Light-Activated Nanobumps: Photoresponsive Low-Density Azobenzene Self-Assembled Monolayers for Smart Surfaces

High Resolution Image Download MS PowerPoint Slide Controlling the interfacial properties of organic materials using external stimuli is crucial for applications in adhesion, tribology, and electrochemistry. In particular, light irradiation provides a precise, noncontact method for dynamically manipulating molecular structures and surface morphology. Azobenzene-based photoresponsive self-assembled monolayers (SAMs) undergo reversible trans – cis isomerization, inducing configurational changes. However, conventional SAMs form densely packed structures that restrict the free volume required for the efficient isomerization of individual azobenzene molecules. To address this limitation, reducing molecular packing density is essential for achieving functional surface responses to light irradiation. This study presents a strategy for constructing azobenzene-based photoresponsive low-density SAMs by controlling their formation kinetics. By combining sterically bulky substituents in azobenzenethiol with the kinetic modulation of SAM formation, we form low-density SAMs capable of reversible nanoscale morphological changes upon photoisomerization. Unlike conventional high-density SAMs, in which azobenzene moieties remain static, our SAMs experience the reversible emergence and disappearance of nanobumps in response to light. According to the results of X-ray photoelectron spectroscopy and atomic force microscopy analyses, this behavior arises from a specific combination of the molecular packing density and distinct thiolate binding states, which are absent in conventional SAMs. Our findings provide a conceptual mechanistic framework for the engineering of smart surfaces, which are defined as interfaces exhibiting reversible light-induced morphological transformations based on molecular-level control. The proposed design strategy advances the development of light-controllable interfacial systems with promising applications in molecular switches, responsive nanodevices, and nanotribological materials.