Evaluating the contributions to conductivity in room temperature ionic liquids.

The conductivity of room temperature ionic liquids is not described adequately by the Nernst-Einstein equation, which accounts only for Brownian motion of the ions. We report on the conductivity of the ionic liquid 1-butyl-3-methylimidazolum bis(trifluoromethylsulfonyl) imide (BMIM TFSI), comparing the known conductivity of this RTIL to the diffusion constants of the cationic and anionic species over a range of length scales, using time-resolved fluorescence depolarization and fluorescence recovery after photobleaching (FRAP) measurements of chromophores in the RTIL. Our data demonstrate that the diffusional contribution to molar conductivity is ca. 50%. Another mechanism for the transmission of charged species in RTILs is responsible for the "excess" molar conductivity, and we consider possible contributions.

The effect of dilution on induced free charge density gradients in room temperature ionic liquids.

We report on changes in the magnitude and length scale of the induced free charge density gradient, ρf, in three imidazolium room temperature ionic liquids (RTILs) with dilution by methanol and acetonitrile. Using depth- and time-resolved fluorescence measurements of cresyl violet rotational diffusion, we find that ρf persists in RTILs to varying degrees depending on RTIL and diluent identity, and in all cases the functional form of ρf is not a smooth monotonic diminution in either magnitude or persistence length with increasing diluent, but a stepwise collapse. This finding is consistent with changes in the bulk RTIL as a function of dilution seen using rotational diffusion measurements that show the rotating entity in bulk RTILs exhibits a larger effective hydrodynamic volume than would be expected based on bulk viscosity data for the diluted RTILs. This excess hydrodynamic volume can be understood in the context of aggregation of RTIL ion pairs in the diluted RTIL system. The size of the aggregates is seen to depend on RTIL identity and diluent, and in all cases aggregate size increases with increasing dilution. This finding is consistent with the ρf dependence on dilution data. The collapse of ρf is seen to correlate with the onset of RTIL ion pair dimer formation, a condition that may facilitate dissociated RTIL ion mobility in the binary system.

Cation structure-dependence of the Pockels effect in aprotic ionic liquids.

We report on the dependence of surface charge-induced birefringence (the Pockels effect) in room temperature ionic liquids (RTILs) with different cation constituents. The induced birefringence is related to the induced free charge density gradient (ρf) in the RTIL. The RTILs are confined in a lens-shaped cell and the surface charge density of the concave cell surface is controlled by the current passed through the surface ITO film. We find that, in all cases, the induced birefringence is proportional to the surface charge density and that the change in refractive index nearest the ITO surface can be on the order of 20%. Our findings indicate that the induced birefringence depends more sensitively on the cation aliphatic substituent length than on the identity of the charge-carrying headgroup.

Cation structure-dependence of the induced free charge density gradient in imidazolium and pyrrolidinium ionic liquids.

We report on the structure-dependence and magnitude of the induced free charge density gradient (ρf) seen in room-temperature ionic liquids (RTILs) with imidazolium and pyrrolidinium cations. We characterize the spatially-resolved rotational diffusion dynamics of a trace-level cationic chromophore to characterize ρf in three different pyrrolidinium RTILs and two imidazolium RTILs. Our data show that the magnitude of ρf depends primarily on the alkyl chain length of RTIL cation and the persistence length of ρf is independent of RTILs' cation structure. These findings collectively suggest that mesoscopic structure in RTILs plays a significant role in allowing charge density gradients to form.

Translational Diffusion Dynamics in Divalent Metal-Phosphonate Monolayers.

Self-assembled monolayers are attractive for surface modification due to their ease of synthesis and the range of chemical functionality that can be applied. Metal-phosphonate monolayer properties can be controlled through the metal ions that can be used in their formation. The organization and fluid properties of these monolayers can be understood in the context of their thermodynamic properties and the association and dissociation kinetics that proceed at the metal-phosphonate complex. In this work, four different M(II)-phosphonate monolayers were synthesized and the diffusional behavior of free and tethered chromophores was evaluated using fluorescence recovery after photobleaching measurements. The ω-terminal group identity of the metal-phosphonate monolayer was varied to determine its effect on monolayer dynamics.

Controlling Quantum Interference between Virtual and Dipole Two-Photon Optical Excitation Pathways Using Phase-Shaped Laser Pulses.

