Direct observation of photoinduced sequential spin transition in a halogen-bonded hybrid system by complementary ultrafast optical and electron probes.

A detailed understanding of the ultrafast dynamics of halogen-bonded materials is desired for designing supramolecular materials and tuning various electronic properties by external stimuli. Here, a prototypical halogen-bonded multifunctional material containing spin crossover (SCO) cations and paramagnetic radical anions is studied as a model system of photo-switchable SCO hybrid systems using ultrafast electron diffraction and two complementary optical spectroscopic techniques. Our results reveal a sequential dynamics from SCO to radical dimer softening, uncovering a key transient intermediate state. In combination with quantum chemistry calculations, we demonstrate the presence of halogen bonds in the low- and high-temperature phases and propose their role during the photoinduced sequential dynamics, underscoring the significance of exploring ultrafast dynamics. Our research highlights the promising utility of halogen bonds in finely tuning functional properties across diverse photoactive multifunctional materials.

Photoinduced dynamics during electronic transfer from narrow to wide bandgap layers in one-dimensional heterostructured materials.

Electron transfer is a fundamental energy conversion process widely present in synthetic, industrial, and natural systems. Understanding the electron transfer process is important to exploit the uniqueness of the low-dimensional van der Waals (vdW) heterostructures because interlayer electron transfer produces the function of this class of material. Here, we show the occurrence of an electron transfer process in one-dimensional layer-stacking of carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs). This observation makes use of femtosecond broadband optical spectroscopy, ultrafast time-resolved electron diffraction, and first-principles theoretical calculations. These results reveal that near-ultraviolet photoexcitation induces an electron transfer from the conduction bands of CNT to BNNT layers via electronic decay channels. This physical process subsequently generates radial phonons in the one-dimensional vdW heterostructure material. The gathered insights unveil the fundamentals physics of interfacial interactions in low dimensional vdW heterostructures and their photoinduced dynamics, pushing their limits for photoactive multifunctional applications.

A-Site Columnar-Ordered Perovskite CaZnV2O6 as a Pauli-Paramagnetic Metal.

In this study, we successfully synthesized a novel A-site columnar-ordered perovskite CaZnV2O6. This compound features a square-planar-coordinated Zn2+ disorder, which is the same characteristic as the centrosymmetric paraelectric CaMnTi2O6. Unlike CaMnTi2O6, which shows a centrosymmetric paraelectric-noncentrosymmetric ferroelectric transition, CaZnV2O6 retains Pauli-paramagnetic metallicity arising from itinerant V4+ d1 electrons and centrosymmetry down to 5 K. Based on analogous compounds, we expect CaZnV2O6 to provide a new playground for the electronic and magnetic states of V4+.

Nonlinear Optical Properties in an Epitaxial YbFe2O4 Film Probed by Second Harmonic and Terahertz Generation.

An epitaxial film of YbFe2O4, a candidate for oxide electronic ferroelectrics, was fabricated on yttrium-stabilized zirconia (YSZ) substrate by magnetron sputtering technique. For the film, second harmonic generation (SHG), and a terahertz radiation signal were observed at room temperature, confirming a polar structure of the film. The azimuth angle dependence of SHG shows four leaves-like profiles and is almost identical to that in a bulk single crystal. Based on tensor analyses of the SHG profiles, we could reveal the polarization structure and the relationship between the film structure of YbFe2O4 and the crystal axes of the YSZ substrate. The observed terahertz pulse showed anisotropic polarization dependence consistent with the SHG measurement, and the intensity of the emitted terahertz pulse reached about 9.2% of that emitted from ZnTe, a typical nonlinear crystal, implying that YbFe2O4 can be applied as a terahertz wave generator in which the direction of the electric field can be easily switched.

Bi0.5Pb0.5FeO3 with Unusual Pb Charge Disproportionation: Indication of a Systematic Charge Distribution Change in Bi0.5Pb0.5MO3 (M: 3d Transition Metal).

