Brightly shining nanoparticles: Lipophilic perylene bisimides in aqueous phase

Article abstract of DOI:10.1039/b717795j

The dispersion of lipophilic perylene bisimides down to nanoparticle dimensions opens the aqueous phase to these highly fluorescent, water insoluble materials. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b717795j

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LETTER
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Brightly shining nanoparticles: lipophilic perylene bisimides in aqueous
phasew
Heinz Langhals*
Received (in Montpellier, France) 16th November 2007, Accepted 27th November 2007
First published as an Advance Article on the web 6th December 2007
DOI: 10.1039/b717795j
The dispersion of lipophilic perylene bisimides down to nano-
particle dimensions opens the aqueous phase to these highly
fluorescent, water insoluble materials.
preparations 1a form strongly yellow fluorescent liquids,
remaining unchanged with storage, where the dispersion of
the dye is so fine that it is not retained by a D5 glass filter, not
1
3
even in traces; see Fig. 1. The porosity of D5 glass filers is
1
Perylene dyes 1, perylene tetracarboxylic bisimides, char-
1.0–1.7 mm and particles can be retained that are smaller by a
1
4
acterised by their high stability, are used as high performance
2
pigments. The attachment of solubility increasing groups
3
such as long-chain secondary alkyl groups (‘‘swallow-tail
factor of three or even more. The complete passing of
particles leaving the glass filter colourless indicates a size of
the order of nanometres. The dispersion of the dye must have
8
,9
groups’’) to the nitrogen atoms of 1 results in readily soluble,
progressed far beyond the size of the previously reported
highly fluorescent dyes for lipophilic solvents with fluorescence
4
quantum yields close to 100%.
particles because the latter could be completely collected by a
D4 glass filter with pores of 9–15 mm size.
The small size of the particles was verified by dynamic
light scattering (DLS) measurements where a maximum of
the distribution was found at 60 nm; see Fig. 2. The
comparably narrow distribution is as remarkable as the result
that no particles were found larger than 600 nm.
Thus, complete dispersion down to nano-dimensions
proceeded. Information about the direct environment of
the nano-dispersed dye molecules was obtained by UV/Vis
However, the application of such dyes in aqueous solution
turned out to be appreciably more difficult. Strongly hydro-
spectroscopy; the chromophore of
but characteristically, solvatochromic.
1
15
is very weakly,
Surprisingly, the
5
philic groups such as sulfonic acid groups, polyether groups,
6
7
and pyridyl substituents, basically induce water solubility of
UV/Vis spectra of nano-dispersed 1b in water are identical
within 2 nm with the spectra in homogeneous solution in
the lipophilic chloroform; see Fig. 1. The unaltered spectral
shape indicates isolated chromophores in the nanoparticles
1; however, these materials exhibit a strong tendency to
aggregate in the highly hydrophilic medium. Moreover, such
dyes exhibit a high aptitude for the collection of heavy metal
6
ions and this interferes with the preparation of the dyes in
16
where exciton interaction with neighbouring chromophores
high purity.
8
A dispersion of a special derivative of 1 with a labile crystal
can be excluded. Such a type of nano-dispersion is
very different from previously described microcrystalline
particles with interacting chromophores; see for example
ref. 17. Moreover, rather high concentrations of dyes in water
can be obtained and are only limited by the solubility of
lattice was only successful for low concentrations of particles
9
and a spontaneous dispersion of a crown ether derivative of 1
is limited to this special dye. A direct application of the
lipophilic, water-insoluble materials would bring about an
appreciable improvement.
We have applied a dispersing detergent of maleinated lin-
1
0
11
seed oil as an alternative; commercial name Bomol-4N.
The lipophilic dyes 1 were dissolved in the dispersant and
spread in more than a hundred times the quantity of water; for
the preparation of nanoparticles by the application of deter-
gents see ref. 12. Water-insoluble dyes were therefore required
that are both lipophilic and readily soluble in the dispersant.
High solubility of 1 was attained by the attachment of long-
3
chain secondary alkyl groups to the nitrogen atoms. Such dye
