Excited-state dynamics of water-soluble polythiophene derivatives: Temperature and side-chain length effects

Article abstract of DOI:10.1021/jp304526h

We report synthesis and detailed spectroscopic study of three water-soluble polythiophene derivatives with distinct homologous oligo(ethylene oxide) side-chain lengths and lower critical solution temperatures (LCSTs). The linear absorption spectra exhibit reversible shifts and broadening with the variation of their aqueous solution temperature, whereas the corresponding steady-state fluorescence emission spectra were found to show negligible shifts and only minor changes in their line shape. Measurements of picosecond time-resolved fluorescence at chosen emission wavelengths reveal a strong dependence of the isotropic decays on both side-chain length and aqueous solution temperature. With lengthening of the side chain, the isotropic decays become not only remarkably slow but also increasingly complex. Except for the polymer with the shortest side chain, significant acceleration of the isotropic decays was found when the solution temperature was raised to the corresponding LCSTs and beyond, which further causes formation of large aggregates as evident by the physical appearance change from clear solutions to turbid suspensions. Direct evidence for a temperature-induced change of polymer chain conformation was obtained through measurements of time-resolved fluorescence anisotropies, which are characterized by a substantial increase of the initial values from ～0.2 to 0.4 and the appearance of a pronounced fast decay component with an estimated lifetime of 36 ps. The high initial anisotropy of ～0.4 observed for the two polymers with longer side-chains above their LCSTs suggests that the polymer chains are highly ordered in the aggregates. The observed effects of side-chain length and solution temperature are discussed by considering the conformational relaxation of the polymer backbones and occurrence of interchain energy transfer.

Full text of DOI:10.1021/jp304526h

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Article
Excited-State Dynamics of Water-Soluble Polythiophene
Derivatives: Temperature and Side-Chain Length Effects
Yingzhong Ma, Robert W. Shaw, Xiang Yu, Hugh M. O'Neill, and Kunlun Hong
J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 09 Nov 2012
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Excited-State Dynamics of Water-Soluble
Polythiophene Derivatives: Temperature and Side-
Chain Length Effects
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Ying-Zhong Ma1*, Robert W. Shaw1, Xiang Yu1, Hugh M. O'Neill3, and Kunlun Hong2*
1Chemical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831
2Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge,
TN 37831
3Center for Structural Molecular Biology, Biology & Soft Matter Division, Oak Ridge National
Laboratory, P.O. Box 2008, Oak Ridge, TN 37831
Notice: This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC05-
00OR22725 with the U.S. Department of Energy. The United States Government retains and the
publisher, by accepting the article for publication, acknowledges that the United States Government
retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the
published form of this manuscript, or allow others to do so, for United States Government purposes.
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Abstract
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We report synthesis and detailed spectroscopic study of three water-soluble polythiophene
derivatives with distinct homologous oligo(ethylene oxide) side-chain lengths and lower critical
solution temperatures (LCSTs). The linear absorption spectra exhibit reversible shifts and
broadening with the variation of their aqueous solution temperature, whereas the corresponding
steady state fluorescence emission spectra were found to show negligible shifts and only minor
changes in their line shape. Measurements of picosecond time-resolved fluorescence at chosen
emission wavelengths reveal a strong dependence of the isotropic decays on both side-chain length
and aqueous solution temperature. With lengthening side-chain, the isotropic decays become not
only remarkably slow but also increasingly complex. Except for the polymer with the shortest side-
chain, significant acceleration of the isotropic decays were found when the solution temperature
was raised to the corresponding LCSTs and beyond, which further causes formation of large
aggregates as evident by the physical appearance change from clear solutions to turbid suspensions.
Direct evidence for a temperature-induced change of polymer chain conformation was obtained
through measurements of time-resolved fluorescence anisotropies, which are characterized by a
substantial increase of the initial values from ∼0.2 to 0.4 and the appearance of a pronounced fast
decay component with an estimated lifetime of 36 ps. The high initial anisotropy of ∼0.4 observed
for the two polymers with longer side-chains above their LCSTs suggests that the polymer chains
are highly ordered in the aggregates. The observed effects of side-chain length and solution
temperature are discussed by considering the conformational relaxation of the polymer backbones
and occurrence of inter-chain energy transfer.
