Research

Ultrafast Interfacial Charge Transfer Initiates Mechanical Stress and Heat Transport at the Au–TiO2 Interface 

Interfacial charge transfer plays a critical role in determining the energy transport properties of heterojunction systems, but direct experimental observation of the coupled structural and thermal dynamics has been lacking. Professor Ihee and his research group employed femtosecond ultrafast electron diffraction (fs-UED) to capture the atomic-scale dynamics occurring at the Au/TiO2 interface upon 800-nm photoexcitation. The analysis uncovered rapid lattice expansion in Au and simultaneous lattice contraction in TiO2, followed by the emergence of anticorrelated coherent vibrational motions. These results demonstrate that photoinduced stress and heat can propagate across the interface through enhanced phonon coupling, revealing a transient, mechanically driven pathway for interfacial energy transport in metal–semiconductor systems.

Capturing the generation and structural transformations of molecular ions 

Molecular ions are central to many chemical and biological reactions, yet their structural evolution is difficult to observe directly. Professor Ihee and his team have achieved real-time visualization of the ionization and subsequent structural transformations of 1,3-dibromopropane (DBP) using megaelectronvolt ultrafast electron diffraction (MeV-UED). By combining [2+1] resonance-enhanced multiphoton ionization with MeV-UED at SLAC, they captured a rare induction period in which the DBP+ ion remains in a structurally indistinguishable “dark state” for 3.6 ps. Following this delay, the ion undergoes sequential transformations: first forming a four-membered iso-DBP+ intermediate with a loosely bound bromine atom, and subsequently rearranging into a bromonium ion featuring a characteristic three-membered ring. These dynamics were confirmed through diffraction data analysis, surface-hopping simulations, and ab initio calculations, providing a new window into gas-phase ion chemistry.

Serial X-ray liquidography: multi-dimensional assay framework for exploring biomolecular structural dynamics with microgram quantities 

We developed serial X-ray liquidography (SXL), a powerful method for probing biomolecular structural dynamics with only microgram-scale sample quantities. Traditional time-resolved X-ray liquidography (TRXL) has been limited to reversible photoreactions of photoactive proteins because of high sample demands and inflexible setups. SXL overcomes these barriers by using a chip with serially arranged microchambers, enabling efficient sample utilization and extending time-resolved studies to both reversible and irreversible reactions, even for non-photoactive proteins.

We successfully demonstrated the versatility of SXL in three representative systems. Specifically, we tracked the irreversible dissociation of the plant photoreceptor AtUVR8 into monomers upon UV-B exposure, observed the reversible conformational dynamics of the AsLOV2 domain, and captured pH-induced structural changes in lysozyme triggered by a photocaged proton. Compared with conventional TRXL, SXL achieves nearly a 100-fold improvement in sample efficiency while maintaining high temporal and structural resolution. This innovation establishes SXL as a flexible, multi-dimensional assay platform, paving the way to explore biomolecular dynamics that were previously inaccessible.

Structural dynamics of protein-protein association in the light-induced transition of Avena sativa LOV2 protein 

We investigated the structural dynamics of the light-oxygen-voltage-sensing domain (LOV2) from Avena sativa phototropin 1, a key photosensory protein that converts blue-light signals into biological responses. While time-resolved spectroscopy has previously revealed the electronic transitions of LOV-domain chromophores, the protein-level mechanism underlying light-induced dimerization had remained unclear.

Using time-resolved X-ray liquidography (TRXL), we captured transient intermediates of LOV2 during its photocycle. They discovered that unfolding of the A’α and Jα helices on the microsecond timescale exposes the β-scaffold region, enabling protein–protein interactions that drive dimerization. Within milliseconds, these interactions stabilize a dimeric interface between the β-scaffolds of two light-activated subunits. Complementary molecular dynamics simulations and mutational studies confirmed the essential roles of helix unfolding and electrostatic interactions in mediating this association. This work provides the first direct structural insights into how local conformational changes trigger global dimerization in LOV proteins, clarifying the molecular basis of light-induced oligomerization and suggesting a general mechanism by which photoreceptors regulate downstream signaling through controlled protein association.

Cerium photocatalyst in action: structural dynamics resolved with time-resolved X-ray liquidography 

Photocatalysts are molecules that use light to drive chemical reactions, but until now, their structural dynamics had not been directly elucidated under substrate conditions—the real environment where reactions occur. Professor Ihee’s research team achieved this breakthrough using time-resolved X-ray liquidography (TRXL), a state-of-the-art ultrafast X-ray technique. They tracked the behavior of the cerium-based photocatalyst [Ce(III)Cl6]3− after photoexcitation and compared its dynamics with and without substrate present. Their observations, spanning femtoseconds to microseconds, revealed how Ce–Cl bonds contract and how the system evolves to the oxidized form ([Ce(IV)Cl6]2−). Importantly, these measurements not only uncovered distinct differences between the two cases but also demonstrated how substrate presence reshapes the photocatalytic reaction pathway. The study represents a first demonstration of capturing a photocatalyst’s structural dynamics “in action” with its substrate in solution, marking an important step toward understanding photocatalytic mechanisms in realistic conditions. This insight opens the door to designing more efficient photocatalysts for sustainable energy and green chemistry, providing a foundation for tailoring light-driven reactions at the atomic scale. 

