A surfactant-assisted unimolecular platform for multicolor emissions

Article abstract of DOI:10.1039/c2sm26678d

We present here a simple molecular assembly approach to multicolor emissions based on a unimolecular platform of a terthiophene-containing amphiphile TTC4L. The amphiphiles self-assemble into vesicles in solution which exhibit a blue emission. Upon controlling the distance between the fluorescent terthiophene groups by transformation of the self-assembly of TTC4L molecules into their co-assembly with surfactants, the color of the emissions can be continuously tailored which covers most of the visible region. Since the multi-colors were obtained without any structural modification on the fluorescent molecules, we have demonstrated a real unimolecular platform for fabricating multicolor emissions. In contrast, only dual emissions can be obtained from TTC4L using host-guest chemistry. As a simple approach of 'tunable emissions', this surfactant-assisted unimolecular platform opens a new vista for the application of molecular assemblies in advanced light emitting materials.

Full text of DOI:10.1039/c2sm26678d

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PAPER
A surfactant-assisted unimolecular platform for multicolor emissions†
Li Zhao, Xinhao Cheng, Yi Ding, Yun Yan* and Jianbin Huang*
Received 20th July 2012, Accepted 9th August 2012
DOI: 10.1039/c2sm26678d
We present here a simple molecular assembly approach to multicolor emissions based on a
unimolecular platform of a terthiophene-containing amphiphile TTC4L. The amphiphiles self-
assemble into vesicles in solution which exhibit a blue emission. Upon controlling the distance between
the fluorescent terthiophene groups by transformation of the self-assembly of TTC4L molecules into
their co-assembly with surfactants, the color of the emissions can be continuously tailored which covers
most of the visible region. Since the multi-colors were obtained without any structural modification on
the fluorescent molecules, we have demonstrated a real unimolecular platform for fabricating
multicolor emissions. In contrast, only dual emissions can be obtained from TTC4L using host–guest
chemistry. As a simple approach of ‘tunable emissions’, this surfactant-assisted unimolecular platform
opens a new vista for the application of molecular assemblies in advanced light emitting materials.
Introduction
group lies in its interesting optical and electronic properties that
7,21–25
have been verified applicable in many fields.
Moreover, the
Multicolor emissions have been the subject of intense academic
research because of their potential applications in full color
oligothiophenes exhibit unique versatile emissions depending on
26–28
the intermolecular distance.
If the distance between the ter-
1–4
displays and white lightening devices. So far, this has been
ꢀ
thiophene groups is around 3–4 A, p–p stacking may occur
27,29
achieved by molecular engineering endeavors via attachment of
5–11
various substituent groups to the central fluorescent groups.
which allows a blue emission;
ꢀ
to 4–7 A, one may obtain green emissions from the excimers.
when the distance is increased
26,30
However, this chemical approach has limitations in terms of
ꢀ
At distances larger than 7 A, there will be no interaction between
laborious, multistep processes, and a lack of generality for a
the terthiophene groups so that one obtains a violet emission
6,30,31
from the monomers.
12
single synthesis. Therefore, it is highly desired to create versatile
fluorescent emissions from exactly the same molecule which
should not suffer any chemical modifications. To this end, host–
However, so far, the intermolecular distance dependant fluo-
rescent behavior of oligothiophenes hasn’t been sufficiently
exploited to produce multiple emissions. This is partly limited by
the poor solubility of oligothiophene derivatives due to the
1
3–20
guest chemistry was employed to obtain tunable emissions.
It
has been found that many fluorescent groups may exhibit distinct
emissions before and after host–guest complexation, which
13,18
provides a protocol to achieve dual fluorescence,
yet it
remains a challenge to obtain multiple colors from this strategy.
