Direct Observation of Photoinduced trans-cis Isomerization on Azobenzene Single Crystal

Photoexcitation can lead to either homogeneous or heterogeneous transformations of a reactive surface. Homogeneous transformations result in a statistical mixture of reactants and products, whereas the outcome of heterogeneous transformations is a coexistence of macroscopic reactant and product domains, separated by a phase boundary. Heterogeneous photoinduced changes are also typically restricted to the surface, have individual phase structures that are inaccessible with classical diffraction methods, and possess surface properties that cannot readily be measured by the traditional wetting (water contact angle) technique. In this study, we demonstrate application of Atomic Force Microscopy (AFM) to obtain high spatial resolution surface energy distribution in the trans and cis domains on the surface of azobenzene single crystal. UV excitation of single crystals of 3′,4′-dimethyl-4-(dimethylamino)azobenzene results in domino-like trans-to-cis isomerization on their surface. In the AFM phase channel, this affords contrasting domains with different physicochemical properties. Small amplitude small set point (SASS) method and bimodal AFM operated in the attractive regime provide maps of the tip-sample adhesion force and the Hamaker constant, respectively. The results show that the Hamaker constant of the cis domains (∼1 × 10^-19 J) is higher than that of the trans domains (∼7 × 10^-20 J). After UV irradiation, the calculated surface energies of the domains were ∼40% higher based on the Hamaker constant. Within a broader context, the results presented here demonstrate the potency of AFM-based surface-sensitive techniques for probing of the dynamic changes in surface properties upon photoinduced isomerization of molecular switches.