Vitamin A is adequately distributed within the body to maintain the biological function of retinoids in the peripheral tissues and the production of the visual chromophore, 11-cis-retinal, in the eye. One of the mysteries in our vision is that humans recognize color by use of a single chromophore molecule (11-cis-retinal), meaning that the chromophore is identical even between blue-absorbing and red-absorbing sensors. Humans have two different types of retinal containing light-sensitive proteins expressed in the retina, rhodopsin (Rh) achieving the twilight vision and three cone pigments, which mediate color vision. Each different chromophore-protein interaction allows preferential absorption of a selected range of wavelengths. While the structural basis for photoreaction and signal transduction of Rh has been well understood by the determination of its atomic-level structure, structural studies of cone pigments lag far behind those of Rh, mainly because of difficulty in sample preparation and lack of suitable methods in structural analysis.

We thus attempted to express monkey cone pigments in HEK293 cell lines for structural analysis using light-induced difference Fourier-transform infrared (FTIR) spectroscopy. The first structural information successfully elicited from the highly accurate spectra for each cone pigment showed that the retinal chromophore is structurally similar between Rh and cone pigments, but the hydrogen-bonding network around the retinal chromophore is entirely different between them. In addition, some spectral differences are observed between cone pigments, including protein-bound water molecules. These differences could be interpreted to play a role in spectral tuning.

Interfaces play important roles in various fields such as catalytic chemistry, electrochemistry, biochemistry, and atmospheric chemistry. However, molecular-level understanding of structure and dynamics of their interfaces is still limited due to technical difficulties. Vibrational sum frequency generation (VSFG) spectroscopy is based on a second-order nonlinear optical process and provides interface-specific information. While conventional homodyne-detected VSFG measures the square of second-order nonlinear susceptibility (|χ⁽²⁾|²), heterodyne-detected (HD-) VSFG enables us to directly measure the imaginary part of χ⁽²⁾ (Imχ⁽²⁾), which corresponds to Imχ⁽¹⁾ obtained with absorption spectroscopy in bulk. We realized the first time-resolved (TR-) HD-VSFG measurement by combining HD-VSFG spectroscopy and pump-probe technique. This review introduces the principle of TR-HD-VSFG spectroscopy and its applications to interfacial dynamics at the metal, water, and lipid monolayer interfaces.

One of the frontiers in modern chemical science is to observe nuclear rearrangements during a chemical reaction in real time and unveil structure-function interplay underlying the sophisticated functions of complex molecular systems. In this quest, various time-resolved techniques have been developed in the last decades. Nevertheless, it has not yet been trivial to track structural changes of the molecules proceeding on the time scale of the nuclear motion, i.e., femto-to-picosecond time scale. Recently, we developed femtosecond time-resolved time-domain Raman spectroscopy using <7-fs pulses, which allows us to track structural changes of the molecules on the femtosecond time scale with exquisite sensitivity. With this technique, we realized real-time observation of the ultrafast structural dynamics in the primary photochemical/photophysical processes in condensed-phase complex molecular systems. In this article, we overview the principle and a brief history of time-domain Raman spectroscopy and then describe the apparatus and recent applications to the femtosecond dynamics of molecules as complex as photoreceptor proteins and molecular assemblies.

Lanthanide (Ln) systems have been widely applied for optical materials because they have bright emissions originating from f–f transitions. To design better optical materials, the information of emission and quenching mechanism is indispensable. However, it is too demanding to apply ab initio calculations for Ln systems due to their strong electron correlation and spin-orbit interaction. To overcome this problem, I proposed a new approximation, the energy shift method, which enabled us to perform the exhaustive exploration of critical points, such as local minima, transition states, and crossing points, in all internal degrees of freedom on and between potential energy surfaces of large Ln systems. This review briefly outlines the basic concepts of the Ln luminescence, followed by the overview of the energy shift method and its application studies for thermo-sensors and strong emitters.

It is commonly accepted that the theory mentioned in the following for chiral materials hold usually: chiral materials are optically active, and optical activity appears only when the material is chiral. However, is this always true? We discuss here whether (and how) this theory holds for molecules and nanomaterials. We describe macroscopic chiro-optical effects of ideal and pseudo two-dimensional materials, and nanoscopic version (local chiro-optical effects) of them, where we will find that the commonly accepted theory mentioned above sometimes breaks down. We also briefly discuss some unique chiral features of plasmonic materials relevant to the local chiro-optical effects.

We reviewed the single-molecule science based on single-molecule measurements using tunneling current and ionic current as probes. Single-molecule measurements using tunneling currents can determine the number of molecules connected to a nanogap electrode. In addition, single-molecule measurements enable measuring the molecular vibration, local temperature, thermoelectric power, and electrode-molecule binding energy of a single molecule connected between electrodes. In addition, as a physical quantity, the phase information of the frontier molecular orbital of single molecules is measured. On the other hand, using an ionic current, single-molecule measurements enable highly accurate identification of a bacterium or virus that passes through a nanopore having a through-hole with a diameter of several μm or less. Nanopores are also a stage for elucidating the flow dynamics of a single substance transported in a liquid confined in a nanospace. Single-molecule science, which is growing as a fundamental discipline, is advancing to applied research targeting biomolecules. Furthermore, the fusion of single-molecule measurements and artificial intelligence will enable data analysis methods that are different from conventional ones. It is also becoming possible to investigate the properties of a single molecule rather than the statistical average molecular behavior.