ATR-far-ultraviolet spectroscopy: a challenge to new σ chemistry.

This review reports the recent progress on ATR-far ultraviolet (FUV) spectroscopy in the condensed phase. ATR-FUV spectroscopy for liquids and solids enables one to explore various topics in physical chemistry, analytical chemistry, nanoscience and technology, materials science, electrochemistry, and organic chemistry. In this review, we put particular emphasis on the three major topics: (1) studies on electronic transitions and structures of various molecules, which one cannot investigate via ordinary UV spectroscopy. The combined use of ATR-FUV spectroscopy and quantum chemical calculations allows for the investigation of various electronic transitions, including σ, n-Rydberg transitions. ATR-FUV spectroscopy may open a new avenue for σ-chemistry. (2) ATR-FUV spectroscopy enables one to measure the first electronic transition of water at approximately 160 nm without peak saturation. Using this band, one can study the electronic structure of water, aqueous solutions, and adsorbed water. (3) ATR-FUV spectroscopy has its own advantages of the ATR method as a surface analysis method. ATR-FUV spectroscopy is a powerful technique for exploring a variety of top surface phenomena (∼50 nm) in adsorbed water, polymers, graphene, organic materials, ionic liquids, and so on. This review briefly describes the principles, characteristics, and instrumentation of ATR-FUV spectroscopy. Next, a detailed description about quantum chemical calculation methods for FUV and UV regions is given. The recent application of ATR-FUV-UV spectroscopy studies on electronic transitions from σ orbitals in various saturated molecules is introduced first, followed by a discussion on the applications of ATR-FUV spectroscopy to studies on water, aqueous solutions, and adsorbed water. Applications of ATR-FUV spectroscopy in the analysis of other materials such as polymers, ionic liquids, inorganic semiconductors, graphene, and carbon nanocomposites are elucidated. In addition, ATR-FUV-UV-vis spectroscopy focusing on electrochemical interfaces is outlined. Finally, FUV-UV-surface plasmon resonance studies are discussed.

Voltammetric and In Situ Spectroscopic Investigations on the Redox Processes of Trioxotriangulene Neutral Radicals on Graphite Electrodes.

To investigate the microscopic electrochemical dynamics of a stable trioxotriangulene (TOT) organic neutral π-radical on a graphite electrode surface, voltammetric and in situ infrared (IR) spectroelectrochemical studies were conducted using electrolyte solutions containing TOT monoanions. Upright columnar crystals (face-on alignment) of the TOT neutral radical were preferentially formed and dissolved in a rather reversible manner in the electrolyte with a low concentration of TOT monoanion under electrochemical conditions; however, more flat-lying columnar crystals (edge-on alignment) were formed in a higher concentration electrolyte. The flat-lying crystals remained on the graphite surface even at a fully reduced potential, owing to the lack of direct π-π interactions between the molecules and the graphite electrode. In situ IR attenuated total reflectance spectroscopy analyses successfully characterized the alignment of the columnar crystals of the TOT neutral radicals and their electrochemical behaviors, including the possible origins of the irreversible redox reaction of TOT on the graphite electrode.

Spectroscopic analysis focusing on ionic liquid/metal electrode and organic semiconductor interfaces in an electrochemical environment.

The solid-liquid interface forms an electric double layer that enables the function of electronic devices and, thus, represents an important area of electrochemical research. Because ionic liquids (ILs) are becoming prominent candidates for new high-performing electrolytes, their interface with solid substrates (e.g., metal electrodes or organic semiconductors) attracts substantial attention. An example of improvement achieved using ILs as electrolytes is a decrease in the operating voltage of transistors from >10 V in traditional SiO2-gated transistors to <1 V in IL-gated electronic double-layer organic field-effect devices. This perspective discusses the investigation of poorly accessible IL/substrate interfaces using both attenuated total reflectance ultraviolet (ATR-UV) spectroscopy and a newly developed electrochemical setup combined with ATR-UV (EC-ATR-UV), which allows analysis of the interfacial area under the application of varying electric potential. The recent EC-ATR-UV applications in interfacial analytical chemistry are overviewed and compared to other spectroscopic methods described in the recent literature. Lastly, the supplementation of experimental data with theoretical calculations (e.g., quantum chemical calculations and molecular dynamics simulations) is also addressed.

