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

Abstract

SummaryThe 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 isomerization of the azo group in such molecules have been examined macroscopically by a number of techniques, modulations of the elastic modulus upon isomerization in self-assembled films were not yet measured directly. Here, we examine the mechanical response upon optical switching of bis[(1,1'-biphenyl)-4-yl]diazene organized in a self-assembled film on Au islands, using atomic force microscopy. Analysis of higher harmonics by means of a torsional harmonic cantilever allowed real-time extraction of mechanical data. Quantitative analysis of elastic modulus maps obtained simultaneously with topographic images show that the modulus of the cis-form is approximately twice that of the trans-isomer. Quantum mechanical and molecular dynamics studies show good agreement with this experimental result, and indicate that the stiffer response in the cis-form comprises contributions both from the individual molecular bonds and from intermolecular interactions in the film. These results demonstrate the power and insights gained from cutting-edge AFM technologies, and advanced computational methods.

First-principle-based MD description of azobenzene molecular rods

“…Lennard-Jones (LJ) parameters for the benzene backbone are taken from the standard OPLS/ AA set of parameters. They have been originally derived to simulate liquid benzene and have been successfully tested [11]. For the remaining LJ parameters, we refer to the work of Schäfer [19].…”

“…Molecular modeling at the classical atomistic level has already proved to be useful in the interpretation of the behavior of a SAM [9,10]; in particular, it has been successfully applied to the study of elastic properties of azobenzene SAMs at the nanoscale [11].…”

Dynamic nanoindentation by instrumented nanoindentation and force microscopy: a comparative review

“…Operating the AFM under dynamic mode has distinct advantages in reducing the sample damage, particularly for delicate samples [ 81 – 82 ]. Also, the volume needed for probing is reduced even further, which allows for the analysis of small areas and thin films down to a single monolayer [ 83 ]. A recent example is the application of AFM imaging together with the mechanical measurement to give a detailed insight on cellular membrane mechanics, which is only meaningful when the viscoelastic response is accounted for [ 84 ].…”

Reactive molecular dynamics simulations of switching processes of azobenzene-based monolayer on surface

“…However, the nano-fabrication of the molecule-based electronic devices requires the deposition of numerous moleca) majing@nju.edu.cn ular switches on surfaces. [19][20][21][22][23][28][29][30][31] For example, a series of recent studies focused on the terminally thiol functionalized azobiphenyls (AZOs), which is one kind of the azobenzene derivatives and can be coated on the Au surface to form self-assembled monolayers (SAMs), as shown in Scheme 1. 15,[21][22][23][24][28][29][30] This SAM has a potential application in the stimuli-induced switching devices, such as the molecular machine acting as a cargo lifter.…”