Burrows, H. D.

Lobo, V. M. M.

Melo, J. Seixas de

Pina, J.

Pradhan, S.

Ramos, M. L.

Scherf, U.

Tapia, M. J.

Valente, A. J. M.

Macromolecules

Estudo Geral

Fluorescence Enhancement of the Water- Soluble Poly{1,4-phenylene-[9,9-bis- (4-phenoxybutylsulfonate)]fluorene-2,7-diyl} Copolymer in n-Dodecylpentaoxyethylene Glycol Ether Micelles

Communications to the Editor
Fluorescence Enhancement of the Water-
Soluble Poly{1,4-phenylene-[9,9-bis-
(4-phenoxybutylsulfonate)]fluorene-2,7-diyl}
Copolymer in n-Dodecylpentaoxyethylene
Glycol Ether Micelles
H. D. Burrows,*,† V. M. M. Lobo,† J. Pina,†
M. L. Ramos,† J. Seixas de Melo,† A. J. M. Valente,†
M. J. Tapia,*,‡ S. Pradhan,§ and U. Scherf§
Departamento de Quı´mica, Universidade de Coimbra,
3004-535 Coimbra Portugal; Departamento de Quı´mica,
Universidad de Burgos, Plaza Misael Ban˜uelos,
Burgos 09001, Spain; and Makromolekulare Chemie,
Bergische Universita¨t Wuppertal, D-42097, Germany
Received June 20, 2004
Interest is increasing in water-soluble conjugated
polymers in areas such as chemical/biological sensors1,2
and self-assembly systems.3 In addition, introduction of
ink jet printing for fabricating light-emitting diode
(LED) displays4 will increase demand for these water-
based systems. Marked changes in fluorescence occur
on adding surfactants.1b,2c Both surfactant complexation1b
and breakup of polymer aggregates2c have been pro-
posed as explanations. The high fluorescence yields and
blue emission of polymers involving 2,7-linked fluorene
units make them among the most attractive classes of
conjugated polymers for devices5 and good candidates
for the above applications. We report the effect of the
nonionic n-dodecylpentaoxyethylene glycol ether (C12E5)
on properties of the water-soluble poly{1,4-phenylene-
[9,9-bis(4-phenoxybutylsulfonate)]fluorene-2,7-diyl} co-
polymer (PBS-PFP, Figure 1B). The polymer (Mh n)
6500 g mol-1) was synthesized by condensation of 2,7-
dibromo-9,9-bis(4-sulfonylbutoxyphenyl)fluorene (A) and
1,4-phenylenediboronic acid using Pd(PPh3)4 as catalyst.
Details of synthesis and characterization are given in
the Supporting Information.
Aqueous solutions of PBS-PFP showed a broad
absorption around 381 nm and a structured fluorescence
(ìmax 424, 448, 475 nm (shoulder)). Band shapes are
similar to the related copolymer poly[2,7-(9,9-bis(2′-
ethylhexylfluorene)-alt-1,4-phenylene] (PFP).6 Upon ad-
dition of C12E5 (3.3  10-8-5.34  10-4 M) to PBS-
PFP (6 mg/L, 9.2  10-7 M),7 blue shifts in absorption
(11 nm) and emission (13 nm, Figure 2) maxima, and
marked increases in absorbance and fluorescence in-
tensity were observed.
With a poly(ethylene oxide) of similar size to C12E5
headgroup (PEG 200 e 2.3  10-4 M), only a decrease
in fluorescence intensity was observed, with no change
in band shape, showing the importance of the surfac-
tant. Fluorescence quantum yields (…f), lifetimes, and
shifts in emission maxima are shown as functions of
surfactant concentration (Figure 3).7 Although the decay
should strictly be treated as biexponential, with the time
resolution of our system and the relatively small dif-
ference in the lifetimes, it was not possible to obtain a
statistically significant separation into two exponentials,
and the decay was taken as monoexponential.
All these properties change at surfactant concentra-
tions greater than the C12E5 critical micelle concentra-
tion (cmc, (5 ( 2)  10-5 M),8,9c suggesting they result
from incorporation of polymer into micelles. PBS-PFP
† Universidade de Coimbra.
‡ Universidad de Burgos.
§ Bergische Universita¨t Wuppertal.
* Corresponding authors. E-mail: burrows@ci.uc.pt; mjtapia@
ubu.es.
Figure 1. Synthetic scheme and structure of poly{1,4-
phenylene-[9,9-bis(4-phenoxybutylsulfonate)]fluorene-2,7-
diyl}.
Volume 37, Number 20 October 5, 2004
© Copyright 2004 by the American Chemical Society
10.1021/ma048780+ CCC: $27.50 © 2004 American Chemical Society
Published on Web 09/10/2004
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is a polyelectrolyte, and this behavior contrasts with
that of many surfactant-polyelectrolyte systems,9 in-
cluding some water-soluble conjugated polymers,1 where
interaction occurs at surfactant concentrations (critical
aggregation concentrations, cac) below the cmc.9
These effects can be explained either by changes in
environment resulting from surfactant complexation1b
or by surfactant-induced breakup or formation of poly-
mer aggregates.2c Fluorescence quantum yields in water
may be an order of magnitude lower than in nonpolar
solvents,10 and the effect of changing polarity on the
polymer nonradiative processes by incorporation into
micelles may explain these differences. However, we feel
this is not the dominant effect, since the lowest energy
absorption band of fluorene is relatively insensitive to
solvent polarity,11 in contrast to the observed pro-
nounced blue shift, and we feel breaking up of aggre-
gates explains both this shift and the increased quan-
