The dynamics of carbene solvation: an ultrafast study of p-biphenylyltrifluoromethylcarbene

Ultrafast photolysis (lambda(ex) = 308 nm) of p-biphenylyltrifluoromethyl diazomethane (BpCN2CF3) releases singlet p-biphenylyltrifluoromethylcarbene (BpCCF3) which absorbs strongly at 385 nm in cyclohexane, immediately after the 300 fs laser pulse. The initial absorption maximum shifts to longer wavelengths in coordinating solvents (nitrile, ether, and alcohol). In low viscosity coordinating solvents, the initial absorption maximum further red shifts between 2 and 10 ps after the laser pulse. Similar effects are observed upon ultrafast photolysis of 2-fluorenyltrifluoromethyl diazomethane (FlCN2CF3) and therefore cannot be associated with torsional motion around the two phenyl rings of the biphenyl compound. Instead, the effect is attributed to the dynamics of solvation of the singlet carbene. The time constant of solvation in normal alcohols lengthens with solvent viscosity in a linear manner. Furthermore, the time constants of the red shift in methanol-O-d (16 ps), ethanol-O-d (26 ps), 2-propanol-OD (40 ps), and 2,2,2-trifluoroethanol-O-d (14 ps) are longer than those recorded in methanol (9.6 ps, KIE = 1.7), ethanol (14.3 ps, KIE = 1.8), 2-propanol (28 ps, KIE = 1.4), and 2,2,2-trifluoroethanol (4.4 ps, KIE = 3.2), which indicates that the solvent reorganization involves formation of hydrogen bonds. The kinetic data are consistent with motion of the solvent to achieve a specific interaction with the carbene, with the creation of a new hydrogen bond. The solvated carbene reacts with the solvent over tens, hundreds, and thousands of ps, depending upon the solvent.     

Microenvironment in the corona region of triblock copolymer micelles: temperature dependent solvation and rotational relaxation dynamics of coumarin dyes

Dynamic Stokes' shift and fluorescence anisotropy measurements using coumarin-153 (C153) and coumarin-151 (C151) as the fluorescence probes have been carried out in aqueous poly(ethylene oxide)20-poly(propylene oxide)70-poly(ethylene oxide)20 (P123) and poly(ethylene oxide)100-poly(propylene oxide)70-poly(ethylene oxide)100 (F127) block copolymer micelles with an aim to understand the water structures and dynamics in the micellar corona region. It has been established that the probes reside in the micellar corona region. It is indicated that the corona regions of P123 and F127 micelles are relatively less hydrated than the Palisade layers of neutral micelles like Triton-X-100 and Brij-35. From the appraisal of total Stokes' shift values for the probes in the two block copolymer micelles, it is inferred that the F127 micelle is more hydrated than the P123 micelle. It is observed that the dynamic Stokes' shift values for both of the probes remain more or less similar at all the temperatures studied in the P123 micelle. For C153 in F127, however, the observed Stokes' shift is seen to decrease quite sharply with temperature, though it remains quite similar for C151. Moreover, the fraction of the unobserved initial dynamic Stokes' shift is appreciably higher for both the probes in the F127 micelle compared to that in P123. Over the studied temperature range of 293-313 K, the spectral shift correlation function is described adequately by a bi-exponential function. Rotational relaxation times for C153 in both the micelles show a kind of transition at around 303 K. These results have been rationalized assuming collapse of the poly(ethylene oxide) (PEO) blocks and formation of water clusters in the corona region due to dehydration of poly(ethylene oxide) blocks with an increase in temperature. A dissimilar probe location has been inferred for the differences in the results with C153 and C151 probes in F127. Comparison of the microviscosity and the hydration of the block copolymer micelles has also been made with those of the other commonly used neutral micelles, for a better understanding of the results in the block copolymer micelles.     

