Photocontrol over self-assembled nanostructures of π-π Stacked dyes supported by the parallel conformer of diarylethene

Article abstract of DOI:10.1002/anie.201310773

Diarylethenes (DAEs) have rarely been used in the design of photoresponsive supramolecular assemblies with a well-defined morphology transition owing to rather small structural changes upon photoisomerization. A supramolecular design based on the parallel conformation of DAEs enables the construction of photoresponsive dye assemblies that undergo remarkable nanomorphology transitions. The cooperative stacking of perylene bisimide (PBI) dyes was used to stabilize the parallel conformer of DAE through complementary hydrogen bonds. Atomic force microscopy, UV/Vis spectroscopy, and molecular modeling revealed that our DAE and PBI building blocks coassembled in nonpolar solvent to form well-defined helical nanofibers featuring J-type dimers of PBI dyes. Upon irradiating the coassembly solution with UV and visible light in turn, a reversible morphology change between nanofibers and nanoparticles was observed. This system involves the generation of a new self-assembly pathway by means of photocontrol. Leading light: Photoresponsive dye aggregates exhibiting remarkable morphological changes were constructed through complexation of the parallel conformer of a diarylethene receptor with dimeric stacks of perylene bisimide through multiple hydrogen bonds. The aggregates can be reversibly transformed between helical nanofibers and granular nanoaggregates upon exposure to visible and UV light, respectively.

Full text of DOI:10.1002/anie.201310773

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Angewandte
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DOI: 10.1002/anie.201310773
Photoresponsive Assemblies
Photocontrol Over Self-Assembled Nanostructures of p–p Stacked
Dyes Supported by the Parallel Conformer of Diarylethene
Shiki Yagai,* Kazunori Iwai, Mitsuaki Yamauchi, Takashi Karatsu, Akihide Kitamura,
Shinobu Uemura, Masakazu Morimoto, Hao Wang, and Frank Wꢀrthner
Abstract: Diarylethenes (DAEs) have rarely been used in the
design of photoresponsive supramolecular assemblies with
a well-defined morphology transition owing to rather small
structural changes upon photoisomerization. A supramolec-
ular design based on the parallel conformation of DAEs
enables the construction of photoresponsive dye assemblies
that undergo remarkable nanomorphology transitions. The
cooperative stacking of perylene bisimide (PBI) dyes was used
to stabilize the parallel conformer of DAE through comple-
mentary hydrogen bonds. Atomic force microscopy, UV/Vis
spectroscopy, and molecular modeling revealed that our DAE
and PBI building blocks coassembled in nonpolar solvent to
form well-defined helical nanofibers featuring J-type dimers of
PBI dyes. Upon irradiating the coassembly solution with UV
and visible light in turn, a reversible morphology change
between nanofibers and nanoparticles was observed. This
system involves the generation of a new self-assembly pathway
by means of photocontrol.
reactivity and photofatigue resistance and because they show
large changes in their optical and electronic properties upon
photoisomerization.[3] Nevertheless, they have rarely been
used in the design of photoresponsive supramolecular assem-
blies,[4] a situation in stark contrast to the many examples of
azobenzene-based photoresponsive assemblies.[5] This is
attributed to the fact that azobenzenes show large structural
changes upon photoisomerization whilst those observed upon
DAE photoswitching are rather small, thus enabling even
photoinduced ring-closing/opening reactions of DAEs in the
bulk state.[6] Most supramolecular assemblies based on DAEs
therefore utilize the difference in stacking propensity
between the flexible open-ring isomers and the rigid closed-
ring isomers to control the assembly and disassembly of one-
dimensionally stacked aggregates.[7,8]
A less investigated strategy for inducing drastic morpho-
logical changes in the assemblies is the use of the photo-
chemically unreactive parallel conformers in equilibrium with
the reactive antiparallel conformers in solution (Figure 1a). If
well-defined supramolecular architectures can be designed
based on the parallel conformers, the dynamic nature of
solution assemblies could allow equilibration with the anti-
parallel conformers through aggregate–monomer exchange,
thereby leading to a ring-closing photoreaction that would
result in a completely different supramolecular structure. The
involvement of the parallel conformers has been proposed for
several supramolecular systems but the morphologies of those
assemblies were not well-defined.[9] We herein demonstrate
that the parallel conformation of diarylethenes can be
stabilized through the cooperative p–p stacking of noncova-
lently bound p-conjugated molecules through specific inter-
molecular interactions, thereby producing supramolecular
polymers with well-defined one-dimensional nanostructures.
The nanostructures dispersed in solution can be reversibly
converted into granular aggregates upon the ring-closing/
opening photoreaction of the DAE moieties.
