Photoreversible supramolecular polymerisation and hierarchical organization of hydrogen-bonded supramolecular Co-polymers composed of diarylethenes and oligothiophenes

Article abstract of DOI:10.1002/chem.201103465

Diarylethene 1 equipped with two monotopic melamine hydrogen-bonding sites and oligothiophene-functionalized ditopic cyanurate (OTCA) were mixed in a nonpolar solvent to form AA-BB-type supramolecular co-polymers (SCPs) bearing photoswitchable moieties in their main chains and extended π systems as side chains. UV/Vis, fluorescence, dynamic light scattering (DLS), TEM, and AFM studies revealed that the two functional co-monomers formed flexible quasi-one-dimensional SCPs in solution that hierarchically self-organized into helical nanofibers through H-aggregation of the oligothiophene side chains. Upon irradiating the SCPs with UV light, a transition occurred from the H-aggregated state to non-aggregated monomeric oligothiophene side chains, as shown by spectroscopic studies, which indicates the formation of small oligomeric species held together only by hydrogen-bonding interactions. TEM and AFM visualized unfolded fibrils corresponding to elongated single SCP chains formed upon removal of solvent. The helical nanofibers were regenerated upon irradiating the UVirradiated solution with visible light. These results demonstrated that the supramolecular polymerisation followed by hierarchical organization can be effectively controlled by proper supramolecular designs using diarylethenes and π-conjugated oligomers. Copyright

Full text of DOI:10.1002/chem.201103465

DOI: 10.1002/chem.201103465
Photoreversible Supramolecular PoACTHNUGTRNEUNGlymerisation and Hierarchical
Organization of Hydrogen-Bonded Supramolecular Co-polymers Composed
of Diarylethenes and Oligothiophenes
Shiki Yagai,*[a] Keisuke Ohta,[a] Marina Gushiken,[a] Kazunori Iwai,[a] Atsushi Asano,[b]
Shu Seki,[b] Yoshihiro Kikkawa,[c] Masakazu Morimoto,[d] Akihide Kitamura,[a] and
Takashi Karatsu[a]
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FULL PAPER
Abstract: Diarylethene
with two monotopic melamine hydro-
gen-bonding sites and oligothiophene-
1
equipped
cal nanofibers through H-aggregation
of the oligothiophene side chains.
Upon irradiating the SCPs with UV
light, a transition occurred from the
H-aggregated state to non-aggregated
monomeric oligothiophene side chains,
as shown by spectroscopic studies,
which indicates the formation of small
oligomeric species held together only
by
hydrogen-bonding
interactions.
TEM and AFM visualized unfolded fi-
brils corresponding to elongated single
SCP chains formed upon removal of
solvent. The helical nanofibers were re-
generated upon irradiating the UV-
functionalized
ditopic
cyanurate
(OTCA) were mixed in a nonpolar sol-
vent to form AA-BB-type supramolec-
ular co-polymers (SCPs) bearing pho-
toswitchable moieties in their main
chains and extended p systems as side
chains. UV/Vis, fluorescence, dynamic
light scattering (DLS), TEM, and AFM
studies revealed that the two functional
co-monomers formed flexible quasi-
one-dimensional SCPs in solution that
hierarchically self-organized into heli-
AHCTUNGTRENNiGUN rradiated solution with visible light.
