A liquid arylazopyrazole derivative is newly synthesized as molecular solar thermal fuel. It shows efficient trans-to-cis photoisomerization in the neat liquid state, with the cis-isomer having a significantly longer thermal half-life (t(1/2) = 3,069 h, at 25 degrees C). It overcomes the limited thermal stability of the corresponding liquid cis-azobenzene derivative (t(1/2) = 16 h) and marked a gravimetric energy density of 41 kJ.mol(-1), which meets the requirement for thermophysical heat storage applications.