Direct dynamics study of ultrafast vibrational energy relaxation in ice Ih

Car-Parrinello molecular dynamics (CPMD) and a previously developed wave packet model are used to study ultrafast relaxation in water clusters. Water clusters of 15 water molecules are used to represent ice Ih. The relaxation is studied by exciting a symmetric or an asymmetric stretch mode of the central water molecule. The CPMD results suggest that relaxation occurs within 100 fs. This is in agreement with experimental work by Woutersen and Bakker and the earlier wave packet calculations. The CPMD results further indicate that the excitation energy is transferred both intramolecularly and intermolecularly on roughly the same time scale. The intramolecular energy transfer occurs predominantly between the symmetric and asymmetric modes while the bend mode is largely left unexcited on the short time scale studied here.     

Absorption and emission lineshapes and ultrafast solvation dynamics of NO in parahydrogen

We perform a theoretical study on the electronic spectroscopy of dilute NO impurity embedded in parahydrogen (p-H(2)). Absorption and emission lineshapes for the A (2)Sigma(+)<--X (2)Pi Rydberg transition of NO in parahydrogen have been previously measured and simulated, which yielded results for the NO/p-H(2) ground and excited state pair potentials [L. Bonacina et al., J. Chem. Phys. 125, 054507 (2006)]. Using these potentials, we performed molecular dynamics simulation, theoretical statistical mechanical calculations of absorption and emission lineshapes, and both equilibrium and nonequilibrium solvation correlation functions for NO chromophore in parahydrogen. Theory was shown to be in good agreement with simulation. Linear response treatment of solvation dynamics was shown to break down due to a dramatic change in the solute-solvent microstructure upon solute excitation to the Rydberg state and the concomitant increase of the solute size.     

Quantitative distinction between competing intramolecular bond twisting and solvent relaxation dynamics: an ultrafast study

Often an intramolecular relaxation process takes place in a time scale similar to that of the solvent relaxation process. Under these circumstances the dynamic Stokes' shift of the probe can be modulated by the combined effect of these two relaxation processes. In the present article we have studied ultrafast solvent relaxation using three different coumarin dyes and proposed a methodology for the quantitative separation of the dynamics of two competing processes, namely, solvent relaxation and bond twisting, that take place simultaneously in the present systems.     

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.     

Aqueous solutions of amphiphilic polysaccharides: Concentration and temperature effect on viscosity

Amphiphilic polymers have been prepared by chemical modification of a neutral bacterial polysaccharide, dextran, consisting of α-1,6 linked glucose units. Various amounts of aliphatic hydrocarbon groups with six carbon atoms are attached along the dextran chains by ether links. The viscosities of their aqueous solutions in the dilute domain and in the beginning of the semi-dilute domain are considered with particular interest for the variation with temperature and polymer concentration.The equations of Fedors and Martin are shown to conveniently depict the effect of polymer concentration over the whole investigated range. A semi-empirical equation is established between the critical concentration deduced from Fedors equation and both the Huggins coefficient and the intrinsic viscosity (parameters deduced from the results in the dilute domain).The variation of solution viscosity with temperature is also examined. Andrade equation, combined with Martin or Fedors equation, is shown to account for the temperature effect. Particularly, the variation of the activation energy with polymer concentration is conveniently depicted. The validity of the derived semi-empirical equations is extended to other polymer solutions using literature data.     

Two-dimensional measurements of the solvent structural relaxation dynamics in dipolar solvation

Resonant-pump polarizability response spectroscopy (RP-PORS) is based on an optical heterodyne detected transient grating (OHD-TG) method with an additional resonant pump pulse. In RP-PORS, the resonant pump pulse excites the solute-solvent system and the subsequent relaxation of the solute-solvent system is monitored by the OHD-TG spectroscopy. RP-PORS is shown to be an excellent experimental tool to directly measure the solvent responses in solvation. In the present work, we extended our previous RP-PORS (Park et al., Phys. Chem. Chem. Phys., 2011, 13, 214-223) to measure time-dependent transient solvation polarizability (TSP) spectra with Coumarin153 (C153) in acetonitrile. The time-dependent TSP spectra showed how the different solvent intermolecular modes were involved in different stages of the solvation process. Most importantly, the inertial and diffusive components of the solvent intermolecular modes in solvation were found to be spectrally and temporally well-separated. In a dipolar solvation of C153, high-frequency inertial solvent modes were found to be driven instantaneously and decay on a subpicosecond timescale while low-frequency diffusive solvent modes were induced slowly and decayed on a picosecond timescale. Our present result is the first experimental manifestation of frequency-dependent solvent intermolecular response in a dipolar solvation.     