Light-induced protein structural dynamics in bacterio- phytochrome revealed by time-resolved x-ray solution scattering

Professor Ihee and his research group elucidated the structural dynamics of bacteriophytochrome, a type of photosensitive protein, using time-resolved X-ray liquidography (TRXL). In the TRXL experiment, the photoresponse of the protein was initiated by light and sequent structural changes were monitored by time-resolved X-ray scattering signals. We extracted the structural information of intermediates occuring in the process of the photoreaction by utilizing a structural modeling technique aided by molecular dynamics simulations. In this study, we confirmed that the subunits of bacteriophytochrome are arranged in a linear and horizontal conformation in the ground state. Upon the exposure to infrared light, these subunits gradually undergo conformational changes, eventually forming an “O”-shaped structure. Based on the structural dynamics information of bacteriophytochrome, we could explain why the activity of bacteriophytochrome increases after receiving infrared light compared to its pre-exposure state.

Determining the charge distribution and the direction of bond cleavage with femtosecond anisotropic x-ray liquidography

Professor Ihee and his research group revealed the charge distribution within molecules in solution using femtosecond time-resolved X-ray liquidography (fs-TRXL) with X-ray free electron lasers (XFELs). In this work, we analyzed the X-ray scattering signal arising from the correlation between solute and solvent molecules in order to identify the arrangement of the solvent molecules around the solute. Based on this information, we determined the charge distribution within the solute molecule with the aid of molecular dynamics simulations. Furthermore, we determined the correlation between the charge distribution and the dissociation properties of the molecules. We investigated the charge distribution and reaction dynamics of a triiodide ion (I3-). Using fs-TRXL, we determined the charge distribution of I3- in solution and elucidated the direction of bond cleavage upon the photoexcitation, thereby elucidating the relationship between the charge distribution and corresponding reaction pathway.

Ultrafast coherent motion and helix rearrangement of homodimeric hemoglobin visualized with femtosecond X-ray solution scattering

The movement of molecules occurring in the ultrafast time domain is one of the significant factors determining the reaction pathways of molecules. However, studies on the ultrafast molecular motions have predominantly focused on small molecules, and research on macromolecules such as proteins remains rare due to experimental difficulties and challenges in analyzing the structure of these large molecules. Professor Ihee and his research group succeeded in revealing ultrafast structural change of homodimeric hemoglobin (HbI) protein using femtosecond time-resolved X-ray liquidography (fs-TRXL). In this study, we conducted research on the movement of proteins in the ultrafast time domain. As a result, we successfully elucidated the specific movements of Hb. We observed the coherent motion of HbI involved in the protein quake process as well as the rearrangement of helices. Furthermore, by utilizing the information in the small-angle X-ray scattering region, we successfully unveiled not only the protein but also the temporal variations in the hydration shell.

Optical Kerr Effect of Liquid Acetonitrile Probed by Femtosecond Time-Resolved X-ray Liquidography

Professor Ihee and his research group utilized femtosecond time-resolved X-ray liquidography (fs-TRXL) to study the ultrafast dynamics of molecular liquids, Optical Kerr effect (OKE). With fs-TRXL, we monitored the microscopic structural motions related to the OKE response. By applying fs-TRXL to acetonitrile and a dye solution in acetonitrile, different types of molecular motions around photoaligned molecules could be resolved selectively, based on the anisotropy of two-dimensional scattering patterns as well as the additional structural information manifested in the scattering data. As a result, different types of reorientational (libration and orientational diffusion) and translational (interaction-induced motion) motions are captured separately through anisotropic and isotropic scattering signals, respectively. Excellent agreement between the measured and simulated scattering signals obtained from molecular dynamics simulations, provides a detailed structural description of the OKE response in liquid acetonitrile, without relying on theoretical modeling.

Filming ultrafast roaming-mediated isomerization of bismuth triiodide in solution 

Isomerization and dissociation processes of molecules play a crucial role in chemical reactions. Despite numerous studies on the products and reaction kinetics of these chemical reactions, direct observation of the molecular motion involved in these reactions has been exceedingly rare. Professor Ihee and his research group investigated the photoisomerization and photodissociation reactions of bismuth triiodide (BiI3) using femtosecond X-ray liquidography (fs-TRXL) to directly observe the molecular motion associated with the recently proposed roaming reaction, which is one of the major mechanisms driving the photoisomerization. With the operation of PAL-XFEL in Pohang, the temporal resolution was significantly improved, enabling us to successfully obtain the experimental data arising from the ultrafast structural change during the roaming reaction. The analysis on the data revealed important chemical processes accompanied in the photoisomerization such as bond formation, dissociation, and motion of the dissociated iodine atom.