Bi0.5Pb0.5FeO3 with 1:1 mixture of Bi and Pb having charge degrees of freedom at the A-site of perovskite oxide ABO3 is obtained for the first time by high-pressure synthesis. Comprehensive synchrotron X-ray powder diffraction, optical second harmonic generation, Mössbauer spectroscopy, and hard X-ray photoemission spectroscopy measurements revealed that Bi0.5Pb0.5FeO3 is a canted antiferromagnetic insulator crystalizing in a nonpolar tetragonal I4/mcm structure with √2a × âˆš2a × 2a unit cell and has unusually Pb charge disproportionated Bi3+0.5Pb2+0.25Pb4+0.25Fe3+O3 charge distribution. The valence of transition metal M in Bi0.5Pb0.5MO3 changes from 3.5+ to 3+ and finally to 2+ from Mn to Fe and to Ni, from left to right in the periodic table as the 3d-level becomes deeper. The valences of Bi and Pb increase to compensate for the decrease in the M's valence, and Pb changes from 6s2 (2+) to 6s0 (4+) before Bi changes.

Enhanced Spontaneous Polarization by V4+ Substitution in a Lead-Free Perovskite CaMnTi2O6.

Spontaneous polarization (Ps) of novel order-disorder type lead-free ferroelectric CaMnTi2O6 was successfully enhanced by partial V4+ substitution for Ti4+. A synchrotron X-ray diffraction study revealed that the polar displacement of octahedrally coordinated (Ti, V) in CaMn(Ti1-xVx)2O6 (0 ≤ x ≤ 0.4) increases with V4+ substitution having Jahn-Teller activity owing to the d1 electronic configuration. Our magnetic study suggested the presence of antisite disorder between Ca2+ and square planar coordinated Mn2+ associated with Mn-V intermetallic charge transfer for x ≥ 0.4, resulting in decreases in spontaneous polarization and the ferroelectric-paraelectric transition temperature. This is the first report on the enhanced polarization owing to the Jahn-Teller distortion of V4+ without stereochemical Pb2+ or Bi3+.

Robust Giant Tetragonal Distortion Coupled with High-Spin Co3+ in Electron-Doped BiCoO3.

BiCoO3 is a PbTiO3 type of perovskite oxide with a giant tetragonal distortion (c/a = 1.27) that shows a pressure-induced transition from tetragonal to orthorhombic phases accompanied by a large volume shrinkage at 3 GPa. In this study, we carried out electron doping of BiCoO3 by substituting Ti4+ for Co3+ in order to destabilize the tetragonal phase and observe a giant negative thermal expansion (NTE) at ambient pressure. BiCo1-xTixO3 (x = 0, 0.1, 0.2, and 0.25) was successfully obtained by using high-pressure synthesis. However, the c/a ratio of the tetragonal phase was almost constant against x (≤0.2), and NTE was not observed at any x, suggesting that the tetragonal distortion coupled with high-spin Co3+ is robust against electron doping. In x = 0.25, a metastable orthorhombic phase was obtained by the high-pressure synthetic process, while it partially transformed into a tetragonal phase after annealing at 600 K. The stability of the giant tetragonal phase is strongly connected with the spin state of Co3+.

Enhanced Negative Thermal Expansion Induced by Simultaneous Charge Transfer and Polar-Nonpolar Transitions.

Negative thermal expansion (NTE) induced by simultaneous mechanisms, that is, charge transfer and polar-nonpolar transitions, was observed for the first time in BiNi1-xFexO3 (0.25 ≤ x ≤ 0.5). The low-temperature phase was found to have a polar structure (space group of R3c) with a Bi3+0.5(1+x)Bi5+0.5(1-x)Ni2+1-xFe3+xO3 charge distribution and short-range ordering of Bi3+ and Bi5+. The volume reduction upon heating that was induced by charge transfer between Bi5+ and Ni2+ decreased with increasing x because of the reduction in the amount of Ni2+. Simultaneous polar-nonpolar transition also contributed to NTE, and a composition-independent enhanced volume reduction of ∼2% was observed.

Ultrafast isomerization-induced cooperative motions to higher molecular orientation in smectic liquid-crystalline azobenzene molecules.

The photoisomerization of molecules is widely used to control the structure of soft matter in both natural and synthetic systems. However, the structural dynamics of the molecules during isomerization and their subsequent response are difficult to elucidate due to their complex and ultrafast nature. Herein, we describe the ultrafast formation of higher-orientation of liquid-crystalline (LC) azobenzene molecules via linearly polarized ultraviolet light (UV) using ultrafast time-resolved electron diffraction. The ultrafast orientation is caused by the trans-to-cis isomerization of the azobenzene molecules. Our observations are consistent with simplified molecular dynamics calculations that revealed that the molecules are aligned with the laser polarization axis by their cooperative motion after photoisomerization. This insight advances the fundamental chemistry of photoresponsive molecules in soft matter as well as their ultrafast photomechanical applications.

