Photoliquefaction and phase transition of m -bisazobenzenes give molecular solar thermal fuels with a high energy density.

A series of m -bisazobenzene chromophores modified with various alkoxy substituents (1; methoxy, 2; ethoxy, 3; butoxy, 4; neopentyloxy) were developed for solvent-free molecular solar thermal fuels (STFs). Compounds ( E , E )-1-3 in the crystalline thin film state exhibited photoliquefaction, the first example of photo-liquefiable m -bisazobenzenes. Meanwhile, ( E , E )-4 did not show photoliquefaction due to the pronounced rigidity of the interdigitated molecular packing indicated by X-ray crystallography. The m -bisazobenzenes 1-4 exhibited twice the Z -to- E isomerization enthalpy compared to monoazobenzene derivatives, and the latent heat associated with the liquid-solid phase change further enhanced their heat storage capacity. To observe both exothermic Z -to- E isomerization and crystallization in a single heat-up process, the temperature increase of differential scanning calorimetry (DSC) must occur at a rate that does not deviate from thermodynamic equilibrium. Bisazobenzene 1 showed an unprecedented gravimetric heat storage capacity of 392 J g -1 that exceeds previous records for well-defined molecular STFs.