Electronic Decoupling in C3-Symmetrical Light-Responsive Tris(Azobenzene) Scaffolds: Self-Assembly and Multiphotochromism

Article abstract of DOI:10.1021/jacs.8b06324

We report the synthesis of a novel C₃-symmetrical multiphotochromic molecule bearing three azobenzene units at positions 1, 3, 5 of the central phenyl ring. The unique geometrical design of such a rigid scaffold enables the electronic decouplin

Full text of DOI:10.1021/jacs.8b06324

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Article
Electronic Decoupling in C3-Symmetrical Light-Responsive
Tris(Azobenzene) Scaffolds: Self-Assembly and Multi-Photochromism
Agostino Galanti, Valentin Diez-Cabanes, Jasmin Santoro, Michal
Valášek, Andrea Minoia, Marcel Mayor, Jerome Cornil, and Paolo Samorì
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b06324 • Publication Date (Web): 31 Oct 2018
Downloaded from http://pubs.acs.org on November 1, 2018
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Electronic Decoupling in C3-Symmetrical Light-Responsive
Tris(Azobenzene) Scaffolds: Self-Assembly and Multi-
Photochromism
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Agostino Galanti,†,≠ Valentin Diez-Cabanes,‡,≠ Jasmin Santoro,§ Michal Valášek,§,* Andrea Minoia,‡
Marcel Mayor,§,±,* Jérôme Cornil,‡,* Paolo Samorì†,*
†Université de Strasbourg, CNRS, ISIS UMR 7006, 8 allée Gaspard Monge, 67000 Strasbourg, France
‡Laboratory for Chemistry of Novel Materials, University of Mons, Place du Parc 20, B-7000 Mons, Belgium
§Karlsruhe Institute of Technology KIT, Institute for Nanotechnology, P.O. Box 3640, 76021 Karlsruhe, Germany
±Department of Chemistry, University of Basel, St. Johannsring 19, 4056 Basel, Switzerland
KEYWORDS azobenzene • in-situ photoswitching • interfaces • scanning tunnelling microscopy • self-assembly
ABSTRACT: We report the synthesis of a novel C3-symmetrical multiphotochromic molecule bearing three azobenzene units at
positions 1,3,5 of the central phenyl ring. The unique geometrical design of such a rigid scaffold enables the electronic decoupling
of the azobenzene moieties to guarantee their simultaneous isomerisation. Photoswitching of all azobenzenes in solution was
demonstrated by means of UV-Vis absorption spectroscopy and high performance liquid chromatography (HPLC) analysis.
Scanning tunnelling microscopy investigations at the solid-liquid interface, corroborated by molecular modelling, made it possible
to unravel the dynamic self-assembly of such systems into ordered supramolecular architectures, by visualising and identifying the
patterns resulting from three different isomers, thereby demonstrating that the multi-photochromism is retained when the molecules
are confined in two-dimensions.
within crystalline or liquid crystalline matrices, it is possible to
INTRODUCTION
amplify its molecular-scale isomerisation into macroscopically
observable effects such as photodeformation,10 light-controlled
phase transition,11 and/or guest release.11a, 12 While interrupting
the electronic coupling in polyaromatic systems was seen to be
essential in order to retain the photoresponsive properties in
multi-azobenzene compounds,13 the use of an exemplary
molecular design to cast light onto this aspect for a system
containing up to three azobenzene units has never been
reported.
Stimuli-responsive molecular systems have attracted a
great interest because their physicochemical properties can be
tuned when triggered by external chemical, electrochemical or
optical inputs.1 Among them, light probably represents the
most appealing way to enable complex function as a result of a
stimulus because of its spatio-temporal resolution.2 Recently,
with the aim to increase the functional complexity of
molecular-scale events which can be attained with synthetic
molecular systems, an increasing effort has been devoted to
the integration of more than one photochromic unit into a
single molecule to interconvert systems between multiple
states in contrast to a simple photochromic molecule, which
can be seen as a conventional binary switch.3
Figure 1. Chemical structure of tris(azobenzene) compound 1 and
Among
the
various
photochromic
compounds,
azobenzenes are of particular interest because of the large
geometrical rearrangement occurring in the molecular
backbone upon its E-Z isomerisation, together with the dipole
moment variation between the two isomers. Such a difference
between the two forms can be used to execute mechanical
work,4 and by extension to realise light-activated molecular
actuators.5 With the implementation of such photoswitchable
units into increasingly complex (supra)molecular systems, it
was proven possible to optically gate various phenomena such
as reversible supramolecular self-assembly,6 assembly of
colloidal particles,7 current tunnelling within molecular
junctions8 which led to the realisation of optically switchable
electrical devices.9 Interestingly, by embedding azobenzene
its non-photoactive analogue 2.
