Surface-induced selection during in situ photoswitching at the solid/liquid interface

Article abstract of DOI:10.1002/anie.201412215

Here we report for the first time a submolecularly resolved scanning tunneling microscopy (STM) study at the solid/liquid interface of the in situ reversible interconversion between two isomers of a diarylethene photoswitch, that is, open and closed form, self-assembled on a graphite surface. Prolonged irradiation with UV light led to the in situ irreversible formation of another isomer as by-product of the reaction, which due to its preferential physisorption accumulates at the surface. By making use of a simple yet powerful thermodynamic model we provide a quantitative description for the observed surface-induced selection of one isomeric form.

Full text of DOI:10.1002/anie.201412215

Angewandte
Chemie
DOI: 10.1002/anie.201412215
Self-Assembly
Surface-Induced Selection During In Situ Photoswitching at the Solid/
Liquid Interface**
Sara Bonacchi, Mohamed El Garah, Artur Ciesielski, Martin Herder, Simone Conti,
Marco Cecchini,* Stefan Hecht,* and Paolo Samorꢀ*
Abstract: Here we report for the first time a submolecularly
resolved scanning tunneling microscopy (STM) study at the
solid/liquid interface of the in situ reversible interconversion
between two isomers of a diarylethene photoswitch, that is,
open and closed form, self-assembled on a graphite surface.
Prolonged irradiation with UV light led to the in situ irrever-
sible formation of another isomer as by-product of the
reaction, which due to its preferential physisorption accumu-
lates at the surface. By making use of a simple yet powerful
thermodynamic model we provide a quantitative description
for the observed surface-induced selection of one isomeric
form.
tures with a nanoscale control over their mechanical move-
ment, aiming at the development of molecular machines and
switches.
Molecular switches, when designed ingeniously, can be
fuelled at surfaces by natureꢀs most abundant and powerful
energy source—light. The respective photochromic systems
are small organic molecules, which are capable of undergoing
efficient and reversible photochemical isomerization between
two or more (meta)stable states featuring markedly different
properties. Among photochromic systems, diarylethenes have
been extensively studied because the two isomers (open and
closed) are both thermodynamically stable and the cyclization
reaction is very fast as it occurs on the picoseconds time-
scale.[4] In addition, upon photoisomerization diarylethenes
exhibit not only a dramatic change in their electronic
properties (i.e., orbital energies and electronic transitions)
suitable for (opto)electronic devices, such as memories[5] and
switches,[6] but they also show a significant change in their
conformational flexibility/rigidity, which provides remote
control over their molecular organization at surfaces.
Scanning tunneling microscopy (STM) is an established
tool to investigate structures and numerous physical and
chemical properties of molecules at surfaces with a sub-
nanometer spatial resolution.[7] Its use to explore molecular
physisorption at interfaces is particularly appealing as it
makes it possible to explore in situ and in real-time dynamic
processes including chemical reactions[8] and switching pro-
cesses.[9] Diarylethene-based self-assembled monolayers in
their open and closed forms were previously explored by
STM;[10] however, to the best of our knowledge, the in situ
reversible interconversion between the two switching states,
that is, ring-open and ring-closed isomers, has never been
resolved by STM with high-resolution imaging.
