A stable single piece of unimolecularly π-stacked porphyrin aggregate in a thixotropic low molecular weight gel: A one-dimensional molecular template for polydiacetylene wiring up to several tens of micrometers in length

Article abstract of DOI:10.1021/ja042869d

An amide-type copper porphyrin gelator having alkyldiacetylene tethers gives very transparent gel in Decalin, and the gel shows unique thixotropic behavior. One-dimensional aggregates generated in the gel act as templates for unimolecularly segregated pol

Full text of DOI:10.1021/ja042869d

Published on Web 03/03/2005
A Stable Single Piece of Unimolecularly π-Stacked Porphyrin Aggregate in a
Thixotropic Low Molecular Weight Gel: A One-Dimensional Molecular
Template for Polydiacetylene Wiring up to Several Tens of
Micrometers in Length
Michihiro Shirakawa, Norifumi Fujita, and Seiji Shinkai*
Department of Chemistry & Biochemistry, Graduate School of Engineering, Kyushu UniVersity,
Fukuoka 812-8581, Japan
Received November 26, 2004; E-mail: seijitcm@mbox.nc.kyushu-u.ac.jp
Molecules with one-dimensional (1-D) projection are the most
promising materials for nanodevices. Carbon nanotubes1 have been
already raised as a candidate for such purposes, which show unique
electron conductivity and physical strength indispensable to those
1-D chemical and electronic structures.2 On the other hand, a
programmed supramolecular approach to design and construct such
1-D nanodevices has received much attention in the last couple of
Figure 1. Photographs showing the thixotropic behavior of the Decalin
decades; however, it is still a challenging subject since most of the
gel of 3b and its TEM image: (a) the Decalin gel of 3b ([3b] ) 10 g
examples have been carried out in the solution state where these
supramolecules have a character of time-averaged factors. Recently,
it has been recognized that low molecular weight gels give well-
defined 1-D assemblies with a high structural aspect ratio, which
can be directly imaged by microscopic observations.3
dm-3), (b) shaking the gel with a vibrator, (c) leaving the gel for several
seconds, (d) the regenerated gel state, and (e) TEM image of the Decalin
gel of 3b ([3b] ) 1.0 g dm-3).
extremely thin, extended, and stable, and have low crystallinity.
The fact that the Decalin gel of 3b shows this unique “thixotropy”
phenomenon implies that this thin and stable 1-D aggregate is very
suitable for template-directed synthesis of polydiacetylens.
In the UV-vis spectra, the Soret band observed in the Decalin
gel of 3b shows a blue shift compared with that of the solution
state of 3b (418 nm f 404 nm).6 As expected, 3b molecules having
a 1-D motivated skeleton adopt an H aggregation mode in the gel
phase. IR spectral analyses of the xerogel prepared from the
cyclohexane gel of 3b allowed us to evaluate the hydrogen bonding
fashion of the amide groups in the gel phase. In both amide I and
amide II regions, several split peaks appeared, which means that
there are a few patterns of hydrogen bondings.6 This observation
is well-consistent with the low crystallinity of the 3b gel as well
as with the high transparency and the thixotropic behavior.
An XRD analysis of the xerogel prepared from the cyclohexane
gel of 3b shows a strong peak at 2θ ) 3.6° (2.5 nm), which is
ascribable to the distance between the carbon atoms that neighbor
the amide groups at the distal position of porphyrin.6 This
observation further confirms that the gel consists of H-aggregated
porphyrin stacks. The foregoing findings based on spectroscopic
analyses and XRD studies consistently support the view that 3b
has a strict nature to grow up into a 1-D aggregate.
To obtain a direct image of the gel tissue, we performed electron
microscopic analyses.6 Figure 1e shows an image of the Decalin
gel of 3b taken by transmission electron microscopy (TEM). It is
clearly seen from this TEM picture that well-developed fibers,
whose diameter is almost consistent with the size of a few 3b
molecules, are entangled. The elongated 1-D tissues reach to several
tens of micrometers in length, which is ascribed to several 10 000
3b molecular stacks into the 1-D direction. Scanning electron
microscopic (SEM) observation of the Decalin gel of 3b also shows
the linear morphology of the gel tissue.6
We have found that alkylamide group-tethered tetraphenyl
porphyrins including 1 and 2 act as excellent gelators with4a or
without guest molecules.4b In the 1-D 2 gel tissue, 2 molecules
adopt an H aggregation mode where amide-amide hydrogen
bonding and porphyrin π-stacking are working efficiently. The
structural insights have fully been characterized by UV-vis and
IR spectroscopy and X-ray crystallographic analyses.4 In the course
of this study, we have recognized that the alkylamide group-tethered
tetraphenyl porphyrin skeleton is a promising candidate of a 1-D
aggregative template for nanostructuring. On the basis of these lines
of information, we introduced diacetylene units as polymerizable
monomers into the 1-D aggregative core, expecting creation of a
novel supramolecular architecture, in which the porphyrin π-stack-
ing column is fabricated by conductive polymers.
The gelation properties of 3 containing diacetylene units have
been tested for 12 kinds of solvents.5 Although 3a does not act as
a gelator for the solvent tested herein, 3b successfully triggers
molecular aggregation leading to formation of very transparent gels
in cyclohexane and decahydronaphthalene (Decalin). This transpar-
ency offers a suitable condition for efficient photochemical reactions
without any scattering of the light.
Interestingly, the Decalin gel of 3b (10 g dm-3) shows very
unique physical behaviors called thixotropy (Figure 1a-d). Gener-
ally, once low molecular weight gels are shaken to break these gel
states, they turn to solution with or without precipitates and never
return to the gel states unless the broken gels are heated to give
