Light-Induced Contraction/Expansion of 1D Photoswitchable Metallopolymer Monitored at the Solid–Liquid Interface

Article abstract of DOI:10.1002/smll.201701790

The use of a bottom-up approach to the fabrication of nanopatterned functional surfaces, which are capable to respond to external stimuli, is of great current interest. Herein, the preparation of light-responsive, linear supramolecular metallopolymers constituted by the ideally infinite repetition of a ditopic ligand bearing an azoaryl moiety and Co(II) coordination nodes is described. The supramolecular polymerization process is followed by optical spectroscopy in dimethylformamide solution. Noteworthy, a submolecularly resolved scanning tunneling microscopy (STM) study of the in situ reversible trans-to-cis photoisomerization of a photoswitchable metallopolymer that self-assembles into 2D crystalline patterns onto a highly oriented pyrolytic graphite surface is achieved for the first time. The STM analysis of the nanopatterned surfaces is corroborated by modeling the physisorbed species onto a graphene slab before and after irradiation by means of density functional theory calculation. Significantly, switching of the monolayers consisting of supramolecular Co(II) metallopolymer bearing trans-azoaryl units to a novel pattern based on cis isomers can be triggered by UV light and reversed back to the trans conformer by using visible light, thereby restoring the trans-based supramolecular 2D packing. These findings represent a step forward toward the design and preparation of photoresponsive “smart” surfaces organized with an atomic precision.

Full text of DOI:10.1002/smll.201701790

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Smart Surfaces
Light-Induced Contraction/Expansion of 1D
Photoswitchable Metallopolymer Monitored at the
Solid–Liquid Interface
Mohamed El Garah, Etienne Borré, Artur Ciesielski, Arezoo Dianat,
Rafael Gutierrez, Gianaurelio Cuniberti, Stéphane Bellemin-Laponnaz,*
Matteo Mauro,* and Paolo Samorì*
The use of a bottom-up approach to the fabrication of nanopatterned functional
surfaces, which are capable to respond to external stimuli, is of great current interest.
Herein, the preparation of light-responsive, linear supramolecular metallopolymers
constituted by the ideally infinite repetition of a ditopic ligand bearing an
azoaryl moiety and Co(II) coordination nodes is described. The supramolecular
polymerization process is followed by optical spectroscopy in dimethylformamide
solution. Noteworthy, a submolecularly resolved scanning tunneling microscopy
(STM) study of the in situ reversible trans-to-cis photoisomerization of a
photoswitchable metallopolymer that self-assembles into 2D crystalline patterns onto
a highly oriented pyrolytic graphite surface is achieved for the first time. The STM
analysis of the nanopatterned surfaces is corroborated by modeling the physisorbed
species onto a graphene slab before and after irradiation by means of density
functional theory calculation. Significantly, switching of the monolayers consisting of
supramolecular Co(II) metallopolymer bearing trans-azoaryl units to a novel pattern
based on cis isomers can be triggered by UV light and reversed back to the trans
conformer by using visible light, thereby restoring the trans-based supramolecular 2D
packing. These findings represent a step forward toward the design and preparation
of photoresponsive “smart” surfaces organized with an atomic precision.
Dr. M. E. Garah, Dr. E. Borré, Dr. A. Ciesielski,
Dr. M. Mauro, Prof. P. Samorì
Université de Strasbourg
Dr. A. Dianat, Dr. R. Gutierrez, Prof. G. Cuniberti
Institute for Materials Science and Max Bergmann
Center of Biomaterials
CNRS, Institut de Science et d’Ingénierie
Supramoléculaires (ISIS)
Dresden University of Technology
01062 Dresden, Germany
8 Allée Gaspard Monge, 67000 Strasbourg, France
E-mail: mauro@unistra.fr; samori@unistra.fr
Prof. G. Cuniberti
Center for Advancing Electronics Dresden
Dresden Center for Computational Materials Science
Dresden University of Technology
01062 Dresden, Germany
Dr. E. Borré, Dr. S. Bellemin-Laponnaz
Département des Matériaux Organiques
Institut de Physique et Chimie des Matériaux de Strasbourg
Université de Strasbourg
CNRS UMR 7504, 23 rue du Loess, 67034 Strasbourg, France
E-mail: bellemin@unistra.fr
The ORCID identification number(s) for the author(s) of this arti-
cle can be found under https://doi.org/10.1002/smll.201701790.