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Scheme 1. Isomerisation of tris(azobenzene) 1.
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In the past two decades, scanning tunnelling microscopy
(STM) has been widely employed as a powerful tool to study
structure and dynamics of molecules at surfaces with a sub-nm
resolution.14 In particular, STM imaging at the solid-liquid
interface offers direct and detailed insight into the
phenomenon of molecular self-assembly.15 Such a technique is
increasingly employed for the study of host-guest, or stimuli
responsive systems.16 Investigation on the isomerisation of
azobenzene with STM has always been an appealing task for
surface scientists,17 albeit the visualisation of the Z isomer at
the solid-liquid interface was found to be particularly
challenging because of its non-planar, kinked geometry, which
yields unfavourable van der Waals interactions with the
underlying substrate surface.18 Therefore, the use of several
artifices such as the decoration of the azobenzene with ad-hoc
functional groups, or molecular geometries enabling the self-
assembly of both E and Z isomers on graphite have
demonstrated to be effective, although sometimes making their
visualisation a cumbersome task.18-19 The STM mapping at the
solid-liquid interface of various isomers of a photochromic
system containing multiple azobenzene units in the same
molecular backbone was achieved by embedding the switches
in a host-guest network.20 Such a finding suggests that the
isomerisation of multi-photochromic systems could be also
monitored in mono-component ultrathin films by attaining an
exquisite control over the interplay between intermolecular
and interfacial interactions via an ad-hoc molecular design. In
this regard, a rigid scaffold possessing a C3 symmetry around a
central benzene core appears ideal to address the key question
of electronic decoupling on the isomerization of
multiazobenzene systems and seems particularly suitable for
the potential integration of such switches as building blocks
for the generation of light-responsive 2D and 3D metal-
organic materials and covalent-organic frameworks.21
analogue molecule 2 in which the diazene-1,2-diyl groups
have been substituted with ethyn-1,2-diyl units, thus
suppressing the photochromic nature of the system.
By design molecule 1 combines four structural features: (i)
a noticeable conformational rigidity determined by the use of
aromatic units, (ii) a central 1,3,5-trisubstituted benzene ring-,
which dictates the peculiar geometry of this molecule; (iii) the
three azobenzene moieties which can respond to light stimuli;
(iv) the carboxylic acid-terminated azobenzene arms to enable
the use of intermolecular H-bonding for controlling the self-
assembly. The symmetry and peripheral functional groups of
choice are motivated by the thorough investigation conducted
in the past over the self-assembly of rigid aromatic carboxylic
acids,22 especially C3-symmetric rigid tricarboxylic acids on
HOPG (using heptanoic, octanoic or nonanoic acid as
22a, 22c, 22d, 22f,
solvents) with increasingly large aromatic cores.
22i, 22j, 23
Significantly, to cast light onto the role of the
electronic coupling in polyaromatic systems containing
multiple azobenzenes, we have devised a prototypical
molecular design characterised by the reciprocal connection of
the azobenzene moieties in the meta-position on the central
phenyl ring to yield a partial interruption of the electronic
communication between the photoactive units, yet keeping the
system sufficiently rigid for allowing its self-assembly in 2D.
