liquid and supercooled RTILs, such as 1-alkyl-3-methyl imidazolium-based salts, using X-ray diffraction. These heterogeneities are of the order of a few nanometers and their size is proportional to the alkyl chain length. These results provide strong support to the findings from recent molecular dynamics simulations, which proposed the occurrence of nanostructures in RTILs, as a consequence of alkyl chains segregation. Moreover, our study addresses the issue of the temperature dependence of the heterogeneities size, showing a behavior that resembles the density one only below the glass transition, thus suggesting a complex behavior above this temperature. These results will provide a novel interpretation approach for the unique chemical physical properties of RTILs.     

Decoupling charge transport from the structural dynamics in room temperature ionic liquids

Light scattering and dielectric spectroscopy measurements were performed on the room temperature ionic liquid (RTIL) [C4mim][NTf2] in a broad temperature and frequency range. Ionic conductivity was used to estimate self-diffusion of ions, while light scattering was used to study structural relaxation. We demonstrate that the ionic diffusion decouples from the structural relaxation process as the temperature of the sample decreases toward T(g). The strength of the decoupling appears to be significantly lower than that expected for a supercooled liquid of similar fragility. The structural relaxation process in the RTIL follows well the high-temperature mode coupling theory (MCT) scenario. Using the MCT analysis we estimated the dynamic crossover temperature in [C4mim][NTf2] to be T(c) ~ 225 ± 5 K. However, our analysis reveals no sign of the dynamic crossover in the ionic diffusion process.     

Excited-state proton-relay dynamics of 7-hydroxyquinoline controlled by solvent reorganization in room temperature ionic liquids

The excited-state triple proton relay of 7-hydroxyquinoline (7HQ) along a hydrogen-bonded methanol chain in room temperature ionic liquids (RTILs) has been investigated using picosecond time-resolved fluorescence spectroscopy. The rate constant of the proton relay in a methanol-added RTIL is found to be slower by an order of magnitude than that in bulk methanol and to have unity in its kinetic isotope effect. These suggest that the excited-state tautomerization dynamics of 7HQ in methanol-added RTILs is mainly controlled by the solvent reorganization dynamics to form a cyclically hydrogen-bonded complex of 7HQ·(CH(3)OH)(2) upon absorption of a photon due to high viscosity values of RTILs. Because the cyclic complex of 7HQ·(CH(3)OH)(2) at the ground state is unstable in RTILs, the collision-induced slow formation of the cyclic complex should take place upon excitation prior to undergoing subsequent intrinsic proton transfer rapidly.     

室温离子液体催化正戊烷异构化反应

采用无水三氯化铝与多种有机氯盐按照不同摩尔比合成了具有不同酸度和阴、阳离子结构的室温离子液体,考察了离子液体的结构和反应条件对正戊烷异构化反应的影响。结果表明,合成离子液体过程中,AlCl3与Et3NHCl摩尔比对离子液体的酸强度和催化性能有较大的影响。实验确定了正丁醇为异构化反应适宜的引发剂,其用量为正戊烷的2.5%;正戊烷异构化的优化反应条件为反应温度30℃,反应10h,搅拌器转速1500r/min。在此优化反应条件下,正戊烷的转化率与异构化率分别达到85.66%和92.86%。     

One- and two-color photon echo peak shift studies of photosystem I

Wavelength-dependent one- and two-color photon echo peak shift spectroscopy was performed on the chlorophyll Qy band of trimeric photosystem I from Thermosynechococcus elongatus. Sub-100 fs energy transfer steps were observed in addition to longer time scales previously measured by others. In the main PSI absorption peak (675-700 nm), the peak shift decays more slowly with increasing wavelength, implying that energy transfer between pigments of similar excitation energy is slower for pigments with lower site energies. In the far-red region (715 nm), the decay of the peak shift is more rapid and is complete by 1 ps, a consequence of the strong electron-phonon coupling present in this spectral region. Two-color photon echo peak shift data show strong excitonic coupling between pigments absorbing at 675 nm and those absorbing at 700 nm. The one- and two-color peak shifts were simulated using the previously developed energy transfer model (J. Phys. Chem. B 2002, 106, 10251; Biophysical Journal 2003, 85, 140). The simulations agree well with the experimental data. Two-color photon echo peak shift is shown to be far more sensitive to variations in the molecular Hamiltonian than one-color photon echo peak shift spectroscopy.     

A comparative study of solvation dynamics in room-temperature ionic liquids

The solvation dynamics of ionic liquids have been the subject of many experimental and theoretical studies but remain poorly understood. We analyze these dynamics by modeling the time-resolved fluorescence response of coumarin 153 in two room-temperature ionic liquids: 1-butyl-1-methylpyrrolidinium bromide and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide. Our results demonstrate that phenomena such as electrostatic screening operate significantly differently in the two liquids, and the relative importance of translational and rovibrational components of the ionic response depends significantly on the character of the ions involved. However, collective motion dominates the response of both ionic liquids, and the qualitative features of this collective behavior are strikingly similar in both cases.     

Electrochemistry of room temperature protic ionic liquids

Eighteen protic ionic liquids containing different combinations of cations and anions, hydrophobicity, viscosity, and conductivity have been synthesized and their physicochemical properties determined. In one series, the diethanolammonium cations were combined with acetate, formate, hydrogen sulfate, chloride, sulfamate, and mesylate anions. In the second series, acetate and formate anions were combined with amine bases, triethylamine, diethylamine, triethanolamine, di-n-propylamine, and di-n-butylamine. The electrochemical characteristics of the eight protic ionic liquids that are liquid at room temperature (RTPILs) have been determined using cyclic, microelectrode, and rotating disk electrode voltammetries. Potential windows of the RTPILs have been compared at glassy carbon, platinum, gold, and boron-doped diamond electrodes and generally found to be the largest in the case of glassy carbon. The voltammetry of IUPAC recommended potential scale reference systems, ferrocene/ferrocenium and cobaltocenium/cobaltocene, have been evaluated and found to be ideal in the case of the less viscous RTPILs but involve adsorption in the highly viscous ones. Other properties such as diffusion coefficients, ionic conductivity, and double layer capacitance also have been measured. The influence of water on the potential windows, viscosity, and diffusion has been studied systematically by deliberate addition of water to the dried ionic liquids. The survey highlights the problems with voltammetric studies in highly viscous room temperature protic ionic liquids and also suggests the way forward with respect to their possible industrial use.