D
ynamic control over the morphologies of molecular
assemblies through external stimuli is the key to developing
dynamic soft-matter devices.[1] By virtue of various photo-
chromic molecules, light is now regarded as the most useful
external stimulus for dynamically controlling the morphology
and functionality of supramolecular assemblies.[2] Among
synthetic photoswitches, diarylethenes (DAEs) are particu-
larly appealing because of their remarkably high photo-
[*] Prof. Dr. S. Yagai, K. Iwai, M. Yamauchi, Prof. Dr. T. Karatsu,
Prof. Dr. A. Kitamura
Department of Applied Chemistry and Biotechnology
Graduate School of Engineering, Chiba University
1-33 Yayoi-cho, Inage-ku, Chiba 263-8522 (Japan)
E-mail: yagai@faculty.chiba-u.jp
Prof. Dr. S. Yagai
CREST, JST
Chiyoda-ku 102-0075 (Japan)
Following our earlier work on DAE-photocontrolled
merocyanine aggregation[9d] and photochromic DAE–pery-
lene bisimide dyads,[10] we herein report photoresponsive
Prof. Dr. S. Uemura
Graduate School of Science and Technology, Kumamoto University
Kurokami 2-39-1, Chuo-ku, Kumamoto 860-8555 (Japan)
supramolecular polymers that make use of the tetratopic
[11]
Prof. Dr. M. Morimoto
receptor 1o
(Figure 1b) and the tetraphenoxy perylene
Department of Chemistry and Research Center for Smart Molecules
Rikkyo University, Nishi-Ikebukuro 3-34-1, Toshima-ku
Tokyo 171-8501 (Japan)
bisimide ditopic guest PBI, which has a strong propensity to
form p–p stacked slipped dimer aggregates.[12] The parallel
conformer of 1o can thus be stabilized by dimeric stacks of
PBI (Figure 1b), and supramolecular polymerization of the
resulting 1:2 (1o/PBI) complex is anticipated to take place
even at low concentrations. Because the closed-ring isomer 1c
cannot adopt the specific conformation required to bind the
PBI-dimer aggregates, UV irradiation of the supramolecular
Dr. H. Wang, Prof. Dr. F. Wꢀrthner
Institut fꢀr Organische Chemie and Center for Nanosystems
Chemistry, Universitꢁt Wꢀrzburg
Am Hubland, 97074 Wꢀrzburg (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201310773.
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Figure 2. UV/Vis titration of PBI (c=3.0ꢂ10À4 m) with increasing
concentrations of receptors 2o (a; 0–2 equiv) and 1o (b; 0–1 equiv) in
toluene. The red, blue, and green solid curves show the spectra at
receptor/guest ratios of 0:1, 0.5:1, and 1:1, respectively. Insets:
changes in e at 605 nm versus equiv of 2o (a) and at 610 nm versus
equiv of 1o (b).
Figure 1. a) Conformational and photochemical isomerism of diaryl-
ethene. b) Structures of the open-ring isomer of the tetratopic diaryl-
ethene receptor 1o (1c denotes the closed-ring isomer), the ditopic
perylene bisimide guest PBI, and the expected binding motif between
the parallel conformer of 1o and a PBI dimer. c) Structure of the ditopic
diarylethene receptor 2o.
studies. The aromatic protons of the thiophene and phenylene
moieties of 2o and the perylene core of PBI showed ill-defined
resonances, thus indicating the presence of a dynamic equi-
librium in which supramolecular species are slowly exchang-
ing with monomeric species (Figure S1 in the Supporting
Information). The particular broadening of the PBI aromatic
resonances is consistent with the UV/Vis results shown in
Figure 2a, which suggest the aggregation of these dyes. More
importantly, the resonance of the central methyl groups of 2o
appeared at d = 2.43 ppm upon complexation with PBI (Fig-
ure S2b). This resonance appears at higher magnetic field in
the absence of PBI.[15] This finding indicates that the
population of the parallel conformer of 2o prevails over that
of the antiparallel conformer , a situation consistent with an
earlier observation by Irie et al.[16] Upon irradiation of the
solution with UV light (303–323 nm) for 10 min, the signals of
the aromatic protons sharpened and almost quantitative
formation of 2c was observed. Because the chemical shifts are
only slightly displaced compared to those of the individual
molecules, we can conclude that the interaction between the
rigid 2c and PBI is only weak at the given concentration and
the degree of association is rather low, that is, monomeric
species prevail. These NMR results led us to expect a large
morphology change for assemblies of 1o and PBI upon
photoirradiation.
