Professor Yukihiro Ozaki has developed molecular spectroscopy markedly by his comprehensive and pioneering investigations in molecular spectroscopy (vibrational spectroscopy and electronic spectroscopy) in condensed phase through the exploration of novel methods and basic principles, the development of instruments, and the applications to a wide research area in chemistry. He has made extensive achievements in the wide range covering infrared (IR), Raman, near-infrared (NIR) and far-ultraviolet (FUV) spectroscopy. Among his pioneering achievements, the following three are chosen here and their scientific significance is described.
1. Investigation of mechanism of surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS) and their applications to physical chemistry, analytical chemistry, and nanomaterials chemistry
Prof. Ozaki focused on electromagnetic enhancement mechanism as general mechanism of SERS and carried out its research. Before he started his research on this topic SERS mechanism had been investigated by using an ensemble nanoparticles system. In such system, the cause of the enhancement (plasmon resonance) is averaged, and thus, it was difficult to explore directly the relation between the cause and the results (SERS). Prof. Ozaki solved this problem by use of a single nanoparticle system. He designed a novel experimental system that enabled him to carry out electron microscope measurement, plasmon resonance measurement, and SERS measurement for the same single nano particle dimer. Using the experimental results obtained by these experimental setups as the calculation conditions for electromagnetic analysis, he reproduced SERS cross section. By comparing the experimental SERS spectra with the calculated SERS spectra, he succeeded in quantitative verification of electromagnetic enhancement effect. By this study he contributed markedly to the advances in SER spectroscopy. He also contributed to studies on chemical mechanism of SERS and semiconductor-enhanced Raman scattering.
As for the application of SERS, his studies of charge-transfer-induced enantiomer selective discrimination of chiral molecules by SERS and three dimensional SERS imaging studies are particularly noted. Prof. Ozaki also carried out unique studies of TERS. For example, he reported nanoscale quantitative measurement of pH profile at a solid-liquid interface by TERS and studies on local nanostructure of graphenes.
2. Development of attenuated total reflection (ATR)-FUV spectroscopy - Pioneering in novelσ chemistry.
In the FUV region a number of electronic transitions are observed, however, the analysis of absorption spectra in the FUV region of condensed phase was an unexplored area of molecular spectroscopy. Prof. Ozaki developed a novel FUV spectrometer (145-300 nm) based on an ATR technique, making possible the easy measurement of FUV spectra of molecules in condensed phase. Since in the FUV region one can expect transitions due to π electrons, lone pair electrons, and σ electrons, the ATR-FUV method has allowed to investigate electronic states of almost all kinds of molecules in condensed phase. Prof. Ozaki established an assignment method of FUV spectra based on comparison of an observed FUV spectrum in condensed phase and the corresponding spectrum in gas phase and electronic transitions calculated by quantum chemical calculations. For example, in the study of electronically excited states of alkanes he assigned a band near 153 nm to a transition from HOMO-2 or HOMO-1 to 3p Rydberg. Since alkanes are formed only by single bonds and do not have lone pair electrons, this transition reflects the state of σ electrons of alkanes.
Moreover, he observed FUV spectral changes of tetradecane before and after its phase transition, and found that the 153 nm band shifts to a longer wavelength, showing thatσelectrons are affected by an intermolecular interaction in condensed phase.
Furthermore, he has promoted investigations of water, aqueous solutions, surface adsorbed water, polymers, ionic liquids, and TiO2 and TiO2 modified with metal nanoparticles including photocatalytic activities of TiO2 by use of ATR-FUV spectroscopy.
In this way he has demonstrated the potential of FUV spectroscopy.
3. Establishment of near-infrared spectroscopy as a molecular spectroscopy.
To develop NIR spectroscopy on one hand, Prof. Ozaki performed basic studies of NIR spectroscopy such those of overtones and combination modes of molecular vibrations, and anharmonicity of vibrational potentials, and on the other hand he carried out applications of NIR spectroscopy to hydrogen bonding studies and solution chemistry and proposed surface plasmon resonance (SPR)- NIR spectroscopy. Furthermore he made pioneering contributions in instrument development and spectral analysis of NIR spectroscopy.
He demonstrated that by using NIR spectroscopy one can detect a change in vibrational energy of alkyl group induced by a weak intermolecular interaction that was not easy to be detected by IR spectroscopy. He also found that changes in vibrational energy and absorption intensity induced by an intermolecular interaction observed in NIR spectroscopy have different tendencies from those in IR spectroscopy. To disclose this finding he investigated frequencies and intensities of overtones and combination modes in solutions by using experiments and quantum chemical calculations. By using quantum chemical calculations, in which mechanical and electrical anharmonicities are taken into account, he succeeded in analyzing spectral variations in NIR spectroscopy caused by an intermolecular interaction that could not be reproduced by an extension of IR spectroscopy. As the result he showed that it is possible to differentiate effects of hydrogen bonding effects from solvent effects by comparing a change in absorption intensity of a fundamental with that of an overtone.
Moreover, Prof. Ozaki developed a new NIR spectroscopic method based on surface plasmon resonance called light absorption responsive SPR-NIR spectroscopy to aim at dramatic sensitivity improvement. He explored optimum conditions of light absorption responsive SPR in the NIR region for the establishment of this technique.
As stated above Prof. Ozaki has challenged to unexplored undeveloped fields of molecular spectroscopy with original idea and methods and developed markedly them. He has contributed significantly to modern molecular spectroscopy through the establishment of basic principles, the development of instruments, the proposals for the novel spectral analysis methods and the applications of quantum chemistry to molecular spectroscopy. Prof. Ozaki's achievements have been internationally recognized and have had a strong impact on a wide research areas in chemistry particularly in physical chemistry, analytical chemistry, nanomaterials chemistry, and biology relevant chemistry. His achievements are, therefore, deemed worthy of the Chemical Society of Japan (CSJ) Award.