I
n the last two decades the design and synthesis of
sophisticated building blocks programmed to interact through
noncovalent forces made it possible to construct with a sub-
nanometer precision various self-assembled systems and
materials possessing tunable chemical and physical proper-
ties.[1] In particular, supramolecular chemistry provides
exquisite control over molecular self-assembly,[2] which com-
bines reversibility, directionality, specificity, and cooperativi-
ty.[1d,3] In the past few years there has been an increasing effort
toward the fabrication of functional supramolecular architec-
[*] Dr. S. Bonacchi,[+] Dr. M. El Garah,[+] Dr. A. Ciesielski,
Prof. Dr. P. Samorꢀ
Nanochemistry Laboratory, ISIS & icFRC
Universitꢁ de Strasbourg & CNRS
8 allꢁe Gaspard Monge, 67000 Strasbourg (France)
E-mail: samori@unistra.fr
M. Herder, Prof. Dr. S. Hecht
Department of Chemistry, Humboldt-Universitꢂt zu Berlin
Brook-Taylor-Str. 2, 12489 Berlin (Germany)
E-mail: sh@chemie.hu-berlin.de
Here, we report for the first time a submolecularly
resolved STM investigation of the in situ light-induced
interconversion between both ring-open and ring-closed
diarylethenes at the solid/liquid interface. We have focused
our attention on 1,2-bis(2-methyl-5-(4-octadecyloxycarbonyl-
phenyl)thien-3-yl)cyclopent-1-ene (DAE, Figure 1a). The
presence of long aliphatic side chains in the para-positions
of both terminal phenyl rings was expected to promote the
molecular physisorption on highly oriented pyrolitic graphite
(HOPG). Since the interactions between molecules and the
HOPG surface are of van der Waals type,[7a,9g] they offer an
interesting dynamic scenario characterized by potential
exchange based on desorption and re-adsorption.[8h]
S. Conti, Dr. M. Cecchini
Laboratory of Molecular Function and Design, ISIS & icFRC
Universitꢁ de Strasbourg & CNRS
8 allꢁe Gaspard Monge, 67000 Strasbourg (France)
E-mail: mcecchini@unistra.fr
[+] These authors contributed equally to this work.
[**] This work was supported by the EC through the Marie Curie IEF
GALACTIC (PIEF-GA-2014-628563), the ERC projects SUPRA-
FUNCTION (GA-257305) and Light4Function (GA-308117), the
Agence Nationale de la Recherche through the LabEx project
Chemistry of Complex Systems (ANR-10-LABX-0026_CSC), the
International Center for Frontier Research in Chemistry (icFRC) as
well as the German Research Foundation (DFG through SFB 658).
S.C. thanks the University of Strasbourg (IdEX program Nr. 16141)
for his PhD fellowship.
To verify that the intrinsic photoswitching ability of the
DAE was maintained, absorption spectra of the ring-open
isomer DAE-o (for synthesis and characterization see the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201412215.
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Figure 1. a) Chemical structures and UV/Vis spectra of investigated diarylethene in its open (DAE-o), closed (DAE-c), and closed containing 60%
of (irreversible) by-product (DAE-b) form. Height STM images of monolayers of b) DAE-o, d) DAE-c, and f) DAE-b at the liquid/graphite interface
self-assembled from solution in 1-phenyloctane (tunneling parameters: average tunneling current (It)=25–30 pA, bias voltage
À650 mVꢀVt ꢀÀ400 mV). Minimized molecular packing motifs of c) DAE-o, e) DAE-c, and g) DAE-b (see molecular modeling in SI for details).
Supporting Information, SI) were recorded in 1-phenyloctane
solution (Figure 2a,c). UV irradiation (310 nm) generates
a new absorption band in the visible part of the spectrum (at
ca. 560 nm), which constitutes a characteristic footprint of the
closed-ring isomer (DAE-c). Upon visible-light irradiation
(530 nm), the DAE-c was converted back to DAE-o and its
original spectrum was restored. Noteworthy, the isosbestic
point at 363 nm remains throughout the entire irradiation,
ruling out the appearance of side products in solution during
the photoisomerization cycle upon light exposure up to about
several tens of minutes (see Figures S1 and S2).
Initially, we investigated the self-assembly of DAE-o by
applying a drop of a 0.5 mm solution of DAE-o in 1-
phenyloctane onto the HOPG surface. The STM image of
the obtained monolayer (Figures 1b and S3) showed a crys-
talline structure that consists of lamellar architectures. The
observed crystalline domains exhibit a unit cell: a = (1.5 Æ
0.1) nm, b = (3.4 Æ 0.1) nm, a = (79 Æ 2)8 leading to an area
A = (5.0 Æ 0.1) nm2, in which each unit cell contains one
molecule DAE-o (Figure 1c). In this 2D crystal, because of
the steric hindrance between thienyl groups, the cores of
DAE-o are partially physisorbed on the surface and adopt U-
shape conformation. Furthermore, the octadecyl side chains
are most likely physisorbed flat on the surface adopting
a parallel conformation, that is, with both alkyl chains
pointing toward the same direction, although due to their
high conformational dynamics they could not be resolved
with STM.
