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visualized coexisting with the disordered β phase, thus unambigu-
ously proving the presence of all of the three different types of
packing described earlier.
the γ phase to the β phase; synthesis and characterization of
PQPClO4; experimental details of the STM measurements. This
org.
Having established their existence, we begin to understand the
relative stabilities of the three phases as follows. The relative
occurrence of the three phases on the surface of HOPG reflects that
the β network of molecules is the most preferred arrangement,
followed by the Rphase, whereas the γphase is a metastable phase. It
must be noted at this juncture that both R and γ phases consist of
ordered arrays of molecules, which is entropically unfavorable. In
contrast, the β network is rather disordered and hence entropically
favored. On the other hand, the calculated packing densities of R, β,
and γ phases are 0.52, 0.61, and 0.58 molecules/nm2, respectively.
On the basis of the molecular densities, the binding enthalpy is
expected to be highest for the β phase. Thus, considering both the
enthalpic and entropic contributions, the preference for the β phase
appears to be reasonable. However, one should also consider the
contribution from the long-range electrostatic interactions in the self-
assembly of these charged molecules. Such repulsive interactions will
prevent the molecules from coming too close to each other. As a
consequence, it is not unreasonable to expect a lower surface density
phase to survive on the surface, which readily explains the stability of
the R phase. In a nutshell, the preference for the β phase arises due to
a complex interplay between long-range electrostatic interactions,
packing density, and entropic considerations.
’ AUTHOR INFORMATION
Corresponding Author
muellen@mpip-mainz.mpg.de; Steven.DeFeyter@chem.kuleuven.be
’ ACKNOWLEDGMENT
This work is supported by the Fund of Scientific ResearchÀ
Flanders (FWO), K.U. Leuven (GOA 2006/2), and Belgian
Federal Science Policy Office (IAP-6/27).
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Experimental evidence suggests that switching is possible at
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’ ASSOCIATED CONTENT
S
Supporting Information. Additional STM images of the
b
three phases; STM images depicting the instantaneous nature of the
phase transformation as well as time -dependent transformation of
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dx.doi.org/10.1021/ja111699n |J. Am. Chem. Soc. 2011, 133, 5686–5688