Activation Energy of Ion Desorption at Ionic Liquid/Pt Electrode Interfaces: A Sum-Frequency Generation Vibrational Spectroscopic Study.

Electrolyte/electrode interfaces of room-temperature ionic liquids (RTILs) exhibit hysteretic responses to different applied potentials owing to the differences in the ion adsorption/desorption processes; the ion desorption requires excess potential, which reflects the activation energy of ion desorption. Thus far, the contributions of the ion adsorption energy and the activation barrier for ion desorption toward the ion-dependent excess potential have not been quantified. Herein, we report on our infrared-visible sum-frequency generation vibrational spectroscopy study of the hysteretic responses of the anion adsorption/desorption at Pt electrode interfaces using neat, binary, and diluted RTILs composed of 1-butyl-3-methylimidazolium cations ([C4mim]+) and bis(trifluoromethanesulfonyl)amide ([TFSA]-) and trifluoromethanesulfonate ([OTf]-) anions. Experimental results are compared to the theoretical calculations for the electric double layer model. The hysteretic response of the RTIL/Pt interface derives predominantly from the activation energy of anion desorption, which causes the negative excess potential required for anion desorption. A comparison of the anion adsorption/desorption behaviors of neat RTILs with those of binary and diluted RTILs reveals that the large activation energy of anion desorption at the neat RTIL/Pt interface originates largely from the activation barrier for restructuring ionic layering in the diffuse layer.

Electronic structure of n-cycloparaphenylenes directly observed by photoemission spectroscopy.

A series of n-cycloparaphenylenes ([n]CPP, n = 8, 9, and 12) were studied by ultraviolet photoemission, inverse photoemission, ultraviolet-visible absorption, and X-ray photoemission spectroscopy to detect their unique electronic structures. [n]CPP has a cyclic structure in which both ends of n-poly(p-phenylene)s (nP) are connected. The molecular size dependence of the HOMO-LUMO gap of [n]CPP was investigated by direct observation and was found to increase as the molecular size increased. This trend is opposite to that of typical π-conjugated systems. Highly strained molecular structures, especially of small [n]CPPs, significantly impact their electronic structure. Insights into the electronic structure of [n]CPP obtained here will aid the design of electronic functionality of non-planar π-conjugation systems.

Reply to the 'Comment on "Bi-layering at ionic liquid surfaces: a sum-frequency generation vibrational spectroscopy- and molecular dynamics simulation-based study"' by M. Deutsch, O. M. Magnussen, J. Haddad, D. Pontoni, B. M. Murphy and B. M. Ocko, Phys. Chem. Chem. Phys., 2021, DOI.

In our recent paper titled "Bi-layering at ionic liquid surfaces: a sum-frequency generation vibrational spectroscopy- and molecular dynamics simulation-based study" co-authored by T. Iwahashi, T. Ishiyama, Y. Sakai, A. Morita, D. Kim, and Y. Ouchi, Phys. Chem. Chem. Phys., 2020, 22, 12565 (hereafter referred to as IW), the sum-frequency (SF) spectra for a homologous series of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([Cnmim][TFSA] n = 4, 6, 8, 10, and 12) were reported. In particular, a clear decrease in the SF signals from the [TFSA]- anions with increasing chain length of the [Cnmim]+ cation (Fig. 5 of IW) was explained in terms of "head-to-head" bi-layer formation at the air/ionic liquid (IL) interface. A comment by M. Deutsch et al. (hereafter referred to as DE) questioned this report, claiming that our proposed structure is not consistent with a multilayered electron density (ED) profile obtained by X-ray reflectivity (XR).

Bi-layering at ionic liquid surfaces: a sum-frequency generation vibrational spectroscopy- and molecular dynamics simulation-based study.

Room-temperature ionic liquids (RTILs) are being increasingly employed as novel solvents in several fields, including chemical engineering, electrochemistry, and synthetic chemistry. To further increase their usage potential, a better understanding of the structure of their surface layer is essential. Bi-layering at the surfaces of RTILs consisting of 1-alkyl-3-methylimidazolium ([Cnmim]+; n = 4, 6, 8, 10, and 12) cations and bis(trifluoromethanesulfonyl)amide ([TFSA]-) anions was demonstrated via infrared-visible sum-frequency generation (IV-SFG) vibrational spectroscopy and molecular dynamics (MD) simulations. It was found that the sum-frequency (SF) signal from the [TFSA]- anions decreases as the alkyl chain length increases, whereas the SF signal from the r+ mode (the terminal CH3 group) of the [Cnmim]+ cations is almost the same regardless of chain length. MD simulations show the formation of a bi-layered structure consisting of the outermost first layer and a submerged second layer in a "head-to-head" molecular arrangement. The decrease in the SF signals of the normal modes of the [TFSA]- anions is caused by destructive and out-of-phase interference of vibrations of corresponding molecular moieties oriented toward each other in the first and second layers. In contrast, the r+ mode of [Cnmim]+ does not experience destructive interference because the peak position of the r+ mode differs marginally at the surface and in the bulk. Our conclusions are not limited to the system presented here. Similar bi-layered structures can be expected for the surfaces of conventional RTILs, which necessitates the consideration of bi-layering in the design and application.

