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

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, non-contact method for dynamically manipulating molecular structures and surface morphology. Azobenzene-based photoresponsive self-assembled monolayers (SAMs) undergo reversible trans–cis isomerization, inducing conformational changes. However, conventional SAMs form densely packed structures that restrict the free volume required for efficient isomerization. To address this limitation, reducing molecular packing density is essential to achieving functional surface responses to light irradiation. This study addressed the challenge of constructing azobenzene-based photoresponsive low-density SAMs by controlling their formation kinetics. We demonstrate that combining bulky substituents in azobenzenethiol with kinetic control of SAM formation enables the creation of low-density SAMs capable of dynamic surface morphological changes upon photoisomerization. Unlike conventional densely packed SAMs, where the azobenzene moieties remain static, the proposed approach facilitates the reversible emergence and disappearance of nanoscale "nanobumps.” X-ray photoelectron spectroscopy and atomic force microscopy analyses confirm this behavior results from a combination of molecular packing density and thiolate binding states, which are absent in conventional high-density SAMs. This study offers new insights into the design of functional photoresponsive surfaces and presents a strategy for developing light-controllable interfacial systems with potential applications in molecular switches and adaptive coatings.