Controllable heat release of phase-change azobenzenes by optimizing molecular structures for low-temperature energy utilization

Phase-change azobenzene derivatives can store and release heat upon isomerization. The amount and rate of heat output are affected by the azobenzene crystallization and isomerization, which are in turn governed by molecular structure and interactions. Thus, optimizing molecular structure is a promising method to control heat release at different temperatures. Herein, we prepared three asymmetric alkoxy-substituted azobenzene molecules (s-Azo) with similar molecular weights but different substituents to investigate the trade-off between crystallization and isomerization. Temperature-dependent crystallizability and photo-induced iso-merization kinetics of all s-Azo were studied. Results indicate that n-alkoxy substitution endows s-Azo with high crystallization enthalpy (ΔHCE) due to strong van der Waals forces, but steric hindrance lowers the degree of isomerization. Short branched alkyl substitution reduces intermolecular interactions and favors the isomerization, which leads to an increase in isomerization enthalpy (ΔHIE) but decreases ΔHCE. The n-alkoxy-substituted s-Azo exhibits photoinduced high-energy heat release with an enthalpy of up to 343.3 J g−1 and a power density of 413 W kg−1 at a wide temperature range from −60.49 to 34.76°C. The synchronous heat release in a distributed energy utilization annular device achieves a temperature rise of 6.3°C at a low temperature environment (−5°C). Results demonstrate that phase-change azobenzene derivatives can be designed and developed for ideal energy-storage systems by optimizing molecular structures and interactions.