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.

Spectroscopic mapping of the gold complex oligomers (dimer, trimer, tetramer, and pentamer) by excited-state coherent nuclear wavepacket motion in aqueous solutions.

We investigate the excited-state dynamics of the [Au(CN)2-] oligomers following photo-initiated intermolecular Au-Au bond formation by carrying out femtosecond time-resolved absorption and emission measurements at various concentrations (0.080-0.6 mol dm-3) with different photoexcitation wavelengths (290-340 nm). The temporal profiles of the time-resolved absorption signals exhibit clear oscillations arising from the Au-Au stretch coherent wavepacket motion of the excited-state oligomers, which is initiated with the photo-induced Au-Au bond formation. The frequency of the observed oscillation is changed with the change of the concentration, excitation wavelength, and wavelength of the excited-state absorption monitored, reflecting the change in the size of the oligomers detected. Fourier transforms (FTs) of the oscillations provide 2D plots of the FT amplitude against the oscillation frequency versus the detected wavelengths. Because the FT amplitude exhibits a node at the peak wavelength of the absorption of the species that gives rise to the oscillation, the 2D plots enabled us to determine the peak wavelength of the excited-state absorption of the dimer, trimer, tetramer, and pentamer. We also performed femtosecond time-resolved absorption measurements for the 0.3 mol dm-3 solution with 260 nm photoexcitation, which is the condition employed in previous time-resolved X-ray studies (e.g., K. H. Kim et al. Nature, 2015, 518 (7539), 385-389). It was found that various excited-state oligomers, including the dimer, were simultaneously generated under this condition, although the analysis of the previous time-resolved X-ray studies was made by assuming that only the excited-state trimer was generated. The obtained results show that the excited-state dynamics of the trimer claimed based on the time-resolved X-ray data is questionable and that re-analysis and re-examining of its data are necessary.

Complex phase of the nonresonant background in sum frequency generation spectroscopy.

Sum frequency generation (SFG) spectroscopy is an interface-selective spectroscopic technique that enables us to selectively observe the vibrational or electronic resonances of molecules within a very thin interface layer. The interfacial properties probed by SFG are contained in a complex quantity called the second-order nonlinear susceptibility (χ2). It is usually believed that the imaginary part of χ2 (Im χ2) exhibits the resonant responses of the system, whereas the nonresonant responses appear solely in the real part of χ2 (Re χ2). However, it was recently theoretically pointed out that a portion of the nonresonant responses actually contributes to the observed Im χ2 spectra when the finite thickness of the interface layer is taken into account. In this study, by considering a simple air/liquid interface without any solutes as a model system, we theoretically evaluate the nonresonant contribution to experimentally accessible Im χ2 as well as to Re χ2, from which the complex phase of the nonresonant background is estimated. It is shown that the deviation of the complex phase from 0° or 180° is less than 1° even if the thickness of the interface layer is taken into account. This means that the nonresonant contribution to Im χ2 is practically negligible, and it is a very good approximation to think that the nonresonant background appears solely in Re χ2 in the case of air/liquid interfaces. This result implies that Im χ2 practically contains only the resonant responses of the system, and molecular resonances at the interface can be conveniently studied using Im χ2 spectra at such interfaces.

Why the Photochemical Reaction of Phenol Becomes Ultrafast at the Air-Water Interface: The Effect of Surface Hydration.

Photochemical reactions at the air-water interface can show remarkably different rates from those in bulk water. The present study elucidates the reaction mechanism of phenol characteristic at the air-water interface by the combination of molecular dynamics simulation and quantum chemical calculations of the excited states. We found that incomplete hydrogen bonding to phenol at the air-water interface affects excited states associated with the conical intersection and significantly reduces the reaction barrier, resulting in the distinctively facilitated rate in comparison with the bulk phase. The present study indicates that the reaction dynamics can be substantially different at the interfaces in general, reflecting the difference in the stabilization energy of the electronic states in markedly different solvation at the interface.

General Design Strategy to Precisely Control the Emission of Fluorophores via a Twisted Intramolecular Charge Transfer (TICT) Process.

Fluorogenic probes for bioimaging have become essential tools for life science and medicine, and the key to their development is a precise understanding of the mechanisms available for fluorescence off/on control, such as photoinduced electron transfer (PeT) and Förster resonance energy transfer (FRET). Here we establish a new molecular design strategy to rationally develop activatable fluorescent probes, which exhibit a fluorescence off/on change in response to target biomolecules, by controlling the twisted intramolecular charge transfer (TICT) process. This approach was developed on the basis of a thorough investigation of the fluorescence quenching mechanism of N-phenyl rhodamine dyes (commercially available as the QSY series) by means of time-dependent density functional theory (TD-DFT) calculations and photophysical evaluation of their derivatives. To illustrate and validate this TICT-based design strategy, we employed it to develop practical fluorogenic probes for HaloTag and SNAP-tag. We further show that the TICT-controlled fluorescence off/on mechanism is generalizable by synthesizing a Si-rhodamine-based fluorogenic probe for HaloTag, thus providing a palette of chemical dyes that spans the visible and near-infrared range.

