Fluorous biphase synthesis of a poly(p-phenyleneethynylene) and its fluorescent aqueous fluorous-phase emulsion

Article abstract of DOI:10.1002/anie.201003111

It's just a phase: A highly fluorinated compound with a rigid three-dimensional architecture was synthesized as a monomer for poly(p-phenyleneethynylene)s (PPEs). Fluorous biphase reactions were applied for the synthesis of a PPE that is soluble only in fluorous solvents. A fluorous solution of the polymer could be processed into aqueous emulsions that display high fluorescence quantum yields (see picture).

Full text of DOI:10.1002/anie.201003111

Communications
DOI: 10.1002/anie.201003111
Conjugated Polymers
Fluorous Biphase Synthesis of a Poly(p-phenyleneethynylene) and its
Fluorescent Aqueous Fluorous-Phase Emulsion**
Jeewoo Lim and Timothy M. Swager*
Highly fluorinated materials display a variety of interesting
properties such as thermal and chemical stability, low surface
energy, and high resistance to oxidation.[1] These materials
can display orthogonal solubility by dissolving in fluorous
solvents with limited solubility in organic solvents, thus
allowing for facile purification by liquid-to-liquid extraction
and/or fluorous solid-phase extraction.[2] Fluorous small
molecules, materials, and solvents have been utilized in
areas such as fluorous biphase chemistry,[3] liquid crystals,[4]
electronics,[5] and as arrays for biosample screening.[6] Con-
jugated polymers that are soluble in fluorous phases have
merit both in that they would allow for facile purification and
processing and in that they could provide nontoxic platforms
for fluorescence-based bioimaging and detection. Further-
more, the introduction of rigid perfluoroalkyl chains to the
backbone of a conjugated polymer is expected to enhance the
quantum yield of the material both in solution and in the solid
Scheme 1. Chemical structures of the PPE polymers P1 and P2.
state. Herein we report the syntheses and the properties of
two highly fluorinated poly(p-phenylene ethynelene)s
(PPEs), P1 and P2 (Scheme 1). Both polymers are highly
fluorescent in solution and in thin films. Furthermore, P1,
which displays a selective solubility in fluorous solvents, can
be synthesized by fluorous biphase polymerization, thus
allowing for facile isolation and purification of the polymer
after polymerization (Figure 1). The alkoxylated counterpart,
P2, shows good solubility in organic solvents but is not soluble
in the fluorous phase. The selective solubility of P1 in fluorous
solvents allows for the creation of a highly fluorescent and
stable emulsion in water.
The design principle of P1 was based on the qualitative
guidelines of fluorous compatibility outlined by Horvꢀth
et al.[3] and consists of 1) a majority fluorine content by weight
and 2) long perfluoroalkyl chains that form a sheath around
the PPE backbone. Furthermore, a rigid, bulky architecture
Figure 1. Fluorous biphase synthesis of fluorescent fluorous polymers.
was desired to discourage aggregation of the polymer both in
solution and in solid state. The syntheses of monomer 1 and of
polymers P1 and P2 from 1 are outlined in Scheme 2.
When the pentacene derivative 5 was treated with an
excess of perfluoro(7-tetradecyne) in xylenes at 1458C, no
reaction was observed after 4 days. This lack of reactivity was
attributed to the poor solubility of perfluoro(7-tetradecyne)
in xylenes, even at elevated temperatures. When the reaction
mixture was homogenized with a high-sheer mixer at 808C,
before raising the temperature to 1458C, the desired twofold
Diels–Alder reaction took place to afford the corresponding
di-adduct in 86% yield. The reaction gave the syn isomer as
the major product, with a 6:1 syn/anti ratio. This selectivity is
in sharp contrast to the Diels–Alder reactions of 5 with
hexafluorobutyne and dimethyl acetylenedicarboxylate
(DMAD), where the anti isomers are observed as the major
[*] J. Lim, Prof. Dr. T. M. Swager
Department of Chemistry, and Institute for Soldier Nanotechnology
Massachusetts Institute of Technology
Cambridge, MA 02139 (USA)
Fax: (+1)617-253-7929
E-mail: tswager@mit.edu
[**] This work was supported by the Army Research Office, through the
Institute for Soldier Nanotechnologies. We thank Trisha L. Andrew
for helpful discussions and photophysical measurements.
