Sunlight Is Changed by Landmark Solid Material

Sunlight Is Changed by Landmark Solid Material ...

Prof. Yoichi Murakami's work involves low-intensity visible blue light or low-energy photons being converted into higher-energy UV photons by a solid film formed on a round glass substrate.

Solar power is becoming increasingly important as a renewable energy source. Through a process known as "artificial photosynthesis," high-energy UV light with a wavelength shorter than 400 nm can be utilized in a variety of ways.

Another important use is the ability of UV light to effectively kill viruses and bacteria by photocatalytic reactions. However, only a small part of sunlight falls within the UV range, leaving much of the spectrum unattended for this purpose.

Photon upconversion (UC) may be the answer to this concern. It is a process of converting long-wavelength, low-energy photons (such as those present in visible light) to short-wavelength, high-energy photons (such as those present in ultraviolet light) using a technique called "triplet-triplet annihilation."

Previous investigations in this field investigated visible-to-UV UC using organic solvent solutions that required the solution to be deoxygenated first and sealed in an airtight container to avoid exposure to oxygen that activated and degraded TTA-based photon UC samples. These limitations posed challenges in the practical use of photon UC.

Prof. Yoichi Murakami and his graduate student Mr. Riku Enomoto have developed a novel solid film that can match ultraviolet light for a brief period in the air while still being photonally stable for an unprecedented amount of time.

Prof. Murakami explains the value of their work: "Our invention will enable the practical use of low-intensity light, such as sunlight and LED room light, for applications that are effectively done with UV light." And its photostability, which was demonstrated to be at least over 100 hours, is the highest ever found in any TTA-based photon UC material, regardless of the material form."

This material is unique due to its extremely low excitation threshold (less than 0.3x the Sun's intensity) and high UC quantum yield of 4.3% (normalized UC emission efficiency of 8.6%) in the presence of air.

The researchers first mixed together a sensitizer (i.e., a molecular chromophore that can absorb longer-wavelength photons) with a much larger quantity of an annihilator (i.e., an organic molecule that received the triplet excited energy from the sensitizer and then caused the TTA process); this bi-component melt was then cooled over a temperature gradient-controlled surface to form a solid-state visible-to-UV photon

Prof. Murakami explains that this novel technique—temperature gradient solidification—is highly controllable and reproducible, implying that it is compatible with real industrial processes. It is possible to construct advanced photon UC films using this technique for the first time.

The researchers used a 1-Sun-intensity simulated sunlight to cure and solidify a resin that would otherwise require UV light for the same process.

Prof. Murakami discusses a novel UV-light-generating material for the very first time.

Riku Enomoto and Yoichi Murakami, 20 December 2022, Journal of Materials Chemistry C. DOI: 10.1039/D2TC04578H

The Japan Society for the Promotion of Science provided funding for the study.

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