Rational design of light-directed dynamic spheres

Article abstract of DOI:10.1039/c2cc36443c

We created light-directed dynamic spheres based on simple azobenzene monomers showing (i) a high yield of reversible trans ? cis photoisomerization and (ii) noticeable phase transition from crystalline to isotropic states under UV light irradiation at amb

Full text of DOI:10.1039/c2cc36443c

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Rational design of light-directed dynamic spheresw
Yumi Okuia and Mina Han*ab
Received 4th September 2012, Accepted 23rd October 2012
DOI: 10.1039/c2cc36443c
We created light-directed dynamic spheres based on simple
azobenzene monomers showing (i) a high yield of reversible
trans 2 cis photoisomerization and (ii) noticeable phase transition
from crystalline to isotropic states under UV light irradiation at
ambient temperature.
showing not only high yield of reversible trans 2 cis photo-
conversion but also phase transition from crystalline to
isotropic states under UV light irradiation. Moreover, when
a hydrophobic fluorescent dye is encapsulated inside the
azobenzene sphere in aqueous solution, it can be released
through disassembly of the sphere by UV light. The resulting
solution becomes transparent and highly blue fluorescent,
which allows us to distinguish between the sphere suspension
and the UV-exposed solution.
The design and creation of dynamic aggregates (vesicles,
micelles, bilayers, and spheres) capable of light-driven reversible
self-assembly and disassembly have attracted extensive
attention in photochemistry, biology, and medical fields for
purposes including drug delivery and separation systems.1
Most of the studies in this area have been concentrated on
the laborious synthesis of block copolymers (BCP)2 and
surfactants3 consisting of photochromic azobenzenes. In general,
cis-azobenzene (m = B3 D) has a larger dipole moment than
trans-azobenzene (m = B0 D),4 but cis-azobenzene itself is not
polar enough to dissolve in aqueous solution. Thus, efficient
light-induced disassembly of self-assembled systems remains
challenging.
We designed an amphiphilic azobenzene monomer
(EtO-Az-EG, see Fig. 1) consisting of a hydrophobic azobenzene
rod and a hydrophilic flexible tetra(ethylene glycol) chain.
Interestingly, crystalline trans-EtO-Az-EG possessing its melting
temperature at 50 1C becomes liquid at ambient temperature
upon UV light irradiation (Fig. 1a and b, Fig. S1 and Movie S1,
ESIw). The distorted ortho-alkylated azobenzene unit in EtO-
Az-EG would experience weak intermolecular p–p stacking
interactions between trans-azobenzenes,7,8 which could be
responsible for the low phase transition temperature from
crystalline to isotropic states. The cis form is expected to become
more hydrophilic with the aid of the tetra(ethylene glycol) chain,
which may help to diffuse in water and other polar solvents.
Azobenzene spheres were produced simply by adding water
into EtO-Az-EG THF solution (4.0 Â 10À4 M, 1.5 Â 10À3 M,
4.0 Â 10À3 M, filtered through a PTFE syringe filter prior to
use) under mild shaking conditions. With gradually increasing
the water content, the THF–H2O mixed solution turned
orange-yellow turbid. Each suspension was well-dispersed
with no evident precipitate over the 1 week storage period.
Transmission electron microscopy (TEM) images displayed
spherical aggregates of approximately 0.4–2.0 mm in diameter
The rod-shaped trans form stabilizes crystalline and liquid
crystalline (LC) phases, whereas the bent-shaped cis form
destabilizes the crystalline and LC phases. Accordingly, irra-
diation with UV light can cause an isothermal LC-to-isotropic
phase transition in an aggregated state.5 Nevertheless, there
are only two reports on solid azobenzene compounds which
liquefy under UV light even at room temperature.6 Akiyama
and Yoshida have demonstrated that fluidic orange droplets
generated by UV light turned back to solid by visible light.6b
The rational molecular design strategy considering an important
balance between a homogeneous molecular structure without
polydispersity and a steric factor inducing appropriate inter-
molecular interactions is desirable for light-induced transformation
between assembled (crystalline and LC) and disassembled
(isotropic) states.
Our purpose of this study is to design photoresponsive
dynamic spheres based on simple azobenzene monomers
a Department of Chemistry and Department of Electronic Chemistry,
Tokyo Institute of Technology, Yokohama 226-8502, Japan
b Department of Molecular Design & Engineering, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan.
E-mail: hanmin@apchem.nagoya-u.ac.jp; Fax: +81 52 789 4669;
Tel: +81 52 789 3199
w Electronic supplementary information (ESI) available: Experimental
procedures and characterization data for EtO-Az-EG along with DSC
(differential scanning calorimetry) data, UV-vis absorption spectra,
additional TEM and OM images, DLS data of azobenzene spheres.
See DOI: 10.1039/c2cc36443c
Fig. 1 (upper panel) Chemical structure of EtO-Az-EG and its
photoisomerization. (lower panel) OM images of UV-induced
liquefaction of EtO-Az-EG. (a) Before and (b) after UV light
irradiation at ambient temperature (see Movie S1, ESIw).
c
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Chem. Commun., 2012, 48, 11763–11765 11763