Ionic Liquid-Based Electrolytes Containing Surface-Functionalized Inorganic Nanofibers for Quasisolid Lithium Batteries.

In the present study, surface amino-functionalized silica nanofibers (f-SiO2NFs, average diameter = 400 and 1000 nm) are used as one-dimensional (1-D) fillers of ionic liquid (IL)-based quasisolid electrolytes. On adding f-SiO2NFs to an IL (1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide, EMITFSA) containing lithium bis(trifluoromethanesulfonyl)-amide (LiTFSA), the well-dispersed 1-D nanofillers easily form a three-dimensional network structure in the IL, function as physical cross-linkers, and increase the viscosity of the composites, consequently providing a quasisolid state at a 3.5 wt % fraction of the NFs. Rheological measurements demonstrate that the prepared composites exhibit "gel-like" characteristics at 40-150 °C. All prepared composites show high ionic conductivities, on the order of 10-3 S cm-1, around room temperature. To investigate the additive effect of f-SiO2NFs in the composites, the lithium transference numbers are also evaluated. It is found that thinner NFs enhance the transference numbers of the composites. In addition, quasisolid lithium-ion cells containing the prepared composites demonstrate relatively high rate characteristics and good cycling performance at high temperature (125 °C).

Liquid/liquid interface layering of 1-butanol and [bmim]PF6 ionic liquid: a nonlinear vibrational spectroscopy and molecular dynamics simulation study.

IR-visible sum-frequency generation (IV-SFG) vibrational spectroscopy and a molecular dynamics (MD) simulation were used to study the local layering order at the interface of 1-butanol-d9 and 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6), a room-temperature ionic liquid (RTIL). The presence of a local non-polar layer at the interface of the two polar liquids was successfully demonstrated. In the SFG spectra of 1-butanol-d9, we observed significant reduction and enhancement in the strength of the CD3 symmetric stretching (r(+)) mode and the antisymmetric stretching (r(-)) mode peaks, respectively. The results can be well explained by the presence of an oppositely oriented quasi-bilayer structure of butanol molecules, where the bottom layer is strongly bound by hydrogen-bonding with the PF6(-) anion. MD simulations reveal that the hydrogen-bonding of butanol with the PF6(-) anion causes the preferential orientation of the butanols; the restriction on the rotational distribution of the terminal methyl group along their C3 axis enhances the r(-) mode. As for the [bmim](+) cations, the SFG spectra taken within the CH stretch region indicate that the butyl chain of [bmim](+) points away from the bulk RTIL phase to the butanol phase at the interface. Combining the SFG spectroscopy and MD simulation results, we propose an interfacial model structure of layering, in which the butyl chains of the butanol molecules form a non-polar interfacial layer with the butyl chains of the [bmim](+) cations at the interface.

Structures of ionic liquid-water mixtures investigated by IR and NMR spectroscopy.

Imidazolium-based ionic liquids having different anions 1-butyl-3-methylimidazolium ([BMIM]X: X = Cl(-), Br(-), I(-), and BF4(-)) and their aqueous mixtures were investigated by IR absorption and proton NMR spectroscopy. The IR spectra of these ionic liquids in the CHx stretching region differed substantially, especially for C-H bonds in the imidazolium ring, and the NMR chemical shifts of protons in the imidazolium ring also varied markedly for ILs having different anions. Upon the introduction of water to screen the electrostatic forces and separate the ions, both IR and NMR spectra of [BMIM]X (X = Cl(-), Br(-), I(-)) showed significant changes, while those of [BMIM]BF4 did not change appreciably. H-D isotopic exchange rates of C(2)-H in [BMIM]X-D2O mixtures exhibited an order: C(2)-HCl > C(2)-HBr > C(2)-HI, while the C(2)-H of [BMIM]BF4 was not deuterated at all. These experimental findings, supported by DFT calculations, lead to the microscopic bulk configurations in which the anions and the protons of the cations in the halide ionic liquids have specific, hydrogen-bond type of interaction, while the BF4(-) anion does not participate in the specific interaction, but interacts less specifically by positioning itself more above the ring plane of the imidazolium cation. This structural change dictated by the anion type will work as a key element to build the structure-property relationship of ionic liquids.