Mapping the emergence of molecular vibrations mediating bond formation

Professor Ihee and his research group achieved the visualization of wavepacket motions, the fully mapped time-resolved all-nuclear motions, in a gold trimer complex, [Au(CN)2–]3, with atomic resolution using femtosecond X-ray liquidography at an X-ray free electron laser (XFEL). This groundbreaking research surpassed prior achievements, the direct observation of chemical bond formation in chemical reactions in solution phase, and served as a remarkable demonstration of the direct tracking of the atomic motions via femtosecond X-ray liquidography.

Direct observation of bond formation in solution with femtosecond X-ray liquidography 

Professor Ihee and his research group visualized the real-time progress of bond formation in a gold trimer complex, [Au(CN)2–]3, with atomic resolution using femtosecond X-ray liquidography at an X-ray free electron laser (XFEL). The making and breaking of chemical bonds are essential processes in chemical reactions, and the ultrafast dynamics of bond breaking processes have been well studied using time-resolved techniques. However, it is extremely difficult to study the dynamics of bond making because the bond making is a bimolecular process and thus is limited by slow diffusion in solution phase. This work was the first example of directly observing the structural dynamics of bond formation process in solution phase and demonstrated the full power of femtosecond X-ray liquidography equipped with femtosecond time resolution and atomic-level structural resolution. 

Volume-conserving trans-cis isomerization pathways in photoactive yellow protein visualized by picosecond X-ray crystallography  

Professor Ihee and his research group revealed the atomic-level structural changes involved in the signaling process of photoactive yellow protein using time-resolved X-ray crystallography. From time-resolved X-ray crystallography measurement of the isomerization process in photoactive yellow protein, the Ihee group revealed that the early stage of the isomerization on the nanosecond time scale proceeds via bifurcated pathways with different chromophore structures. This result was highly valued because it provided the atomic-level structural change of the chromophore, which cannot be probed by time-resolved spectroscopy, with the progress of the protein structural transition.

Protein structural dynamics in solution revealed by pump-probe X-ray protein liquidography and time-resolved X-ray Laue crystallography; Hemoglobin, Myoglobin, and Photoactive yellow protein 

Professor Ihee demonstrates tracking of protein structural changes with pump-probe X-ray protein liquidography with picosecond time resolution. We investigated the tertiary and quaternary conformational changes of hemoglobin and myoglobin under nearly physiological conditions triggered by laser-induced ligand photolysis. By providing insights into the structural dynamics of proteins functioning in their natural environment, pump-probe X-ray protein liquidography complements and extends results obtained with time-resolved optical spectroscopy and X-ray Laue crystallography. We also identify a complex chemical mechanism and all atomic structures of five distinct structural intermediates of the blue light photoreceptor photoactive yellow protein (PYP) using time-resolved X-ray Laue crystallography.

Revealing an unknown reaction pathway of photocatalyst; Ru3(CO)12 

TRXL was used to probe the photolysis of Ru3(CO)12 in cyclohexane, and a new intermediate (Ru3(CO)10 with terminal CO only) was identified besides the two μ-CO intermediates known from ultrafast IR spectroscopy. Three intermediates, Ru3(CO)10 with terminal CO only, Ru3(CO)11(μ-CO) and Ru3(CO)10(μ-CO), undergo different photodissociation pathways at 260 nm and 390 nm.

The reaction mechanism of metathesis probed by time-dependent fluorescence quenching; Metal-based catalysts 

Quantitative catalyst-substrate association relationships of diverse metathesis Mo and Ru catalysts to their substrates are determined directly by a general method based on FRET principle. The determined substrate preferences exhibit the order of alkyne > alkene > allene for Schrock or Schrock-Hoveyda Mo catalysts, allene > alkene > alkyne for Grubbs or Crubbs-Hoveyda 1st generation Ru catalysts, and alkyne > allene > alkene for Grubbs or Grubbs-Hoveyda 2nd generation Ru catalysts.

Determining molecular structures of reaction intermediates in solution: Capturing the bridged structure of C2H4I 

Professor Ihee and his research group have used time-resolved X-ray Liquidography (TRXL) to identify the intermediates of the elimination reaction of haloethane (C2H4I2). Upon irradiation at 267 nm, haloethane dissociates into a haloethyl radical and an iodine atom. Of particular interest is the molecular structure of the haloethyl radical because a bridged structure rather than open classical structure (anti and gauche) had been proposed to explain the observed stereoselectivity of certain chemical processes but had never been directly observed. According to the TRXL result, the bridged structure is strongly favored rather than open classical structure and is also predicted by quantum mechanical calculation.