Self-Assembled Monolayers at the Solid–Liquid Interface
COMMUNICATION
Importantly, such a dimeric motif was visualized only upon
illumination of the monolayer with UV light. Finally, such
observations, together with the non-photoresponsive charac-
ter of the bare platform bearing no azobenzene (see mole-
cule 1’ in the Supporting Information, Figure S4), strongly
support our notion that the monolayer observed after UV
light irradiation is based on cis-1 molecules.
Importantly, the reversible structural reorganization of
the monolayer of 1 from trans to cis motif could be moni-
tored in real-time over several tens of minutes, taking ad-
vantage of the slow nature of the process. Figure 2 shows se-
lected snapshots of these measurements where the trans do-
mains are gradually converted into cis domains over a time
scale of 55 min (Figure 2a-d), presumably driven by the
more stable packing of the cis form, accumulating over time.
The total coverage of the surface by the cis assembly is com-
pleted within 55 min of irradiation (Figure 2d). The fuzzy
parts on the STM images correspond to areas where the
considerably high mobility of the molecules, triggered by
the irradiation process, hinders high resolution STM imag-
ing.
The present results provide the first examples of sub-mo-
leculary resolved vertically oriented switchable photo-chro-
mophores in physisorbed monolayers as well as vertically
physisorbed azobenzenes monitored in the two switching
states at the solid–liquid interface. We are currently investi-
gating the possibility to pin this type of building blocks on
carbon-based surfaces via a two-step process involving first
the physisorption of the molecules with controllable spacing
on the surface followed by their chemisorption via reaction
with the underlying HOPG or graphene. Suitable reactive
groups can be attached to the C9 positions of the fluorene
units using well-established nucleophilic substitution strat-
egies or connected to the ends of the alkyl chains. By offer-
ing the possibility to control switching of azobenzenes when
placed at tunable distance of one another, our scaffold could
in principle allow us to explore fundamental photochemical
properties of non-interacting azobenzenes at surfaces and to
develop azobenzene-based unirotational switch assemblies
by performing the isomerization inside a chiral medium, or
using polarized light.[25]
The formation of cis-1 monolayers, from a solution in
which 66% of molecules 1 remain in their trans configura-
tion upon UV illumination, as established by UV/Vis spec-
troscopy, is interesting. This observation provides indirect
evidence for the enhanced stability of the supramolecular
architectures composed of cis-1 as compared to ones formed
from trans-1. This increased stability should be reflected by
a significantly higher packing density of cis-1 molecules
when compared to trans-1, that is, a smaller area occupied
by a single molecule physisorbed on the HOPG surface.
Indeed, the areas occupied by single molecule in the trans
and cis form amount to (4.88ꢁ0.47) and (4.35ꢁ0.34) nm2,
respectively. Such a difference indicates that the switching
process has occurred by desorption of trans-1 and subse-
quent re-adsorption of cis-1 molecules present in the super-
natant solution (as established by solution studies, after one
hour irradiation at 357 nm the supernatant solution contains
34% of cis-1 molecules, see Supporting Information).
Upon irradiation of the cis-1 monolayer with visible light
(white light bulb) the trans monolayer 2D motif is reformed
again (Figure 2e,f). A short illumination (Figure 2e) leads
to monolayers exposing the two coexisting isomers, that is,
trans (marked in red) and cis (marked in green). Further ir-
radiation resulted in desorption of the molecules, which
after some time (ca. 40 min) re-adsorb in the trans-type of
assembly (Figure 2 f), proving the reversible nature of the
process.
Acknowledgements
The authors thank Dr. Steffen Weidner (BAM) for acquiring MALDI-
TOF mass spectra. Generous support by the German Research Founda-
tion (DFGvia SFB 658), the EC Marie Curie project SUPERIOR
(PITN-CT-2009-238177), the EC FP7 ONE-P large-scale project no.
212311, the NanoSci-E+ project SENSORS, and the International
Center for Frontier Research in Chemistry (FRC) is gratefully acknowl-
edged. Wacker Chemie AG, BASF AG, Bayer Industry Services, and
Sasol Germany are thanked for generous donations of chemicals.
Keywords: azobenzenes · photochromism · scanning probe
microscopy · self-assembly · surfaces
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In summary, the self-assembly of a properly designed 3D
building block (1) exposing a vertically oriented azobenzene
when physisorbed on HOPG in its trans form has been in-
vestigated by STM. The switching of the monolayer consist-
ing of upright oriented trans-azobenzenes to the correspond-
ing monolayer based on cis-azobenzenes has been triggered
by light, leading to a significant structural change of the
self-assembly in the third dimension (decrease of height) as
well as in the plane of the substrate (different 2D packing).
[8] Diarylethene can reversibly isomerized in the solid state revealing
the small geometrical difference between the structure of the ring-
opened and ring-closed forms which translate into macroscopic
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