White-light emitting dye micelles in aqueous solution

Article abstract of DOI:10.1039/c3cc44875d

Biscarbazole-loaded perylene bisimide (PBI) dye micelles in water afforded a fluorescence resonance energy transfer (FRET)-based white-light emitting nano luminary system with biscarbazoles as energy donors and PBI micelles as acceptors.

Full text of DOI:10.1039/c3cc44875d

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White-light emitting dye micelles in aqueous solution†
Xin Zhang, Daniel Gorl and Frank Wurthner*
¨
¨
Cite this: DOI: 10.1039/c3cc44875d
Received 29th June 2013,
Accepted 26th July 2013
DOI: 10.1039/c3cc44875d
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Biscarbazole-loaded perylene bisimide (PBI) dye micelles in water
afforded a fluorescence resonance energy transfer (FRET)-based
white-light emitting nano luminary system with biscarbazoles as
energy donors and PBI micelles as acceptors.
Water-soluble luminescent assemblies of organic building blocks
have attracted much attention because of their potential applica-
tions as fluorescence sensors and probes in biological systems,
and as energy collecting and transfer materials for artificial
photosynthesis.1,2 Among such luminescent assemblies, white-
light emitting systems are of particular interest for a wide range of
applications.3 On the basis of the Commission Internationale de
l’Eclairage (CIE) chromaticity diagram, white light can be achieved
by careful tuning of each colour contribution4 and energy transfer
between dye components in an assembling system.5 In the past
years, various organic composites have been reported that emit
white light in thin-films,6–11 organogels,12 and vesicles.13,14
For a few years, one of our research interests has been self-
assembly of perylene bisimide (PBI) dyes in water.15 Previously,
we reported on the precise control of morphology of PBI dye
aggregates to obtain nanorods, bilayer vesicles and spherical
micelles in water.16 Moreover, we developed luminescent vesi-
cular nanocapsules of 30–40 nm diameter based on PBI dyes
which displayed pH-dependent emissive colours including
white light.14 In this work, we report on functionalization of
much smaller nanosystems of o10 nm diameter, i.e. PBI dye
micelles, by loading biscarbazoles with appropriate spacer length
into the micelle interior to obtain nano-sized fluorescence reso-
nance energy transfer (FRET) systems. To the best of our knowl-
edge, we demonstrate here for the first time white-light emitting
dye micelles that are created from molecular building blocks in
aqueous solution.
Scheme 1 Chemical structures of BCzs 1, 2, 3, and PBI 4.
lengths17 as energy donor molecules and amphiphilic PBI 4
(ref. 16) as an energy acceptor building block in the exterior
of morphologically distinct micelles created by self-assembly
in water.
Prior to the investigations employing PBI micelles, we first
studied loading of BCzs into a model micelle system using the
well-known surfactant sodium dodecyl sulfate (SDS), which readily
forms micelles in water (Scheme 2, for details see ESI†).18 Since
the micelle interior is highly hydrophobic,18 hydrophobic bis-
carbazoles are adsorbed into the hydrophobic interior of SDS
micelles in aqueous solution (Scheme 2) as confirmed by fluores-
cence spectroscopy (Fig. 1b and Fig. S5, ESI†).
Our choice of a,o-alkyl tether-linked biscarbazoles was based
on earlier work by De Schryver and coworkers who showed that
We have employed biscarbazoles (BCzs) 1–3 (Scheme 1, for
the synthesis and characterization see ESI†) with varied spacer
¨
¨
¨
Universitat Wurzburg, Institut fur Organische Chemie and Center for Nanosystems
¨
Chemistry, Am Hubland, 97074 Wurzburg, Germany.
E-mail: wuerthner@chemie.uni-wuerzburg.de
† Electronic supplementary information (ESI) available: Synthetic details, addi-
tional NMR, UV/vis, fluorescence and TEM data. See DOI: 10.1039/c3cc44875d
Scheme 2 Schematic illustration of the loading of BCz 1, 2 or 3 into sodium
dodecyl sulfate (SDS) micelles in aqueous solution.
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with the emission spectra of BCz 1–3 donor units in the
hydrophobic environment of SDS micelles (Fig. 1 and Fig. S5,
ESI†). The emission band of BCz 1 within the confined interior
space of SDS micelles does not overlap with the absorption
band of pure PBI micelles (Fig. S5, ESI†) suggesting that no
FRET can occur between BCz 1 donors and PBI dye acceptors.
On the other hand, the broad emission bands from the excited
states of stacked BCz 2 or 3, i.e. intramolecular excimers, lead to
spectral overlap with the absorption of PBI micelles (Fig. 1b and
Fig. S5, ESI†). These results suggest that FRET can occur between
BCz 2 or 3 donors and PBI micelle acceptors.
Following the studies with the model micelle, we then explored
the functional PBI micelles. Like SDS micelles, the interior of
PBI micelles is hydrophobic and thus can load hydrophobic
biscarbazoles into their interior. Functional PBI 4 micelles were
prepared as follows: biscarbazole 1, 2 or 3 (5.0 mg of each) was
Fig. 1 (a) Fluorescence spectrum of BCz
2 in THF and UV-vis absorption
spectrum of PBI 4 micelles in aqueous solution; [BCz 2] = 5.0 Â 10À6 M, [PBI 4] = dissolved in THF (1.0 mL) and a portion of 200 mL of the
