Ultrafast vibrational control of organohalide perovskite optoelectronic devices using vibrationally promoted electronic resonance.

Vibrational control (VC) of photochemistry through the optical stimulation of structural dynamics is a nascent concept only recently demonstrated for model molecules in solution. Extending VC to state-of-the-art materials may lead to new applications and improved performance for optoelectronic devices. Metal halide perovskites are promising targets for VC due to their mechanical softness and the rich array of vibrational motions of both their inorganic and organic sublattices. Here, we demonstrate the ultrafast VC of FAPbBr3 perovskite solar cells via intramolecular vibrations of the formamidinium cation using spectroscopic techniques based on vibrationally promoted electronic resonance. The observed short (~300 fs) time window of VC highlights the fast dynamics of coupling between the cation and inorganic sublattice. First-principles modelling reveals that this coupling is mediated by hydrogen bonds that modulate both lead halide lattice and electronic states. Cation dynamics modulating this coupling may suppress non-radiative recombination in perovskites, leading to photovoltaics with reduced voltage losses.

Elucidation of the pH-Dependent Electric Double Layer Structure at the Silica/Water Interface Using Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy.

The silica/water interface is one of the most abundant charged interfaces in natural environments, and the elucidation of the water structure at the silica/water interface is essential. In the present study, we measured the interface-selective vibrational (χ(2)) spectra in the OH stretch region of the silica/water interface in a wide pH range of pH 2.0-12.0 while changing the salt concentration by heterodyne-detected vibrational sum-frequency generation spectroscopy. With the help of singular value decomposition analysis, it is shown that the imaginary part of the χ(2) (Imχ(2)) spectra can be decomposed into the spectra of the diffuse Gouy-Chapman layer (DL) and the compact Stern layer (SL), which enables us to quantitatively analyze the spectra of DL and SL separately. The salt-concentration dependence of the DL spectra at different pH values is analyzed using the modified Gouy-Chapman theory, and the pH-dependent surface charge density and the pKa value (4.8 ± 0.2) of the silica/water interface are evaluated. Furthermore, it is found that the pH-dependent change of the SL spectra is quantitatively explained by three spectral components that represent the three characteristic water species appearing in different pH regions in SL. The quantitative understanding obtained from the analysis of each spectral component in the Imχ(2) spectra provides a clear molecular-level picture of the electric double layer at the silica/water interface.

Ultrafast vibrational dynamics of the free OD at the air/water interface: Negligible isotopic dilution effect but large isotope substitution effect.

Vibrational relaxation dynamics of the OH stretch of water at the air/water interface has been a subject of intensive research, facilitated by recent developments in ultrafast interface-selective nonlinear spectroscopy. However, a reliable determination of the vibrational relaxation dynamics in the OD stretch region at the air/D2O interface has not been yet achieved. Here, we report a study of the vibrational relaxation of the free OD carried out by time-resolved heterodyne-detected vibrational sum frequency generation spectroscopy. The results obtained with the aid of singular value decomposition analysis indicate that the vibrational relaxation (T1) time of the free OD at the air/D2O interface and air/isotopically diluted water (HOD-H2O) interfaces show no detectable isotopic dilution effect within the experimental error, as in the case of the free OH in the OH stretch region. Thus, it is concluded that the relaxation of the excited free OH/OD predominantly proceeds with their reorientation, negating a major contribution of the intramolecular energy transfer. It is also shown that the T1 time of the free OD is substantially longer than that of the free OH, further supporting the reorientation relaxation mechanism. The large difference in the T1 time between the free OD and the free OH (factor of ∼2) may indicate the nuclear quantum effect on the diffusive reorientation of the free OD/OH because this difference is significantly larger than the value expected for a classical rotational motion.

In situ observation of the potential-dependent structure of an electrolyte/electrode interface by heterodyne-detected vibrational sum frequency generation.

Elucidating the structure of electrolyte/electrode interfaces is of essential importance not only for understanding of the fundamental process of electrochemistry but also for developing next-generation rechargeable batteries. In this study, we applied HD-VSFG spectroscopy to study a prototypical non-aqueous electrochemical interface of a platinum electrode in 0.1 M LiCF3SO3 acetonitrile (CH3CN) solution, and measured Im χ(2) spectra by changing the applied potential in the range of -0.8 V to 2.0 V. In the positive potential region, the positive bands assignable to acetonitrile appear in the CH3 and CN stretch regions, and their positive signs indicate the CH3-down orientation of acetonitrile at the interface. We also observed an SO3- stretch band of the anion of the electrolyte and found that the potential dependence of its intensity is similar to those of the CH3 and CN bands of acetonitrile. These observations indicate that the CF3SO3- anion is adsorbed at the platinum surface in the positive potentials, which induces CH3-down orientation of acetonitrile at the interface. The present study demonstrates the advantages of HD-VSFG spectroscopy for studying electrochemical systems, and it opens a new way to investigate electrolyte/electrode interfaces at the molecular level.