Characteristics of visible fluorescence from ionic liquids.

The observation of fluorescence in the visible spectral range in imidazolium-based ionic liquids, in which the peak of the fluorescence spectrum shifts with the change in the excitation wavelength by over 200 nm, was reported by Samanta and co-workers (Paul et al. J. Phys. Chem. B 2005, 109, 9148; Chem. Phys. Lett. 2005, 402, 375), and the aggregate structure in the bulk ionic liquid was suggested to explain this unique phenomenon. In this work, by employing 2D-scan fluorescence spectroscopy, we identified the long- and short-wavelength fluorescence components of the fluorescence spectrum of 1-butyl-3-methylimidazolium tetrafluoroborate ([C4MIM][BF4]), of which only the long-wavelength fluorescence component was found to be responsible for the reported fluorescence properties. The fluorescence intensity of the long-wavelength component decreased much faster upon dilution in aqueous mixtures than the short-wavelength component, supporting the conclusion that the long-wavelength fluorescence is from molecular aggregates in the bulk ionic liquid. Fluorescence correlation spectroscopy (FCS), which was used to accurately account for the number density of the long-wavelength fluorescent species in aqueous solutions of the ionic liquid, also suggested that the fluorescence came from aggregate structures of molecules in ionic liquids.

Surface nanocrystallization of an ionic liquid.

Surface crystallization at the vapor-liquid interface of the ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate) is observed in synchrotron x-ray diffraction studies. Sharp Bragg reflections emerge in grazing-angle x-ray diffraction patterns 37 °C above the bulk melting temperature, indicating the presence of a long-range ordered phase at the surface in coexistence with the bulk parent liquid. The unique structure of the vapor-liquid interface where butyl chains attached to the cations are expelled to the vapor side facilitates interionic electrostatic interactions that lead to the crystallization. Our results demonstrate the complexity of ionic-liquid structure with their tendency to spontaneously phase separate into nanodomains with finite correlation lengths in coexistence with the liquid phase. By virtue of interfacial boundary conditions, these nanodomains grow laterally to form quasi-two-dimensional crystals.

Nonlinear vibrational spectroscopic studies on waterlionic liquid([C(n)mim]TFSA: n = 4, 8) interfaces.

The interfaces of water/room temperature ionic liquids (RTIL) (1-alkyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide ([C(n)mim]TFSA): n = 4, 8) are investigated by infrared-visible sum frequency generation (IV-SFG) vibrational spectroscopy and molecular dynamics (MD) simulation. SFG spectra taken within the SO stretch region drastically differ between air/RTIL and water/RTIL interfaces. When a RTIL surface is in contact with water, a broadened and blue-shifted SO2-ss mode peak is observed in the SFG spectra, indicating an inhomogeneous intermolecular interaction due to hydrogen bonding of the [TFSA]- anions and water molecules at the water/[C(n)mim]TFSA interface. MD simulations show the SO2 groups of the anion are preferentially orientated toward the water phase, which is consistent with the SFG spectral features. Polar orientation of the [TFSA] anion originates from the ordered structure of the alkyl chains of [C(n)mim]+ cations.

Crystal structure, spin polarization, solid-state electrochemistry, and high n-type carrier mobility of a paramagnetic semiconductor: vanadyl tetrakis(thiadiazole)porphyrazine.

We report the synthesis, crystal structure, and magnetic, electrochemical, and carrier-transport properties of vanadyl tetrakis(thiadiazole)porphyrazine (abbreviated as VOTTDPz) with S = ½. X-ray crystal analysis reveals two polymorphs, the α and ß forms; the former consists of a 1D regular π stacking, while the latter forms a 2D π network. Molecular orbital calculations suggest a V(4+)(d(1)) ground state and a characteristic spin polarization on the whole molecular skeleton. The temperature dependence of the paramagnetic susceptibility of the α form clearly indicates a ferromagnetic interaction with a positive Weiss constant of θ = 2.4 K, which is well-explained by McConnell's type I mechanism. VOTTDPz forms amorphous thin films with a flat and smooth surface, and their cyclic voltammogram curves indicate a one-electron reduction process, which is highly electrochromic, because of a reduction of the porphyrazine π ring. Thin-film field-effect transistors of VOTTDPz with ionic-liquid gate dielectrics exhibit n-type performance, with a high mobility of µ = 2.8 × 10(-2) cm(2) V(-1) s(-1) and an on/off ratio of 10(4), even though the thin films are amorphous.

