Spontaneous generation of highly emissive RGB organic nanospheres

Article abstract of DOI:10.1002/anie.201101945

Over the rainbow: Three highly luminescent compounds use hydrogen-bonding interactions to spontaneously generate hollow nanospheres when dropcast from anhydrous solvents (see picture). Together, they cover more than 75% of the gamut of a conventional liqu

Full text of DOI:10.1002/anie.201101945

Communications
DOI: 10.1002/anie.201101945
Self-Assembly
Spontaneous Generation of Highly Emissive RGB Organic
Nanospheres**
Kuo-Pi Tseng, Fu-Chuan Fang, Jing-Jong Shyue, Ken-Tsung Wong,* Guillaume Raffy,
Andrꢀ Del Guerzo, and Dario M. Bassani*
The rational design of mesoscopic structures endowed with
tunable electronic properties is a milestone towards bridging
the bottom-up and top-down approach to molecule-based
electronics that could herald an important step forward in
attaining the levels of miniaturization needed for future
generations of electronic devices. Aggregation can be induced
by the interplay of hydrophobic forces and offers access to a
variety of well-defined architectures that span multiple orders
of scale in size, from a few nanometers (e.g. micelles), to
hundreds of microns (e.g. giant vesicles, lamellar assemblies,
or fibers) into which photo- or electroactive functionalities
can be incorporated.[1] Numerous examples of luminescent
fibrillar aggregates in which the color of the emitted light is
controlled by the energy gap of the material have been
reported. Further tuning of the emission envelope, particu-
larly important for white-light emission, can be achieved
through Fꢀrster resonant energy transfer by incorporating
luminescent guests whose absorption spectrum overlaps with
the emission spectrum of the material.[2] On the other hand,
the size and shape of spherical aggregates make them
attractive candidates for the fabrication of soft light-emitting
devices in which the size of the luminophore is determined by
the self-assembly properties of the molecular constituent.
These can be one or more orders of magnitude smaller than
current printable domains and would be valuable as point
light sources and in high-resolution displays. However, the
spontaneous self-assembly of nanometer-sized spherical
aggregates larger than micelles is uncommon.[3]
solution of the material in water or by cooling in a hydro-
carbon solvent. The formation of spherical aggregates is
driven by the presence of long hydrocarbon or ethylene-oxide
chains and can be sensitive to minor variations in struc-
ture.[2b,4] Furthermore, such vesicles are generally obtained as
aqueous suspensions[5] that are not ideal for incorporation
into electronic devices, for which anhydrous conditions are
preferred. We now report a series of intensely fluorescent
compounds that spontaneously generate vesicle-like nano-
spheres (200–600 nm in diameter) when dropcast from an
anhydrous organic solvent. The compounds are neutral,
relying on the presence of self-complementary hydrogen
bonding (H-B) rather than hydrocarbon chains to drive the
self-assembly process. Their color can be tuned from blue to
red by adjusting the energy of the singlet excited state, and the
compounds can be mixed in varying proportions without
affecting aggregate morphology. Because the system allows
blending of not just two, but all three primary colors, a large
spectrum (including pure white) is readily available by simply
mixing the compounds. The palette of colors each individual
aggregate can generate is unprecedented and covers more
than 75% of the gamut of current liquid crystalline displays
(LCDs).