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Keywords: anisotropy, conjugated polymers, excited-state dynamics, low critical solution
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temperature, polymer aggregates, time-resolved fluorescence
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Introduction
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Polythiophenes have emerged as one of the most versatile family of conjugated polymers because
of their excellent optoelectronic properties, high charge carrier mobility, good chemical/thermal
stability, and tunable solubility in organic solvents.1-3 These unique properties have enabled a wide
range of applications such as organic field-effect transistors,4-6 organic solar cells,7 chemical and
optical sensors,8 and electrochromic devices.9 A particularly exciting advance involves use of a
poly(3-hexylthiophene) (P3HT) polymer as both light absorber and electron-donating molecule in
bulk heterojunction solar cells, which has enabled a high power conversion efficiency of ∼ 5%.10
These novel properties and exciting applications have stimulated extensive efforts in recent years to
understand their optical properties,11-13 as well as the nature of their elementary photoexcitations,14
electronic excited-state dynamics,15-18 and exciton migration and photoinduced charge separation
in the presence of a fullerene electron acceptor such as [6,6]-phenyl C60 butyric acid methyl ester
(PCBM).19-23
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The insolubility of polythiophene polymers in water and aqueous media hinders their
applications in biological environments24 and prevents environmentally friendly processing.25
general strategy to enable their water solubility is to attach hydrophilic side chains with either
charged or neutral groups to the hydrophobic backbones of polythiophenes.26 The hydrogen-
A
bonding interactions between these side-chains and surrounding water molecules serve as the
driving-force for aqueous solubilization of these synthetic polythiophene derivatives.27-29 An even
more remarkable thermoresponsive property with a material-specific lower critical solution
temperature (LCST) can further arise from the amphiphilic character of these synthetic polymers,
i.e., coexistence of hydrophobic backbones and hydrophilic side-chains. Owing to a rapid
conformational change with increasing temperature,28 these polymers are soluble in water below
their characteristic LCSTs but aggregate and/or precipitate above the LCSTs. The corresponding
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conformational changes have been studied extensively for various thermoresponsive polymers (see
refs. 29,30 and relevant references cited therein). For instance, through application of a dynamic
light scattering technique, Lutz and coworkers found that the thermoresponsive copolymers of 2-(2-
methoxyethoxy)ethyl methacrylate and oligo(ethylene glycol) methacrylate adopt mostly a coiled
conformation in water with a hydrodynamic radius of ~3.7 nm below the LCST, but form large
aggregates with a hydrodynamic radius of ~300 nm above the LCST. These large aggregates are
further assumed to form stable mesoglobules that result from aggregation of collapsed dehydrated
chains.28,29 Similar sizes of aggregates have also been observed above the LCST for poly(N-
isopropylacrylamide), poly(N,N´-diethl acrylamide) and ploy(vinylmethyl ether).31-33 In contrast,
little is known about the molecular organization in aggregates, specifically, whether the polymer
molecules adopt backbone conformations that are significantly different from their solubilized
forms. Our understanding about the spectral characteristics and electronic excited-state dynamics
of the aggregates, and how they are distinct from the solubilized polymers below the LCST is also
limited. Picosecond time-resolved fluorescence and polarization measurements can provide useful
insight into these poorly understood questions; the knowledge so gained will likely not only further
our fundamental understanding about water-soluble polythiophene polymers in both solubilized
and aggregated forms, but also benefit the potential applications based on either their
thermoresponsive properties or/and water solubility.
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In this paper, we report synthesis of three poly(3-oligo(ethylene oxide)) thiophenes with distinct
side-chain lengths, denoted hereafter as EOx with x = 2, 3, and 4, which represents the number of
repeating EO groups. The different side-chain lengths provide control of both the water solubility
and LCSTs. We found that the linear absorption spectra undergo fully reversible changes (spectral
shift and broadening) with the variations of the aqueous solution temperature. The most drastic
spectral changes were observed at the corresponding LCSTs with a concomitant appearance of
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solution turbidity. Picosecond time-resolved fluorescence anisotropy measurements at
temperatures above LCSTs provide direct evidence for polymer backbone ordering in the
aggregates and occurrence of inter-chain energy transfer. Significant effects of the side-chain length
and solution temperature on the isotropic fluorescence decays were also identified.
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Materials and Methods
Polymer synthesis and sample preparation for spectroscopic experiments: The polythiophene
derivatives were synthesized based on the Kumada catalyst transfer polymerization, and the whole
reaction is outlined in Scheme 1.34 The polymers obtained were characterized using 1H-NMR and gel
permeation chromatography (GPC), and the results are summarized in Table 1. For all spectroscopic
measurements, the polymers were dissolved in deionized water, and fluorescence cuvettes with a 1-
cm path length were used. Control of the sample temperature was realized using a thermostatted
cuvette holder connected to either a thermoelectric heater/cooler or a water-circulating chiller. In
either case, the sample temperature was monitored directly inside the cuvette holder using a
thermocouple thermometer.
Measurements of linear absorption and steady-state fluorescence emission spectra: A Varian 5000 UV-
VIS-NIR absorption spectrometer was used to record the linear absorption spectra at different
temperatures. The corrected fluorescence emission spectra were measured using a Fluorolog-3
spectrofluorometer upon excitation at 445, 427 and 410 nm for EO2, EO3 and EO4, respectively. Typical
bandwidths for the excitation and emission monochromators were 5 and 2 nm, respectively.
