Photoresponsive self-assembly and self-organization of hydrogen-bonded supramolecular tapes

Article abstract of DOI:10.1002/chem.200501468

Self-assembling building blocks that are readily functionalizable and capable of achieving programmed hierarchical organization have enabled us to create various functional nanomaterials. We have previously demonstrated that N,N′-disubstituted 4,6-diamino

Full text of DOI:10.1002/chem.200501468

DOI: 10.1002/chem.200501468
Photoresponsive Self-Assembly and Self-Organization of Hydrogen-Bonded
Supramolecular Tapes
Shiki Yagai,* Tomoyuki Iwashima, Keiki Kishikawa, Shoichiro Nakahara,
Takashi Karatsu, and Akihide Kitamura*[a]
1
Abstract: Self-assembling building
blocks that are readily functionalizable
and capable of achieving programmed
hierarchical organization have enabled
us to create various functional nanoma-
terials. We have previously demonstrat-
ed that N,N’-disubstituted 4,6-diamino-
azobenzene side chains. H NMR, UV/
Vis, and dynamic light scattering stud-
ies confirmed the presence of nanome-
ter-scale tapelike supramolecular poly-
mers in alkane solvents at micromolar
regimes. At higher concentrations (mil-
limolar regimes), the supramolecular
polymers hierarchically organized into
lamellar superstructures to form orga-
nogels, as shown by X-ray diffraction
and polarized optical microscopy. Re-
markably, the azobenzene side chains
are photoisomerizable even in the su-
pramolecular polymers, owing to their
loosely packed state supported by the
rigid hydrogen-bonded scaffold, ena-
bling us to establish photocontrollable
supramolecular polymerization and
higher order organization of the tape-
like supramolecular polymers into la-
mellar superstructures.
pyrimidin-2ACHTREUNG(1H)-one (DAP), a gua-
nine–cytosine hybridized molecule, is a
versatile building block for the creation
of tapelike supramolecular polymer
species in solution. In the current
study, DAP was functionalized with
Keywords: azobenzene
gels · photoisomerization · self-as-
sembly · supramolecular chemistry
· organo-
Introduction
tion of supramolecular polymers, due to their high direction-
ality and selectivity.[4,5] Another attractive feature of the use
of this type of noncovalent interaction is that the hydrogen-
bonded planes created by heterocyclic aromatic compounds
are capable of hierarchically organizing through p–p stack-
ing and van der Waals interactions to provide diverse meso-
scopic assemblages such as columnar[4e,6] and lamellar struc-
tures,[7,8] depending on the shape of the primary hydrogen-
bonded species. The resulting hierarchically organized meso-
scopic assemblages often exist in a gel phase in the presence
of appropriate solvents and a liquid crystalline mesophase in
the bulk state. Such spontaneous formation of highly organ-
ized superstructures plays a pivotal role in the fabrication of
organic materials requiring high levels of molecular aniso-
tropy.
Functionalization of supramolecular building blocks with
photoswitching molecules provides photoresponsive self-as-
semblies, which have recently attracted considerable atten-
tion.[9,10] While most systems utilize morphological changes
in photochromic chromophores to control inter- or intramo-
lecular hydrogen-bonding interactions on the formation of
supramolecular species, their application to control the hier-
archical organization of supramolecular species is very rare.
We recently reported photochemical control over the stack-
ing of hydrogen-bonded cyclic oligomers (rosettes) through
Supramolecular polymers are fascinating materials exhibit-
ing self-repairing and stimuli-responsive properties arising
from their reversible natures.[1] A spontaneous chain-elonga-
tion process based on noncovalent interactions permits the
polymerization of supramolecular building blocks possessing
chemically labile functional groups under ambient condi-
tions, thus paving the way for the preparation of polymeric
materials featuring various functional side chains such as op-
tically and electronically active dyes.[2] These supramolecular
polymers would thus be expected to exhibit novel photo-
chemical or electrochemical activities, with the individual
properties of the chromophoric units being amplified at the
supramolecular levels.
DNA base-type multiple hydrogen-bonding interactions[3]
should be the most beneficial noncovalent glue for the crea-
[a] Dr. S. Yagai, T. Iwashima, Prof. Dr. K. Kishikawa, S. Nakahara,
Prof. Dr. T. Karatsu, Prof. Dr. A. Kitamura
Department of Applied Chemistry and Biotechnology
Faculty of Engineering, Chiba University, 1–33 Yayoi-cho
Inage-ku, Chiba 263–8522 (Japan)
Fax : (+81)43-290-3039
E-mail: yagai@faculty.chiba-u.jp
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FULL PAPER
the use of photoisomerization
of azobenzene chromophor-
es.[6t] As another example of
photoresponsive hierarchical
organizations of supramolecu-
lar species, here we report
photoresponsive tapelike su-
pramolecular polymers based
on multiple hydrogen-bonding
interactions, which can hier-
archically organize into lamel-
lar superstructures.
Scheme 1. Supramolecular polymerization of N,N’-disubstituted 4,6-diaminopyrimidin-2CAHTRE(UNG 1H)-one (DAP) units.
