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We study the molecular-scale structure of the surface of Reline, a DES made from urea and choline chloride, using heterodyne-detected vibrational sum frequency generation (HD-VSFG). Reline absorbs water when exposed to the ambient atmosphere, and following structure-specific changes at the Reline/air interface is crucial and difficult. For Reline (dry, 0 wt %, w/w, water) we observe vibrational signatures of both urea and choline ions at the surface. Upon increase of the water content, there is a gradual depletion of urea from the surface, an enhanced alignment, and an enrichment of the surface with choline cations, indicating surface speciation of ChCl. Above 40% w/w water content, choline cations abruptly deplete from the surface, as evidenced by the decrease of the vibrational signal of the −CH2– groups of choline and the rapid rise of a water signal. Above 60% w/w water content, the surface spectrum of aqueous Reline becomes indistinguishable from that of neat water.

Hyaluronan is a biopolymer that is essential for many biological processes in the human body, like the regulation of tissue lubrication and inflammatory responses. Here, we study the behavior of hyaluronan at aqueous surfaces using heterodyne-detected vibrational sum-frequency generation spectroscopy (HD-VSFG). Low-molecular-weight hyaluronan (similar to 150 I(Da) gradually covers the water-air interface within hours, leading to a negatively charged surface and a reorientation of interfacial water molecules. The rate of surface accumulation strongly increases when the bulk concentration of low-molecular-weight hyaluronan is increased. In contrast, high-molecular-weight hyaluronan (>1 MDa) cannot be detected at the surface, even hours after the addition of the polymer to the aqueous solution. The strong dependence on the polymer molecular weight can be explained by entanglements of the hyaluronan polymers. We also find that for low-molecular-weight hyaluronan the migration kinetics of hyaluronan in aqueous media shows an anomalous dependence on the pH of the solution, which can be explained from the interplay of hydrogen bonding and electrostatic interactions of hyaluronan polymers.

Antifreeze glycoproteins (AFGPs) are able to bind to ice, halt its growth, and are the most potent inhibitors of ice recrystallization known. The structural basis for AFGP’s unique properties remains largely elusive. Here we determined the antifreeze activities of AFGP variants that we constructed by chemically modifying the hydroxyl groups of the disaccharide of natural AFGPs. Using nuclear magnetic resonance, two-dimensional infrared spectroscopy, and circular dichroism, the expected modifications were confirmed as well as their effect on AFGPs solution structure. We find that the presence of all the hydroxyls on the disaccharides is a requirement for the native AFGP hysteresis as well as the maximal inhibition of ice recrystallization. The saccharide hydroxyls are apparently as important as the acetyl group on the galactosamine, the α-linkage between the disaccharide and threonine, and the methyl groups on the threonine and alanine. We conclude that the use of hydrogen-bonding through the hydroxyl groups of the disaccharide and hydrophobic interactions through the polypeptide backbone are equally important in promoting the antifreeze activities observed in the native AFGPs. These important criteria should be considered when designing synthetic mimics.

The molecular configuration and chemistry at the zinc/zinc oxide-polyester interface were studied by using two complementary spectroscopic techniques: attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) and sum-frequency generation (SFG) spectroscopy. It was shown that ATR-FTIR should be considered as a (3D) interphase-sensitive technique with probing depths of 250-400 nm in the headgroup region (2000-1200 cm-1). On the other hand, SFG is known to be a (2D) interface-sensitive technique. The ATR-FTIR measurements showed that carboxylate groups are formed within the near-interface region of the polyester phase. SFG measurements showed that the carboxylic acid groups are stable at the polymer-zinc/zinc oxide interface. In addition, in situ ATR-FTIR and SFG measurements have been conducted when exposing the polyester-zinc/zinc oxide system to D2O. The exposure to D2O is observed to lead to an additional conversion of ester and carboxylic acid groups to carboxylate groups. The comparison of the SFG and ATR-FTIR measurements shows that this conversion occurs much slower at the polyester-zinc/zinc oxide interface than in the bulk of the polyester. Finally, the strengths and limitations as well as the complementarity of both techniques are discussed.

We use two-dimensional infrared spectroscopy to study the interactions between the amide and carboxylate anion groups of hyaluronan polymers at neutral pH. The spectra reveal the presence of intrachain hydrogen bonds between the amide and carboxylate anion groups in aqueous solution. We determine the relative orientation of the amide and carboxylate anion groups when forming this hydrogen bond and quantify the fraction of amide groups that participate in hydrogen bonding. We find that a variation of the pH and/or temperature has a negligible effect on this fraction, whereas the persistence length of the hyaluronan chains and the associated viscosity of hyaluronan solutions are known to change significantly. We conclude that the hydrogen bonding between the amide and carboxylate anion groups does not significantly contribute to the chain rigidity of hyaluronan polymers.