These results demonstrated that the
supramolecular polymerisation fol-
lowed by hierarchical organization can
be effectively controlled by proper
supramolecular designs using di-
Keywords: diarylethenes
·
oligo-
thiophenes photoresponsive as-
·
AHCTUNGTRENNUNG
semblies · self-assembly · supra-
molecular polymers
Introduction
ration of secondary interaction sites, such as extended p sys-
tems, organize supramolecular polymer chains into complex
higher-order nanostructures.[6,7] Although many of the
supramolecular polymers feature dynamic and reversible
monomerisation/polymerisation triggered by changes in the
external environment, such as temperature, solvent, and
concentration, the design of the supramolecular polymers
for which higher-level organization could be controlled with
accessible external stimuli may open new possibilities in
their applications.[8]
Main-chain supramolecular polymers, in which noncovalent
interactions are the driving force for polymerisation, are
probably one of the most promising applications of supra-
molecular chemistry owing to their extreme sensitivity to ex-
ternal stimuli arising from the reversible nature of these in-
teractions.[1] Among the many supramolecular polymers
based on various noncovalent interactions, those based on
multiple hydrogen bonds, pioneered by Lehn[2] and Meijer,[3]
are within the main stream of research investigations.[1] The
high directionality and selectivity of multiple hydrogen
bonds allow fine control over polymer architectures and
properties.[4] Another distinct advantage of multiple hydro-
gen bonds in supramolecular polymer research is that more
than one functional molecular element can be readily incor-
porated into a polymer chain, affording complex and multi-
functional polymeric materials.[5] Furthermore, the incorpo-
Light is arguably the most useful external stimulus, be-
cause of its non-invasive character, high spatial and tempo-
ral resolution, and its ability to excite selected chromo-
phores absorbing at a certain wavelength.[9] Thus, various
photochromic molecules have been functionalized with spe-
cific noncovalent interaction sites, and the impact of photo-
induced mechanical motion on their aggregation has been
investigated.[10,11] While many of these systems were charac-
terized by the photoinduced disassembling/assembling of
molecular building blocks, some of them displayed photore-
versible transformations between two well-defined self-
[a] Prof. Dr. S. Yagai, K. Ohta, M. Gushiken, K. Iwai,
Prof. Dr. A. Kitamura, Prof. Dr. T. Karatsu
Department of Applied Chemistry and Biotechnology
Graduate School of Engineering, Chiba University
1-33 Yayoi-cho, Inage-ku, Chiba 263-8522 (Japan)
Fax : (+81)43-290-3039
AHCTUNGTRENNUNG
assembled nanostructures.[12] However, there are no exam-
ples of phototransformable nanostructures composed of
electronically active p-conjugated molecular elements, de-
spite their potential utility in smart nanoelectronics and
nanophotonics.[13]
E-mail: yagai@faculty.chiba-u.jp
[b] A. Asano, Prof. Dr. S. Seki
Department of Applied Chemistry
To photochemically transform the robust supramolecular
nanostructures that are stabilized by strong p–p stacking in-
teractions, a rational co-assembly of proper photochromic
molecules and functional p-conjugated molecules is re-
quired. We have previously exploited azobenzene/perylene
bisimide bichromophoric supramolecular co-polymers con-
nected through melamine–imide multiple hydrogen-bonding
interactions to fabricate supramolecular dye assemblies, the
nanostructures of which could be controlled by external
light input.[14] Owing to the attractive interchain J-type p–p
stacking interaction between the perylene chromophores
embedded in the main chain, the hydrogen-bonded co-poly-
Graduate School of Engineering, Osaka University
2-1, Yamadaoka, Suita, Osaka 565-0871 (Japan)
[c] Dr. Y. Kikkawa
Photonics Research Institute
National Institute of Advanced Industrial Science and Technology
(AIST)
1-1-1 Higashi, Tsukuba, Ibaraki 305-8562 (Japan)
[d] Prof. Dr. M. Morimoto
Department of Chemistry and Research Center for Smart Molecules
Rikkyo University, Nishi-Ikebukuro 3-34-1
Toshima-ku, Tokyo 171-8501 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103465.
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S. Yagai et al.
mers afforded unique coil-shaped nanostructures. Despite
the favourable chromophore packing within these nano-
structures, the photoisomerisation of their constituent azo-
benzene moieties was severely suppressed due to tight mo-
lecular packing (H-aggregation). In this context, di-
ACHTUNGTRENNUNGarylethenes (DAEs) were particularly appealing, because of
their known ability to photoisomerise even in the crystalline
state.[15] Several DAE derivatives suitable for supramolec-
ular assembly have already been designed and shown to ex-
hibit unique photoresponsive aggregation properties.[16] Such
photoresponsive aggregation of these supramolecular DAE
systems is based on conformational changes upon ring-clos-
ing/ring-opening reactions and/or modification of the elec-
tron density of DAE moieties.
Here we report a supramolecular polymer system in
which DAEs form part of the main chain of hydrogen-
bonded supramolecular polymers[17] bearing p-electronic
side chains.[18] For this supramolecular polymer system, it is
possible to modulate the conformational flexibility of the
main chain by reversible ring-closing and ring-opening pho-
toreactions of the DAE moieties. The p-electronic side
chains enable higher-order organization of the supramolec-
ular polymers by interchain p–p stacking, thus rendering the
impact of the flexibility change detectable by spectroscopy
and microscopy, as well as imparting fruitful optical and
electronic properties to the supramolecular polymers. As a
consequence of distinct functional properties in the main
chain and the side chain, a high level of control over the
supramolecular polymerisation, that is, a reversible transi-
tion between helically intertwined secondary structures and
small oligomeric species,[19,20] was achieved by modifying the
external light input.