Selective
formation
of
linear, tapelike supramolecular
polymers (supramolecular tapes) is of profound importance
for the creation of rigid nanostructures exhibiting high mo-
lecular anisotropy. Multiple hydrogen-bonding interactions
represent a convenient tool for the construction of such
tapelike assemblies, as exemplified by complementary mela-
mine-barbiturate/cyanurate systems.[11] Gottarelli et al.[7] and
Kato et al.[8] have reported supramolecular tapes generated
from guanine and folic acid derivatives, respectively. The
planar and rigid structures of the supramolecular tapes
eventually give rise to the formation of lamellar superstruc-
tures to form gels and (lyotropic) liquid crystalline meso-
phases. We recently reported the synthesis and the self-as-
sembly of N,N’-disubstituted
zation under dilute conditions and the hierarchical organiza-
tion of the resulting supramolecular tapes into lamellar su-
perstructures under concentrated conditions can be control-
led by external light input.
Results and Discussion
Synthesis: We designed and synthesized azobenzene-func-
tionalized DAPs 1 (Scheme 2) in which two photoresponsive
azobenzene units tailed with solubilizing chains are attached
to the 4- and 6-amino groups through propoxy linkers. An
4,6-diaminopyrimidin-2ACHTER(UNG 1H)-
ones (DAPs, see Figure 1) fea-
turing donor–donor–acceptor
(DDA) and acceptor–accept-
or–donor (AAD) hydrogen-
bonding arrays.[12] These self-
complementary guanine–cyto-
sine hybridized supramolecular
building blocks are based on 5-
octyl-4,6-diaminopyrimidin-2-
ACHTREUNG(1H)-one, reported by Lehn
et al. in 1992,[13] but they can
be functionalized even more
easily. DAP building blocks
possessing long aliphatic chains
have been shown to form
robust supramolecular tapes in
solution and in the solid state,
so a lot of functionalized su-
pramolecular tapes based on
DAP building blocks are avail-
able.
In this work the photores-
ponsive supramolecular tape
was constructed from azoben-
zene-pendent DAP building
blocks (Scheme 1). Through
Scheme 2. Synthesis of 1. a) Pd/C, THF, H2, 658C; b) NaNO2, aq. HCl, then phenol, NaOH, Na2CO3, H2O;
c) K2CO3, 1-bromododecane (for 1a), 1-bromo-2-ethylhexane (for 1b), or 3,4,5-tris(dodecan-1-yloxy)benzyl
photoisomerization
of
the
pendent azobenzene moieties,
both supramolecular polymeri-
chloride (for 1c), DMF, 658C; d) Hydrazine monohydrate, EtOH; e) sodium 4,6-dichloropyrimidin-2
onate, dioxane, diisopropylethylamine, 1058C, 24 h.
ACHTREUNG(1H)-
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S. Yagai, A. Kitamura et al.
attempt to introduce aminoazobenzene derivatives directly
onto the DAP unit was hampered by the low reactivity of
the aminoazobenzene derivatives in the nucleophilic substi-
tution with 4,6-dichloropyrimidin-2ACHTRE(UNG 1H)-one. The key com-
pound for the synthesis of 1 was the monoetherified 4,4’-di-
hydroxyazobenzene derivative 4, which was prepared by the
diazonium coupling of aniline 3—possessing a phthalimide-
protected w-aminopropoxy group at its 4-position—with
phenol. Etherification of the hydroxy group of 4 with the
appropriate halogenated compound and subsequent removal
of the phthalimide protecting group gave azobenzene deriv-
atives 6a–c, each possessing a reactive aliphatic amino
group. Finally, compounds 6a–c were treated with 4,6-di-
chloropyrimidin-2ACHTREUNG(1H)-one to give the crude DAP-azoben-
zene compounds 1a–1c.
Compound 1c, possessing tridodecyloxyphenyl groups,
shows good solubility in organic solvents, allowing its purifi-
cation by silica gel column chromatography. Compound 1a,
however, is almost insoluble in common organic solvents,
whilst compound 1b is soluble only in chlorinated solvents
such as chloroform and dichloromethane with gentle heat-
ing, the solutions turning to gels on cooling. Because of
their poor solubilities arising from supramolecular polymeri-
zation, 1a and 1b could not be isolated in the high purities
necessary to allowdetailed investigation into their self-ag-
gregation and hierarchical organization. Study of self-aggre-
gation was therefore carried out only for 1c, which was char-
acterized by 1H NMR, FAB-MS, and elemental analysis.
1
Figure 1. Parts of the H NMR spectra of 1c (c = 5”10ꢀ3 m) in a) CDCl3
at 258C, and b) in [D12]cyclohexane at 258C and c) at 758C.
Self-assembly: Compound 1c is highly soluble in THF and
chloroform and even in nonpolar alkane media such as
hexane and cyclohexane. The H NMR spectrum of 1c dis-
from the length of molecular-modeled hexadecamer
(11.5 nm), as shown below.