Measurements of picosecond time-resolved fluorescence and anisotropy: A time-correlated single-
photon-counting (TCSPC) apparatus based on an actively quenched single photon avalanche
photodiode (PDM 50CT module, Micro Photon Devices) connected to a PicoQuant TCSPC system
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(PicoHarp 300, Picoquant) was employed. An optical parametric amplifier pumped by a 250 kHz
Ti:Sapphire regenerative amplifier was used as the excitation source. Excitation was at 490 nm with
typically 150 fs pulse duration and < 8 nJ pulse energy. Fluorescence emission was selected by using
a 10 nm (FWHM) bandpass filter centered at 620 nm for EO2 and EO3 and at 589 nm for EO4, which
were chosen as close to the peak wavelengths of the fluorescence emission spectra as possible. The
instrument response function (IRF) showed a FWHM of ∼ 40 ps as recorded at the excitation
wavelength using a dilute water suspension of coffee creamer. A 4.0-ps channel time was chosen
and typically more than 10,000 counts were collected in the peak channel in order to obtain an
acceptable signal-to-noise ratio. Isotropic fluorescence data were collected by setting the
polarization of the laser excitation at the magic angle (54.7°) with respect to an emission linear
polarizer. Measurements of time-resolved fluorescence anisotropy involves collecting data for four
polarization combinations of an excitation half-wave plate and an emission linear polarizer, and
calculating according the following relation,
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IVV (t) − GIVH (t)
r(t) =
IVV (t) + 2GIVH (t)
I (t)
I (t)
VH
where
and
are the fluorescence decays measured with parallel and perpendicular
I (t)
VV
polarizations, respectively. The G-factor was obtained by measuring two additional decays,
(t)
HV
I
G = I (t)dt
I
(t)dt
.
HH
and
, and calculating according to the relation
∫
∫
HH
HV
Results
Linear absorption and steady-state fluorescence emission spectra
Figure 1a shows the linear absorption spectra recorded for EO4 at multiple aqueous solution
temperatures ranging from 32 to 80 °C. It is clearly evident that the linear absorption spectra are
very sensitive to temperature. Even for this moderate temperature change, a redshift as large as 35
nm and a clear spectral broadening occurred. We further noticed that both the spectral shift and
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broadening could be fully reversed by simply lowering the temperature to the initial value (Figure
1b). This reversible spectral change can be better seen through plotting the peak wavelengths of the
absorption spectra as a function of aqueous solution temperature (Figure 1c). The two traces
corresponding to the heating (open circles) and cooling (filled squares) processes are
indistinguishable from each other within experimental uncertainty. It should be also pointed out
that the spectral shift, as can be clearly seen from Figure 1c, is not linear with respect to the
temperature change. It begins with either no or very little shift until the temperature reaches a
characteristic value. While qualitatively similar shifts and broadening were also observed for the
linear absorption spectra measured for both EO2 and EO3 at different temperatures, a hysteresis was
found in both cases in the cooling processes even though the spectral changes are fully reversed
eventually at the starting temperature of the corresponding heating-to-cooling circle (data not
shown). The characteristic temperatures at which significant spectral shifts occur are obviously
sample dependent. A more reliable comparison of these distinct characteristic temperatures can be
made using the temperatures at which 10% of the overall shift occurs, which were estimated to be
47, 51 and 60 °C for EO2, EO3 and EO4, respectively. We will consider these values as approximate
LCSTs of these three polymers in the following.
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When the aqueous solutions of these three polymers were heated to above their corresponding
LCSTs, large aggregates form as evident by the physical appearance change from clear solutions to
turbid suspensions. Except the redshift and broadening, the linear absorption spectra remain
essentially structureless as those observed below the LCSTs (see Figure 1a and b). These
structureless spectra are strikingly distinct from the absorption spectra reported for the crystalline
nanofibers formed through self-assembly of P3HT or poly(3-alkythiophene) in organic solvents,
where vibronic transitions such as 0-0 and 0-1 were clearly seen.11,35-37 Upon closer visualization of
the absorption spectra measured above the LCSTs (see Figure 2a for those recorded at 70 °C as an
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example), we further noticed that the spectral broadening increases with decreasing side-chain
length. This result indicates that the shorter the side-chain, the greater the spectral inhomogeneity.
In stark contrast to the strong temperature dependence of the absorption spectra, the
corresponding fluorescence emission spectra show not only negligible spectral shifts but also very
minor line shape changes within experimental uncertainty. As an example, we show in Figure 2b the
fluorescence emission spectra recorded for these polymers at 30 °C. However, the emission
intensity decreases with increasing temperature, with a dramatic reduction observed at
temperatures above the LCSTs. We further found that the Stokes shift decreases with increasing
side-chain length; at 30 °C the values determined for EO2, EO3 and EO4 are approximately 8300,
7900, and 7400 cm-1, respectively. Moreover, because an increase of solution temperature causes
red-shifted linear absorption spectra but does not affect the corresponding fluorescence emission
spectra, this would lead to decreased Stokes shifts with increasing temperature for all three
polymers.
