Fs-ps Exciton dynamics in a stretched tetraphenylsquaraine polymer

Article abstract of DOI:10.1039/c9cp02900a

A tetraphenylsquaraine was synthesized whose structure was elucidated by single crystal X-ray structure analysis. Unlike all known indolenine squaraines, the tetraphenylsquaraine shows an unusual nonplanar structure with the four phenyl groups pointing aw

Full text of DOI:10.1039/c9cp02900a

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fs–ps Exciton dynamics in a stretched
tetraphenylsquaraine polymer†
Cite this:DOI: 10.1039/c9cp02900a
Maximilian H. Schreck, Lena Breitschwerdt, Henning Marciniak, Marco Holzapfel,
David Schmidt, Frank Wurthner
and Christoph Lambert
*
¨
A tetraphenylsquaraine was synthesized whose structure was elucidated by single crystal X-ray structure
analysis. Unlike all known indolenine squaraines, the tetraphenylsquaraine shows an unusual nonplanar
structure with the four phenyl groups pointing away from the squaric acid core in order to avoid steric
congestion. This tetraphenylsquaraine was polymerized by a Yamamoto coupling to form a conjugated
polymer with Xn = 38. The absorption spectra of this polymer are red-shifted compared to that of the
monomer and show a J-type absorption band due to exciton coupling. Transient absorption spectra
with fs-time resolution display a strong ground state bleaching signal with a peak on the red side rising
concurrently with the decay of a peak on the blue side of an isosbestic point at 12 000 cmÀ1. This
behavior is caused by energy transfer between two slightly different sections of the polymer with time
constants of 0.3 and 2.6 ps. According to semiempirical calculations these different sections are
stretched and slightly bent conformations of the polymer strand. Power dependent transient absorption
measurements indicate exciton annihilation which also proves the excitons to be very mobile.
Received 22nd May 2019,
Accepted 22nd June 2019
DOI: 10.1039/c9cp02900a
rsc.li/pccp
play an important role. The latter squaraine dyes are somewhat
exceptional because of their intense and narrow absorption band in
Introduction
Conjugated polymers are one of the key players in organic the red to near infrared spectral region and their high fluorescence
electronics1–4 as they can be used as emitters in OLEDs,5 as light quantum yield.24,25 Unlike cationic cyanine dyes, which show
absorbing materials in organic photovoltaic (OPV) devices,6–8 in similar optical properties, squaraine dyes are electrically neutral
bioelectronics9 and as transport materials in organic field effect and soluble in common organic solvents. Between the two
transistors (OFET).10,11 Here the monomeric constituents of the extremes, conjugated polymers on the one hand and molecular
polymers are usually small aromatic or heteroaromatic units dyes on the other, oligomers or polymers formed by covalent
such as benzene, thiophene or pyrrole or even more complex linking of molecular dyes represent an attractive alternative
heterocycles such as benzothiadiazole, either annulated, with often more predictable electronic properties.26,27 In these
directly linked via single bonds, or via other p-bridges such as covalently linked ‘‘aggregates’’, the optical properties can success-
conjugated CC-double or triple bonds. In these polymers the fully be understood by simple exciton coupling of localized
optical and electrical properties are largely determined by the transition dipole moments which yields an exciton manifold to
conjugative interaction of the monomers which leads to quite which transitions may be allowed or forbidden, depending on
distinct optoelectronic properties of the polymers compared to the relative orientation of the transition dipole moments.28 For
their monomeric constituents. On the other hand, molecular example, linear BODIPY oligomers in which the relative orientation
conjugated materials12 and dyes such as diketopyrrolopyrroles, of the chromophores is locked by appropriate alky substituents,
merocyanines, perylenedimides, BODIPYs or squaraine dyes13–18 show distinct exchange narrowing of the J-type absorption band
may substitute conjugated polymers in the above mentioned appli- indicative of a strong delocalization of the exciton29 while oligomers
cations or may even be superior as for example in OFETs.19–23 Here, of other BODIPYs show helix formation with H-aggregate
the electronic and optical properties are inherent to the chromo- behavior.30 Similar effects were observed for homo and hetero-
phore unit but supramolecular arrangement and morphology also polymers of squaraine dyes.13,31,32 Such oligomers and polymers
have also successfully been used in OPV13 and NIR OLEDs.33
In general, polymerization of suitable monomeric squaraine
¨
Institute of Organic Chemistry, Center for Nanosystems Chemistry, Universitat
dyes leads to polysquaraines whose optical properties not only
depend on the chemical nature of the squaraine monomers but
also on the mutual arrangement of the squaraine dyes within
the polymer strand, that is, the polymer folding structure.34
¨
¨
Wurzburg, Am Hubland, D-97074 Wurzburg, Germany.