Compounds 1–3 (Scheme 1) are composed of a p-con-
jugated core appended with self-complementary H-B biuret
groups at the extremities and were synthesized by Suzuki
coupling of the corresponding aryl bromide and a phenyl-
boronic biuret derivative.[6] These compounds are not amphi-
philic, and appear freely soluble in common organic solvents
such as chloroform and THF. While investigating the self-
A limited number of luminescent vesicles based on
conjugated organic chromophores and polymers have been
reported to date, obtained either by dispersion of an organic
[*] G. Raffy, Prof. A. Del Guerzo, Dr. D. M. Bassani
Institut des Sciences Molꢀculaires
CNRS UMR 5255, Universitꢀ Bordeaux 1
351, Cours de la Libꢀration, 33405 Talence (France)
E-mail: d.bassani@ism.u-bordeaux1.fr
K.-P. Tseng, Dr. F.-C. Fang, Prof. K.-T. Wong
Department of Chemistry, National Taiwan University
1, Sec. 4, Roosevelt Rd., Taipei 10617 (Taiwan)
E-mail: kenwong@ntu.edu.tw
Dr. J.-J. Shyue
Research Center for Applied Sciences, Acamedia Sinica 128
Academia Road, Nankang, Taipei 115 (Taiwan)
[**] Financial support for this work was provided by the Agence
Nationale de la Recherche (ANR-08-BLAN-016101), the Rꢀgion
Aquitaine, and the National Science Council of Taiwan (NSC-99-
2923M-002-002-MY3).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201101945.
Scheme 1. Structures of compounds 1–3.
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assembly of 1, we discovered that it spontaneously lead to the
deposition of hollow spheres when dropcast from dilute
(10À4 m) THF solutions. The aggregates range in size from
approximately 200 nm to 0.5 mm and are similar in morphol-
ogy to previously reported artificial vesicles, as evidenced by
scanning electron microscopy (SEM, Figure 1). Evidence for
energy gap. Compared to the highly fluorescent bisfluorene
1, which emits in the blue region of the visible spectrum,
compounds 2 and 3 respectively cover the green-yellow and
red regions of the spectrum (Figure 2). The photolumines-
cence efficiencies of all three compounds in THF solutions
are high, with FF = 0.96, 0.85, and 0.64 for compounds 1, 2,
Figure 2. Normalized fluorescence spectra (1 mm solutions,
lex =350 nm) of 1 (c), 2 (a), and 3 (g) in THF.
and 3, respectively.[8] More importantly, both 2 and 3 share the
ability of compound 1 to spontaneously generate hollow
spheres when dropcast from dilute THF solutions.[7]
We observe that the vesicle-like aggregates are obtained
from THF solutions regardless of the absence of water (THF
dried by refluxing over Na/benzophenone), or if water is
intentionally added to the solution prior to deposition (see
Figure 1b and S2). Therefore, the process does not appear to
be driven by the presence of nonmiscible solvent mixtures. No
heating or sonication is required to obtain clear solutions of 1,
2, or 3 in THF at concentrations up to 1 mm. However,
dynamic light scattering (DLS) analysis of THF solutions of 1,
2, or 3 revealed that, even at 10À4 m, aggregates whose average
hydrodynamic radius is 250 nm are present.[7] Although DLS
does not provide direct information on their morphology, this
value is very close to the typical diameter of the vesicles that
are deposited and suggests that aggregates commensurate in
size are present prior to deposition. In agreement with the
hypothesis that H-B interactions are important for aggrega-
tion, replacing THF by DMSO, a solvent known to break up
H-B interactions, leads to the disappearance of the DLS
signal.
Figure 1. SEM images of the spherical aggregates formed by com-
pound 1 upon dropcasting from dilute anhydrous THF solution
([1]=0.1 mm) onto SiO2 substrates at various magnification ratios (a–
d). In (b), water (30%) was intentionally added to the solution prior to
deposition. The presence of holes in some of the aggregates (c, d) is
indicative of their hollow nature, which is further confirmed by TEM (e,
f). Tapping mode AFM images of the aggregates on SiO2 substrates
((g), (i): height; (h), (j): phase) shows the presence of flattened or
partially collapsed aggregates.