RESULTS AND DISCUSSION
To verify the occurrence of an efficient photochemical
isomerisation of 1, UV-Vis absorption spectroscopy and
HPLC analysis were used (for synthesis and characterisation
see SI). The absorption spectrum of (E,E,E)-1 in DMSO
shows the typical two bands of an azobenzene chromophore:
π-π* and n-π* located respectively at ca. 370 nm and 455 nm
Here, we report on the design and synthesis of a novel C3-
symmetrical multi-photochromic molecule 1 containing three
azobenzene units branching out in the 1,3,5 positions of a
central benzene ring. Optical characterisations by absorption
spectroscopy combined with HPLC analysis have been
performed in solution in order to explore the response of 1 to
light at different wavelengths. STM investigation of the
molecular self-assembly at the solid-liquid interface offered an
in-depth insight into the responsive nature of the
supramolecular assemblies of molecule 1 when in-situ
irradiated with ultraviolet and visible light. The subtle
interpretation of sub-molecularly resolved patterns was
achieved with the aid of Molecular Mechanics/ Dynamics
(MM/MD) simulations. In order to demonstrate that the
dynamic self-assembly upon light irradiation at different
wavelengths is due to the isomerisation of the three
azobenzene moieties, we have extended our study to an
(Figure 2).
Figure 2. Absorption spectral variation of 1 upon UV (λmax = 365
nm) irradiation in DMSO, c = 5.0×10-6 M. Full black line no
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irradiation, full red line PSS UV, dashed blue line, PSS Vis (irr.
λmax = 451 nm).
retain their photoreactivity, a mixture of the four possible
(E,E,E)-1, (E,E,Z)-1, (E,Z,Z)-1 and (Z,Z,Z)-1 isomers would
1
2
3
be observed at the PSS. We used HPLC to separate the
photogenerated isomers at the various steps of
1
photoisomerisation (Figure 3). The appearance of three
additional peaks at lower elution time is observed upon UV
irradiation. This is consistent with the generation of the
expected photoproducts, possessing one, two and three
azobenzene units in the Z form. The absorption spectra
recorded at the peaks of the chromatograms (Figure 3, inset)
revealed that the spectra of the photoproducts display a
progressive decrease of the π-π* band around 360 nm and an
increase of the n-π* band with respect to (E,E,E)-1, in
consistency with the expected absorption spectra of the
azobenzene Z isomer. The insight offered by HPLC analysis is
corroborated by the computed absorption spectra of the four
possible isomers of 1 at the Time-Dependent Density
Functional Theory (TD-DFT) level, which nicely correspond
to those found experimentally (Figure S19). In agreement with
the UV-Vis absorption spectroscopy, Z → E isomerisation of 1
with Vis light (λmax = 451 nm) yields a PSS in which the
population of the Z-isomers of 1 is partially converted to the E
ones, while the all-(E) situation is obtained by thermal Z → E
isomerisation (Figure S13). Further photochemical
characterisation about the composition of the photostationary
state(s) and the role of conjugation in the present systems goes
beyond the scope of this work.
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Figure 3. HPLC traces recorded upon injecting irradiated
solutions of 1. Full black line no irradiation, full red line PSS UV.
Inset, UV-Vis absorption spectra corresponding to the four peaks
of PSS UV chromatogram. Black line, (E,E,E)-1 (retention time: 3
min 38 s), green line, (E,E,Z)-1 (retention time: 3 min 22 s), light
blue line, (E,Z,Z)-1 (retention time: 3 min 6 s), blue line, (Z,Z,Z)-1
(retention time: 2 min 51 s).
Upon UV irradiation at 365 nm of 1 in DMSO solution, a
decrease in the strong π-π* transition was observed, together
with an increase in absorption of the n-π* band. These features
are typical for the E → Z isomerisation of azobenzene.
Interestingly, the data exhibit sharp isosbestic points at ca. 312
nm and 430 nm which typically indicate electronic decoupling
between azobenzene chromophores.13b Upon subsequent
irradiation with visible light at ca. 450 nm, a progressive
recovery of the original spectral features was observed, while
the full recovery to the initial scenario was achieved by storing
the solution at room temperature in the dark over four days (or
alternatively for ca. 12 hours at 50 °C). The latter evidence
confirms the full reversibility of the isomerisation of 1, i.e., a
behaviour which is typical for azobenzene-based compounds.
