Light-induced ostwald ripening of organic nanodots to rods

Article abstract of DOI:10.1021/ja301002g

Ostwald ripening allows the synthesis of 1D nanorods of metal and semiconductor nanoparticles. However, this phenomenon is unsuccessful with organic π-systems due to their spontaneous self-assembly to elongated fibers or tapes. Here we demonstrate the uses of light as a versatile tool to control the ripening of amorphous organic nanodots (ca. 15 nm) of an azobenzene-derived molecular assembly to micrometer-sized supramolecular rods. A surface-confined dipole variation associated with a low-yield (13-14%) trans-cis isomerization of the azobenzene moiety and the consequent dipole-dipole interaction in a nonpolar solvent is believed to be the driving force for the ripening of the nanodots to rods.

Full text of DOI:10.1021/ja301002g

Communication
pubs.acs.org/JACS
Light-Induced Ostwald Ripening of Organic Nanodots to Rods
Sankarapillai Mahesh, Anesh Gopal, Rajasekaran Thirumalai, and Ayyappanpillai Ajayaghosh*
Photosciences and Photonics Group, Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and
Technology, CSIR, Trivandrum-695019, India
S
* Supporting Information
π-interactions.8 Therefore, we have designed a molecule 4,
ABSTRACT: Ostwald ripening allows the synthesis of 1D
nanorods of metal and semiconductor nanoparticles.
However, this phenomenon is unsuccessful with organic
π-systems due to their spontaneous self-assembly to
elongated fibers or tapes. Here we demonstrate the uses
of light as a versatile tool to control the ripening of
amorphous organic nanodots (ca. 15 nm) of an
azobenzene-derived molecular assembly to micrometer-
sized supramolecular rods. A surface-confined dipole
variation associated with a low-yield (13−14%) trans−cis
isomerization of the azobenzene moiety and the
consequent dipole−dipole interaction in a nonpolar
solvent is believed to be the driving force for the ripening
of the nanodots to rods.
combining the features of phenyleneethyneylene and azo
moieties. Thus, the azobenzene derivative 4, by virtue of its
weak π-stacking ability, was reluctant to form spontaneous 1D
assemblies; instead it formed nanodots. Upon UV light
irradiation, in contrast to the usually expected deaggregation,9
length-controlled supramolecular rods were formed. This
phenomenon is attributed to a low-yield trans−cis photo-
isomerization leading to surface polarity change, resulting in
coalescence of the aggregates to rods through a process akin to
the Ostwald ripening of metals and semiconductor nano-
particles.
The trans-azobenzene derivative 4 was prepared by the
Sonogashira coupling protocol in 45% yield (Scheme 1).10 The
Scheme 1. Synthesis and Photoisomerization of 4
ormation of one-dimensional (1D) nano- and micro-
F
structures is a topic of current interest in the field of
advanced materials research.1 Ostwald ripening is one of the
reasons for the formation of 1D nanostructures from
nanoparticles of metals and semiconductors.2 For example,
CdTe, CdSe, and ZnO nanoparticles have been shown to form
nanorods by an Ostwald-type ripening process.3 However,
controlled ripening of amorphous organic self-assemblies to
nano- or microsized supramolecular rods has not been
reported. Even though template-assisted synthesis of organic
rods is known,4 template-free preparation of rod-shaped
structures of organic molecules with controlled aspect ratio
remains challenging. Light is a powerful tool to manipulate the
size, shape, and properties of molecules and materials. For
example, light has been shown to control the self-assembly of
semiconductor nanoparticles into twisted ribbons, which is a
rare observation.5 More widely, light-induced changes of
photochromic molecules have been exploited to control
properties of molecular, macromolecular, and supramolecular
architectures.6 Among various photochromic molecules,
azobenzene has received much attention due to its reversible
trans−cis photoisomerization that proceeds with large dipole
moment and volume change, leading to significant modulation
of the macroscopic properties.7
product was characterized by 1H NMR and MALDI-TOF mass
spectrometry. The absorption spectrum of 4 in chloroform (1