tum yield. With aggregates, similar interchain interac-
tions are present to those in polyfluorene films, and
breaking these up by incorporation into micelles is
equivalent to going from thin films to isolated polymer
chains. The differences between emission spectra and
fluorescence quantum yields with PBS-PFP in water
and C12E5 micelles are similar to those seen with fluor-
ene copolymers between thin films and cyclohexane
solutions,6 supporting this model. Breakup of an ag-
gregate of a water-soluble poly(p-phenylenevinylene) on
adding an oppositely charged surfactant has been
confirmed by small-angle neutron scattering.12
Since PBS-PFP is a polyelectrolyte, electrical con-
ductivity may be informative. This was studied in
water,7 and the molar conductivity (⁄) was calculated
using
where  and 0 are electrolytic conductivities of solution
of molar concentration c and solvent, respectively. The
experimental electrolytic conductivity is around 3.5 
10-5 S m-1. The molar conductivity depends on the
square root of concentration, in agreement with the
Kohlrausch equation13
(⁄0 ) 0.362 ( 0.011 S m2 mol-1, A ) 8.50 ( 0.27 S
mol-3/2 m7/2). The molar limiting conductivity ⁄0 is
similar to that of other polyelectrolytes in water.14
On adding C12E5, ionic conductivity increases with
concentration (Figure 4). Two distinct changes were
observed. At very low surfactant concentrations (see
inset), there was a change in the slope of the plot, which
we believe is associated with initial interactions between
PBS-PFP and the surfactant. The second change occurs
Figure 2. Normalized fluorescence spectra of PBS-PFP (6
mg L-1) in aqueous solution: alone (solid line) and with C12E5
(5.3  10-4 M, dotted line).
Figure 3. Changes in (a) quantum yield (bold circles) and
emission maxima (open circles) and (b) fluorescence lifetime
for aqueous solutions of PBS-PFP with C12E5 concentration.
For quantum yields, solutions were adjusted to have absorp-
tion e0.1 at excitation wavelength.
Figure 4. Molar conductivity of PBS-PFP solutions as a
function of surfactant concentration. Note the change in slope
at the cmc. Inset: changes observed at very low surfactant
concentrations.
⁄ ) ( - 0)/(c  1000) (1)
⁄ ) ⁄0 - Ac1/2 (2)
7426 Communications to the Editor Macromolecules, Vol. 37, No. 20, 2004
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around the surfactant cmc, where an increase in the
slope is observed. While the mobility of the polymer may
be expected to be reduced on micellization, PBS-PFP
molecules will be distributed between the different
micelles, until at the limit there will be a maximum of
one polymer per micelle. Associated with each polymer/
surfactant micelles will be the corresponding counter-
ions, and the equilibrium between these and bulk
solution is expected to lead to an increase of the free
counterion concentration and, hence, molar conductivity.
In support, qualitative studies using both ultrafiltration
and ion selective electrodes do show increased free
sodium ion concentrations above the cmc.7
In water, C12E5 forms elongated cylindrical micelles,
which grow with concentration, temperature, or solute
incorporation.15 Support for incorporation of polymer
into such micelles comes from 1H NMR spectra. The
aromatic protons appear in the region ä 6.8-7.9 ppm,
and with PBS-PFP in D2O solutions (0.152 g/L) these
appear as relatively sharp signals. However, on adding
C12E5 (3.4  10-4 M), they become broadened (see
Supporting Information). This broadening is not ob-
served in spherical micelles but is common in large,
rodlike micelles and results from slow tumbling.16
We have shown that dramatic increases in the fluo-
rescence quantum yield occur in PBS-PFP on incorpo-
ration into C12E5 micelles and at low surfactant con-
centrations provide a stable medium for the polymer in
aqueous solutions. Further, the surfactant C12E5 forms
a variety of phases in two- and three-component sys-
tems, including lamellar, hexagonal, and cubic bicon-
tinuous structures.17 Incorporation of PBS-PFP into
these structures may be important for sensor applica-
tions and can produce structures, e.g. columnar meso-
phases, with interesting properties.
Acknowledgment. The Universidad de Burgos
(M.J.T.), POCTI, FCT, and FEDER are thanked for
financial support.
Supporting Information Available: Details of synthesis
and characterization and NMR spectra of PBS-PFP in D2O
solutions. This material is available free of charge via the
Internet at http://pubs.acs.org.
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MA048780+
Macromolecules, Vol. 37, No. 20, 2004 Communications to the Editor 7427
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http://estudogeral.sib.uc.pt/jspui/bitstream/10316/10454/1/Fluorescence%20Enhancement%20of%20the%20Water.pdf

Fluorescence Enhancement of the Water- Soluble Poly{1,4-phenylene-[9,9-bis- (4-phenoxybutylsulfonate)]fluorene-2,7-diyl} Copolymer in n-Dodecylpentaoxyethylene Glycol Ether Micelles

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