DOI: 10.1002/smll.201701790
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of only one component with higher affinity toward the solid
substrate is expected under thermodynamic equilibrium.[12]
Upon titration, the formation of the supramolecular
1. Introduction
The synthesis of sophisticated building blocks designed to
interact through noncovalent forces is widely explored as it polyelectrolyte was followed in solution by means of
allows construction of various self-assembled systems and UV–visible absorption spectroscopy. Noteworthy, when the
materials with a sub-nanometer precision.[1] In particular, metallopolymer was prepared in situ at 1-phenyloctane/
supramolecular chemistry offers numerous straightforward highly oriented pyrolytic graphite (HOPG) interface, the
solutions toward the bottom-up functionalization of surfaces formation of a 2D pattern of self-assembled arrays could
and interfaces with ad hoc molecules. Such strategy makes be monitored by STM that showed azoaryl moieties in their
possible to generate functional nanostructures for various trans form. A photonic stimulus was used to reversibly switch
application in materials science. By judicious molecular azobenzene between trans and cis conformers by using UV
design, a range of different functionalities can be chemi- and visible light, respectively, resulting in a neat variation of
cally encoded at the molecular level, thus yielding organized the surface pattern as also supported by theoretical modeling
nanostructures that are capable to respond to external inputs at density functional theory (DFT) level on HOPG surface.
such as thermal,[2] electrical,[3] chemical,[4] and optical[5] In this manner, nanopatterned photoresponsive surfaces
stimuli. The latter is of major interest because light-induced could be fabricated.
excitation allows remote and spatiotemporal control of the
applied stimulus. In addition, introduction of photorespon-
sive moieties might allow orthogonal selection of both excita-
tion wavelength and output response, such as luminescence,
2. Results and Discussion
energy and/or electron transfer processes, reactivity and cata- 2.1. Synthesis and Characterization
lytic properties, to cite some.
Nowadays, a number of photoinduced reactions and pro- The chemical structure of the ditopic ligand containing
cesses are known and among those, trans-to-cis isomerization
a photoswitchable azoaryl moiety, namely Azo-bistpy,
in a series of derivatives such as stilbenes,[6] azobenzenes,[7] is depicted in Scheme 1 along with the corresponding
as well as opening/closing processes in diarylethenes[5e,8] cobalt(II) metallopolymer, namely [Azo-bistpy-Co]n. The
have been extensively studied both from fundamental and ligand Azo-bistpy has been synthesized in its thermodynami-
application points of view. Among photoswitches, systems cally stable trans configuration, namely trans-Azo-bistpy,
based on azobenzene scaffolds hold a leading position due by reaction of 4′-(4-aminophenyl)-2,2′:6,2″-terpyridine with
to their peculiar characteristics including relatively straight- potassium tert-butoxide (tBuOK) in dimethyl sulfoxide/tert-
forward synthetic procedures, thermal and photochemical butanol (DMSO/t BuOH) solvent mixture. The experi-
reversibility, selection of the excitation wavelength that can mental details of the synthetic procedures are reported
be modulated by electronic and steric effects upon variation elsewhere.[13] In a first attempt, synthesis of the metallopoly-
of the substituents,[9] as well as large geometrical and dipole mers containing Co(II) coordination nodes was carried out
reorganization that accompany the photoinduced isomeriza- as bulk reaction by using a cobalt salt and Azo-bistpy in
tion. In this respect, some of us have recently reported on a CHCl3. Although Azo-bistpy can form metallopolymers with
novel photoresponsive class of supramolecular metallopoly- various metal ions such as Fe(II) or Zn(II),[10] Co(II) has
mers bearing diazoaryl moieties that are suitable for the been chosen in this study as a coordination node because it
preparation of self-healable and photoresponsive gels.[10]
offers low kinetic lability, which allows in situ generation of
Scanning tunneling microscopy (STM) is an established elongated 1D metallopolymers at the solid/liquid interface.