[Image: see text] We investigate the molecular geometry of the carboxyl group of formic acid in acetonitrile and aqueous solutions at room temperature with two-dimensional infrared spectroscopy (2D-IR). We found that the carboxyl group adopts two distinct configurations: a configuration in which the carbonyl group is oriented antiparallel to the hydroxyl (anti-conformer), and a configuration in which the carbonyl group is oriented at an angle of ∼60° with respect to the hydroxyl (syn-conformer). These results constitute the first experimental evidence that carboxyl groups exist as two distinct and long-living conformational isomers in aqueous solution at room temperature.

We use surface-specific heterodyne-detected vibrational sum-frequency generation spectroscopy (HD-VSFG) and surface tension measurements to investigate the molecular structure of the surface of aqueous solutions of poly(vinyl alcohol) (PVA) polymers with average molecular weights of 10000 and 125000 g/mol. We find that the interfacial water molecules have a preferred orientation with their hydrogen-bonded O-H groups pointing away from the bulk, for both PVA10000 and PVA125000. This observation is explained from the ongoing hydrolysis of the acetyl impurities on the PVA polymer chains. This hydrolysis yields negatively charged acetate ions that have a relatively high surface propensity. For both PVA10000 and PVA125000 the strong positive signal vanishes when the pH is decreased, due to the neutralization of the acetate ions. For solutions with a high concentration of PVA10000 the interfacial water signal becomes very small, indicating that the surface gets completely covered with a disordered PVA polymer film. In contrast, for high concentrations of PVA125000, the strong positive water signal persists at high pH, which shows that the water surface does not get completely covered. The HD-VSFG data combined with surface tension data indicate that concentrated PVA125000 solutions form a structured surface layer with pores containing a high density of interfacial water.

Antifreeze proteins (AFPs) and antifreeze glycoproteins (AFGPs) inhibit ice growth via an adsorption-inhibition mechanism that assumes irreversible binding of AF(G)Ps to embryonic ice crystals and the inhibition of further growth. The irreversible binding of antifreeze glycoproteins (AFGPs) to ice has been questioned and remains poorly understood. Here, we used microfluidics and fluorescence microscopy to investigate the nature of the binding of small and large AFGP isoforms. We found that both AFGP isoforms bind irreversibly to ice, as evidenced by microfluidic solution exchange experiments. We measured the adsorption rate of the large AFGP isoform and found it to be 50% faster than that of AFP type III. We also found that the AFGP adsorption rate decreased by 65% in the presence of borate, a well-known inhibitor of AFGP activity. Our results demonstrate that the adsorption rate of AFGPs to ice is crucial for their ice growth inhibition capability.

We study the secondary structure of the blood protein fibrinogen using two-dimensional infrared spectroscopy. With this technique, we identify the amide I' vibrational modes of the antiparallel β-sheets and turns of fibrinogen. We observe ultrafast energy flow among these amide I' vibrational modes with a time constant of ∼7 ps. This energy transfer time constant does not change significantly upon fibrin fiber formation, indicating that the secondary structure of the fibrinogen monomers remains largely unchanged in the polymerization process.

We study the resonant interaction of the OH stretch vibrations of water molecules at the surfaces of liquid water and ice using heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy. By studying different isotopic mixtures of H2O and D2O, we vary the strength of the interaction, and we monitor the resulting effect on the HD-SFG spectrum of the OH stretch vibrations. We observe that the near-resonant coupling effects are weaker at the surface than in the bulk, for both water and ice, indicating that for both phases of water the OH vibrations are less strongly delocalized at the surface than in the bulk.

We use heterodyne-detected vibrational sum-frequency generation (HD-VSFG) to determine the orientation of the molecular plane of methylguanidinium ions at the surface of aqueous solutions. We measure the VSFG response of the symmetric and antisymmetric methyl stretch vibrations of the methylguanidinium ion with different polarization combinations. We find that for at least 50% of the methylguanidinium ions the molecular plane is at an angle >20° with respect to the surface plane. Hence, for only a minor fraction of the ions does the molecular plane have an orientation (near-)parallel to the surface plane, in contrast to the predictions of recent molecular dynamics simulation studies.

