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

A series of -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 (,)-1-3 in the crystalline thin film state exhibited photoliquefaction, the first example of photo-liquefiable -bisazobenzenes. Meanwhile, (,)-4 did not show photoliquefaction due to the pronounced rigidity of the interdigitated molecular packing indicated by X-ray crystallography. The -bisazobenzenes 1-4 exhibited twice the -to- 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 -to- 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 that exceeds previous records for well-defined molecular STFs.