Two-photon excitation (TPE) proceeds via a "virtual" pathway, which depends on the accessibility of one or more intermediate states, and, in the case of non-centrosymmetric molecules, an additional "dipole" pathway involving the off-resonance dipole-allowed one-photon transitions and the change in the permanent dipole moment between the initial and final states. Here, we control the quantum interference between these two optical excitation pathways by using phase-shaped femtosecond laser pulses. We find enhancements by a factor of up to 1.75 in the two-photon-excited fluorescence of the photobase FR0-SB in methanol after taking into account the longer pulse duration of the shaped laser pulses. Simulations taking into account the different responses of the virtual and dipole pathways to external fields and the effect of pulse shaping on two-photon transitions are found to be in good agreement with our experimental measurements. The observed quantum control of TPE in the condensed phase may lead to an enhanced signal at a lower intensity in two-photon microscopy, multiphoton-excited photoreagents, and novel spectroscopic techniques that are sensitive to the magnitude of the contributions from the virtual and dipole pathways to multiphoton excitations.

Steric effects in light-induced solvent proton abstraction.

The significance of solvent structural factors in the excited-state proton transfer (ESPT) reactions of Schiff bases with alcohols is reported here. We use the super photobase FR0-SB and a series of primary, secondary, and tertiary alcohol solvents to illustrate the steric issues associated with solvent to photobase proton transfer. Steady-state and time-resolved fluorescence data show that ESPT occurs readily for primary alcohols, with a probability proportional to the relative -OH concentration. For secondary alcohols, ESPT is greatly diminished, consistent with the barrier heights obtained using quantum chemistry calculations. ESPT is not observed in the tertiary alcohol. We explain ESPT using a model involving an intermediate hydrogen-bonded complex where the proton is "shared" by the Schiff base and the alcohol. The formation of this complex depends on the ability of the alcohol solvent to achieve spatial proximity to and alignment with the FR0-SB* imine lone pair stabilized by the solvent environment.

Isoenergetic two-photon excitation enhances solvent-to-solute excited-state proton transfer.

Two-photon excitation (TPE) is an attractive means for controlling chemistry in both space and time. Since isoenergetic one- and two-photon excitations (OPE and TPE) in non-centrosymmetric molecules are allowed to reach the same excited state, it is usually assumed that they produce similar excited-state reactivity. We compare the solvent-to-solute excited-state proton transfer of the super photobase FR0-SB following isoenergetic OPE and TPE. We find up to 62% increased reactivity following TPE compared to OPE. From steady-state spectroscopy, we rule out the involvement of different excited states and find that OPE and TPE spectra are identical in non-polar solvents but not in polar ones. We propose that differences in the matrix elements that contribute to the two-photon absorption cross sections lead to the observed enhanced isoenergetic reactivity, consistent with the predictions of our high-level coupled-cluster-based computational protocol. We find that polar solvent configurations favor greater dipole moment change between ground and excited states, which enters the probability for TPE as the absolute value squared. This, in turn, causes a difference in the Franck-Condon region reached via TPE compared to OPE. We conclude that a new method has been found for controlling chemical reactivity via the matrix elements that affect two-photon cross sections, which may be of great utility for spatial and temporal precision chemistry.

Effects of Cu(II) on the Formation and Orientation of an Arachidic Acid Langmuir-Blodgett Film.

The influence of the copper(II) ion on the formation, morphology, and organization of an arachidic acid monolayer was investigated using Langmuir-Blodgett (LB) monolayers, Π-A isotherms, and Brewster angle microscopy (BAM). Our findings indicate that a Cu2+-complexed LB film exhibits an order that depends on the subphase pH, analogous to other metal ions. Yazdanian , M. ; et al. Ionic Interactions of Fatty Acid Monolayers at the Air-Water Interface . Langmuir 1990 , 6 , 1093 - 1098 . Kurnaz , M. L. ; et al. Morphology of Microphase Separation in Arachidic Acid-Cadmium Arachidate Langmuir-Blodgett Multilayers . J. Phys. Chem. 1996 , 100 , 11113 - 11119 . The metal ion facilitates the formation of solid-phase films at surface pressures as low as 5 mN/m. The films exhibit a rigid, ordered phase, evidenced by the absence of a collapse point and an increase in surface pressure rather than the typical sharp decrease in surface pressure, indicative of film failure. Amphiphile ionic charge vs pH (i.e., the extent of arachidic acid protonation) plays a role in the observed absence of collapse and the ability of the films to maintain order and cohesion at high surface pressures (ca. 65 mN/m). Additionally, film thickness data suggest that the incorporation of Cu2+ ions induces a change in orientation of the aliphatic chains of the amphiphiles and that amphiphile solubility in the subphase may play a role in the observed film behavior at low surface areas and high pH.

Measuring Competing Equilibria at a Silica Surface through the Contact Angle of a Nonpolar Liquid.

Characterizing the composition of an oxide surface is critical to a range of applications in separations and reaction chemistry. For silica surfaces, among the key issues is the competition for occupancy of the siloxide (SiO-) sites by H+ and other cations. We studied the competition between H+ and Na+ for surface siloxide occupancy as a function of the concentrations of each cation using the contact angle of a droplet of CCl4 on a silica surface immersed in aqueous solution as the probe of surface energy. We find the dissociation constant for SiONa to be larger than the SiOH dissociation constant, and with knowledge of the surface energy for CCl4 in contact with H2O, we are able to determine the surface energies of the SiOH-water, SiONa-water, and CCl4-SiOX interfaces.