We study the signatures of the OH stretch vibrations at the basal surface of ice using heterodyne-detected sum-frequency generation and molecular dynamics simulations. At 150 K, we observe seven distinct modes in the sum-frequency response, five of which have an analogue in the bulk, and two pure surface-specific modes at higher frequencies (∼3530 and ∼3700 cm–1). The band at ∼3530 cm–1 has not been reported previously. Using molecular dynamics simulations, we find that the ∼3530 cm–1 band contains contributions from OH stretch vibrations of both fully coordinated interfacial water molecules and water molecules with two donor and one acceptor hydrogen bond.

We use surface-specific intensity vibrational sum-frequency generation and attenuated total reflection spectroscopy to probe the ionization state of the amino-acids l-alanine and l-proline at the air/water surface and in the bulk. The ionization state is determined by probing the vibrational signatures of the carboxylic acid group, representing the nondissociated acid form, and the carboxylate anion group, representing the dissociated form, over a wide range of pH values. We find that the carboxylic acid group deprotonates at a significantly higher pH at the surface than in the bulk.

Water in contact with lipids is an important aspect of most biological systems and has been termed “biological water”. We used time-resolved infrared spectroscopy to investigate the vibrational dynamics of lipid-bound water molecules, to shed more light on the properties of these important molecules. We studied water in contact with a positively charged lipid monolayer using surface-specific two-dimensional sum frequency generation vibrational spectroscopy with subpicosecond time resolution. The dynamics of the O–D stretch vibration was measured for both pure D2O and isotopically diluted D2O under a monolayer of 1,2-dipalmitoyl-3-trimethylammonium-propane. It was found that the lifetime of the stretch vibration depends on the excitation frequency and that efficient energy transfer occurs between the interfacial water molecules. The spectral diffusion and vibrational relaxation of the stretch vibration were successfully explained with a simple model, taking into account the Forster transfer between stretch...

We use intensity and heterodyne-detected vibrational sum-frequency generation (VSFG and HD-VSFG) to study the structure of water at the surface of aqueous tetra-n-alkylammonium bromide (TAABr) solutions. We compare the water structure for four different n-alkyl chains (n = 1, 2, 3, 4). For solutions of tetra-n-alkylammonium bromides with short n-alkyl chains (n = 1, 2), we observe the structure of the surface water to be similar to the structure observed for simple inorganic salt solutions. For these solutions, the presence of Br– at the interface is observed to lead to a small decrease in the average strength of the hydrogen bonds. For solutions of tetra-n-alkylammonium bromides with long n-alkyl chains (n = 3, 4), we observe a strong ordering of the water molecules at the solution surface. The water molecules show a net orientation of their O–H group toward the bulk, which can be explained from the high surface propensity of positively charged tetra-n-alkylammonium ions with long alkyl chains (n = 3, 4)...

We study the ultrafast O-H stretch vibrational relaxation dynamics of protonated water clusters embedded in a matrix of deuterated acetonitrile, using polarization-resolved mid-IR femtosecond spectroscopy. The clusters are produced by mixing triflic (trifluoromethanesulfonic) acid and H2O in molar ratios of 1:1, 1:2, and 1:3, thus varying the degree of hydration of the proton. At all hydration levels the excited O-H stretch vibration of the hydrated proton shows an ultrafast vibrational relaxation with a time constant T1100 fs, leading to an ultrafast local heating of the protonated water cluster. This excess thermal energy, initially highly localized to the region of the excited proton, first re-distributes over the aqueous cluster and then dissipates into the surrounding acetonitrile matrix. For clusters with a triflic acid to H2O ratio of 1:3 these processes occur with time constants of 320 ± 20 fs and 1.4 ± 0.1 ps, respectively. The cooling of the clusters reveals a long-living, underlying transient absorption change with high anisotropy. We argue that this feature stems from the vibrational predissociation of a small fraction of the proton hydration structures, directly following the ultrafast infrared excitation.

Understanding the structure and dynamics of water’s constituent ions, proton and hydroxide, has been a subject of numerous experimental and theoretical studies over the last century. Besides their obvious importance in acid-base chemistry, these ions play an important role in numerous applications ranging from enzyme catalysis to environmental chemistry. Despite a long history of research, many fundamental issues regarding their properties continue to be an active area of research. Here, we provide a review of the experimental and theoretical advances made in the last several decades in understanding the structure, dynamics, and transport of the proton and hydroxide ions in different aqueous environments, ranging from water clusters to the bulk liquid and its interfaces with hydrophobic surfaces. The propensity of these ions to accumulate at hydrophobic surfaces has been a subject of intense debate, and we highlight the open issues and challenges in this area. Biological applications reviewed include proton transport along the hydration layer of various membranes and through channel proteins, problems that are at the core of cellular bioenergetics.