× 10−5 M) exhibited two distinct maxima, around 280 nm,
corresponding to the π−π* transition of the phenyleneethyny-
lene backbone, and 350 nm, corresponding to the π−π*
transition of the azo moiety (Figure 1a). The absorption
spectrum in cyclohexane (1 × 10−5 M) after heating and
subsequent cooling to 25 °C showed a low molar extinction
coefficient (ε) at 334 nm (27 200 M−1 cm−1) when compared
to that in chloroform (66 700 M−1 cm−1). The intensity of the
absorption maximum in cyclohexane is enhanced (ε = 55 900
Herein we report the light-induced reversible morphology
change from the initially formed nanodots of a trans-
azobenzene derivative to supramolecular rods. For the self-
assembly of π-systems to nano- and microrods, it is important
to prevent the usually occurring spontaneous extended
aggregation of molecules. π-Systems derived from phenyl-
eneethynylene units are appropriate for this purpose since they
are known to form spherical or circular assemblies due to weak
M
−1 cm−1) upon heating to 60 °C. The decrease in the ε value
in cyclohexane and the subsequent increase with increase in
Received: January 31, 2012
Published: April 14, 2012
© 2012 American Chemical Society
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Communication
Figure 1. (a) UV/vis absorption spectral changes of 4 (1 × 10−5 M) in
chloroform (red), in cyclohexane at 20 °C (black), and in cyclohexane
at 60 °C (blue). Inset shows the plot of fraction of aggregates (α)
versus temperature (in cyclohexane, λ = 330 nm). (b) Spectral changes
of 4 upon photoirradiation in cyclohexane (1 × 10−5 M) using 350 nm
light at 25 °C. The inset zooms in on the region between 400 and 525
nm.
Figure 2. TEM images of the self-assemblies in cyclohexane (1 × 10−4
M). (a) Nanodots of 4 before irradiation. The size distribution is
shown in the inset. (b) Transformation of nanoparticles to rods upon
irradiation for 10 min and (c) after 1 h with 350 nm light. (d) Re-
formation of nanodots after heating and cooling of the rods (1 °C/
min) followed by irradiation with visible light. TEM images were
obtained without staining.
temperature indicate aggregation and deaggregation, respec-
tively, of the molecules. The melting transition temperature
(Tm) of the aggregates was 34 °C in cyclohexane.
powder X-ray diffraction patterns of the nanodots and the rods
reveal no long-range periodicity of molecular packing,
indicating the absence of any crystalline phase (Figure S4).
For an insight into the photoinduced morphology transition,
dynamic light scattering (DLS) analysis was performed in
cyclohexane (1 × 10−5 M) before and after irradiation. Initially,
aggregates with a hydrodynamic radius (Rh) of around 15 nm
were observed (Figure 3a). The corresponding autocorrelation
function as a function of the delay time is shown in the inset.
After irradiation (λband‑pass = 350 nm, intensity 0.1 W/cm2 for
45 min), an increase in the Rh value is observed, with an average
size of 698 nm (Figure 3a). The autocorrelation function of
rods exhibited higher relaxation time when compared to that of
the nanodots, reflecting a lower diffusion coefficient for the
former. The relatively slower decay of the autocorrelation
function with significant deviation indicates considerable
changes in the size and morphology of the aggregates.
Monitoring of the size variation and size distribution with
irradiation shows the time-dependent growth of the particles
(Figure 3b). After 10 min irradiation, the particle size has
increased significantly with a temporary bimodal distribution, as
indicated by the weak signals of the residual particles of the
original size, the intensity of which further decreased with
continued irradiation.
Irradiation of 4 at 350 nm for 45 min using a band-pass filter
(λband‑pass = 350 nm, intensity 0.1 W/cm2) exhibited a slow
decrease in the intensity of the absorption maximum at 335 nm,
with the formation of a weak band at 450 nm (Figure 1b). The
weak change in the absorption spectrum indicates the low
percentage conversion of the trans to cis isomer. The
1
photoisomerization was further confirmed by the H NMR
spectral changes in cyclohexane-d12 and CDCl3 solvent mixture
(1:1 v/v) before and after UV irradiation (Figures S1 and S2).
