Contact Ion Pair Formation between Hard Acids and Soft Bases in Aqueous Solutions Observed with 2DIR Spectroscopy

Does Viscosity Drive the Dynamics in an Alcohol-Based Deep Eutectic Solvent?

Deep eutectic solvents, consisting of heterogeneous nanodomains of hydrogen-bonded networks, have evolved into a range of viscous fluids that find applications in several fields. As viscosity is known to influence the dynamics of other neoteric solvents like ionic liquids, understanding the effect of viscosity on deep eutectic solvents is crucial to realize their full potential. Herein, we combine polarization-selective pump-probe spectroscopy, two-dimensional infrared spectroscopy, and molecular dynamics simulations to elucidate the impact of viscosity on the dynamics of an alcohol-based deep eutectic solvent, ethaline. We compare the solvent fluctuation and solute reorientation time scales in ethaline with those in ethylene glycol, an ethaline constituent. Interestingly, we find that the solute's reorientation apparently scales the bulk viscosity of the solvent, but the same is not valid for the overall solvation dynamics. Using the variations in the estimated intercomponent hydrogen bond lifetimes, we show that a dissolved solute does not sense the bulk viscosity of the deep eutectic solvent; instead, it senses domain-specific local viscosity determined by the making and breaking of the hydrogen bond network. Our results indicate that understanding the domain-specific local environment experienced by the dissolved solute is of utmost importance in deep eutectic solvents.

Ligand Dynamics Time Scales Identify the Surface-Ligand Interactions in Thiocyanate-Capped Cadmium Sulfide Nanocrystals

The nanocrystal surface, which acts as an interface between the semiconductor lattice and the capping ligands, plays a significant role in the attractive photophysical properties of semiconductor nanocrystals for use in a wide range of applications. Replacing the long-chain organic ligands with short inorganic variants improves the conductivity and carrier mobility of nanocrystal-based devices. However, our current understanding of the interactions between the inorganic ligands and the nanocrystals is obscure due to the lack of experiments to directly probe the inorganic ligands. Herein, using two-dimensional infrared spectroscopy, we show that the variations in the inorganic ligand dynamics within the heterogeneous nanocrystal ensemble can identify the diversities in the inorganic ligand-nanocrystal interactions. The ligand dynamics time scale in SCN^- capped CdS nanocrystals identifies three distinct ligand populations and provides molecular insight into the nanocrystal surface. Our results demonstrate that the SCN^- ligands engage in a dynamic equilibrium and stabilize the nanocrystals by neutralizing the surface charges through both direct binding and electrostatic interaction.

Water Dynamics and Structure of Highly Concentrated LiCl Solutions Investigated Using Ultrafast Infrared Spectroscopy

In highly concentrated salt solutions, the water hydrogen bond (H-bond) network is completely disrupted by the presence of ions. Water is forced to restructure as dictated by the water-ion and ion-ion interactions. Using ultrafast polarization-selective pump-probe (PSPP) spectroscopy measurements of the OD stretch of dilute HOD, we demonstrate that the limited water-water H-bonding present in concentrated lithium chloride solutions (up to four waters per ion pair) is, on average, stronger than that occurring in bulk water. Furthermore, information on the orientational dynamics and the angular restriction of water H-bonded to both water oxygens and chloride anions was obtained through analysis of the frequency-dependent anisotropy decays. It was found that, when the salt concentration increased, the water showed increasing restriction and slowing at frequencies correlated with strong H-bonding. The angular restriction of the water molecules and strengthening of water-water H-bonds are due to the formation of a water-ion network not present in bulk water and dilute salt solutions. The structural evolution of the ionic medium was also observed through spectral diffusion of the OD stretch using 2D IR spectroscopy. Compared to bulk water, there is significant slowing of the biexponential spectral diffusion dynamics. The slowest component of the spectral diffusion (13 ps) is virtually identical to the time for complete reorientation of HOD measured with the PSPP experiments. This result suggests that the slowest component of the spectral diffusion reflects rearrangement of water molecules in the water-ion network.

