Synthesis and single chain fluorescence of a sulfonated conjugated polymer

Article abstract of DOI:10.1071/CH09348

A water-soluble sulfonated conjugated phenylene vinylene polymer (Alt-Co-DPS-PPV) has been prepared by a HornerEmmons reaction involving the condensation of a disulfonyl aryl bisphosphonate and terephthaldehyde. The photoluminescence from single chains of

Full text of DOI:10.1071/CH09348

Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2009, 62, 1577–1582
Synthesis and Single Chain Fluorescence of a Sulfonated
Conjugated Polymer
Daniel E. Gómez,A Tina A. T. Tan,A Jonathan M. White,A Toby D. M. Bell,A
and Kenneth P. GhigginoA,B
ASchool of Chemistry, Bio21 Institute, The University of Melbourne, Parkville,
Vic. 3010, Australia.
BCorresponding author. Email: ghiggino@unimelb.edu.au
A water-soluble sulfonated conjugated phenylene vinylene polymer (Alt-Co-DPS-PPV) has been prepared by a
Horner–Emmons reaction involving the condensation of a disulfonyl aryl bisphosphonate and terephthaldehyde. The
photoluminescence from single chains of the polymer dispersed in a poly(vinyl alcohol) film at room temperature has
been studied by confocal imaging techniques. Fluorescence intensity trajectories from Alt-Co-DPS-PPV single chains
exhibit intermittencies or ‘blinking’ on both short (sub-millisecond) and long (seconds) time scales. The short time-scale
intensity fluctuation data has been successfully analyzed using a simple model involving excursions from the fluorescent
excited singlet state of the polymer chromophore to a non-emitting triplet state. It is found that the triplet state lifetime is
molecule dependent and can be perturbed by the presence of the added heavy atom, iodide.These single chain fluorescence
studies provide an insight into the distribution of excited state behaviours that make up the bulk polymer photochemical
properties.
Manuscript received: 19 June 2009.
Manuscript accepted: 28 August 2009.
Introduction
the quasi-chromophores that make up the chain) to the popula-
tionofexcitedsingletstatesthatcanthenfluoresce.[5,6] However,
it is known that triplet electronic states typically exist at lower
energies than the corresponding singlet state in π-electron con-
taining molecules. A triplet electronic state is populated by a
non-radiative intersystem crossing process from the singlet state
that competes with fluorescence, and results from electron spin
inversion. There have been relatively few studies reported on
conjugated polymer triplet states because their yield of for-
mation is typically low and they are usually non-emitting at
room temperature.[7] However, due to their longer excited state
lifetimes and higher reactivity, triplet states may play an impor-
tant role in the overall photochemistry and performance of
conjugated polymers.
A polymer sample is inherently heterogeneous at the molecu-
lar level usually being polydisperse (i.e. consisting of a distribu-
tion of molecular weights) arising from the synthetic procedures
required, and characterized by a distribution of conformations
due to the conformational flexibility of polymer chains. Bulk
polymer properties measured by conventional methods are thus
an average of individual molecular behaviours that make up the
sample (often described, for example, by an average molecular
weight and average molecular dimensions). It can be expected
that bulk photophysical properties will also be an average
of some distribution of individual single chain excited state
behaviours.
Organic semiconductors and, in particular, conjugated polymers,
are a class of materials that due to their low cost, ease of fab-
rication and processing, are realizing their potential to become
the building blocks of new generation displays and photovoltaic
devices.[1–4] These applications rely on the special optical and
photophysical properties of these materials, which in turn are
intrinsically related to their electronic structure. Understand-
ing the electronic excited state behaviour of these interesting
polymers is thus a key step in developing optimized chemical
structures and conditions for their use in practical applications.