tool to investigate structures and numerous physical and Moreover, Co(II)/bistpy metallopolymers and complexes
chemical properties of molecules at surfaces with a sub- are in general thermodynamically more stable than Zn(II)
nanometer spatial resolution.[11] Its use to explore mole- analogs.
cular physisorption at interfaces is particularly appealing, as
It is important to note, that the nature of the solvent,
it makes possible to explore dynamic processes in situ and ligand:cobalt ratio, and the nature of the Co counteranion
in real-time, including switching processes and chemical are crucial points for determining the final outcome in a reac-
reactions.[4b,c,7e]
tion between bis-terpyridine type of ligands and a cobalt(II)
Seeking for the possibility to monitor photoswitching salt.[14] Thus, we have chosen to perform the reaction using
processes and construct photoresponsive surfaces, we herein CoCl2 as the cobalt source and in a 1:1 ratio Azo-bistpy:Co2+
report on the preparation of a linear metallopolymer based since quantitative coordination of the N-based ligand onto
on a photoswitchable bis-terpyridine ditopic ligand and con- metal ion is expected to occur under our reaction condition,
taining Co(II) coordination nodes, namely [Azo-bistpy-Co]n. giving the target metallopolymer [Azo-bistpy-Co]n. Unfortu-
Noteworthy, some of us studied in the past the self- nately, this synthetic procedure afforded an insoluble solid,
assembly of three-component metallopolymers, in which thus hampering any deeper chemical characterization of the
Azo-bistpy was used as one out of the two organic ligands.[10] obtained product. Such finding is in agreement with related
However, such systems are not suitable for studying the reactions carried out under similar condition and reported
photoswitching processes of the self-assembled structures, previously.[14]
as the two organic components have very different adsorp-
Thus, we decided to carry out an in situ complexa-
tion energies. Therefore, phase segregation or self-assembly tion between the ditopic trans-Azo-bistpy ligand and the
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Scheme 1. Chemical structure of the ditopic ligand Azo-bistpy in its trans conformation and the corresponding linear Co(II)-based metallopolymer
[Azo-bistpy-Co]n prepared at 1:1 ligand:CoCl2 ratio.
CoCl2 and follow the polymerization process by UV–visible it is ascribed to the subsequent depolymerization process
spectrophotometry in dilute DMF solution. A sample of of [trans-Azo-bistpy-Co]n into the dimetallic species with
Azo-bistpy ligand at concentration of 2.0 × 10−5 mol was trans-Azo-bistpy:Co2+ 1:2 stoichiometry. Subsequently, in the
titrated with 20 µL aliquots of a solution of CoCl2 at concen- above 2 equiv. of CoCl2, no substantial change in the absor-
tration of 2.5 × 10−4 mol. Formation of Co(II) coordination bance is observed, indicating that complete depolymeriza-
polymer was monitored by UV–vis upon stepwise addition tion has occurred. Although supramolecular polymerization
of metal cation. The UV–vis titration curves and the cor- processes of ditopic ligands of terpyridine and other types of
responding plot displaying the variation of the absorbance N-based ligands have been already shown to afford a mix-
recorded at 335 nm upon different addition of CoCl2 aliquots ture of metallo-macrocyclic species and linear polymeric
are depicted in Figure 1.
counterparts at variable ratio depending on the counterion,
In Figure 1c, two steep changes in the absorbance trend solvent, and even more importantly concentration, we confi-
at around 1 and 2 equiv. of CoCl2 can be clearly identified dentially assume that [trans-Azo-bistpy-Co]n metallopolymer
corresponding to three different regimes. The first regime possesses a linear arrangement on the basis of the rigid and
(between 0 and 1 equiv. of CoCl2) is indicative of a poly- rod-like nature of the divergent ditopic ligand trans-Azo-
merization process between molecules of trans-Azo-bistpy bistpy.[15] Such linear and ideally infinite arrangement of
ligands and metal center, which yields the metallopoly- the [trans-Azo-bistpy-Co]n is further corroborated by direct
meric species [trans-Azo-bistpy-Co]n. The second regime visualization via scanning probe technique at the solid/liquid
corresponds to a trans-Azo-bistpy:Co2+ 1:1 to 1:2 ratio and interface (vide infra).
