Angewandte
Chemie
size of hexagonal C6 rings and the substrate lattice.[11] The
molecule corannulene (C20H10) is a curved, C5v-symmetric
aromatic molecule (Figure 1a) with a pentagonal ring cen-
tered among hexagonal rings and thus represents a segment of
C60 buckminsterfullerene.[12] At room temperature, a regular
(4 0, 0 4) array is observed for the complete corannulene
monolayer (Figure 1e).[13] Upon cooling, the 2D crystal
undergoes two different phase transitions. A denser (4 0, 3
7) lattice forms at 225 K, and a (4 2, 0 7) structure forms upon
a further decrease in temperature. Each structure is stable in
its respective temperature interval, as confirmed by the
reversibility of the transitions: The medium-temperature
phase forms again when the low-temperature phase is
heated, and is then converted completely into the (4 0, 0 4)
phase at room temperature. To our knowledge, this is the first
observation of an enantiotropic phase transition in a 2D
organic molecular crystal. Both transitions show a hysteresis
in temperature. The (4 0, 3 7) structure exists at temperatures
between 201 and 248 K. The highest temperature observed
for the (4 2, 0 7) structure was 236 K upon cooling and 271 K
during heating (see Figure S1 in the Supporting Information).
This overlap of temperature ranges indicates that it should be
possible to convert the (4 0, 0 4) phase directly into the (4 2, 0
7) phase and vice versa, as occasionally observed in this study.
However, in these cases the transition could have passed too
quickly through the (4 0, 3 7) phase to be resolved by STM.
Nevertheless, coexistence of the two low-temperature
phases—separated by a phase boundary containing highly
mobile molecules—was observed regularly upon careful
cooling (see Figure S2 in the Supporting Information).
Single molecules in all three phases appear asymmetrical
by STM. This apparent asymmetry indicates a substantial tilt
of the molecular bowl with respect to the surface plane. This
geometry was confirmed by dispersion-corrected density
functional theory (DFT-D) calculations, which showed that
one of the five C6 rings is oriented parallel to the surface
above a threefold hollow site (see Figure S3 in the Supporting
Information). Such an orientation has also been reported for
C60 in its (4 0, 0 4) lattice on Cu(111).[14] With increasing
intramolecular resolution at lower temperatures, a simple
assignment of the upper and middle part of the molecules
becomes possible (Figure 1b–d). In contrast to the (4 0, 0 4)
structure, in which all molecules in a single domain have
identical orientations (Figure 1e), the unit cells of the two
low-temperature phases contain two molecules, each with
different azimuthal orientations. The (4 0, 3 7) intermediate
structure was identified by detailed statistical analysis of next-
neighbor distances and lattice-vector directions in many STM
images (see Figure S4 in the Supporting Information). The
two molecules of the unit cell are located on different sites,
namely, hexagonal-close-packed (hcp) and face-centered-
cubic (fcc) threefold hollow sites.
14.3%. Density changes in 3D solid-state phase transitions
are usually of the order of a few percent.[15] One consequence
of this lattice contraction is that other surface areas have a
lower molecule density. Figure 2 shows an STM image taken
at the boundary between such a low-density area and the
Figure 2. “Frozen 2D gas”. The STM image shows the condensed (4 2,
0 7) lattice (top right) and a low-density disordered phase containing
molecules that appear as perfect fivefold-symmetric doughnuts (red
arrows), some of which are surrounded by six tilted molecules (inset).
Scale bar: 5 nm. T=69 K.
ordered crystal phase at 69 K. This coexistence is a result of a
dynamic equilibrium between the crystal phase and a 2D gas.
The new phase grows until the attachment–detachment
equilibrium is reached. Molecules in the 2D gas phase are
too mobile to be resolved with STM, but they freeze upon
further cooling. When the sample is heated, the disordered
area melts first at about 102 K. In strong contrast to the
appearance of the molecules in the ordered crystal phases,
many molecules in the disordered area appear as fivefold-
symmetric doughnuts (Figure 2, red arrows, inset). This
orientation agrees well with that found for corannulene on
Cu(110) in the ordered lattice, in which the central pentagonal
ring is essentially oriented parallel to the surface, and the
bowl opening is pointing upwards.[16] These differences
between the appearance of molecules in the lattice and the
appearance of molecules in the disordered area strongly
support our conclusion that the molecules in the ordered
structures are substantially tilted (Figure 3a).
Two lattice directions and one intermolecular distance are
identical in the (4 0, 0 4) and (4 0, 3 7) structures. The fact that
two intermolecular distances became shorter in the transition
to the (4 0, 3 7) structure is only compatible with compression
along one direction of close-packed molecules (see Figure S4
in the Supporting Information).
The lattice density changes substantially during the (4 0, 0
4)/(4 0, 3 7) transition, but not for the (4 0, 3 7)/(4 2, 0 7)
transition. The unit-cell area for the two lower-temperature
phases is 1.94 nm2 (28 Cu atoms) for two molecules and for
the room-temperature phase 1.11 nm2 (16 Cu surface atoms)
for one molecule. Thus, upon cooling, with the same number
of molecules on the surface, the lattice density is increased by
When the (4 0, 0 4) phase becomes unstable upon slight
supercooling below the equilibrium temperature, wiggling
motions of molecules in every second row take place with a
period of a few seconds (Figure 4a–d). That is, the STM
images show double rows that form periodically in equivalent
substrate lattice directions. The different contrast of the
molecules in adjacent rows suggests that the motion involves a
Angew. Chem. Int. Ed. 2009, 48, 1966 –1969
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1967