A surface science approach to ultrafast electron transfer and solvation dynamics at interfaces

Excess electrons in polar media, such as water or ice, are screened by reorientation of the surrounding molecular dipoles. This process of electron solvation is of vital importance for various fields of physical chemistry and biology as, for instance, in electrochemistry or photosynthesis. Generation of such excess electrons in bulk water involves either photoionization of solvent molecules or doping with e.g. alkali atoms, involving possibly perturbing interactions of the system with the parent-cation. Such effects are avoided when using a surface science approach to electron solvation: in the case of polar adsorbate layers on metal surfaces, the substrate acts as an electron source from where photoexcited carriers are injected into the adlayer. Besides the investigation of electron solvation at such interfaces, this approach allows for the investigation of heterogeneous electron transfer, as the excited solvated electron population continuously decays back to the metal substrate. In this manner, electron transfer and solvation processes are intimately connected at any polar adsorbate-metal interface. In this tutorial review, we discuss recent experiments on the ultrafast dynamics of photoinduced electron transfer and solvation processes at amorphous ice-metal interfaces. Femtosecond time-resolved two-photon photoelectron spectroscopy is employed as a direct probe of the electron dynamics, which enables the analysis of all elementary processes: the charge injection across the interface, the subsequent electron localization and solvation, and the dynamics of electron transfer back to the substrate. Using surface science techniques to grow and characterize various well-defined ice structures, we gain detailed insight into the correlation between adsorbate structure and electron solvation dynamics, the location (bulk versus surface) of the solvation site, and the role of the electronic structure of the underlying metal substrate on the electron transfer rate.     

Femtosecond spectroscopic study of the solvation of amphiphilic molecules by water

We use polarization-resolved mid-infrared pump-probe spectroscopy to study the aqueous solvation of proline and N-methylacetamide. These molecules serve as models to study the solvation of proteins. We monitor the orientational dynamics of partly deuterated water molecules (HDO) that are present at a low concentration in the water. We find that the OD vibration of HDO relaxes via an intermediate level, that is characterized by a hydrogen-bond that is stronger than in the ground state. With increasing concentration the lifetime of the excited state increases from 1.8 ps to 2.4 ps and the lifetime of the intermediate level from 0.6 ps to 1.0 ps. Regarding the orientational dynamics we observe biexponential behavior, which finds its origin in the presence of two classes of water molecules. There is a fraction of water molecules that has bulk-like orientational dynamics (τ_rot = 2.5 ps) and a fraction of immobilized water molecules (τ_rot > 10 ps). The relative abundance of the two fractions is determined by the nature and concentration of the solute. We find that the hydrophobic solute groups are responsible for the immobilization of water molecules. Every methyl group causes the immobilization of approximately 4 water OH groups. The hydrophilic solute groups, on the other hand, do not hinder the reorientation and the water molecules solvating them reorient with the same rate as in the bulk liquid.     

Dielectric friction and solvation dynamics: Novel results on relaxation in dipolar liquids

In this article we present a new, general but simple, microscopic expression for time-dependent solvation energy of an ion. This expression is surprisingly similar to the expression for the time-dependent dielectric friction on a moving ion. We show that both the Chandra-Bagchi and the Fried-Mukamel formulations of solvation dynamics can be easily derived from this expression. This expression leads to an almost perfect agreement of the theory with all the available computer simulation results. Second, we show here for the first time that the mobility of a light solute ion can significantly accelerate its own solvation, specially in the underdamped limit. The latter result is also in excellent agreement with the computer simulations.     

Theory for the solvation of nonpolar solutes in water

We recently developed an angle-dependent Wertheim integral equation theory (IET) of the Mercedes-Benz (MB) model of pure water [Silverstein et al., J. Am. Chem. Soc. 120, 3166 (1998)]. Our approach treats explicitly the coupled orientational constraints within water molecules. The analytical theory offers the advantage of being less computationally expensive than Monte Carlo simulations by two orders of magnitude. Here we apply the angle-dependent IET to studying the hydrophobic effect, the transfer of a nonpolar solute into MB water. We find that the theory reproduces the Monte Carlo results qualitatively for cold water and quantitatively for hot water.     