Scheme 1. Structure of melamine-appended diarylethene 1 in its open
(1o) and closed forms (1c) and oligothiophene-functionalized cyanurate
(OTCA) and dodecylated cyanurate (dCA).
Results and Discussion
Design and synthesis: We have designed a supramolecular
co-polymer (SCP) system consisting of diarylethene 1 func-
tionalized at its ends with two monotopic melamine hydro-
gen-bonding modules and oligothiophene-functionalized di-
topic cyanurate OTCA (Scheme 1).[21] Complementary triple
hydrogen-bonding interactions between these ditopic co-
monomers were expected to afford quasi-one-dimensional
AA+BB-type SCPs possessing photoswitchable DAE moi-
eties in their main chains with oligothiophene (OT) moieties
as side chains (Scheme 2).[22] It was anticipated that photo-
cyclisation of the DAE moieties residing in the main chains
would decrease the conformational flexibility of the SCPs
by restricting the conformational freedom associated with
rotation of the thiophene rings.[15a] The OT moieties not
only imparted favourable optical and electronic properties
to the SCPs, but also enabled their higher-order nanostruc-
turisation, thus amplifying the impact of photoswitching.
Diarylethene 1 was synthesized from 1,2-bis[5-(4-amino-
phenyl)-2-methylthien-3-yl]perfluorocyclopentene[23] by re-
action with 1,3,5-trichloro-2,4,6-triazine and subsequent re-
actions with n-dodecylamine and di-n-octylamine. The syn-
Scheme 2. Schematic representation of the photoresponsive hierarchical
organization of supramolecular co-polymers of 1 and OTCA.
thesis of oligothiophene-functionalized cyanurate OTCA
has been reported previously.[21]
Photoisomerisation of melamine-appended DAE 1 and its
aggregation with non-chromophoric cyanurate dCA:
1H NMR spectra of 1 in CDCl3 (Figure 1A) revealed that 1
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ring-closing. In contrast, lower-field shifts were observed for
the inner NH protons (marked by green star) and the
methyl protons of the DAE core (marked by blue star). In
the photostationary state (PSS), no proton resonances of 1o
were observed, demonstrating the quantitative formation of
1c. The UV/Vis spectrum of the PSS state of 1 in CHCl3 thus
corresponded to that of pure 1c (Figure S1 in the Supporting
Information), allowing the determination of isomerisation
yields by UV/Vis measurements in the following experi-
ments.
The photochromic reaction of 1 also occurred in nonpolar
solvents such as cyclohexane as shown by UV/Vis measure-
ments (Figure 1B). The quantum yields of the ring-closing
and the ring-opening reactions in cyclohexane were mea-
sured to be 0.58 and 0.0021, respectively. The relatively low
quantum yield of the ring-opening reaction might be due to
a substituent effect.[24] The reversible photochromic reaction
in cyclohexane could be repeated at least ten times without
degradation.