Considering the analogies of host–guest chemistry and
molecular self-assembly, we are inspired to tune the fluorescence
of the ‘guest’ molecules with the assistance of amphiphiles. To
make multicolor emissions based on a unimolecular platform a
reality, we designed a terthiophene amphiphile TTC4L as illus-
trated in Scheme 1. The choice of terthiophene as the fluorescent
Beijing National Laboratory for Molecular Sciences (BNLMS), State Key
Laboratory for Structural Chemistry of Unstable and Stable Species,
College of Chemistry and Molecular Engineering, Peking University,
Beijing, 100871, P. R. China. E-mail: yunyan@pku.edu.cn; jbhuang@
pku.edu.cn
†
Electronic supplementary information (ESI) available: the synthesis of
TTC4L; species distributions and turbidity curve of TTC4L; fluorescence
lifetime of TTC4L and TTC4L–DEAB systems; normalized emission
spectra of TTC4L–DEAB system; emission spectra and photographs of
1
H
TTC4L–C12
C
4
C12, TDPS, C14DMAO, TX100 and SDS systems;
NMR spectra of TTC4L–bCD systems; fluorescence spectra of
TTC4L–bCD systems. See DOI: 10.1039/c2sm26678d
Scheme 1 Synthesis of TTC4L.
1
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6
,32–34
strong aggregation tendency driven by p–p stacking.
To
cylindrical scattering cell. The samples were filtered through
ꢀ
overcome this problem, a pH sensitive bulky ionic head group
was introduced to TTC4L, as demonstrated in Scheme 1. This
design allows for the desired solubility as well as the ability of
TTC4L to self-assemble at a suitable pH. Upon introduction of
amphiphiles to the TTC4L aqueous system, we expect that the
self-assembly of TTC4L may be replaced by the co-assembly
formed by TTC4L and the added amphiphiles. At suitable
amphiphilic concentrations, there would be oligothiophenes of
various distances, so that multicolors can be obtained simply by
variation of the concentration of surfactants. Herein we show
that the addition of simple amphiphiles such as surfactants
indeed allows for tunable multicolor emissions which may cover
most of the visible range.
450 nm filters. The scattering angle was 90 .
TEM observation
A JEOL-100 CX II transmission electron microscope (TEM) was
employed to observe the morphology of the vesicles. Drops of
samples were put onto 230 mesh copper grids coated with For-
mvar film. Excess water was removed followed by staining the
film negatively with uranyl acetate. After removal of the excess
staining liquid by filter paper, samples were then allowed to dry
in ambient air at room temperature, before TEM observation.
Spectroscopic measurements
A Hitachi F-4500 fluorescence spectrometer was used to measure
the fluorescence emission of TTC4L–surfactant solutions. The
time-resolved fluorescence spectrum measurement was per-
formed with a Lifespec Red spectrofluorometer of Edinburgh
Instruments equipped with a Hamamatsu picosecond light pulser
C8898 using a 372 nm laser with a repetition rate of 1 MHz as the
light source. For each measurement, at least 3000 photon counts
were collected in the peak channel to ensure the decay quality.
The quality of fit for the decay curves was supported by the fact
Experimental
Materials
TTC4L was designed and synthesized in our lab as demonstrated
in Scheme 1. Details for this synthesis and characterization are
given in the ESI.† Tetradecyl dimethylammonium propane
sulfonate (TDPS, Sigma-Aldrich) and sodium dodecyl sulfate
(
SDS, Acros) were used as received. Dodecyltriethyl ammonium
bromide (DEAB) and cetyl trimethyl ammonium bromide
CTAB) were prepared by reactions of 1-bromododecane with
2
that the fitting parameter c was less than 1.3. UV-vis spectra for
all the samples were recorded on a HITACHI spectrophotometer
(
in the visible range of 200–800 nm. The scan rate for each
À1
corresponding amines, followed by recrystallizing five times from
ethanol–acetone. The Gemini surfactant alkanediyl-1,12-bis-
measurement was 10 nm min
.
(
dodecyldiethylammonium
bromide)
([C H (CH CH ) -
2 2
1
2
25
3
N(CH ) N(CH CH ) C H ]Br , abbreviated as C C C (Et))
4 12
Results and discussion
2
4
2
3 2 12 25
2
12
35
was synthesized according to the procedure reported. Tetra-
decyl dimethyl amine oxide (C14DMAO, Clariant) was prepared
by freeze drying 70% aqueous solution, followed by recrystall-
izing more than two times from acetone. The purities of the
surfactants were examined by the lack of minima in their surface
tension curves. The water used was re-distilled from potassium
permanganate. The other reagents were of analytical reagent
grade.