As for DAEs only the antiparallel conformation is
photoactive;[11] ring-closure cannot be induced within the
monolayer but only in solution. The stability of the crystalline
monolayer, on the time scale of several minutes, and the
2
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Chemie
monolayers with green light resulted
in desorption followed by the read-
sorption of DAE-o in its favored self-
assembly structure.
According to the previous
reports[12] and in line with recent
investigations by some of us,[13] a pro-
longed UV irradiation of diaryl-
ethenes can induce the formation of
a photoirreversible by-product, com-
posed of an annulated polycyclic
core (as shown for DAE-b in Fig-
ure 1a, right).[10c,d] Interestingly,
a longer in situ UV irradiation of
both the DAE-o and DAE-c mono-
layers (t > 20 min and 6 min, respec-
tively) resulted in an unexpected and
different 2D assembly (Figures 1 f,
S5, and S6) on graphite. This new
assembly pattern exhibits unit cell
parameters: a = (0.9 Æ 0.3) nm, b =
(4.0 Æ 0.5) nm, a = (79 Æ 2)8 leading
to an area A = (3.5 Æ 0.5) nm2, in
which each unit cell contains one
diarylethene molecule. In this 2D
crystal, interdigitated octadecyl
chains are clearly visible in the
STM image. The brighter parts of
the STM image can be assigned to
the polycyclic core of the by-product
molecules; the fuzzy contrast
strongly suggests that these cores do
Figure 2. Photochromic behavior of DAE in solution: UV/Vis absorption spectra of DAE
(c=0.5 mm) in cyclohexane/1-phenyloctane (1:5) during a) the course of UV and visible irradiation
(lirr =310 nm and 530 nm, respectively) from the ring-open isomer DAE-o to the photostationary
state, consisting almost exclusively of the ring-closed isomer DAE-c, and back. b) Formation of the
by-product DAE-b upon continuing UV irradiation (lirr =310 nm) of DAE-c. c) and d) time-depen-
dent absorption changes at 330 nm (empty squares) and at 560 nm (full squares) during the (a)
and (b) isomerization studies, respectively.
possibility to record highly resolved images indicates that the
molecules are immobilized on the surface, that is, the
supramolecular motif is stabilized by relatively strong mole-
cule–graphite van der Waals interactions.
not interact strongly with the underlying HOPG surface. It is
indeed most likely that the core of the molecules is located
out-of-plane with respect to the octadecyl side chains;
therefore their conformational mobility is extremely high
during STM mapping resulting in fuzzy areas in STM pictures.
Noteworthy, irradiation of this new type of monolayer with
either blue and/or green light did not alter the packing motif,
indicating that the molecules cannot be switched back either
to the open or to the closed form. In light of these findings, we
can ascribe the observed 2D motif in the STM images
(Figure 1 f) to the by-product form of diarylethene, that is,
DAE-b.
To confirm the presence of the DAE-b on the HOPG
substrate, we investigated, ex situ, the photophysical proper-
ties of the DAE-o solution under UV light (Figures 2b, S1,
and S2). As expected,[10c,d] the absorption maximum of the by-
product is slightly blue-shifted (540 nm) and broader when
compared to the DAE-c form. The quantity of by-product
formed starting from a DAE-c solution depends on the time
of irradiation, being up to a 60% in content after 10 h
(Figure 2d). Noteworthy, the self-assembled motif in 2D
monitored on samples prepared by applying a drop of the
ex situ prepared DAE-b to HOPG perfectly matches with the
one observed after a prolonged UV irradiation in situ (Fig-
ure S9).
