Where do the Fluorine Atoms Go in Inorganic-Oxide Fluorinations? A Fluorooxoborate Illustration under Terahertz Light.

The substitution of fluorine atoms for oxygen atoms/hydroxyl groups has emerged as a promising strategy to enhance the physical and chemical properties of oxides/hydroxides in fluorine chemistry. However, distinguishing fluorine from oxygen/hydroxyl in the reaction products poses a significant challenge in existing characterization methods. In this study, we illustrate that terahertz (THz) spectroscopy provides a powerful tool for addressing this challenge. To this end, we investigated two fluorination reactions of boric acid, utilizing MHF2 (M=Na, C(NH2)3) as fluorine reagents. Through an interplay between THz spectroscopy and solid-state density functional theory, we have conclusively demonstrated that fluorine atoms exclusively bind with the sp3-boron but not with the sp2-boron in the reaction products of Na[B(OH)3][B3O3F2(OH)2] (NaBOFH) and [C(NH2)3]2B3O3F4OH (GBF2). Based on this evidence, we have proposed a reaction pathway for the fluorinations under investigation, a process previously hindered due to structural ambiguity. This work represents a step forward in gaining a deeper understanding of the precise structures and reaction mechanisms involved in the fluorination of oxides/hydroxides, illuminated by the insights provided by THz spectroscopy.

Slow spectral diffusion of the NO stretching mode of [RuCl5(NO)]2- in D2O studied by 2D-IR spectroscopy and molecular dynamics simulations.

The vibrational dynamics of the NO stretching mode of [RuCl5(NO)]2- in D2O were investigated by nonlinear infrared (IR) spectroscopy. We performed IR pump-probe measurements to obtain the vibrational lifetime of this molecule. The lifetime is 31 ps, which is sufficiently long enough to study the vibrational frequency fluctuation on a slower time scale with high precision. By two-dimensional IR spectroscopy, the frequency-frequency time correlation function (FFTCF) of the NO stretching mode was characterized with a delta function plus a double-exponential function. The time constant of the slower component was ∼10 ps. We also found that the time constant does not strongly depend on temperature. In order to investigate the microscopic origin of this component, we performed classical molecular dynamics simulations. It was found that the hydration structure around the NO group was influenced by the negatively charged Cl ligands. To calculate the FFTCF decay, we employed an approximate theoretical model based on the vibrational solvatochromism theory. It was demonstrated that water fluctuations around the Cl ligands projected on the NO group correspond to the 10 ps decay component in the FFTCF. The fluctuation is related to the orientational dynamics of the water molecules attracted by the Cl ligands. By comparing the FFTCF parameters of the present solute with those of previously reported metal complexes and SCN- in D2O, we conclude that the presence of different electrostatic environments around the vibrational probe and the other interaction sites of the solute is important for understanding the slow decay component in the FFTCFs.

Microscopic Structures, Dynamics, and Spin Configuration of the Charge Carriers in Organic Photovoltaic Solar Cells Studied by Advanced Time-Resolved Spectroscopic Methods.

Organic photovoltaics (OPVs) are promising solutions for renewable energy and sustainable technologies and have attracted much attention in recent years. Two types of organic semiconductors are used as donor materials to fabricate OPV cells. One type is a photoconductive polymer, and the other type is a small-molecule-based compound. The discovery of a bulk-heterojunction (BHJ) structure using a mixture of p- and n-type organic semiconductors has dramatically increased the power conversion efficiency (PCE) of OPV cells. In this feature article, we review our recent studies on organic BHJ thin films and OPVs by using advanced time-resolved spectroscopic techniques. Two topics regarding the microscopic behaviors of the charge carriers are discussed. The first topic is focused on how to quantify the local mobility of the charge carriers. Here, we discuss charge carrier dynamics in diketopyrrolopyrrole-linked tetrabenzoporphyrin (DPP-BP) BHJ thin films studied by time-resolved terahertz spectroscopy on a subpicosecond to several tens of picoseconds time scale and by transient photocurrent measurements on a microsecond time scale. The second topic concerns the spin configuration and interaction of the electron and hole of the polaron pairs in polymer-based BHJ thin films and OPV cells studied by the time-resolved electron paramagnetic resonance method, time-resolved simultaneous optical and electrical detection, and measurement of the magnetoconductance effect.

