Effect of Oligoethylene Chains on the Formation of Photoresponsive Nanotubes by Azobenzene Dyad

Article abstract of DOI:10.1002/ejoc.202000019

Azobenzene dyad 1 possessing hydrophobic side chains has previously been reported to assemble into toroidal nanostructures, which can further stack to afford nanotubes in nonpolar media. In this study, amphiphilic dyad 2 was synthesized to obtain nanotubes with the hydrophilic interior. Dyad 2 directly assembled into nanotubes without forming isolatable toroidal nanostructures. UV light irradiation to the nanotubes resulted in monomerization by trans→cis photoisomerization, but following reconstruction by visible light irradiation afforded amorphous films, suggesting a kinetic trap of trans-2 by residual cis-2 through co-aggregation.

Full text of DOI:10.1002/ejoc.202000019

DOI: 10.1002/ejoc.202000019
Communication
Nanotubes | Very Important Paper |
Effect of Oligoethylene Chains on the Formation of
Photoresponsive Nanotubes by Azobenzene Dyad
Takuho Saito[a] and Shiki Yagai*[a,b]
Abstract: Azobenzene dyad 1 possessing hydrophobic side toroidal nanostructures. UV light irradiation to the nanotubes
chains has previously been reported to assemble into toroidal
nanostructures, which can further stack to afford nanotubes in
nonpolar media. In this study, amphiphilic dyad 2 was synthe-
sized to obtain nanotubes with the hydrophilic interior. Dyad 2
directly assembled into nanotubes without forming isolatable
resulted in monomerization by trans→cis photoisomerization,
but following reconstruction by visible light irradiation afforded
amorphous films, suggesting a kinetic trap of trans-2 by resid-
ual cis-2 through co-aggregation.
of 2 with unique hysteresis between UV light-induced dissocia-
tion and visible light-induced reconstruction of nanoaggre-
gates.
Highly-organized nanostructures consisting of photochromic
molecules are promising as smart materials that can show re-
versible changes in their morphology and functions upon expo-
sure to light of a specific wavelength.[1] In particular, tubular
nanostructures are very promising in view of their capability of
encapsulation and releasing function of small molecules.[2] Of
particular importance are different properties of outer and inte-
rior surfaces of the nanotubes because of selective accommo-
dation of guest-nanomaterials.[3] However, such nanotubes with
photoresponsive properties have rarely been reported so far.[4]
As a unique nanotube-forming supramolecular building
block, we previously reported azobenzene dyad 1 possessing
hydrophobic side chains (Figure 1a).[5] In methylcyclohexane
(MCH), this scissor-shaped dyad[6] self-assembles to toroidal
nanostructures at 20 °C, and then stacks into nanotubes upon
cooling the solution to 0 °C. This unique self-assembly process
of 1 is ascribable to the intramolecular folding through the π–π
stacking of two azobenzene moieties and hydrogen-bonding
between amide groups to produce spontaneous curvature (Fig-
ure 1b). The nanotube of 1 can be reversibly dissociated and
reconstructed through the photoisomerization of azobenzene
moieties. To further modify the properties of this nanotube, in
the present study we newly designed and synthesized dyad 2
possessing hydrophilic triethylene glycol (TEG) side chains at
the azobenzene termini (Figure 1a). We found that this amphi-
philic dyad 2 also self-assemble into nanotubes in MCH media,
but without forming isolatable toroidal intermediates (Fig-
ure 1c). Furthermore, we show photoresponsive self-assembly
Figure 1. a) Molecular structures of azobenzene dyads 1 and 2. b, c) Sche-
matic representation of self-assembly of 1 (b) and 2 (c).
Amphiphilic azobenzene dyad 2 was synthesized according
1
to Scheme S1 and characterized by H and 13C NMR spectro-
scopy and ESI mass spectroscopy (see the Supporting Informa-
tion). Supramolecular assemblies of 2 were prepared by cooling
a hot MCH solution (c = 300 μM) from 90 °C to 20 °C. At 90 °C,
[a] Division of Advanced Science and Engineering, Graduate school of Science
and Engineering, Chiba University
1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
[b] Institute for Global Prominent Research (IGPR), Chiba University
1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
2 displayed an absorption band with an absorption maximum
at 358 nm, which is derived from the π–π* transition of trans-
azobenzene moieties in the molecularly dissolved state (Figure
S1). Upon cooling, this band slightly red-shifted to show the
absorption maximum at 361 nm, with the emergence of an
absorption shoulder at ca. 380 nm. The small absorption shift
indicates a weak electronic interaction between azobenzene
E-mail: yagai@faculty.chiba-u.jp
http://chem.tf.chiba-u.jp/gacb18/index.html
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.202000019.
