Revealing the activation barrier for the on-surface thermal-induced <i>cis</i> → <i>trans</i> isomerization of azobenzene derivatives

Azobenzene (AB) and its derivatives are known for their reversible <i>trans</i> ↔ <i>cis</i> isomerization in solution and the solid phase in response to UV/Vis radiation. While the forward <i>trans</i> → <i>cis</i> isomerization is triggered by UV radiation, the backward <i>cis</i> → <i>trans</i> isomerization can be triggered either by visible light or by temperature. The function of these molecules is highly dependent on the activation barrier for the forward and backward isomerization and is the key for governing the properties of photo-switchable molecules. Typically when the molecules are deposited on a surface, they are stabilized in their equilibrium state, <i>trans</i>. So it is feasible to understand the dynamics and energy barriers of <i>trans</i> → <i>cis</i> isomerization experimentally. However, the energy barrier for the reverse process (<i>cis</i> → <i>trans</i> isomerization) on surfaces is generally elusive and not accessible due to the fact that AB derivatives do not condense into the phase of non-equilibrium state, <i>cis</i>. Though the energy barriers may be computed theoretically, the experimental determination is not straightforward. Here, we demonstrate a novel approach where the <i>cis</i> isomers of two AB derivatives are condensed onto the highly oriented pyrolytic graphite (HOPG) surface from a <i>cis</i> dominant solution of the molecules. Further, by analysing the percentage of on-surface thermal-induced <i>cis</i> → <i>trans</i> isomerization, we have determined the corresponding energy barrier. Temperature dependent atomic force micrographs of monolayer and submonolayer films of the molecules are used for the experiment.