Synthesis and characterization of polymethacrylates containing conjugated oligo(phenylene ethynylene)s as side chains

Article abstract of DOI:10.1002/pola.26105

In order to form suitable systems designed for resonance energy transfer, a series of monodisperse methacrylate-based monomers containing rigid π-conjugated oligo(phenylene ethynylenes) with different sizes of the conjugated systems (M1-M3), and therefore different optoelectronic properties, were synthesized and subsequently polymerized using the reversible addition-fragmentation chain transfer polymerization technique (P1-P3). In addition, these oligomers were also copolymerized with methyl methacrylate. The obtained polymers were characterized by ¹H NMR spectroscopy, size exclusion chromatography, and analytical ultracentrifugation. The photophysical properties of the polymers were studied by UV-vis absorption and emission spectroscopy in diluted solutions as well as in thin films and compared to the photophysics of the corresponding monomers. Thereby, changes going from monomeric to polymeric systems could be detected in fluorescence quantum yields and lifetimes pointing to energy trapping, e.g., energy transfer. Donor-acceptor copolymers containing different numbers of monomeric units within the side chain exhibit differences in the emission spectra, indicating that energy trapping in polymers is very sensitive to structural properties such as the chain length. UV-vis absorption spectroscopy as well as time-resolved lifetime studies indicate intrapolymer and interpolymer energy transfer.

Full text of DOI:10.1002/pola.26105

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Synthesis and Characterization of Polymethacrylates Containing
Conjugated Oligo(phenylene ethynylene)s as Side Chains
Alexander M. Breul,1,2 Johann Schafer, Georgy M. Pavlov,1,2 Anke Teichler,1,2,4
2,3
€
1,2,4
Stephanie Hoppener,
Christine Weber,1,2,4 Ju¨ rgen Nowotny,1,2 Lars Blankenburg,5
€
Ju¨ rgen Popp,2,3,6 Martin D. Hager,1,2,4 Benjamin Dietzek,2,3,6 Ulrich S. Schubert1,2,4
1Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich-Schiller-University Jena, Humboldtstr. 10, 07743 Jena,
Germany
2Jena Center for Soft Matter (JCSM), Friedrich-Schiller-University Jena, Humboldtstr. 10, 07743 Jena, Germany
3Institute of Photonic Technology Jena e.V. (IPHT), Albert-Einstein-Straße 9, 07745 Jena, Germany
4Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands
5Thuringisches Institut fur Textil- und Kunststoff-Forschung e.V. (TITK), Breitscheidstr. 97, D-07407 Rudolstadt, Germany
¨
¨
6Institute of Physical Chemistry (IPC) and Abbe Center of Photonics (ACP), Friedrich-Schiller-University Jena, Helmholtzweg 4,
07743 Jena, Germany
Correspondence to: M. D. Hager (E-mail: martin.hager@uni-jena.de) or B. Dietzek (E-mail: benjamin.dietzek@uni-jena.de) or U. S.