In order to clearly demonstrate the photochemical robustness
of compound 1, we have performed a test over 10
photoswitching cycles, which showed negligible degradation
of its photochromic activity (Figure S14). As previously seen,
UV-Vis absorption spectroscopy does not allow to rationalise
the isomeric composition of the mixture at the photostationary
states (PSS), since all possible isomers of 1 absorb in the same
spectral region. In principle, if all azobenzene units within 1
Initially, we targeted at investigating the self-assembly of
all-(E)-1 in the dark at the interface between its solution in 1-
heptanoic acid (c = 10 µM) and highly ordered pyrolytic
graphite (HOPG). Towards this end, to make sure that all three
azobenzenes moieties of molecule 1 were in their all-(E) state,
we applied to the surface a drop of a non-irradiated solution of
1, in order to benefit from the thermodynamic stability of (E)-
azobenzene isomer. The STM images recorded in-situ display
a tightly packed 2D crystalline lamellar structure consisting of
(E,E,E)-1 arranged in a zig-zag fashion (Figure 4, Figure 5a).
The structure observed displays a unit cell: a = 4.1 ± 0.2 nm, b
= 3.0 ± 0.3 nm, α = 41 ± 5 ° with an area A = 8.7 ± 0.3 nm2,
each containing two molecules. A careful image analysis
revealed the absence of polymorphs of such a crystalline
packing, also upon varying the concentration of 1 solution
used for the experiments: 10 µM was found to be the optimal
value
Figure 4. STM images of 1 recorded at the interface between an HOPG substrate and a 10 µM solution of 1 in 1-heptanoic acid. Left, no
light irradiation (average tunnelling current (IT) = 30 pA, tip bias voltage (VT) = +800 mV). Centre, in-situ UV (λmax = 365 nm) light
irradiation (IT = 30 pA, VT = +800 mV). Right, subsequent in-situ Vis (λmax = 451 nm) light irradiation (IT = 20 pA, VT = +800 mV).
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Figure 5. High resolution STM images of ordered domains of (a) (E,E,E)-1, (c) (E,Z,Z)-1, and (e) (E,E,Z)-1 self-assembled at the HOPG-
solution interface using 1-heptanoic acid as solvent. Supramolecular packing models obtained by MM/MD simulations for (b) (E,E,E)-1,
(d) (E,Z,Z)-1, and (f) (E,E,Z)-1. The yellow rectangles indicate the formation of hydrogen bonded carboxylic acid dimers between (E)-
azobenzene arms. Orange rectangles indicate hydrogen bonds between carboxylic moieties positioned on (Z)-azobenzene arms. Tunneling
parameters: (a) average tunnelling current (IT) = 40 pA, tip bias voltage (VT) = +800 mV), (c) IT = 30 pA, VT = +800 mV, (e) IT = 20 pA,
VT = +800 mV.
for attaining higher spatial resolution (for details see SI). The
total absence of the ideal “honeycomb network” H-bonded
pattern which should arise from the formation of the
intermolecular 2-fold cyclic O-H···O bonding between
carboxylic groups is not surprising. This consideration comes
from the large dimension of the rigid aromatic core of 1, in
line with the reported tendency of large C3-symmetric
tricarboxylic acids to form more densely packed structures.23
The reason for this evidence was explained by the higher
adsorption energy contribution obtained by forming a more
densely packed crystal with a non-ideal H-bonding pattern
compared to the corresponding “ideal” honeycomb structure
expected from the generation of the two-fold H-bonding
dimers, leading to a looser crystalline structure. In other
words, the most prominent term driving the assembly is not
associated to intermolecular interactions, but rather to the
molecule-substrate adsorption energy per unit area, thus
yielding the “tightest” assembly and not the “ideal” H-binding
motif.23 In order to confirm the assignment of the molecular
packing given by experimental data, we simulated the
assembly of (E,E,E)-1 by Molecular Dynamics (MD, see SI
for details), which yielded an average unit cell: a = 4.3 nm, b =
2.8 nm, α = 41 °, with two molecules per unit cell (Figure 5b).