To confirm the hydrogen-bonding capability of 1 with di-
topic cyanurates, we first investigated the complexation of
1o with monododecylated cyanurate (dCA) as
a non-
AHCTUNGTREGcNNNU hromophoric co-monomer. Upon increasing the amount of
dCA in a solution of 1o (c=1ꢁ10ꢀ5 m) in cyclohexane, a hy-
pochromic effect was observed for the p–p transition of 1o
at 321 nm (Figure 1B). This indicates that the electron den-
sity of the amino groups is affected by hydrogen-bonding in-
teractions and/or complexation with dCA induces p–p stack-
ing of the DAE moieties. Plots of the spectral change versus
the equivalents of dCA displayed a kink at molar ratio 1:1
(inset in Figure 1B), confirming the quantitative complexa-
tion of these two ditopic co-monomers.[25] Such quantitative
complexation in DAD–ADA (D=donor; A=acceptor) hy-
drogen-bonding systems at micromolar concentrations, the
binding constants of which are only about 2ꢁ105 mꢀ1, are
often observed for those containing p-conjugated systems,
because additional p–p stacking interactions and/or the for-
mation of cyclic species stabilize the resulting co-assem-
blies.[7c,d,h,i,14,22g,26]
Figure 1C shows progress of the ring-closing photoreac-
tion of 1o in cyclohexane in the presence of varying amounts
of dCA. The ring-closing reaction was suppressed with in-
creasing content of dCA, which can be attributed to the in-
duced p–p stacking interaction and/or the conformational
fixation upon hydrogen bonding.[15a] This suppression level-
led off when the amount of dCA reached one equivalent,[25]
further confirming the 1:1 complexation. The quantum yield
of the ring-closing reaction for the 1:1 mixture then dimin-
ished to 0.16. The absorption spectrum at the PSS state also
showed a hypsochromic effect compared to pure 1c, indicat-
ing the interaction with dCA through hydrogen bonding
(Figure 1B). The quantum yield of the ring-opening reaction
for the 1:1 mixture was 0.0020, almost identical to that of 1c
alone. Despite the quantitative complexation of 1 with dCA,
atomic force microscopy showed no well-defined nanostruc-
tured aggregates, either in the open or in the closed state, in
films spin-coated from a 1ꢁ10ꢀ4 m solution.
Figure 1. A) 1H NMR spectra of 1o (top) in CDCl3 (5ꢁ10ꢀ3 m), and of 1c
(bottom), obtained by photoirradiation of 1o at 313 nm for 1 h. B) Red to
blue solid curves: UV/Vis spectral changes of 1o (1ꢁ10ꢀ5 m) in cyclohex-
ane upon addition of dCA (0, 0.25, 0.5, 0.75 and 1 equiv). The dotted
arrow indicates the direction of increasing dCA. Inset shows the plot of e
at 321 nm versus equivalents of dCA (0–2 equiv). Red and blue dotted
curves: UV/Vis spectra of solutions with 0 and 1 equiv of dCA recorded
after photoirradiation at 313 nm for 80 (0 equiv) or 300 s (1 equiv).
C) Time-course changes of the absorbance at 600 nm relative to that
after reaching the photostationary state during photoirradiation of 1o (1ꢁ
10ꢀ5 m) in cyclohexane in the presence of varying amounts of dCA (0–
2 equiv). Photoirradiation was performed in a f=5 mm NMR tube or a
1 cm quartz cuvette using a fluorimeter equipped with a 150 W xenon
lamp (band width=20 nm).
underwent a reversible photochromic ring-opening (1o!1c;
subscripts o and c denote the open and closed forms, respec-
tively) and ring-closing reaction (1c!1o) upon irradiation
with UV and visible light, respectively. In the NMR spectra,
a prominent higher-field shift was observed for the proton
resonance of the thiophene rings (marked by red star) upon
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S. Yagai et al.
Photoresponsive supramolecular co-polymerisation of 1 and
OTCA: Oligothiophene-functionalized cyanurate (OTCA)
was previously shown to form H-aggregates in nonpolar sol-
vents.[21] The formation of H-aggregates was evidenced by a
prominent hypsochromic shift (414!378 nm) of the absorp-
tion band and a bathochromic shift (513!540 nm) of the
emission band originating from the p—p* transition of the
OT moiety upon decreasing the solvent polarity. When ali-
quots of 1o were added to a solution of H-aggregated
OTCA (c=1ꢁ10ꢀ5 m) in cyclohexane, the absorption band
of the OT moiety at 378 nm decreased and a new band ap-
peared at 415 nm (Figure 2A). In the fluorescence spectra,
dependent changes in the UV/Vis and fluorescence spectra
(Figure 3). Because of the slow progress of this higher-order
organization towards equilibrium (Figure S2 in Supporting
Figure 3. A) UV/Vis and B) normalized fluorescence spectra of a 1:1 mix-
ture of 1o and OTCA in cyclohexane for concentrations from 1ꢁ10ꢀ5
m
(blue) to 1ꢁ10ꢀ4 m (green). The arrows indicate the changes upon in-
creasing the concentration. Insets show changes in e at 389 nm in A) and
the ratio of fluorescence intensities at 536 nm (H-aggregate) and 481 nm
(monomer) in B) with increasing concentration of the mixture.
Information), the spectra were recorded after aging for 3 h.