Selective Reduction Mechanism of Graphene Oxide Driven by the Photon Mode versus the Thermal Mode.

A two-dimensional nanocarbon, graphene, has attracted substantial interest due to its excellent properties. The reduction of graphene oxide (GO) has been investigated for the mass production of graphene used in practical applications. Different reduction processes produce different properties in graphene, affecting the performance of the final materials or devices. Therefore, an understanding of the mechanisms of GO reduction is important for controlling the properties of functional two-dimensional systems. Here, we determined the average structure of reduced GO prepared via heating and photoexcitation and clearly distinguished their reduction mechanisms using ultrafast time-resolved electron diffraction, time-resolved infrared vibrational spectroscopy, and time-dependent density functional theory calculations. The oxygen atoms of epoxy groups are selectively removed from the basal plane of GO by photoexcitation (photon mode), in stark contrast to the behavior observed for the thermal reduction of hydroxyl and epoxy groups (thermal mode). The difference originates from the selective excitation of epoxy bonds via an electronic transition due to their antibonding character. This work will enable the preparation of the optimum GO for the intended applications and expands the application scope of two-dimensional systems.

Bond Dissociation Triggering Molecular Disorder in Amorphous H2O.

We developed a system to deposit H2O molecules onto ultrathin silicon nitride substrates in situ using time-resolved transmission electron diffraction apparatus and performed ultrafast time-resolved electron diffraction measurements in the noncrystalline (amorphous) H2O under near-ultraviolet photoexcitation. The observed dynamics directly represent O-H bond dissociation via multiphoton absorption and charge transfer, which trigger ionization and intermolecular disorder in the amorphous H2O. Our results illustrate the intriguing nature of light-matter and matter-matter interactions in H2O molecules.

Bandgap modulation in photoexcited topological insulator Bi2Te3 via atomic displacements.

The atomic and electronic dynamics in the topological insulator (TI) Bi2Te3 under strong photoexcitation were characterized with time-resolved electron diffraction and time-resolved mid-infrared spectroscopy. Three-dimensional TIs characterized as bulk insulators with an electronic conduction surface band have shown a variety of exotic responses in terms of electronic transport when observed under conditions of applied pressure, magnetic field, or circularly polarized light. However, the atomic motions and their correlation between electronic systems in TIs under strong photoexcitation have not been explored. The artificial and transient modification of the electronic structures in TIs via photoinduced atomic motions represents a novel mechanism for providing a comparable level of bandgap control. The results of time-domain crystallography indicate that photoexcitation induces two-step atomic motions: first bismuth and then tellurium center-symmetric displacements. These atomic motions in Bi2Te3 trigger 10% bulk bandgap narrowing, which is consistent with the time-resolved mid-infrared spectroscopy results.

Direct observation of collective modes coupled to molecular orbital-driven charge transfer.

Correlated electron systems can undergo ultrafast photoinduced phase transitions involving concerted transformations of electronic and lattice structure. Understanding these phenomena requires identifying the key structural modes that couple to the electronic states. We report the ultrafast photoresponse of the molecular crystal Me4P[Pt(dmit)2]2, which exhibits a photoinduced charge transfer similar to transitions between thermally accessible states, and demonstrate how femtosecond electron diffraction can be applied to directly observe the associated molecular motions. Even for such a complex system, the key large-amplitude modes can be identified by eye and involve a dimer expansion and a librational mode. The dynamics are consistent with the time-resolved optical study, revealing how the electronic, molecular, and lattice structures together facilitate ultrafast switching of the state.

Diverse photoinduced dynamics in an organic charge-transfer complex having strong electron-phonon interactions.