Figure 1. a,b) UV–visible spectra and c) plot of the variation of the absorbance monitored at 335 nm versus Co2+:Azo-bistpy ligand ratio recorded
in DMF ([Azo-bistpy] = 2.0 × 10−5 mol). Black arrows indicated the variation of the absorbance upon addition of CoCl2 aliquots in the range 0–1
and 1.1–2.4 equiv. for panels (a) and (b), respectively. In (c), dotted gray lines are only intended to guide the eye.
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Figure 2. Kineticstudiesofthe photoisomerization processfor [trans-Azo-bistpy-Co]n in DMF(1 ×10−5 mol) carried outbyUV–visspectrophotometry.
a) Trans-to-cis photoisomerization upon irradiation at 365 nm. Spectra were recorded between 0 (black trace) and 180 s (red trace) light exposure.
b) Cis-to-trans back isomerization obtained upon irradiation with a LED at 455 nm. UV–visible spectra were recorded between 0 (red trace) and
180 s (black trace) light exposure. Black arrows indicate the variation of absorbance over photoirradiation time.
Our molecular design was made in such a way to intro- unoccupied MOs, namely HOMO and LUMO, respectively,
duce a unit responsive to light input, i.e., a diazoaryl moiety, computed for Azo-bistpy at its electronic ground state. As far
capable to undergo trans–cis photoisomerization, while as the trans-Azo-bistpy conformer is concerned, the HOMO
keeping a certain extent of molecular rigidity of the 1D is localized on the azoaryl group whereas the LUMO is delo-
linear architecture. To investigate the photoresponsive- calized on the azobenzene and on the two central pyridyl
ness of the cobalt-containing metallopolymer, a DMF moieties of the terpyridyl groups. For cis-Azo-bistpy, while
solution of [trans-Azo-bistpy-Co]n at concentration of HOMO is delocalized on the azobenzene and the two central
1.0 × 10−5 mol was irradiated with a continuous-wave light- pyridyl groups, the LUMO is delocalized on the entire mole-
emitting diode (LED) light source at 365 nm under an irradi- cule. Such orbitals are computed at −4.96 and −3.24 eV, for
ance of 21 mW cm−1 and the process monitored by UV–vis trans-Azo-bistpy, and −5.09 and −3.03 eV for cis-Azo-bistpy,
spectroscopy. As displayed in Figure 2, upon UV photoirra- respectively.
diation, absorption spectra show a sizeable decrease of the
According to our previous investigations,[18] the com-
symmetry-allowed π–π* transition centered at 357 nm over bination of the CoCl2 and the pyridyl/terpyridyl moie-
irradiation-time and an isosbestic point at λabs = 322 nm. This ties should generate a coordinated 1D polymeric structure
finding is indicative of a trans→cis photoisomerization pro- (see Scheme 1). In the gas phase and as displayed in
cess involving the diazoaryl moiety and the photostationary Figures S4a and S5a (Supporting Information) for trans-Azo-
state is reached within 60 s of irradiation.[9a] However, the bistpy-Co and cis-Azo-bistpy-Co, respectively, both con-
parallel increase of the n–π* absorption band, which becomes formers show an idealized octahedral geometry around
less symmetry-forbidden in the cis conformation, is not the metal center (tpy-Co-tpy), i.e., the two terpyridines
clearly visible. Nonetheless, this is most likely due to the large coordinated to Co(II) adopt an angle of 90°
7°. How-
spectral overlap with the much more intense metal to ligand ever, such geometrical arrangement becomes sizably dis-
charge transfer band between 450 and 550 nm corresponding torted upon adsorption of the tpy-Co-tpy moiety onto the
to the [(tpy)2Co]2+ fragment.[16] Noteworthy, the isomeriza- graphite surface, where the angle of tpy-Co-tpy moiety
tion process can be readily reversed, thus yielding back the become (90 15)° and (90 10)° for trans-Azo-bistpy-Co
thermodynamically stable trans conformer upon irradiation and cis-Azo-bistpy-Co, respectively, as shown in Figures S4b
into the diazoaryl n–π* absorption band with a 455 nm LED and S5b (Supporting Information). In Table S1 (Supporting
(irradiance 31 mW cm−1).
