The desire to understand chemical reactions is probably a sentiment shared by researchers involved in molecular science. In this account, methods for qualitatively discussing chemical reactions using Walsh diagrams, orbital correlation diagrams, and orbital interaction diagrams are described. These methods are suitable for the analysis of reactions of organic molecules and organometallic complexes. These methods have been further applied by the author to the analysis of surface reactions. This account illustrates how the Walsh diagrams, orbital correlation diagrams, and orbital interaction diagrams can be applied to surface reaction analysis, using as examples the C-H bond activation of methane on the surface of IrO₂, H-H and C-C bond formation processes on the surface of a typical Pt catalyst. Based on the findings obtained by applying these methods to surface systems, hints for designing new catalysts are also given.

Electron diffraction has been a powerful tool to investigate atomistic structures of molecules, liquids, and solids. Time-resolved diffraction with pulsed electron beams is capable of visualizing transient structural changes with the combined fast temporal and atomic-scale spatial resolutions. Considerable effort has been devoted to producing ultrashort electron pulses for the imaging of atomic and electronic motions occurring on the time scale of femtoseconds and attoseconds, respectively. This account briefly describes technologies for producing ultrashort electron pulses including our recent works where we produced and detected attosecond electron pulses by manipulating electron beams with laser light. A qualitative discussion is given on the modulation of a diffraction pattern due to a change of electron density distribution in a molecule. Electron diffraction with attosecond pulses will allow to visualize how electron clouds in a molecule or material evolve in space and time during a reaction.

The establishment of guidelines for their control is essential for the functionalization of molecules. In this paper, the author describes the work on the effect of vibrational strong coupling of electrocatalytic and ionic conductor functions. In particular, the formation of polariton states when the vibrational levels of molecules interact with the optical modes of resonators and the control of ionic conduction through the control of their hydration environment are described, as well as their utilization as electrocatalysts. Finally, we will describe the utilization of polaritons for the exploration of the research areas.

As the demands for diverse and complex functionality, the development of photo-functional molecular materials is advanced. In molecules without heavy metals,the electronic excited states can be largely classified into luminescent singlet excited states and non-luminescent triplet excited states. The ability to transition between these two states with vastly different properties through intra/intermolecular electron spin conversion,coupled with the diversity of molecular design, provides a unique advantage in designing photo-functions. Thus, understanding electron spin conversion in molecular systems is fundamentally important from both fundamental science and application perspectives. However, electronic functions are heavily modulated by vibrational degrees of freedom inherent in molecules. Also, study of their photo-functions requires extensive use of time-resolved spectroscopy using ultrafast lasers for capturing the short-lived excited states. Here, several attempts to disentangle the complex molecular excited state dynamics based on ultrafast spectroscopy were introduced. In particular, reverse intersystem crossing and singlet fission are the main topics.

The molecular shape distinguishes molecular systems from simple atomic (metallic) and inorganic systems. This review describes its significant effects on the formed structure and macroscopic physical properties of molecular crystals, mainly based on the author's research history. In the studies, the unique character of entropy, the macroscopic quantity with clear microscopic meaning, plays an essential role in counting the number of states. The topics cover the phase transition involving internal conformational degrees of freedom, glass transitions in organic conductors, stability of real liquid crystals, and aggregation structures of layered and cubic liquid crystals. Introductory summaries are also included for thermodynamics, phase transition, glass transition, melting of molecular crystals, and diffraction.

Our earlier studies on the adhesion between epoxy resin and aluminum surface (carbon fiber) are reviewed. Mechanistic aspects of the adhesion interaction has been studied by using first-principles calculations. Stable structures of resin-surface complex, adhesion energies, and details about interaction sites are obtained from geometry optimizations for some reasonable models based on first-principles calculations with a plane wave basis set and periodic boundary conditions. Calculations reveal that hydroxyl groups of the epoxy resin interact with the surface of aluminum oxide to form hydrogen bonds, which work as a main force for the adhesion. Plots of energy versus vertical distance of the resin from the surface are nicely approximated by the Morse potential. The force required for detachment of the resin from the surface can be estimated from the maximum value of the force-distance curve, which is obtained from the derivative of the potential energy curve. Surface structures of carbon fiber are modeled by the armchair-edge structure of graphite functionalized with OH and COOH groups. Adhesive properties of the model interfaces are also evaluated based on the binding energy and the maximum adhesive force acting at the interface. Calculated adhesive properties in the two aluminum surface and carbon fiber systems strongly suggest that hydrogen bonds between the oxygen-containing functional groups of epoxy resin and the surfaces play a crucial role in the adhesive interactions.

Probe microscopes are now one of the essential observation tools for materials science research. Scanning tunneling microscope is not limited to surface morphology observations, keeping the highest spatial resolution as an analytical tool with sub-Å spatial resolution in the plane and the availability as a tool for spectroscopy provides the STM a unique standpoint. Furthermore, the possibility of using it as a tool for manipulating surface atoms and molecules was thoroughly pursued. After 40 years since its invention, I would provide some of the amazing innovations that this analytical instrument has brought to the field of surface science.