Conjugated polymers, such as the commercially important
poly[2-methoxy-5-(2ꢀ-ethyl-hexyloxy)-1,4-phenylene vinylene]
(MEH-PPV, see Fig. 1), contain delocalized π-electrons along
segments of the polymer chain that constitute the polymer
chromophores. Absorption of light by these polymer chromo-
phores ultimately leads (usually via rapid energy transfer among
ϩKϪO3S
O
O
n
O
n
Advances in imaging and photon detection methods have
enabled the observation of fluorescence from single molecules
by optical microscopy techniques.[8–10] In these experiments
molecules of a sample are typically dispersed at very low con-
centration (typically 10−9 mol dm−3 or less) in a transparent
SOϪ3 Kϩ
OMe
MEH-PPV
Alt-Co-DPS-PPV
Fig. 1. Structures of MEH-PPV and Alt-Co-DPS-PPV.
© CSIRO 2009
10.1071/CH09348
0004-9425/09/121577

1578
D. E. Gómez et al.
1.2
1
1.2
1
polymer matrix so they are immobilized and spatially separated
on a micron scale.The fluorescence from individual molecules is
then observed by confocal fluorescence microscopy, a technique
that provides the required spatial resolution. When combined
with spectral dispersion of the fluorescence, pulsed excitation
sources and photon counting detection methods, the method has
developed into a powerful tool for studying individual molecule
photophysical behaviour.
Single molecule fluorescence methods have been applied
previously to the study of conjugated polymers.[11–16] How-
ever most studies have concentrated on the major commercially
available conjugated polymer (MEH-PPV) dispersed in organic
solvent soluble polymer matrices such as poly(methyl methacry-
late) or poly(styrene). Many such studies have reported that
fluorescence from single polymer chains show intermittencies or
‘blinking’ under steady illumination.[11–16] The blinking occur-
ringontimescalesofsecondshasbeenattributedtothereversible
formation of fluorescent quenching species in the polymer
molecule arising from oxidation and/or formation of quenching
polymer polarons following charge separation.[11,14]
Our studies are concerned with the development of water-
soluble conjugated polymers that may find application as novel
fluorophores, water-soluble fluorescent sensors, and in thin film
optoelectronic devices. As part of this work we have examined
their single molecule fluorescence properties when dispersed in
water-soluble polymer matrices and describe here an analysis
of fast (sub-millisecond) time scale fluorescence ‘blinking’ that
is observed. This behaviour is attributed to excursions to non-
fluorescent triplet states of the polymer and it is shown that this
process is both molecule and environment dependent.
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
270 320 370 420 470 520 570 620 670
Wavelength [nm]
Fig. 2. Normalized absorption (left) and fluorescence (right) spectra of
Alt-Co-DPS-PPV in water.
1200
1000
800
600
400
200
0
0
2
2
µm
The structures of MEH-PPV and the polymer synthesized
(see Experimental section) and studied in the present work
are shown in Fig. 1. This water-soluble PPV derivative has
an alternating arrangement of dipropoxysulfonated and non-
sulfonated aryl moieties as part of the conjugated backbone
and is designated as Alt-Co-DPS-PPV (an alternating copoly-
mer of poly[p-(phenylene vinylene)-(2,5-dipropoxysulfonate-p-
phenylene vinylene)]).
µm
4
4
Fig. 3. Scanned fluorescence intensity image of a 5 × 5 micron area
of an Alt-Co-DPS-PPV/PVA film showing intensity peaks arising from
fluorescence of single polymer chains.
attributed to weakly emitting contaminants in the PVA matrix).
Fig. 4 illustrates a representative fluorescence intensity-time tra-
jectory of a single Alt-Co-DPS-PPV molecule. It is evident in
Fig. 4 (lower trace) that the fluorescence from the single poly-
mer chain ‘turns off’ for quite long periods of time (e.g. for
∼10 s in Fig. 4) and then ‘turns on’. These long ‘blinks’ have
been attributed in studies of other conjugated polymers, such
as MEH-PPV to the reversible formation of species, for exam-
ple, oxidation products or hole-polarons that strongly quench
the fluorescence.[11,14] The origin of these long blinks in the
Alt-Co-DPS-PPV/PVA sample will be the topic of a separate
investigation and not discussed further here.