1
solved in CDCl3 (c = 5”10ꢀ3 m) is well resolved, indicating
the molecularly dissolved state (Figure 1a). The azobenzene
moieties are almost entirely trans (>99%) under ordinary
conditions, as shown by the resonances of the aromatic pro-
tons. In contrast, the spectrum of a [D12]cyclohexane solu-
tion (c = 5”10ꢀ3 m) is greatly broadened in all resonances,
indicating the formation of extended polymeric species (Fig-
ure 1b). The spectrum recorded at 758C is somewhat re-
solved yet still broadened, indicating the high stability of the
supramolecular polymers (Figure 1c).
Compound 1c is soluble on gentle heating even in hep-
tane or higher alkane solvents. In these solvents the aggre-
gation of 1c was dramatically enhanced, DLS showing that
the freshly prepared heptane solution contained aggregated
species with the average Dh of 15 nm at a relatively lowcon-
centration (2”10ꢀ4 m) at 258C. Interestingly, the aggregates
slowly grew, with a rate of roughly 8 nmhꢀ1, giving large ag-
gregated species exceeding 150 nm after 12 h. The slow
growth of the aggregates implies that the supramolecular
polymerization of 1c in heptane is driven not only by the
strong DDA:AAD hydrogen bonding interaction (Kassoc
reaches 104 mꢀ1 even in chloroform) but also by weak cohe-
sive forces competing with the solvation by heptane (i.e.,
van der Waals and p–p stacking interactions). With increases
in the concentration up to millimolar regimes, the solution
became turbid with time with the propagation of filamen-
tous aggregates as detailed below.
Figure 2a compares the UV/Vis absorption spectra of 1c
(c = 2”10ꢀ4 m) in THF (dashed line) and in heptane (solid
line; the steady state obtained after the sample was left to
stand for 12 h), corresponding to the monomeric and the ag-
gregated states, respectively, whilst Figure 2b shows the
spectra of a synthetic intermediate 5c lacking the DAP hy-
drogen-bonding unit. The absorption band at lmax = 280 nm
is only observed in the spectra of 1c, and is ascribable to the
In dynamic light scattering (DLS) analyses of 1c dissolved
in THF and CHCl3 at concentrations ranging from 1”10ꢀ5
to 6”10ꢀ3 m, no large particles with definitive scattering
light intensity detectable by our DLS instrument were ob-
servable at room temperature, indicating the absence of
1
large aggregated species. As expected from the H NMR re-
sults, the cyclohexane solution of 1c (c = 5”10ꢀ3 m) gave
moderate scattering intensity in DLS analysis owing to the
formation of polymeric species. The cumulative analysis of
the autocorrelation function resulted in a broad distribution
of the hydrodynamic diameter (Dh) centered at 50 nm,
which reflects the polydisperse nature of the linear supramo-
lecular polymers. If it is assumed that the observed Dh corre-
sponds to the gyration diameter of the rigid supramolecular
polymers, the average aggregation number of 70 is estimated
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Hydrogen-Bonded Supramolecular Tapes
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Figure 3. a) Molecular modeling structure of hexadecameric 1c. b) Pro-
posed packing motif for a supramolecular tape (side view).
the space prerequisite for photoisomerizarion within the su-
pramolecular polymers.[14]
Hierarchical organization: When heptane or higher alkane
solutions of 1c were allowed to stand, at a concentration of
1”10ꢀ3 m, yellowfilamentous precipitates started to appear
within several hours. Optical microscopic observation of the
precipitated solution revealed macroscopic fibers with sub-
micrometer diameters and lengths over 10 mm (Figure 4a).
Figure 2. UV/Vis spectra of a) 1c (c = 2”10ꢀ4 m), and b) 5c (c = 4”
10ꢀ4 m) in heptane (solid line) and THF (dashed line).
transition related to the DAP unit. The p–p transition band
of the azobenzene moieties of 1c in heptane is blue-shifted
by 13 nm and less structured than that of the molecularly
dissolved state in THF. Compound 5c, without a DAP hy-
drogen-bonding unit, also shows a 6-nm hypsochromic shift
with decreasing solvent polarity (Figure 2b), indicating that
the solvent-dependent hypsochromic shift observed for 1c
involves inherent solvatochromism of the azobenzene chro-
mophores. Therefore, the spectral change of the azobenzene
chromophores of 1c upon supramolecular polymerization is
relatively small, in contrast with other azobenzene-based
self-assemblies that are considerably stabilized by p–p stack-
ing interactions,[14] suggesting that the azobenzene chromo-
phores in this supramolecular polymer are free of extended
aggregation. This is clear evidence of the rigid nature of the
DAP-based supramolecular polymer scaffold. The mismatch
between the optimum p–p stacking distance (typically 3–
4 Š) and the intervals of the nitrogen atoms of the amino
groups bearing the azobenzene side chains (4.8–4.9 Š) may
prevent extended p–p stacking of the azobenzene chromo-
phores. A molecular modeling-derived hexadecamer of 1c
(MacroModel 9.0, MMFF force-field calculation), shown in
Figure 3a, illustrates this situation: the azobenzene side
chains were optimized as a partially overlapped dimeric
state with adjacent chromophores. Such a loose molecular
packing would provide the azobenzene chromophores with
Figure 4. a) Optical microscopic image of a precipitated solution of 1c in
heptane. b) FE-SEM image of the dried precipitates of 1c.
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S. Yagai, A. Kitamura et al.