Impact of macronutrient composition and palatability in wet diets on food selection in cats.

Cats are obligate carnivores adapted to high-protein diets, but are commonly fed diets rich in carbohydrate. The aim of this study was to examine the food intake choices of cats when diets with different protein and carbohydrate contents were offered. Thirty-nine cats participated in voluntary dietary intake studies. Four foods were formulated to provide between 24% and 53% of metabolizable energy as protein, between 43% and 11% as carbohydrate and holding dietary fat constant with a contribution of approximately 36%. Foods were offered either singly to evaluate voluntary food intake or in pairs to compare food intake between pairs of diets. Cats regulated their macronutrient intake to attain an overall diet composition that provided 53% of metabolizable energy as protein, 11% as carbohydrate and 36% as fat. The protein contribution corresponded to approximately 6 g of protein/kg body weight/day. High-protein/low-carbohydrate diets were always eaten preferentially over low-protein/high-carbohydrate foods. When low-protein/high-carbohydrate diets were offered, cats limited their food intake to limit daily carbohydrate intake to less than 3 g of carbohydrate/kg body weight. This carbohydrate ceiling may limit protein and even energy intake when only low-protein/high-carbohydrate diets were offered. The inclusion of palatability enhancer in the diets increased food intake but did not change protein or carbohydrate intake patterns, indicating that macronutrient intake can be regulated regardless of the use of palatability enhancers in cats. We conclude that cats can discriminate between diets based on macronutrient composition and regulate their intake to maintain maximal protein intake but limit carbohydrate intake.

Interface-mediation of lipid bilayer organization and dynamics.

We report on the morphology and dynamics of planar supported lipid bilayer structures as a function of pH and ionic strength of the aqueous overlayer. Supported lipid bilayers composed of three components (phosphocholine, sphingomyelin and cholesterol) are known to exhibit phase segregation, with the characteristic domain sizes dependent on the amount and identity of each constituent, and the composition of the aqueous overlayer in contact with the bilayer. We report on fluorescence anisotropy decay imaging measurements of a rhodamine chromophore tethered to the headgroup of a phosphoethanolamine, where anisotropy decay images were acquired as a function of solution overlayer pH and ionic strength. The data reveal a two-component anisotropy decay under all conditions, with the faster time constant being largely independent of pH and ionic strength and the slower component depending on pH and ionic strength in different manners. For liposomes of the same composition, a single exponential anisotropy decay was seen. We interpret this difference in terms of bilayer curvature and support surface-bilayer interactions, and the pH and ionic strength dependencies in terms of ionic screening and protonation in the bilayer headgroup region.

Controlling S2 Population in Cyanine Dyes Using Shaped Femtosecond Pulses.

Fast population transfer from higher to lower excited states occurs via internal conversion (IC) and is the basis of Kasha's rule, which states that spontaneous emission takes place from the lowest excited state of the same multiplicity. Photonic control over IC is of interest because it would allow direct influence over intramolecular nonradiative decay processes occurring in condensed phase. Here we tracked the S2 and S1 fluorescence yield for different cyanine dyes in solution as a function of linear chirp. For the cyanine dyes with polar solvation response IR144 and meso-piperidine substituted IR806, increased S2 emission was observed when using transform limited pulses, whereas chirped pulses led to increased S1 emission. The nonpolar solvated cyanine IR806, on the other hand, did not show S2 emission. A theoretical model, based on a nonperturbative solution of the equation of motion for the density matrix, is offered to explain and simulate the anomalous chirp dependence. Our findings, which depend on pulse properties beyond peak intensity, offer a photonic method to control S2 population thereby opening the door for the exploration of photochemical processes initiated from higher excited states.

Detection and characterization of liquid|solid and liquid|liquid|solid interfacial gradients of water nanodroplets in wet N-octyl-2-pyrrolidone.

We report on the rotational diffusion dynamics and fluorescence lifetime of lissamine rhodamine B sulfonyl chloride (LRSC) in two thin-film experimental configurations. These are liquid|solid interfaces, where N-octyl-2-pyrrolidone (NOP) containing water and ethylene glycol (EG) thin films are each supported on glass, and a liquid|liquid|solid interface where thin films of water and NOP, both supported on glass, are in contact with one another, forming an NOP|water interface. The reorientation dynamics and fluorescence lifetime of LRSC are measured as a function of distance from the NOP|glass and EG|glass interfaces and from the NOP|water and NOP|glass interfaces in the liquid|liquid|solid experimental configuration. Fluorescence anisotropy decay data from the liquid|solid systems reveal a liquid film depth-dependent gradient spanning tens of micrometers from the NOP|glass interface into the wet NOP phase, while this gradient is absent in EG. We interpret these findings in the context of a compositional gradient in the NOP phase. The spatially resolved fluorescence lifetime and anisotropy decay data for an NOP|water|glass interfacial structure exhibits the absence of a gradient in the anisotropy decay profile normal to the NOP|water interface and the presence of a fluorescence lifetime gradient as a function of distance from the NOP|water interface. The compositional heterogeneity for both interfacial systems is in the form of water nanodroplets in the NOP phase. We understand this compositional gradient in the context of the relative surface energies of the water, NOP, and glass components.