The morphology of the aggregates was characterized by
environmental SEM, TEM, and AFM, and proved independ-
ent of the substrate onto which the aggregates are deposited
(SiO2, Si, or ITO).[7] We found that the presence of solvent
vapors in the SEM chamber reduced the proportion of
ruptured aggregates, in contrast to TEM images, which
showed a large percentage of fractured aggregates and that
the shell is very thin (ca. 5-10 nm, Figure 1 f). AFM micros-
copy revealed that the aggregates collapse to flat discs or
doughnuts upon evaporation of the solvent (Figure 1g–j). The
discs present an indentation of varying depth and size,
indicating they are being deformed by the pressure exerted
by the AFM tip, as already observed in artificial vesicles
assembled from polymers[9] or from discotic liquid crystalline
molecules.[4b]
the hollow nature of the aggregates is provided by the
identification of pores or holes in which some of the material
was lost during sample preparation, and from TEM and AFM
studies (see below). The formation of these aggregates is
independent of the nature of the substrate, whether semi-
conducting (silicon), insulating (SiO2), or conducting trans-
parent metal oxide (ITO).[7]
Anticipating that the combination of a rigid core and the
presence of the strong self-complementary H-B motifs is
responsible for the spontaneous generation of spherical
aggregates, we proceeded to prepare compounds 2 and 3, in
which a benzothiadiazole moiety or the juxtaposition of
electron donor–acceptor units is used to tailor the S1–S0
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The luminescence properties of individual aggregates
deposited on SiO2 substrates were investigated using confocal
fluorescence microscopy. Although their size is near the
diffraction limit, the aggregates are clearly identifiable as they
are relatively dispersed and highly luminescent. The results,
summarized in Figure 3, indicate that the emission collected
Figure 4. Fluorescence quenching of the emission of 1 (0.1 mm in
THF, lex =350 nm) by 2 and corresponding Stern–Volmer analysis
(lem =420 nm, KSV =2.2ꢁ105 LmolÀ1, see inset). It is calculated that at
the highest concentration of the acceptor used (7.5 mm), less than 5%
of the light is directly absorbed by 2.
quenching behavior characteristic of aggregation in solution.
The quenching process is assigned to singlet energy transfer
on the basis of the favorable overlap between the emission
spectrum of 1 and the absorption shell of 2 or 3, and the
presence of electronic transitions of 1 in the excitation
spectrum of 2 or 3 acquired at a wavelength at which 1 does
not emit.[7]
Figure 3. Confocal fluorescence microscopy images (40ꢁ40 mm,
lex =385 nm) and averaged corrected fluorescence spectra from these
areas of aggregates of 1 ((a), solid line), 2 ((b), dashed line), and 3
((c), dotted line). The emission of compound 1 is cut off below
410 nm by a long-pass filter. See Figure S14 for corresponding
fluorescence lifetime images.
Along similar lines, we found it possible to tune the
emission spectrum of the individual vesicle-like aggregates by
adjusting the composition of the solution from which they are
drop-cast. To demonstrate this, the color coordinates of a
series of samples in which the proportion of 1, 2, and 3 are
varied were measured, and the vesicle-like aggregates thus
formed gave the expected progression of colors from blue to
yellow (Figure 5).[10] Thanks to the possibility of combining
three different colors, the gamut obtainable is very large: as
shown in Figure 5, the overlap between the gamut of colors
covered by our system overlaps substantially (> 75%) with
that of a standardized red-green-blue (RGB) color display. In
fact, a very good match would be obtained by blue-shifting
the emission of 2 by 10 nm (less than 0.04 eV). A special case
is obtained when a THF solution composed of 10À4 m of 1,
0.2 mm (0.20 mol%) of 2, and 0.25 mm (0.25 mol%) of 3 is
used, as it gives rise to vesicles that emit a very clean white
light that corresponds precisely to the D65 standard when
excited at 385 nm (Figure 5). Other shades of white (D50 and
warm white) were also obtained by slightly adjusting the
proportions of 2 and 3. From the relative molar extinction
coefficients, it can be calculated that > 99.85% of the incident
radiation is absorbed by 1 at these concentrations. However,
the observed emission corresponds to a composite of 1
(84.5%), 2 (5.0%), and 3 (10.5%), in agreement with energy
transfer within each aggregate from excited 1 to 2 or 3. This
result is further supported by time-resolved confocal micros-
copy fluorescence measurements: in the blue spectral region,
the average decay time of 1 in the aggregates decreases upon
addition of 2 (0.2 mol%) or 2 and 3 (0.2 and 0.4 mol%,
respectively) from 0.7 ns to 0.6 and 0.5 ns, respectively.