Information) are summarized the computed adsorption ener-
gies (Ed) for all the investigated structures optimized on a
graphene slab. The Ed computed for [trans-Azo-bistpy-Co]n
assembly onto a graphene surface (−4.2 eV) was found lower
than for the cis counterpart (−3.81 eV), supporting the higher
2.2. Density Functional Theory Studies
To gain deeper insights into the structural and electronic stability of trans assemblies on graphite when compared to its
properties of the Azo-bistpy and its Co(II)-metallopolymer, cis conformer.
theoretical investigations were performed at their electronic
ground state geometry using DFT as implemented in the
CP2K software package.[17] The geometry optimization of 2.3. Scanning Tunneling Microscopy Analysis
Azo-bistpy and its complexes, namely trans-Azo-bistpy-Co,
cis-Azo-bistpy-Co, were performed in gas phase and on Azo-bistpy ligand offers peripheral and divergent coordi-
a graphene surface. All the structures are displayed in nation points based on two terdentate terpyridyl moieties.
Figures S1–S5 (Supporting Information).
The combination of such ditopic ligand in a twofold sym-
Figure S2 (Supporting Information) displays the frontier metry with an octahedral metallic center is thus expected to
molecular orbitals (MO) as the highest occupied and lowest lead to the formation of a 1D extended periodic molecular
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architecture. The formation of the coordinated complexes
The STM images were acquired by applying a posi-
was achieved at the 1-phenyloctane/graphite interface. The tive sample bias. Therefore, bright protrusions observed in
azoaryl group offers the possibility to switch between trans Figure 3b can be assigned to the LUMO of the molecules
and cis conformer under UV–vis irradiation. Interestingly, adsorbed on the HOPG surface. Since the LUMO of trans-
this process can be now exploited to create dynamic changes Azo-bistpy displays a rectangular shape (see Figure S2 in the
of the molecular packing of Azo-bistpy on graphite surface Supporting Information), we assign each bright feature vis-
in an unprecedented manner.
ible on STM images to a single trans-Azo-bistpy molecule.
With this goal in mind, STM was used to probe in situ Also, such LUMO orbital is delocalized along the core of the
the self-assembly behavior of the neat Azo-bistpy molecule molecule, which corroborates the fact that the two external
at the HOPG/liquid interface. The process was carried out pyridyl rings of terpyridyl groups cannot be seen in the STM
by drop-casting 4 µL solution of Azo-bistpy in 1-phenyloc- images. The analysis of the unit cell parameters and the con-
tane at concentration of 0.1 mmol onto the HOPG surface. trast of the STM images suggest that all trans-Azo-bistpy
Figure 3a displays a survey STM image of the obtained molecules are oriented along the same direction. The for-
molecular monolayer providing evidence for the formation mations of such self-assembled structure can be expected to
of a 2D crystalline structure extended over a scale of several be thermodynamically favored since it can be stabilized by
hundreds of nm2.
the minimization of dipolar interactions.[18a,b] A proposed
For all crystalline patterns the unit cell parameters, i.e., molecular model is presented in Figure 3c. The molecular
the length of the vectors (a) and (b), the angle between the structure was also computed by means of DFT computations
vectors (α), the unit cell area (A), the number of molecules in and the result is presented in Figure 3d. A good agreement is
the unit cell (Nmol), and the area occupied by a single mole- found between the experimental and theoretical data as evi-
cule in the unit cell (Amol, with Amol = A/Nmol) are reported denced by experimental and computed dimension of the unit
in Table 1.
cell (see Table 1).
Figure 3. STM images of the trans-Azo-bistpy networks formed at the 1-phenyloctane solution/HOPG interface. a) Survey image, average tunneling
current (It) = 25 pA, tip bias (Vt) = 550 mV. b) Height STM image, zoom-in, It = 25 pA, Vt = 800 mV. c) Proposed molecular model. d) DFT simulated
model on graphene surface.
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Table 1. Experimental and theoretical unit cell parameters of the structure Azo-bistpy and [Azo-bistpy-Co]n.