Advances, challenges and perspectives of quantum chemical approaches in molecular spectroscopy of the condensed phase.

The purpose of this review is to demonstrate advances, challenges and perspectives of quantum chemical approaches in molecular spectroscopy of the condensed phase. Molecular spectroscopy, particularly vibrational spectroscopy and electronic spectroscopy, has been used extensively for a wide range of areas of chemical sciences and materials science as well as nano- and biosciences because it provides valuable information about structure, functions, and reactions of molecules. In the meantime, quantum chemical approaches play crucial roles in the spectral analysis. They also yield important knowledge about molecular and electronic structures as well as electronic transitions. The combination of spectroscopic approaches and quantum chemical calculations is a powerful tool for science, in general. Thus, our article, which treats various spectroscopy and quantum chemical approaches, should have strong implications in the wider scientific community. This review covers a wide area of molecular spectroscopy from far-ultraviolet (FUV, 120-200 nm) to far-infrared (FIR, 400-10 cm-1)/terahertz and Raman spectroscopy. As quantum chemical approaches, we introduce several anharmonic approaches such as vibrational self-consistent field (VSCF) and the combination of periodic harmonic calculations with anharmonic corrections based on finite models, grid-based techniques like the Numerov approach, the Cartesian coordinate tensor transfer (CCT) method, Symmetry-Adapted Cluster Configuration-Interaction (SAC-CI), and the ZINDO (Semi-empirical calculations at Zerner's Intermediate Neglect of Differential Overlap). One can use anharmonic approaches and grid-based approaches for both infrared (IR) and near-infrared (NIR) spectroscopy, while CCT methods are employed for Raman, Raman optical activity (ROA), FIR/terahertz and low-frequency Raman spectroscopy. Therefore, this review overviews cross relations between molecular spectroscopy and quantum chemical approaches, and provides various kinds of close-reality advanced spectral simulation for condensed phases.

Plasmonic Biosensing with Aluminum Thin Films under the Kretschmann Configuration.

Aluminum has recently attracted considerable interest as a plasmonic material due to its unique optical properties, but most work has been limited to nanostructures. We report here SPR biosensing with aluminum thin-films using the standard Kretschmann configuration that has previously been dominated by gold films. Electron-beam physical vapor deposition (EBPVD)-prepared Al films oxidize in air to form a nanofilm of Al2O3, yielding robust stability for sensing applications in buffered solutions. FDTD simulations revealed a sharp plasmonic dip in the visible range that enables measurement of both angular shift and reflection intensity change at a fixed angle. Bulk and surface tests indicated that Al films exhibited superb sensitivity performance in both categories. Compared to Au, the Al/Al2O3 layer showed a marked effect of suppressing nonspecific binding from proteins in human serum. Further characterization indicated that Al film demonstrated a higher sensitivity and a wider working range than Au films when used for SPR imaging analysis. Combined with its economic and manufacturing benefits, the Al thin-film has the potential to become a highly advantageous plasmonic substrate to meet a wide range of biosensing needs in SPR configurations.

Attenuated total reflectance far-ultraviolet and deep-ultraviolet spectroscopy analysis of the electronic structure of a dicyanamide-based ionic liquid with Li.