As water is an essential ingredient in protein structure, dynamics, and functioning, knowledge of its behavior near proteins is crucial. We investigate water dynamics around bovine α-lactalbumin by combining molecular dynamics simulations with polarization-resolved femtosecond infrared (fs-IR) spectroscopy. We identify slowly reorienting surface waters and establish their hydrogen-bond lifetime and reorientation dynamics, which we compare to the experimentally measured anisotropy decay. The calculated number of slow surface waters is in reasonable agreement with the results of fs-IR experiments. While surface waters form fewer hydrogen bonds than the bulk, within the hydration layer water is slower when donating more hydrogen bonds. At concave sites the protein-water hydrogen bonds break preferably via translational diffusion rather than via a hydrogen-bond jump mechanism. Water molecules reorient slower near these sites than at convex water-exposed sites. Protein misfolding leads to an increased exposure of hydrophobic groups, inducing relatively faster surface water dynamics. Nevertheless, the larger exposed surface slows down a larger amount of water. While for native proteins hydrating water is slower near hydrophobic than near hydrophilic residues, mainly due to stronger confinement, misfolding causes hydrophobic water to reorient relatively faster because exposure of hydrophobic groups destroys concave protein cavities with a large excluded volume.

Water molecules in hydrated salts often have a well-defined geometrical arrangement and form an excellent model system for studying the effects of the hydrogen-bond environment on vibrational energy relaxation. Hydrated lithium nitrate contains two distinct types of crystal water molecules. One water molecule makes strong and weak hydrogen bonds; the other water molecule makes two bifurcated hydrogen bonds. We use femtosecond two-dimensional infrared spectroscopy to probe the vibrational relaxation dynamics of the OD stretch vibration of dilute HDO molecules in lithium nitrate trihydrate. In the temperature range from 22 to 295 K we observe a decrease in vibrational lifetime from 3.8 ± 0.2 to 2.8 ± 0.1 ps for the strongly hydrogen-bonded species, from 5.41 ± 0.08 to 4.14 ± 0.05 ps for the bifurcated hydrogen-bonded species, and from 10.4 ± 0.2 to 8.8 ± 0.4 ps for weakly hydrogen-bonded species. This temperature dependence is opposite to that of the OD stretch vibration of dilute HDO:H2O ice, for which the...

We present an overview of the studies of ion pairing in aqueous media of the past decade. In these studies, interactions between ions, and between ions and water, are investigated with relatively novel approaches, including dielectric relaxation spectroscopy, far-infrared (terahertz) absorption spectroscopy, femtosecond mid-infrared spectroscopy, and X-ray spectroscopy and scattering, as well as molecular dynamics simulation methods. With these methods, it is found that ion pairing is not a rare phenomenon only occurring for very particular, strongly interacting cations and anions. Instead, for many salt solutions and their interfaces, the measured and calculated structure and dynamics reveal the presence of a distinct concentration of contact ion pairs (CIPs), solvent shared ion pairs (SIPs), and solvent-separated ion pairs (2SIPs). We discuss the importance of specific ion-pairing interactions between cations like Li(+) and Na(+) and anionic carboxylate and phosphate groups for the structure and functioning of large (bio)molecular systems.

Proteins perform specific biological functions that strongly depend on their three-dimensional structure. This three-dimensional structure, i.e. the way the protein folds, is strongly determined by the interaction between the protein and the water solvent. We study the dynamics of water in aqueous solutions of several globular proteins at different degrees of urea-induced unfolding, using polarization-resolved femtosecond infrared spectroscopy. We observe that a fraction of the water molecules is strongly slowed down by their interaction with the protein surface. By monitoring the slow water fraction we can directly probe the amount of water-exposed protein surface. We find that at mild denaturing conditions, the water-exposed surface increases by almost 50%, while the secondary structure is still intact. This finding indicates that protein unfolding starts with the protein structure becoming less tight, thereby allowing water to enter.