Properties of TTC4L aqueous system
1
The synthesis of TTC4L was illustrated in Scheme 1. H NMR
and elemental analysis verified the success of this synthesis
(
Details of the synthesis and characterization of structure of
TTC4L can be found in the ESI†). TTC4L is soluble in water at
concentrations below 1 mM at pH > 8, and a strong Tyndall
effect, which is caused by the scattering of colloidal particles,
may be observed at concentrations down to 0.01 mM in the pH
range of 8–12, demonstrating the strong self-assembling ability
of TTC4L. For the convenience of measurements, a solution of
pH 9.0 was chosen for further analysis (See Fig. S1† for the
component distribution in TTC4L solutions at various pH).
DLS measurements indicated the presence of particles with
average hydrodynamic radii of about 100 nm, which didn’t show
obvious concentration dependence (Fig. 1a). TEM observations
verified that these particles are vesicles (Fig. 1b). The diameters
of the vesicles are in the range of 100–300 nm, which is in good
agreement with the DLS measurements.
Sample preparation
The solutions were obtained by dissolving the solid TTC4L
powder in ultrapure water (Milli-Q, 18.2 MU cm). The pH of
TTC4L solutions was adjusted to about 9.0 by adding a small
enough amount of concentrated KOH aqueous solution to
ignore the volume change. Corresponding solutions were
prepared by mixing TTC4L solution and concentrated surfactant
aqueous solutions of negligible quantity in ultrapure water.
ꢀ
These samples were vortex mixed and kept at 25 C. The pH
values were measured using a SevenMulti type pH meter with
InLab Semi-Micro electrodes (Mettler Toledo, Switzerland). All
The vesicular suspension of TTC4L exhibits intense blue
fluorescence which red shifts with increasing concentration. It
can be seen from Fig. 2a that upon increasing the concentration
from 0.01 to 1 mM, the fluorescence emission increases from 456
to 475 nm. The red-shift of the fluorescence indicates parallel
packing of the terthiophene groups in the form of H-aggre-
ꢀ
analytical measurements were performed at 25 C.
Dynamic light scattering (DLS) measurements
32,36
DLS measurements were performed with an ALV/DLS/SLS-
gates.
Such a conclusion was confirmed further by the
33,37–40
5
022F light scattering-apparatus, equipped with a 22 mW He–Ne
occurrence of a blue shift in the UV spectra.
It is shown in
laser operating at a wavelength of 632.8 nm with a refractive
index matching bath of filtered toluene surrounding the
Fig. 2b that the p–p* transition band occurs at 343 nm for the
aqueous TTC4L vesicular suspensions but at 360 nm in ethanol
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27,41
where TTC4L forms a molecular solution.
The occurrence of
H-aggregates suggests that the terthiophene groups are
33
p-stacked, which allows for a characteristic blue emission, as
demonstrated in Fig. 2a.
Tuning fluorescence emission by forming co-assembly
The emissions of TTC4L can be changed upon the addition of
42–44
oppositely charged amphiphiles to break the self-assembly.
As demonstrated in Fig. 3a (and Fig. S2†), upon addition of
2 mM cationic surfactant DEAB (terethylammonium bromide)
to 0.1 mM TTC4L, the peak of the spectrum shifts from 455
Fig. 3a, curve 1) to 506 nm (Fig. 3a, curve 2), corresponding to
1
(
an emission change from blue to green (Inset in Fig. 3a). At 12
mM DEAB, the emission spectrum is rather broad and struc-
45,46
tureless, which is the characteristic emission from excimers.
This broad feature of the spectrum occurs in wide DEAB
concentrations of 5–18 mM. Upon further increasing the
concentration of DEAB to over 20 mM, the spectrum blue-shifts
again, and the emission of monomeric terthiophene occurs at 439
31,47
nm with a shoulder peak at around 420 nm (Fig. 3a, curve 3).
Fig. 1 (a) DLS results of TTC4L aqueous solution at various concen-
trations. (b) TEM image of 1 mM TTC4L aqueous solutions at pH 9.0.
