A physical operation of hydrodynamic orientation of an azobenzene supramolecular assembly with light and sound

Article abstract of DOI:10.1039/c4cc02078b

Photoisomerizations of a newly designed azobenzene derivative reversibly change its self-assembly in a solution to form twisted supramolecular nanofibers and amorphous aggregates, respectively. When irradiating the sample solution with audible sound, the former assembly exhibits a LD response due to its hydrodynamic orientation, but the latter one is LD silent, in the sound-induced fluid flows. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02078b

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A physical operation of hydrodynamic orientation
of an azobenzene supramolecular assembly with
light and sound†
Cite this: Chem. Commun., 2014,
50, 5615
Received 20th March 2014,
Accepted 2nd April 2014
Yasuhisa Hotta,a Shunsuke Suiko,a Jin Motoyanagi,b Hiroshi Onishi,a
Taisuke Ihozaki,c Ryuichi Arakawac and Akihiko Tsuda*a
DOI: 10.1039/c4cc02078b
www.rsc.org/chemcomm
Photoisomerizations of a newly designed azobenzene derivative
reversibly change its self-assembly in a solution to form twisted
supramolecular nanofibers and amorphous aggregates, respectively.
When irradiating the sample solution with audible sound, the former
assembly exhibits a LD response due to its hydrodynamic orientation,
but the latter one is LD silent, in the sound-induced fluid flows.
Functional materials composed of macroscopic molecular
assemblies that bring about reversible changes of their shape
and/or physical properties upon external physical stimuli have
attracted much attention in material sciences.1 Light is used for
bringing about photoreactions and structural isomerizations of
molecules, leading to changes of matter. Sound is also available for
controlling orientations of nanoscale objects, such as polymers and
molecular assemblies in solutions.2–4 However, no example has yet
been reported of matter capable of reacting to both these physical
stimuli. In the present study, we succeeded in synthesizing a novel
photo- and/or sound-responsive supramolecular nanoarchitecture.
Visible and UV light reversibly turn their capability for acoustic
alignment in a solution on and off through photoconversions
between fibrous and amorphous nanostructures. The sample
solution exhibits a linear dichroism (LD) response with inputs
Fig. 1 (a) Chemical structures of trans-AZ(n) and cis-AZ(n) (n = 6, 12, and
16), and (b) their possible self-assemblies.
of visible light and audible sound, which has potential as a Brownian motions of the solvent molecules, they begin to react to
molecular-based AND Boolean logic gate.5
weak sheared flows generated by sound vibration.6 With the expecta-
We recently reported that supramolecular nanofibers, tion that photoisomerization of the molecular components changes
composed of a porphyrin or an anthracene derivative, hydro- hydrodynamic interactions of their assemblies with the fluid flows,
dynamically align in solutions under exposure to audible sound we designed azobenzene derivatives AZ(n) (n = 6, 12, and 16), bearing
of 100–1000 Hz frequency.4 When molecules or molecular N-phenyl amide groups with long alkyl chains (Fig. 1a).7
assemblies are long enough to average local effects of the
AZ(n) was synthesized starting from 4-aminophenylacetic acid,
which was successfully converted to 4,40-azobenzenediacetic acid,
condensed with 3,4,5-(tris-alkoxy)benzenamine, and characterized
by means of NMR, IR, and electronic absorption spectroscopies,
together with MALDI-TOF mass spectrometry (see ESI†). A repre-
a Department of Chemistry, Graduate School of Science, Kobe University,
1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan. E-mail: tsuda@harbor.kobe-u.ac.jp
b Department of Chemistry and Materials Technology, Graduate School of Science and
Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585,
Japan
c Department of Chemistry and Materials Engineering, Faculty of Chemistry,
Materials and Bioengineering, Kansai University, Suita, Osaka 564-8680, Japan
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cc02078b
1
sentative H NMR spectrum of trans-AZ(12) dissolved in CDCl3 at
20 1C showed characteristic peaks owing to its molecular structure
(Fig. 2a). The spectrum obtained in low polar cyclohexane-d12
solvent showed notably broadened peaks, but they became sharper
at an elevated temperature of 70 1C. Thus, trans-AZ(12) molecules
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Fig. 2 (a) 1H NMR spectra of trans-AZ(12) at 20 and 70 1C and cis-AZ(12)
at 20 1C in CDCl3 or cyclohexane-d12 with a concentration of 1.2 ꢀ 10ꢁ2 M.