The electronic states of N-butyl-N-methylpyrrolidinium dicyanamide ([BMP][DCA]), a solvated ionic liquid, around Li+ were investigated using attenuated total reflectance far-ultraviolet and deep-ultraviolet (ATR-FUV-DUV) spectroscopy. The absorption bands ascribed to the [DCA]- were blue-shifted as the Li+ concentration increased, and the origin of the shift was explained by the energetic destabilization of the final (excited) molecular orbital using time-dependent density functional theory (TD-DFT) calculations. Using the multivariate curve resolution-alternating least squares (MCR-ALS) algorithm, the obtained spectra were decomposed into two types of [DCA]- at electronic state level, which were categorised as pure [BMP][DCA] and [DCA]- affected by Li+. Our results revealed that the number of [DCA]- with electronic states affected by a Li+, which was termed the electronic coordination number, was ∼5. This value was different from the coordination number within the first solvation layer, which was ∼4. Combining the TD-DFT with molecular dynamics simulations, we demonstrated that one [DCA]- outside the first solvation layer had a different electronic state from that of pure [BMP][DCA]. This is the first successful study that combines ATR-FUV-DUV spectroscopy with MCR-ALS calculations to build a solvation model that describes the electronic states.

Correlation between mobility and the hydrogen bonding network of water at an electrified-graphite electrode using molecular dynamics simulation.

Focusing on the electric double layer formed at aqueous solution/graphite electrode interfaces, we investigated the relationship between the mobility of interfacial water and its hydrogen bonding networks by using molecular dynamics simulations. We focused on the mobility of the first hydration layer constructed nearest to the electrode. The mobility was determined by calculating the diffusion coefficient which showed an opposite trend to that of the applied potential polarity. The mobility decreased upon positive potentials while showing an increase upon negative potentials, which is rationalized by the strength of the interfacial hydrogen bonding networks.

Potential Dependence of Electronic Transition Spectra of Interfacial Ionic Liquids Studied by Newly Developed Electrochemical Attenuated Total Reflectance Spectroscopy.

Recently, ionic liquids at the electrode/ionic liquid interface have been intensively studied because they are promising as novel alternatives to traditional electrolyte solutions that are both safe and functional. In this study, we constructed an attenuated total-reflectance spectroscopic system that operates under electrochemical conditions in order to investigate the electronic states of ionic liquids near the electrode surface. Upon application of voltage to an ionic liquid consisting of imidazolium cations and iodide anions, electronic transition spectra in the 150-450 nm range varied. In particular, absorbance due to charge transfer from the anion to the cation drastically increased at positive potentials. The extent of spectral change and contact area between the electrode and the ionic liquid were positively correlated, and thus spectral variations reflected the behavior of the interfacial ionic liquid on the electrode. In addition to potential dependence, time dependence and hysteresis were also investigated. The newly developed system can be applied not only for ionic liquids but foreseeably also for various electrochemical materials such as organic semiconductors.

Far- and Deep-Ultraviolet Surface Plasmon Resonance Sensor.

Plasmonics in the UV region has been widely focused because of the higher energy and the abundant electronic resonances compared to the conventional visible plasmonics. Recently, we have investigated the surface plasmon resonance (SPR) properties of the Al film, aiming for the application as refractive index sensors. Utilizing the UV lights, we expect three advantages: high sensitivity, material selectivity, and surface selectivity. By using an original attenuated total reflectance spectroscopic instrument, Al-SPR angle and wavelength were investigated with changing environments on the Al film. Al film thickness and materials of prisms on which Al was evaporated were also important factors for the SPR properties. By optimizing the conditions, the Al film worked as a sensor both in air and in liquids. In addition, our established system expands the plasmonics into an even higher energy region than 200 nm, while the UV-plasmonics have been studied in the wavelength region longer than 200 nm.

Systematic analysis of various ionic liquids by attenuated total reflectance spectroscopy (145-450 nm) and quantum chemical calculations.