Observation of the freeze-dried precipitates by field-emis-
sion scanning electron microscopy (FE-SEM) also con-
firmed the presence of entangled fibrous entities with diam-
eters of several hundred nanometers (Figure 4b).
transparent film is strongly birefringent (inset in Figure 6a),
indicating the presence of molecular anisotropy. The X-ray
diffraction pattern of the film at room temperature is shown
in Figure 6a. Most importantly, the weak Bragg diffraction
Interestingly, use of more concentrated heptane or higher
alkane solutions of 1c resulted in the formation of two-di-
mensional sheet-like macroscopic entities on standing, as
again observed by optical microscopy (Figure 5a) and FE-
Figure 6. X-ray diffraction patterns and cross-polarized optical micro-
graphs (insets) of self-assembled sheet of 1c at a) room temperature, and
b) 1558C upon cooling from the isotropic liquid.
corresponding to a spacing of 9.62 Š matches very well with
the periodic distance between the neighboring DAP units in
the molecular modeling-derived tape (ꢁ9.7 Š, side viewin
Figure 3a) as well as that in the crystal structure of 4,6-dia-
Figure 5. a) Optical microscopic image of the heptane gel of 1c. b) FE-
SEM image of the dried gel. c)Magnified image of b).
mino-5-octylpyrimidin-2ACHTER(UNG 1H)-one (9.68 Š) reported by
Lehn et al.[13] The broad diffuse halo in the wide-angle
region indicates the liquid-like nature of the aliphatic tales.
The sole intense peak observed in the small angle region
suggests the presence of a lamellar structure with a spacing
of 45.6 Š, which is commensurate neither with the width of
the molecular modeling-derived tape of 1c (ca. 77 Š, Figur-
e 3a) nor with the molecular length of 1c with extended
alkyl chains (ca. 38 Š). Interdigitation of the aliphatic tails
between the lamella is unlikely because of the steric crowd-
ing of aliphatic tails. Instead, tilted stacking (q = 36.38) of
the tapes in the lamella, as shown in Figure 3b, is strongly
suggested for a hierarchical structure. An almost identical
lamellar structure was found for the cast film prepared from
SEM after freeze-drying (Figure 5b). The resulting sheet-
suspended liquids obtained after 12 h standing were opaque
organogels that showed stability against gravitational flow
(see Figure 10a and c).[15] FE-SEM observation with high
magnification revealed that the sheet was not composed of
assembled fibers (Figure 5c). The supramolecular polymer
of 1c therefore hierarchically organizes into distinct macro-
scopic entities with variation of concentration.
The heptane organogel made up of the self-assembled
sheet of 1c was dried and analyzed by polarized optical mi-
croscopy (POM) and X-ray diffraction (XRD). The resulting
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Hydrogen-Bonded Supramolecular Tapes
FULL PAPER
the homogeneous cyclohexane solution of 1c (data not
shown). On the basis of these results, it can be suggested
that the supramolecular tape of 1c had two-dimensionally
self-organized in the hydrocarbon media to form a lamellar
superstructure in the self-assembled sheet.
The lamellar superstructure persisted up to isotropization
(2208C) as judged by the XRD analysis: no significant
change in the sole observed diffraction peak at around 46 Š
was observed with increasing temperature. In addition, no
defined transition was detected by differential scanning calo-
rimetry measurement until isotropization. Upon cooling
slowly from the isotropic phase (18Cminꢀ1), however, the
presence of a columnar liquid crystalline mesophase was
suggested from the focal conic textures observed between
92–2208C (inset in Figure 6b). XRD analysis showed an in-
tense peak corresponding to a spacing of 40.2 Š and a weak
peak corresponding to a spacing of 9.50 Š. The presence of
the latter peak corroborates the persistence of the supramo-
lecular tape of 1c in the mesophase. The former spacing is
rather shorter than those observed for the lamellar struc-
tures, indicating the presence of a mesoscopic structure dif-
ferent from that observed before the isotropization. Com-
bined with the focal conic texture, it is most likely that col-
umns composed of several supramolecular tapes are ar-
ranged in two-dimensional ordering by micro-segrega-
tion[16,17] due to the mismatched volume between the rigid
aromatic core and the liquid-like aliphatic part mobile at
high temperature.[18] No definite assignment of the meso-
phase could be made, due to the absence of higher-order X-
ray diffractions. Detailed investigation of the liquid crystal-
line behavior of DAP-based supramolecular tapes with the
aid of DAP molecules possessing different optically active
side chains is nowunderway.
Figure 7. UV/Vis spectral changes (upon irradiation with 350 nm light) of
solutions of 1c (c = 2”10ꢀ5 m) in a) THF (0, 1, 2, 6 and 10 min), and
b) heptane (0, 1, 2, 4, and 10 min).
Photoisomerization: The azobenzene moieties in both mon-
omeric and aggregated 1c undergo reversible photoisomeri-
zation on irradiation with UV light at around 350 nm
(trans!cis) and subsequent irradiation with visible light at
around 450 nm (cis!trans). Figure 7 shows the UV/Vis
spectral changes of dilute THF (a) and heptane solutions
(b) of 1c (c = 2”10ꢀ5 m) upon irradiation with UV light.