E-mail: christoph.lambert@uni-wuerzburg.de
† Electronic supplementary information (ESI) available. CCDC 1916888. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c9cp02900a
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Scheme 1 Polymer structure of [SQB]n and of [TP-SQB]n.
This is because within the exciton coupling theory the relative the samples was approx. 250 mm in radius, estimated from pump
orientation of localized transition dipole moments is decisive for energy measurements before and after a pinhole. The spot size
the resulting states of the exciton manifold and their transition of the probe pulses was significantly smaller with a radius of
probability.35 Recently we showed that a polymer [SQB]n (see approx. 125 mm.
Scheme 1) built up from a cis-squaraine may form stretched or
helical foldamers, depending on the solvent environment.13,32
While the former shows typical J-type aggregate behavior, that is,
Results and discussion
an intense and red-shifted absorption band, the helices display
typical H-type aggregate bands with the main absorption blue-
shifted. In order to control the polymer structure, and thereby
also the optical properties, we designed a new tetraphenyl-cis-
squaraine monomer which, owing to four sterically demanding
phenyl substituents, should form a stretched polymer strand
[TP-SQB]n (see Scheme 1) rather than a helical foldamer, irrespec-
tive of the used solvent. The optical properties, in particular the
photoinduced dynamics of this polymer are in focus of this work.
Synthesis and structure
The monomer TP-SQB and its bromo derivative TP-SQB-1-Br2
were synthesized as presented in Scheme 2. Also a bromo
derivative with hexyl side chains instead of the dimethyloctyl
chains was prepared.
By acidic activation of isatin 1 with TfOH an electrophilic
substitution at benzene yielded the diphenyloxindoles 2a and
2b which were alkylated subsequently by the appropriate alkyl
halide at the nitrogen to give lactams 3a–3c. Reaction with a
methyl Grignard reagent and subsequent elimination gave the
iminium salts which proved to be unstable37 and were therefore
used without further purification. Deprotonation with a weak
base (pyridine) and reaction with dicyanomethylene squaric
Experimental
The synthetic protocols can be found in the ESI.†
Absorption spectra were measured with a Jasco V-570 or acid ethyl ester gave the desired squaraine dyes. In this context
V-670 UV/vis/NIR spectrometer in 1 mm or 10 mm quartz it is worth mentioning that debromination is a severe side
cuvettes and referenced against the pure solvent.
An Edinburgh Instruments FLS980 fluorescence lifetime inert gas atmosphere and short reaction times (ca. 4 h).
spectrometer was used for steady state fluorescence measurements, Polymerization of TP-SQB-1-Br2 was achieved by Yamamoto
reaction for 4b and 4c which could be suppressed by applying
determination of fluorescence quantum yields and fluorescence homocoupling reaction with Ni(COD), see Scheme 2. The crude
lifetime measurements. The observed fluorescence quantum yields polymer was purified by Soxhlet extraction with hexane, MeOH
were determined with an integration sphere and afterwards and finally acetone. The acetone extract was fractionated by
corrected for self-absorption applying the method of Bardeen preparative GPC and afforded a batch with Mn = 36 600 and
et al.36 Fluorescence lifetimes were determined by time-correlated PDI = 2.35 with a degree of polymerization Xn = 38.
single-photon counting (TCSPC).