Structure
Azo-bistpy
[Azo-bistpy-Co]n
Experimental
Computed
Experimental
Computed
trans
cis
NA
NA
NA
NA
NA
NA
trans
2.7
1.6
42
cis
NA
NA
NA
NA
NA
NA
trans
cis
trans
2.2
1.3
53
cis
2.2
1.7
90
3.7
1
a [nm]
b [nm]
α [°]
2.6 0.1
1.6 0.1
2.1 0.1
1.4 0.1
2.2 0.1
1.7 0.1
43
2
51
2
90
2
A [nm2]
2.8 0.2
1
2.89
1
2.3 0.2
1
3.7 0.2
1
2.3
1
Nmol
Amol [nm2]
2.8 0.2
2.89
2.3 0.2
3.7 0.2
2.3
3.7
The monolayer structure of trans-Azo-bistpy on HOPG containing azoaryl moieties as trans conformation can often
was then exposed to UV light irradiation (λexc = 365 nm) be visualized by the means of STM, monolayers composed
for 10 min. As a result, desorption of Azo-bistpy molecules entirely of cis isomers are rarely observed.[7e,19] Indeed, upon
from the surface and formation of disordered aggregates irradiation with UV light and consequent trans-to-cis photo-
were observed. Interestingly, by exposing the same sample isomerization, desorption of molecules typically occurs due
to visible light (λexc = 455 nm) for an irradiation time as to the inability of the resulting nonplanar cis conformation
short as 5 min, the initial molecular packing of trans-Azo- to pack efficiently onto a surface.[20] As discussed in the com-
bistpy was restored on graphite surface owing to the back putational section (vide infra), this behavior is further sup-
cis-to-trans photoisomerization process. At this stage, it is ported by the much lower Ed computed for cis-Azo-bistpy
worth to point out that although supramolecular assemblies with respect to its trans counterpart.
Figure 4. STM images of [trans-Azo-bistpy-Co]n networks formed at 1-phenyloctane solution/HOPG interface. a) Survey image, It = 25 pA,
Vt = 550 mV. b) Height STM image, zoom-in, It = 30 pA, Vt = 500 mV. c) Proposed molecular model. d) DFT simulated model on graphene.
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To shed light onto the ability of this class of photorespon- the simulated unit cell parameters match perfectly those
sive metallopolymers to physisorb at surfaces into ordered determined experimentally (Table 1).
assemblies and monitor their photoswitching behavior,
Surprisingly, exposure of the existing 2D molecular layer
STM investigation at the solid–liquid interface has been of [trans-Azo-bistpy-Co]n to UV light (λexc = 365 nm) for
performed both on the linear supramolecular polymers an irradiation period of 15 min resulted in clean rearrange-
[Azo-bistpy-Co]n using CoCl2 as the octahedral coordina- ment of [Azo-bistpy-Co]n metallopolymers and formation of
tion node, formed in situ and ex situ. In situ formation of a new pattern onto the HOPG surface. The STM image of
[Azo-bistpy-Co]n has been achieved by the addition of the interface after in situ irradiation is displayed in Figure 5a.
1 equiv. of CoCl2 × 2H2O (i.e., 4 µL drop of 0.1 × 10−3 m solu- The crystalline pattern features a different structural motif
tion of isopropanol:1-phenyloctane 1:99 V/V) on the top of a if compared to that before irradiation (see Figure 4). Similar
pre-existing monolayer of trans-Azo-bistpy molecule. STM to the [trans-Azo-bistpy-Co]n monolayers, the 2D crystalline
images of the assembled crystalline patterns are shown in structure shown at in Figure 5a,b consists of linear arrays
the Figure 4a. It is worth to notice that combination of Azo- composed by bright protrusions, yet, different to the case of
bistpy and Co(II) resulted in the formation of linear arrays [trans-Azo-bistpy-Co]n. The interspacing between them was
physisorbed on the graphite surface. Within those linear measured as (1.7 0.1) nm, which is 0.3 nm larger than that
architectures that appear periodically distributed, bright found in trans-based monolayers (1.4 0.1) nm.