Molecular Mechanism of Acceleration and Retardation of Collective Orientation Relaxation of Water Molecules in Aqueous Solutions.

The collective orientation relaxation (COR) of water molecules in aqueous solutions is faster or slower with an increase in the concentration of the solutions than that in pure water; for example, acceleration (deceleration) of the COR is observed in a solution of sodium chloride (tetramethylammonium chloride) with increasing concentration. However, the molecular mechanism of the solution and concentration dependence of the relaxation time of the COR has not yet been clarified. We theoretically investigate the concentration dependence of the COR of water molecules in solutions of tetramethylammonium chloride (TMACl), guanidinium chloride (GdmCl), and sodium chloride (NaCl). Based on the Mori-Zwanzig equation, we identify two opposing factors that determine the COR of water molecules in any aqueous solution: the correlation of dipole moments and the single-molecule orientation relaxation. We reveal the molecular mechanism of the concentration dependence of the relaxation time of the COR in the TMACl, GdmCl, and NaCl solutions in terms of these two factors.

Vibrational Spectroscopic Map, Vibrational Spectroscopy, and Intermolecular Interaction.

Vibrational spectroscopy is an essential tool in chemical analyses, biological assays, and studies of functional materials. Over the past decade, various coherent nonlinear vibrational spectroscopic techniques have been developed and enabled researchers to study time-correlations of the fluctuating frequencies that are directly related to solute-solvent dynamics, dynamical changes in molecular conformations and local electrostatic environments, chemical and biochemical reactions, protein structural dynamics and functions, characteristic processes of functional materials, and so on. In order to gain incisive and quantitative information on the local electrostatic environment, molecular conformation, protein structure and interprotein contacts, ligand binding kinetics, and electric and optical properties of functional materials, a variety of vibrational probes have been developed and site-specifically incorporated into molecular, biological, and material systems for time-resolved vibrational spectroscopic investigation. However, still, an all-encompassing theory that describes the vibrational solvatochromism, electrochromism, and dynamic fluctuation of vibrational frequencies has not been completely established mainly due to the intrinsic complexity of intermolecular interactions in condensed phases. In particular, the amount of data obtained from the linear and nonlinear vibrational spectroscopic experiments has been rapidly increasing, but the lack of a quantitative method to interpret these measurements has been one major obstacle in broadening the applications of these methods. Among various theoretical models, one of the most successful approaches is a semiempirical model generally referred to as the vibrational spectroscopic map that is based on a rigorous theory of intermolecular interactions. Recently, genetic algorithm, neural network, and machine learning approaches have been applied to the development of vibrational solvatochromism theory. In this review, we provide comprehensive descriptions of the theoretical foundation and various examples showing its extraordinary successes in the interpretations of experimental observations. In addition, a brief introduction to a newly created repository Web site (http://frequencymap.org) for vibrational spectroscopic maps is presented. We anticipate that a combination of the vibrational frequency map approach and state-of-the-art multidimensional vibrational spectroscopy will be one of the most fruitful ways to study the structure and dynamics of chemical, biological, and functional molecular systems in the future.

Theoretical Modeling of Electronic Structures of Polyiodide Species Included in α-Cyclodextrin.

The molecular mechanism of blue color formation in an iodine-starch reaction is studied by employing the iodine-α-cyclodextrin (α-CD) complex as a practical model system that resembles the structural properties of the blue amylose-iodine complex. To this end, we construct, using the quantum chemistry method, a molecular model of the complex (I5-/Li+/2α-CD) that consists of one I5-, two molecules of α-CD, and a lithium cation, and this model is employed as a basic unit in constructing the structural models of polyiodide ions (I5-)n. The initial structure in the geometry optimization is adopted from the α-CD-iodine complex structure obtained from the X-ray crystallography study. The structural models of (I5-)n are built by adding the basic unit n times along the crystal axis and by optimizing the structure using quantum mechanics/molecular mechanics (QM (iodine)/MM (α-CD)) calculations. The electronic absorption spectra of the resulting model structures are calculated by time-dependent density functional theory (TD-DFT). We find that I5- acts as a basic unit of coloration in the visible region. The visible color originates from the electronic transition within the I5- molecule, and any charge transfer between the I5- ion and either of α-CD or a coexisting counter cation is not involved. We also reveal that the electronic transitions of (I5-)n are delocalized, which accounts for the well-known observation that the color of the iodine-starch reaction becomes bluish with an increase in the chain length of amylose. Furthermore, the preresonance Raman spectra calculated from the model suggest that the vibrational motions are localized in the I5- subunit dominantly. A comparison between an experimental absorption spectrum feature of the α-CD-iodine complex and the calculated ones of (I5-)n ions with various n values suggests that (I5-)4 polyiodide ions tend to be populated dominantly in the α-CD-iodine complex under aqueous conditions.