For the monomeric hexyl derivative TP-SQB-2-Br2 we were
Transient absorption measurements were performed using able to grow green crystals from a highly concentrated DCM
a Helios transient absorption spectrometer from Ultrafast solution by layering with hexane. Much to our surprise the X-ray
Systems. The pump pulses were generated with a noncollinear crystal structure analysis of these crystals revealed an unusual
optical parametric amplifier (NOPA), and compressed with a geometry of the squaraine dye (Fig. 1). While all known cis- and
fused silica prism compressor to durations of under 25 fs. White trans-indolenine squaraines feature a planar geometry with the
light generation in 3 mm sapphire plates in the Helios spectro- alkyl-nitrogen groups pointing away from the squaric acid
meter as well as in the NOPA was seeded with 900 nm pulses oxygen,38–55 in TP-SQB-2-Br2 both N-alkyl chains point towards
from a TOPAS-C (Light Conversion) to achieve pump and probe the squaric acid core. Moreover, because of steric reasons the
wavelengths near the fundamental wavelength of the laser source indolenine groups are twisted out of the plane thereby avoiding
(800 nm). A Solstice amplified Ti:sapphire laser from Newport steric congestions of the N-alkyl chains with the squaric acid
Spectra-Physics served as pulse source for the whole setup. The core. This leads to a strongly nonplanar p-system with dihedral
sample solutions in CHCl3 were pumped through a flow cell angles of the indolenine ring plane and the central squaric acid
(Starna) with 0.2 mm thick quartz windows and 0.2 mm path ring plane of 37.31 and 48.31, respectively (see Scheme 3). Two of
length during the measurements, using a micro annular gear the four phenyl groups point above the squaric acid p-system. The
pump (HNP Mikrosysteme). The spot size of the pump pulses on highly twisted p-system also shows some significant deviations of
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Scheme 2 Synthesis of squaraines and polysquaraine [TP-SQB]n.
Fig. 1 Molecular structure (top and side view) of TP-SQB-2-Br2. Hydrogen atoms are omitted for clarity; grey: C, blue: N, red: O, yellow: Br.
the C–C and C–N bond lengths in comparison to ‘‘normal’’ cis- Accordingly, the optimized structure corresponding to the crystal
squaraines, as can be seen from Table S1 (ESI†). Obviously, the structure is 22.7 kJ molÀ1 (0.236 eV) lower in energy than the
reason for this unexpected structure is to avoid steric repulsion expected optimized normal configuration reflecting the amount of
of the four phenyl substituents with each other and with the steric repulsion in the latter. According to the X-ray crystal structure,
squaric acid oxygen in the expected ‘‘normal’’ squaraine dye there are no intermolecular p–p interactions between two different
configuration (see Scheme 3). Indeed, DFT computations at squaraine chromophores highlighting the steric shielding of the
B3LYP/6-31G* level of theory support these findings (see ESI†). The squaraine p-system by the four phenyl substituents.
optimized ‘‘normal’’ configuration is also slightly nonplanar (see
The structure of the polymer was simulated using the
ESI†) because the four phenyl groups try to avoid steric repulsion. semiempirical AM1 Hamiltonian. We optimized two different
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Scheme 3 Schematic drawings of different squaraine stereoisomers.