Alkyl-chain dividing layer at an alcohol/ionic liquid buried interface studied by sum-frequency generation vibrational spectroscopy.

We demonstrate for the first time the formation of a non-polar alkyl-chain dividing layer between a room-temperature ionic liquid (RTIL) and an n-alcohol. This newly described non-polar interfacial layer, which should be more hydrophobic than both RTIL and alcohol phases, might find applications in liquid/liquid reaction systems, or serve as a soft nano-functional space.

Electronic structure of delocalized singlet biradical Ph2-IDPL solid film.

The film structure and electronic structure of a biradical hydrocarbon, diphenyl derivative of s-indacenodiphenalene (Ph(2)-IDPL) solid film has been investigated. A small energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) compared with that of typical π-conjugated small molecules was observed even for the amorphous film of Ph(2)-IDPL. This result indicates that the small HOMO-LUMO gap is an important characteristic of the singlet biradical electronic structure and well explains the previously reported ambipolar field effects of amorphous Ph(2)-IDPL film by Chikamatsu et al., Appl. Phys. Lett. 2007, 91, 043506. It was found that the gas-deposition method substantially improved the crystallinity of the film where Ph(2)-IDPL molecules form quasi one-dimensional (1D) molecular chains normal to the substrate surface. An extremely small HOMO-LUMO gap was observed in the polycrystalline Ph(2)-IDPL film, which is possibly caused by strong intermolecular coupling. The photon energy dependence of ultraviolet photoemission spectra shows that the stacked Ph(2)-IDPL molecular chain in the polycrystalline film develops an energy band structure in the direction of the surface normal of the film. The intermolecular covalency therefore evolves into the quasi 1D energy band along the molecular stacking direction.

Highly ordered and stable layered "polymer nanosheets" constructed with amorphous side chains and pi-pi stacking of functional groups in ternary comb copolymers.

We have developed a highly stable, layered structure for ternary copolymers in Langmuir-Blodgett (LB) films at a nanometer scale, with substantial durability over the long-term. In these ternary copolymer LB films, amorphous side chains support the layered structure, and the distance between the layers is controlled at the nanometer scale by the composition of hydrogenated and fluorinated side chains. In the present study, the fine structures of newly synthesized ternary comb copolymers with a carbazole ring in the solid state and molecular orientations in the LB films were investigated using wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), surface pressure-area (pi-A) isotherms, in-plane and out-of-plane X-ray diffraction (XRD), and atomic force microscopy (AFM). The WAXD results identified two short-spacing peaks related to the formation of the subcells for both the fluorinated and hydrogenated side chains. Further, SAXS measurements indicated that these ternary copolymers formed a highly ordered layer structure. In addition, monolayers on the water surface of these ternary copolymers were highly condensed. From the results of in-plane XRD and AFM, it was determined that the side chains and side-chain crystals could not form phase-separated structures in two-dimensional films. These structural features may result from enhancement of pi-pi interactions between the arranged carbazole rings. The side chains of the copolymers in the two-dimensional films are apparently in a miscible state, and monolayers form a homogeneous amorphous surface because of cancellation of differences in van der Waals forces between the two types of side chains. As a result, formation of a highly ordered layer structure in copolymer films having substantial durability over the long term is realized because amorphous side chains support the layer structure in the LB multilayers. Further, control of long spacing at a subnanometer level becomes possible due to changes in the tilt angle of the side chains, depending on their fluorocarbon content.

Effect of annealing on the electronic structure of poly(3-hexylthiophene) thin film.

The electronic structure and film structure of poly(3-hexylthiophene) (P3HT) have been studied by X-ray diffraction (XRD) measurements, ultraviolet-visible (UV-vis) absorption spectroscopy, near-edge X-ray absorption fine structure (NEXAFS) measurements, ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). As reported in previous works, XRD results show that the crystallinity of the film with regioregular P3HT is significantly improved by annealing at 170 degrees C. The effects of annealing on the electronic structure strongly depend on the substrate and the degree of regioregularity of the P3HT polymer backbone. Even in the case of the regiorandom sample, annealing considerably changes the vacuum level energy, which is the result of changes in the conformation of the hexyl groups at the free surface of the film. The pi- and pi*-band onsets uniformly shift downward by the annealing resulting in an increased hole-injection barrier at the electrode interface. The effects of annealing on the electronic structure of regioregular samples are more complex and depend on multiple factors. It is necessary to take into account variations in the pi- and pi*-band widths and the polarization energy to determine the effects of annealing. The former is associated with the conformation of the backbones of the polymer chains, and the latter is associated with the packing density of the conjugated polymer planes. The combination of these variations determines the effects of annealing on the electronic structure of the regioregular film. This is a possible reason for the strong dependence of the effects of annealing on the surface roughness of the substrate, since substrate roughness has a considerable effect on the morphology and crystallinity of regioregular P3HT films.