Charge separation and recombination are key processes determining the performance of organic optoelectronic devices. Here we combine photoluminescence and photovoltaic characterisation of organic solar cell devices with ultrafast multi-pulse photocurrent spectroscopy to investigate charge generation mechanisms in the organic photovoltaic devices based on a blend of an alternating polyquinoxaline copolymer with fullerene. The combined use of these techniques enables the determination of the contributions of geminate and bimolecular processes to the solar cell performance. We observe that charge separation is not a temperature-activated process in the studied materials. At the same time, the generation of free charges shows a clear external-field and morphology dependence. This indicates that the critical step of charge separation involves the non-equilibrium state that is formed at early times after photoexcitation, when the polaronic localisation is not yet complete. This work reveals new aspects of molecular level charge dynamics in the organic light-conversion systems.

We have studied the properties of water molecules at the surface of salt solutions containing hydrophobic anions like triflate (CF3SO3–), ethanesulfonate (C2H5SO3–), and butanesulfonate (C4H9SO3–) using vibrational sum-frequency generation (VSFG) spectroscopy. The VSFG spectra reveal a surprisingly strong isotope effect in the intra- and intermolecular mixing of the water molecules contained in the hydration shells of the hydrophobic anions. The O–H stretch vibrations of H2O molecules in the hydration shell are strongly mixed, whereas the O–D stretch vibrations of hydrating D2O molecules are decoupled. This isotope effect is not observed for other ions like perchlorate (ClO4–), and can be explained from the structure of the hydration shells of the hydrophobic ions.

We study the vibrational relaxation dynamics and the reorientation dynamics of HDO molecules in binary water-dimethyl sulfoxide (DMSO) and water-acetone mixtures with polarization-resolved femtosecond mid-infrared spectroscopy. For low solute concentrations we observe a slowing down of the reorientation of part of the water molecules that hydrate the hydrophobic methyl groups of DMSO and acetone. For water-DMSO mixtures the fraction of slowed-down water molecules rises much steeper with solute concentration than for water-acetone mixtures, showing that acetone molecules show significant aggregation already at low concentrations. At high solute concentrations, the vibrational and reorientation dynamics of both water-DMSO and water-acetone mixtures show a clear distinction between the dynamics of water molecules donating hydrogen bonds to other water molecules and the dynamics of water donating a hydrogen bond to the S═O/C═O group of the solute. For water-DMSO mixtures both types of water molecules show a very slow reorientation. The water molecules forming hydrogen bonds to the S═O group reorient with a time constant that decreases from 46 ± 14 ps at XDMSO = 0.33 to 13 ± 2 ps at XDMSO = 0.95. The water molecules forming hydrogen bonds to the C═O group of acetone show a much faster reorientation with a time constant that decreases from 6.1 ± 0.2 ps at Xacet = 0.3 to 2.96 ± 0.05 ps at Xacet = 0.9. The large difference in reorientation time constant of the solute-bound water for DMSO and acetone can be explained from the fact that the hydrogen bond between water and the S═O group of DMSO is much stronger than the hydrogen bond between water and the C═O group of acetone. We attribute the strongly different behavior of water in DMSO-rich and acetone-rich mixtures to their difference in molecular shape.

We study the energy relaxation and structural relaxation dynamics of hydrated protons in Nafion membranes at different hydration levels using femtosecond infrared transient absorption spectroscopy. At low hydration levels we observe that the excitation of the proton vibration of an Eigen-like proton hydration structure leads to a structural relaxation process in which the Eigen-like structure evolves to a Zundel-like proton hydration structure. This reorganization leads to a transfer of the proton charge and closely follows the mechanism of infrared-induced adiabatic proton transfer that has been proposed by S. Hammes-Schiffer, J. T. Hynes, and others. At high hydration levels, the spectral dynamics are dominated by vibrational energy relaxation and subsequent cooling of the proton hydration structures and the surrounding water molecules. Using a kinetic analysis of the transient spectral data, we determine the rates of proton transfer, vibrational energy relaxation, and cooling as a function of hydration level. We find that infrared-induced proton transfer occurs at all hydration levels but becomes less observable at high hydration levels due to the increasingly dominant influence of the vibrational energy relaxation.

We study the properties of aqueous solution/air interfaces with time-resolved surface sum-frequency generation (SFG) spectroscopy. Using this technique, we monitor the change in SFG signal followin...