Fig. 3 (a) Normalized emission spectra of 0.1 mM TTC4L aqueous
solution excited at 360 nm in the presence of 0 (curve 1), 12 (curve 2), and
3
0 (curve 3) mM DEAB, respectively. Curves 4 and 5 are the normalized
emission spectra for the 12 mM DEAB system excited at 440 and 520 nm,
respectively. The inset images 1, 2, and 3 demonstrate the color of the
emissions under 365 nm UV light induced by 0, 12, and 30 mM DEAB,
respectively. Image 5 shows the yellow beam pass in the 12 mM DEAB
system when irradiated with 532 nm laser. (b) Emission color of 0.1 mM
TTC4L solutions in a CIE 1931 chromaticity diagram (The dots con-
nected by the black line show the location of 0.1–12 mM solution irra-
diated at 365, 400, 440, and 520 nm, respectively).
Fig. 2 Emission (a) and absorption (b) spectra of TTC4L aqueous
solutions at various concentrations. The inset in (a) is an image of 0.1 mM
TTC4L solution under 365 nm UV light.
1
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So far we have successfully obtained blue, green, and violet
emissions from the same TTC4L molecule. However, the colors
available in the TTC4L solution are not limited to these. We
noticed that at medium DEAB concentrations such as 12 mM,
the emission peak is rather broad, and actually extends beyond
6
00 nm. Therefore, it is possible to generate more narrow colors
simply by varying the excitation wavelength. Curves 4 and 5 in
Fig. 3a demonstrate that the emissions keep red-shifting upon
increasing the excitation wavelength. We may even obtain yellow
and orange emissions when exciting the same solution at 520 nm.
Inset image 5 in Fig. 3 demonstrates the yellow-orange Tyndall
light pass obtained by excitation of the solution with a 532 nm
green laser. The actual color of the light pass is orange which
looks like yellow in the photo. The location of this orange color
was demonstrated in the CIE plot (Fig. 3b) with coordinates of
(
0.507, 0.491). It can be clearly visualized in the CEI plot that the
colors obtained in the TTC4L system may cover most of the
visible light region except red and deep purple.
As already discussed, the emission spectra is rather broad in
the wide DEAB concentration range of 5–20 mM, which allows
continual tuning of the emissions from violet to orange by simply
increasing the concentration of DEAB gradually.
Molecular mechanism of the multiple color emissions
In order to gain more insight into the emission, time-correlated
45
single-photon-counting measurements were carried out for the
TTC4L systems with and without DEAB to provide information
about the molecular states of TTC4L. Two lifetimes were
obtained from the fluorescence decay curves for the TTC4L
aqueous suspensions without DEAB, suggesting the presence of
two states of TTC4L. The shorter one around 0.3 ns can be
ascribed to the non-aggregated single TTC4L molecules
Fig. 4 (a) Percentage of TTC4L molecules in different states determined
from fluorescence lifetime measurements. (b) Variation of the percentage
of different fluorescent species in the TTC4L solutions with increasing
DEAB concentration.
(
monomers) owing to a fast intersystem crossing to the triplet
state, whereas the longer one around 2.7 ns (ESI, Table S1†)
47–50
absorption centered at 343 nm, which is caused by the p–p*
originates from the H-aggregated TTC4L molecules.
The
31
electron transition of terthiophene groups in the aggregated
states. Upon the addition of DEAB to this solution, the intensity
of this absorption increases significantly and the position shifts
toward red. It can be seen from Fig. 5 that in the presence of 12
mM DEAB, the peak is further broadened and an additional
time-correlated single-photon-counting measurements also
suggest that the amounts of TTC4L in the monomeric state and
in aggregates are about 15 and 85%, respectively, (Fig. 4a)
regardless of the concentrations. This means that in the self-
assembled systems, the emission from the monomers is
predominantly overwhelmed by that of TTC4L in the aggregate
states.