Association-Dissociation Dynamics of Ionic Electrolytes in Low Dielectric Medium

Ionic electrolytes are known to form various complexes which exist in dynamic equilibrium in a low dielectric medium. However, structural characterization of these complexes has always posed a great challenge to the scientific community. An additional challenge is the estimation of the dynamic association-dissociation time scales (lifetime of the complexes), which are key to the fundamental understanding of ion transport. In this work, we have used a combination of infrared absorption spectroscopy, two-dimensional infrared spectroscopy, molecular dynamics simulations, and density functional theory calculations to characterize the various ion complexes formed by the thiocyanate-based ionic electrolytes as a function of different cations in a low dielectric medium. Our results demonstrate that thiocyanate is an excellent vibrational reporter of the heterogeneous ion complexes undergoing association-dissociation dynamics. We find that the ionic electrolytes exist as contact ion pairs, dimers, and clusters in a low dielectric medium. The relative ratios of the various ion complexes are sensitive to the cations. In addition to the interactions between the thiocyanate anion and the countercation, the solute-solvent interactions drive the dynamic equilibrium. We have estimated the association-dissociation dynamics time scales from two-dimensional infrared spectroscopy. The exchange time scale involving the cluster is faster than that between a dimer and an ion pair. Moreover, we find that the dynamic equilibrium between the cluster and another ion complex is correlated to the solvent fluctuations.

Ion Pair Supramolecular Structure Identified by ATR-FTIR Spectroscopy and Simulations in Explicit Solvent*

The present work uses ATR-FTIR spectroscopy assisted by simulations in explicit solvent and frequency calculations to investigate the supramolecular structure of carboxylate alkali-metal ion pairs in aqueous solutions. ATR-FTIR spectra in the 0.25-4.0 M concentration range displayed cation-specific behaviors, which enabled the measurement of the appearance concentration thresholds of contact ion pairs between 1.9 and 2.6 M depending on the cation. Conformational explorations performed using a non-local optimization method associated to a polarizable force-field (AMOEBA), followed by high quantum chemistry level (RI-B97-D3/dhf-TZVPP) optimizations, mode-dependent scaled harmonic frequency calculations and electron density analyses, were used to identify the main supramolecular structures contributing to the experimental spectra. A thorough analysis enables us to reveal the mechanisms responsible for the spectroscopic sensitivity of the carboxylate group and the respective role played by the cation and the water molecules, highlighting the necessity of combining advanced experimental and theoretical techniques to provide a fair and accurate description of ion pairing.

Solubilities in aqueous nitrate solutions that appear to reverse the law of mass action

Non-ideal aqueous electrolyte solutions have been studied since the start of the application of thermodynamics to chemistry in the late 19th century. The present study examines some of the most extreme non-ideal behavior ever observed: solubilities of alkali and NH₄⁺ nitrate salts in water that appear to behave the opposite of how the Law of Mass Action would predict. A literature review discovered that the solubilities of NH₄NO₃ and many alkali nitrate salts increases when another nitrate-bearing electrolyte is added to solution. These occurrences were in concentrated solutions with insufficient water to provide all ions their preferred hydration number without sharing waters between ions. This water deficit results in the formation of contact ion-pairs as well as larger ion-clusters. These ion-clusters may be favored when there is more than one type of monovalent cation present.

Ultrafast vibrational dynamics of a trigonal planar anionic probe in ionic liquids (ILs): A two-dimensional infrared (2DIR) spectroscopic investigation