Despite providing rich information on electronic states, the far-ultraviolet (FUV, <200 nm) and deep-ultraviolet (DUV, <300 nm) absorption spectra of ionic liquids (ILs) are difficult to obtain without saturation due to very strong analyte absorbance. Herein, FUV-DUV spectra of selected ILs were systematically and easily recorded using an attenuated total reflectance spectrometer and rationalized based on quantum chemical calculations. ILs containing pyrrolidinium or ammonium cations and fluorine-containing anions exhibited weak absorbance below 200 nm that could not be measured by conventional UV-Vis spectroscopy, whereas the corresponding imidazolium-based ILs showed distinct absorption bands that could be reproduced by single-cation-model calculations. On the other hand, imidazolium-based ILs with halide anions showed characteristic charge transfer (CT)-related absorbances. Thus, the above spectroscopic investigations contribute to a fundamental understanding of the electronic processes (e.g., intramolecular excitations and CT transitions) and molecular designs used in electrochemical devices.

Rydberg transitions as a probe for structural changes and phase transition at polymer surfaces: an ATR-FUV-DUV and quantum chemical study of poly(3-hydroxybutyrate) and its nanocomposite with graphene.

We investigated the surface (<50 nm) of poly(3-hydroxybutyrate) (PHB) and its nanocomposite with graphene by attenuated total reflection far- and deep-ultraviolet (ATR-FUV-DUV; 145-300 nm; 8.55-4.13 eV) spectroscopy and quantum mechanical calculations. The major absorption of polymers occurs in FUV and is related to Rydberg transitions. ATR-FUV-DUV spectroscopy allows for direct measurements of these transitions in the solid phase. Using ATR-FUV-DUV spectroscopy, periodic density functional theory (DFT) and time-dependent DFT (TD-DFT), we explained the origins of the FUV-DUV absorption of PHB and provided insights into structural changes of PHB which occur upon formation of a graphene nanocomposite and upon heating of the pure polymer. The structural changes cause specific and gradual spectral variations in FUV-DUV. We systematically studied the relaxation of the polymer helix and concluded that the common feature of all models of the unfolded helix lies in a specific and consistent FUV-DUV spectral signature. Relaxed structures feature a blue-shift of the major FUV transition (non-bonding molecular orbital to Rydberg 3p and π to π*) as compared with crystalline PHB. The FUV absorption of the relaxed structures was determined to be significantly stronger than that of the crystalline state. These results are consistent with the observed temperature-dependent spectra of the pure PHB. The simulation of the thermal expansion of the crystalline polymer by a periodic-DFT study allows us to exclude the possibility that spectral variations observed experimentally are influenced by changes in the crystalline phase. We concluded that the crystallinity of PHB at the sample surface increases with an increase in graphene content in the nanocomposite. However, it is unlikely that the polymer structure inside the crystal is affected; instead the FUV-DUV spectral variations result from changes in the polymer morphology that occur at the sample surface. The phase transition of PHB is affected by temperature and addition of graphene content. These changes are likely to be the opposite of those occurring in the bulk sample.

Direct optical measurements of far- and deep-ultraviolet surface plasmon resonance with different refractive indices.

The surface plasmon resonance (SPR) of Al thin films was investigated by varying the refractive index of the environment near the films in the far-ultraviolet (FUV, 120-200 nm) and deep-ultraviolet (DUV, 200-300 nm) regions. An original FUV-DUV spectrometer that adopts an attenuated total reflectance (ATR) system was used. The measurable wavelength range was down to the 180 nm, and the environment near the Al surface could be controlled. The resultant spectra enabled the dispersion relationship of Al-SPR in the FUV and DUV regions to be obtained. In the presence of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) on the Al film, the angle and wavelength of the SPR became larger and longer, respectively, compared to those in air. These shifts correspond well with the results of simulations performed using Fresnel equations.

Far- and Deep-UV Spectroscopy of Semiconductor Nanoparticles Measured Based on Attenuated Total Reflectance spectroscopy.