The situation will be different when the cationic surfactant
DEAB is added to the system. The percentage of monomers
increases with increasing DEAB concentration (Fig. 4b), which
amounts to nearly 95% at 30 mM DEAB. Meanwhile, the
percentage of aggregated TTC4L keeps decreasing and become
negligible at 30 mM DEAB. The most remarkable phenomenon
is the occurrence of excimers with a lifetime longer than 5 ns at
26
medium DEAB concentrations. There is approximately 19% of
TTC4L in the form of excimers at 12 mM DEAB. Although this
percentage is not dominant in the system, the emission is so
overwhelming that the system gives a characteristic green
emission.
The change of the molecular state of TTC4L upon addition of
DEAB was also reflected in the UV spectra. In Fig. 5 we show the
change of the UV spectra of TTC4L with increasing DEAB
concentration. Without DEAB, 0.1 mM TTC4L gives a broad
Fig. 5 UV spectra of 0.1 mM TTC4L in the presence of DEAB at
various concentrations.
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2
9
peak at around 360 nm is roughly identified. This suggests that
at this stage, TTC4L in the monomeric state is present. It is also
very clear that the UV spectra exhibit a featureless shoulder peak
TTC4L molecule is about 2.2 nm, these smaller particles are
probably micellar clusters co-assembled from TTC4L and
DEAB. This suggests that the addition of DEAB into the TTC4L
suspension has transformed the self-assembly of TTC4L into a
co-assembly of TTC4L and DEAB, thus the intermolecular
distance between the terthiophene groups is effectively increased.
We can exclude the possibility that the smaller particles might be
the micelles of DEAB itself as the extending length of DEAB is
about 1.5 nm, so that the radius of DEAB micelles should not
exceed this value. This was indeed the case. As demonstrated in
Fig. 6, at a concentration far beyond the critical micellar
concentration (CMC), the average hydrodynamic radius of
DEAB micelles is roughly 1 nm. Thus, the control experiments
have clearly verified that the smaller particles in the TTC4L–
DEAB mixed solutions are indeed the co-assembly from these
two components. However, the fluorescence lifetime measure-
ments (Fig. 4b) suggest that there are still self-aggregated
domains of TTC4L in the small aggregates, but the size of the
aggregated domains are considerably decreased which is inferred
26
at around 390–400 nm, indicating the presence of excimers. At
much higher DEAB concentrations, such as 20 and 30 mM, the
intensity for the monomeric TTC4L absorption is further
enhanced, whereas the shoulder become less impressive. These
features are in perfect agreement with the results obtained from
the fluorescence lifetime measurements in Fig. 4b, which unam-
biguously confirmed the change of molecular states of TTC4L
with addition of DEAB at different concentrations. However,
both the fluorescence and the UV measurements suggest that in
the wide range of medium DEAB concentrations, it is difficult to
get narrowly distributed p–p distances. We may obtain a
ꢀ
narrowly distributed p–p distance larger than 7 A at very high
DEAB concentrations, for example, over 20 mM in this case.
Otherwise, the addition of a small amount of DEAB always
triggered polydisperse p–p distances. However, such multiple
possibilities of the p–p distance are not undesired in this system,
because it is this polydispersed p–p distance that allows the
continuous generation of multicolor fluorescence.
48
from the shorter fluorescence lifetimes (1.3 ns, Table S2†).
According to Fig. 4b, the fraction of TTC4L in these domains is
about 49%. We believe that such ‘aggregated TTC4L’ is probably
a result of irregular overlap of the terthiophene groups in the
The change of molecular states of TTC4L with increasing
surfactants is also a reflection of the evolution of the self-
assembled structures in the system. To verify this, dynamic light
scattering measurements were carried out. As demonstrated in
Fig. 6, the size of the particles in the 0.5 mM TTC4L system is
progressively decreased with increasing DEAB concentration.
Without addition of DEAB, the TTC4L self-assemble into
particles with an average hydrodynamic radius of around
ꢀ
micellar core, which leads to 4–7 A separations between the
terthiophene groups. This allows for the formation of excimers,
and a green emission occurs. At a DEAB concentration greater
than 20 mM, each micellar cluster may contain only one TTC4L
molecule so that only a monomeric emission is observed. The
whole process of aggregate transition is illustrated in Scheme 2.