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Isotropic fluorescence decays at different temperatures
Representative time-resolved fluorescence data collected with the magic angle polarization at
four different temperatures are shown in Figure 3. The isotropic fluorescence decays measured for
both EO3 and EO4 exhibited significant acceleration when the sample solution temperature was
raised to about 60 °C. The exact temperature appears dependent on the length of the side-chain. For
EO3, an obviously faster decay was already noticeable at 52 °C, but this was not obvious for EO4 until
the temperature reached 61 °C. A further temperature increase led to an even faster decay, whereas
below these temperatures (52 °C for EO3 and 61 °C for EO4) either no change or a minor effect was
observed (Figure 3b, c). The temperatures at which clearly accelerated isotropic decays were
observed for these two polymers correspond fairly well to their LCSTs. In contrast, the fluorescence
data collected for EO2 at all temperatures exhibit an essentially indistinguishable decay behavior
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from each other upon normalizing at the maximum fluorescence intensities, because all the decays
were too rapid to be resolved with the available time resolution of our TCSPC apparatus (Figure 3a).
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Quantitative analysis of the isotropic fluorescence decays were performed by employing a least
squares deconvolution fitting algorithm with explicit consideration of the finite instrumental
response. Table 2 summarizes all the fitting parameters, including decay lifetimes, relative
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2
2
amplitudes and reduced chi-squares (χ ). Inclusion of the χ values is intended to provide a
quantitative measure of the fit quality. We found that the fluorescence decays measured for the EO2
sample at all temperatures exhibited a bi-exponential decay behavior. The major component has a
lifetime ranging from 28 to 33 ps and a relative amplitude of ∼98%, whereas the minor component
is characterized by a lifetime between 370 and 480 ps. The variation of the extracted lifetimes,
especially for the major decay component, is likely to be caused by both sample temperature
uncertainty and the limited signal-to-noise ratios of the experimental data. It should be emphasized
that the similar fitting parameters extracted from these EO2 data do not mean that this polymer is
insensitive to temperature changes. Rather, the similarity arises from a very rapid decay at all
temperatures, which is too fast to be resolved with our current experimental setup. Satisfactory
fitting of the isotropic data collected for the EO3 sample required a model function consisting of
three exponents. The lifetime (relative amplitude) of the major decay component decreases from 50
ps (78 %) at 28 °C to 31 ps (97 %) at 68 °C, and the corresponding changes for the other two
components are from 378 ps (15 %) and 950 ps (7 %) to 280 ps (2 %) and 690 ps (1 %),
respectively. The isotropic fluorescence decays of the EO4 sample are even more complicated, and
the data collected at 589 nm can be satisfactorily fitted only with a model function consisting of four
exponents. The lifetimes (relative amplitudes) of two major components vary from 29 ps (47 %)
and 126 ps (28 %) at 28 °C to 10 ps (71 %) and 44 ps (27 %) at 68 °C. The corresponding slower
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decay components have lifetimes (relative amplitudes) ranging from 536 ps (17 %) and 1.03 ns (8
%) to 386 ps (2 %) and 850 ps (1 %).
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By comparing the isotropic fluorescence decays measured at a given temperature, we further
found that the length of the side-chain plays a markedly large role on the excited-state relaxation
dynamics of these polythiophene polymers. An increase of the side-chain length leads to
significantly slower isotropic fluorescence decays (see, as an example, Figure 4a for the data
recorded at 28 °C). This side-chain length effect can also be seen in a quantitative manner from a
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mean lifetime calculated using
τ = ∑τiAi /∑ Ai , where τ and A represent the lifetimes and relative
i i
amplitudes extracted from the deconvolution data fitting and i is the number of exponents in a given
fitting function. For the data collected at 44 °C, we obtained a mean lifetime of 36 ps for EO2, 155 ps
for EO3 and 209 ps for EO4. Furthermore, the decay rate difference observed for the three polymer
samples decreases with increasing temperature, and at the highest sample temperature of our
measurements (68 °C) the difference becomes only marginal (see Figure 4b). It is pertinent to point
out that similar side-chain length effects were reported previously for amine-end-capped p-
phenylenevinylene trimers with different side-chains and end-groups,38 as well as their parent
trimers 2,5-di-[(2-ethylhexyl)-oxy] p-phenylenevinylene and 2,5-di-[(3′-methylbutyl)-oxy] p-
phenylenevinylene.39 A noteworthy distinction between these previous reports and our current
study is that the change of side-chain length influences mainly the fastest decay component in the
former but the overall decay here. On the contrary, a significant reduction of fluorescence lifetime
was observed when a thienylene-vinylene side-chain of a polythiophene derivative was increased.
