Controlling Molecular Switching via Chemical Functionality; Ethyl vs. Methoxy Rotors
2019
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.
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