The result obtained by MD simulations matches very well the
experimental values obtained by STM, therefore confirming
the validity of our model. The azobenzene units in molecule 1
present a kink in the CNNC bond, it thus follows that upon the
adsorption of 1 on the HOPG surface the resulting structures
could show two isomers of the compound, together with the
possibility of the emergence of their chirality. Nevertheless,
from the images obtained, we could not attain the level of
detail needed to neither assign unambiguously which specific
isomer the crystalline domains belonged to, nor to comment
about the chirality of the 2D structures.
As a blank experiment, the same study was also performed
on the non-photochromic compound 2, displaying a similar
geometry to (E,E,E)-1, but having tolane moieties instead of
azobenzenes in each of the three “arms”. The experiment was
performed after confirming the absence of photoreactivity of 2
by UV-Vis absorption spectroscopy (Figure S15). Compound
2 was found to self-assemble in a crystalline structure with the
same symmetry displayed by (E,E,E)-1 (Figure S17). Such
structure is characterised by the following unit cell: a = 4.2 ±
0.2 nm, b = 2.9 ± 0.1 nm, α = 46 ± 1 ° with an area A = 8.8 ±
0.4 nm2, each containing two molecules. The parameters are
substantially unvaried when compared with (E,E,E)-1, within
experimental error. A blank test performed by irradiating 2
solutions in-situ with both UV and visible light did not lead to
any perceivable variation in the supramolecular packing
(Figure S18).
Interestingly, the in-situ photoirradiation of a solution of
(E,E,E)-1 with ultraviolet light is generally accompanied with
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a loss of ordered crystalline packing, indicating a decrease in Moreover, it is striking to observe how the ordered
1
2
3
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5
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7
8
concentration of the (E,E,E)-1 isomer. The general
disappearance of the initial packing was seen logical knowing
the lower stability of the azobenzene Z-isomer when adsorbed
on a surface, due to its non-planar geometry.18 Surprisingly, in
such a situation it was also possible to visualise domains of 1
displaying a periodical assembly (Figure 4). From our
interpretation, each domain is composed by one isomer:
(E,Z,Z)-1 and (E,E,Z)-1.
domains of both the photoproducts (E,Z,Z)-1 and (E,E,Z)-1
show a less dense crystalline packing compared to (E,E,E)-1
and 2 (Figure 5, Table 1) which is in line with the larger
stability in the STM imaging of the monolayers of the (E,E,E)-
1. One explanation for this observation could come from the
non-planar conformation of the Z-isomer of the azobenzene
units, resulting in less favourable molecule-substrate
interactions compared to the E form. This lower stabilisation is
balanced by the formation of stronger intermolecular hydrogen
bonds, such as carboxylic acid dimers, as evidenced by the
larger spacing between rows of (E,Z,Z)-1 and (E,E,Z)-1,
For (E,Z,Z)-1, the estimated unit cell parameters are the
following: a = 7.6 nm, b = 2.7 nm, α = 69 ° with an area A =
20 nm2, each containing four molecules (Figure 5c).