The trans isomer showed characteristic peaks at δ = 8.09 (s,
ArH, 2H), 7.90 (d, J = 8.4 Hz, ArH, 2H), 7.63 (d, J = 7.9 Hz,
ArH, 2H), 7.53 (m, ArH, 2H), 7.01 (s, ArH, 2H), 6.92 (s, ArH,
2H), 4.70 (s, ArCH2, 4H), and 4.03 (m, OCH3, 8H). Upon UV
irradiation, new peaks were formed at δ = 7.36, 7.01, 6.91, and
6.59 due to the formation of the cis isomers. The percentage of
trans−cis conversion was calculated from the change in the
integration values of the peak at δ = 8.09 with respect to a
reference peak δ = 4.03 ppm (m, -OCH2, 8H), yielding nearly
13% of the cis isomers at the photostationary state (PSS).
Transmission electron microscopy (TEM) analysis of the
molecule 4 dissolved in cyclohexane (1 × 10−4 M) by heating
and subsequent cooling showed the formation of nanodots
when the solution was drop-cast on carbon-coated grids
(Figure 2a). The size distribution histogram showed an average
size of 15 nm (Figure 2a, inset). Surprisingly, after irradiation,
the TEM images revealed the complete transformation of the
nanodots to supramolecular rods having diameter of 200−400
nm and length of nearly 500 nm to 2 μm (Figure 2b,c). While
the width is comparable among different rods, the length
showed significant variation with time of irradiation, indicating
a pseudo-1D growth process (Figure 2b,c). The high-resolution
TEM and the electron diffraction pattern of the rods indicate
that they are amorphous in nature (Figure S3). Similarly,
For further information on the light-induced growth of the
dots to rods, zeta potentials were measured before and after
photoirradiation in cyclohexane (1 × 10−5 M). It is known that
ζ-potential of colloidal particles is a measure of the stability and
the size variation, since high ζ-potential value favors better
stability and low value indicates rapid coagulation.11 Surpris-
ingly, the nanodots exhibited an initial average ζ-potential of 30
3 mV (Figure 4a), suggesting that the aggregates have
moderate stability in solution. However, upon irradiation with
350 nm light, the average ζ-potential value decreased to −5
3
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Communication
confirmed by TEM analysis (Figure 2d). Similarly, the ζ-
potential values exhibited significant variation upon heating and
cooling followed by visible light irradiation. These changes are
reversible for several cycles of UV and visible light irradiations
(Figure S6).
The above observations were surprising since, in the case of
azo-linked molecules, trans−cis isomerization is known to
disrupt or weaken the self-assembly, as reported in several
cases.9 In the present case, the low photoisomerization yield
(13%) has become advantageous for the observed morphology
transition. The situation would have been different in the case
of an efficient isomerization of the trans to the cis form. Our
attempt to isolate the cis isomer to study its aggregation
behavior was not successful due to the fast thermal back-
isomerization to the trans form. The reason for the low yield of
the cis isomer could be related to structural as well as
morphological features of 4. The phenyleneethynylene moieties
may partly interfere with the light absorption by the azo
moieties in 4, which may decrease the efficiency of isomer-
ization. In addition, light filtration by the phenyleneethynylene
moieties may decrease the light absorption by molecules
present in the inner part of the nanodots, preventing their
isomerization.
We hypothesize that the mechanism of the rod formation
may involve a surface interaction pathway associated with a
time-dependent, light-driven process as shown in Figure 5.
Figure 3. (a) DLS profiles showing the intensity-averaged hydro-
dynamic radius (Rh) of the self-assemblies before (blue) and after
(red) photoirradiation with 350 nm light. The corresponding
autocorrelation functions are shown in the inset. (b) DLS histogram
showing the particles’ size growth upon intervals of photoirradiation.