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.

A Unified View on Varied Ultrafast Dynamics of the Primary Process in Microbial Rhodopsins.

All-trans to 13-cis photoisomerization of the protonated retinal Schiff base (PRSB) chromophore is the primary step that triggers various biological functions of microbial rhodopsins. While this ultrafast primary process has been extensively studied, it has been recognized that the relevant excited-state relaxation dynamics differ significantly from one rhodopsin to another. To elucidate the origin of the complicated ultrafast dynamics of the primary process in microbial rhodopsins, we studied the excited-state dynamics of proteorhodopsin, its D97N mutant, and bacteriorhodopsin by femtosecond time-resolved absorption (TA) spectroscopy in a wide pH range. The TA data showed that their excited-state relaxation dynamics drastically change when pH approaches the pKa of the counterion residue of the PRSB chromophore in the ground state. This result reveals that the varied excited-state relaxation dynamics in different rhodopsins mainly originate from the difference of the ground-state heterogeneity (i.e., protonation/deprotonation of the PRSB counterion).

Tracking Ultrafast Structural Dynamics by Time-Domain Raman Spectroscopy.

In traditional Raman spectroscopy, narrow-band light is irradiated on a sample, and its inelastic scattering, i.e., Raman scattering, is detected. The energy difference between the Raman scattering and the incident light corresponds to the vibrational energy of the molecule, providing the Raman spectrum that contains rich information about the molecular-level properties of the materials. On the other hand, by using ultrashort optical pulses, it is possible to induce Raman-active coherent nuclear motion of the molecule and to observe the molecular vibration in real time. Moreover, this time-domain Raman measurement can be combined with femtosecond photoexcitation, triggering chemical changes, which enables tracking ultrafast structural dynamics in a form of "time-resolved" time-domain Raman spectroscopy, also known as time-resolved impulsive stimulated Raman spectroscopy. With the advent of stable, ultrashort laser pulse sources, time-resolved impulsive stimulated Raman spectroscopy now realizes high sensitivity and a wide detection frequency window from THz to 3000 cm-1, and has seen success in unveiling the molecular mechanisms underlying the efficient functions of complex molecular systems. In this Perspective, we overview the present status of time-domain Raman spectroscopy, particularly focusing on its application to the study of femtosecond structural dynamics. We first explain the principle and a brief history of time-domain Raman spectroscopy and then describe the apparatus and recent applications to the femtosecond dynamics of complex molecular systems, including proteins, molecular assemblies, and functional materials. We also discuss future directions for time-domain Raman spectroscopy, which has reached a status allowing a wide range of applications.

Microsecond Folding of preQ1 Riboswitch and Its Biological Significance Revealed by Two-Dimensional Fluorescence Lifetime Correlation Spectroscopy.

Riboswitches are regulatory elements of bacterial mRNA which function with conformational switching upon binding of specific cellular metabolites. In particular, transcriptional riboswitches regulate gene expression kinetically through the conformational change of the aptamer domain. In this study, we investigate the conformational dynamics and ligand binding mechanisms of the aptamer domain of a transcriptional prequeuosine (preQ1) riboswitch from Bacillus subtilis using two-dimensional fluorescence lifetime correlation spectroscopy (2D FLCS) with microsecond time resolution. The obtained time-resolved single-molecule data indicate that the aptamer domain undergoes folding/unfolding including three forms, which are attributed to hairpin (O), pseudoknot-like (pF), and H-type pseudoknot (fF) structures. It is found that a cofactor, Mg2+, binds only to the fF form with the conformational selection mechanism. In contrast, it is indicated that the ligand, preQ1, binds to the O form with the induced-fit mechanism and significantly accelerates the microsecond O → pF folding process. It is also shown that the binding with preQ1 substantially stabilizes the fF form that is generated from the pF form with a long time constant (>10 ms). Combining these results with the results of a former smFRET study on the slower time scale, we obtain an overall picture of the folding/unfolding dynamics of the aptamer domain as well as its energy landscape. On the basis of the picture obtained, we discuss the significance of the microsecond folding/unfolding of the aptamer domain for biological function of the riboswitch and propose the molecular mechanism of the gene expression controlled by the structural dynamics of the aptamer domain.

Mode-Specific Vibrational Analysis of Exciton Delocalization and Structural Dynamics in Conjugated Oligomers.

Exciton delocalization in organic semiconducting polymers, affected by structures at a molecular level, plays a crucial role in modulating relaxation pathways, such as charge generation and singlet fission, which can boost device efficiency. However, the structural diversity of polymers and broad signals from typical electronic spectroscopy have their limits when it comes to revealing the interplay between local structures and exciton delocalization. To tackle these problems, we apply femtosecond stimulated Raman spectroscopy in archetypical conjugated oligothiophenes with different chain lengths. We observed Raman frequency dispersions of symmetric bond stretching modes and mode-specific kinetics in the S1 Raman spectra, which underpins the subtle and complex interplay between exciton delocalization and bond length alternation along the conjugation coordinate. Our results provide a more general picture of exciton delocalization in the context of molecular structures for conjugated materials.