CONSPECTUS: Phenomena that occur in nonequilibrium states created by photoexcitation differ qualitatively from those that occur at thermal equilibrium, and various physical theories developed for thermal equilibrium states can hardly be applied to such phenomena. Recently it has been realized that understanding phenomena in nonequilibrium states in solids is important for photoenergy usage and ultrafast computing. Consequently, much effort has been devoted to revealing such phenomena by developing various ultrafast observation techniques and theories applicable to nonequilibrium states. This Account describes our recent studies of diverse photoinduced dynamics in a strongly correlated organic solid using various ultrafast techniques. Solids in which the electronic behavior is affected by Coulomb interactions between electrons are designated as strongly correlated materials and are known to exhibit unique physical properties even at thermal equilibrium. Among them, many organic charge-transfer (CT) complexes have low dimensionality and flexibility in addition to strong correlations; thus, their physical properties change sensitively in response to changes in pressure or electric field. Photoexcitation is also expected to drastically change their physical properties and would be useful for ultrafast photoswitching devices. However, in nonequilibrium states, the complicated dynamics due to these characteristics prevents us from understanding and using these materials for photonic devices. The CT complex (EDO-TTF)2PF6 (EDO-TTF = 4,5-ethylenedioxytetrathiafulvalene) exhibits unique photoinduced dynamics due to strong electron-electron and electron-phonon interactions. We have performed detailed studies of the dynamics of this complex using transient electronic spectroscopy at the 10 and 100 fs time scales. These studies include transient vibrational spectroscopy, which is sensitive to the charges and structures of constituent molecules, and transient electron diffraction, which provides direct information on the crystal structure. Photoexcitation of the charge-ordered low-temperature phase of (EDO-TTF)2PF6 creates a new photoinduced phase over 40 fs via the Franck-Condon state, in which electrons and vibrations are coherently and strongly coupled. This new photoinduced phase is assigned to an insulator-like state in which the charge order differs from that of the initial state. In the photoinduced phase, translations of component molecules proceed before the rearrangements of intramolecular conformations. Subsequently, the charge order and structure gradually approach those of the high-temperature phase over 100 ps. This unusual two-step photoinduced phase transition presumably originates from steric effects due to the bent EDO-TTF as well as strong electron-lattice interactions.

Femtosecond time-resolved photoemission electron microscopy for spatiotemporal imaging of photogenerated carrier dynamics in semiconductors.

We constructed an instrument for time-resolved photoemission electron microscopy (TR-PEEM) utilizing femtosecond (fs) laser pulses to visualize the dynamics of photogenerated electrons in semiconductors on ultrasmall and ultrafast scales. The spatial distribution of the excited electrons and their relaxation and/or recombination processes were imaged by the proposed TR-PEEM method with a spatial resolution about 100 nm and an ultrafast temporal resolution defined by the cross-correlation of the fs laser pulses (240 fs). A direct observation of the dynamical behavior of electrons on higher resistivity samples, such as semiconductors, by TR-PEEM has still been facing difficulties because of space and/or sample charging effects originating from the high photon flux of the ultrashort pulsed laser utilized for the photoemission process. Here, a regenerative amplified fs laser with a widely tunable repetition rate has been utilized, and with careful optimization of laser parameters, such as fluence and repetition rate, and consideration for carrier lifetimes, the electron dynamics in semiconductors were visualized. For demonstrating our newly developed TR-PEEM method, the photogenerated carrier lifetimes around a nanoscale defect on a GaAs surface were observed. The obtained lifetimes were on a sub-picosecond time scale, which is much shorter than the lifetimes of carriers observed in the non-defective surrounding regions. Our findings are consistent with the fact that structural defects induce mid-gap states in the forbidden band, and that the electrons captured in these states promptly relax into the ground state.

Crystal melting by light: X-ray crystal structure analysis of an azo crystal showing photoinduced crystal-melt transition.

Trans-cis photoisomerization in an azo compound containing azobenzene chromophores and long alkyl chains leads to a photoinduced crystal-melt transition (PCMT). X-ray structure analysis of this crystal clarifies the characteristic coexistence of the structurally ordered chromophores through their π···π interactions and disordered alkyl chains around room temperature. These structural features reveal that the PCMT starts near the surface of the crystal and propagates into the depth, sacrificing the π···π interactions. A temporal change of the powder X-ray diffraction pattern under light irradiation and a two-component phase diagram allow qualitative analysis of the PCMT and the following reconstructive crystallization of the cis isomer as a function of product accumulation. This is the first structural characterization of a compound showing the PCMT, overcoming the low periodicity that makes X-ray crystal structure analysis difficult.

Infrared vibrational spectroscopy of [Ru(bpy)2(bpm)]2+ and [Ru(bpy)3]2+ in the excited triplet state.