Structure of water and polymer at the buried polymer/water interface unveiled using heterodyne-detected vibrational sum frequency generation.

The structure of the prototypical acrylic polymer (poly(methyl methacrylate): PMMA)/water interface is elucidated at the molecular level using heterodyne-detected sum-frequency generation. Two distinct OH groups of interfacial water are found at the interface: one forms hydrogen bonds with the carbonyl group and the other weakly interacts with the ester methyl group of the polymer surface.

Hidden Isolated OH at the Charged Hydrophobic Interface Revealed by Two-Dimensional Heterodyne-Detected VSFG Spectroscopy.

Water around hydrophobic groups mediates hydrophobic interactions that play key roles in many chemical and biological processes. Thus, the molecular-level elucidation of the properties of water in the vicinity of hydrophobic groups is important. We report on the structure and dynamics of water at two oppositely charged hydrophobic ion/water interfaces, that is, the tetraphenylborate-ion (TPB- )/water and tetraphenylarsonium-ion (TPA+ )/water interfaces, which are clarified by two-dimensional heterodyne-detected vibrational sum-frequency generation (2D HD-VSFG) spectroscopy. The obtained 2D HD-VSFG spectra of the anionic TPB- interface reveal the existence of distinct π-hydrogen bonded OH groups in addition to the usual hydrogen-bonded OH groups, which are hidden in the steady-state spectrum. In contrast, 2D HD-VSFG spectra of the cationic TPA+ interface only show the presence of usual hydrogen-bonded OH groups. The present study demonstrates that the sign of the interfacial charge governs the structure and dynamics of water molecules that face the hydrophobic region.

Ultrafast Dynamics at Water Interfaces Studied by Vibrational Sum Frequency Generation Spectroscopy.

We present an overview of studies on the ultrafast dynamics of water at aqueous interfaces carried out by time-resolved vibrational sum frequency generation (VSFG) spectroscopies. This research field has been growing rapidly, stimulated by technical developments achieved recently. In this review, first, the principles and instrumentations are described for conventional VSFG, heterodyne-detected VSFG, and various IR-pump/VSFG-probe techniques, namely, time-resolved conventional VSFG, time-resolved heterodyne-detected VSFG, and their extension to two-dimensional spectroscopy. Second, the applications of these time-resolved VSFG techniques to the study of the femtosecond vibrational dynamics of water at various interfaces are discussed, in the order of silica/water, charged monolayer/water, and the air/water interfaces. These studies demonstrate that there exists water dynamics specific to the interfaces and that time-resolved VSFG spectroscopies can unambiguously detect such unique dynamics in an interface-selective manner. In particular, the most recent time-resolved heterodyne-detected VSFG and two-dimensional heterodyne-detected VSFG unveil the inhomogeneity of the hydrogen bond and relevant vibrational dynamics of interfacial water through unambiguous observation of hole-burning in the OH stretch band, as well as the subsequent spectral diffusion in the femtosecond time region. These time-resolved VSFG studies have also left several issues for discussion. We describe not only the obtained conclusive physical insights into interfacial water dynamics but also the points left unclear or controversial. A new type of experiment that utilizes UV excitation is also described briefly. Lastly, the summary and some future perspectives of time-resolved VSFG spectroscopies are given.

Effect of hydrogen-bond on ultrafast spectral diffusion dynamics of water at charged monolayer interfaces.

Ultrafast hydrogen-bond fluctuation dynamics of water at charged monolayer interfaces were studied by the use of steady-state and 2D heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy. Specifically, the effect of hydrogen-bond ability of the interface on the dynamics was investigated by comparing two monolayer interfaces that provide different hydrogen bond abilities: hydrogen bonding octadecylammonium (ODA) monolayer (pH = 2) and non-hydrogen bonding 1,2-dipalmitoyl-3-trimethyl-ammonium propane (DPTAP) monolayer. The steady-state HD-VSFG spectra and their ionic strength dependence revealed that water molecules at both of ODA and DPTAP interfaces are H-down oriented, pointing their H away from the interface, and that the contributions of the electrical double layer in the interfacial spectra of these interfaces are comparable to each other. However, 2D HD-VSFG data clearly indicated that the ultrafast hydrogen-bond fluctuation of water at the ODA interface is significantly suppressed, compared to that at the DPTAP interfaces. The obtained results suggest that the hydrogen-bond fluctuation of the topmost interfacial water at a positively charged interface is significantly affected by the hydrogen-bonding ability of the interface even in the case that the interfacial water molecules act as a hydrogen-bond acceptor to the head group of the monolayer.