Far- and deep-ultraviolet spectra (150-300 nm) of semiconductor nanoparticles (zinc oxide and zinc sulfide) are successfully measured by using attenuated total reflectance (ATR) spectroscopy, and analyzed using finite-difference time-domain (FDTD) simulations. The obtained spectra show good consistency with earlier synchrotron-radiation spectra and with theoretical calculations. The FDTD simulation results show that the present system collected the correct spectra. In the present system, the obtained spectra are affected by the real part n of the complex refractive index more strongly than the imaginary part k. It is also revealed both experimentally and theoretically that spectral intensities of the semiconductor nanoparticles are approximately one tenth those of liquid samples. These results provide insights into the far- and deep-ultraviolet spectroscopy based on the ATR system, and show the general applicability of our original ATR spectroscopy to semiconductor nanoparticles. The system needs neither high vacuum nor much space, and enables rapid and systematic investigation of the electronic states of various materials.

Far-ultraviolet spectroscopy of solid and liquid states: characteristics, instrumentation, and applications.

Recently, far-ultraviolet (FUV) spectroscopy, which is the spectroscopy of wavelengths in the region 140-200 nm, of solid and liquid states has received significant attention as a novel spectroscopic method. FUV spectroscopy provides new possibilities for studying electronic structures and transitions in almost all types of molecules, from water to polymers. It also shows great promise for a variety of applications. It is well known that wavelengths below 200 nm are rich in information regarding the electronic structure and states of molecules. However, absorptivity is so high in the FUV region, that it has not been employed to investigate solids and liquids. Another problem for FUV region analysis was the instrumentation: FUV spectrometers required vacuum evacuation. Moreover, it was difficult to find applications for FUV spectroscopy. Recently, we introduced the attenuated total reflection (ATR) technique to FUV spectroscopy, which overcomes these issues. ATR-FUV spectroscopy enables the measurement of FUV spectra for solid and liquid samples, establishing a new spectroscopic research area. Using ATR-FUV spectroscopy, electronic transitions that cannot be observed with ordinary UV spectroscopy (200-380 nm) are accessible; Rydberg transitions are just one example. FUV spectroscopy has been demonstrated to have unique and versatile applications. A variety of extensive application studies are now in progress, ranging from applications to fundamental science, such as studies of hydrogen bonding, hydration, and adsorption of water and aqueous solutions, to practical applications such as online, geochemical, environmental, and polymer film analyses. This review provides an introduction to, and brief history of, FUV spectroscopy, and describes the development of new FUV spectrometers, studies on electronic structure and transitions, its various applications, and future prospects.

Electronic absorption spectra of imidazolium-based ionic liquids studied by far-ultraviolet spectroscopy and quantum chemical calculations.

Electronic absorption spectra of imidazolium-based ionic liquids were studied by far- and deep-ultraviolet spectroscopy and quantum chemical calculations. The absorption spectra in the 145-300 nm region of imidazolium-based ionic liquids, [Cnmim](+)[BF4](-) (n = 2, 4, 8) and [C4mim](+)[PF6](-), were recorded using our original attenuated total reflectance (ATR) system spectrometer. The obtained spectra had two definitive peaks at ∼160 and ∼210 nm. Depending on the number of carbon atoms in the alkyl side chain, the peak wavelength around 160 nm changed, while that around 210 nm remained at almost the same wavelength. Quantum chemical calculation results based on the time-dependent density functional theory (TD-DFT) also showed the corresponding peak shifts. In contrast, there was almost no significant difference between [C4mim](+)[BF4](-) and [C4mim](+)[PF6](-), which corresponded with our calculations. Therefore, it can be concluded that the absorption spectra in the 145-300 nm region are mainly determined by the cations when fluorine-containing anions are adopted. In addition, upon addition of organic solvent (acetonitrile) to [C4mim](+)[BF4](-), small peak shifts to the longer wavelength were revealed for both peaks at ∼160 and ∼210 nm. The peak shift in the deep-ultraviolet region (≤200 nm) in the presence of the solvent, which indicates the change of electronic states of the ionic liquid, was experimentally observed for the first time by using the ATR spectrometer.

