Liquid and Photoliquefiable Azobenzene Derivatives for Solvent-free Molecular Solar Thermal Fuels

Abstract

A series of liquid and photoliquefiable azobenzene (Azo) derivatives (Azo-Cn-Br) have been synthesized for molecular solar thermal fuels. Each of the liquid and photoliquefiable azo derivatives shows a high degree of isomerization, a fast isomerization rate, a long half-life, an appropriate energy storage density, and a solvent-free “charging” and “discharging” process. The photoliquefied azo derivatives can isomerize upon UV light irradiation at low temperatures to give the “UV-charged” azo ones. Therefore, the phase transition enthalpy is stored simultaneously along with the isomerization enthalpy. The “UV-charged” azo derivatives are capable of releasing heat under the manipulation of blue light.

Photoswitches with different numbers of azo chromophores for molecular solar thermal storage

“…In addition, we compared the energy density of charged 1 with those of some liquid and photoliquefiable azobenzene derivatives reported in the literature (Table S2, ESI †). [52][53][54] Although they could be charged in solventfree state, the relatively high molecular weight resulted in a low energy density. By contrast, although 2 did undergo photoisomerization in the solid state, the Z isomer content was so low that DSC could not detect the released heat (Fig.…”

Visible-Light-Photomeltable Azobenzenes as Solar Thermal Fuels

“…1,2,5−7 Reflecting its application potential, photomelting of different kinds of azobenzene derivatives has already been applied in diverse fields, 1,2 such as reworkable adhesives, 8,9 gas storage, 10 self-healing materials, 11 photoresists, 6,12 photoactuators, 13−15 and solar thermal fuels (STFs). 7,16,24 The STF technology is an emerging technology that converts and stores light energy from the sun as chemical energy using photochemical reactions that result in products with higher enthalpies (Figure 1). 16−26 In the early stage, a STF was developed as an in-solution system, 17 which compromises the overall energy density.…”

“…16−26 In the early stage, a STF was developed as an in-solution system, 17 which compromises the overall energy density. In order to increase the energy density, condensed-phase STFs have recently drawn increased attention, 7,16,[18][19][20][21][22][23][24][25][26]36 and photomeltable azobenzene is a promising STF candidate. 7,16 In addition to eliminating the need for a solvent, photomeltable azobenzenes have another advantage as STFs: the energy released during the discharging state is a combination of the isomerization and recrystallization enthalpies (Figure 1a).…”

“…16,18 However, all previously reported azobenzene derivatives that show photomelting require UV light (Chart 1). 1,2,7,16,24,25,27 This limits the applicability of photomelting-driven photofunctional materials, including STFs, because UV light can damage the materials. 30 Furthermore, UV light only comprises a small fraction (4− 5%) of the solar spectrum.…”

“…Azobenzene is one such photoisomerizable compound. Although azobenzene itself does not undergo photomelting from its crystal phase because of its crystal packing, , several photomeltable azobenzene derivatives can be realized through suitable molecular designs. ,,− Reflecting its application potential, photomelting of different kinds of azobenzene derivatives has already been applied in diverse fields, , such as reworkable adhesives, , gas storage, self-healing materials, photoresists, , photoactuators, − and solar thermal fuels (STFs). ,, …”