stretched hexamer model structures which differ in the dihedral three chromophores32 which also leads to the formation of
angle of the biaryl axis between two different squaraine dyes cyclic structures.56
(see Fig. 2). This leads to structure A with a large turn of the
Steady state optical spectroscopy
polymer chain and to structure B where the alternating orientation
leads to an overall stretched polymer structure. In reality, a mixture
of the two structural types within one polymer strand may be
present in solution. As we show below, we have evidence that at
least two different structural sections exist within one polymer
strand which thus is supported by the semiempirical com-
putations. Unlike other polysquaraines, a closely spaced helical
structure cannot be formed in the present polymer because the
steric repulsion of the four phenyl groups forces the squaraine
into the present unusual configuration. The consequence is
a much wider angle between two biaryl axes in the polymer
which impedes any helix formation with a small number of
chromophores per helix turn. From the sketch in Fig. 2 one can
estimate that about 10 chromophores would be necessary for
one helix turn. However, this is quite unlikely to be formed
because disorder will occur before one turn is completed. In
comparison in [SQB]n one helix turn consists of only about
The steady state absorption and fluorescence spectra of the
TP-SQB monomer and polymer in CHCl3 solution are depicted
in Fig. 3 (for data see Table 1). The monomer exhibits typical
characteristics of squaraine compounds: an intense main absorption
peak at 13 400 cmÀ1 and a weak shoulder from a vibronic
progression on the high energy side with the fluorescence
spectra showing a red shift and mirror symmetry. Integration
of the absorption band gives a squared transition moment of
85.9 D2. The lowest energy absorption band of this squaraine
monomer appears to be distinctly redshifted compared to the
one of SQB which displays an absorption at 14 600 cmÀ1 with
e = 1.96 Â 105 MÀ1 cmÀ1 and m2 = 98.6 D2. Thus, the distortion
of the p-system from planarity leads to a bathochromic shift
and a reduced extinction coefficient.57
The absorption spectrum of the [TP-SQB]n polymer shows a
maximum in the near infrared at 12 400 cmÀ1 with a decent red
Fig. 2 AM1 computed model hexamers with two different structures caused by the relative dihedral angle between neighboring squaraine dyes. The
blue arrows indicate estimated localized transition dipole moments for the localized S1 ’ S0 transition.
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absorption band shape of the monomer is hardly affected by the
solvent and the maximum varies only between 13 100 cmÀ1
(toluene) and 13 500 cmÀ1 (DMF). Quite similar observations
were made for the polymer in a series of solvents (toluene,
oDCB, PhCN, CHCl3, DCM, acetone, DMF, see ESI†). These little
changes are attributed to stabilization of the excited state by
more polarizable solvents.
A closer look at the absorption band of the polymer reveals a
double peak structure at ca. 12 500 cmÀ1, which is visible as two
merging peaks forming a plateau at the peak maximum, which
has to be discussed in further detail below. A possible explanation
would be the existence of two different, though energetically very
similar, monomer–monomer conformers within the polymer
strands. These conformers may be formed by a rotation around
the central biaryl axis connecting the individual squaraine dyes as
anticipated by the AM1 calculations, see above. Indeed, CNDO
S2 calculations based on these structural models predict J-type
exciton bands for both conformers but the absorption maximum of
Fig. 3 Absorption and fluorescence spectra of TP-SQB monomer and
[TP-SQB]n polymer in CHCl3.
shift of 1000 cmÀ1 compared to the monomer, which is mainly the exciton manifold is 500 cmÀ1 lower energy for structure B than
caused by the exciton splitting of the first excited state. This red for structure A (see ESI†).
shift is similar to the shift observed for covalently linked SQB
The fluorescence spectrum of the [TP-SQB]n polymer roughly
dimers in toluene compared to their monomers (920 cmÀ1, see resembles the shape of the monomer fluorescence and seems
Rohr et al.58) and therefore speaks against a delocalization to originate exclusively from the lowest lying exciton states,
¨
length beyond that of ca. two monomers because an exciton that showing a Stokes shift of 560 cmÀ1. This is more than the
is delocalized over several monomers would generate a larger monomer Stokes shift of 340 cmÀ1 and is therefore in contradiction
splitting of the first excited state according to exciton coupling with results from other J-aggregates, where the delocalization of the
theory.35 Unlike polymers derived from SQB, the absorption exciton leads to smaller Stokes shift compared to the respective
oscillator strength of [TP-SQB]n polymer is apparently con- monomers. The full width at half maximum (fwhm) of the polymer
centrated in the lower energy edge of the exciton band, while fluorescence band (720 cmÀ1) is smaller than that of the monomer
no significant absorption emerges from the upper edge of the (860 cmÀ1). According to the NÀ1/2 relation,60–63 this indicates a
exciton band. This hints at a J-type aggregate configuration of delocalization length of only B1.4 monomers and, thus also
the polymers, where the transition moments of the monomers points against a long delocalization length. On the other hand,
are in a head-to-tail arrangement as expected for a stretched this criterion is not very reliable because of the inhomogeneity
polymer structure. Possible helix structures, as they were (polydispersity) of the polymer sample.