Schubert (E-mail: ulrich.schubert@uni-jena.de)
Received 28 March 2012; accepted 4 April 2012; published online 17 May 2012
DOI: 10.1002/pola.26105
ABSTRACT: In order to form suitable systems designed for reso-
nance energy transfer, a series of monodisperse methacrylate-
based monomers containing rigid p-conjugated oligo(pheny-
lene ethynylenes) with different sizes of the conjugated sys-
tems (M1–M3), and therefore different optoelectronic
properties, were synthesized and subsequently polymerized
using the reversible addition–fragmentation chain transfer po-
lymerization technique (P1–P3). In addition, these oligomers
were also copolymerized with methyl methacrylate. The
obtained polymers were characterized by 1H NMR spectros-
copy, size exclusion chromatography, and analytical ultracen-
trifugation. The photophysical properties of the polymers were
studied by UV–vis absorption and emission spectroscopy in
diluted solutions as well as in thin films and compared to the
photophysics of the corresponding monomers. Thereby,
changes going from monomeric to polymeric systems could
be detected in fluorescence quantum yields and lifetimes
pointing to energy trapping, e.g., energy transfer. Donor–
acceptor copolymers containing different numbers of mono-
meric units within the side chain exhibit differences in the
emission spectra, indicating that energy trapping in polymers
is very sensitive to structural properties such as the chain
length. UV–vis absorption spectroscopy as well as time-
resolved lifetime studies indicate intrapolymer and interpoly-
C
V
mer energy transfer. 2012 Wiley Periodicals, Inc. J Polym Sci
Part A: Polym Chem 50: 3192–3205, 2012
KEYWORDS: dyes/pigments; energy transfer; fluorescence;
monodisperse oligo(phenylene ethynylene)s; reversible addition
fragmentation chain transfer polymerization (RAFT); p-stacking
INTRODUCTION During the past decade, the field of comb
polymers attracted significant attention in various areas, e.g.,
(hybrid) nanostructures,1–4 biomedicine,5–7 super soft elasto-
mers,8,9 photonics,10 and thermoresponsive polymers with
lower critical solution temperature behavior.11–13 The dense
packing of the side chains covalently bound to a linear back-
bone causes confined and compact structures, in particular,
worm-like conformations.14 Hence, notable chain end effects
can be observed. The incorporation/attachment of conju-
gated oligomers to nonconjugated backbones leads to a vari-
ety of different structures due to the wide range of possible
pendant molecules revealing chemical stability, photoconduc-
tivity of the attached p-electron systems, and an invariance
of the standard oxidation/reduction potential with the
degree of substitution.15 In order to take advantage of these
possibilities, conjugated oligomers were attached either to al-
iphatic polymers as side chains or in dendritic structures.16
The resulting polymers combine the typical polymer proper-
ties (e.g., film forming ability, mechanical stability, and proc-
essing advantages)16 with the well-defined electronic, pho-
tonic, and morphological properties of the monodisperse
oligomer moieties. In addition, the generally low solubility of
€
Alexander Breul and Johann Schafer equally contributed to this work.
Additional Supporting Information may be found in the online version of this article.
C
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2012 Wiley Periodicals, Inc.
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SCHEME 1 Schematic representation of the monomer synthesis.
the oligomers can be improved significantly. For this pur-
pose, large variety of p-conjugated oligomers for
nium complexes.28 Similar to the natural light-harvesting
units, where fast RET and funneling to a reaction center
occurs because of short interchromophore distance and
adequate spatial arrangement,37 the polymers at hand allow
for RET and energy funneling from a donor to an acceptor.
a
–
instance, carbazoles,17 triphenylamines,15 perylenes,18 diphe-
nylacetylene units,19 and oligothiophenes20 – were attached
to aliphatic backbones (e.g., methacrylates) as side chains.
Diverse block copolymers published by Thelakkat and co-
workers21–25 revealed good phase separation behavior fea-
turing nanodomain sizes between 5 and 20 nm. Thus, they
became attractive for applying them in bulk hetero-junction
organic solar cells. This defined distance lies not only within
the range of the exciton diffusion length which plays a signif-
icant role for organic photovoltaics but also in the range of
€
The Forster radius of 43 Å for the donor–acceptor pair M2/
M3 illustrates this potential and is comparable to other tai-
lor-made donor–acceptor pairs.38 The homopolymers P2 and
P3 as well as the polymers P2-stat-P3 and P2-stat-PMMA-
stat-P3 are potentially light-harvesting units for an enhanced
excitation of reaction centers by light to use their photosyn-
thetic capacity.
€
€
the Forster radius of the most suitable dyes for the Forster
resonant energy transfer (FRET). So far, the light-harvesting
properties of selected dyes, e.g., coumarins26,27 or 4-hydroxy
thiazoles,28 were used to transfer the energy to Ru(II) tris-
bipyridine complexes along the polymer chain.
RESULTS AND DISCUSSION
Synthesis
Three monodisperse oligomeric conjugated monomers based
on phenylene ethynylene units were synthesized in moderate
to good yields by multiple sequential Sonogashira cross-cou-
pling reactions (Scheme 1). The donor monomer 1, the do-
nor dimer 2, and the acceptor trimer 3 were coupled with
the functional methacrylate 4. By this manner, the conju-
gated system was extended and the polymerizable function –
i.e., the methacrylate – was introduced. Noteworthy, no side
reactions of the double bond or polymerization of the meth-
acrylate occurred during the Sonogashira cross-coupling
reaction. Further details are summarized in Supporting Infor-
mation. The rigid, rod-like character of the monomers is
expected to promote p-stacking and ordering by its close
spatial arrangement along the polymer chain.