Closer inspection of the data at a higher time resolution
(where the fluorescence signal is accumulated in smaller time
intervals, see upper trace in Fig. 4) reveals that superimposed
on the events where the intensity appears above the background
level, there are in fact much faster fluctuations occurring in the
emission count rate. The temporal scale on which these fluctua-
tions take place can be analyzed by generating the second-order
emission intensity autocorrelation function (ACF, Eqn 1).[13]
Results and Discussion
The bulk absorption and emission spectra of Alt-Co-DPS-PPV
in water are shown in Fig. 2. Under these conditions we have
determined the fluorescence quantum yield to be 0.12 with
excitation at the absorption spectrum maximum (365 nm).
A very dilute solution (∼10−9 mol dm−3) of Alt-Co-DPS-
PPV in water with 6% wt/wt poly(vinyl alcohol)(PVA) was spin-
coated onto a clean glass microscope coverslip. The resulting
PVA film, ∼100 nm in thickness, containing the dispersed Alt-
Co-DPS-PPV molecules, was then placed on a piezo-scanning
stage of a fluorescence microscope operating in a confocal
arrangement. The instrument constructed in our laboratories for
the single molecule fluorescence measurements is described in
the Experimental section. Diffraction limited fluorescent ‘spots’
from single chains of the polymer could be readily observed
while scanning the microscope stage. The position of the stage
could be computer-controlled with sub-micron resolution so that
the intensity of fluorescence from an individual chain could be
isolated and monitored by the avalanche photodiode detector of
the instrument.
ꢁI(t)I(t + tꢀ)ꢂ
C2(tꢀ) =
(1)
ꢁI(t)2ꢂ
Fig. 3 is an image of a 5 × 5 micron scanned area of the sam-
ple clearly showing two strong intensity peaks associated with
emission from two individual single Alt-Co-DPS-PPV poly-
mer chains (the much lower intensity background peaks can be
The fluorescence intensity as a function of time (I(t)) and
a time-shifted version (I(t + tꢀ)) are used to generate the ACF

Synthesis and Single Chain Fluorescence of a Sulfonated Conjugated Polymer
1579
18
12
6
0
26.93
26.94
26.92
26.95
26.91
26.96
26.97
600
400
200
0
0
10
20
Time [s]
30
40
Fig. 4. Fluorescence intensity-time trace of a single Alt-Co-DPS-PPV chain embedded in a PVA film. The sample
was kept under a constant flow of dry nitrogen gas and was excited with 2 µW of laser power (466 nm). The upper
trace is an expanded region of the lower trace (over the time region 26.91 to 26.97 s) showing the faster intensity
fluctuations that are present during the apparent ‘on’ periods in the lower trace data.
−1
where τ−1 = (τoff
)
+ (τon)−1 and A = τoff · (τon)−1, where τoff
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
and τon are the ‘off’ time constant (i.e. duration of interruptions
to fluorescence) and ‘on’ time constant (i.e. duration between
interruptions) respectively.
Exp
Model
A reasonable hypothesis is that the fast fluctuations in the
emission count rate arise from transitions to a non-emitting
triplet state of the polymer.[13] In this case τoff can be associated
with the triplet state lifetime (276 µs for the single molecule
dataset in Fig. 5). Formation of a non-fluorescent triplet state
from the singlet state upon some excitation cycles of the polymer
would be expected to lead to transient blinking as the polymer
fluorescence switches ‘off’ and cannot recover until the ground
state is repopulated by normal triplet state decay process and
excitation of the chromophore occurs again. It can be expected
that the presence of a triplet state in a particular chromophore
of the polymer chain is also likely to lead to quenching of any
other excited singlet state that might be formed in the poly-
mer during the triplet state lifetime due to rapid triplet-singlet
annihilation.[13]
10Ϫ6
10Ϫ5
10Ϫ4
Time lag [s]
10Ϫ3
10Ϫ2
Fig. 5. Autocorrelation function of the segment of the time trace from 14 to
33 s in Fig. 4.The line through the data points is the fit to a single exponential
function with an amplitude of 1.76 and a time constant of 0.1 ms.