(b) Absorption spectra of trans-AZ(12) at 20 and 70 1C and cis-AZ(12) in
cyclohexane with a concentration of 4.0 ꢀ 10ꢁ5 M.
Fig. 3 Scanning electron micrographs of air-dried samples of (a) trans-
AZ(12) and (c) cis-AZ(12) prepared from a cyclohexane solution with a
concentration of 4.0 ꢀ 10ꢁ5 M deposited on a specimen grid covered with
a thin carbon support film. (b) FM-AFM image and height profile (positions
A to B) of a trans-AZ(12) fiber on the HOPG surface immersed in pure
water. Cantilever oscillation amplitude: 0.2 nm. Frequency-shift setpoint:
+80 Hz.
most likely self-assemble at the lower temperature. The broadened
peaks observed in both aromatic and aliphatic regions at 20 1C
indicate that trans-AZ(12) molecules are tightly held in the self-
assembled structure. In the electronic absorption spectrum, the
cyclohexane solution of trans-AZ(12) provided the lowest energy
absorption band with lmax at 326 nm (Fig. 2b, black curve),
originating from the p–p* transition of the azobenzene group.8 intermolecular multiple hydrogen bonding interactions of
Since the spectrum is blue-shifted in comparison with that amide groups and p–p interactions of the aromatic components
measured at 70 1C (Fig. 2b, red curve), trans-AZ(12) likely forms (Fig. 1b, left). Both trans-AZ(6) and trans-AZ(16), having shorter
an H-aggregate.9
and longer alkyl chains, respectively, showed dominant forma-
SEM of the sample, prepared from a cyclohexane solution tions of amorphous aggregates under the same conditions in
with a concentration of [trans-AZ(12)] = 4.0 ꢀ 10ꢁ5 M, shows SEM (Fig. S2, ESI†). Van der Waals interactions of the attached
that nanofibers formed under those conditions have fibrous alkyl chains may also play an important role in bundling the
structures with diameters of 20–60 nm. The nanofibers may single supramolecular polymers.
be formed through the bundling of the trans-AZ(12) supra-
Photoisomerization of trans-AZ(12) to cis-AZ(12) in chloroform at
molecular polymers (Fig. 3a). Frequency-modulation atomic 20 1C occurred upon photoirradiation by 365 nm light using a
force microscopy (FM-AFM) was then performed on a HOPG 150 W xenon lamp for 20 min. The absorption spectrum showed
substrate with a solution of the self-assembled trans-AZ(12) in that the lowest energy absorption band of trans-azobenzene with
cyclohexane (4.0 ꢀ 10ꢁ5 M), after drying under flowing N2.
lmax at 332 nm decreased while that of cis-azobenzene at 256 and
Topographic images observed in pure water (Fig. 3b and Fig. S1, 433 nm increased (Fig. S3, ESI†). The 1H NMR spectrum in CDCl3
ESI†) revealed that the formed nanofibers are actually bundles also showed characteristic peak shifts, and the resulting trans/cis
of supramolecular polymers. The height profile (B4 nm) in the ratio was estimated to be 92: 8 in the photostationary state
cross section image and the calculated size of the trans-AZ(12) (Fig. 2a).11 In the cyclohexane solution, where trans-AZ(12)
molecule (2–3 nm) indicate that the nanofiber observed in formed supramolecular nanofibers, the photoisomerization
Fig. 3b is a double-stranded supramolecular polymer.10 These slowly proceeded at 20 1C, but was accelerated upon increasing
micrograph images and the obtained spectral features the temperature to 40 1C due to thermal dissociation of the
described above indicate that the trans-AZ(12) molecules self- molecular assembly.12 The resulting cyclohexane solution con-
assemble to form the supramolecular polymer through possible taining cis-AZ(12) also provided the characteristic absorption
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spectrum of cis-azobenzene with lmax at 256 and 431 nm (Fig. 2b,
Hydrodynamic orientations of polymers and molecular
blue curve). However, the peaks observed in the 1H NMR spectrum, assemblies having anisotropic structures give an LD response.4–6
obtained in cyclohexane-d12 solution at 20 1C, are broad as observed We conducted LD spectral measurements for the cyclohexane