The cis/trans ratio in heptane at the photostationary state
(PS) is 0.55, lower than that in THF (0.98). The lower cis/
trans ratio in heptane is indicative of supramolecular poly-
merization of 1c occurring at lowconcentrations. This was
supported by a control experiment performed with synthetic
intermediate 5c, without the DAP unit, which showed a
very high cis/trans ratio (0.99) upon UV irradiation even in
heptane. The value of 0.55 for the cis/trans ratio corresponds
to isomerization of one of the two azobenzene side chains
of 1c in the supramolecular tape.
Figure 8. Changes in the cis/trans ratio of the azobenzene moiety (left
axis, open circles) and the DLS-determined average aggregate size of 1c
(right axis, closed circles) in heptane (c = 2”10ꢀ4 m) versus irradiation
time (350-nm light). The average aggregate size after irradiation for
25 min could not be measured because of the lowscattering intensity.
To explore the effect of photoisomerization on the supra-
molecular polymerization of 1c, UV irradiation of the
steady-state heptane solution (c = 2”10ꢀ4 m, average aggre-
gate size is 150 nm) was followed by DLS and UV/Vis meas-
urements. Figure 8 shows the changes in aggregate size and
cis/trans ratio upon UV irradiation. Interestingly, the first
20 min irradiation resulted in the dramatic diminution of the
aggregate size, to roughly 30 nm, with an increase in the cis/
trans ratio to 0.43. Further irradiation achieved a PS state
(cis/trans ratio = 0.48), but the light scattering intensity in
DLS measurement was too weak to be detectable by our
DLS instrument, so we were not able to judge directly
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S. Yagai, A. Kitamura et al.
whether the photoirradiation had induced complete disrup-
tion of the supramolecular polymers into the monomeric
state. In viewof the low cis/trans ratio (0.48) in the PS state,
however, 1c molecules in the PS state seem to remain ag-
gregated.
Irradiation of the UV-irradiated solution with visible light
(450 nm) rapidly restored a trans-rich PS state (cis/trans
ratio = 0.90), within 5 min. DLS analysis showed the recov-
ery of the aggregated species with an average size of 30 nm,
and this again grewfurther with time, as in the case of the
freshly prepared heptane solution of 1c (vide supra). The
original aggregate size (ca. 150 nm) recovered within 24 h,
so the supramolecular polymerization of 1c under dilute
conditions is a photoreversible process. Figure 9 shows a
molecular modeling-derived hexadecamer of 1c, with one
azobenzene moiety adopting a cis conformation (trans,cis-
Figure 10. Photoinduced collapse of the heptane gel of 1c (c
= 1”
10ꢀ2 m) in a 1-mm cuvette (a!b, 1 h) and in a 1-cm cuvette (c!d, 4 h)
upon irradiation with 350-nm light.
below). Prolonged irradiation
did not cause further disrup-
tion of the polymeric species,
as judged from the DLS analy-
sis. The persistence of the pol-
ymeric species in the PS state
is convincing in viewof the
high concentration of 1c (c =
1”10ꢀ2 m), which increases the
degree of polymerization.
The cast films of the above
photogenerated sol phases
showed neither fibrous nor
sheet-like structures on FE-
SEM observation (Figure 11).
Figure 9. Molecular modeling-derived structure of hexadecameric 1c, one azobenzene moiety of which is
adopting the cis conformation (trans,cis-1c). Arrows indicate the cis-azobenzene moieties.
1c). While force-field calculations showed the hydrogen-
bond-directed polymerization of trans,cis-1c to be an ener-
getically favorable process, the resulting supramolecular
tape lacks the crowding of the aliphatic tails contributing to
the stabilization of the tape through van der Waals interac-
tion. Thus, under relatively dilute conditions, as in this DLS
experiment (c = 2”10ꢀ4 m), the degree of polymerization
may decrease with the trans!cis isomerization of the azo-
benzene side chains.
The azobenzene moieties of 1c are highly photoreactive
even in the organogel state. Figure 10 shows the changes un-
dergone by the heptane gels of 1c (c = 1”10ꢀ2 m) in 1-mm
(a!b) and 1-cm cuvettes (c!d) upon UV light irradiation
(150-W xenon lamp, 350 nm, bandwidth = 20 nm). The irra-
diation slowly dissolved macroscopic aggregates, resulting in
the collapse of the gel within 1 h in the 1 mm cuvette and in
4 h in the 1-cm cuvette.[19–21] The cis/trans ratio was roughly
0.4, as estimated from UV/Vis measurement upon 1000-fold
dilution, indicating that one of the two azobenzene side
chains of 1c photoisomerized. DLS analysis of the photo-
generated sol phase showed the presence of large aggregat-
ed species with the average size of 188 nm. These findings
indicate that the trans!cis isomerization of the azobenzene
moieties induces the collapse of the lamellar structures into
soluble supramolecular tapes (see XRD study described
Figure 11. FE-SEM image of the cast film of the photogenerated heptane
sol of 1c.
The absence of extended lamellar structure was shown by
POM and XRD analyses: the resulting film showed neither
birefringence between crossed polarizers nor defined XRD
peaks characteristic of the lamellar structure (Figure 12).