Broadband dielectric spectroscopy from sub GHz to THz of hydrated lipid bilayer of DMPC.

In order to study the dynamics of a phospholipid and its hydration water, we measured complex dielectric spectra of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) from the sub-GHz to the THz frequency region with varying the temperature and hydration level of the sample. Spectra obtained from a vector network analyzer and two terahertz time-domain spectrometers are adjusted, which enables us to analyze the dielectric spectra from the sub-GHz region to the THz region by a model function. We confirmed a fast relaxational mode in the sub-THz region, which was suggested by the previous work which only used a THz spectrometer.

Terahertz Fingerprints of Short-Range Correlations of Disordered Atoms in Diflunisal.

This work proposes a terahertz (THz) spectroscopy approach to the investigation of one of the outstanding problems in crystallography-the structure analysis of a crystal with disorder. Form I of diflunisal, in which the two ortho sites on one phenyl ring of diflunisal show occupational disorder, was used for an illustration. THz radiation interacts with the collective vibrations of correlated disorder, thus providing a promising tool to examine the symmetry of short-range correlations of disordered atoms. Through a thorough examination of the selection rule of THz vibrations in which the disordered atoms are involved to different extents, we deduced that only four short-range correlation possibilities of disorder exist and all of them display unambiguous fingerprint peaks in the 50-170 cm-1 frequency region. We finally proposed an alternating packing model in which the correlation lengths of disorder are on the nanometer scale.

Effect of Temperature and Hydration Level on Purple Membrane Dynamics Studied Using Broadband Dielectric Spectroscopy from Sub-GHz to THz Regions.

To investigate the effects of temperature and hydration on the dynamics of purple membrane (PM), we measured the broadband complex dielectric spectra from 0.5 GHz to 2.3 THz using a vector network analyzer and terahertz time-domain spectroscopy from 233 to 293 K. In the lower temperature region down to 83 K, the complex dielectric spectra in the THz region were also obtained. The complex dielectric spectra were analyzed through curve fitting using several model functions. We found that the hydrated states of one relaxational mode, which was assigned as the coupled motion of water molecules with the PM surface, began to overlap with the THz region at approximately 230 K. On the other hand, the relaxational mode was not observed for the dehydrated state. On the basis of this result, we conclude that the protein-dynamical-transition-like behavior in the THz region is due to the onset of the overlap of the relaxational mode with the THz region. Temperature hysteresis was observed in the dielectric spectrum at 263 K when the hydration level was high. It is suggested that the hydration water behaves similarly to supercooled liquid at that temperature. The third hydration layer may be partly formed to observe such a phenomenon. We also found that the relaxation time is slower than that of a globular protein, lysozyme, and the microscopic environment in the vicinity of the PM surface is suggested to be more heterogeneous than lysozyme. It is proposed that the spectral overlap of the relaxational mode and the low-frequency vibrational mode is necessary for the large conformational change of protein.

Rotational Dynamics of Solutes with Multiple Single Bond Axes Studied by Infrared Pump-Probe Spectroscopy.

To investigate the relationship between the structural degrees of freedom around a vibrational probe and the rotational relaxation process of a solute in solution, we studied the anisotropy decays of three different N3-derivatized amino acids in primary alcohol solutions. By performing polarization-controlled IR pump-probe measurements, we reveal that the anisotropy decays of the vibrational probe molecules in 1-alcohol solutions possess two decay components, at subpicosecond and picosecond time scales. On the basis of results showing that the fast relaxation component is insensitive to the vibrational probe molecule, we suggest that the anisotropy decay of the N3 group on a subpicosecond time scale results from a local, small-amplitude fluctuation of the flexible vibrational probe, which does not depend on the details of its molecular structure. However, the slow relaxation component depends on the solute: with longer alkyl chains attached to the N3 group, the anisotropy decay of the slow component is faster. Consequently, we conclude that the slow relaxation component corresponds to the reorientational motion of the N3 group correlated with other intramolecular rotational motions (e.g., rotational motions of the neighboring alkyl chain). Our experimental results provide important insight into understanding the rotational dynamics of solutes with multiple single bond axes in solution.

