Design of structure and electronic properties of organic molecules have been achieved through organic synthesis. Organic molecules with interesting molecular and electronic structure and molecular assemblies have been reported. On the other hand, connection such unique structure and assemblies to unique novel function is still difficult. Bulk physical properties of organic molecules have been mainly studied in the static molecular assemblies. Utilization of molecular motion of organic molecule in the assembly could develop new functional organic material.
Herein, the author introduces unique functional organic materials based on molecular motion in the condensed phases developed by the author and his co-workers. A variety of molecular motions is possible in organic molecules, which has different mode of motions and activation energies. With consideration of such factors, materials with new function could be proposed through the assemblies of molecules with appropriate arrangement and degree of freedom of molecular motion. Coupling molecular motion with its electronic, photophysical properties in the assemblies could develop new multifunctional organic molecular materials. Specifically, fluorescent ferroelectric liquid crystal, jumping crystals and thermoresponsive amphipathic fluorescent liquid are discussed.
Development of novel molecular materials has been a central issue in molecular science. In this study, we have successfully developed a new type of molecular conductor, where the π-electrons in the conducting layers are coupled to hydrogen dynamics in hydrogen bonds. This unique feature has enabled us to control the π-electron structures and properties by using the hydrogen dynamics. In this paper, the synthesis, structures, and properties of this new type of molecular conductor are summarized, especially focusing on the H/D isotope effect, phase transition behavior, and pressure and electric-field effects.
For deeper understanding and better use of a dynamic nature of molecules, it is essential to clearly visualize molecular motions of interest. Because motions of molecules are generally described by wave packets, coherent superpositions of stationary state wave functions, it is of great importance to experimentally characterize wave packets. Time-resolved Coulomb explosion imaging is one of the powerful approaches for achieving this objective. However, there have been several difficulties in existing imaging techniques. High-precision measurements of molecular movies were, therefore, still limited. Here, we developed a conceptually new, space-slice ion imaging apparatus to overcome existing difficulties, offering a way to capture molecular snapshots in the polarization plane of an incident laser pulse. We applied our method to real-time imaging of laser-kicked rotational wave packet dynamics. In the observed molecular movies, wave packet dynamics including localization and dispersion during the classical-like rotational motion are clearly visualized. Such an experimental characterization of a spatiotemporal pattern of wave packet dynamics leads to direct physical insight into molecular motions.
Understanding of the electronic structure and photoexcited state dynamics at well-prepared layered functional materials on substrates is essentially important in order to precisely design and control the future electronic or optical nanodevices. I have been so far engaged in this research field experimentally from the view point of molecular science, where the electronic states and dynamics at nanoscale functional films fabricated with organic molecules and/or nanocluster superatoms are investigated by probing photoelectrons, combining with a femtosecond light source and with a nanocluster deposition system. In this account, firstly, I show the electronic structures and photoexcited state dynamics at two-dimensional (2D) molecular monolayer systems of alkanethiolate self-assembled monolayers by two-photon photoemission spectroscopy which clarifies novel ultrafast phenomena characteristic to the 2D assembly of the functional molecules. Secondly, I present the visualization of local photoexcited states in the organic films by changing the probe system into two-photon photoelectron emission microscopy. Finally, the electronic states and chemical properties of nanocluster superatoms as a new class of functional nanomaterials are explained, which are non-destructively landed onto the substrates.
Finding transition states are important for analyzing chemical reactions, but revealing the reaction dynamics can be also essential in understanding many realistic reactions. For instance, recent experiments have shed light on the importance of protein's heterogeneous dynamics in the native state and during function, and ultrafast dynamics in photo-triggered chemical reactions have been studied over decades. However, most efforts in theoretical studies have been devoted to characterizing transition states and calculating ensemble-averaged properties, e.g. free energy profiles, whereas the dynamics have been of less focus. In this account, we review our recent efforts toward revealing the dynamics of reactions under diverse conditions from the theoretical perspective. We discuss three cases, i.e. photo-isomerization reaction in gas phase, and protein folding and enzyme catalysis in condensed phase. The key in these studies has been to shed light on the individual events occurring during reactions, rather than focusing only on the characteristic states and ensemble averages. These studies show that dynamics play a fundamental role in all three cases, and demonstrate how the dynamics analyses can deepen our understanding of the reactions under various conditions.
Metal surfaces are a playground for heterogeneous reactions including catalysis and electrochemistry. They also serve as a template for thin film growth and an electrode in various devices. Thus, metal surfaces are important in both fundamental and applied sciences. This review presents two topics regarding the structure and dynamics of adsorbates on metal surfaces probed with sum frequency generation (SFG) spectroscopy. First, the directional orientation of water molecules in the ice crystalline thin film grown on a Pt(111) surface is described. Heterodyne detection of SFG makes it possible to determine the direction of water at the metal surface: they are preferentially oriented such that one of hydrogen atoms is directed toward the metal surface. This directional configuration propagates in the bulk of ice crystalline film through hydrogen bond network. Second, the ultrafast dynamics in the early stage of photo-stimulated desorption of CO on Cu(100) is described. Here the heterodyne detection of SFG is employed in pump-and-probe measurements. The phase and amplitude of SFG optical field obtained with this method are used for retrieving the perturbed free induction decay of CO stretch vibration polarization. This allows us to probe adsorbate dynamics leading to desorption induced by irradiation of an intense pump pulse. The ultrafast dynamics of adsorbates are the manifestation of coupling between hot electrons in metal and frustrated motions of CO at the surface, which provide a clue for understanding nonadiabatic processes accompanying adsorbate motions, which are ubiquitous in metal and at its surface.
Ligand-protected gold clusters with specific compositions exhibit high stability due to symmetrical geometric structures, closed electronic structures of the gold cores, and complete passivation of the surface gold atoms by the ligands. In this regard, they can be viewed as chemically-modified superatoms in which valence electrons are accommodated into atomic-like discrete orbitals. This review article highlights the geometric and electronic structures of superatoms with reference to those of conventional atoms and their bonding scheme to form quasi-molecules (superatomic molecules). Our recent efforts to characterize their structures in the gas phase are also described.