Direct monitoring of opto-mechanical switching of self-assembled monolayer films containing the azobenzene group

The potential for manipulation and control inherent in molecule-based motors holds great scientific and technological promise.Molecules containing the azobenzene group have been heavily studied in this context. While the effects of the cis–trans isomerizationof the azo group in such molecules have been examined macroscopically by a number of techniques, modulations of the elasticmodulus upon isomerization in self-assembled films were not yet measured directly. Here, we examine the mechanical responseupon optical switching of bis[(1,1'-biphenyl)-4-yl]diazene organized in a self-assembled film on Au islands, using atomic forcemicroscopy. Analysis of higher harmonics by means of a torsional harmonic cantilever allowed real-time extraction of mechanicaldata. Quantitative analysis of elastic modulus maps obtained simultaneously with topographic images show that the modulus of thecis-form is approximately twice that of the trans-isomer. Quantum mechanical and molecular dynamics studies show good agreementwith this experimental result, and indicate that the stiffer response in the cis-form comprises contributions both from the individualmolecular bonds and from intermolecular interactions in the film. These results demonstrate the power and insights gainedfrom cutting-edge AFM technologies, and advanced computational methods.