Department of Chemistry, LMU University of Munich, Butenandtstr.
13, D-81377 Munich, Germany. E-mail: Langhals@lrz.uni-
muenchen.de; Fax: +49 89-2180-77700; Tel: +49 89-2180-77699
w Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b717795j
Fig. 1 UV/Vis absorption and fluorescence spectra of 1. Left: ab-
sorption spectra from bottom to top: 1b in chloroform (red), 1b
nanoparticles in water (black), 1a nanoparticle in water (blue). Right:
fluorescence spectrum of 1b nanoparticles in water (black).
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Fig. 3 UV/Vis absorption spectra of 2 (blue) and 3 (black). Upper,
thick lines: nano-dispersions in water; lower, thin lines: reference
spectra in chloroform.
Fig. 2 Particle distribution of 1b nanoparticles in water: Intensity I of
counts versus the size d of particles in nm.
the dye in the dispersant and the ratio between the dispersant
and water. Flocculation does not occur, even not for the
highest attainable dye concentrations. Appreciably coloured
aqueous phases were obtained from 1a despite the
rather short swallow-tail substituent. Even more deeply co-
loured phases can be obtained with the longer and
more efficiently solubilizing substituents in 1b. The absorption
of the dispersed dyes is overlaid by the absorption and
light-scattering of the dispersant and the baseline of the
spectrum deviates to higher values for shorter wavelengths;
see Fig. 1. This deviation is strongest for 1a because its rather
low solubility requires more dispersant than the more soluble
dye 1b where matrix effects are essentially concentrated in the
UV. Thus, 1b can be applied in the visible in the same way as a
homogeneously dissolved dye.
an indication of some aggregation of chromophores in
the particles.
The very similar results with the completely different chro-
mophores 1, 2, and 3 indicate a broad application of the
dispersion of dyes to form nanoparticles. The aqueous phase is
of central importance for biochemical systems; thus, the novel
concept opens up such applications for lipophilic dyes.
We thank Professor Dr. T. Bein for the particle measure-
ments and Dr. H. Mottl from ISP Switzerland AG for a gift of
Bomol-4N.
The pronounced fluorescence of the aqueous phases of 1 is
remarkable. The fluorescence spectra of the aqueous phases of
Experimental
1
resemble the spectra in chloroform; see Fig. 1. The fluores-
cence quantum yields of 1a and 1b are close to 100% as in
4
solution in chloroform. The high fluorescence quantum yields
The perylene dyes 1 were prepared according to literature
3
procedures. Saturated solutions of these dyes in Bomol-
1
0,11
are additionally indicators for isolated chromophores in the
4
N
were prepared, left in contact with the solid material
nanoparticles because interaction of chromophores such as in
1
aggregates causes a weakening of fluorescence.
for 16 h, decanted, poured into 100 times the quantity of
distilled water or even more and filtered through a D5 glass
filter. The indigo derivatives 2 and 3 were prepared according
8
Completely novel applications become possible for strongly
lipophilic dyes which were otherwise inhibited by the insolu-
bility in polar media. In particular, the high heat capacity of
water allows applications with high radiation densities. On the
other hand, the aqueous phase is of central importance for any
use of dyes in biochemistry.
1
9,20
to the literature
. The dispersions were diluted with distilled water to obtain
and dispersed completely analogously to
1
optical densities between 0.5 and 1.0 in 1 cm cuvettes and the
UV/Vis spectra were recorded with a Cary 5000 spectrometer
from Varian. The fluorescence spectra were recorded with an
SP3000 fluorescence spectrometer from Perkin Elmer and
The dispersion of dyes demonstrated with perylene
derivatives can be extended to other classes of dyes. Lipophilic
and readily soluble dyes are therefore required. For example,
4
quantum corrected. Fluorescence quantum yields were deter-
mined analogously to refs 4 and 22 with 1b as the standard.
The particle distribution was measured with a DLS counter:
Malvern Nano-ZS ZEN 3600.
0
19
0
20
indigo was substituted in the positions 5,5 (2) and 6,6 (3)
with tert-butyl groups in order to increase the solubility.
2
1
Saturated solutions in the dispersant were prepared with
these dyes and spread in water as was described for 1.
More dispersant than for 1 is necessary for 2 and 3,
respectively, because of the lower solubility. Analogously to
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