A major impediment limiting the widespread application of ionic liquids (ILs) is their high shear viscosity. Incorporation of a tricyanomethanide (TCM^-) anion in ILs leads to low shear viscosity and improvement of several characteristics suitable for large scale applications. However, properties including interactions of TCM^- with the local environment and dynamics of TCM^- have not been thoroughly investigated. Herein, we have studied the ultrafast dynamics of TCM^- in several imidazolium ILs using linear IR and two-dimensional infrared spectroscopy techniques. The spectral diffusion dynamics of the CN stretching modes of TCM^- in all ILs exhibit a nonexponential behavior with a short time component of ∼2 ps and a long time component spanning ∼9 ps to 14 ps. The TCM^- vibrational probe reports a significantly faster relaxation of ILs compared to those observed previously using linear vibrational probes, such as thiocyanate and selenocyanate. Our results indicate a rapid relaxation of the local ion-cage structure embedding the vibrational probe in the ILs. The faster relaxation suggests that the lifetime of the local ion-cage structure decreases in the presence of TCM^- in the ILs. Linear IR spectroscopic results show that the hydrogen-bonding interaction between TCM^- and imidazolium cations in ILs is much weaker. Shorter ion-cage lifetimes together with weaker hydrogen-bonding interactions account for the low shear viscosity of TCM^- based ILs compared to commonly used ILs. In addition, this study demonstrates that TCM^- can be used as a potential vibrational reporter to study the structure and dynamics of ILs and other molecular systems.

Vibrational Relaxation Dynamics of a Semiconductor Copper(I) Thiocyanate (CuSCN) Film as a Hole-Transporting Layer

The semiconductor CuSCN film, which is typically used as the hole-transporting layer (HTL) in solar cell studies, has been investigated by Fourier transform infrared (FTIR) spectroscopy and ultrafast transient infrared (IR) spectroscopy. A sharp peak at 2175 cm^-1 corresponding to the CN vibrational stretching mode in CuSCN was observed, and the peak frequency remained unchanged by varying the thickness of the CuSCN thin film. Vibrational relaxation measurements showed that the 0-1 and 1-2 transitions of CN stretching can be observed at 2175 and 2140 cm^-1, respectively. The heat-induced absorption and bleaching peaks (2167 and 2175 cm^-1) can be clearly seen at a waiting time of 40 ps. The vibrational relaxation of the CN stretching mode determined from the 1-2 transition exhibited a biexponential decay with time constants of 7.4 ± 0.5 (90%) and 158 ± 50 ps (10%). Importantly, the abnormal anisotropy decay of the CN stretching mode in the CuSCN thin film was also observed for the first time. A detailed analysis showed that the distinct anisotropy decay curve could be described using a triexponential decay function, which was explained by three different processes: resonance energy transfer (∼8 ps), a thermalization process (∼40 ps), and molecular rotation (∼150 ps). The time scale of the thermalization process caused by the vibrational relaxation in CuSCN is at a time scale of 40 ps, which is important for us to understand the thermally activated charge-transport property of the CuSCN film employed as the HTL. Further UV pump-IR probe measurement revealed that the carrier scattering and relaxation processes in the CuSCN film are strongly associated with the vibrational excitation and relaxation dynamics of the CN stretching mode. It is expected that the fundamental understanding of the vibrational relaxation dynamics of the CuSCN thin film should provide helpful insight to elucidate its role as the HTL in solar cell studies at the molecular level.

C-D Vibration at C2 Position of Imidazolium Cation as a Probe of the Ionic Liquid Microenvironment

Unlike molecular solvents, imidazolium-based ionic liquids are entirely made of ions with spatial heterogeneity. There is a need for spectroscopic probes that can assess the microenvironment near the cations of these complex liquids. In this manuscript, we describe simple chemical procedures to label the C2 position of imidazolium cation with a C-D vibrational probe and show, through linear and nonlinear vibrational spectroscopies, that this C-D stretching mode can be a useful analytical tool to assess both the solvent microenvironment and solute-solvent interactions in imidazolium-based ionic liquids from the cation point of view. It is expected that this C-D vibration probe on the cation will lead to the development of innovative experimental strategies that can provide a better understanding of such ionic liquids.