Probing Charge Carrier Dynamics in Porphyrin-Based Organic Semiconductor Thin Films by Time-Resolved THz Spectroscopy.

To improve the power conversion efficiency of solar cells, it is important to understand the underlying relaxation mechanisms of photogenerated charge carriers in organic semiconductors. In this work, we studied the charge carrier dynamics of diketopyrrolopyrrole-linked tetrabenzoporphyrin thin films where the diketopyrrolopyrrole unit has two n-butyl groups, abbreviated as C4-DPP-BP. We used time-resolved terahertz (THz) spectroscopy to track charge carrier dynamics with excitations at 800 and 400 nm. Compared with tetrabenzoporphyrin (BP), the extension of π-electron delocalization to the diketopyrrolopyrrole peripherals leads to an increase in absorption in the near-infrared region. Following the excitation at 800 nm, we found that the transient THz signals in C4-DPP-BP thin films decay with time constants of 0.5 and 9.1 ps, with small residual components. With excitation at 400 nm, we found that the transient THz signals decay with time constants of 0.4 and 7.5 ps. On the basis of the similarity of the decay profiles of the transient THz signals obtained with excitations at 400 and 800 nm, we considered that the decaying components are due to charge carrier recombination and/or trapping at defect sites, which do not depend on the excess energy of the photoexcitation. In contrast to BP, even without an electron acceptor, we observed the finite offset of the transient THz signals at 100 ps, demonstrating the existence of long-lived charge carriers. We also measured the photoconductivity spectra of C4-DPP-BP thin films with the excitation at both 800 and 400 nm. It was found that the spectra can be fitted by the Drude-Smith model. From these results, it was determined that the charge carriers are localized right after photoexcitation. At 0.4 ps, the product of the quantum yield of charge generation and mobility of charge carriers at 400 nm is approximately twice that obtained at 800 nm. We discuss the implications of the excess excitation energy in organic semiconductor-based devices.

Application of THz Vibrational Spectroscopy to Molecular Characterization and the Theoretical Fundamentals: An Illustration Using Saccharide Molecules.

This work illustrates several theoretical fundamentals for the application of THz vibrational spectroscopy to molecular characterization in the solid state using two different types of saccharide systems as examples. Four subjects have been specifically addressed: (1) the qualitative differences in the molecular vibrational signatures monitored by THz and mid-IR vibrational spectroscopy; (2) the selection rules for THz vibrational spectroscopy as applied to crystalline and amorphous systems; (3) a normal mode simulation, using α-l-xylose as an example; and (4) a rigorous mode analysis to quantify the percentage contributions of the intermolecular and intramolecular vibrations to the normal mode of interest.

Elucidation of Chiral Symmetry Breaking in a Racemic Polymer System with Terahertz Vibrational Spectroscopy and Crystal Orbital Density Functional Theory.

The conservation of chiral symmetry has been used as a fundamental rule to determine polymer packing conformations in racemic systems. We have illustrated, through the interplay of polarization terahertz (THz) spectroscopy and solid-state density functional theory, that the chiral symmetry is not conserved in a poly(lactic acid) stereocomplex (scPLA) system. Poly(l-lactic acid) (PLLA) displays a weaker violation of the 31 screw symmetry than poly(d-lactic acid) (PDLA) and possesses a stronger intramolecular vibrational energy, on average, in the low-frequency gamma-point phonon modes than does PDLA. Polarization THz spectroscopy adds a new dimension to polymer crystallography through which new phenomena are expected to be revealed.

Broadband Dielectric Spectroscopy on Lysozyme in the Sub-Gigahertz to Terahertz Frequency Regions: Effects of Hydration and Thermal Excitation.