Noteworthy here is that the weak diffraction peak at 9.59 Š
corresponding to the periodic distance of the dimeric DAP
units persists, demonstrating that the photogenerated sol
phase indeed contains supramolecular tapes consisting of
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Hydrogen-Bonded Supramolecular Tapes
FULL PAPER
Conclusion
Self-assembling building blocks that are readily functionaliz-
able and capable of forming desired superstructures in solu-
tion and in the solid state are of great importance for the
creation of functional nanomaterials. Herein, we have dem-
onstrated the functionalization of N,N’-disubstituted 4,6-dia-
minopyrimidin-2ACHTRE(UNG 1H)-one (DAP), a versatile supramolecu-
lar building block for the creation of functional supramolec-
ular polymers, using azobenzene photoresponsive side
chains. The resulting functional DAP building blocks formed
tapelike supramolecular polymers in nonpolar media, and
these hierarchically organize into lamellar superstructures to
form fiber- and sheet-like macroscopic entities. An especial-
ly interesting aspect of this compound is its photoresponse
in the polymeric state, allowing photoinduced disruption
and reformation of the lamellar superstructures in heptane,
as demonstrated by the photochemically reversible sol–gel
transition. We are nowinvestigating the application of these
photoresponsive supramolecular polymers in photopattern-
ing techniques.[20] Furthermore, several DAP derivatives
possessing optoelectronically active substituents appear to
showthe versatility of DAP supramolecular building block
as novel functional nanomaterials.
Figure 12. X-ray diffraction pattern of the cast film prepared from the
photogenerated heptane sol of 1c.
photoisomerized 1c (i.e., trans,cis-1c). This is in agreement
with the DLS result that showed the presence of polymeric
species in the photogenerated sol phase. However, such su-
pramolecular tapes contain roughly 40% of cis-azobenzene
side chains as judged from the cis/trans ratio of 0.4, they
might be randomly deposited on the surface in the air-
drying process because of their unfavorable morphologies
for the hierarchical organization. The molecular modeling-
derived hexadecamer constructed from trans,cis-1c shown in
Figure 9 again represents one of these morphologies well:
the tridodecyloxyphenyl substituents attached to the cis-azo-
benzene moieties fully cover the aromatic surface of the su-
pramolecular tapes. Obviously, such a morphology is unfav-
orable for extended stacking, so it can be concluded that the
photoirradiation of the extended lamellar structures com-
posed of stacked supramolecular tapes of 1c resulted in the
disruption of the lamellar structures into soluble supramo-
lecular tapes (from left to right in Figure 13).
Experimental Section
General: 1H NMR spectra were recorded on a JEOL LA400 spectrome-
ter, and chemical shifts are reported in ppm with the TMS signal as inter-
nal standard. Variable-temperature 1H NMR spectra were recorded on a
JEOL LA500 spectrometer, and chemical shifts are reported in ppm with
the signals of residual solvents as internal standard. UV spectra were
measured on a JASCO V570 spectrophotometer, FAB-MS spectra on a
JEOL JMS-AX500 mass spectrometer. Elemental analyses were per-
formed at the Analytical Center of Chiba University. Electron micro-
scopic observation was carried out by field emission scanning electron
microscopy (JEOL JSM-6330F). The freeze-dried gels were sputtered
with Os by use of a Meiwafosis Neoc Pure Osmium Coater. Dynamic
light scattering measurements were conducted on a Beckmann Coulter
N5 particle analyzer fitted with a 25-mW He-Ne laser. The hot sample
solutions were filtered with Millipore membrane filter (pore size
=
0.2 mm) before measurements to remove dust. Photoirradiation experi-
ments were performed on a fluorimeter fitted with a 150-W xenon lamp
(20 nm excitation bandwidth). Molecular modeling calculations were per-
formed with MacroModel version 9.0. MMFF force field calculation was
applied for minimization of the hexadecamer of 1c (solvent: chloroform).
For the use of this calculation method, the conformation of the two
phenyl rings in the azobenzene unit was constrained to coplanar with re-
Figure 13. Schematic representation of the photoresponsive hierarchical
organization for the supramolecular tapes (side views).
ꢀ
ꢀ
spect to the N=N double bond as obtained by MM2 calculation.
Compound 7 was prepared by the previously reported procedures.[12]
Column chromatography was performed on 63–210 mm silica gel. The sol-
vents for the spectroscopic measurements and the gelling experiments
were all spectral grade and were used without further purification. All
other commercially available reagents and solvents were of reagent grade
and were used without further purification.
The UV-generated sol phase restored the original gel
state (lamellar superstructures) upon irradiation with visible
light (450 nm) and subsequent standing over 24 h (from
right to left in Figure 13). This process includes photochemi-
cal cis!trans isomerization followed by the slow hierarchi-
cal association of the supramolecular tapes into extended la-
mellar structures. Photochemical gel!sol conversion and
subsequent reformation of gel could be repeated at least
five times without decomposition of 1c.