We study the hydrogen-bond and reorientation dynamics of weakly hydrogen-bonded water molecules by studying their spectral diffusion and anisotropy dynamics with polarization-resolved two-color femtosecond mid-infrared spectroscopy. We selectively excite weakly hydrogen-bonded water molecules by tuning a relatively narrow band excitation pulse far into the high-frequency wing of the O-D stretch vibration of HDO molecules in H2O water. We observe that the spectral diffusion and the anisotropy both show pronounced biexponential dynamics. On the basis of previous work, the fast component of the spectral dynamics with a time constant of ∼100 fs is assigned to rapid hydrogen-bond switching events. We observed that these switching events lead to a pronounced effect on the anisotropy of the excited O-D groups, which shows that the spectral relaxation is accompanied by a large change of the orientation of the O-D groups. The slow component of the spectral relaxation can be assigned to the collective structural reorganization of the hydrogen-bond network of liquid water. With increasing temperature, the spectral relaxation shows a similar acceleration as the average molecular reorientation, showing that these processes are intimately connected.

In this work, we present evidence for ion pair formation of cations with a high surface charge density (Na(+) and Ca(2+)) and phosphate groups of phospholipids. We used femto-second infrared pump-probe and dielectric spectroscopy to probe the dynamics of water molecules in solutions of phosphorylethanolamine and different types of cations. We find that sodium and calcium cooperatively retard the dynamics of water in solutions of phosphorylethanolamine, implying the formation of solvent separated ion pairs. This ion-specific interaction is absent for potassium, cesium and ammonium. We compare our results to dielectric spectroscopy experiments, which probes the rotation of all dipolar molecules and ions in solution. The rotation of the dipolar phosphorylethanolamine ion shows that long-lived ion-pairs are only formed with calcium and not with ammonium, cesium, potassium, and sodium. This finding implies that the association between calcium and the phosphate is strong with lifetimes exceeding 200 ps, while the interaction with sodium is relatively short-lived (∼20-100 ps).

We studied the effects of tetra-n-alkylammonium bromide (N(C(n)H(2n+1))(4)(+)Br(-)) salts on the dynamics of water using polarization-resolved femtosecond infrared spectroscopy. With this technique, we are capable of distinguishing the response of water solvating the hydrophobic cations from that of water solvating the bromide anion. We observe that both types of ions slow down the orientational dynamics of the water molecules in their solvation shells. However, the nature of this slowdown is different for both ions. For the hydrophobic cation, we find an increasing number of retarded water molecules, scaling with the alkyl chain length. Water in the bromide solvation shell experiences a partial decay of its orientation by a fast wobbling motion, after which the remaining anisotropy decays much slower. The dynamics of the wobbling motion are observed to be dependent on the nature of the cation. For Me(4)NBr, the slow reorientation time is not concentration-dependent, and no aggregation is observed. This is in contrast to the tetra-n-alkylammonium salts with longer alkyl chains, for which the slow reorientation time of bromide-bound water molecules increases dramatically with concentration, and clusters of cations and anions appear to be formed.

International audience; The efficiency of solution-processed colloidal quantum dot (QD) based solar cells is limited by poor charge transport in the active layer of the device, which originates from multiple trapping sites provided by QD surface defects. We apply a recently developed ultrafast electro-optical technique, pump-push photocurrent spectroscopy, to elucidate the charge trapping dynamics in PbS colloidal-QD photovoltaic devices at working conditions. We show that IR photoinduced absorption of QD in the 0.2-05 eV region is partly associated with immobile charges, which can be optically detrapped in our experiment. Using this absorption as a probe, we observe that the early trapping dynamics strongly depend on the nature of the ligands used for QD passivation, while it depends only slightly on the nature of the electron-accepting layer. We find that weakly bound states, with a photon-activation energy of 0.2 eV, are populated instantaneously upon photoexcitation. This indicates that the photogenerated states show an intrinsically bound-state character, arguably similar to charge-transfer states formation in organic photovoltaic materials. Sequential population of deeper traps (activation energy 03-05 eV) is observed on the similar to 0.1-10 ns time scales, indicating that most of carrier trapping occurs only after substantial charge relaxation/transport. The reported study disentangles fundamentally different contributions to charge trapping dynamics in the nanocrystal-based optoelectronic devices and can serve as a useful tool for QD solar cell development.

The structure and autoionization of water at the water–phospholipid interface are investigated by ab initio molecular dynamics and ab initio Monte Carlo simulations using local density approximation (LDA) and generalized gradient approximation (GGA) for the exchange–correlation energy functional. Depending on the lipid headgroup, strongly enhanced ionization is observed, leading to the dissociation of several water molecules into H+ and OH– per lipid. The results can shed light on the phenomena of the high proton conductivity along membranes that has been reported experimentally.