features can be seen with their interchain distance corre-
Such large variation of the unit cell parameters upon UV
sponding to the length of a single trans-Azo-bistpy molecule irradiation is being associated to the trans-to-cis isomeriza-
(see Table 1). According to the simulated LUMO of [trans- tion of azoaryl moieties, which undergo large conformational
Azo-bistpy-Co]n structures, those bright protrusions can be changes upon photoinduced isomerization. As a result, the
attributed to a Co(II) node and two terpyridyl moieties of area of the unit cell of cis-metallopolymer is 1.3 nm larger
two consecutive trans-Azo-bistpy molecules (see Figure 4c than the trans (Table 1).Although we cannot provide detailed
and Figure S7b in the Supporting Information). Moreover, and unambiguous insight into the geometry adopted by the
Figure 5. STM images of [cis-Azo-bistpy-Co]n formed at 1-phenyloctane solution/HOPG interface after UV irradiation using a wavelength of 365 nm.
a) Survey image, It = 25 pA, Vt = 600 mV. b) Current STM image, zoom-in, It = 25 pA, Vt = 600 mV. c) Proposed molecular model. d) DFT simulated
model on graphene.
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cis-Azo units within the supramolecular polymers, the mole-
cular model simulated with DFT (Figure 5d and Figure S7d
(Supporting Information)), and in particular the unit cell
parameters, fits very well with the experimental results. Based
on these findings, these linear supramolecular architectures
can be ascribed to metallopolymers of [cis-Azo-bistpy-Co]n.
Interestingly, upon irradiation of monolayer sample
of [cis-Azo-bistpy-Co]n with a visible light (white light
bulb), the 2D pattern corresponding to the metallopolymer
[trans-Azo-bistpy-Co]n is completely restored. Indeed, an
illumination period as short as 1 min leads to immediate
desorption of the molecules, followed by readsorption onto
the HOPG surface of the trans conformer of the supramolec-
ular polymer assembly within about 15 min, confirming the
reversible nature of the light-induced processes at play and
formation of the photoresponsive surface.
Acknowledgements
E.B., S.B.-L., and M.M. gratefully acknowledge the University of
Strasbourg Institute for Advanced Study (USIAS) for funding this
work with an USIAS Fellowship. The authors gratefully acknowl-
edge financial support from the European Commission through the
Marie Sklodowska-Curie project ITN project iSwitch (GA-642196),
the Agence Nationale de la Recherche through the Labex projects
CSC (ANR-10-LABX-0026 CSC) within the Investissement d’Avenir
program (ANR-10-120 IDEX-0002-02), and the International Center
for Frontier Research in Chemistry (icFRC). Computational resources
were provided by the Center for Information Services and High
Performance Computing (ZIH) of the Technische Universität Dresden.
Conflict of Interest
The authors declare no conflict of interest.
3. Conclusion
In summary, in situ preparation of supramolecular metal-
lopolymers bearing an azoaryl units on the backbone and
Co(II) coordination nodes was achieved and its photos-
witching ability in solution has been investigated in DMF
solution by optical spectroscopy. In addition, 1D directional
metallopolymer networks resulting from interconnection of
such photofunctional molecules bearing two terpyridyl coor-
dinating poles with CoCl2 were generated on a HOPG sur-
face by combining the supramolecular approach at the solid/
liquid interface with in situ STM nanoscale-resolved imaging.
The photoisomerization of the monolayers consisting of trans
conformer of the bare ditopic ligand to the corresponding
assemblies based on cis isomers has been triggered by UV
light. In the case of the Azo-bistpy assemblies, the confor-
mational changes accompanying the trans-to-cis transition as
well as the computed low interaction energy of cis-Azo-bistpy
with graphite resulted in formation of disordered supramo-
lecular structures. However, the embedment of Azo-bistpy in
metallopolymeric chain [Azo-bistpy-Co]n limits the degree
of molecular freedom and prevents the Azo-bistpy from fast
desorption. Therefore, Azo-bistpy moieties could be revers-
ibly isomerized within the metallopolymeric patterns, leading
to a significant structural change of the in-plane self-assembly
as a result of the contraction/expansion process of the supra-
molecular metallopolymeric structure associated with the
trans–cis photoisomerization processes. To the best of our
knowledge, these results provide the first examples of submo-
lecularly resolved switchable metallopolymers in physisorbed
monolayers and offer the possibility to prepare photorespon-
sive nanopatterned surfaces by controlling the switching of
azobenzenes in supramolecular linear arrays.
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