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chromophores. The formation of assemblies was confirmed by sociated with hydrophilic TEG chains, and cannot be detectable
dynamic light scattering (DLS) measurement. At 90 °C, no by usual AFM and TEM analysis.
assemblies with a detectable hydrodynamic diameter (DH
≤
We next investigated the photoresponsive property of nano-
tubes of 2. Irradiation to a nanotube solution of 2 with UV light
at 365 nm resulted in a smooth decrease of the π–π* transition
absorption band of trans-azobenzene moieties and an increase
of the n-π* transition absorption band of cis-azobenzene
1 nm) were observed, indicating the molecularly dissolved state
(Figure S2). Upon cooling the solution to 20 °C, however, DH of
ca. 1 μm was immediately detected, suggesting the formation
of large assemblies.
Atomic force microscopy (AFM) and transmission electron around 450 nm (Figure 3a). Under a given condition, a photo-
microscopy (TEM) showed that the above assemblies were well- stationary state (PSSUV) was achieved within 10 min. By compar-
defined bundled cylindrical nanostructures (Figure 2a, b, and ing the absorption spectral change in CHCl3 with that of the
S3). Their width and height measured by AFM cross-sectional monomer dissolved in CDCl3 wherein the trans:cis ratio of 6:94
analysis were approximately 28 nm and 6.4 nm (Figure 2d), re- has been directly determined by 1H NMR (Figure S6), the
spectively, suggesting an ellipsoidal cross-section. It is interest- trans:cis ratio at PSSUV was estimated to be 4:96. DLS measure-
ing to note that the previous nanotube of 1 could be imaged ments showed a gradual decrease in the average DH upon UV
by AFM to show a circular cross-section with identical width light irradiation, which well corresponds to the trans→cis isom-
and height of 12.7 nm (Figure 2c, d). This difference, therefore, erization (Figure 3b, c). AFM imaging of a sample prepared by
implies a mechanical fragility of the nanotubes of 2 due to the spin-coating the PSSUV solution displayed amorphous films
soft TEG interior. Indeed, the nanotubes of 2 were found to be (Figure 3d). These results demonstrate that cis-2 cannot form
deformable during AFM scanning (Figure S4) while the nano- nanotubes due to its bent conformation.[1,7]
tubes of 1 with the aliphatic interior were mechanically stable.
Because TEM observation of the nanotubes of 2 did not show
hollow tubular structures as observed for those of 1 (Figure 2b),
the nanotubes of 2 are thus deformed upon drying.
Figure 3. a) UV/Vis absorption spectral change of 2 in MCH (c = 300 μM) at
20 °C upon UV light irradiation (365 nm) from 0 sec to 600 sec. The time-
interval was set to 20 sec. b) Change of DLS size distribution of 2 in MCH
(c = 300 μ
and the cis-content (%, right axis) vs. the UV light irradiation time. d) AFM
image of amorphous film of 2 obtained from a MCH solution (c = 300 μ ) at
M) by UV light irradiation. c) Plot of the average DH (<DH>, left axis)
M
the PSSUV. The sample was prepared by spin-coating the solution onto HOPG.
Scale bar, 500 nm.
Figure 2. a) AFM and b) TEM images of nanotubes of 2 obtained by cooling
a MCH solution of 2 (c = 300 μ ) from 90 °C to 20 °C. The samples were
M
prepared by spin-coating the solution onto HOPG (AFM) or carbon-coated
copper grid (TEM). Scale bar, 500 nm. c) AFM image of nanotubes of 1 pre-
pared by cooling a MCH solution of 1 (c = 300 μM) from 90 °C to 0 °C. Scale
bar, 100 nm. The sample was prepared by spin-coating the solution onto
HOPG. d) AFM cross-sectional analysis along the orange lines in the AFM
images (a) and (c). h1 and h2 are the average nanotube heights of 1 and 2,
respectively.
Subsequent irradiation of the PSSUV solution with visible
light at 470 nm induced a smooth cis→trans isomerization, as
shown by a recovery of the absorption band at 361 nm
(Figure 4a). Another photostationary state (PSSVis) was thus
achieved by irradiation for 8 min, at which the trans:cis ratio
was estimated to be 79:21. Despite this sufficient back isomeri-
It is worth noting that previous dyad 1 forms toroidal inter- zation of azobenzene moieties, DLS measurements showed
mediates at 20 °C (Figure S5), and only upon cooling to 0 °C only the presence of particles with 50 nm in the average size
they can hierarchically organize into the nanotubes.[5] In con- (Figure 4b). By AFM observation, only island-like films with a
trast, amphiphilic dyad 2 forms the nanotubes already at 20 °C, thickness of approximately 2 nm, which are perhaps created
and any toroidal intermediates cannot be detected even upon by dewetting of the hydrophilic substance on the hydrophobic
moderately heating the solution. These results demonstrate HOPG substrate were visualized (Figure 4c, d). Only after 2 days
that 2 can directly organize into nanotubes. It is assumed that from the visible light irradiation at which trans:cis ratio was
the toroidal intermediates of 2 are probably unstable as a dis- 92:8, we have confirmed the reconstruction of nanotubes
crete object in apolar media due to solvophobic interaction as- (Figure S7). These observations suggest that a photoisomeric
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mixture of 2 at PSSVis cannot organize into well-defined nano-
structures.