This result was explained by assuming that more conformational defects were generated along the
polymer main chain because of enhanced steric hindrance and strong twisting.40
Time-resolved fluorescence anisotropy
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The isotropic fluorescence data described above are insensitive to the orientation changes of
electronic transition dipole moments, which may arise from several rather rapid dynamical
processes including a) energy transfer from one conjugation segment to another either within the
same polymer backbone or between different backbones, i.e., intra- or inter-chain excitation energy
transfer, b) the conformational changes of polymer backbones such as twisting or bending, and c)
overall polymer rotations in solution. To assess these changes directly in the timescales of their
occurrence, we performed time-resolved fluorescence anisotropy measurements at the same
detection wavelengths as employed for the isotropic experiments. Figure 5 shows the anisotropic
decays obtained for EO4 at four different temperatures; the results obtained for EO3 share
substantial similarities (data not shown). At a low temperature such as at 28 °C, the anisotropies
obtained for both EO3 and EO4 were characterized by an initial value of ∼0.2 and a relatively slow
decay to a residual value of ∼0.1 at about 1.0 ns. A temperature increase led to several striking
changes. First, the initial anisotropy increased substantially near and above the LCSTs, and at the
highest temperature of our experiments (68 °C) the initial anisotropy almost reached the theoretical
limit of 0.4, which is expected only when the absorption and emission transition dipole moments
are parallel.41 This observation suggests that a temperature-induced, remarkable change of the
polymer backbone conformation occurs during aggregate formation. Following the collapse of the
EO side-chains, which causes the observed physical appearance change, such a conformational
change of polymer backbones is obviously needed in order to bring different polymer molecules
into close spatial proximity for aggregation. According to previous experimental and theoretical
studies,42-45 both the transition dipole moments associated with photon absorption and fluorescence
emission should orient along the backbones. Consequently, the observed high initial anisotropy of
∼0.4 means that the polymer backbones must be highly ordered in the aggregates. Second, a fast
decay component was clearly noticeable at 44 °C, which became more pronounced when the
temperature was raised to 61 and 68 °C. The lifetime of this component shortened significantly with
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increasing temperature, and was estimated to be 36 ps at 68 °C. Note that this lifetime was extracted
without deconvolution using the IRF, and therefore it should be viewed as a rough estimate. This
rapid anisotropic decay can be straightforwardly assigned to an inter-chain energy transfer process,
i.e., from one polymer chain to another within the same aggregate. This inter-chain transfer is
possible only when the polymer chains are spatially adjacent, which can further explain why no
such a decay was found in the solubilized case at temperatures below the LCSTs. Third, the
subsequent slower decay also accelerated clearly near and above the LCSTs, which varies from
roughly few nanoseconds at 28 °C to about 870 ps at 68 °C. Furthermore, at any given temperature
the anisotropic decays proceed until the corresponding isotropic fluorescence signals cease. As
already seen from the isotropic decays, the anisotropic data recorded for EO2 again are an exception.
Due to the rapid excited-state relaxation, the fluorescence signals at delay times greater than 200 ps
are low for any combination of the excitation and emission polarizations, and therefore the
anisotropy data show poor signal-to-noise ratios. Nevertheless, these data exhibit initial
anisotropies between 0.25 and 0.3, but no clear trend with temperature changes can be ascertained
with confidence.
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Discussion
The key findings of our time-resolved fluorescence experiments can be summarized as follows.
First, the isotropic fluorescence decay depends strongly on the polymer side-chain length. An
increase of the side-chain length leads to not only significantly slower decay rates but also more
complex decay behavior, which is evident from the greater number of exponents needed for
satisfactory description of the decay. Second, the fluorescence decays of EO3 and EO4 measured near
and above their LCSTs depend markedly on the aqueous solution temperature, manifested by more
rapid decay rates at higher temperatures. However, the complexity of the decay behavior remains
unchanged for each polymer. No such temperature dependence could be resolved for EO2 owing to
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its extremely rapid decay with respect to our available time resolution at all measured
temperatures. Third, a similar temperature effect was also observed for the anisotropic decays
obtained for EO3 and EO4, which are characterized by clearly temperature-dependent initial
anisotropies and decay timescales. Of particular interest is the observation of an initial anisotropy of
∼0.4 above the LCSTs, which is only expected when the transition dipole moments associated with
photon absorption and fluorescence emission are parallel. This result suggests that the polymer
chains within the aggregates are highly ordered. Finally, for either polymer at any given
temperature, the anisotropic decay was found to persist for essentially the same time as the
corresponding isotropic decay. In the following, we will discuss separately the effects of side-chain
length and aqueous solution temperature on time-resolved fluorescence isotropic decays, as well as
how the aggregates formed near and above LCSTs differ from those crystalline polythiophene
nanofibers discovered previously in organic solvents.
