Controlling Molecular Switching via Chemical Functionality; Ethyl vs. Methoxy Rotors

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.