In order to confirm that excursions to the triplet state
are indeed the origin of the rapid blinking observed, Alt-Co-
DPS-PPV was dispersed in a PVA matrix containing a high
concentration (50 mg g−1 PVA) of potassium iodide in order to
incorporate the ‘heavy atom’I− in the substrate. Iodide ions have
been shown[17,18] to enhance intersystem crossing rates between
singlet and triplet states in solutions of aromatic molecules by
perturbing spin-orbit coupling of the luminescent species. As
shown in Fig. 6, the time constant (τ) of the ACF was indeed
found to dramatically decrease in value with the addition of
iodide ions to the PVA matrix. Addition of chloride ions at
the same concentration had no influence on the blinking rate
confirming that specific iodide perturbations were responsible
for the effects observed. There was negligible effect on the
blinking rate when the films were exposed to air rather than
(C2(tꢀ)) and the outer diagonal brackets represent a time average.
The ACF reflects the magnitude of the repetitive behaviour in
the intensity fluctuations as a function of the time scale of the
process. An example of the ACF generated from the intensity
fluctuation data for the polymer chain fluorescence observed
during the time region of ∼14–33 s of Fig. 4, is shown in Fig. 5
(noting that this time region of the trace does not include any of
the long ‘off-time’ blinking events).
The solid line drawn through the data in Fig. 5 is a fit to a
single exponential function of the form (Eqn 2):
ꢀ
ꢁ
t
C2(t) = A exp
−
+ 1,
(2)
τ

1580
D. E. Gómez et al.
1.0
It is likely that the distribution of fluorescence blinking
behaviour (and associated triplet state decay rates) among
polymer chains of the sample reflects a distribution of chromo-
phore environments within the PVA matrix as well as a distri-
bution in the electronic energy states of the chromophores from
which the emission arises. The chromophores within MEH-PPV
have been reported to be composed of lengths of conjugated
backbone of ∼10–15 repeat units.[14] The distribution of conju-
gated sequence lengths is determined by the presence of polymer
conformational defects that break the conjugation of the poly-
mer backbone. It is not surprising that the electronic energy
states of the chromophores in particular polymer chains can
be different and thus the associated photophysics (including
triplet state lifetimes) can also be expected to exhibit distributed
behaviour.
IϪ
PVA
0.8
0.6
0.4
0.2
0.0
10Ϫ5
10Ϫ4
τ [s]
10Ϫ3
10Ϫ2
The single molecule fluorescence of the Alt-Co-DPS-PPV
sulfonated conjugated polymer studied in this work can be
compared with related studies of organic soluble conjugated
polymers that have found application in photovoltaic and light
emitting devices.[11–16] Rapid (sub-millisecond) fluorescence
blinking observed in MEH-PPV (Fig. 1) single polymer chains
incorporated in 100 nm thick poly(methyl methacrylate) films
(overlaid with a 200 nm gold film to reduce the oxygen concen-
tration) has also been attributed to triplet exciton formation,[13]
while the longer (seconds) blinking reported for the same poly-
mer is thought to arise from singlet state quenching by oxidation
products and hole-polarons.[11–16] The close correspondence
with the emission behaviour of the sulfonated conjugated poly-
mer in the PVA matrix suggests similar excited processes are
likely to be operative. The sulfonated conjugated polymers may
thus be useful materials for optoelectronic device fabrication and
this aspect is currently being explored.
Fig. 6. Normalized autocorrelation function as calculated from fluo-
rescence intensity-time trajectories of individual Alt-Co-DPS-PPV chains
embedded in PVA matrices in the absence (squares) and presence (connected
circles) of added KI (concentration 50 mg g−1 PVA).
22
20
20
18
16
15
14
12
10
10
8
6
Conclusion
5
4
Fluorescence from single chains of the water-soluble conju-
gated polymer Alt-Co-DPS-PPV shows fluorescence blinking
on sub-millisecond time scales. The time scale of the intensity
fluctuations and their susceptibility to perturbation by the heavy
atom, iodide, provide good evidence that excursions to the triplet
state are responsible for the rapid blinking.The triplet state decay
timeisshowntovarybetweenpolymerchainsdemonstratingthat
the ensemble photophysical properties of this polymer, like other
conjugated polymers, is composed of a distribution of individual
polymer chain behaviours.