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Table 1. Experimental and modelled unit cell parameters for 1 and 2, and estimated thermodynamic quantities
a [nm]
b [nm]
α [°]
A [nm2]
Na H-bondsb
Eads
BE [kcal/mol]d
-6.83
[kcal/mol]c
experimental
theoretical
(4.1 ± 0.2) (3.0 ± 0.3) (41 ± 5)
4.3 2.8 41
(4.2 ± 0.2) (2.9 ± 0.1) (46 ± 1)
(8.7 ± 0.7)
(E,E,E)-1
2
2
8.0
-101.91
2
experimental
experimental
(8.8 ± 0.4)
(20)
26
2
(7.6)
8.1
(2.7)
3.7
(69)
61
I
1.5
-80.50
-78.84
-78.59
-6.63
-10.13
-8.63
(E,Z,Z)-1
(E,E,Z)-1
4
2
theoretic
al
II
7.6
2.8
74
20
III 7.8
3.3
56
21
2.5
experimental
(4.0)
3.8
4
(3.3)
3.3
(55)
56
(11)
11
I
1
-90.38
-89.96
-90.59
-8.87
-6.36
-7.83
2
2
theoretic
al
II
3.8
55
13
III 5.5
3.2
50
13
2
b
c
aNumber of molecules per unit cell, Number of H-bonds with neighbouring molecules, Adsorption energy, average interaction
energy of an individual molecule adsorbed on graphite, dBinding energy, average interaction energy between neighbouring molecules
Conversely, for (E,E,Z)-1 we estimate the following unit cell:
a = 4.0 nm, b = 3.3 nm, α = 55 ° with an area A = 11 nm2, each
containing two molecules (Figure 5e). Both assemblies display
notably different geometry and unit cell parameters compared
to (E,E,E)-1 and 2. Moreover, their stability appears lower
compared to the one of the (E,E,E)-1, being evidenced by the
smaller size of the ordered domains, and the sometimes fuzzy
contrast visible in the STM images. Interestingly it was not
possible to visualise the formation of ordered domains formed
by (Z,Z,Z)-1; this observation can be ascribed to the non-
planar conformation of the three (Z)-azobenzene units,
lowering the energy of adsorption of the molecules on the
basal plane of graphite and hence providing unfavourable
geometry for stabilisation via H-bonding with neighbouring
molecules. Upon subsequent visible light irradiation, a radical
change in the supramolecular assembly was evidenced,
yielding a scenario in which the only ordered domains
monitored at the interface were those containing (E,E,Z)-1 (as
mentioned above, Figure 4, Figure 5e).
compared to (E,E,E)-1. The formation of intermolecular
carboxylic acid dimers between two (E)-azobenzene branches
in the crystalline domains of (E,Z,Z)-1 and (E,E,Z)-1 is nicely
supported by MM/MD simulations (see details in SI). For both
(E,Z,Z)-1 and (E,E,Z)-1, the complicated H-bonding pattern
forced us to perform the simulation over multiple different
possible assemblies, in order to interpret correctly the
experimental data (Table 1, Figure S22). In this context, it is
important to mention that the majority of the starting
geometries used to model (E,Z,Z)-1 and (E,E,Z)-1 ended up in
amorphous structures after the MM/MD run. Only a few of
them presenting motion constrained by additional H-bonds
showed a clear assembly pattern. In all cases, the H-bonding
between two carboxylic groups takes place (yellow rectangles
in Figure 5d, f, Figure S22), thus confirming its crucial role for
the stabilisation of the supramolecular packing. In some
models, surprisingly, it was necessary to consider also the
occurrence of hydrogen bonds between carboxylic moieties
positioned on (Z)-azobenzene arms in order to obtain a stable
structure (orange rectangles in Figure 5d, Figure S22b, c). For
(E,Z,Z)-1, the structure reproducing best the experimental
pattern consists of model II (Table 1, Figure S22), with the
following parameters: a = 7.6 nm, b = 2.8 nm, α = 74 ° with
an area A = 20 nm2, each containing four molecules.
Regarding this isomer, it is important to point out that several
types of assemblies were considered and analysed, but only
the assemblies presenting H-bonds between (Z)-azobenzene
arms were able to form stable ordered assemblies, thus
confirming the importance of these bonds in the stability of the
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assembly. For (E,E,Z)-1, the chosen model I yields an unit variation of the inter-row spacing compared to the non-
cell: a = 3.8 nm, b = 3.3 nm, α = 56 ° with an area A = 11 nm2,
each containing two molecules. In this case, the computed
structures highlight the presence of π-π interactions between
two out-of-plane phenyl rings of adjacent (E,E,Z)-1 molecules
dominating over the formation of additional H-bonds (Figure
5f, Figure S22b I).