Structure at the air/water interface in the presence of phenol: a study using heterodyne-detected vibrational sum frequency generation and molecular dynamics simulation.

Many kinds of organic compounds pollute the aquatic environment, and they change the properties of the water surface due to their high surface affinity. Chemical reactions at the water surface are key in environmental chemistry because, for instance, reactions occurring at the surface of aqueous aerosols play essential roles in the atmosphere. Therefore, it is very important to elucidate how organic compounds affect the properties of water surfaces. Here, we choose phenol as an organic pollutant prototype and report how phenol affects the molecular-level structure of the air/water interface. Interface-selective vibrational spectra, i.e., the imaginary part of second-order nonlinear susceptibility (Im χ(2)), of the air/water-phenol mixture interface in the OH stretch region were collected using heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy, and the observed Im χ(2) spectra were interpreted with the aid of molecular dynamics (MD) simulation. The Im χ(2) spectra observed via HD-VSFG drastically change as a function of phenol concentration in water, and exhibit two isosbestic points. In the spectra, a positive OH band appears at 3620 cm-1, which is assigned to an OH group of water that forms an OHπ hydrogen-bond (H-bond) with the aromatic ring of phenol, and a strong negative OH band appears around 3200 cm-1, which is attributed to a water that accepts a H-bond from the phenol OH, while pointing its OH groups toward the bulk water side. It was concluded that two types of unique water molecules hydrate a phenol molecule: (1) water that forms an OHπ H-bond; and (2) water that accepts a H-bond from a phenol OH group. Each phenol molecule adsorbed at the air/water forms a specific hydration structure, which causes a large change in the interfacial water structure. The present study provides a clear example demonstrating that even such a simple organic pollutant as phenol can drastically alter the interfacial water structure.

Molecular mechanism of charge inversion revealed by polar orientation of interfacial water molecules: A heterodyne-detected vibrational sum frequency generation study.

"Charge inversion" is a phenomenon in which multivalent counterions overcompensate for interfacial charges and invert the sign of the net charge near a surface. This phenomenon is believed to be relevant to biologically important processes such as DNA condensation, and hence it has attracted much attention. We investigated the polar orientation of interfacial water molecules at two different negatively charged interfaces in the absence and presence of La3+ using heterodyne-detected vibrational sum frequency generation spectroscopy, which can directly determine the up/down orientation of interfacial molecules. It was found that the orientations of water molecules at a bio-relevant phospholipid interface change from the hydrogen-up to the hydrogen-down with the addition of 10 µM La3+. This change of water orientation indicates that the net charge at the phospholipid interface is inverted by adsorption of La3+ to the phosphate headgroup. By contrast, at an alkylsulfate interface, the majority of the interfacial water molecules remain hydrogen-up orientated even in the presence of 25 mM La3+, indicating that the sulfate headgroup is still solvated by up-oriented water. The observed headgroup specificity suggests that charge inversion at the phospholipid interface originates primarily from the chemical interaction between the phosphate and La3+ ion.

Change of the isoelectric point of hemoglobin at the air/water interface probed by the orientational flip-flop of water molecules.

Elucidation of the molecular mechanisms of protein adsorption is of essential importance for further development of biotechnology. Here, we use interface-selective nonlinear vibrational spectroscopy to investigate protein charge at the air/water interface by probing the orientation of interfacial water molecules. We measured the Im χ(2) spectra of hemoglobin, myoglobin, serum albumin and lysozyme at the air/water interface in the CH and OH stretching regions using heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy, and we deduced the isoelectric point of the protein by monitoring the orientational flip-flop of water molecules at the interface. Strikingly, our measurements indicate that the isoelectric point of hemoglobin is significantly lowered (by about one pH unit) at the air/water interface compared to that in the bulk. This can be predominantly attributed to the modifications of the protein structure at the air/water interface. Our results also suggest that a similar mechanism accounts for the modification of myoglobin charge at the air/water interface. This effect has not been reported for other model proteins at interfaces probed by conventional VSFG techniques, and it emphasizes the importance of the structural modifications of proteins at the interface, which can drastically affect their charge profiles in a protein-specific manner. The direct experimental approach using HD-VSFG can unveil the changes of the isoelectric point of adsorbed proteins at various interfaces, which is of major relevance to many biological applications and sheds new light on the effect of interfaces on protein charge.