2
0
0
0
2
4
6
0.2
0.4
τ [ms]
0.6
Amplitude [a.u.]
Fig. 7. Histograms showing the distribution of time constants (left) and
amplitudes (right) extracted from fits to Eqn 1 of the fast fluctuations in
fluorescence intensity for Alt-Co-DPS-PPV chains in PVA.
a nitrogen atmosphere and this can be attributed to the well
known impermeability of PVA films to oxygen, a known triplet
quencher, and its low concentration in the substrate. In Fig. 6 the
amplitude of the ACFs determined in the presence and absence
of KI were normalized in order to highlight the reduction in the
characteristic time of the fluctuations in the emission intensity
that occur as a consequence of the interactions of the polymer
chromophores with nearby added iodide.
The rate of blinking was found to vary among different poly-
mer chains within a sample. The histograms presented in Fig. 7
summarize the distribution of amplitudes and time constants
extracted from fitting the ACF of the fluorescence trajectories
of 55 individual chains to Eqn 2. The ‘average’ triplet state life-
time extracted from this distribution is 250 µs, while a similar
analysis of polymer chain blinking in the presence of iodide
was 50 µs emphasizing the role of the additive in enhancing the
non-radiative intersystem crossing process to the ground state.
Experimental
Alt-Co-DPS-PPV was synthesized using a Horner–Emmons
reaction involving the condensation of the appropriate disulfonyl
aryl bisphosphonate and an aryl bisaldehyde (terephthaldehyde)
(see Fig. 8 and with full details in the Accessory Publica-
tion). The aqueous soluble polymer was purified using dialysis
against water with dialysis tubing of molecular weight cut-off
12400 g mol−1. Gel-permeation chromatography of the poly-
mer yielded an average molecular weight of 22000 g mol−1 (see
Accessory Publication).
For single chain spectroscopy, the samples were prepared
by first adjusting the concentration of Alt-Co-DPS-PPV to
∼1 nM by serial dilution with water (milliQ) that contained
PVA (at a concentration of ∼6% wt/wt), and later by spin-
coating onto clean glass coverslips. The final dry polymer films

Synthesis and Single Chain Fluorescence of a Sulfonated Conjugated Polymer
1581
O
SO2Cl
SO2Cl
SOϪ3 Naϩ
OH
O
O
S
O
O
SOCl2/DMF
NaOH/MeOH
N2
O
SOϪ3 Naϩ
OH
O
1
2
O
SO2F
SO2F
O
SO2F
HCl(g)/HCHO
1,4-Dioxane
P(OEt)3
Reflux
KF, 18-crown-6 ether
Acetonitrile
Cl
Cl
O
O
SO2F
3
4
O
SO2F
ϩKϪO3S
O
O
CHO
OHC
n
PO(OEt)2
tBuOK, DMF
(EtO)2OP
SOϪ3 Kϩ
O
SO2F
Alt-Co-DPS-PPV
Fig. 8. Reaction scheme for the synthesis of Alt-Co-DPS-PPV.
5
Diode laser
were ∼100 nm thick. This film thickness minimizes molecule-
interface effects (due to location of molecules near the film
surface) that can be apparent for thinner films, while maintaining
a low probability of polymer chain overlap within the con-
focal imaging volume. For the studies of the effect of added
ions, solutions of PVA were prepared with the addition of KI at
concentrations of 50 mg g−1 PVA.
