Prof. Koichi Iwata, University of Tokyo

Tracing "loose electrons" - New physical chemistry with femtosecond time-resolved near-infrared spectroscopy

Abstract

We examine "elementary processes" of solution-phase chemical reactions with various time-resolved spectroscopies. In this presentation, I report on femtosecond time-resolved fluorescence spectroscopy and picosecond time-resolved Raman spectroscopy.

By using femtosecond time-resolved near-infrared spectrometer that covers the spectral rage of 900 to 1500 nm, we trance the dynamics of "loose electrons", electrons loosely bound to the nuclei, with a femtosecond time- resolution. Because the probe wavelength is longer, it is possible to measure the time-resolved spectra of scattering powder samples with the transmission geometry, by keeping the femtosecond time-resolution in tact. We measure the migration and decay dynamics of charge carriers generated in photocatalytic TO2 powders. The carrier dynamics changes largely depending on the crystal form (anatase or rutile) and the diameter. When Pt is loaded as a co-catalyst, the photogenerated electrons are transferred to Pt in 2 ps. When the TiO2 powders are immersed in an ionic liquid, the decay process of the generated carriers is accelerated from those in vacuum, in air, or in water. We also examine the intramolecular charge transfer reaction in 9,9'-bianthryl. We discuss the detailed mechanism of the photoinduced charge transfer reaction in polar and nonpolar environments based on the experimental results from femtosecond time-resolved near-infrared spectroscopy.

When a molecule in solution is photoexcited to one of its electronically excited states with larger energy than required for the 0-0 transition, the excess energy is deposited to the vibrational manifold. The solute molecule then starts cooling down by dissipating the excess energy to the surrounding solvent molecules. By monitoring the cooling process of the solute, it is possible to trance the solute- solvent energy transfer process. We trace the cooling kinetics of the solute by measuring the picosecond time- resolved Raman spectra of the first excited singlet (S1) state of trans-stilbene. The trans-stilbene molecule serves as a good thermometer with a time-resolving capability of a few picoseconds. We found that there was a good correlation between the cooling rate of S1 trans-stilbene and the thermal diffusivity of the solvent in ten molecular solvents. Little is known, however, about the energy transfer mechanism in room-temperature ionic liquids. We examine the cooling kinetics of S1 trans-stilbene in the ionic liquids. It is interesting to know how the possible "local structures" in the ionic liquids, an assumption not proved in a rigorous sense but supported by increasing number of researchers, affect the microscopic solute-solvent or solvent-solvent energy transfer.

Biography

Name: Koichi IWATA
Title: Associate Professor
Institute: Research Centre for Spectrochemistry, School of Science, The University of Tokyo
Address: 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
E-mail: iwata@chem.s.u-tokyo.ac.jp
Telephone: 81-3-5841-4600
FAX: 81-3-5800-4540

Education:
The University of Tokyo, Japan B.S. 1984 Chemistry
The University of Tokyo, Japan M.S. 1986 Physical Chemistry
The University of Tokyo, Japan Dr.Sci. 1989 Physical Chemistry
Title of the doctoral thesis, "Construction of a Microsecond Time-resolved Infrared Spectrometer and Its Applications"

Professional Experience:
Postdoctoral Fellow, Department of Chemistry, The Ohio State University, Columbus, Ohio, USA 1989-1990
Research Associate, Molecular Spectroscopy Laboratory, Kanagawa Academy of Science and Technology, Kawasaki, Japan 1991-1997
Associate Professor, Department of Chemistry, School of Science, The University of Tokyo, Tokyo, Japan 1997-2001
Associate Professor, Research Centre for Spectrochemistry, School of Science, The University of Tokyo, Tokyo, Japan 2001-present

For more information contact Prof. Rob Dickson (404-894-4007).