A hypsochromic shift of the absorption and a bathochromic
shift of the emission of the OT moieties upon increasing the
concentration from 1ꢁ10ꢀ5 to 1ꢁ10ꢀ4 m unequivocally indi-
cate a transition from the free to the H-aggregated state.
For this spectral change, the transition from the co-assem-
bled state (complementarily hydrogen-bonded 1o and
OTCA) to the self-sorted state (simple mixture of 1o and
H-aggregated OTCA) was clearly excluded by the AFM
study described later. For a 1ꢁ10ꢀ4 m solution, the formation
of large aggregates with average size of 600 nm was con-
firmed by dynamic light scattering (DLS) measurements
(Figure S3 in the Supporting Information). If only comple-
mentary hydrogen-bonding interactions between melamine
and
cyanurate moieties, the binding constants of which are about
2ꢁ105 mꢀ1,[22d] are responsible for the co-assembling of 1o
and OTCA, the degree of polymerisation (DP) for (1o +
OTCA)n at 1ꢁ10ꢀ4 m is only 2.6. Thus, cooperative H-aggre-
gation of OT moieties promotes the supramolecular co-poly-
merisation, leading to the formation of the higher-order ag-
gregates.[7d,26]
Figure 2. A) UV/Vis and B) fluorescence titration experiments of OTCA
(1ꢁ10ꢀ5 m) with 1o (0 to 2 equiv) in cyclohexane. Only the spectra at
OTCA:1o =1:0 (yellow), 1:0.25 (black), 1:0.5 (black), 1:0.75 (black) and
1:1 (blue) are shown for clarity. The arrows indicate the direction of spec-
tral change upon increasing 1o. Insets show changes in e at 415 nm for
A) and relative fluorescence intensity at 481 nm for B) with increasing
[1o].
the emission of the H-aggregates at 540 nm shifted to a
more bluish band at 513 nm with more significant vibronic
components, characteristic of the free OT moiety (Fig-
ure 2B). These observations demonstrated that 1o disrupted
the H-aggregates of OTCA by its complementary triple hy-
drogen-bonding interaction. Both these spectral changes
levelled off close to a 1:1 molar ratio (insets). The quantita-
tive complexation suggests that the resulting 1:1 co-assem-
blies are further stabilized by p–p stacking interactions be-
tween the DAE moieties and also probably by the formation
of cyclic species.
Upon increasing concentration of the 1:1 mixture, their
higher-order organization was indicated by concentration-
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Upon irradiating
OTCA)n with UV light, 1o was almost completely converted
to 1c, as confirmed by UV/Vis measurements (Figure 4A
a
1ꢁ10ꢀ4
m
solution of SCPs (1+
TEM and AFM Studies of photoreponsive supramolecular
co-polymers: The change in the nanostructure of the SCPs
of 1 and OTCA upon UV irradiation was investigated by
transmission electron microscopy (TEM). For this purpose,
aliquots were taken from the 1ꢁ10ꢀ4 m solution of the SCPs
in cyclohexane before and after exposure to UV light to
achieve PSS state (photoconversion yield=87%), and de-
posited on TEM grids. TEM images of the SCPs without ir-
radiation showed bundled nanofibers, the longest of which
had lengths on the order of micrometers (Figure 5A). The
Figure 4. A) UV/Vis and C) normalized fluorescence spectra of a 1:1 mix-
ture of 1o and OTCA (1ꢁ10ꢀ4 m) in cyclohexane upon photoirradiation at
313 nm (green to red spectra). The photoirradiation was performed in a
1 mm quartz cuvette using a 150 W xenon lamp equipped with fluorime-
ter (band width=20 nm). Fluorescence spectra were recorded with the
front-face geometry. The arrows indicate the changes upon photoirradia-
tion. In C), the un-normalized spectrum recorded after 90 min of UV ir-
radiation is shown by the red dotted curve. B) Time-course changes in e
at 600 nm in A). D) Time-course changes in fluorescence intensity at
536 nm in C).
Figure 5. TEM images of A,B) (1o +OTCA)n and C,D) (1c +OTCA)n.