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Side-chain Length Effect
Because the hydrophilic side-chains collapse and large aggregates form near and above the LCSTs
due to presumably a change of the hydrogen-bonding interactions with water molecules, we will
restrict our discussion only to the data collected below the LCSTs to ensure that the polymer chains
are homogeneously dispersed in water. In this case, the favorable hydrogen-bonding interactions
between the hydrophilic side-chains and water molecules must be strong enough to balance the
unfavorable interactions between water molecules and the polymer backbones,28 which may adopt
either an extended or a twisted conformation. While a highly twisted, coiled conformation has been
reported for a nonionic water-soluble conjugated polymer, poly[2,5-bis(diethylaminetetraethylene
glycol)phenylene vinylene] in water,26 we do not expect any of three polymers examined in this
work will adopt such a conformation. This is because thiophene based polymers, just like other
conjugated polymers, usually adopt rod-like conformation due to their extensive main chain
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conjugation.46,47 Absence of such coiled conformations is further supported by our time-resolved
fluorescence anisotropy data. If such coiled conformations with tight backbone folding were
formed, the different segments of a same polymer backbone would become spatially close and then
intra-chain excitation energy transfer could occur.48 Since this transfer takes typically a few tens of
picoseconds,18 its occurrence would lead to fast depolarization and the anisotropy should reach zero
in a short time. This is clearly inconsistent with the much slower anisotropy decays we observed
(Figure 5), which proceed until the corresponding isotropic decays end. Based on these arguments,
we will consider only extended backbone conformations in the following discussion.
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It has been widely accepted that, upon photoexcitation, conjugated polymers generally undergo
conformational changes, such as from aromatic towards quinoidal geometries.49-51 Consequently,
similar changes are also expected in the relaxation process from the electronic excited-state to the
corresponding ground state. The relaxation timescales should therefore depend on how fast these
conformational changes take place, which in turn depend on the rigidness of the backbones and the
strength of the hydrogen-bonding interactions between the attached hydrophilic side-chains and
the water molecules. Given the same backbones, longer side-chains and concomitant stronger
hydrogen-bonding interactions should make these conformational changes more difficult and
consequently slow down the excited-state relaxation as compared to polymers with shorter side-
chains. As a result, we can expect slower fluorescence decay with increasing side-chain length,
which is qualitatively consistent with our experimental observations. This consideration can also
explain why the isotropic decays become more complex with lengthening the side-chain. Because of
the stronger hydrogen-bonding interactions, the longer side-chains will be capable of
accommodating a greater number of backbone conformations. According to previous
experimental38,39,52-56,57 and theoretical studies,58-60 the spectroscopic and photophysical properties
of conjugated polymers depend strongly on their backbone conformations. This would indicate that
presence of different number of backbone conformations should give rise to distinct excited-state
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decay behavior. Based on the consideration given above, we attribute the observed isotropic decays
below the LCSTs to the conformational relaxation of their backbones in their electronically excited
first singlet states.
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So far, we have assumed that the solubilized polymer molecules exist mainly as single chains with
no aggregation. However, this may not be true in view of the shoulder on the red-side of the EO2 and
EO3 emission spectra (Figure 2b). Formation of small aggregates such as dimers or trimers will not
alter the physical appearance of the aqueous solutions, but could be visible in the fluorescence
emission. Presence of these small aggregates could further lead to an accelerated isotropic decay as
observed previously for poly[3-(4-octyl-phenyl)-2,2´-bithiophene].61 The absence of such a shoulder
in EO4 emission is most probably because this polymer has much higher solubility than either EO3 or
EO2, and therefore has the least tendency of forming aggregates. An examination of the relative
contents of these small aggregates and their precise influence on the fluorescence isotropic decays
requires more detailed study with detection of time-resolved fluorescence data at multiple
wavelengths and for different solution concentrations in combination with global lifetime analyses.
Such a study, however, is beyond the scope of this work.
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Temperature Effect
We now turn to the temperature effect. As shown in Figure 3b and 3c, the isotropic decays of both
EO3 and EO4 become clearly faster when the solution temperature was raised to near or above their
LCSTs. At these temperatures, the hydrophilic side-chains collapse and should no longer impact the
excited-state relaxation. Upon aggregation, the spatially adjacent polymer chains will enable inter-
chain energy transfer from the excited molecules to those unexcited ones. This transfer should
accelerate as the chain ordering improves with raising temperature, which is evident by the
increased initial anisotropies (see Figure 5). An improved ordering may bring different polymer
chains even closer and/or give rise to a more favorable orientation for the energy transfer.
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Occurrence of this inter-chain transfer and its acceleration with temperature can qualitatively
explain the temperature observed.
