Abstract
Solar energy can be transformed into heat energy using molecular solar thermal (MOST) energy storage materials, which can then store the heat energy and release it in response to an external stimulus like light or heat. Arylazoisoxazoles, unlike analogous arylazopyrazoles, fail to draw much attention as MOST materials due to poor photoisomerization yields in both directions and utilization of UV light for trans-cis isomerization. With the para-thioalkyl substitution in the phenyl ring, we have demonstrated that the photoswitching yields in both directions can be drastically improved under visible light irradiations. Additionally, the long thioalkyl chain permitted solid-liquid phase transition followed by very high trans-cis photoisomerization and liquid-solid transition associated with high cis-trans photoconversion at various temperatures (10-35oC). The solid liquefication enabled the storage of the latent heat along with the isomerization energy in the charged state. The compound with a C6 alkyl chain could store ̴ 196 J/g of thermal energy for nearly three weeks at 10 oC and three months at 0 oC; the liquidity of the charged state can be retained at lower temperature down to -15 oC. We further showed that the reverse cis-trans isomerization-related heat release triggered by green light can be monitored by directly recording the sample surface temperature with a high-resolution IR camera. Our results demonstrate that arylazoisoxazoles have the potential to serve as MOST materials.
Supplementary materials
Title
Visible light sensitive photoliquefiable arylazoisoxazoles for the molecular solar thermal energy storage under solvent-free conditions at room temperature and lower temperatures
Description
Reagents from generally accessible suppliers, such as Sigma-Aldrich, TCI, Avra, and Spectrochem, were used without further purification. Solvents had to be distilled before column chromatography could be used. Anhydrous condition reactions were carried out in oven-dried glassware and an argon environment. HPLC-grade solvent (Acetonitrile, DMSO, or Toluene) was used for UV-Vis spectroscopy and photoswitching. Column chromatography and silica gel with a mesh size of 100–200 were used to purify the compounds. The reactions were monitored using TLC and Merck silica gel 60 F254 plates (0.25 mm). A high-vacuum pump was used to further dry the compounds after the solvent had been evaporated using a rotary evaporator. For TLC plate viewing, UV chambers (254 or 365 nm) were used. A HITACHI UH4150 spectrophotometer or a Shimadzu UV-1900 UV-Vis spectrophotometer was used to measure UV-Vis absorption spectra before and after exposure, as well as for kinetics studies.
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