Copper
phthalocyanine (CuPc) is a small molecule often used in
organic light emitting diodes where it is deposited on a conducting
electrode. Previous scanning tunneling microscopy (STM) studies of
CuPc on Cu(111) have shown that inelastic tunneling events can cause
CuPc to switch between a ground state and two symmetrically equivalent
metastable states in which the molecule is rotated. We investigated
CuPc on Cu(111) and Ag(111) with STM and lateral force microscopy
(LFM). Even without inelastic events, the presence of the tip can
induce rotations and upon closer approach, causes the rotated states
to be favored. Combining STM measurements at various temperatures
and LFM measurements, we show that the long-range attraction of the
tip changes the potential energy landscape of this molecular switch.
We can also determine the geometry of the rotated and ground states.
We compare our observations of CuPc on Cu(111) to CuPc on Ag(111).
On Ag(111), CuPc appears flat and does not rotate. Stronger bonding
typically involves shorter bond lengths, larger shifts of energy levels,
and structural stability. Although the binding of CuPc to Cu(111)
is stronger than that on Ag(111), the nonplanar geometry of CuPc on
Cu(111) is accompanied by two metastable states which are not present
on the Ag(111) surface.