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absorption spectral shift originating from H- or J-aggregation
was found.9 This result suggests that the ortho-alkylated
azobenzene unit is significantly distorted from coplanarity,
resulting in modest intermolecular p–p stacking interactions
between aromatic rings, as mentioned earlier.
Exposure of the azobenzene suspension to UV light for
inducing trans-to-cis photoisomerization led to a drastic
decrease in the p–p* absorption band. Simultaneously, the
scattered light intensity at 650 nm decreased substantially by
B98% (Abs = 0.003), and the suspension turned transparent
(inset photograph in Fig. 3). The cis content was then estimated
1
to be approximately 87%, based on the H NMR and UV-vis
absorption spectral results of EtO-Az-EG (Fig. 3 and Fig. S4,
ESIw). In addition to the high photoisomerization yield, not
only a large difference in the dipole moment between the trans
and cis forms (mcis À mtrans = B5.0)10 but also a hydrophilic
tetra(ethylene glycol) chain covalently attached to the azobenzene
unit becomes an important factor in making the cis form more
hydrophilic. Therefore, the cis-azobenzene should efficiently
diffuse in the THF–H2O solution. However, we could
occasionally observe largely disassembled objects as well as
shrunken aggregates from TEM images of UV-exposed transparent
aliquots (Fig. 2b). Their size, as measured by DLS, was
calculated to be 0.2–0.5 mm (Fig. 2d). The existence of such
shrunken objects seems to be due to the remaining small
amount of trans-azobenzene (B13%) which stabilizes aggregated
states.
Fig. 2 Unstained TEM images of (a) EtO-Az-EG spheres obtained
from 1.3 Â 10À3 M THF/H2O (4.0/8.2, v/v). (b) Exposure to UV light
at 365 nm. (c) Subsequent exposure to visible light at 436 nm. (d) DLS
measurements of size changes of azobenzene spheres upon UV and
subsequent visible light irradiation.
for 1.3 Â 10À3 M THF/H2O (4.0/8.2, v/v) suspension,
consistent with optical microscopy (OM) images (Fig. 2a
and Fig. S2, ESIw). Dynamic light scattering (DLS) experiments
also provided convincing evidence for generation of spheres
with the size of 0.5–1.5 mm (Fig. 2d). The measured average size
was independent of the azobenzene concentrations varying
from 9.0 Â 10À5 M to 1.3 Â 10À3 M (Fig. S3, ESIw).
Amazingly, subsequent irradiation with visible light for
5 min resulted in a prominent increase in the absorbance at
354 nm as well as a considerable recovery of turbidity (Fig. S5,
ESIw), strongly indicating that rapid regeneration of spheres
occurred through reverse cis-to-trans photoisomerization. The
ratio of the cis to trans forms was then estimated to be 30/70 at
the photostationary state. Unambiguously, TEM and DLS
measurements provided strong evidence for the recovery of
spherical aggregates with diameters of 0.3–1.5 mm (Fig. 2c and d).
As shown in Fig. S5 (ESIw), the excellent assembly and
disassembly process was repeatedly achieved together with
reversible trans 2 cis photoisomerization, without serious
absorption spectral degradation.
To examine light-driven reversible changes in the molecular
conformation of the azobenzene unit and the resulting
morphological transformation of azobenzene spheres under
UV and visible light irradiation, a dilute azobenzene suspension
(8.9 Â 10À5 M THF/H2O (2.0/7.0, v/v)) suitable for UV-vis
