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

Abstract

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 (ΔH_isom.) as well as crystallization enthalpy (ΔH_cryst.). 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.