solution once. The thixotropic behavior in low molecular weight
gel7 is best explained by the specific nature that the gel fibers are
Given such 1-D arrays of the polymerizable units in hand, we
conducted photopolymerization of the diacetylene units covalently
attached to the porphyrin template. The Decalin gel of 3b (4.0 g
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10.1021/ja042869d CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
polymers. An AFM image of the sample without photoirradiation
lost its original image after chloroform washing. A height profile
of this photoirradiated sample revealed that each fiber is about 3-nm
thick, which is consistent with the molecular width of 3b. These
AFM observations strongly support the nature of this templated
polymerization, that is, 3b molecules are arranged in a 1-D structure,
the diacetylene units of which are photopolymerized in situ. Further
striking is the finding that each unimolecularly polymerized
polydiacetylene fiber is elongated to more than several micrometers
in length without any defect. This implies that the distance between
1-D arranged 3b molecules is suitable for the topochemical reaction
among the peripheral diacetylene groups.
In conclusion, this work shows that the low molecular weight
gel is a specific medium for generating highly elongated materials
with a unimolecularly stacked array. By using this material as a
photopolymerization template, unimolecularly segregated polydi-
acetylene nanowires are easily obtained. Despite many attempts
concerning template-directed polymerization of diacetylene deriva-
tives in gels, unimolecularly segregated polymerization several
micrometers in length has thus far been unsuccessful.9 We believe
that this is the first example of such a programmed bottom-up
system.
Figure 2. (a) Time dependence of the UV-vis spectral change of the
Decalin gel of 3b upon UV irradiation ([3b] ) 4.0 g dm-3) by a 500 W
high-pressure mercury lamp at 25 °C. AFM images of (b) the Decalin gel
of 3b and (c) the Decalin gel of 3b after UV irradiation and chloroform
rinsing; these processes were conducted on HOPG, [3b] ) 1.0 g dm-3, a
500 W high-pressure mercury lamp, 25 °C. Edges of the AFM images are
2-µm long.
dm-3) in a 0.1-mm quartz cell was subjected to photoirradiation
with a 500 W high-pressure mercury lamp at 25 °C from a distance
of 8 cm. The time-dependence of the UV-vis spectral change in
this photoirradiation process (Figure 2a) shows that the Soret band
at 404 nm gradually disappears concomitant with an appearance
of a new Soret band at 420 nm. This observation evidences that
assembled porphyrinic monomers undergo the structural perturba-
tion as photopolymerization proceeds at the periphery of the
porphyrins. The spectral change shows that all diacetylene units
should join the polymeryzation along the extremely long aggregate
template. In addition, two new absorption bands appeared at 580
and 620 nm, which are assigned to those of π-conjugated polydi-
acetylene. Judging from the absorption maxima, the effective
conjugation length seems to be very long.8
IR spectra of the 3b gel sample before and after photoirradiation
also confirmed completion of photopolymerization of the polydi-
acetylene units. When the Decalin gel of 3b was photoirradiated,
the mass of the gel was gradually broken and purple insoluble solid
precipitated. IR analysis of this solid material6 shows that the
absorption band at 2250 cm-1 characteristic of diacetylene mono-
mers completely disappeared. Unfortunately, this insolubility of this
photopolymerized solid material makes TEM and SEM observations
difficult.
To obtain structural images by microscopic analysis, we con-
ducted photopolymerization of the 1-D 3b array directly on the
surface for atomic force microscopic (AFM) measurements. First,
we tested whether the surface photopolymerization proceeds
completely with the aid of UV-vis spectroscopic analysis. The
Decalin gel of 3b (1.0 g dm-3) was cast on the glass surface and
left for 12 h under the ambient conditions. This treatment resulted
in a filmlike material. This material shows the blue-shifted Soret
band which is similar to that of the bulk gel material. Upon UV
irradiation for 4 h, the red shift of the Soret band (404 f 420 nm)
and the appearance of 580 and 620 nm absorption bands were
observed. These spectral changes are consistent with the previous
bulk experiments (Figure 2a), which show successful photopoly-
merization on the surface.6 Second, the Decalin gel of 3b (1.0 g
dm-3) was cast on highly oriented pyrolytic graphite (HOPG). The
cast gel was washed with cyclohexane (a poor solvent for the 3b)
to remove the solvent Decalin. An AFM image of this sample shows
dense accumulations of straight tissues on the surface (Figure 2b).
Next, the cast gels on the HOPG surface with or without 4 h of
UV irradiation were washed with chloroform (a good solvent for
3b) to remove unreacted monomers 3b. As seen in Figure 2c, an
AFM image of the photoirradiated sample shows that straight fiber
structures still remain on the HOPG surface without dissolution
by chloroform washing because of the insolubilized diacetylene
Acknowledgment. This work was partially supported by Grant-
in-Aid for Young Scientists (B) (No. 16750122) and the 21st
Century COE Program, “Functional Innovation of Molecular
Informatics” from the Ministry of Education, Culture, Science,
Sports and Technology of Japan. We would like to thank Ms. M.
Fujita and Ms. E. Okasaki of Kyushu University for AFM and FAB-
MS (HR) measurements, respectively.
Supporting Information Available: Experimental details. IR and
UV spectra and XRD profile of assembled 3b before and after
photopolymerization. Magnified pictures of Figures 1e and 2. This
material is available free of charge via the Internet at http://pubs.acs.org.
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