The samples were prepared by drop-casting of their cyclohexane solu-
tions (c=1ꢁ10ꢀ4 m) onto carbon-coated TEM grid and staining with
uranyl acetate.
and 4B). In contrast to the spectral changes accompanying
the ring-closing reaction of 1o alone, no well-defined isosbes-
tic point was observed at around 350 nm (inset in Fig-
ure 4A), implying that the ring-closing of the DAE moieties
in the SCPs induced a change in the overlapping absorption
band of the OT moiety. A more impressive change could be
seen in the corresponding fluorescence spectra (Figure 4C
and 4D). Significant quenching of the OT emission was ob-
served upon UV irradiation due to Fçrster resonance
energy transfer (FRET) to the closed DAE moiety, the visi-
ble absorption band of which overlaps considerably with the
emission band of the OT moiety. More importantly, normal-
ized fluorescence spectra clearly showed the unstructured
emission band of the H-aggregated OT moiety gradually
changing to that with the vibronic progression of the mono-
meric OT moiety (Figure 4C). This finding implies that the
ring-closing reaction of the DAE moieties in the SCPs pre-
vent their higher-order organization. Without additional p–p
stacking interaction between OT moieties, the co-assemblies
cannot elongate to form SCPs, and may exist as small hydro-
gen-bonded oligomers. Indeed, for the UV-irradiated solu-
tion, no large aggregates could be detected by DLS analysis.
widths of the nanofibers were quite uniform at 20 nm (Fig-
ure 5B), suggesting that the SCPs form a well-defined sec-
ondary structure. Interestingly, these nanofibers were com-
pletely absent in the TEM images of UV-irradiated SCPs,
and neuron-like clumps of thinner fibrils were seen instead
(Figure 5C). These fibrils were at most about 200 nm long,
with a width of 5 nm (Figure 5D). The width is consistent
with the extended molecular length of OTCA (ca. 5 nm),
suggesting that the observed fibrils correspond to the linear-
ly extended supramolecular co-polymer chains of 1c and
OTCA. Since the distance between the two melamine hy-
drogen-bonding sites of 1c (based on the nitrogen atoms of
the outer hydrogen-bonding NH groups) is about 2.5 nm,
the DP for the 200 nm length fibril is roughly estimated to
be 80. If such extended SCPs exist in solution, we should
detect them by DLS. The absence of any large aggregates in
the DLS experiments of 1c and OTCA thus suggests that
these fibrils visualized by TEM are formed by an increase of
the DP by solvent evaporation.
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S. Yagai et al.
To examine the photoinduced nanostructure transition of
our SCPs, they were imaged with atomic force microscopy
(AFM) after different periods of exposure to UV light. The
SCPs were spin-coated onto highly oriented pyrolytic graph-
ite (HOPG). Without UV irradiation, SCPs (1o +OTCA)n
formed many bundled nanofibers (Figure S4 in the Support-
ing Information), consistent with TEM observations. Photo-
conversion of 50% of 1o to 1c resulted in the dispersion of
the nanofibers, most likely due to the decrease in the con-
centration of (1o +OTCA)n (Figures S5 and S6 in the Sup-
porting Information). AFM imaging of the isolated nanofib-
ers with a phase mode sensitive to the mechanical and
chemical properties of materials revealed a helical morphol-
ogy with an average helical pitch of (10ꢁ2) nm (Figure 6A
and 6B). The helical sense appears to be unbiased, which is
reasonable in the absence of any chiral sources. The average
height of the nanofibers, estimated by cross-sectional analy-
sis, was (8.2ꢁ0.1) nm (see Figure 6D, described later), con-
siderably shorter than the width measured by TEM
(20 nm).[27] The significant difference between these two
measurements was attributed to the presumably hollow
structure of the helices, which made them susceptible to de-
formation through evaporation processes.[14] The dimensions
of the nanofibers are unambiguously different from those of
previously reported nanostructures of self-aggregated
OTCA,[21] indicating the formation of SCPs (1o +OTCA)n.
The observed helical morphology is reminiscent of that
found by Wꢂrthner and co-workers for merocyanine supra-
molecular polymer nanorods,[28] suggesting helical folding
and intertwining of the SCP chains.
The images obtained at 75% photoconversion to 1c dis-
played short, curved fibrils (Figure 6C). The average height
of the fibrils was (3.2ꢁ0.1) nm (Figure 6D). At 87% photo-
conversion to 1c, almost all the nanofibers disappeared, leav-
ing only curved fibrils and small particles (Figure 6E and
6F). The lengths of fibrils are about 100 nm, shorter than
those observed by TEM. This is reasonable, because spin-
coating enables a faster evaporation of solvent compared to
drop-casting employed for the preparation of the TEM sam-
ples. When extensively UV-irradiated solutions were irradi-
ated with visible light for the ring-opening reaction, helical
nanofibers of (1o +OTCA)n were regenerated, accompany-
ing the nearly complete reconversion of 1c to 1o (Figure S7
in the Supporting Information), thus demonstrating the pho-
toreversibility of the present system.