copolymers should disrupt the unique supramolecular archi-
tecture. To emphasize the specific complexation between 1o
and PBI, we also examined the complexation of PBI with the
ditopic receptor 2o (Figure 1c), which would provide discrete
2+2 complexes by taking the parallel conformation. Both 1o
and 2o were functionalized with tri(dodecyloxy)phenyl groups
to make the supramolecular assemblies soluble in low-polar-
ity solvents, thus ensuring nearly quantitative complexation
through complementary triple hydrogen bonding.[13]
UV/Vis titration experiments of PBI (c = 3.0 ꢀ 10À4 m)
with the ditopic DAE 2o were performed in toluene. Upon
increasing the concentration of 2o, a transition from mono-
meric PBI (lmax = 572 nm) to aggregates was indicated by
a bathochromically shifted absorption band (J-band) at lmax
=
605 nm (Figure 2a).[14] A nearly stoichiometric 1:1 complex-
ation was demonstrated by an almost linear increase of
absorption at 605 nm upon increasing the concentration of 2o
to 1 equiv (Figure 2a, inset).
Unlike the polymeric 1:2 assemblies of 1o and PBI
described later, the 1:1 complexation of 2o or 2c with PBI is
expected to provide small oligomeric species that are
accessible by NMR characterization techniques. We thus
recorded the 1H NMR spectrum of the 1:1 mixture in
[D8]toluene at the same concentrations as in the UV/Vis
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The titration of PBI with the tetratopic receptor 1o showed
more pronounced growth of the J-band (Figure 2b).
Although the wavelength of the absorption maxima of the
J-band did not differ significantly between the complexes of
PBI with 1o (610 nm) and PBI with 2o (605 nm), the molar
extinction coefficient (e) at the maximum exceeded that of
the monomer band when the concentration of 1o approached
the stoichiometric amount (0.5 equiv).[17] The more pro-
nounced J-band is indicative of the formation of supramolec-
ular copolymers denoted [1o·(PBI)2]n, where conformational
freedom of the repeating units could be suppressed by chain
elongation. Upon the addition of excess 1o, a broadening of
the J-band was observed (Figure 2b, inset), thus indicating
a decrease in the degree of polymerization.[18]
The morphology of the supramolecular copolymers
[1o·(PBI)2]n was studied by atomic force microscopy (AFM)
of spin-coated samples on highly oriented pyrolytic graphite
(HOPG). Well-defined fibrous nanostructures with lengths
approaching the micrometer scale were visualized (Fig-
ure 3a). A closer inspection of the nanofibers revealed that
they have a helical morphology with a height of approx-
imately 2 nm and a helical pitch of approximately 20 nm
(Figure 3b). The helical sense appears to be unbiased, which
is reasonable in the absence of any chiral source. Such
extended one-dimensional nanostructures were not found for
the samples prepared from a 1:1 mixture of 2o and PBI
(Figure S4), PBI alone, or a 1:1 mixture of 1o and PBI
(Figure S5), thus demonstrating that only the specific 1:2
complexation between 1o and PBI leads to structurally
defined helical supramolecular copolymers.
Force-field molecular mechanics calculations for the
supramolecular copolymer [1o·(PBI)2]12 support the spectro-
scopic and morphological features of the supramolecular
polymers described above (Figure 3c).[19] In this modeled
structure, quasi one-dimensionality of the supramolecular
polymers was provided by a zigzag arrangement of dimeric
stacks of PBI connected through 1o in the parallel conforma-
tion.[20] The modeled structure displays helicity provided by
the twisting of the perylene cores (Figure 3d), thus corrob-
orating the experimental results from AFM (Figure 3b). The
helical morphology observed in the AFM images likely
originates from the higher-order helical organization of one-
dimensional supramolecular polymers with this intrinsic
molecular-level helicity. The modeled structure also repre-
sents a slipped cofacial (J-type) stack of two PBI chromo-
phores, which is supported by a staggered conformation of
cofacial melamine moieties connected by the folded dithi-
enylethene linker (Figure 3d).
With photoresponsive units comprised of well-defined
multichromophoric supramolecular polymers in hand, photo-
irradiation experiments were performed. Irradiation of a tol-
uene solution of [1o·(PBI)2]n ([1o] = 1.5 ꢀ 10À4 m; [PBI] = 3.0 ꢀ
10À4 m) with UV light resulted in the smooth progression of
the ring-closing reaction of 1o to give 1c as shown by an
increase of the absorption in the range of 550–700 nm and at
389 nm with bleaching of the band at 330 nm (Figure 4a). The
quantum yield of the ring-closing reaction (Frc) was deter-
mined to be 22%. This value is nearly half of that of free 1o in
toluene (Frc = 38%, Figure S7), thus suggesting an increase in
the concentration of the parallel conformer of 1o through
hydrogen-bond-directed complexation with PBI.[16] A photo-
stationary state (PSS) was achieved after 120 s irradiation
under the applied conditions,[19] where photoconversion of
over 95% was confirmed by comparison with a UV-irradi-
ation experiment of 1o alone. Notably, the difference spec-
trum obtained by subtracting the spectrum of 1c from that of
UV-irradiated [1o·(PBI)2]n revealed that the PBI J-band did
not disappear after the ring-closing reaction (Figure S8),
a result that further corroborates the tight stacking of PBI
dimers in low-polarity media.