We performed sum-frequency generation (SFG) spectroscopic measurements on water in contact with supported thin metal and metal-oxide films. We employed an internal reflection configuration and varied the angles of incidence of the visible and infrared beams and measured the SFG signals using different polarization combinations. While SFG is a surface-specific vibrational spectroscopy, the shape of the SFG spectra can be fully accounted for by the bulk response of the materials through the frequency-dependent enhancement of the local incident infrared fields at the interface, i.e., Fresnel effects. We find that the dispersion of the refractive index of the bulk water phase leads to a strong enhancement of the electric field at the interface at specific infrared frequencies. These local, frequency-dependent fields act on the frequency-independent, nonresonant SFG response of the electrons at the surfaces of the metal or metal-oxide films. As a result, the measured SFG spectra closely follow this infrared fr...

We study the molecular rotation of the protic room-temperature ionic liquid ethylammonium nitrate with dielectric relaxation spectroscopy and femtosecond-infrared spectroscopy (fs-IR) of the ammonium N-H vibrations. The results suggest that the rotation of ethylammonium ion takes place via large angular jumps. Such nondiffusive reorientational dynamics is unique to strongly hydrogen-bonded liquids such as water and indicates that the intermolecular interaction is highly directional in this class of ionic liquids.

The potential-dependent water adsorption on gold surfaces in perchloric and sulfuric acid solutions has been studied by surface-enhanced infrared absorption spectroscopy (SEIRAS). It is found that ...

We studied the pathways of vibrational energy relaxation of the amide I (~1660 cm-1) and amide II (~1560 cm-1) vibrational modes of N-methylacetamide (NMA) in CCl4 solution using two-color femtosecond vibrational spectroscopy. We measured the transient spectral dynamics upon excitation of each of these amide modes. The results show that there is no energy transfer between the amide I (AI) and amide II (AII) modes. Instead we find that the vibrational energy is transferred on a picosecond time scale to a common combination tone of lower-frequency modes. By use of polarization-resolved femtosecond pump−probe measurements we also study the reorientation dynamics of the NMA molecules and the relative angle between the transition dipole moments of the AI and AII vibrations. The spectral dynamics at later times after the excitation (>40 ps) reveal the presence of a dissociation process of the NMA aggregates, trimers, and higher order structures into dimers and monomers. By measuring the dissociation kinetics at different temperatures, we determined the activation energy of this dissociation Ea = 35 ± 3 kJ mol-1.

We report vibrational lifetime measurements of the OH stretch vibration of interfacial water in contact with lipid monolayers, using time-resolved vibrational sum frequency (VSF) spectroscopy. The dynamics of water in contact with four different lipids are reported and are characterized by vibrational relaxation rates measured at 3200, 3300, 3400, and 3500 cm(-1). We observe that the water molecules with an OH frequency ranging from 3300 to 3500 cm(-1) all show vibrational relaxation with a time constant of T(1) = 180 ± 35 fs, similar to what is found for bulk water. Water molecules with OH groups near 3200 cm(-1) show distinctly faster relaxation dynamics, with T(1)80 fs. We successfully model the data by describing the interfacial water containing two distinct subensembles in which spectral diffusion is, respectively, rapid (3300-3500 cm(-1)) and absent (3200 cm(-1)). We discuss the potential biological implications of the presence of the strongly hydrogen-bonded, rapidly relaxing water molecules at 3200 cm(-1) that are decoupled from the bulk water system.

We study the rate and mechanism of deuteron transfer from the photoacids 1-naphtol-4-sulfonate (1-NPS) and 2-naphtol-3,7-disulphonate (2-NPS) to acetate base in aqueous (D2O) solution. The photoacids are activated by excitation with 100 fs laser pulses at 267 nm. The electronic absorption and stimulated emission spectra of the photoacid and the conjugate photobase and the vibrational absorption spectra of the hydrated deuteron and the acetate base are probed with broad-band delayed 100 fs pulses at visible and mid-infrared wavelengths, respectively. A significant fraction of the deuteron transfer events are observed to occur on a timescale of

