Structure-property relationship of p-alkoxyazobenzenes as molecular solar thermal phase change material energy storage systems (MOST-PCM)

Journal article, 2025

In recent years, azobenzene (AB) has been investigated for thermal energy storage applications. One approach to enhance the function of AB is to combine photoisomerization with a solid ↔ liquid phase transition enabling harvesting solar energy and ambient heat simultaneously. Thus, the photoisomerization of a crystalline (E)-isomer can be used to obtain a liquid (Z)-isomer. Upon reversible photoisomerization, the (E)-liquid → (Z)-crystal back reaction releases the stored energy as isomerization enthalpy (ΔHisom.) as well as crystallization enthalpy (ΔHcryst.). In order to optimize such MOST-PCM, a systematic series of p-alkoxy-AB with increasing chain lengths were prepared to investigate the structure-property relationship of the melting points, kinetics, photoisomerizability and optical absorption in the solid state and in solution. It could be shown that with increasing chain length the half-lives in solution increase by almost 30% depending on the chain length. However, the neat compounds show a 50% shorter half-life, probably due to autocatalysis. Both parameters exhibit an odd-even effect. Powder-XRD revealed a two stage kinetic behavior. Interestingly, the highest energy densities could be observed with medium chain lengths of about six to eight carbon atoms.