observed for SQB derived polymers in some solvents, are there-
Both monomer and polymer display a rather small fluorescence
fore excluded as they would produce H-type aggregate bands quantum yield of 0.02 and 0.04, respectively, which is distinctly
with the largest oscillator strengths at the high energy exciton smaller than that of e.g. SQB with 0.55. Using the lifetime and the
bands. Such helix foldamers with a small pitch are disfavoured quantum yield we also calculated the squared fluorescence transi-
because of steric repulsion of the phenyl groups extending tion dipole moment59 (see Table 1) which is smaller for the
almost perpendicularly from the p-plane (see Fig. 1) which monomer but larger for the polymer. The former may hint towards
induces the present distorted squaraine conformation leading structural distortion in the monomer excited state which reduces
to a much wider biaryl–biaryl angle in the polymer. Integration the effective overlap of excited and ground state wave function and
of the lowest energy absorption band and taking the degree of the latter confirms the delocalization of the exciton over a few
polymerization into account yields the squared transition monomers because the transition dipole moment should ideally
dipole moment59 m2 = 89.3 D2 per monomer in very good agreement be additive with the number of chromophores involved in the
2
with the monomer. Unlike [SQB]n polymer, the solvatochromic excited state. In fact, the ratio of mfl2/mabs B 3.7 is larger than
response of TP-SQB and [TP-SQB]n is rather weak, that is, the the delocalization length estimated above from the exchange
Table 1 Absorption (n~abs) and fluorescence (n~fl) maxima, fluorescence lifetimes (tfl) amplitude (a) and quantum yield (Ffl) of TP-SQB monomers and
[TP-SQB]n polymer in CHCl3
n~abs1/cmÀ1 [e/MÀ1 cmÀ1
]
n~abs2/cmÀ1
mabs2/D2
n~fl/cmÀ1
tfla/ns (a)
tfl2a/ns (a)
Ffl
mfl2/D2
TP-SQB
[TP-SQB]n
13 400 [1.37 Â 105]
13 700
12 700
85.9
89.3
13 100
11 900
0.20 (0.97)
0.12 (0.94)
1.81 (0.03)
0.33 (0.06)
0.02 Æ 0.014
0.04 Æ 0.014
57.8
216.5
12 400 [7.66 Â 104]
a
Measured by TCSPC.
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narrowing. However, we have to stress that the accuracy is and a very small component of 1.8 ns. This contrasts the lifetime of
limited in this case because the low fluorescence quantum yield ‘‘normal’’ SQB with t = 2.40 ns and explains the rather small
is afflicted with a major error (see Table 1).
fluorescence quantum yield of the former. The polymer also has
an even shorter lifetime of 0.12 ns as the principal component.
In order to get a closer look into the photoinduced dynamics
at shorter times we performed transient absorption spectro-
scopy with fs-time resolution in CHCl3. The transient absorp-
tion measurements of the monomer (see Fig. 4) are dominated
Time resolved optical spectroscopy
The fluorescence lifetimes of both monomer and polymer were
measured in CHCl3 solution using time correlated single photon
counting method (TCSPC) with excitation at 15 200 cmÀ1 (IRF =
0.27 ns). The monomer shows a predominant lifetime of t = 0.20 ns
by a strong negative feature peaking at around 13 200 cmÀ1
,
Fig. 4 Top panel: Transient absorption spectra of TP-SQB monomer in
CHCl3 after excitation at 13 400 cmÀ1, probed under magic angle. Middle
panel: Decay associated difference spectra (DADS) from a global fit of the
transient absorption spectra of TP-SQB monomer in CHCl3 with associated
time constants t. Lower panel: Transient absorption time trace at
13 200 cmÀ1 and corresponding fit curve from the global fit.