Experimental and theoretical study of temperature dependent exciton delocalization and relaxation in anthracene thin films

The spectroscopy of solid anthracene is examined both experimentally and theoretically. To avoid experimental complications such as self-absorption and polariton effects, ultrathin polycrystalline films deposited on transparent substrates are studied. To separate the contributions from different emitting species, the emission is resolved in both time and wavelength. The spectroscopic data are interpreted in terms of a three-state kinetic model, where two excited states, a high energy state 1 and a low energy state 2, both contribute to the luminescence and are kinetically coupled. Using this model, we analyze the spectral lineshape, relative quantum yield, and relaxation rates as a function of temperature. For state 1, we find that the ratio of the 0-0 vibronic peak to the 0-1 peak is enhanced by roughly a factor of 3.5 at low temperature, while the quantum yield and decay rates also increase by a similar factor. These observations are explained using a theoretical model previously developed for herringbone polyacene crystals. The early-time emission lineshape is consistent with that expected for a linear aggregate corresponding to an edge-dislocation defect. The results of experiment and theory are quantitatively compared at different temperatures in order to estimate that the singlet exciton in our polycrystalline films is delocalized over about ten molecules. Within these domains, the exciton's coherence length steadily increases as the temperature drops, until it reaches the limits of the domain, whereupon it saturates and remains constant as the temperature is lowered further. While the theoretical modeling correctly reproduces the temperature dependence of the fluorescence spectral lineshape, the decay of the singlet exciton appears to be determined by a trapping process that becomes more rapid as the temperature is lowered. This more rapid decay is consistent with accelerated trapping due to increased delocalization of the exciton at lower temperatures. These observations suggest that exciton coherence can play an important role in both radiative and nonradiative decay channels in these materials. Our results show that the spectroscopy of polyacene solids can be analyzed in a self-consistent fashion to obtain information about electronic delocalization and domain sizes.     

A volumetric study of two related amphiphilic beta-blockers as a function of temperature and electrolyte concentration

The effects of electrolyte concentration and temperature on aqueous solutions of propranolol and acebutolol hydrochlorides have been investigated using density and ultrasound velocity measurements. The electrolyte range was 0.0–0.5 and 0.4–1.0 m for propranolol and acebutolol, respectively. For each electrolyte concentration the temperature range was 288.15–313.15 K. Critical concentrations were obtained from plots of ultrasound velocity against drug concentration. Experimental results yielded the apparent molar volume and the apparent molar adiabatic compressibility for both beta-blockers, measured over a wide concentration range. Negative deviations of the apparent molar volume from the Debye–Hückel limiting law in dilute solutions indicate the absence of premicellar aggregation. A negative slope was found for ΔV_m against temperature for both drugs. This negative value suggests that the expansibility of the surfactant in the micellar state is less than that in the aqueous phase. Changes in molar volume and adiabatic compressibility accompanying aggregate formation were smaller than those of typical surfactants, suggesting a more tightly packed aggregate.     

Excited-state absorption and ultrafast relaxation dynamics of porphyrin, diprotonated porphyrin, and tetraoxaporphyrin dication

The relaxation dynamics of unsubstituted porphyrin (H2P), diprotonated porphyrin (H4P2+), and tetraoxaporphyrin dication (TOxP2+) has been investigated in the femtosecond-nanosecond time domain upon photoexcitation in the Soret band with pulses of femtosecond duration. By probing with spectrally broad femtosecond pulses, we have observed transient absorption spectra at delay times up to 1.5 ns. The kinetic profiles corresponding with the band maxima due to excited-state absorption have been determined for the three species. Four components of the relaxation process are distinguished for H2P: the unresolvably short B --> Qy internal conversion is followed by the Qy --> Qx process, vibrational relaxation, and thermalization in the Qx state with time constant approximately 150 fs, 1.8 ps, and 24.9 ps, respectively. Going from H2P to TOxP2+, two processes are resolved, i.e., B --> Q internal conversion and thermal equilibration in the Q state. The B --> Q time constant has been determined to be 25 ps. The large difference with respect to the B --> Qy time constant of H2P has been related to the increased energy gap between the coupled states, 9370 cm-1 in TOxP2+ vs 6100 cm-1 in H2P. The relaxation dynamics of H4P2+ has a first ultrafast component of approximately 300 fs assigned as internal conversion between the B (or Soret) state and charge-transfer (CT) states of the H4P2+ complex with two trifluoroacetate counterions. This process is followed by internal CT --> Q conversion (time constant 9 ps) and thermalization in the Q state (time constant 22 ps).     