Phosphate-Magnesium Ion Interactions in Water Probed by Ultrafast Two-Dimensional Infrared Spectroscopy

Electric interactions between ions and ionic molecular groups in aqueous solution play a fundamental role in chemistry and biology. While Mg²⁺ ions are known to strongly affect the structure and folding dynamics of biomolecules, the relevance of different solvation geometries and the underlying interactions are mainly unresolved. We study dynamics and couplings between the hydrated Mg²⁺ and the dimethylphosphate anion, an established model system for the DNA and RNA backbone. The asymmetric (PO₂^-) stretching vibration serves as a sensitive noninvasive probe of phosphate-ion interactions. Femtosecond two-dimensional infrared (2D-IR) spectroscopy directly maps Mg²⁺ ions in contact with the phosphate groups via a distinct blue-shifted signature in the 2D spectrum. Data for different Mg²⁺ concentrations are analyzed by microscopic density functional theory modeling of cluster geometries and associated spectroscopic features, providing spatial assignments of the observed 2D-IR signatures. Phosphate-ion interactions arising from electrostatic Coulomb forces and exchange repulsion are the predominant origin of the observed frequency shifts.

Ultrafast structural molecular dynamics investigated with 2D infrared spectroscopy methods

Ultrafast, multi-dimensional infrared (IR) spectroscopy has been advanced in recent years to a versatile analytical tool with a broad range of applications to elucidate molecular structure on ultrafast timescales, and it can be used for samples in a many different environments. Following a short and general introduction on the benefits of 2D IR spectroscopy, the first part of this chapter contains a brief discussion on basic descriptions and conceptual considerations of 2D IR spectroscopy. Outstanding classical applications of 2D IR are used afterwards to highlight the strengths and basic applicability of the method. This includes the identification of vibrational coupling in molecules, characterization of spectral diffusion dynamics, chemical exchange of chemical bond formation and breaking, as well as dynamics of intra- and intermolecular energy transfer for molecules in bulk solution and thin films. In the second part, several important, recently developed variants and new applications of 2D IR spectroscopy are introduced. These methods focus on (i) applications to molecules under two- and three-dimensional confinement, (ii) the combination of 2D IR with electrochemistry, (iii) ultrafast 2D IR in conjunction with diffraction-limited microscopy, (iv) several variants of non-equilibrium 2D IR spectroscopy such as transient 2D IR and 3D IR, and (v) extensions of the pump and probe spectral regions for multi-dimensional vibrational spectroscopy towards mixed vibrational-electronic spectroscopies. In light of these examples, the important open scientific and conceptual questions with regard to intra- and intermolecular dynamics are highlighted. Such questions can be tackled with the existing arsenal of experimental variants of 2D IR spectroscopy to promote the understanding of fundamentally new aspects in chemistry, biology and materials science. The final part of the chapter introduces several concepts of currently performed technical developments, which aim at exploiting 2D IR spectroscopy as an analytical tool. Such developments embrace the combination of 2D IR spectroscopy and plasmonic spectroscopy for ultrasensitive analytics, merging 2D IR spectroscopy with ultra-high-resolution microscopy (nanoscopy), future variants of transient 2D IR methods, or 2D IR in conjunction with microfluidics. It is expected that these techniques will allow for groundbreaking research in many new areas of natural sciences.

High-Throughput Two-Dimensional Infrared (2D IR) Spectroscopy Achieved by Interfacing Microfluidic Technology with a High Repetition Rate 2D IR Spectrometer

The precision control of microfluidic technology was successfully interfaced with a 100 kHz two-dimensional infrared (2D IR) spectrometer to observe the sensitivity of the anion cyanate (OCN^-) to the surrounding solvent environment in a high-throughput manner. Producing high-throughput 2D IR spectroscopy measurements allows us to observe the vibrational response of cyanate in mixed solvent environments. Changes in solvation environment around the cyanate ion yield frequency shifts from 2150 to 2165 cm^-1 when moving from a pure dimethylformamide solvent environment to a pure methanol environment. 2D IR spectra were captured laterally across microfluidic devices tailored to produce a tunable gradient to observe the OCN^- vibrational response to mixed solvent environments. These experiments reveal that there is no preferential solvation of cyanate in this system; instead, a more complex local solvent environment is observed.