Water is, of course, a fascinating and important substance. For such a simple molecule, its condensed phase properties are surprisingly complex. Here we might mention the many different solid phases, the higher density of the liquid as compared to ice Ih, and the density maximum (as a function of temperature) in the liquid phase. Moreover, for such a light molecule, many of the liquid-state properties are anomalous: the boiling point, freezing point, heat capacity, surface tension, and viscosity are all unusually high. Even so, it is perhaps surprising that we still do not fully understand the properties of the liquid state.1-3 From a theoretical point of view, this can probably be attributed to two features of liquid water: cooperative hydrogen bonding (H-bonding) and nuclear quantum effects. The former refers to the fact that the binding energy of two H-bonded molecules is modified by the presence of a third molecule.4-9 In terms of simulating the liquid, then, it follows that the potential energy cannot be written as a sum of twomolecule terms. This means that simple two-body simulation models cannot completely describe reality, and attempts to capture the effects of these many-body interactions with polarizable models are not fully satisfactory either.8,10 Nuclear quantum effects occur because the hydrogen nucleus is sufficiently light that classical mechanics for the nuclear motion is simply not adequate. Thus, classical mechanics cannot describe such important properties as spatial dispersion of the hydrogen positions, nuclear tunneling, zero-point energy, and quantization of nuclear motions. Much energy has recently been expended toward the simulation of liquid water using ab initio electronic-structure methods (which, in priniciple, will produce the correct Born-Oppenheimer potential surface, including the effects of many-body interactions),11-19 together with methods for quantum dynamics,19-25 but still more work needs to be done before we have a complete and accurate description.

We study the hydrogen bond dynamics of solutions of LiBr and NaBr in isotopically diluted water (2% HDO:D2O) with femtosecond spectral hole-burning spectroscopy. We study the frequency fluctuations of the O-H stretch vibrations of the HDO molecules and observe spectral dynamics with time constants of 0.8 +/- 0.1 ps and 4.3 +/- 0.3 ps. The slow process we assign to the hydrogen bond fluctuations of the O-H center dot center dot Br- hydrogen bonds of the hydration shell of the Br- anion. We find that the time scale of the hydrogen bond fluctuations of the hydration shell of Br- is independent of the nature of the cation and the concentration.

We study the mechanism of proton transfer (PT) between the photoacid (8-hydroxy-1,3,6-pyrenetrisulfonic acid (HPTS)) and the base acetate in aqueous solution using femtosecond vibrational spectroscopy. By probing the vibrational resonances of the photoacid, the accepting base, and the hydrated proton we find that intermolecular PT in this model system involves the transfer of the proton across several water molecules linking the donor-acceptor pair by hydrogen bonds (H-bonds). We find that at high base concentration the rate of PT is not determined by the mutual diffusion of acid and base but rather by the rate of Grotthuss-like conduction of the proton between molecules. This long-range PT requires an activated solvent configuration to facilitate the charge transfer.

Preparation of Cu/ZnO catalysts is discussed in which mechanochemical reactions are involved. These reactions include oxidation of copper and copper oxides during milling in the presence of oxygen, reduction under vacuum, and formation of carbonates in CO2. Oxidation and reduction are promoted by the presence of ZnO. Formation of Cu2O-like intermediates is suggested in oxidic mixtures, which are not observed when milling occurs without ZnO. This effect may be partly due to defect formation. The various resulting copper species reduce at different temperatures in H2 atmosphere and this gives an indication for the intimacy of the Cu−Zn interaction. When milling is carried out in the presence of CO2, mixed carbonates are formed. Intimate mixtures of Cu and Zn are thus obtained, exhibiting high surface areas after reduction that can be explained by evolved porosity. A linear relation between Cu0 surface area and activity for methanol synthesis at 2 bar is observed. Irreversible deactivation is explained by ov...

Femtosecond two-color mid-infrared pump−probe spectroscopy is used to study the vibrational relaxation and the hydrogen-bond dynamics of HDO dissolved in liquid H2O. By looking at the spectral dyna...

We have studied the equilibration dynamics of liquid water and alcohols following a local deposition of energy using time-resolved femtosecond mid-infrared pump−probe spectroscopy. The equilibration dynamics is monitored via the spectral response of the OH-stretch vibration. It is found that the equilibration leads to complicated changes of the absorption band of the OH-stretch vibration including a shift of the absorption band and a decrease of the absorption cross section. Interestingly, these spectral changes do not occur simultaneously, which indicates that they are associated with the equilibration dynamics of different low-frequency modes. For water, we find an equilibration time constant of 0.55 ± 0.05 ps. We observe that the equilibration time strongly increases going from water to alcohols such as methanol, ethanol, and propanol which means that water molecules can adapt much faster to a local deposition of energy than other hydrogen-bonding liquids.