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The change of chain ordering in the aggregates can further affect the relaxation of the excited
polymers to their ground states due to the involvement of distinct conformational changes at
different temperatures. Here, the temperature change should affect mainly the ground state
conformations as judged from the large and continuous shift and broadening of the linear
absorption spectra with raising temperature. According to the density functional theory calculations
of Zade and Bendikov, a polymer band-gap is strongly dependent on chain twisting.59 Thus, a red-
shifted linear absorption spectrum, or equivalently a reduced band-gap, must be accompanied by a
conformational change of polymer chain. This is further supported by the substantial increase of
initial anisotropies observed for EO3 and EO4 near or above their LCSTs. A higher initial anisotropy
means a smaller angle between the transition dipole moments associated with absorption and
emission, and thus more similar conformations in the excited-state and ground state. Consequently,
the relaxation should be faster at a higher temperature, as less conformational change is needed.
This temperature-dependent relaxation process can be visualized through a schematic description
depicted in Figure 6. At a lower temperature, different polymer chain conformations are
represented by clearly distinct potential energy curves with relatively larger displacement between
the E0 and E1 minima. Both photon absorption and fluorescence emission, depicted by vertical
arrows, will be accompanied by a substantial conformational change. As a result, a larger Stokes
shift as observed experimentally is expected. At a higher temperature, the two curves share more
similarities and a smaller displacement, and therefore little conformational change is required
between the photon absorption and fluorescence emission transitions. The corresponding Stokes
shift reduces as well, which is again consistent with our experimental data. We would like to
emphasize that this is again a conformational relaxation, but from an excited-sate to a ground state
with a temperature-dependent chain conformation.
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The occurrence of this inter-chain energy transfer and/or conformational relaxation at
temperatures near and above the LCST was also supported by the isotropic data that was collected
for EO4 at 620 nm (data not shown), which is located at the low energy region of the steady-state
fluorescence emission spectrum (Figure 2b). Satisfactory fitting of the data measured at 61 and 68
°C is only possible when a rise component is also included in the model function, and then the
lifetimes extracted are 11 and 14 ps, respectively. This lifetime corresponds fairly well to the fastest
10 ps decay component resolved from the data collected at 589 nm at the same temperatures (see
the last two rows in Table 2). No such a rise component could be resolved from the data measured
below the LCST. However, based on these data alone, we can assign neither this rapid rise seen at
620 nm nor its corresponding decay at 589 nm exclusively to an inter-chain transfer or a
conformational relaxation because both of these dynamical processes can result in the same kinetic
rise and decay. A possible way of distinguishing these processes is to vary the viscosity of the
aqueous solution and thus slow down the conformational change, which will be an interesting topic
for future study.
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Obviously, our argument about the temperature influence on the relaxation of an excited polymer
to its ground state is valid only when the interactions between the polymer chains in the same
aggregate are sufficiently weak so that they behave as individual molecules. As will be discussed in
the following subsection, the weak interactions are indeed supported by the structureless
absorption spectra seen near and above the LCSTs.
Comparison of the aggregates formed near and above LCSTs with crystalline polythiophene
nanofibers
The high initial anisotropy of ∼0.4 observed for EO3 and EO4 above their LCSTs indicates that the
polymer chains became highly ordered in contrast to the randomly oriented chains in the
solubilized case below the LCSTs. This may not be surprising in view of the long-range structural
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order discovered for the polythiophene nanfibers in organic solvents.11,35,37,62 However, their linear
absorption spectra are strikingly distinct. While vibronic transitions such as 0-0 and 0-1 are clearly
evident in the nanofibers,11,35-37 these transitions are entirely absent in the aggregates formed near
and above LCSTs here (see Figure 2a). We believe that these distinct spectral features arise from the
different nature of inter-chain interactions: excitonic in the nanofibers versus van der Waals in the
aggregates. Owing to the presence of side-chains and possibly some “residual” water molecules, the
aggregates may be somewhat “swollen”, similar to those suspended nanofibers prior to solvent
evaporation.28 The weaker interactions in the later case ensure that the aggregates can be converted
entirely into the solubilized form even when only a small temperature drop to below the LCSTs
occur, and therefore produce the fully reversible linear absorption spectra as observed
experimentally.
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Conclusions
In this paper, we report synthesis of three thermoresponsive poly(3-oligo(ethylene oxide))
thiophenes with different side-chain length and LCSTs. The linear absorption spectra of these
polymers exhibit fully reversible changes (spectral shift and broadening) with variation of the
aqueous solution temperature, whereas their steady-state fluorescence emission spectra show
negligible shifts in combination with very minor line shape changes. Consequently, a decrease in the
Stokes shifts with increasing temperature was observed for all three polymers. Near and above the
LCSTs, the aggregates formed were further found to exhibit no vibronic transitions in contrast to
those observed for crystalline polythiophene nanofibers. We believe that this difference arises from
the distinct nature of inter-chain interactions in the two cases.