We have performed dielectric spectral measurements of lysozyme in a solid state to understand the effects of hydration and thermal excitation on the low-frequency dynamics of protein. Dielectric measurements were performed under changing hydration conditions at room temperature in the frequency region of 0.5 GHz to 1.8 THz. We also studied the temperature dependence (83 to 293 K) of the complex dielectric spectra in the THz frequency region (0.3 THz to 1.8 THz). Spectral analyses were performed using model functions for the complex dielectric constant. To reproduce the spectra, we found that two relaxational modes and two underdamped modes are necessary together with an ionic conductivity term in the model function. At room temperature, the two relaxational modes have relaxation times of ∼20 ps and ∼100 ps. The faster component has a major spectral intensity and is suggested to be due to coupled water-protein motion. The two underdamped modes are necessary to reproduce the temperature dependence of the spectra in the THz region satisfactorily. The protein dynamical transition is a well-known behavior in the neutron-scattering experiment for proteins, where the atomic mean-square displacement shows a sudden change in the temperature dependence at approximately 200 K, when the samples are hydrated. A similar behavior has also been observed in the temperature dependence of the absorption spectra of protein in the THz frequency region. From our broadband dielectric spectroscopic measurements, we conclude that the increase in the spectral intensities in the THz region at approximately 200 K is due to a spectral blue-shift of the fast relaxational mode.

Characteristics of Low-Frequency Molecular Phonon Modes Studied by THz Spectroscopy and Solid-State ab Initio Theory: Polymorphs I and III of Diflunisal.

THz absorption spectra of two polymorphs of diflunisal, form I and form III, exhibit distinct features due to the influence of packing conformations on the frequency distributions and IR activities of gamma point phonon modes within the 100 cm(-1) region. In order to understand the origins of these THz modes, we perform a detailed mode analysis. The result shows that although the spectral features are different, these low-frequency phonon modes of the two molecular polymorphs have similar vibrational characteristics in terms of harmonic couplings of intermolecular and intramolecular vibrations.

Vibrational dynamics of azide-derivatized amino acids studied by nonlinear infrared spectroscopy.

Recently, biomolecules which are labeled by azide or thiocyanate groups in solutions and proteins have been studied to examine microscopic environment around a solute by nonlinear infrared (IR) spectroscopy. In this study, we have performed two-dimensional infrared (2D-IR) spectroscopy to investigate the vibrational frequency fluctuations of two different azide-derivatized amino acids, Ala (N3-Ala) and Pro (N3-Pro), and N3(-) in water. From the 2D-IR experiments, it was found that the frequency-frequency time correlation function (FFTCF) of solute can be modeled by a delta function plus an exponential function and constant. FFTCF for each probe molecule has a decay component of about 1 ps, and this result suggests that the stretching mode of the covalently bonded azide group is sensitive to the fluctuations of hydrogen bond network system, as found in previous studies of N3(-) in water. In contrast to FFTCF of N3(-), FFTCF of the azide-derivatized amino acids contains static component. This static component may reflect dynamics of water affected by the solutes or the structural fluctuations of the solute itself. We also performed the IR pump-probe measurements for the probe molecules in water in order to investigate vibrational energy relaxation (VER) and reorientational relaxation. It was revealed that the charge fluctuations in the azide group are significant for the VER of this mode in water, reflecting that the VER rate of N3(-) is faster than those of the azide-derivatized amino acids. While the behaviors of the anisotropy decay of N3-Ala and N3(-) are similar to each other, the anisotropy decay of N3-Pro contains much slower decaying component. By considering the structural difference around the vibrational probe between N3-Ala and N3-Pro, it is suggested that the structural freedom of the probe molecules can affect the reorientational processes.

Vibrational dynamics of the CO stretching of 9-fluorenone studied by visible-pump and infrared-probe spectroscopy.

We studied the effects of hydrogen bonds on the vibrational structures and vibrational dynamics of the CO stretching mode of 9-fluorenone (FL) in the electronically excited state in aprotic and protic solvents using sub-picosecond visible-pump and IR-probe spectroscopy. The transient IR spectrum of the CO stretching band in methanol-d4 has two bands at 1529.9 cm(-1) and 1543.4 cm(-1), which are assigned to an FL-solvent complex and free FL, respectively. In the aprotic solvents, the CO stretching bands show blue-shifts in time. This shift is due to vibrational cooling, which is derived from anharmonic couplings with some low-frequency modes. Interestingly, a red-shift is observed at later delay time for the band at 1529.9 cm(-1) in methanol-d4. A possible mechanism of this spectral shift is related to the hydrogen bond dynamics between the solute and solvent.