Fig. 5 Top panel: Transient absorption spectra of [TP-SQB]n polymer in
CHCl3 after excitation at 12 500 cmÀ1 with a pump pulse energy of 60 nJ,
probed under magic angle. Middle panel: DADS from a global fit of the
transient absorption spectra of [TP-SQB]n polymer in CHCl3 with associated
time constants t. Lower panel: Transient absorption time traces at selected
wavenumbers and corresponding fit curves from the global fit.
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between the absorption and the fluorescence maximum (13 400
The transient absorption spectra of the polymer show a
and 13 100 cmÀ1, respectively, see Table 1). This is the sum of strong negative signal around 12 000 cmÀ1, stemming from
ground state bleaching (GSB) and stimulated emission (SE). At GSB and SE (see Fig. 5). At higher wavenumbers, the spectra are
higher wavenumbers, excited state absorption (ESA) can be dominated by ESA. The small peak at 13 200 cmÀ1 is caused by
observed with a maximum at around 17 400 cmÀ1
.
a two-photon state as observed before in such polymers.31 At
A global exponential fit of the transient data of the monomer short times, the negative signal around 12 000 cmÀ1 exhibits
yields decay associated difference spectra (DADS)64 revealing a double peak structure with an energy difference (12 200–
small spectral shifts in the first picoseconds, parametrized with 11 900 cmÀ1 = 300 cmÀ1) which is in reasonable agreement
two time constants of 1.64 and 9.5 ps, and a monoexponential with the one calculated with the CINDO S2 method (see above).
decay to the ground state with a time constants of 155 ps. The Here the peak on the red side rises concurrently with the decay
latter is the dominant decay time and agrees nicely with the of the peak on the blue side. Thereby, an isosbestic point seems
fluorescence lifetime of 200 ps (see Table 1). A time constant to appear between the peaks at 12 000 cmÀ1 (see inset, Fig. 5).
with 1 Â 106 ps, which serves as an offset on the timescale This is an indication for energy transfer between two species (or
of the measurement, was added to compensate for improper two states) rather than for vibrational or geometric relaxation
scatter correction. This component shows a very small ampli- processes in the excited state, which are the common cause for
tude in the spectral region of the excitation pulse.
a Stokes shift. A continuous shift of the red side of the joint GSB
Fig. 6 Left column: Transient absorption spectra of [TP-SQB]n polymer in CHCl3 after excitation at 12 500 cmÀ1 with a pump pulse energies of 30, 60
and 120 nJ, probed under magic angle. Middle panel: Transient absorption time traces at selected wavenumbers and corresponding fit curves from the
global fit. Right column: EADS from the global fits of the transient absorption spectra with associated time constants t.
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and SE signal along with a broadening would be expected instead
of the occurrence of a second peak. The second peak, on the
other hand, could support the notion of different monomer–
monomer configurations which also serves to explain the double
peak in the steady state absorption spectra. That might also be
the reason for the relatively large Stokes shift. Hence, a section
within the polymer strand is postulated which possesses a
specific configuration with an excited state being lower in energy
than the surrounding polymer sections and which therefore acts
as an excitation trap. A prerequisite for this exciton trapping
effect would be that the excitations within the polymer strands
are mobile.