Figure 5. Schematic representation of the light-driven ripening of
nanodots to rods. The surface-confined trans−cis isomerization is
indicated by the red color on the dots and rods. At the PSS, rods have
13−14% cis content.
Upon UV irradiation of the nanodots, the 4trans is converted to
the 4cis in low yields (13−14%), which is restricted to the
surface of the dots. Therefore, the local concentration of 4cis on
the surface of the dots will be high, which in turn enhances the
surface dipole moment since each trans−cis isomerization of the
azobenzene may contribute a dipole moment change of μ ≈
0.52 D for the trans to μ ≈ 5 D for the cis. Such a large increase
in the dipole moment after irradiation changes the ζ-potential
of the nanodots (30 3 mV) toward an instability regime (−5
3 mV), which facilitates interparticle association, leading to a
rod-shaped structural transformation. The cis content in the
rods obtained after 40 min irradiation was ca. 13−14%, as
estimated from the change in the UV−vis spectra. The
observed morphology evolution of the dots to rods can
therefore be corroborated to the surface-confined dipole
moment change associated with the trans−cis isomerization
and the consequent ζ-potential variation of the initially formed
molecular aggregates.
Figure 4. (a) Zeta potential variation of 4 before (green) and after
(red) irradiation. (b) Time-dependent ζ-potential variation of the
molecules with respect to irradiation time.
mV. The decrease in the ζ-potential value upon the light-
induced isomerization, as evident in Figure 4b, may facilitate
agglomeration of the individual aggregate, leading to the growth
of rods, as supported by the TEM images.
The rods could be converted back to dots by heating the
former to 60 °C followed by cooling at a rate of 1 °C/min
under irradiation with visible light. DLS analysis of the
reversible process showed a decrease of the hydrodynamic
radii (Figure S5) due to nanodots formation which is further
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Journal of the American Chemical Society
Communication
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The above suggested mechanism is well supported by recent
reports on the effect of the surface polarity difference of
azobenzene monolayers on gold nanoparticles, as reported by
Grzybowski and co-workers.12 In another report, Ikegami et al.
showed that passivated ultrathin Nb films containing azo
chromophores exhibit significant changes in their super-
conductivity upon photoirradiation.13 In both cases, the
observed property changes are attributed to the change in the
surface dipole moment as a result of the photoinduced trans−cis
isomerization. Thus, the weak electrostatic repulsion of the
aggregates is overcome by the relatively strong dipole−dipole
interaction. The preferential 1D growth may be controlled by
the mass transport through the surface equilibrium of the cis
and the trans isomers. The large surface area on the top and
bottom faces of the nanodots allows longitudinal growth,
leading to pseudo-1D rods, similar to the Ostwald ripening of
metal and semiconductor nanoparticles. It is believed that the
initially formed nanodots morphology and the polarity of the
solvents play crucial roles in the formation of rods. To prove
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tetrahydrofuran. In both solvents, even though aggregates are
formed before and after irradiation, they were not exactly the
nanodots or rods as observed in cyclohexane (Figures S7).
However, in chloroform-cyclohexane mixture (1:1, v/v)
nanodots and rods were formed before and after irradiation,
indicating that the addition of a nonpolar solvent facilitates the
photoinduced ripening process (Figure S8).
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This method allows the preparation of organic supra-
molecular rods without using templates, which can be
assembled and disassembled by light of appropriate wave-
lengths. The new observation described here reveals yet
another property of the versatile azobenzene chromophore,
which may inspire further studies en route to stimuli-responsive
hierarchical structures with controlled morphological features.
ASSOCIATED CONTENT
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AUTHOR INFORMATION
Corresponding Author
ajayaghosh62@gmail.com
■
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
A.A. is grateful to the Department of Atomic Energy,
Government of India, for a DAE-SRC Outstanding Researcher
Award. S.M is thankful to Universities Grants Commission for a
Fellowship. A.G. and R.T. are thankful to CSIR for fellowships.
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