Partially Hydrated Electrons at the Air/Water Interface Observed by UV-Excited Time-Resolved Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy.

Hydrated electrons are the most fundamental anion species, consisting only of electrons and surrounding water molecules. Although hydrated electrons have been extensively studied in the bulk aqueous solutions, even their existence is still controversial at the water surface. Here, we report the observation and characterization of hydrated electrons at the air/water interface using new time-resolved interface-selective nonlinear vibrational spectroscopy. With the generation of electrons at the air/water interface by ultraviolet photoirradiation, we observed the appearance of a strong transient band in the OH stretch region by heterodyne-detected vibrational sum-frequency generation. Through the comparison with the time-resolved spectra at the air/indole solution interface, the transient band was assigned to the vibration of water molecules that solvate electrons at the interface. The analysis of the frequency and decay of the observed transient band indicated that the electrons are only partially hydrated at the water surface, and that they escape into the bulk within 100 ps.

Femtosecond Hydrogen Bond Dynamics of Bulk-like and Bound Water at Positively and Negatively Charged Lipid Interfaces Revealed by 2D HD-VSFG Spectroscopy.

Interfacial water in the vicinity of lipids plays an important role in many biological processes, such as drug delivery, ion transportation, and lipid fusion. Hence, molecular-level elucidation of the properties of water at lipid interfaces is of the utmost importance. We report the two-dimensional heterodyne-detected vibrational sum frequency generation (2D HD-VSFG) study of the OH stretch of HOD at charged lipid interfaces, which shows that the hydrogen bond dynamics of interfacial water differ drastically, depending on the lipids. The data indicate that the spectral diffusion of the OH stretch at a positively charged lipid interface is dominated by the ultrafast (<∼100 fs) component, followed by the minor sub-picosecond slow dynamics, while the dynamics at a negatively charged lipid interface exhibit sub-picosecond dynamics almost exclusively, implying that fast hydrogen bond fluctuation is prohibited. These results reveal that the ultrafast hydrogen bond dynamics at the positively charged lipid-water interface are attributable to the bulk-like property of interfacial water, whereas the slow dynamics at the negatively charged lipid interface are due to bound water, which is hydrogen-bonded to the hydrophilic head group.

2D heterodyne-detected sum frequency generation study on the ultrafast vibrational dynamics of H2O and HOD water at charged interfaces.

Two-dimensional heterodyne-detected vibrational sum-frequency generation (2D HD-VSFG) spectroscopy is applied to study the ultrafast vibrational dynamics of water at positively charged aqueous interfaces, and 2D HD-VSFG spectra of cetyltrimethylammonium bromide (CTAB)/water interfaces in the whole hydrogen-bonded OH stretch region (3000 cm(-1) ≤ ωpump ≤ 3600 cm(-1)) are measured. 2D HD-VSFG spectrum of the CTAB/isotopically diluted water (HOD-D2O) interface exhibits a diagonally elongated bleaching lobe immediately after excitation, which becomes round with a time constant of ∼0.3 ps due to spectral diffusion. In contrast, 2D HD-VSFG spectrum of the CTAB/H2O interface at 0.0 ps clearly shows two diagonal peaks and their cross peaks in the bleaching region, corresponding to the double peaks observed at 3230 cm(-1) and 3420 cm(-1) in the steady-state HD-VSFG spectrum. Horizontal slices of the 2D spectrum show that the relative intensity of the two peaks of the bleaching at the CTAB/H2O interface gradually change with the change of the pump frequency. We simulate the pump-frequency dependence of the bleaching feature using a model that takes account of the Fermi resonance and inhomogeneity of the OH stretch vibration, and the simulated spectra reproduce the essential features of the 2D HD-VSFG spectra of the CTAB/H2O interface. The present study demonstrates that heterodyne detection of the time-resolved VSFG is critically important for studying the ultrafast dynamics of water interfaces and for unveiling the underlying mechanism.

Accurate determination of complex χ(2) spectrum of the air/water interface.

Discussion on the structure of the water surface relies on accurate determination of the χ(2) spectrum. For obtaining accurate χ(2) spectrum of the air/water interface in the OH stretch region, we performed heterodyne-detected vibrational sum-frequency generation measurements with a high phase accuracy, and also examined the validity of the phase and amplitude calibration using different non-resonant materials. In contrast to the previous reports, it was concluded that the imaginary part of the χ(2) spectrum of the air/water interface does not exhibit noticeable positive resonance in the low frequency region within the experimental error. This result urges us to reconsider the structure of the air/water interface based on the accurate χ(2) spectrum.

Counterion effect on interfacial water at charged interfaces and its relevance to the Hofmeister series.