Structure-Dependence and Mechanistic Insights into the Piezoelectric Effect in Ionic Liquids.

We reported recently that two imidazolium room-temperature ionic liquids (RTILs) exhibit the direct piezoelectric effect (J. Phys. Chem. Lett., 2023, 14, 2731-2735). We have subsequently investigated several other RTILs with pyrrolidinium and imidazolium cations and tetrafluoroborate and bis(trifluoromethylsulfonyl)imide anions in an effort to gain insight into the generality and mechanism of the effect. All the RTILs studied exhibit the direct piezoelectric effect, with a magnitude (d33) and threshold force that depend on the structures of both the cation and anion. The structure-dependence and existence of a threshold force for the piezoelectric effect are consistent with a pressure-induced liquid-to-crystalline solid phase transition in the RTILs, and this is consistent with experimental X-ray diffraction data.

Interactions of doxorubicin with organized interfacial assemblies. 1. Electrochemical characterization.

Doxorubicin is an anthracycline that has found wide use as a chemotherapeutic agent, with the primary target of its action being nuclear DNA. Despite the large body of knowledge on this family of compounds, the mechanism of doxorubicin penetration through the cellular or nuclear membrane remains understood to a limited extent. The plasma membrane acts as a barrier to the permeation of polar molecules, and this effect is mainly due to the hydrophobicity of membrane interior. The partitioning of DOX molecules into the lipid bilayer must thus be the basis for its passive transport across the biological membrane and therefore a key area of research activity lies in understanding how the structure of the anthracycline influences its interactions with amphiphilic interfaces. We have studied interactions between doxorubicin and Langmuir/Langmuir-Blodgett monomolecular films of octadecylamine (C18NH2), dihexadecylphosphate (DHP) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and DMPC bilayer films (Langmuir-Schaeffer) on a polycrystalline gold surface using ellipsometry, cyclic voltammetry, electrochemical impedance spectroscopy, and quartz crystal microbalance measurements. For all biomimetic films there is a substantial interaction between doxorubicin and the interface, and the extent of this interaction depends on the hydrophobic/hydrophilic properties of the film formed and its organization.

Interactions of doxorubicin with organized interfacial assemblies. 2. Spectroscopic characterization.

Doxorubicin is an anthracycline that has found wide use as a chemotherapeutic agent, with the primary limitation to its use being cardiotoxicity. Depending on the identity and location of pendent side groups, the anthracyclines exhibit varying degrees of chemotherapeutic activity and toxicity, and a key area of research activity lies in understanding how the structure of the anthracycline influences its interactions with amphiphilic interfaces. We have studied interactions between doxorubicin and interfacial adlayers of octadecylamine (C18NH2), dihexadecylphosphate (DHP), and both monolayers and bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) on mica using time- and frequency-resolved spectroscopic measurements. We report surface-enhanced resonance Raman data and fluorescence lifetime and anisotropy imaging data for doxorubicin at these interfaces. For all monolayers, there is a substantial interaction between doxorubicin and the interface. For DMPC bilayers, the extent of the interaction between doxorubicin and the interface depends on how the interface was formed.

Trophic shift in young-of-the-year Mugilidae during salt-marsh colonization.

This study investigated the trophic shift of young-of-the-year (YOY) thinlip grey mullet Liza ramada and golden grey mullet Liza aurata during their recruitment in a salt marsh located on the European Atlantic Ocean coast. Stable-isotope signatures (δ(13) C and δ(15) N) of the fishes followed a pattern, having enrichments in (13) C and (15) N with increasing fork length (LF ): δ(13) C in fishes < 30 mm ranged from -19.5 to -15.0‰, whereas in fishes > 30 mm δ(13) C ranged from -15.8 to -12.7‰, closer to the level in salt-marsh food resources. Large differences between the δ(15) N values of mugilids and those of food sources (6·0‰ on average) showed that YOY are secondary consumers, similar to older individuals, when feeding in the salt marsh. YOY mugilids shift from browsing on pelagic prey to grazing on benthic resources from the salt marsh before reaching 30 mm LF. The results highlight the role of European salt marshes as nurseries for juvenile mugilids.