Controlling the S1 Energy Profile by Tuning Excited-State Aromaticity.

The shape of the lowest singlet excited-state (S1) energy profile is of primary importance in photochemistry and related materials science areas. Here we demonstrate a new approach for controlling the shape of the S1 energy profile which relies on tuning the level of excited-state aromaticity (ESA). In a series of fluorescent π-expanded oxepins, the energy decrease accompanying the bent-to-planar conformational change in S1 becomes less pronounced with lower ESA levels. Stabilization energies following from ESA were quantitatively estimated to be 10-20 kcal/mol using photophysical data. Very fast planarization dynamics in S1 was revealed by time-resolved fluorescence spectroscopy. The time constants were estimated to be shorter than 1 ps, regardless of molecular size and level of ESA, indicating barrierless S1 planarization within the oxepin series.

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.

Coherent Vibration and Femtosecond Dynamics of the Platinum Complex Oligomers upon Intermolecular Bond Formation in the Excited State.

Femtosecond time-resolved absorption and picosecond time-resolved emission measurements were carried out for highly concentrated aqueous solutions of K2 [Pt(CN)4 ] to investigate excited-state dynamics of the [Pt(CN)4 2- ] oligomers formed with metallophilic interactions. Time-resolved absorption spectra exhibit complicated dynamics that are represented with five time constants. Among them, the 90-ps and 400-ps dynamics were assigned to the S1 → T1 intersystem crossing of the trimer and tetramer coexisting in the solution by comparison with the fluorescence decays. Clear oscillations of transient absorption were observed in the first few picoseconds, and the frequency-detected-wavelength 2D analysis revealed that the 135-cm-1 and 65-cm-1 oscillations arise from the Pt-Pt stretch motions of the S1 trimer and S1 tetramer, respectively. The obtained time-resolved spectroscopic data provide a clear view of the excited-state dynamics of the [Pt(CN)4 2- ] oligomers in the femto-/picosecond time region.

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.

Femtosecond Polarization Switching in the Crystal of a [CrCo] Dinuclear Complex.

Capability to control macroscopic molecular properties with external stimuli offers the possibility to exploit molecules as switching devices of various types. However, application of such molecular-level switching has often been limited by its speed and thus efficiency. Herein, we demonstrate ultrafast, photoinduced polarization switching in the crystal of a [CrCo] dinuclear complex by ultrafast pump-probe spectroscopy in the visible and mid-infrared regions. The photoinduced polarization switching was found to have a time constant of 280 fs, which makes the [CrCo] complex crystal the fastest polarization-switching material realized using the metastable state. Moreover, the pump-probe data in the visible region reveal the pronounced appearance of coherent nuclear wavepacket motion with a frequency as low as 22 cm-1 , which we attribute to a lattice vibrational mode. The pronounced non-Condon effect for its resonance Raman enhancement implies that this mode couples the relevant electronic states, thereby facilitating the ultrafast polarization switching.

Flapping Peryleneimide as a Fluorogenic Dye with High Photostability and Strong Visible-Light Absorption.

Flapping fluorophores (FLAP) with a flexible 8π ring are rapidly gaining attention as a versatile photofunctional system. Here we report a highly photostable "flapping peryleneimide" with an unprecedented fluorogenic mechanism based on a bent-to-planar conformational change in the S1 excited state. The S1 planarization induces an electronic configurational switch, almost quenching the inherent fluorescence (FL) of the peryleneimide moieties. However, the FL quantum yield is remarkably improved with a prolonged lifetime upon a slight environmental change. This fluorogenic function is realized by sensitive π-conjugation design, as a more π-expanded analogue does not show the planarization dynamics. With strong visible-light absorption, the FL lifetime response synchronized with the flexible flapping motion is useful for the latest optical techniques such as FL lifetime imaging microscopy (FLIM).

Tracking Photoinduced Au-Au Bond Formation through Transient Terahertz Vibrations Observed by Femtosecond Time-Domain Raman Spectroscopy.

Real-time observation of chemical bond formation and subsequent nuclear rearrangements is an ultimate goal of chemical science. Yet, such attempts have been hampered by the technical difficulty of triggering bond formation at well-defined, desired timing. The trimer of dicyanoaurate complex ([Au(CN)2-]3) is an ideal system for achieving this aim because the tight covalent Au-Au bonds are formed upon photoexcitation. Despite the apparent simplicity of the system, however, recent time-resolved studies failed to construct a consistent picture of its ultrafast dynamics. Here, we report femtosecond time-domain Raman tracking of ultrafast structural dynamics of the [Au(CN)2-] trimer upon photoinduced Au-Au bond formation. The obtained Raman data reveal that the Au-Au breathing vibration at ∼90 cm-1 exhibits a gradual frequency upshift in a few picoseconds, demonstrating a continuous bent-to-linear structural change on the triplet-state potential energy surface upon the Au-Au bond formation. The comprehensive ultrafast spectroscopic study settles the controversy on this prototypical molecular assembly.

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.