By application of picosecond time-resolved fluorescence spectroscopy, we demonstrated that the
excited-state relaxation dynamics of these polymers in aqueous solution depend strongly on the
side-chain length and temperature. With lengthening side-chain, the isotropic fluorescence signals
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exhibit not only remarkably slower decays but also an increasingly complex decay behavior. The
differences between these isotropic decays reduce gradually with increasing temperature and
become minimal at the highest temperature of our measurements (68 °C), but the complexity of the
decay behavior remains unchanged for each polymer. On the other hand, significant acceleration of
the isotropic decays was observed for both EO3 and EO4 when the aqueous solutions were heated to
their LCSTs, 51 °C for EO3 and 60 °C for EO4, and above; at these temperatures large aggregates form
as evident by the physical appearance change from clear solutions to turbid suspensions. We
attribute the observed side-chain length effect to the conformational relaxation of the polymer
backbones. The temperature effect observed near and above the LCSTs, on the other hand, arises
from an inter-chain energy transfer between the spatially adjacent chains in the aggregates and a
change in the excited-state relaxation owing to the involvement of distinct chain conformations in
the ground state. Both these two processes should accelerate with increasing temperature. A
particularly interesting observation is the remarkable changes of time-resolved fluorescence
anisotropies near and above LCSTs, which are manifested by a substantial increase of initial values
from ∼0.2 to 0.4 in combination with the appearance of a pronounced fast anisotropic decay
component with a shortest lifetime estimated to be 36 ps. These different initial anisotropies
provide direct evidence for a temperature-induced change of polymer chain conformations in the
aggregates, especially, for the formation of highly ordered chains at the highest temperature of our
time-resolved experiments (68 °C). No clear temperature dependence could be identified with
confidence for either isotropic or anisotropic decays collected for EO2 due to our limited
instrumental time resolution.
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AUTHOR INFORMATION
*Corresponding Authors
Phone: 865-574-7213. Fax: 865-574-8363. Email: may1@ornl.gov
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Phone: 865-574-4974. Fax: 865-574-1753. Email: hongkq@ornl.gov
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Acknowledgments
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Research sponsored by the Laboratory Directed Research and Development Program of ORNL,
managed by UT-Battelle, LLC, for the U. S. Department of Energy (YZM, FY2010, 2011), and the
Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S.
Department of Energy (YZM and RWS, FY2012). The research conducted at the Center for
Nanophase Materials Sciences of Oak Ridge National Laboratory is sponsored at by the Scientific
User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.
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Figure 1. Linear absorption spectra recorded for EO4 at different temperatures (32 to 80 °C) during
heating (a) and cooling (b) segments. The spectra were measured every 4°C from 32 to 48°C and
every 2°C for higher temperatures. (c) Plot of the peak wavelengths of the absorption spectra
measured during heating (open circles) and cooling (filled squares) versus aqueous solution
temperature.
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Figure 2. Linear absorption (a) and steady-state fluorescence emission spectra (b) recorded at 70
and 30 °C, respectively, for the EO2 (dark green), EO3 (blue), and EO4 (black) polymers. All spectra
are normalized at the peak absorbance or emission intensity.
Figure 3. Isotropic fluorescence data collected at 28, 52, 61 and 68 °C for EO2 (a), EO3 (b) and EO4
(c). For ease of comparing their distinct decays, all the data have been normalized at the maximum
intensities. The dotted line in (a) shows the IRF.
Figure 4. Comparison of isotropic fluorescence decays measured at (a) 28 and (b) 68 °C for EO2, EO3,
and EO4 samples. For ease of comparing their distinct decays, all the data have been normalized at
the maximum intensities.
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Figure 5. Time-resolved fluorescence anisotropic decays recorded for EO4 at (a) 28, (b) 44, (c) 61,
and (d) 68 °C, respectively. The solid lines are bi-exponential decay fits. To avoid possible excitation
light scattering, only the data after 40 ps are plotted.
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Figure 6. Schematic model describing the temperature-dependence of excited-state relaxation
dynamics in water-soluble polythiophene polymers. E0 and E1 represent the ground state and first
singlet excited-state, and q1 and q2 denote the relative shifts of their potential energy curves. The
vertical arrows depict photon absorption and fluorescence emission. The corresponding energy
differences or Stokes shifts reduce when the two potential energy curves are more similar and
possess a smaller q displacement between their minima.
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Scheme 1
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CH2(OC2H4)xOCH3
CH2(OC2H4)xOCH3
(4) t-BuMgCl
CH2Br
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(1) CH3(OCH2CH2)xOTs
(5) Ni(dppp)Cl2
Br
I
(2) NBS
(3) I2/PhI(OAc)2
S
n
(6) iPrMgCl
S
S
x=2, 3, 4
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