Bulk fluorescence spectra were recorded on a Horiba
Jobin-Yvon Fluorolog (FL-1057) fluorimeter and fluorescence
quantum yields determined using quinine bisulfate in 1 N H2SO4
as the fluorescence reference standard (fluorescence quantum
yield of 0.55).[19]
Piezo stage
Pinhole and
beam expander
High NA objective
Dichroic mirror
The detection of individual polymer chains was achieved
by means of sample-scanning confocal microscopy using
instrumentation established in our laboratories. A schematic
of the instrument is provided in Fig. 9. The excitation source
consisted of a pulsed laser diode operating at 40 MHz repeti-
tion rate (λ = 466 nm, LDH-P-C-470 PicoQuant, Gmbh). After
passing through a spatial filter and a beam expander/collimator,
the laser light was sent to the back port of an Olympus IX71
inverted microscope, where it reflects from a dichroic mirror
(z476rdc, Chroma Tech Corp) into an oil-immersion objective
(100x, NA = 1.4, Olympus UPlanSApo). At the focal plane, the
Pinhole
CCD
Beam splitter
SPAD
Beam
splitter
Mirror
Filter
Lens
excitation intensity was estimated to be typically 1.5 kW cm−2
,
Router and
TCSPC card
SPAD
which is well below the excitation saturation intensity for conju-
gatedpolymers.Thefluorescencefromthesampleiscollectedby
the objective, filtered to remove any excitation light (HQ500 LP,
Chroma Tech Corp), and detected by a single photon detecting
avalanche photodiode (SPAD, Perkin–Elmer, SPCM-AQR-14)
after passing through a confocal pinhole (diameter 100 µm,
Thorlabs). The position of the single chains in the film is deter-
mined by raster-scanning the sample with a piezo-driven stage
(P-521.2CL with E-761 driver, Physik Instrumente Gmbh). A
Fig. 9. Schematic diagram of the confocal imaging microscope instrument
used for single polymer chain fluorescence studies.
single-photon counting card (TCSPC card, TimeHarp 200, Pico-
Quant Gmbh), allows the recording of the absolute arrival time
of each photon (with a nominal resolution of 100 ns) and its
detection time relative to the excitation pulse.

1582
D. E. Gómez et al.
Accessory Publication
[6] P. Wang, C. J. Collision, L. J. Rothberg, J. Photochem. Photobiol. A
2001, 144, 63. doi:10.1016/S1010-6030(01)00518-4
[7] H. D. Burrows, J. Seixas de Mello, C. Serpa, L. G. Arnaut, M. da G.
Miguel, A. P. Monkman, I. Hamblett, S. Navaratnam, Chem. Phys.
2002, 285, 3. doi:10.1016/S0301-0104(02)00684-5
The Accessory Publication contains full experimental details
for the synthesis of Alt-Co-DPS-PPV, available on the Journal’s
website.
[8] W. E. Moerner, J. Phys. Chem. B 2002, 106, 910. doi:10.1021/
JP012992G
Acknowledgements
[9] F. Kulzer, M. Orrit, Annu. Rev. Phys. Chem. 2004, 55, 585.
doi:10.1146/ANNUREV.PHYSCHEM.54.011002.103816
[10] F. C. De Schryver, T. Vosch, M. Cotlet, M. Van der Auweraer, K.
Mullen, J. Hofkens, Acc. Chem. Res. 2005, 38, 514. doi:10.1021/
AR040126R
[11] P. F. Barbara, A. J. Gesquiere, S. J. Park, Y. J. Lee, Acc. Chem. Res.
2005, 38, 602. doi:10.1021/AR040141W
[12] T. Pullerits, O. Mirzov, I. G. Schleblykin, J. Phys. Chem. B 2005, 109,
19099. doi:10.1021/JP052189C
T.D.M.B. acknowledges the award of the University of Melbourne Faculty
of Science Centenary Fellowship. K.P.G. and T.D.M.B. thank the Australian
Research Council for financial support of this research under Discovery
Project DP0664163. We also thank Professors Johan Hofkens and Frans
De Schryver, Katholieke University of Leuven, for their advice in estab-
lishing the single molecule instrumentation at the University of Melbourne.
We acknowledge the UNICHE Scheme for scholarship funding support (to
T.A.T.T.) and Ms Emma Hooley and Ms Su Khim Ling for assistance with
the photophysical measurements.
[13] J.Yu, R. Lammi, A. J. Gesquiere, P. F. Barbara, J. Phys. Chem. B 2005,
109, 10025. doi:10.1021/JP0506742
[14] D. Hu, J. Yu, P. F. Barbara, J. Am. Chem. Soc. 1999, 121, 6937.
[15] O. Mirzov, I. G. Scheblykin, Phys. Chem. Chem. Phys. 2006, 8, 5569.
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