Figure 4. a) UV/Vis absorption spectral change of 2 in MCH (c = 300 μM) at
20 °C upon visible light irradiation (470 nm) from 0 sec to 480 sec. The time-
interval was set to 20 sec. The orange curve shows the as-cooled of 2 in MCH
Figure 5. Schematic representation of the photoresponsive self-assembly of
(c = 300 μM) at 20 °C. b) DLS size distribution of 2 in MCH (c = 300 μM) before
2.
UV irradiation (orange peak), at PSSUV (pink peak) and at PSSVis (blue peak).
c) AFM image of amorphous films of 2 obtained from a MCH solution (c =
300 μM) at PSSVis. The sample was prepared by spin-coating the solution onto
HOPG. Scale bar, 500 nm. d) AFM cross-sectional analysis along the blue line
in the AFM image (c).
In summary, we have shown that amphiphilic azobenzene
dyad 2 directly organize into nanotubes in nonpolar solvent
without forming isolatable toroidal intermediates. This self-
assembly process differs from the previously reported hydro-
phobic dyad and illustrates the usefulness of solvophobic inter-
action of hydrophilic substituents in apolar media to control
hierarchical self-assembly. Furthermore, the amphiphilic nano-
tubes exhibited irreversible dissociation and reconstruction
through the trans→cis isomerization of azobenzene moieties
because of the co-aggregation of aggregative tt-2 and non-
aggregative tc-2. The current results illustrate the usefulness of
using hydrophilic chains for designing photoresponsive nano-
structures thorough the self-assembly in nonpolar organic me-
dia.[9] The encapsulation of hydrophilic guests in apolar media
and the release properties of the guests by light[4b] is an ongo-
ing study in our group.
To better understand the unique results after visible light
irradiation, we simulated the concentrations of the three possi-
ble photoisomers of 2 that can be generated by the statistical
cis→trans isomerization of the two azobenzene units in 2, i.e.,
trans-trans (tt-2), trans-cis (tc-2) and cis-cis (cc-2) isomers (Figure
S8 and for detail, see the Supporting Information). The validity
of the simulation was confirmed by direct measurement of the
1
mole fractions of the three isomers by H NMR in CDCl3 upon
visible light irradiation (Figure S9). The simulation thus suggests
that the ratio of the three isomers at PSSVis is tt-2:tc-2:cc-2 =
62:33:5 (Figure S8). Accordingly, the concentration of tt-2 at
PSSVis was calculated to be 182 μ
enough higher than the critical aggregation concentration of
tt-2 (ca. 125 μ ) that has been estimated by AFM (Figure S10).
M. This concentration is
M
This finding implies that self-assembly of tt-2 could be deterio-
rated in the presence of non-aggregative tc-2.
Acknowledgment
To examine another possibility for the formation of the
above island-like film, that is a kinetic trap of the photogener-
ated tt-2 at ambient temperature, we investigated how tt-2 can
assemble at ambient condition. For this purpose, we dissolved
a thin film of 2 (100 % tt-2), which was prepared by drying a
chloroform stock solution, in MCH without heating. AFM analy-
sis of the resulting assemblies displayed sheet-like aggregates
with a thickness of approximately 5 nm (Figure S11), which are
different from not only the nanotubes obtained by cooling in
MCH but also the island-like film obtained at PSSVis. Probably
the sheet-like aggregates are composed of bilayered aggre-
gates of tt-2 that are formed upon drying chloroform.[8] Taking
this observation into account, we concluded that tt-2 cannot
properly organize into nanotubes at ambient temperature, and
under such a situation this isomer co-aggregate with non-
aggregative tc-2 to form amorphous agglomerates, i.e., amor-
phous films (Figure 5).
We are grateful to Prof. Dr. Tomonori Ohba for the TEM meas-
urement.
Keywords: Azobenzene · Photoisomerization · Nanotubes ·
Amphiphilicity · Self-assembly
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Received: January 6, 2020
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Nanotubes
T. Saito, S. Yagai* ............................... 1–5
Effect of Oligoethylene Chains on
the Formation of Photoresponsive
Nanotubes by Azobenzene Dyad
Self-assembly of an amphiphilic azo- persed state through trans/cis photo-
benzene dyad was investigated in isomerization. Visible light irradiation
methylcyclohexane. The dyad formed did not result in the reconstruction of
mechanically fragile nanotubes. UV nanotubes while amorphous films
light irradiation induced dissociation were obtained due to coaggregation
of nanotubes into the molecularly dis- of photoisomers.
DOI: 10.1002/ejoc.202000019
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