Visible-Light-Photomeltable Azobenzenes as Solar Thermal Fuels

Abstract

Photomelting of azobenzene is a phenomenon that is typically achieved by UV-light irradiation. However, many applications based on azobenzene photomelting, including the solar thermal fuel (STF) application, require responsiveness to visible light. Therefore, in this study, we designed and synthesized several azobenzene derivatives that melt under visible-light irradiation and explored their potential application as STFs. By combining molecular design elements that endow azobenzene with visible-light-responsiveness (ortho-halogen) and photomeltability (para-alkoxy), we successfully synthesized four compounds that exhibit visible-light photomelting properties. These compounds also exhibit a long cis-isomer lifetime, which makes them good candidates for STFs in the future.

Optically Controlled Solid-to-Liquid Phase Transition Materials Based on Azo Compounds

“…88 The azo compounds synthesized in that way span the solar spectrum well. They could even be regulated directly by sunlight (Figure 5h,i), 72,88 taking a step toward solar energy usage.…”

“…140 It is important to acquire photoliquefiable azo compounds with visible light responsiveness through modification with specific functional groups, as shown in section 2.1 (Figure 5g,i). 72,88,109 The STFs made up of these compounds can expand the absorption of the solar spectrum and even the response to sunlight directly. 10,72,88 To utilize the full width of the solar spectrum, the combination of multiazo compounds with different absorption peaks 140 or using photon upconversion 141 was also effective.…”

“…First, some asymmetric azo molecules whose trans isomers have MPs close to room temperature appear to be more susceptible to the photoliquefaction process, such as 3,3′-dimethyl-azobenzene and MOAZ (Table , entries 7 and 8). Second, through the substitution of specific groups (Table , entries 3 and 13) , and the adjustment of the alkyl-chain length (Table , entries 10 and 12), ,, the structural symmetry of azo derivatives can be reduced, providing enough free volume for photoisomerization. In some ionic liquid or ionic crystal, the solid-to-liquid phase transition can also be photoinduced by adjusting the positions and types of substitutions (Table , entries 4 and 5). , Third, the synthesis of some stereoregular azo molecules with multi-azo arms can also introduce free volume into the solid (Table , entries 2, 9, and 14). ,,, In addition, some amphiphilic molecules and oligomers also show potential in OPTMs (Table , entries 6 and 11). ,, As a result, when azo compounds can generate enough cis isomers (does not need to reach 100%), it will be helpful to weaken the π–π stacking, van der Waals forces, and other intermolecular forces, so that the MP of the trans / cis mixture decreases below room temperature .…”

“…Upon photoisomerization, the structure tightness was broken significantly, and the MP of mixtures was more easily decreased below room temperature, realizing photoliquefaction. Recently, Shi et al and Kwaria et al reported that an azo derivative with halogen substitutions at the ortho positions can also be photoliquefied by visible light. The responsive visible wavelength could be altered by changing the halogen type (Figure g) .…”

“…Photoliquefiable materials can reach a sufficiently high cis isomer ratio without using solvents, relative to the non-OPTM azo solid, which provides true closed-loop storage. ,, In addition, when an azo material undergoes the transition from a solid to a liquid, it stores not only Δ H iso but also the phase transition energy between the crystal and liquid phase (Δ H cryst ) (Figure b). , Griffiths et al measured the ED of a finely ground powder of photoliquefiable MOAZ. The matter was kept stirring during the charging process to achieve a sufficient phase transition and achieved a gravimetric ED of ≤315.1 J/g …”