2.86 Â 10À5 M. (b) Fluorescence spectrum of BCz 2-loaded SDS micelles in aqueous
respective THF solution was slowly added into PBI micelle
solution (4.0 mL) under stirring, and then ultrasonicated to
dissolve biscarbazoles (Fig. 2b). The morphologies of these
solution and UV-vis absorption spectrum of PBI 4 micelles in aqueous solution;
[BCz 2] = 5.0 Â 10À6 M.
functional PBI micelles were determined by TEM measurements.
these molecules prevail in different conformations depending These BCz-loaded PBI micelles have an average size of 6–9 nm in
on the respective alkyl chain length and the solvent.17 In parti- diameter (Fig. 2a and Fig. S6, ESI†), which is slightly larger than
cular for 1,3-di-N-carbazolylpropanes, e.g. BCz 2, which are unloaded PBI 4 dye micelles (4–6 nm)16 due to BCz encapsula-
model systems for the first commercial organic photoconductor tion. The hydrophobic interior could be visualized in a magni-
poly(N-vinylcarbazole), it was shown that stacked conformations fied TEM image (Fig. 2a, inset) which displays slight lightness
are preferred over extended ones in low polarity environments.17 (low electron density) relative to the hydrophilic exterior staining
While in the extended conformation two carbazole rings stretch with uranyl acetate (relative darkness). Dynamic light scattering
outward as observed in the polar solvent THF for BCz 2 (DLS) analysis showed that the size of these functional micelles
(Fig. 1a), in stacked conformations two carbazole rings are is independent of scattering angles further confirming the
packed face-to-face by intramolecular p–p-stacking interaction, spherical shape of these morphologies (Fig. 2c).
leading to intramolecular excimer fluorescence as observed in
aliphatic solvents,17 or in the interior of SDS micelles as shown show an additional fluorescence band at 360–480 nm (Fig. 3c),
in Fig. 1b. which arose from the emission of stacked conformation of
Compared with pure PBI micelles, these BCz-loaded micelles
UV-vis absorption spectra of BCz 2 are concentration- biscarbazole donors within PBI micelles in aqueous solution.
independent, which further confirms the intramolecular stacking
of BCz 2 (Fig. S4, ESI†). The fluorescence spectra display a large
spectral shift from the UV (320–400 nm, Fig. 1a) in good-solvating
THF solvent to visible cyan-blue (380–550 nm, Fig. 1b) in the low-
polarity interior of SDS micelles. For BCz 3, quite similar to BCz 2,
the intramolecularly stacked conformation is predominant in the
confined interior space of SDS micelles according to fluorescence
spectra (Fig. S5b, ESI†). In contrast, for BCz 1, only the extended
conformation exists in solution as well as in the low-polarity
confined interior space of SDS micelles as revealed by fluores-
cence spectroscopy (Fig. S5a, ESI†). Thus, within SDS micelles,
BCz 1 displays two fluorescence peaks at 335 nm and 360 nm
similarly to those observed in good-solvating solvents for aro-
matic p-systems such as THF. This finding is quite reasonable
since the ethylene spacer between two carbazole rings of 1 is too
short, and thus intramolecular stacking is disfavoured due to the
geometrical restriction.
The FRET from excited donors to acceptors can be judged
Fig. 2 (a) TEM image of BCz 2-loaded PBI micelles prepared in aqueous solution,
by spectral overlap between donor emission and acceptor
inset: a magnified section; [PBI 4] = 0.17 mg mLÀ1. (b) Schematic illustration
absorption.19 Thus, next we prepared pure PBI micelles in aqueous
based on space-filling models for the loading of BCz 2 into PBI micelles. (c) Size
solution according to previously reported procedures,16 and com-
distribution of BCz 2-loaded PBI micelles in aqueous solution determined by DLS
pared the UV-vis absorption spectrum of PBI micelles as acceptors at the scattering angles of 15.71, 30.11 and 62.61.
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within these functional micelles to be 0.46 and 0.70 for BCz
2- and BCz 3-loaded PBI micelles, respectively (for details see
ESI†). Because this value is too large for BCz 3-loaded micelles,
white colour could not be realized, pinpointing the challenge in
the supramolecular design.
In summary, we have demonstrated FRET within dye
micelles by loading biscarbazoles as energy donors into the
hydrophobic interior of PBI micelles that act as acceptors. Our
white-light emitting, water-soluble BCz 2-loaded PBI micelles are
unique because they represent the smallest imaginable bulb-
shaped white-light emitting nano luminary systems accom-
plished from only two fluorescence colour components, i.e. cyan
blue and red.
Financial support from the Deutsche Forschungsgemeinschaft
DFG (grant project: WU 317/11) is gratefully acknowledged.
Fig. 3 (a) Photograph of BCz 2-loaded PBI micelles in aqueous solution under a
UV lamp. (b) CIE 1931 chromaticity diagram. The circles indicate the colour
coordinates for the cyan blue fluorescence of biscarbazole excimers (0.21, 0.27),
the red fluorescence of pure perylene micelles (0.65, 0.35) and white fluores-
cence coordinates (0.34, 0.30) for the functional micelles. (c) Fluorescence spectra
of BCz 2-loaded PBI dye micelles in aqueous solution at the excitation wave-
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