N-[3-(4-Nitrophenoxy)propyl]phthalimide (2): N-(3-Bromopropyl)phthal-
AHCTREiUNG mide (5.78 g, 21.5 mmol) in dry DMF (15 mL) was added dropwise at
658C under N2 to a mixture of p-nitrophenol (3.0 g, 21.5 mmol) and
K2CO3 (6.0 g) in dry DMF (15 mL) and the resulting mixture was stirred
for 4 h. The mixture was cooled to room temperature and poured into ice
water. The resulting precipitate was collected and used for the next step
Chem. Eur. J. 2006, 12, 3984 – 3994
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3991

S. Yagai, A. Kitamura et al.
1
without purification (6.8 g, 96.4% yield). H NMR (400 MHz, CDCl3): d
J = 9.0 Hz, 2H), 6.63 (s, 2H), 5.02 (s, 2H), 4.14 (t, J = 6.3 Hz, 2H), 3.95
(m, 6H), 2.94 (t, J = 6.6 Hz, 2H), 1.97 (m, 2H), 1.77 (m, 6H), 1.55–1.31
(m, 48H), 0.88 (m, 9H) ppm; MS (FAB): 860 [M+H]+.
= 8.15 (d, J = 9.5 Hz, 2H), 7.85 (m, 2H), 7.74 (m, 2H), 6.85 (d, J =
9.3 Hz, 2H), 4.12 (t, J = 6.1 Hz, 2H), 3.93 (t, J = 6.8 Hz, 2H), 2.24 (q, J
= 6.6 Hz, 2H) ppm; MS (FAB): 327 [M]+.
General procedures for compounds 1a–c: Mixtures of 6a–c (0.43 mmol),
the sodium salt of 4,6-dichloropyrimidin-2ACHTREU(NG 1H)-one (0.21 mmol), and dii-
4-{4-[(3-Phthalimidopropyl)oxy]phenylazo}phenol (4): A mixture of com-
pound 2 (2.5 g, 7.66 mmol) and Pd/C (250 mg) in dry THF was stirred at
658C under H2 for 24 h. The mixture was filtered and the filtrate was
evaporated to dryness. The residue was purified by column chromatogra-
phy on silica gel (hexane/ethyl acetate 1:5) to give the corresponding ani-
line derivative 3 (1.78 g, 78.4% yield). The entire product was dissolved
in acetone/water (1:1 mixture) and conc. HCl (2 mL) was added to the
mixture at 08C. NaNO2 (450 mg) in water (7 mL) was added to this solu-
tion, and the mixture was stirred for 15 min at 08C. This solution was
slowly added to an aqueous solution (20 mL) of phenol (611 mg) contain-
ing NaOH (390 mg) and Na2CO3 (1.0 g). The resulting red precipitates
were collected by filtration to give crude compound 4 (1.7 g). This com-
pound was used for following etherification without further purification.
1H NMR (400 MHz, CDCl3): d = 7.85 (m, 2H), 7.81 (m, 4H), 7.72 (m,
2H), 6.92 (d, J = 9.0 Hz, 2H) 6.86 (d, J = 9.0 Hz, 2H), 4.11 (t, J =
6.1 Hz, 2H), 3.95 (t, J = 6.1 Hz, 2H), 2.23 (q, J = 6.4 Hz, 2H) ppm.
sopropylethylamine (0.3 mL) in dry dioxane (20 mL) were stirred at
1058C for 24 h. The mixture was cooled to room temperature and poured
into ice water and extracted with CHCl3. The combined organic phase
was dried over Na2SO4 and concentrated to dryness. The residue was dis-
solved in CHCl3 and adsorbed on silica gel. First, a mixture of ethyl ace-
tate/methanol 9:1 was used as eluent to remove by-products, and the de-
sired compound was then eluted with chloroform/methanol 20:1. Com-
pounds 1a and 1b could not be purified to completely pure states be-
cause of their lowsolubilities in organic solvents.
N,N’-Bis[3-(4-{4-[3,4,5-tris(n-dodecan-1-yloxy)benzyloxy]phenylazo}phen-
(1H)-one (1c, 61% yield): 1H NMR
ACHRTEoUNG xy)propyl]-4,6-diaminopyrimidin-2ACHTREUGN
(400 MHz, CDCl3): d = 7.80 (m, 8H), 7.00 (d, J = 9.0 Hz, 4H), 6.91 (d,
J = 8.3 Hz, 4H), 6.61 (s, 4H), 4.95 (s, 4H), 4.55 (s, 1H), 3.94 (m, 16H,)
3.17 (br, 4H), 1.98 (br, 4H), 1.76 (m, 12H), 1.44 (m, 12H), 1.25 (m,
96H), 0.88 (m, 18H) ppm; MS (FAB): 1922 [M+H]+; elemental analysis
(%) calcd for C12H190N8O11: C 75.03, H 9.97, N 5.83, O 9.16; found: C
74.85, H 9.77, N 5.80.
General procedure for the preparation of compounds 5a–c: The chlori-
nated compound (1.24 mmol) in dry DMF (15 mL) was added dropwise
at 658C under N2 to a mixture of compound 4 (1.24 mmol) and K2CO3
(4.0 g) in dry DMF (15 mL), and the resulting mixture was stirred for 4 h.
