Isomer-Dependent Melting Behavior of Low Molar Mass Azobenzene Derivatives: Observation of a Higher Melting Z-Isomer

Azobenzene compounds are putative solar thermal fuels (STF) due to the excellent photostability and structural control of isomerization rates. Azobenzenes, in which both Z- and E-isomers are liquid at room temperature, are promising candidates for STF flow technology. A literature survey of melting points led to the synthesis and isomer separation of ortho- and meta-monosubstituted azobenzenes with fluoro, methyl, ethyl, trifluoromethyl and methoxy substituents and several dimethyl substituted azobenzenes. Four of the compounds are liquid azobenzenes with higher specific energy than literature work with higher molar mass, liquid compounds. Eight of the compounds unexpectedly displayed a higher melting point for the Z-isomer which is rarely observed. Intermolecular close contacts in the crystal lattice of the Z-isomer are the main factor responsible for the higher melting temperatures.