Electronic structure of disjoint diradical 4,4'-bis(1,2,3,5-dithiadiazolyl) thin films.

The electronic structure of the thiazyl diradical, 4,4'-bis(1,2,3,5-dithiadiazolyl) (BDTDA), has been investigated by ultraviolet photoemission spectroscopy. Stacked BDTDA dimers showed an energy band dispersion of about 0.3 eV for the highest occupied molecular orbital in the direction of the surface normal of the BDTDA solid film. The pi-orbital overlap between the stacked dimers therefore evolves into a quasi one-dimensional energy band along the dimer stacking direction.

Interactive radical dimers in photoconductive organic thin films.

Fully interactive: Overlap between extended unoccupied molecular orbitals leads to the high photoconductivity of interactive radical dimers. Sandwich-type cells (see picture; ITO = indium tin oxide) comprising highly oriented thin films of a disjoint diradical, 4,4'-bis(1,2,3,5-dithiadiazolyl) (BDTDA) exhibit a photocurrent with a high on/off ratio at reverse bias voltages and photovoltaic behavior at zero bias voltage.

Control of arrangement for s-triazine group in comb copolymers by the Langmuir-Blodgett method and its structural estimation by NEXAFS spectroscopy.

We investigated the molecular orientation of organized molecular films with regard to solid-state structures for newly synthesized comb copolymers with 2-vinyl-4,6-diamino-1,3,5-triazine (VDAT) by surface pressure-area (pi-A) isotherms, in-plane and out-of-plane X-ray diffraction (XRD), atomic force microscopy (AFM), and polarized near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Since VDAT has adsorption ability to an adenine-thymine base pair of a DNA molecule, control of orientation for VDAT units in monolayers is possible to form surface patterning of biomolecules and construct candidates of new biochip materials. In the bulk state, hydrogenated and fluorinated comb copolymers containing VDAT form side-chain crystals for a two-dimensional lattice spacing of 4.2 and 5.0 A, respectively. From the results of the differential scanning calorimetric (DSC) measurements, sharp-shaped melting peaks appear on the relatively lower temperature side of the thermograms. This result supports the formation of side-chain crystals in the synthesized comb copolymers. These monolayers of copolymers on the water surface were extremely condensed, except for the VDAT:OA = 5:1 copolymer. From the in-plane XRD measurement of multilayers on solids, changes in the two-dimensional lattice structure of fluorinated comb copolymer films containing VDAT units, as opposed to their bulk state, were confirmed. It seems that these structural changes are caused by the stronger pi-pi interaction between the s-triazine rings rather than the van der Waals interaction between fluorocarbons. Polarized NEXAFS spectroscopy showed highly ordered orientation of s-triazine groups in the films based on the incident angle dependency of C and N1s-pi*(CN) transitions with synchrotron radiation. These experimental findings relate to well-ordered arrangement of functional groups supporting the side-chain rearrangement caused by the pi-pi interaction between the s-triazine rings.

Anion configuration at the air/liquid interface of ionic liquid [bmim]OTf studied by sum-frequency generation spectroscopy.

The air/liquid interface of a room temperature ionic liquid, 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim]OTf), is investigated using infrared-visible sum frequency generation (SFG) spectroscopy. The SFG spectra clearly show low-frequency modes [CF3-symmetric stretching (ss) mode and SO3-symmetric stretching (ss) mode] of the OTf anion, demonstrating the existence of anions polar oriented at the interface. The amplitude of the CF3-ss peak of the OTf anion has the opposite sign with respect to that of the SO3-ss peak, indicating that OTf anions at the surface have polar ordering where the nonpolar CF3 group points away from the bulk into the air, whereas the SO3 group points toward the bulk liquid. The line width of the SFG peak from the submerged SO3 group is appreciably narrower than that from IR absorption, suggesting the environment of the surface OTf anions is much more homogeneous than that of the bulk. The vibrational calculations also suggest that the anions and the cations form a more specific aggregated configuration at the surface as compared to the bulk.

First acid dissociation at an aqueous H2SO4 interface with sum frequency generation spectroscopy.

The vibrational sum frequency generation (SFG) spectra of the air-liquid interface of H2SO4-H2O solutions over a wide range of concentrations are measured in the SO stretching region (1000-1300 cm(-1)). The analogy of the concentration dependence of Raman and SFG is indicative of a nearly identical behavior of the first acid dissociation at the air-liquid interface as in the bulk.