Switching of an Azobenzene-Tripod Molecule on Ag(111)

Abstract

The trans-cis isomerization makes azobenzene (AB) a robust molecular switch. Once adsorbed to a metal, however, the switching is inefficient or absent due to rapid excited-state quenching or loss of the trans-cis bistability. We find that tris-[4-(phenylazo)-phenyl]-amine is a rather efficient switch on Ag(111). Using scanning tunneling and atomic force microscopy at submolecular resolution along with density functional theory calculations, we show that the switching process is no trans-cis isomerization but rather a reorientation of the N-N bond of an AB unit. It proceeds through a twisting motion of the azo-bridge that leads to a lateral shift of a phenyl ring. Thus, the role of the Ag substrate is ambivalent. While it suppresses the original bistability of the azobenzene units, it creates a new one by inducing a barrier for the rotation of the N-N bond.

Light-Induced Spin Crossover in an Fe(II) Low-Spin Complex Enabled by Surface Adsorption

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14] Conversely, for functional (e.g., switchable) molecules, the adsorption of such molecules on surfaces is often detrimental to their intrinsic functionality. 15 Recently, substantial efforts focused on preventing or minimizing the so far unfavorable effect of the substrate on spin-crossover (SCO) complexes, a particular class of functional molecules, whose spin state can be switched by various stimuli including light. 16 Indeed, the spin bistability or even the integrity of the fragile SCO complexes are generally affected by the proximity of metal surfaces.…”

Reversible Photoswitching and Isomer‐Dependent Diffusion of Single Azobenzene Tetramers on a Metal Surface

“…[14] Azobenzene adsorption on metal surfaces typically results in quenching of their light-induced isomerization, owing to electronic coupling to the substrate, [15] which can be weakened by adding bulky side groups to the azobenzene unit, thus reducing the interaction with the surface. [16] Another strategy to enhance the photomechanical activity of molecular switches on as urface is to integrate them into molecular architectures that are inherently rigid and threedimensional, such as tripod structures.F or example,r eversible photoisomerization was observed for (tetra)molecular architectures consisting of three adamantane moieties [17] or thiols [18,19] anchored to the surface and afourth branch with an azobenzene group.T his approach ensures stable adsorption through the tripod linker and an upright arrangement of the molecular switch unit, and thus electronic decoupling from the surface.Whenintegrating two [20] or three [21,22] azobenzene groups into one and the same molecule,the flexible structure of these molecules resulted in flat conformations on the surface,a sa ll azobenzene moieties were adsorbed planar. Hence,avertical orientation and/or the electronic decoupling of the switching units from the surface was not achieved, and molecular architectures that contain several switching units and maintain their three-dimensional conformation upon adsorption at as urface were not realized so far.…”

“…This interpretation is in agreement with scanning probe microscopy studies of self-assembled monolayers (SAMs) [25,26] or tripod molecules, [18,19] resulting in either large or small apparent heights in the images if the upright azobenzene group is in the trans or cis state,r espectively.W et herefore assigned the "bright" and "dark" lobes on the surface to the trans and cis configuration, respectively,o ft he upwards pointing arm of the tetramer.I no ther words:t he trans isomers appear bright, owing to their rather linear structure pointing approximately vertically off the surface,a nd the cis isomers appear darker.T he other three arms (see Figure S5), which are adsorbed at the surface,a re most likely always in the trans state. [21,27] This assignment is on one hand based on previous studies,i nw hich the trans state of azobenzene derivatives is the energetically most stable form in the gas phase, [1] in the solid state, [8] and on various metal surfaces [9,10,14,27] after deposition at room temperature.I mportantly,also tri-azobenzene systems adsorb on Ag(111) with all azobenzene arms in the trans state, [21] ac onfiguration that is Figure 1. Tetra-azo islands on Ag(111).…”

Tuning the contact conductance of anchoring groups in single molecule junctions by molecular design

“…Multipodal platforms were developed recently to establish a directional attachment of molecules to metallic surfaces. 1,2 Tripodal scaffolds are their most common representatives and include triazatriangulene, [3][4][5] trioxatriangulene, 6 cyclohexane trithiol, 7 adamantane, [8][9][10][11] tris(azobenzyl)amine, 12 spirobifluorene, [13][14][15][16] tetraphenylmethane [17][18][19][20][21][22] and tetraphenylsilane 23,24 moieties. The principal function of molecular electronic components is the transport of electric charge.…”