absorption spectroscopy measurements was prepared. As
clearly seen in Fig. 3, the azobenzene suspension shows a broad
p–p* absorption band maximized at 354 nm and a conspicuously
increased absorbance at 650 nm (indicated as turbidity),
demonstrating formation of spherical aggregates. When
compared to the typical monomer-like absorption spectrum
of EtO-Az-EG (Fig. 3 and Fig. S4, ESIw), no noticeable
Encouraged by the high trans 2 cis photoisomerization of
EtO-Az-EG as well as the dynamic transformation between
spherical and disassembled objects, we next examined encap-
sulation and release of hydrophobic fluorescent dyes at
a chosen light wavelength (Fig. 4b). A small amount of 9,10-
diphenylanthracene (DPA, 4.0 Â 10À5 M) was dissolved in
EtO-Az-EG (4.0 Â 10À3 M) THF solution. Slow addition
of water into the mixed solution gave a turbid suspension
(THF/H2O = 4.0/8.2, v/v, see OM and DLS data in Fig. S6
and S7, ESIw). When excited at 365 nm, the suspension
fluoresced very weakly and exhibited two discrete emission
peaks with maxima at 429 nm and 532 nm, obviously different
from the emission spectrum of the dilute single-component
DPA solution11 (Fig. 4c and d). Such striking emission
spectral features seem to be closely associated with encap-
sulation of fluorescent dyes into azobenzene spheres. As the
suspension was continuously exposed to UV light, the blue
fluorescence centered around 407 nm significantly increased
within 15 min. The fluorescence intensity improved by about
Fig. 3 UV-vis absorption spectral changes of 8.9 Â 10À5 M THF/
H2O (2.0/7.0, v/v) as a function of exposure time of UV light. Inset
photograph: red light from a red laser pointer is shone from the right
side of the samples.
c
11764 Chem. Commun., 2012, 48, 11763–11765
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and disassembled states, which could be clearly seen as variations
in turbidity with the naked eye. The dynamic photoswitching
allowed azobenzene spheres to encapsulate fluorescent dyes
which need to be released in a non-contact fashion. The light-
responsive sphere system described in this investigation can be
applied to drug delivery and emerging separation systems
which require on-demand encapsulation/release at a specific
light wavelength.
This work was supported by the Global COE program from
the Ministry of Education, Culture, Sports, Science, and
Technology of Japanese Government. We are grateful to Prof.
T. Iyoda and Prof. K. Kamata for TEM measurements, as
well as Prof. T. Ikeda and Prof. A. Shishido for OM
measurements.
Notes and references
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Fig. 4 (a) Changes of 1.3 Â 10À3 M EtO-Az-EG THF/H2O (4.0/8.2, v/v)
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70 times reduction of blue fluorescence intensity along with a
recovery of scattered light intensity ([2] - [3] in Fig. 4a, c and d).
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changes in spherical size in response to a chosen light wavelength
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In summary, we have demonstrated that a rationally
designed azobenzene self-assembled into spherical aggregates
in a THF–H2O solution. The synthetic spheres underwent
light-directed morphological transformation between spherical
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 11763–11765 11765