Electronic properties of nanostructures: Because our SCPs
contained oligothiophene semiconductors, their intrinsic
charge carrier mobilities were evaluated by flash-photolysis
time-resolved microwave conductivity (FP-TRMC) mea-
AHCTUNGTREGsNNNU urements, which provided information on the short-range
(ca. 10 nm) dynamics of mobile charge carriers.[29] To pre-
pare films capable of exhibiting detectable TRMC signals, a
1:1 mixture of 1o and OTCA was dissolved in cyclohexane at
a high concentration (1ꢁ10ꢀ2 m) with heating. After the re-
sulting homogeneous solution was cooled to room tempera-
ture, it formed a gel on standing for several minutes, as a
result of the formation of elongated nanofibers.[10c,30] UV ir-
radiation of the gel prepared in a 1 mm cuvette converted it
to a solution together with a photochromic reaction (Fig-
ure 7A). Irradiation of the resulting sol with visible light re-
formed the original gel state. Although it was not possible
to obtain images of the nanostructures formed in these con-
densed materials, the observed photoreversible sol–gel tran-
sition is consistent with the photoreversible supramolecular
polymerisation seen in the above-described microscopic
studies. We therefore prepared films from the gel and the
sol for TRMC studies. Both films, upon excitation with a
355 nm laser pulse, exhibited TRMC signals given by fSm
(f, photocarrier generation yield; Sm, sum of the mobilities
of photogenerated charge carriers) with a maximum value
of 1.9ꢁ10ꢀ5 cm2 Vꢀ1 sꢀ1 (Figure 7B). Since films of 1o or 1c
alone showed negligible TRMC signals, the foregoing results
indicate that both the films contained oligothiophene stacks
as pathways for mobile charge carriers. For the film pre-
pared from the sol, nonspecific aggregation of (1c +OTCA)n
might have occurred by p–p stacking of the OT moieties
upon evaporating the solution. This was confirmed by fluo-
Figure 6. AFM images showing photoconversion of (1o +OTCA)n to (1c +
OTCA)n. A) Phase image obtained at 50% photoconversion to 1c.
B) Cross sectional analysis along the white line in A). C) Height image
obtained at 75% photoconversion to 1c. The yellow arrows denote fibrils
of (1c +OTCA)n. D) Superimposed cross-sectional curves along the red
and the blue lines in C). E) Height image and F) phase image, both at
87% photoconversion to 1c.
2250
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Hydrogen-Bonded Supramolecular Co-polymers
FULL PAPER
higher-order organization capability, SCPs of (1c +OTCA)n
are not formed in solution due to weak triple hydrogen-
bonding interaction between melamines and cyanurates, and
only when evaporation of the solvent occurs do they elon-
gate to form fibrils with several hundreds of nanometers in
length. Both the helically organized nanofibers and the pho-
togenerated fibrils showed comparable charge transport ca-
pabilities, as shown by FP-TRMC, due to the nonspecific as-
sociation of the latter in the condensed state. However, the
high level of control over self-assembled nanostructures by
the photochromic reaction of DAE demonstrated here af-
fords a promising prospect of the use of supramolecular
polymer chemistry for the spatiotemporally controlled gen-
eration of novel photo- and electroactive nanoarchitec-
tures.[32]
Acknowledgements
S.Y. thanks Izumi Science and Technology Foundation and The Sumito-
mo Foundation for partial financial support. This work is partially sup-
ported by CREST, JST.
Figure 7. A) UV-induced gel-to-sol transition of the cyclohexane gel con-
taining 1:1 mixture of 1o and OTCA (c=1ꢁ10ꢀ2 m). The resulting sol
could be converted to the gel state by irradiation with visible light as for-
mally shown by the dotted arrow. B) Conductivity transients observed for
thin films of (1o +OTCA)n (black line) and (1c +OTCA)n (red line). The
transients were recorded under 355 nm laser pulse excitation at 9.1ꢁ
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Received: November 4, 2011
Published online: January 31, 2012
Chem. Eur. J. 2012, 18, 2244 – 2253
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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