irradiated situation is the most evident consequence of the
external stimulus given by UV light. It is however evident
how the morphology of the patterns seen upon irradiation at
the solid-liquid interface is different from what obtained in the
latter experiments: in such a scenario, the molecules are not
capable of desorbing from the surface upon isomerisation, thus
to rearrange forming 2D crystalline domains, each formed by
only one Z-isomer, as previously mentioned, and visible in
Figure 5. In the photoswitching experiments performed on 1 at
the graphite-air interface we could on the contrary observe the
neighbouring molecules within the same row being most likely
present in the same configuration, nevertheless the molecular
configuration of the neighbouring rows seems to vary
randomly in the ordered domains. Thus, failing to form regular
crystalline domains containing one, single isomer (see Figure
S16). In light of these experimental evidences, we believe that
we can safely state that in the STM experiments performed at
the solid-liquid interface, the change in the geometry of the
supramolecular 2D assemblies is due to the competitive
adsorption of (E,Z,Z)-1 and (E,E,Z)-1 isomers, following the
isomerisation of (E,E,E)-1 in solution. Even though we cannot
neglect that the molecules can isomerise when adsorbed on the
graphite surface, we envision that in such a case the result
would be largely different.
1
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8
To obtain
a more complete interpretation of our
experimental findings, we casted down the different energies
driving the 2D self-assemblies of (E,E,E)-1, (E,Z,Z)-1 and
(E,E,Z)-1 on graphene. For this purpose, we have computed
two parameters: adsorption energy (Eads) and binding energy
(BE), giving us a hint on the strength of the molecule-substrate
and intermolecular interactions, respectively (Table 1 and SI).
From the data, it is clear that the structural packing of both
(E,Z,Z)-1 and (E,E,Z)-1 gives rise to a lower Eads compared to
(E,E,E)-1, due to the lower π-π interaction caused by the
azobenzene units in the (Z)-conformation. This result
rationalises the fact that no self-assembly for (Z,Z,Z)-1 was
observed at the experimental level. Conversely, for (E,E,Z)-1
and (E,Z,Z)-1 the larger BE values are related to the formation
of a strong H-bonding network stabilising the supramolecular
packing. The oppositive behaviour of Eads and BE when going
from (E,E,E)-1 to its isomers validates our interpretation of the
experimental molecular patterns visualised by STM. By and
large, the computational insights confirm that the self-
assembly of such large aromatic carboxylic acids is driven by
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CONCLUSIONS
a
complicate interplay between intermolecular- and
In summary, a novel rigid multiphotochromic system 1
based on three azobenzene chromophores attached to the 1, 3
and 5 positions of a central benzene ring has been designed
and synthesised in order to explore the role of electronic
decoupling in the isomerization of the individual azobenzene
moieties. Photoswitching of all three azobenzene units
embedded in the C3-symmetrical rigid molecular scaffold was
qualitatively confirmed by UV-Vis absorption spectroscopy
and liquid chromatography in solution. Each photochromic
unit was found to retain its photoresponsive nature, thus
yielding a reversible mixture of four different isomers upon
photoirradiation of a solution of all-(E)-1 with ultraviolet light.
In-situ STM investigation on the self-assembly of 1 at the
graphite-solution interface revealed the formation of patterns
of all-(E)-1. Upon in-situ irradiation with ultraviolet and
visible light we evidenced the variation of the supramolecular
packing, resulting from the formation of crystalline assemblies
of two different partially (Z)-isomers. For the first time, we
were able to identify by STM the existence of multiple
isomeric states of a multiphotochromic compound in single-
component self-assembled networks with a high level of
details. We believe that the present results could be of great
value for further research on multiphotochromic systems, and
could clarify the role of non-covalent interactions in the
supramolecular self-assembly of similar systems. In view of
the intrinsic molecular geometry and rigidity, we envision that
our multi-azobenzene compound could be employed in the
future as building blocks of photo-responsive materials for
various applications such as light-triggered host-guest
systems, or optically-responsive metal-organic frameworks.