1
00 nm, which have been previously verified to be vesicles
(
Fig. 1b). In contrast, upon addition of 2 mM DEAB, another
group of particles with an average hydrodynamic radius of
around 4 nm were found to come into co-existence with the
vesicles. At 4 mM DEAB, the scattering from the smaller parti-
cles becomes dominant, which is at the cost of the lowering of the
scattering from the larger ones. If we consider that the scattering
Generality of the surfactant-assisted multicolor emissions
Surfactant-induced multicolor emissions from TTC4L molecules
can be generalized to a large variety of surfactants. Non-ionic
surfactant such as TX-100, zwitterionic ones like TPPS and
C
14DMAO, as well as other cationic surfactants, such as CTAB,
intensity is proportional to the particle radius with the order of
4
0 , it can be estimated that in the presence of 2–4 mM DEAB,
and even gemini-type cationic surfactants, may have a similar
effect as DEAB (Fig. S3–S6†). In contrast, negatively charged
surfactants such as SDS, which carries the same charge as
TTC4L, are less effective at breaking the self-assembly of TTC4L
into a mixed co-assembly (Fig. S7†). Therefore, the electrostatic
interactions should play a key role in the present strategy, which
1
the number of large vesicles in the 0.5 mM TTC4L system is
almost negligible compared with the quantity of the small
particles. Considering the fact that the extending length of the
Scheme 2 Illustration of the arrangement of TTC4L molecules under
various conditions. (a) represents the self-assembly formed by TTC4L,
whereas (b) and (c) are the co-assemblies between TTC4L and surfactants
with increasing surfactant concentration.
Fig. 6 DLS results of 0.5 mM TTC4L in the presence of different
concentrations of DEAB. The curve with 100 mM DEAB is the result
from the DEAB aqueous solution where no TTC4L is present.
1
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may provide a higher efficiency in triggering the desired emis-
sions. Actually, the stronger the interaction between TTC4L and
surfactants, the less surfactant is required to reach the same
emissions. It was found that if DEAB was replaced by cationic
surfactants with lower CMCs, such as CTAB and
Therefore, the surfactant-assisted strategy shows a clear superi-
ority to host–guest chemistry.
Conclusions
51
In summary, we have provided a surfactant-assisted unim-
olecular strategy to generate multicolor fluorescent emissions
from the same fluorescent terthiophene-amphiphile TTC4L.
Upon tuning the intermolecular distance between the terthio-
phene groups by changing the self-assembly of TTC4L to co-
assembly with various surfactants, we may obtain emissions
from violet to orange which cover most of the visible light region
without changing and modifying the structure of the TTC4L. It is
a general approach which is applicable to many surfactants and
fluorescent molecules with similar properties. We believe that it is
far beyond an interesting example of ‘multicolor emissions from
one molecular platform’, but also an innovative employment of
molecular assemblies in fabricating advanced laminating
materials.
C
12
C
4
C
12(Et), the critical surfactant concentration desired for
triggering fluorescence transition was much lower. For instance,
.5 mM CTAB or C12 12(Et) is enough to alter the fluores-
0
4
C C
cence of TTC4L from blue to green, as demonstrated in Fig. 7a.
The effect of the surfactant on the multicolor emission was
also compared with that obtained by host–guest chemistry. Two
types of pristine cyclodextrins (b- and g-CDs) were used to form
26,52,53
1
complexes with a terthiophene moiety.
Although H NMR
measurements confirmed the successful inclusion of TTC4L into
the cavity of b-CD (Fig. S8†), this only affects the intensity of the
emission, which doesn’t produce a considerable effect on the
52
fluorescence emission (Fig. S9†). In analogy, if g-CD is used, an
obvious red-shift of the emission yet negligible blue-shift can be
triggered, as demonstrated in Fig. 7b. The occurrence of the
green component is attributed to the inclusion of two TTC4L
54
molecules in the larger cavity of g-CD, which is beneficial to the
Acknowledgements
26
formation of excimers. However, it seems that the fraction of
excimers generated in this host–guest chemistry is rather low so
that only a transition color between blue and green was obtained.
This research was supported by the National Natural Science
Foundation of China (20903005, 21173011, 51121091,
2
1073006), and the Doctorial Fund of Ministry of Education of
China. We would like to thank Prof. Zichen Li for his advice on
the synthesis.
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