Simultaneously, the average decay times of the emission
from each single vesicle-like aggregate is only slightly bath-
ochromically shifted (by < 10 nm) with respect to the
emission from THF solution, and that it varies minimally
between aggregates. The emission spectra of the vesicle-like
aggregates of 1, 2, and 3 on SiO2 correspond to the blue,
yellow-green, and red colors of the CIE plot, respectively, and
form a triangle that defines the possible colors (gamut) that
can be generated from a combination of the emission of the
pure compounds.
Energy transfer processes are known to be efficient in
artificial vesicles because of fast exciton migration and
proximity of the chromophores.[4a] The S1–S0 energy gaps of
compounds 2 and 3 overlap well with the emission spectrum
of 1 and are thus well-suited for this purpose. Together,
compounds 1–3 can potentially generate single aggregates the
emission of which can be tuned over the entire visible
spectrum. Given that the compounds form aggregates even in
relatively dilute solutions, we may expect static fluorescence
quenching behavior. This is indeed the case, and emission of 1
(10À4 m solution in THF) is efficiently quenched by the
addition of micromolar amounts of 2 or 3 (Figure 4 and S9).
At these quencher concentrations, the Stern–Volmer plots
show only minimal curvature and give values of KSV = 2.2 ꢁ
105 or 1.0 ꢁ 105 mÀ1 for the quenching of 1 by 2 or 3,
respectively. Because the excited state lifetime of 1 is short
(0.75 ns),[7] such large values of KSV are incompatible with a
diffusion-limited quenching process and indicate static
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Figure 6. Dispersion of colors obtained by analyzing 104 spectra from
the 10ꢁ10 mm image shown on the right. The composition of the
solution used for depositing the aggregates was 0.1 mm in 1, 0.2 mm
in 2, and 0.25 mm in 3 (in THF).
Under the experimental conditions used for the confocal
microscopy studies, we find the emission from the deposited
vesicle-like aggregates to be very stable. For example, under
an argon blanket, the emission from a single aggregate
composed of 1 doped with 2 and 3 was found to decrease with
a half-life of 260 s under pulsed picosecond laser illumina-
tion.[7] The estimated energy at the focal point is approx-
imately 1.6 ꢁ 10À17 J per pulse, which corresponds to an
average excitation irradiance of approximately 0.24 WcmÀ2.
During this time, we observe a shift of the color towards the
blue, in agreement with the loss of the red component from
the composite emission. The preferential bleach of the lowest-
energy acceptor is in agreement with its enhanced excitation
by the light-harvesting donor component through energy
transfer.[4a] Aggregates prepared using a single component,
either compound 1, 2, or 3, are even less susceptible to
Figure 5. Energy migration in tricomponent spherical aggregates
allows a large portion of the visible spectrum to be covered. Open
circles correspond to the chromatic coordinates of the emission from
aggregates of 1, 2, or 3 and define a gamut (dashed line) of available
colors. Filled circles represent the color obtained by doping aggregates
of 1 with varying proportions of 2 and 3. For comparison, the gamut of
a standard RGB display is also shown (solid lines). Bottom: Emission
spectrum of a single vesicle-like aggregate emitting D65 white light. Its
position on the chromatic diagram is indicated by the white triangle,
(x=0.313, y=0.331).
above 605 nm exhibits long-lived components (4.0 and 4.8 ns,
respectively) that are similar to the average decay times of 2
and 3.[11] The observed decrease in the average decay time of 1
is attributed to the presence of a distribution of distances that
separate the quencher molecules from the excited donor at
such low quencher concentrations, as has been previously
observed in doped fluorescent fibers.[2e] The effectiveness of
such low dopant levels to quench the emission of the donor
indicates that energy transfer is highly efficient. In this
respect, the lack of bulky hydrocarbon chains generally used
to promote aggregation may contribute to enhancing the
packing density of the chromophores in the aggregates.
