Converting absorbed photons into twice as many excitons
A research group comprising Associate Professor Taku Hasobe and Assistant Professor Hayato Sakai of the Keio University Faculty of Science and Technology, Toshiyuki Saegusa of the Keio University Graduate School of Science and Technology (completed master’s program in 2019), and Professor Yasuhiro Kobori and postdoctoral researcher Hiroki Nagashima of the Kobe University Molecular Photoscience Research Center found that when light was exposed to the surface of a tetracene alkanethiol-modified gold nanocluster, which they developed themselves, twice as many excitons could be converted compared to the number of photons absorbed by the tetracene molecules. They also found that these excitons have a lifetime that is approximately 10,000 times longer than that of the organic molecules on conventional gold surfaces. Furthermore, they succeeded in converting singlet oxygen (a type of reactive oxygen species) at a highly efficient conversion rate of 160%, far exceeding 100% conversion, in comparison to the number of absorbed photons. Singlet oxygen is used in photodynamic therapy (treatment of cancer with light) and organic synthesis, among other applications.
These findings are expected to contribute to areas such as solar energy conversion, electronics, life sciences, and medical care in the future. The outcomes of this research were published in the online version of the American scientific publication the “Journal of the American Chemical Society” on September 6.
- Normally, when one photon is absorbed by a molecule, only one exciton (a bound state of an electron hole and an electron) is formed. However in recent years, singlet fission (which forms two excitons from the absorption process of a single photon) is gathering much attention worldwide, although significant work remains before it can be used in chemical reactions.
- In general, an organic molecule that has been chemically modified and integrated into the surface of metals loses significant excitation energy when compared to the isolated state of an organic molecule.
- In order to solve all of the above problems at once, a tetracene alkanethiol-modified gold nanocluster was newly designed and synthesized. An increase in lifetime of about 10,000 times was achieved by greatly suppressing the rapid loss of excitation energy on the metal surface. In addition, excitons were formed with high efficiency through singlet fission, and the efficiency of generating singlet oxygen was successfully improved to about 160%.
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- Toshiyuki Saegusa, Hayato Sakai, Hiroki Nagashima, Yasuhiro Kobori, Nikolai V. Tkachenko, Taku Hasobe. Controlled Orientations of Neighboring Tetracene Units by Mixed Self-Assembled Monolayers on Gold Nanoclusters for High-Yield and Long-Lived Triplet Excited States through Singlet Fission. Journal of the American Chemical Society, 2019; 141 (37): 14720 DOI: 10.1021/jacs.9b06567
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Kobe University. “Converting absorbed photons into twice as many excitons.” ScienceDaily. ScienceDaily, 24 September 2019. <www.sciencedaily.com/releases/2019/09/190924104110.htm>.
Kobe University. (2019, September 24). Converting absorbed photons into twice as many excitons. ScienceDaily. Retrieved September 24, 2019 from www.sciencedaily.com/releases/2019/09/190924104110.htm
Kobe University. “Converting absorbed photons into twice as many excitons.” ScienceDaily. www.sciencedaily.com/releases/2019/09/190924104110.htm (accessed September 24, 2019).