A Japanese research group comprised of Dr. Tokushi Sato, Professor Shunsuke Nozawa, and Professor Shin-ichi Adachi of the Institute of Materials Structure Science at KEK, Professor Hiroshi Fujii of the Institute of Molecular Science at the National Institutes of Natural Sciences, and Professor Shin-ya Koshihara of the Tokyo Institute of Technology successfully visualized electron transfer process in a photosensitizer molecule, which is the fundamental process in solar cells and photocatalysts, at a temporal resolution of 100 ps.
Improvements of the efficiency and the lifetime of solar cells and photocatalysts are necessary to solve energy and environmental problems. Understanding the electron motion due to light irradiation, which is common to all these devices, is very important in realizing such developments. The photoexcitation mechanism in Ruthenium(II)-tris-2,2′-bipyridine [RuII(bpy)3]2+, used in solar cells, was observed in this research. The absorption band wavelengths (400500 nm) for charge transfer from metal to ligand (MLCT) in this material are close to the maximum intensity wavelengths of the Sun, and therefore, efficient charge separation induced by solar light occurs in this material. Therefore, it is frequently used in investigating photochemical reactions.
The research group used time-resolved X-ray absorption spectroscopy at the Photon Factory at KEK to observe the changes in the electronic state and the molecular structure. The observation of the electronic state before and after irradiation showed that there is electron transfer from ruthenium to bipyridine ligand within 100 ps, resulting in a change in the oxidation state of ruthenium from II to III. The bipyridine ligand moves toward the ruthenium atom by 0.04 Å with the charge transfer and a structurally distorted state exists in the photoexcited state .
This research elucidated the details of electron transfer, which is a fundamental process in solar cells and photocatalysts, and the change in molecular structure that accompanies electron transfer. This is important information in designing solar cells and photocatalysts, and it is expected to result in advances such as further improvements in device efficiency.
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Coordination and Electronic Structure of Ruthenium(II)-tris-2,2-Bipyridine in the Triplet Metal-to-Ligand Charge Transfer Excited State Observed by Picosecond Time-Resolved Ru K-edge XAFS The Journal of Physical Chemistry C, Vol. 116, No. 25 (June 28, 2012). doi:10.1021/jp3038285 . Authors：T. Sato, et al.