Because the emission of composite colors (including
white) involves multiple energy transfer processes between
identical (exciton hopping) as well as between different
chromophores, we may expect that the emission spectrum
may vary from one aggregate to another. To assess this point,
we investigated the distribution of colors from a 10 ꢁ 10 mm
area containing approximately 200 vesicle-like aggregates
(104 spectra). The results are shown graphically in Figure 6,
giving the standard deviation of the x and y chromatic
coordinates of a population of individual objects to be sx =
0.008 and sy = 0.011. Furthermore, for each compound, the
corrected emission spectra from individual aggregates of
different sizes were collected and compared. In all cases, the
differences in the emission spectra between the aggregates
were minimal (see Figure S12).[7] It is also interesting to note
that, whereas the emission spectra of compounds 2 and 3 are
unchanged when they are included as dopants in aggregates of
1 or 2, respectively, the emission of 3 is shifted hypsochromi-
cally when included in aggregates of 1.
photobleaching and required higher laser powers (3 WcmÀ2
)
to show a significant decline in emission intensity (see
Figure S13).
The system described herein offers an unprecedented
combination of high photostability, with the spontaneous
formation of spherical aggregates that can incorporate up to
three different chromophores giving access to a large
spectrum of colors. Furthermore, the possibility of depositing
mixed-component vesicles on a variety of insulating or
conducting substrates makes this system very promising for
the in-depth study of energy-transfer processes in organic
vesicle-like aggregates and for the development of nano-
meter-sized point sources for light-emitting devices covering
the visible spectrum. Elucidation of the mechanism respon-
sible for the spontaneous formation of spherical aggregates
and their inclusion into organic electroluminescent devices
are underway.
Experimental Section
Preparation of vesicles. Solutions of compounds 1, 2, or 3 were
prepared by adding the appropriate volume of THF (dried over Na/
benzophenone and distilled prior to use) to a glass vial containing a
solid sample of the compound to produce a 0.1 mm solution.
Dissolution was achieved by gentle shaking of the vial. For mixed-
color vesicles, to a sample of 1 in THF (0.5 mL, 0.1 mm) was added an
aliquot (1–5 mL) of a solution of 2 or 3 (0.1 mm in THF), and the
solution was homogenized by gentle swirling. The solutions were
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Communications
dropcast onto the substrates using a syringe to deposit approximately
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Scanning electron microscopy. Samples were prepared by drop-
casting a THF solution of 1 (0.1 mm) onto a flat SiO2/Si substrate as
described above. The samples were imaged using a FEI Nova
NanoSEM 200 in low-vacuum mode with no conductive overcoat.
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TEM. Samples were dropcast onto a 200 mesh copper grid coated
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Sa laser (Coherent) tuned at 385 nm with picosecond pulses (6 ps) at
4.76 MHz repetition rate. Average powers of typically 0.24 WcmÀ2
(peak power of ca. 16 kWcmÀ2), and up to 24 WcmÀ2, have been used
for image and spectra acquisition. The laser beam is injected by 908
reflection on a 80%T/20%R spectrally flat beam splitter into the
microscope objective (100 ꢁ UPLSAPO, N.A. 1.4). The emission is
collected by transmission through the same beam splitter and a
suitable interferential filter before being focused on a 50 mm pinhole.
After the pinhole, light can be diverted into an Andor SR300i
spectrometer equipped with a Newton EM-CCD for spectroscopy
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ꢀAi e(Àt/ti) and the average decay times are defined as ꢀAiti2/ꢀAiti.
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Received: March 18, 2011
Revised: May 30, 2011
Published online: June 20, 2011
Keywords: energy transfer · luminescence · self-assembly ·
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emission spectrum of individual objects.
[11] The average luminescence decay times from vesicle-like aggre-
gates composed of pure 2 or 3 were determined to be 3.2 and
5.3 ns, respectively.
7036 www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 7032 –7036