AFM observations revealed that the ring-closing reaction
leads to a dramatic morphological change in the assemblies.
When the ratio of 1o to 1c reached to 56:44, the elongated
nanofibers observed before UV irradiation were fragmented
into shorter nanofibers with lengths of several hundreds of
nanometers (Figure 4b), as well as granular nanoaggregates
(Figure S9). Further progression of the ring-closing reaction
at 1o/1c = 19:81 resulted in the complete conversion of the
nanofibers into granular nanoaggregates with diameters and
heights of approximately 10 and 1.2 nm, respectively (Fig-
ure 4c–e). Although the granular nanoaggregates precipi-
tated within a few minutes by unspecific agglomeration, the
irradiation of the precipitated solution with visible light
(630 nm) with vigorous stirring enabled a smooth ring-open-
ing reaction of 1c to give 1o (Figure S10), thereby restoring the
Figure 3. a) and b) AFM height images of coaggregates formed by the
1:2 complexation of 1o (c=1.5ꢂ10À4 m) and PBI (c=3.0ꢂ10À4 m) in
toluene. The sample was prepared by spin-coating of the solution onto
HOPG. The inset shows the cross-sectional analysis along the black
line. c) Energy-optimized structures (AMBER force field) of [1o·(PBI)2]12.
d) Extraction of the repeating unit 1o·(PBI)2. For these structures, the
3,4,5-tridodecyloxyphenyl groups of 1o and the p-tert-butylphenoxy
groups of PBI were omitted after calculation for clarity.
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Figure 5. Schematic representation of the photocontrol of the aggrega-
tion pathway of a mixture of the diarylethene receptor 1o/1c and the
perylene bisimide guest PBI.
quite different from those obtained by photoirradiation.
These findings imply the possibility of the photogeneration of
specific supramolecular nanomaterials that are not obtainable
through the self-assembly of ground state molecules under
either kinetic or thermodynamic control, thus indicating
a new self-assembly pathway[21] that is under photocontrol
(Figure 5).
In conclusion, we have demonstrated that the proper
design of supramolecular building blocks based on the
parallel conformers of diarylethene derivatives can produce
soluble supramolecular polymers that exhibit remarkable
morphology changes upon photoirradiation. A key feature of
our approach is the stabilization of the parallel conformer of
diarylethene by strongly p-stacked perylene bisimide chro-
mophores through directional hydrogen-bonding interactions.
The application of a similar strategy to various functional p
systems would expand the potential of photoresponsive
supramolecular nanomaterials.
Figure 4. a) Red and blue spectra: Change in the UV/Vis spectra of the
1:2 mixture of 1o (c=1.5ꢂ10À4 m) and PBI (c=3.0ꢂ10À4 m) in toluene
upon UV irradiation at 313 nm for 2 min at 208C. Arrows indicate the
changes upon UV irradiation. Blue dotted spectrum: UV/Vis spectrum
of pure 1c. b–d) AFM of the UV-irradiated samples of [1o·(PBI)2]n
prepared in toluene solution at 1o/1c ratios of 56:44 (b) and 19:81 (c).
(d) is a magnified image of (c), and (e) is the cross sectional analysis
along the yellow line in (d).
Received: December 12, 2013
Published online: February 7, 2014
Keywords: diarylethenes · perylene dyes · photochromism ·
self-assembly · supramolecular chemistry
fibrous nanostructures (Figure S11). This cycle could be
repeated at least three times (Figure S12), thereby demon-
strating the photoreversibility of the supramolecular system.
Taken together, the experimental results show that the
tetratopic diarylethene receptor 1o can connect J-dimers of
PBI through the formation of multiple hydrogen bonds from
the parallel conformation, thereby providing photoresponsive
one-dimensional supramolecular copolymers. Because such
assemblies are in dynamic exchange with monomeric or small
oligomeric species, 1o can undergo ring closing through the
antiparallel conformation. Because the photogenerated 1c is
a rigid and divergent hydrogen-bond receptor, it may act as
a “dispersant” for the J-dimer of PBI without the formation of
specific and uniform supramolecular architectures (Figure 5).
It should be noted that such a dispersing solution of the J-
dimer could be obtained only through the UV-irradiation of
the supramolecular copolymer solution, whilst the direct self-
assembly of preformed 1c and PBI by cooling the molecularly
dissolved mixture at 908C (Figure S13) did not show the
characteristic sharp J-band of PBI dimers (Figure S14).
Moreover, the resulting aggregates were film-like aggregates
with a thickness of around 0.8 nm (Figure S15), structures
.
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