Supporting information for this article (detailed experimental
procedures and chemical syntheses) is available on the WWW
under http://dx.doi.org/10.1002/anie.201003111.
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Chemie
manner was optically pure and was used without further
purification for photophysical measurements.
When monomer 1 was treated with comonomer 7 under
identical conditions, a complete reversal of the solubility was
observed, with the fluorescence of the product biphasic
mixture localized in the upper organic phase. Removal of the
fluorous layer, followed by precipitation of the organic layer
with ethanol and washing the resulting solid with acetone
gave P2 in 78% yield.
The normalized absorption and emission spectra of P1
and P2 are shown in Figure 2. The fluorous soluble P1
displays a band edge and an emission maximum that are both
Scheme 2. Synthetic routes to PPE polymers P1 and P2. Polymerization
conditions: perfluoro(methylcyclohexane)/toluene/diisopropylamine
3:5:1, 5 mol% [Pd(PPh3)4], 7 mol% CuI, 858C, 4 days. The photo-
graphs show reaction mixtures at the end of the reaction irradiated
with a hand-held long-wavelength UV lamp.
products.[7] The anti isomer had a poor solubility in most
organic solvents, while the syn isomer was highly soluble;
therefore the synthesis was carried out by using only the syn
isomer.
Figure 2. Absorption (dotted line) and emission (solid line) spectra of
P1 (blue, in perfluorodecalin; quantum yield 0.95) and P2 (red, in
toluene; quantum yield 0.84).
Removal of the triisopropylsilyl (TIPS) moieties from 2
gave the corresponding diacetylene 4. Sonogashira–Hagihara
cross-coupling polymerization of 4 with diiodide 6 under
various conditions gave only oligomeric products. The low
degree of polymerization was attributed to the sterically
demanding environment around the acetylene moieties of 4.
It was therefore envisioned that monomer 1, with reduced
steric hindrance around the acetylene functional groups,
would provide polymers with higher molecular weights.
When monomer 1 was subjected to Sonogashira–Hagi-
hara cross-coupling polymerization in toluene/diisopropyl-
amine with 6, higher-molecular-weight products were
obtained, although the products were still soluble in organic
solvents. It was expected that more fluorous solvent condi-
tions for the Sonogashira–Hagihara cross-coupling reaction
would produce polymers with higher molecular weights,
which may subsequently render the material selectively
soluble in fluorous solvents. When a solvent mixture of
toluene/perfluoro(methylcyclohexane)/diisopropylamine
(5:3:2) was heated, it was observed that the solvent mixture
became monophasic at 828C, and, upon cooling, the fluorous
phase separated neatly from the organic (toluene/diisopro-
pylamine) phase. The Sonogashira–Hagihara cross-coupling
polymerization between monomer 1 and diiodide 6 in this
solvent system at 858C gave, upon cooling, a biphasic mixture
in which the bright blue fluorescence was localized in the
fluorous layer (photograph in Scheme 2). Removal of the
organic layer, followed by washing of the fluorous layer with
methanol, acetone, and ethyl acetate gave P1 in 87% yield.
This constitutes a first example of a fluorous biphase synthesis
of a conjugated polymer. The polymer obtained in this
blue-shifted relative to P2. A small (5–6 nm) Stokes shift and
sharp absorption and emission spectra of P1 suggest that the
structure of the polymer in solution is highly rigid. Both P1
and P2 are highly fluorescent. Fluorous P1 has a quantum
yield of 0.95 in perfluorodecalin and P2 has a quantum yield
of 0.84 in toluene. Furthermore, both polymers exhibit high
quantum yields in thin films (0.32 for P1 and 0.42 for P2). The
lower thin-film quantum yield of P1 relative to that of P2
could be associated with the flatter geometry of the comono-
mer 6 relative to 7, which results in a higher degree of
aggregation for P1 in the solid state than for P2.