The mixture was cooled to room temperature and poured into ice water.
The resulting yellowprecipitates was collected and purified by column
chromatography over silica gel with chloroform as eluent to give com-
pounds 5a–c.
Acknowledgements
This work was partially supported by a Grant-in-Aid for Young Scientists
(B) (No. 17750120) from the Ministry of Education, Culture, Sports, Sci-
ence and Technology. S. Y. thanks Iketani Science and Technology Foun-
dation for the financial support.
4-Dodecyloxy-1-{4-[(3-phthalimidopropyl)oxy]phenylazo}benzene
(5a,
1
79% yield): H NMR (400 MHz, CDCl3): d = 7.85 (m, 2H), 7.85 (d, J =
9.0 Hz, 2H), 7.82 (d, J = 9.0 Hz, 2H), 7.73 (m, 2H), 6.98 (d, J = 9.0 Hz,
2H), 6.88 (d, J = 9.0 Hz, 2H), 4.11 (t, J = 6.1 Hz, 2H), 4.03 (t, J =
6.6 Hz, 2H), 3.94 (t, J = 6.8 Hz, 2H), 2.22 (m, 2H), 1.81 (q, J = 8.1 Hz,
2H), 1.57 (m, 2H), 1.26 (m, 16H), 0.88 (m, 3H) ppm; MS (FAB): 571
[M+H]+.
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4-[(2-Ethylhexyl)oxy]-1-{4-[(3-phthalimidopropyl)oxy]phenylazo}benzene
1
(5b, 83% yield): H NMR (400 MHz, CDCl3): d = 7.85 (m, 2H), 7.85 (d,
J = 9.0 Hz, 2H), 7.82 (d, J = 9.0 Hz, 2H), 7.72 (m, 2H), 6.99 (d, J =
9.0 Hz, 2H), 6.88 (d, J = 9.0 Hz, 2H), 4.11 (t, J = 6.1 Hz, 2H), 3.93 (m,
4H), 2.23 (m, 2H), 1.75 (m, 1H), 1.55–1.31 (m, 8H), 0.93 (m, 6H) ppm;
MS (FAB): 513 [M+H]+.
1-{4-[(3-Phthalimidopropyl)oxy]phenylazo}-4-[3,4,5-tris(dodecan-1-yloxy)-
benzyloxy]benzene (5c, 73% yield): 1H NMR (400 MHz, CDCl3): d =
7.85 (m, 2H), 7.85 (d, J = 9.0 Hz, 2H), 7.82 (d, J = 9.0 Hz, 2H), 7.72
(m, 2H), 7.06 (d, J = 9.0 Hz, 2H), 6.88 (d, J = 9.0 Hz, 2H), 6.63 (s,
2H), 5.02 (s, 2H), 4.11 (t, J = 6.1 Hz, 2H), 3.95 (m, 8H), 2.23 (m, 2H),
1.77 (m, 6H), 1.55–1.31 (m, 48H), 0.88 (m, 9H) ppm; MS (FAB): 1045
[M+H]+.
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General procedure for the preparation of compounds 6a–c: Mixtures of
compounds 5a–5c (0.9 mmol) and hydrazine monohydrate (0.4 mL) in
ethanol (40 mL) were heated at reflux for 4 h. The mixture was extracted
with CHCl3 and the combined organic phases were washed with water,
dried over Na2SO4, and evaporated to dryness to give compounds 6a–c in
almost pure states.
1-{4-[(3-Aminopropyl)oxy]phenylazo}-4-dodecyloxybenzene (6a, 99%
1
yield): H NMR (400 MHz, CDCl3): d = 7.86 (d, 4H), 6.99 (m, 4H), 4.13
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1.97 (q, J = 6.6 Hz, 2H), 1.81 (m, 2H), 1.47 (m, 2H), 1.34 (m, 16H), 0.88
(m, 3H) ppm; MS (FAB): 860 [M+H]+.
1-{4-[(3-Aminopropyl)oxy]phenylazo}-4-[(2-ethylhexyl)oxy]benzene (6b,
1
98% yield): H NMR (400 MHz, CDCl3): d = 7.86 (d, 4H), 6.99 (d, 4H),
4.13 (t, J = 6.4 Hz, 2H), 3.92 (d, J = 5.6 Hz, 2H), 2.94 (t, J = 6.8 Hz,
2H), 1.97 (m, 2H), 1.76 (m, 1H) 1.56–1.25 (m, 8H), 0.93 (m, 6H) ppm;
MS (FAB): 860 [M+H]+.
1-{4-[(3-Aminopropyl)oxy]phenylazo}-4-[3,4,5-tris(n-dodecan-1-yloxy)-
benzyloxy]benzene (6c, 99% yield): 1H NMR (400 MHz, CDCl3): d =
7.87 (m, 4H), 7.07 (d, J = 9.0 Hz, 2H), 7.00 (d, J = 9.3 Hz, 2H), 7.06 (d,
3992
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 3984 – 3994

Hydrogen-Bonded Supramolecular Tapes
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Received: November 26, 2005
Revised: February 3, 2006
Published online: March 21, 2006
3994
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Chem. Eur. J. 2006, 12, 3984 – 3994