Tailoring the properties of surface-immobilized azobenzenes by monolayer dilution and surface curvature

Photoswitching in densely packed azobenzene self-assembled monolayers (SAMs) is strongly affected by steric constraints and excitonic coupling between neighboring chromophores. Therefore, control of the chromophore density is essential for enhancing and manipulating the photoisomerization yield. We systematically compare two methods to achieve this goal: First, we assemble monocomponent azobenzene–alkanethiolate SAMs on gold nanoparticles of varying size. Second, we form mixed SAMs of azobenzene–alkanethiolates and “dummy” alkanethiolates on planar substrates. Both methods lead to a gradual decrease of the chromophore density and enable efficient photoswitching with low-power light sources. X-ray spectroscopy reveals that coadsorption from solution yields mixtures with tunable composition. The orientation of the chromophores with respect to the surface normal changes from a tilted to an upright position with increasing azobenzene density. For both systems, optical spectroscopy reveals a pronounced exciton...