To compare these properties to those of a nonfluorinated
polymer, a new polymer, P3, that features a rigid, three-
dimensional architecture and a dialkylaryl moiety in the
backbone, was synthesized (Scheme 3; see Figure S1 in the
Supporting Information for the solution absorption and
emission spectra). This polymer showed a reduced quantum
yield in solution (0.48 in toluene) compared to P1 and P2. The
thin-film emission spectrum of P3 showed a broad, red-shifted
peak, suggesting a large degree of aggregation, whereas the
thin-film emission spectra of P1 and P2 did not display
significant shifts from their respective solution spectra (Fig-
ure S2 in the Supporting Information). Also, P3 was insoluble
in fluorous solvents.
Monomer 1 is soluble in organic solvents including
acetone, hexane, chloroform, ethyl acetate, and THF, and
insoluble in nonpolar fluorous solvents (such as perfluoro-
hexane (FC-72), perfluoromethylcyclohexane, and perfluoro-
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Figure 3. a) Absorption (dotted line) and emission (solid line) spectra
of the emulsion of a perfluorodecalin solution of P1 in pH 7.4 PBS
buffer (quantum yield 0.58), and a schematic representation of the
emulsion particle; b) photograph of the emulsion before (left) and
after (right) irradiation with a hand-held UV lamp.
Scheme 3. Structure and synthesis of PPE polymer P3.
decalin). Polymer P1 is soluble in these fluorous solvents, but
is insoluble in organic solvents. The exclusive solubility of P1
in fluorous solvents made it difficult to determine the degree
of polymerization by gel permeation chromatography (GPC)
analysis. Dynamic light scattering (DLS) analysis showed that
the average length of P1 is 16 nm, a value that corresponds to
the typical persistence length of long PPEs.[8] DLS measure-
ments in perfluorodecalin also showed similar length distri-
butions. When P1 was end-capped with 1-bromo-4-tert-
butylbenzene, no signals corresponding to the tert-butyl
We have synthesized two PPEs from a novel, highly
fluorinated building block 1 and have demonstrated that,
depending on the choice of the comonomer, the solubility
properties of the materials could be changed drastically. Both
polymers were highly fluorescent both in solution and in thin
films. The fluorous-phase-soluble PPE, P1, could be pro-
cessed into a stable emulsion in PBS buffer (pH 7.4). The
emulsion involves a nontoxic fluorous solvent, is highly
fluorescent, and has functional groups on the surface, which
could be further modified. Further work on the attachment of
biologically active molecules to the surface of the emulsion is
under way.
1
group were observed in the H NMR spectrum, indicating a
high degree of polymerization (> 20). Polymer P2, which is
soluble in organic solvents, could be analyzed by GPC, and
was shown to have Mn = 520 kDa, Mw = 2850 kDa, and PDI =
5.48.
Received: May 22, 2010
Revised: July 7, 2010
Published online: September 2, 2010
In order to facilitate the imaging and sensory applications
of our fluorous-phase soluble fluorescent polymer, we
processed fluorous solutions of P1 into a stable emulsion in
water with easily modifiable functional groups on the surface.
Perfluorodecalin,[9] which has been approved by the US Food
and Drug Administration (FDA) for use as a component in
human blood surrogate, was chosen as the fluorous compo-
nent of the emulsion. When a solution of P1 in perfluorode-
calin was added slowly to a hot aqueous solution of
2H,2H,3H,3H-perfluorononanoic acid under probe sonica-
tion, a turbid and strongly fluorescent emulsion formed. Upon
cooling, a relatively monodisperse emulsion was obtained
with an average diameter of 245.8 nm and a polydispersity
index (PdI) of 0.099 as determined by DLS. The emulsion
displayed absorption and emission spectra (Figure 3) identi-
cal to those of P1. The emulsion prepared in phosphate-
buffered saline (PBS) buffer was highly fluorescent, with a
quantum yield of 0.58. The Z-potential is a useful method for
the measurement of the stability of colloids in water, because
a higher surface charge discourages the aggregation of
particles. Colloids with a surface potential of Æ 40 mV and
higher are considered to have a good stability.[10] An emulsion
of the perfluorodecalin solution of P1 in water had a Z-
potential of À57 mV with 13.2 mV deviation, thus indicating
that the surfaces of the emulsion are sufficiently charged to
confer good stability to the overall emulsion.
Keywords: biphasic systems · emulsions · fluorescence ·
perfluorinated solvents · polymerization
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