Excited states in electron-transfer reaction products: ultrafast relaxation dynamics of an isolated acceptor radical anion

The spectroscopy and ultrafast relaxation dynamics of excited states of the radical anion of a representative charge-transfer acceptor molecule, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, have been studied in the gas phase using time-resolved photoelectron spectroscopy. The photoelectron spectra reveal that at least two anion excited states are bound. Time-resolved studies show that both excited states are very short-lived and internally convert to the anion ground state, with the lower energy state relaxing within 200 fs and a near-threshold valence-excited state relaxing on a 60 fs time scale. These excited states, and in particular the valence-excited state, present efficient pathways for electron-transfer reactions in the highly exergonic inverted region which commonly displays rates exceeding predictions from electron-transfer theory.     

Water dynamics at protein interfaces: ultrafast optical Kerr effect study

The behavior of water molecules surrounding a protein can have an important bearing on its structure and function. Consequently, a great deal of attention has been focused on changes in the relaxation dynamics of water when it is located at the protein surface. Here we use the ultrafast optical Kerr effect to study the H-bond structure and dynamics of aqueous solutions of proteins. Measurements are made for three proteins as a function of concentration. We find that the water dynamics in the first solvation layer of the proteins are slowed by up to a factor of 8 in comparison to those in bulk water. The most marked slowdown was observed for the most hydrophilic protein studied, bovine serum albumin, whereas the most hydrophobic protein, trypsin, had a slightly smaller effect. The terahertz Raman spectra of these protein solutions resemble those of pure water up to 5 wt % of protein, above which a new feature appears at ~80 cm(-1), which is assigned to a bending of the protein amide chain.     

Femtosecond infrared study of the dynamics of solvation and solvent caging.

The ultrafast reaction dynamics following 295-nm photodissociation of Re2CO10 were studied experimentally with 300-fs time resolution in the reactive, strongly coordinating CCl4 solution and in the inert, weakly coordinating hexane solution. Density-functional theoretical (DFT) and ab initio calculations were used to further characterize the transient intermediates seen in the experiments. It was found that the quantum yield of the Re-Re bond dissociation is governed by geminate recombination on two time scales in CCl4, approximately 50 and approximately 500 ps. The recombination dynamics are discussed in terms of solvent caging in which the geminate Re(CO)5 pair has a low probability to escape the first solvent shell in the first few picoseconds after femtosecond photolysis. The other photofragmentation channel resulted in the equatorially solvated dirhenium nonacarbonyl eq-Re2(CO)9(solvent). Theoretical calculations indicated that a structural reorganization energy cost on the order of 6-7 kcal/mol might be required for the unsolvated nonacarbonyl to coordinate to a solvent molecule. These results suggest that for Re(CO)5 the solvent can be treated as a viscous continuum, whereas for the Re2(CO)9 the solvent is best described in molecular terms.     

Characterization of temperature dependent modifier effects in SFC using linear solvation energy relationships

Summary Linear solvation energy relationships (LSERs) are used to probe the changes in mobile and stationary phase properties of a carbon dioxide-based mobile phase and a polymeric stationary phase under near-critical conditions. Four mobile phase modifiers are compared with respect to dipolarity/polarizability, hydrogen bond donating and accepting ability, and other intermolecular interactions as a function of temperature. As temperature nears the mixture critical point, the differences in these properties between the mobile and stationary phases change to reflect the growing heterogeneity in mobile phase component distribution at the chromatographic interface. The stationary phase loses many of its original characteristics and takes on characteristics typical of the mobile phase modifier due to preferential adsorption of the modifier at the surface of the stationary phase.     

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.