Exploiting Light Interferences To Generate Micrometer-High Superstructures From Monomeric Azo Materials With Extensive Orientational Mobility

Photochromic azo materials have stirred considerable interest for their ability to mechanically respond to polarized light through large photoinduced migration and orientation processes. In order to apprehend the microscopic dynamics behind the extensive mass transport occurring under interferential illumination, two azo compounds differing by their propensity to form hydrogen bonds are synthesized and processed as nondoped glassy thin films. Interferential irradiation using polarization and intensity patterns reveals fully distinct responses. Regular nanometer-high surface relief gratings transform into micrometer superstructures with an amplitude ten times higher than the initial film thickness when using the latter polarization. Systematic comparisons between the azo materials in terms of thermal properties, photochromism in solution and in the solid state, and photomigration are carried out. The progressive formation of superstructures is ascribed to two successive processes. The first one relates to fast photoinduced migration due to the impinging structured light, and the second one is promoted by slower thermally activated "zig-zag"-like diffusion and Z-E thermal relaxation, which in turn requests high orientational mobility of the azo compounds and causes large nanomechanical changes. Such studies should provide novel structural guidelines in terms of material fluidity to rapidly achieve highly structured and rewritable materials at low light irradiance.