Converting absorbed photons into twice as many excitons: Successful high-efficiency energy conversion with organic monolayer on gold nanocluster surface


IMAGE: Introduction of singlet fission response in a tetracene alkanethiol-modified gold nanocluster.
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Credit: Keio University, Kobe University.

A research study group consisting of Partner Teacher Taku Hasobe and Assistant Teacher Hayato Sakai of the Keio University Professors of Science and Technology, Toshiyuki Saegusa of the Keio University Graduate School of Science and Technology (finished master’s program in 2019), and Teacher Yasuhiro Kobori and postdoctoral scientist Hiroki Nagashima of the Kobe University Molecular Photoscience Proving ground discovered that when light was exposed to the surface of a tetracene alkanethiol-modified gold nanocluster, which they established themselves, twice as many excitons might be transformed compared to the variety of photons absorbed by the tetracene particles. They likewise discovered that these excitons have a life time that is around 10,000 times longer than that of the organic particles on traditional gold surface areas. Additionally, they was successful in converting singlet oxygen (a kind of reactive oxygen types) at an extremely effective conversion rate of 160%, far going beyond 100% conversion, in contrast to the variety of absorbed photons. Singlet oxygen is utilized in photodynamic treatment (treatment of cancer with light) and organic synthesis, to name a few applications.

These findings are anticipated to add to locations such as solar energy conversion, electronic devices, life sciences, and healthcare in the future. The results of this research study were released in the online variation of the American clinical publication the “Journal of the American Chemical Society” on September 6.

1. Bottom line of research study

Typically, when one photon is absorbed by a particle, just one exciton (a bound state of an electron hole and an electron) is formed. Nevertheless in the last few years, singlet fission (which forms 2 excitons from the absorption procedure of a single photon) is collecting much attention worldwide, although considerable work stays prior to it can be utilized in chain reactions.
In basic, an organic particle that has actually been chemically customized and incorporated into the surface of metals loses considerable excitation energy when compared to the separated state of an organic particle.
In order to fix all of the above issues simultaneously, a tetracene alkanethiol-modified gold nanocluster was recently created and manufactured. A boost in life time of about 10,000 times was attained by considerably reducing the quick loss of excitation energy on the metal surface. In addition, excitons were formed with high effectiveness through singlet fission, and the effectiveness of producing singlet oxygen was effectively enhanced to about 160% (fig. 1).

2. Material of research study and outcomes

A research study group consisting of scientists from Keio University, Kobe University, and Tampere University focused on a photoreaction called singlet fission. This is a procedure in which 2 particles placed neighboring interact with each other after among the particles takes in a photon, forming 2 excitons. With the objective of resolving the abovementioned issues at one time, they thought about customizing tetracene (whose chemical structure is made up of 4 benzene rings linked in a straight line) into a metal nanocluster by the self-assembled monolayers (SAMs) technique (fig.2). SAMs are monolayers made by chemically customizing organic ligands such as alkanethiol on the metal surface, and have actually been an essential core technology for current developments in nanotechnology. In singlet fission, where responses happen in between 2 particles situated near to each other, the range and orientation in between the 2 particles need to be strictly managed. When the surface of a gold nanocluster is chemically customized utilizing tetracene homodisulfide (fig.2, right) that is made up of 2 tetracene alkanethiol with the very same alkyl chains of length n, the possibility of tetracene with the very same alkyl chains of length n being put in close distance undoubtedly increases. For that reason, as revealed on the left side of figure 2, tetracene heterodisulfide (Tc-C11-S-S-Cn-Tc) with various alkyl chain lengths (for which one had a length of n = 11 while the other had a length of n = 5, 7, or 9), were recently manufactured, and the gold nanocluster surface was chemically customized. Utilizing tetracene heterodisulfide and tetracene homodisulfide, a series of tetracene alkanethiol-modified gold nanoclusters was manufactured (a block [cluster] of gold with 144 gold atoms whose surface was chemically customized by 60 tetracenes with alkanethiol as the medium). As an outcome, it was possible to produce bimolecule plans with the optimum range and orientation for singlet fission to happen effectively on the gold nanocluster surface (while reducing reverse responses).

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