in the case of trans-AZ(12). The broadened peaks and the relatively solution of trans- and cis-AZ(n) upon exposure to a sinusoidal
sharp peaks, corresponding to the aromatic and alkoxy groups, audible sound, which generates fluid flows including a primary
respectively, are observed in lower and higher magnetic field oscillatory flow and a secondary steady flow in the solution.14 The
regions, respectively. These results indicate that cis-AZ(12) also LD spectrometer in this study was equipped with a 12 ꢀ 12 ꢀ
self-assembles in cyclohexane solution. Here, it assembles through 44 mm quartz optical cuvette, having a +10 ꢀ 10 mm cylindrical
possible intermolecular dipole–dipole interactions of cis-azobenzene neck (outer diameter), composed of 1 mm-thick quartz glass, which
groups, having the polar structure compared with that of the trans- was filled with the solution of AZ(n). The cyclohexane solutions
isomer, and hydrogen bonding interactions of the amide groups, containing the self-assembled AZ(n) (n = 6, 12, and 16) at 20 1C
with weak interactions of the alkyl chains (Fig. 1b, right). In fact, a showed no LD responses in the absence of irradiation by the
SEM image obtained from the cyclohexane solution of cis-AZ(12) audible sound (Fig. 4a, black curve and Fig. S7, ESI†). When the
([AZ(12)] = 4.0 ꢀ 10ꢁ5 M) shows the presence of amorphous samples were irradiated with 120 Hz sound, interestingly, a
aggregates with particle sizes of 250–300 nm (Fig. 3c), whose shapes bisignate LD spectrum was observed only with trans-AZ(12) that
are entirely different from those obtained from trans-AZ(12) (Fig. 3a). formed the supramolecular nanofiber (Fig. 4a, red curve). The
Dynamic light scattering (DLS) measurements of the sample with mechanism of the alignment is such that the nanofiber reacts to
[AZ(12)] = 2.0 ꢀ 10ꢁ4 M also showed the presence of aggregates with velocity gradients of the media occurring in crossing areas of the
an average radius of 171 nm with a size distribution in the range of downward and upward flows, and by the laminar flows generated
70–960 nm (Fig. S4, ESI†).13 The observed conversions of the shapes around the glass surfaces of the vessel due to the sound-induced
of self-assembled trans- and cis-AZ(12) reversibly occurred through fluid flows.4b The observed LD intensity is, thus, highly dependent
photoirradiation with UV or visible light. Thermal isomerization on the frequency and amplitude of the sound (Fig. 4a) and the
from cis-AZ(12) to trans-AZ(12) in the dark occurred very slowly at shape of the optical cuvette used.4b A sound wave with low
20 1C with a half-life of 25 days (Fig. S5, ESI†). Since the Arrhenius frequency, which causes larger vibrations of the fluid media, allows
plot for the thermal isomerization in the range of 20–60 1C provided efficient alignment of the nanofibers in the solution to give an
a non-linear profile (Fig. S6, ESI†), it is also decelerated by the intense LD response. However, the sample becomes LD silent after
aggregation of cis-AZ(12) molecules at the lower temperature.12
photoisomerization from trans-AZ(12) to cis-AZ(12) (Fig. 4b). This
result is reasonably explained in that the cis-AZ(12) aggregates
without the anisotropic fibrous structure cannot orient in response
to the shared flows.
In conclusion, self-assembled AZ(12) reversibly changes its
capability to align acoustically through trans- and cis-photo-
isomerizations with visible and UV light. The alignment occurs
only with trans-AZ(12), forming supramolecular nanofibers,
under sound-irradiation to give the LD response. This system
has the potential to function as a molecular AND logic gate
operated by both light and sound.
The present work was sponsored by Grants-in-Aid for Scientific
Research (B) (No. 25286017) and Challenging Exploratory Research
(No. 24655125) from the Ministry of Education, Science, Sports, and
Culture, Japan, and a Toray Science and Technology Grant. The
AFM observations in this work were supported by a Grant-in-Aid for
Scientific Research (B) (No. 25286009).
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