A global exponential fit of the transient data shows four time
constants (a fifth one of 1 Â 106 ps is used to account for
improper scatter correction) in the range from B0.3 ps up to
B120 ps whereby the latter is in good agreement with the
fluorescence lifetime (see Table 1). Fig. 5 (middle panel) presents
the DADS of the time constants. The DADS show, that especially
the shortest time constant of B0.3 ps is associated with the above
described decay of signal strength at the blue side of the main
transient absorption band and the concurring rise of (negative)
signal strength on the red side, which indicates a transition from
a higher to a lower excited state along with excitation energy
transfer between different sites of the polymer. However, the
second DADS associated with a time constant of 2.6 ps also
shows these characteristics to some extent. Therefore, the DADS
illustrate energy transfer dynamics, which are obviously not
single exponential. This hints at a diffusion controlled process
like exciton trapping, too. This kind of energy transfer is on a
similar time scale as exciton localization processes which are
believed to take place on the order of 100 fs. On the other hand,
intrachain energy transfer should be on the order of tens ps to
hundreds of ps.65
Transient absorption measurements with different pump
pulse energies of 30, 60 and 120 nJ, i.e. different pump intensities
(see Fig. 6), reveal a change in the relative signal strength within
the double peak structure: with increasing pump intensity, the
relative strength of the low energy peak decreases compared to
the strength of the high energy peak (see also Fig. 7). A possible
explanation could be a saturation of the state or species associated
with the low energy peak. Another effect, that can be observed, is a
slight excitation intensity dependence of the dynamics, which
appear to be faster at higher intensities (see Fig. 7). This is
typical of exciton–exciton annihilation speaking again for
mobile excitons.31,66–73 However, the saturation effect alone
might be responsible for the different dynamics. On the other
hand, fast exciton–exciton–annihilation could act as a decay
Fig. 7 Comparison of transient absorption spectra at 0.1 ps (top) and
selected time traces of [TP-SQB]n polymer in CHCl3 after excitation at
12 500 cmÀ1 with different excitation pulse energies (30, 60, and 120 nJ).
The traces are scaled according to the factors given in the legend.
at the low energy peak (see Fig. 7, upper panel). While a detailed
analysis of the possible exciton annihilation process is beyond
the scope of this work we state here that we are confident that our
interpretation of the energy transfer processes between the two
different polymer sections is not afflicted neither quantitatively
nor quantitatively by the annihilation.
Conclusion
channel for the energetically higher lying state, competing with A squaraine polymer was synthesized which, owing to the
relaxation to the lower state and thus increasingly suppressing structure of the monomer, may contain stretched or slightly
SE from the latter with increasing excitation intensity.
bent sections. Within one polymer strand these sections possess
The time constants from a global fit of the three data sets slightly different excited state energies. This leads to a broad
shorten slightly with rising excitation intensity, reflecting the absorption band for the exciton manifold where two peaks can be
intensity dependence of the dynamics (see Fig. 7). Moreover, identified which may be associated with the two conformational
the relative amplitudes of the evolution associated difference structures. Excitation of the higher energy band populated the
spectra (EADS)74 change, probably partly due to the change in excited state of the one section and is followed by a biphasic
dynamics as well as due to the saturation effect that is observable energy transfer (0.3 and 2.6 ps) to the section with the lower
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12 B. Walker, C. Kim and T.-Q. Nguyen, Chem. Mater., 2011, 23,
energy band. Variation of the pump intensity shows saturation of
the section with the lower energy excited state supporting the
energy transfer model between different sections rather than an
internal conversion process within one section. The exciton
annihilation responsible for the intensity dependent decay times
also proves the high mobility of the excitons. The energy transfer
rates are on the same order of magnitude as observed in other
conjugated polymers75–79 such as MEH-PPV but are slower than
in a squaraine copolymer [SQA-SQB]n composed of trans- and cis-
indolenine squaraine dyes for which we estimated exciton trans-
470–482.
¨
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fer rates of about (30 fs)À1 31
The latter also shows significant
.
exchange narrowing of the absorption band which is most likely
caused by a high structural order but is almost absent in the
present polymer owing to a much smaller delocalization length
which may also be responsible for the much slower energy
transfer rate. However, for the [SQB]n homopolymer we found,
depending on the solvent and the specific pathway, energy
18 G. Wei, S. Wang, K. Sun, M. E. Thompson and S. R. Forrest,
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transfer rates between (70 fs)À1 and (4 ps)À1 32
Thus, in terms
.
¨
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
We acknowledge funding by the Deutsche Forschungsgemeinschaft
within the Research Group FOR 1809 and the Bavarian State
Ministry of Science, Research and the Arts within the SolTech
network.
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27 M. H. Schreck, M. I. S. Rohr, T. Clark, V. Stepanenko,
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