Controlling Molecular Switching via Chemical Functionality: Ethyl vs Methoxy Rotors

Abstract

Surface-bound molecular rotors provide a useful way to study the structure and dynamics of molecular motion at the single-molecule level. However, when most molecules adsorb on a metal surface, their interaction with the metal changes their properties dramatically, making a priori design impossible. We report a case in which gas-phase predictions of the stable orientations of a class of molecular rotors hold true when they are attached to a surface. This transferability is achieved by mounting the molecular rotor moiety on a metal–organic complex formed as an intermediate in the surface-catalyzed Ullmann coupling reaction of 1-bromo-4-ethylbenzene versus 1-bromo-4-methoxybenzene. Gas-phase calculations predict that, while the ethyl molecular rotor is most stable when oriented perpendicular to the phenyl ring, the methoxy rotor’s stable orientation is in plane with the phenyl ring. Our STM imaging results confirm this behavior, with the methoxy rotor exhibiting switching in plane with the surface versus the ethyl rotor, which switches out of plane with respect to the surface. Furthermore, the two rotors exhibit different rotational excitation characteristics. Action spectra measurements reveal that, while the threshold voltage for direct excitation of the rotational process of the ethyl rotor is identical to the rotational barrier (45 meV), the methoxy rotors require a significantly larger applied voltage (300 mV) than the 128 meV torsional barrier calculated for methoxybenzene in the gas phase. Density functional theory (DFT) calculations of a methoxybenzene molecule on Cu(111) reveal that, while interaction with the Cu(111) surface does not change the preferred orientations of the methoxy rotor, the barrier for rotation is raised to 246 meV, which is much closer to that observed experimentally. This study offers insight into the factors determining the dynamics of molecular rotors based on both the chemical nature of the rotor and its interaction with the surface.

Rotation of Ethoxy and Ethyl Moieties on a Molecular Platform on Au(111)

“…Rotor molecules deposited on surfaces may be investigated at a submolecular scale using scanning tunneling microscopy (STM). , The compounds thus studied fall into two classes. Either molecular adsorbates serve as a rotor and the substrate is used as a stator − or the stator and the rotor are subunits of the same molecule. − Besides rotors there are also rotary switches that alternate between two rotation angles − and consequently do not enable studies on the directionality of the motion, which is essential for using a rotor in a motor. , …”

Visualizing and Understanding Ordered Surface Phases during the Ullmann Coupling Reaction

“…Our DFT calculations confirmed that the −CH 2 CH 3 /–CH 2 CF 3 tails of intermediates and products 1, 2, and 3 are most stable when oriented perpendicular to the phenyl ring, while the methoxy tail of I 4 and P 4 are most stable when in plane with the phenyl ring (Figure S1). − In Figure A, the I 1 species has relatively small charges and, based on the orientation of the organometallic intermediates, there does not seem to be a significant relationship between any local dipoles and its packing structure. However, in Figure B, the orientation of the I 2 organometallic intermediates appears to suggest a relationship as the highly electronegative fluorine groups, which are aligned with the hydrogens on the phenyl rings of adjacent intermediates.…”

“…Through the use of surface techniques, such as scanning tunneling electron microscopy (STM), the formation and surface diffusion of Ullmann coupling intermediates have been studied by tracking the reaction from the starting aryl reagents, through the intermediates and to the final biaryl products, and this mechanism is summarized schematically in Figure . − On a Cu(111) surface, Ullmann coupling intermediates do spontaneously self-assemble into 2D crystals and these assemblies have demonstrated interesting properties ranging from a diversity of domain types to molecular-scale machines. − Due to current limitations in 2D crystal engineering, it is not yet possible to a priori predict the relationships between the molecular structure of the aryl halide precursor and the crystal packing of the intermediate and final biaryl product; oftentimes, slight changes in the starting reagent result in a huge impact on the packing structures of the intermediate and product. One of the key features of the Ullmann coupling reaction on a copper surface is that, in the intermediate stage, the aryls to be coupled are bonded to a copper atom that is removed from the surface (Figure , middle panel). ,,, Compared to the biaryl product, the intermediates are slightly larger and pack quite differently as this manuscript will show.…”

Hypothetical Efficiency of Electrical to Mechanical Energy Transfer during Individual Stochastic Molecular Switching Events

“…Figure A–C shows representative images in frequency shift, AFM tip tunneling current, and STM modes of the 2D arrays, respectively. We have previously studied the structures of these 2D molecular rotor arrays. − Briefly, when 4-bromo-1-ethyl-2-fluorobenzene is deposited on a Cu(111) surface and annealed to 260 K, the C–Br bond is broken, and organometallic intermediates of the Ullmann coupling reaction form on the surface, as shown in Figure E. These organometallic intermediates spontaneously self-assemble into a variety of 2D crystal structures on the Cu(111) surface.…”