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

Abstract

Azobenezene compounds are putative solar thermal fuels (STF) due to 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‐substituted, monofunctional azobenzenes with fluoro, methyl, ethyl, trifluoromethyl and methoxy substituents. Four of the compounds are liquid azobenzenes with higher energy density 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. The higher‐melting behavior is explained, in part, by intermolecular close contacts in the Z‐isomer packing lattice.