STM was used to probe the in situ photoisomerization of
DAE-o self-assembled structures, by irradiation with UV
light. To this end, the preexisting monolayer of DAE-o was
irradiated for 5 min with UV light (310 nm). In all experi-
ments we have performed, the irradiation of DAE-o mono-
layers resulted in the desorption of DAE-o molecules
followed by the molecular readsorption to form a new type
of 2D crystalline motif (Figure 1d). This new 2D pattern
exhibits unit cell parameters: a = (1.0 Æ 0.1) nm, b = (4.1 Æ
0.1) nm, a = (79 Æ 2)8 leading to an area A = (4.0 Æ 0.1) nm2,
in which each unit cell contains one molecule (Figure 1e).
Noteworthy, the unit cell parameters of this crystalline
structure are in perfect agreement with those extracted
from STM images of monolayers obtained from DAE-c
molecules obtained by ex situ irradiation of DAE-o solution
(see Figures S7 (DAE-o) and S8 (DAE-c)). In the STM
images one can notice three bright features linearly aligned to
the main lamellar axis which can be assigned to the two
phenyl rings and to the core of the DAE-c molecules, the
latter appearing brighter due to its extended p-conjugation.
Importantly, the DAE-c monolayers can be converted in situ
back to the DAE-o motif if irradiated with 530 nm light for
20 min (Figures S2 and S6). Hence, irradiation of DAE-c
The coexistence of the by-product and the ring-closed
isomer in the solution makes it interesting to explore their
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self-assembly on graphite in the framework of competitive
physisorption at the solid/liquid interface. By applying a drop
of a mixture consisting mostly of the ring-closed isomer
(DAE-c/DAE-b = 92:8), only patterns of the by-product were
found by STM, which indicates a surface-induced selection of
the by-product. Importantly, the selective adsorption
observed here allows for the enrichment (and potential
removal) of the minor by-product of the photochemical
reaction in solution.
energy between the open form and the others, that is, the
higher the density the higher the energetic gain per surface
area. The physisorption of the by-product is preferred over
that of the closed isomer as a result of opposing contributions.
On one hand, the adsorption energy per DAE-b molecule is
weaker by 11.4 kcalmolÀ1 if compared to DAE-c. On the
other hand, the higher packing density of DEA-b molecules
results in intermolecular interactions in the monolayer
enhanced by 13.5 kcalmolÀ1 per unit cell. As shown in
Figure 3, the core of the diarylethene derivatives is face-on
in the DAE-c architecture, whereas it is edge-on in the DAE-
To gain a more quantitative understanding of the surface
selectivity for the by-product, we performed a simple ther-
modynamic analysis of the 2D
self-assembly of the open, the
closed, and the by-product iso-
mers on graphene. To this end,
the free energy cost of covering
a given surface area of substrate
was estimated by modeling the
self-assembly of freely diffusing
monomers in the gas phase to
defect-free crystalline architec-
Figure 3. Magnification of architectures for the a) DAE-o, b) DAE-c, and c) DAE-b isomers obtained by
molecular modeling. Note the parallel conformation of DAE-o, the different packing of DAE-c and DAE-b
due to their different alkyl chain conformations, and the overlap of the cores in DAE-b, which is not
present in DAE-c.
tures on graphene.[14] In this
model, the energy of forming
the 2D nanopattern includes
contributions from both molec-
ular adsorption (Eads) and inter-
molecular interaction in the self-assembled monolayer (Einter).
These terms were evaluated here using the GAFF force
field[15] and 2D model architectures with an atomistic
representation of the graphene surface.[16] In addition, the
entropic cost associated to 2D confinement and assembly into
a crystalline architecture was evaluated using a statistical
mechanics approach based on the ideal gas approximation
(see the molecular modeling in SI). In this framework, the
free energy excess on self-assembly normalized per unit cell
area (g) was evaluated and used as a quantitative measure to
compare the (relative) thermodynamic stability of the three
architectures in Figure 1.