This work involved a detailed investigation into the infrared vibrational spectra of ruthenium polypyridyl complexes, specifically heteroleptic [Ru(bpy)2(bpm)](2+) (bpy = 2,2'-bipyridine and bpm = 2,2'-bipyrimidine) and homoleptic [Ru(bpy)3](2+), in the excited triplet state. Transient spectra were acquired 500 ps after photoexcitation, corresponding to the vibrational ground state of the excited triplet state, using time-resolved infrared spectroscopy. We assigned the observed bands to specific ligands in [Ru(bpy)2(bpm)](2+) based on the results of deuterium substitution and identified the corresponding normal vibrational modes using quantum-chemical calculations. Through this process, the more complex vibrational bands of [Ru(bpy)3](2+) were assigned to normal vibrational modes. The results are in good agreement with the model in which excited electrons are localized on a single ligand. We also found that the vibrational bands of both complexes associated with the ligands on which electrons are little localized appear at approximately 1317 and 1608 cm(-1). These assignments should allow the study of the reaction dynamics of various photofunctional systems including ruthenium polypyridyl complexes.

Mapping molecular motions leading to charge delocalization with ultrabright electrons.

Ultrafast processes can now be studied with the combined atomic spatial resolution of diffraction methods and the temporal resolution of femtosecond optical spectroscopy by using femtosecond pulses of electrons or hard X-rays as structural probes. However, it is challenging to apply these methods to organic materials, which have weak scattering centres, thermal lability, and poor heat conduction. These characteristics mean that the source needs to be extremely bright to enable us to obtain high-quality diffraction data before cumulative heating effects from the laser excitation either degrade the sample or mask the structural dynamics. Here we show that a recently developed, ultrabright femtosecond electron source makes it possible to monitor the molecular motions in the organic salt (EDO-TTF)2PF6 as it undergoes its photo-induced insulator-to-metal phase transition. After the ultrafast laser excitation, we record time-delayed diffraction patterns that allow us to identify hundreds of Bragg reflections with which to map the structural evolution of the system. The data and supporting model calculations indicate the formation of a transient intermediate structure in the early stage of charge delocalization (less than five picoseconds), and reveal that the molecular motions driving its formation are distinct from those that, assisted by thermal relaxation, convert the system into a metallic state on the hundred-picosecond timescale. These findings establish the potential of ultrabright femtosecond electron sources for probing the primary processes governing structural dynamics with atomic resolution in labile systems relevant to chemistry and biology.

Structural transitions from triangular to square molecular arrangements in the quasi-one-dimensional molecular conductors (DMEDO-TTF)2XF6 (X = P, As, and Sb).

A series of quasi-one-dimensional molecular conductors (DMEDO-TTF)(2)XF(6) (X = P, As, and Sb), where DMEDO-TTF is dimethyl(ethylenedioxy)tetrathiafulvalene, undergo characteristic structural transitions in the range of 130-195 K for the PF(6) salt and 222-242 K for the AsF(6) salt. The dramatic structural transition is induced by the order of the ethylenedioxy moiety, and the resulting anion rotation leads to the reconstruction of the H···F interaction between the methyl groups and the anions. The unique hydrogen bonds play a crucial role in the transition. As a result, the molecular packing is rearranged entirely; the high-temperature molecular stacks with an ordinary quasi-triangular molecular network transforms to a quasi-square-like network, which has never been observed among organic conductors. Nonetheless, the low-temperature phase exhibits a good metallic conductivity as well, so the transition is a metal-metal (MM) transition. The resistivity measured along the perpendicular direction to the conducting ac-plane (ρ(⊥)) and the calculation of the Fermi surface demonstrate that the high-temperature metal phase is a one-dimensional metal, whereas the low-temperature metal phase has considerable interchain interaction. In the SbF(6) salt, a similar structural transition takes place around 370 K, so that the quasi-square-like lattice is realized even at room temperature. Despite the largely different MM transition temperatures, all these salts undergo metal-insulator (MI) transitions approximately at the same temperature of 50 K. The low-temperature insulator phase is nonmagnetic, and the reflectance spectra suggest the presence of charge disproportionation with small charge difference (0.14).

Ligand migration in myoglobin: a combined study of computer simulation and x-ray crystallography.

A ligand-migration mechanism of myoglobin was studied by a multidisciplinary approach that used x-ray crystallography and molecular dynamics simulation. The former revealed the structural changes of the protein along with the ligand migration, and the latter provided the statistical ensemble of protein conformations around the thermal average. We developed a novel computational method, homogeneous ensemble displacement, and generated the conformational ensemble of ligand-detached species from that of ligand-bound species. The thermally averaged ligand-protein interaction was illustrated in terms of the potential of mean force. Although the structural changes were small, the presence of the ligand molecule in the protein matrix significantly affected the 3D scalar field of the potential of mean force, in accordance with the self-opening model proposed in the previous x-ray study.