Specific counterion effects represented by Hofmeister series are important for a variety of phenomena such as protein precipitations, surface tensions of electrolytes solutions, phase transitions of surfactants, etc. We applied heterodyne-detected vibrational sum-frequency generation spectroscopy to study the counterion effect on the interfacial water at charged interfaces and discussed the observed effect with relevance to the Hofmeister series. Experiments were carried out for model systems of positively charged cetyltrimethylammonium monolayer/electrolyte solution interface and negatively charged dodecylsulfate monolayer/electrolyte interface. At the positively charged interface, the intensity of the OH band of the interfacial water decreases in the order of the Hofmeister series, suggesting that the adsorbability of halide anions onto the interface determines the Hofmeister order as previously proposed by Zhang and Cremer (Curr. Opin. Chem. Biol. 2006, 10, 658-663). At the negatively charged interfaces, on the other hand, the OH band intensity does not depend significantly on the countercation, whereas variation in the hydrogen-bond strength of the interfacial water is well correlated with the Hofmeister order of the cation effect. These results provide new insights into the molecular level mechanisms of anionic and cationic Hofmeister effects.

Structure and dynamics of interfacial water studied by heterodyne-detected vibrational sum-frequency generation.

Vibrational sum-frequency generation (VSFG) spectroscopy is a powerful tool to study interfaces. Recently, multiplex heterodyne-detected VSFG (HD-VSFG) has been developed, which enables the direct measurement of complex second-order nonlinear susceptibility [χ((2))]. HD-VSFG has remarkable advantages over conventional VSFG. For example, the imaginary part of χ((2)) [Imχ((2))] obtained with this interferometric technique is the direct counterpart to the infrared [Imχ((1))] and Raman [Imχ((3))] spectra in the bulk, and it is free from the spectral deformation inevitable in conventional VSFG [|χ((2))|(2)] spectra. The Imχ((2)) signal is obtained with a sign that contains unambiguous information about the up/down orientation of interfacial molecules. Furthermore, HD-VSFG can be straightforwardly extended to time-resolved measurements when combined with photoexcitation. In this review, we describe the present status of experiments and applications of multiplex HD-VSFG spectroscopy, in particular with regard to the orientation and structure of interfacial water at charged, neutral, and biorelevant water interfaces.

Interfacial water in the vicinity of a positively charged interface studied by steady-state and time-resolved heterodyne-detected vibrational sum frequency generation.

To investigate the properties of water in the close vicinity of a positively charged surfactant/water interface, steady-state and femtosecond time-resolved interfacial vibrational spectra were measured in the presence of excess alkali halide salts. The steady-state Imχ((2)) spectra show a drastic intensity decrease with excess salts, indicating that the thickness of the probed water layer is substantially reduced. Fluoride salts do not noticeably affect spectral features in the OH stretch region whereas the chloride and bromide salts induce significant blue shifts of the OH stretch frequency. Femtosecond time-resolved ΔImχ((2)) spectra obtained with fluoride salts exhibit a very broad bleach even at 0 fs as observed without excess salts, while chloride and bromide salts give rise to a narrow spectral hole burning. These results indicate that the excess chloride and bromide ions strongly interact with interfacial water in the vicinity of the charged interface and it suppresses intramolecular coupling (i.e., Fermi resonance) that causes spectral broadening.

Unified picture of vibrational relaxation of OH stretch at the air/water interface.

The elucidation of the energy dissipation process is crucial for understanding various phenomena occurring in nature. Yet, the vibrational relaxation and its timescale at the water interface, where the hydrogen-bonding network is truncated, are not well understood and are still under debate. In the present study, we focus on the OH stretch of interfacial water at the air/water interface and investigate its vibrational relaxation by femtosecond time-resolved, heterodyne-detected vibrational sum-frequency generation (TR-HD-VSFG) spectroscopy. The temporal change of the vibrationally excited hydrogen-bonded (HB) OH stretch band (ν=1→2 transition) is measured, enabling us to determine reliable vibrational relaxation (T1) time. The T1 times obtained with direct excitations of HB OH stretch are 0.2-0.4 ps, which are similar to the T1 time in bulk water and do not noticeably change with the excitation frequency. It suggests that vibrational relaxation of the interfacial HB OH proceeds predominantly with the intramolecular relaxation mechanism as in the case of bulk water. The delayed rise and following decay of the excited-state HB OH band are observed with excitation of free OH stretch, indicating conversion from excited free OH to excited HB OH (~0.9 ps) followed by relaxation to low-frequency vibrations (~0.3 ps). This study provides a complete set of the T1 time of the interfacial OH stretch and presents a unified picture of its vibrational relaxation at the air/water interface.