The results obtained for the open, closed, and by-product
isomers clearly show that at the experimental conditions
(0.5 mm concentration in solution) the DAE-b architecture is
most stable with g = À34.61 kcalmolÀ1 nmÀ2, followed by
DAE-c and DAE-o in this order (Table 1). The density of
packing is primarily responsible for the difference in free
b. Moreover, whereas in the former the octadecyl chains are
physisorbed in an in-plane zig-zag fashion, in the latter they
adopt an out-of-plane conformation. Overall, the interplay
between Eads and Einter, which is quantified by the energy per
unit cell (Euc) in Table 1, is in favor of the DAE-b architecture
by 4.5 kcalmolÀ1. In addition, because the entropic cost of
association at the experimental conditions is similar for these
two architectures (DDS < 1 kcalmolÀ1 nmÀ2, see SI), the
DAE-b nanopattern is predicted to be thermodynamically
preferred. This interpretation rationalizes all STM observa-
tions, yields atomistic models for the DAE self-assembled
architectures, and provides new insight on the nature of the
surface-induced selection and amplification of the photo-
irradiation by-product. Despite the higher free energy of the
open versus the closed isomeric architecture amounting to
À21.04 and À30.83 kcalmolÀ1 nmÀ2, respectively, it is interest-
ing to note that STM imaging reveals that upon illumination
of the closed monolayer with visible light a reorganization
Table 1: Experimental and modeled unit cell parameters for the three DAE isomers and their estimated thermodynamic quantities. Eads is the
adsorption energy, Einter the interaction energy with the neighboring molecules, Euc/A the total energy per molecule normalized by the unit cell area, and
g the free energy excess considering both energetic and entropic contributions.
a [nm]
b [nm]
a [8]
A [nm2]
Eads
E
inter/2
Euc/A
g
[kcalmolÀ1
]
[kcalmolÀ1
]
[kcalmolÀ1 nmÀ2
]
[kcalmolÀ1 nmÀ2
]
theoretical
experimental
1.5
(1.5Æ0.1)
3.4
(3.4Æ0.1)
76
(79Æ2)
4.9
(5.0Æ0.1)
À119.1
À112.7
À101.3
À15.7
À26.38
À38.14
À42.60
À21.04
À30.83
À34.61
DAE-o
DAE-c
DAE-b
theoretical
experimental
0.9
(1.0Æ0.1)
4.2
(4.1Æ0.1)
78
(79Æ2)
3.8
(4.0Æ0.1)
À32.6
À46.1
theoretical
experimental
0.9
(0.9Æ0.3)
3.9
(4.0Æ0.5)
78
(79Æ2)
3.4
(3.5Æ0.5)
4
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In summary, a novel diarylethene derivative has been
designed and synthesized. It exhibits pronounced affinity for
the graphite surface in all its isomeric forms, thereby
providing for the first time a high-resolution evidence of the
in situ reversible light-powered switching of a diarylethene
between the open and the closed form at the solid/liquid
interface. Prolonged irradiation with UV light of both the
“wet” film and the solution led to the irreversible formation
of the annulated isomer (photochemical by-product). A
simple yet insightful thermodynamic analysis of the self-
assembly provided evidence for the capacity of the surface to
act as a selector, thereby amplifying the presence of the minor
constituent on the surface starting from a mixture. Such an
ability of graphite to behave as a filter or selector opens new
perspectives toward the use of graphene as stationary phase in
liquid chromatography and more generally to exploit surface
selectivity to purify multicomponent solutions.
Received: December 19, 2014
Published online: && &&, &&&&
Keywords: diarylethene · in situ photoswitching · interfaces ·
.
scanning tunneling microscopy · self-assembly
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Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Self-Assembly
A diarylethene derivative exhibits pro-
nounced affinity for the graphite surface
in all its isomeric forms. The photoswitch
between its two isomers, that is, the open
and closed form, at the solid/liquid
interface is investigated by submolecu-
larly resolved scanning tunneling mi-
croscopy and atomistic modeling. Pro-
longed irradiation leads to the in situ
irreversible formation of another isomer
as by-product of the reaction.
S. Bonacchi, M. El Garah, A. Ciesielski,
M. Herder, S. Conti, M. Cecchini,*
S. Hecht,* P. Samorꢁ*
&&&& — &&&&
Surface-Induced Selection During In Situ
Photoswitching at the Solid/Liquid
Interface
6
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
These are not the final page numbers!