molecule/substrate- interactions. For the planar all-(E)-1 the
geometry of the 2D crystalline assembly is governed by the
thermodynamics of strong molecule/substrate interactions
leading to a tightly packed unit cell. The photogenerated (Z)-
isomers show less favourable molecule/substrate interactions
due to their non-planar conformation; this is, however,
compensated by more stable hydrogen bonding interactions
between the carboxylic groups, and/or π-π interactions
between adjacent out-of-plane phenyl rings. The
aforementioned intermolecular interactions allow the
formation of stable supramolecular assemblies showing larger
unit cells compared to all-(E)-1, which are particularly unusual
in view of the known tendency of rigid carboxylic acids to
form a dense crystal structure.23
In order to cast further light onto whether the isomerization
takes place or not on the basal plane of the surface, we have
extended our study to the STM visualization of the
isomerisation occurring in dry films, i.e. at the solid-air
interface, of 1 physisorbed on the HOPG surface upon in-situ
irradiation (Figure S16, see SI for details). This was done by
depositing molecule 1 from a non-irradiated solution in THF
on graphite substrates by means of spin-coating. The high
affinity of the large aromatic core of 1 for the basal plane of
HOPG made it possible to obtain a molecular adsorbate
characterised by small crystalline regions alternated by un-
coated substrate regions, thus yielding a sub-monolayer
coverage. From semi-quantitative point of view, the unit cell
of (E,E,E)-1 monitored by STM at the solid-liquid and at the
solid-air interface are similar. Interestingly, upon performing
in-situ irradiation of the sample with ultraviolet light, we could
observe an evident variation of the original pattern visualised
prior to light irradiation (Figure S16) as a result of the
molecular isomerisation. In such a condition, thus in the
absence of a medium capable of solvating 1, the adsorbates
cannot desorb from the graphite substrate. The striking
ASSOCIATED CONTENT
The Supporting Information is available free of charge via the
Internet at http://pubs.acs.org..
Detailed experimental procedures; synthesis and characterisation
of the products, computational methodologies.
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Journal of the American Chemical Society
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(a) Klajn, R.; Wesson, P. J.; Bishop, K. J.; Grzybowski, B.
AUTHOR INFORMATION
A. Writing self-erasing images using metastable nanoparticle "inks".
Angew. Chem., Int. Ed. 2009, 48 (38), 7035-9; (b) Raimondo, C.;
Reinders, F.; Soydaner, U.; Mayor, M.; Samorì, P. Light-responsive
reversible solvation and precipitation of gold nanoparticles. Chem.
Commun. 2010, 46 (7), 1147-9; (c) Manna, D.; Udayabhaskararao, T.;
Zhao, H.; Klajn, R. Orthogonal Light-Induced Self-Assembly of
Nanoparticles using Differently Substituted Azobenzenes. Angew.
Chem., Int. Ed. 2015, 54 (42), 12394-7.
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Corresponding Authors
*samori@unistra.fr,*jerome.cornil@umons.ac.be,
*michal.valasek@kit.edu, *marcel.mayor@unibas.ch
Author Contributions
≠These authors contributed equally.
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(8)
(a) Ferri, V.; Elbing, M.; Pace, G.; Dickey, M. D.;
ACKNOWLEDGEMENTS
Zharnikov, M.; Samorì, P.; Mayor, M.; Rampi, M. A. Light-powered
electrical switch based on cargo-lifting azobenzene monolayers.
Angew. Chem., Int. Ed. 2008, 47 (18), 3407-9; (b) Mativetsky, J. M.;
Pace, G.; Elbing, M.; Rampi, M. A.; Mayor, M.; Samorì, P.
Azobenzenes as Light-Controlled Molecular Electronic Switches in
Nanoscale Metal-Molecule-Metal Junctions. J. Am. Chem. Soc. 2008,
130 (29), 9192-9193.
We gratefully thank Dr. Sara Bonacchi and Dr. Martin Herder for
enlightening discussions. This work was supported by the EC
through the Marie Sklodowska-Curie ITN project iSwitch (GA-
642196), the Agence Nationale de la Recherche through the
LabEx project Chemistry of Complex Systems (ANR-10-LABX-
0026_CSC) and the International Center for Frontier Research in
Chemistry (icFRC). M.V. and M.M. acknowledge financial
support by the Helmholtz Research Programm STN (Science and
Technology of Nanosystems). Computational resources were
provided by the Consortium des Équipements de Calcul Intensif
(CÉCI) funded by the Belgian National Fund for Scientific
Research (F.R.S.-FNRS) under Grant 2.5020.11. J.C. is an FNRS
research director.
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Crivillers, N.; Orgiu, E.; Reinders, F.; Mayor, M.; Samorì,
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Polymer Networks. Angew. Chem., Int. Ed. 2016, 55 (34), 9908-12;
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