Reply to the comment on "Sensitive marker bands for the detection of spin states of heme in surface-enhanced resonance Raman scattering spectra of metmyoglobin".

In our SERRS spectra of metmyoglobin by excitation at 514 nm, the peak at 1510 cm(-1), which is assigned to the 6-coordinated heme in the low spin state, was observed by the addition of imidazole and NaN3. Thus, the SERRS likely originates not from the non-native 5-coordinated heme, which is in the high spin state.

Sensitive marker bands for the detection of spin states of heme in surface-enhanced resonance Raman scattering spectra of metmyoglobin.

Surface-enhanced resonance Raman scattering (SERRS) spectra of myoglobin (Mb) with various ligands were measured. In the resonance Raman scattering (RRS) spectra, peaks at around 1610 and 1640 cm(-1) have so far been used to discriminate between the heme iron in a high or low spin state. In the SERRS spectra, however, the spin state cannot be distinguished by the corresponding peaks. Alternatively, the intensity ratio of the SERRS peak at 1560 cm(-1) to that at 1620 cm(-1) was applied to detect the spin states sensitively (1.5 × 10(5) times compared with the RRS); namely, a high ratio was obtained from met-Mb in the high spin state at pH ≤ 7 except for in a strong acid solution. The different marker bands between the SERRS and RRS spectra may be due to the enhancement order from the surface selection rule.

Significant enhancement of photocatalytic activity of rutile TiO2 compared with anatase TiO2 upon Pt nanoparticle deposition studied by far-ultraviolet spectroscopy.

Absorption spectra of anatase and rutile TiO2 in the 150-300 nm region before and after the deposition of Pt nanoparticles were measured. For anatase TiO2, the spectral intensity in the longer wavelength region decreased (>∼210 nm), while that in the shorter wavelength region increased (<∼210 nm). In particular, spectral band intensity in the far-ultraviolet (FUV) region (∼160 nm) was increased. In contrast, the spectral intensity of rutile TiO2 increased over the entire wavelength region under investigation. Rutile TiO2 showed a spectral band at a longer wavelength region (∼170 nm) than anatase TiO2, and the difference in the band wavelengths in the FUV region was due to the differences in the electronic structures of their phase. The decrease and increase in the intensity upon the Pt nanoparticle deposition suggest electron transfer from the TiO2 to Pt nanoparticles and enhancement of charge-separation, respectively. The photocatalytic activity of rutile TiO2, as evaluated by a photo-degradation reaction of methylene blue, increased more than that of anatase TiO2 upon the deposition of Pt nanoparticles. Thus, we concluded that the charge-separation efficiency of rutile TiO2 is enhanced relative to that of anatase TiO2 upon the deposition of Pt nanoparticles.

Electronic interaction between dimethyl carbonate and Li+ studied by attenuated total reflectance far-ultraviolet spectroscopy.

Organic electrolytes with Li+ were analyzed by far-ultraviolet (≤200 nm) spectroscopy, achieved by an attenuated total reflectance setup. The spectra showed a redshift with Li+ addition, attributed to the charge transfer, as revealed by quantum chemical calculations. Multivariate analysis successfully decomposed the spectra into pure solvent and Li-coordinated solvent components.

Plasmonic manipulation of color and morphology of single silver nanospheres.

Optical control of size, shape, or orientation of a metal nanoparticle is important for development of nanoscale optical devices and elements of photonic circuits. Thus far, however, independent control of two or more parameters has not yet been achieved. Here we place a simple spherical Ag nanoparticle on TiO(2) with high refractive index and separate a plasmon mode localized at the Ag-TiO(2) interface from the other mode distributed over the nanoparticle. Selective excitation of each mode gives rise to a corresponding morphological change and selective suppression of the plasmon mode, resulting in multicolor changes of scattering light from orange to red, green, or a dark color.