In this contribution, the design and synthesis of new comb
polymers with p-conjugated oligomers as side chains are
described to combine the well-defined optoelectronic proper-
ties of the oligomers with typical processing properties of
common polymers. For this purpose, the synthesis of differ-
ent monomers that consist of a monodisperse oligo(pheny-
lene ethynylene) (OPE) moiety and a methacryl end group as
a polymerizable function was targeted (M1–M3). The rod-
like structure of the triple bond results in rigid, linear, and
planar oligomers, enabling the possibility for a supramolecu-
lar arrangement, e.g., via stapling and p-stacking.29,30 Subse-
quently, these monomers were polymerized using a con-
trolled radical polymerization (CRP) technique, specifically
the reversible addition–fragmentation chain transfer (RAFT)
method.31 The precisely defined length, conformation, and
constitution of the monomers allow a reliable prediction of
structure–property relationships.32 In addition, one can gain
further information about the folding properties of the corre-
sponding polymers. The close spatial arrangement of the
conjugated oligomers in the polymer chain should lead to
increased intermolecular interactions compared with the iso-
lated and independent oligomers. As a consequence, the
polymers presented are capable to act as light harvesters
possibly for artificial photosynthetic systems, like, hydrogen
evolving metal complexes33–36 or as sensitizers for ruthe-
For all polymerizations of the three different monomers M1–
M3, a standard procedure of the RAFT polymerizations tech-
nique39 was applied (Scheme 2, Table 1). In the following
paragraphs, the nomenclature for the different substances is
applied as follows: the capital letter M stands for the mono-
mers, whereas P is used for polymers. The monomers and
polymers, respectively, were numbered from 1 to 3 in order
to demonstrate the type of the conjugated system that is
attached to the polymerizable function and backbone,
respectively. 1 symbolizes the donor dimer, 2, donor trimer,
and 3, acceptor trimer. The small indices a–e indicate the
difference of the samples that were synthesized in different
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SCHEME 2 Schematic representation of the RAFT polymerization of M1–M3 (left) and the donor–acceptor copolymers (right).
batches to obtain different molar masses and chain lengths,
respectively. Consequently, the statistical donor–acceptor co-
polymer is abbreviated by P2-stat-P3. Furthermore, copoly-
mers with an increased distance between the single oligom-
ers were synthesized. For this purpose, the dyes were
‘‘diluted’’ with methyl methacrylate (MMA) as comonomer. In
case of these polymers, ‘‘-stat-PMMA’’ was inserted into the
abbreviation. 2,20-Azobis(iso-butyronitrile) (AIBN) was used
as an initiator and 2-cyanobutan-2-yl benzodithioate (CBDB)
as a chain transfer agent.13 Generally, toluene was used as
solvent, but for the polymerizations of M3, tetrahydrofuran
(THF) was applied, due to the low solubility of M3 in tolu-
ene. The chain length of the polymers could be adjusted by
the variation of the [monomer]:[CBDB] ratio. For the poly-
merization of the statistical donor–acceptor polymer P2-
stat-P3, a ratio of M2:M3 of 1:1.5 was used. The resulting
copolymer had a composition of 30% M2 and 70% M3,
which was calculated from the 1H NMR spectrum (Support-
ing Information) indicating a slightly enhanced incorporation
of M3 into the copolymer. Because of the high molar masses
of the monomers (662.9 g/mol for M2 and 520.6 g/mol for
M3), the conversions of the monomers could not be deter-
mined separately by gas chromatography. Alternatively, size
exclusion chromatography (SEC) using a UV-detector could
not be used to determine both monomer conversions,
because both monomers show equal retention times (no
baseline separation) due to the relatively low difference in
molar masses compared to the whole calibration range of
the SEC column with the lowest exclusion limit. Furthermore,
1H NMR shifts are overlapping for the vinyl as well as for
TABLE 1 Summary of the Observed Molar Masses (Mn, SEC), PDI Values, M/CBDB Ratios, Reaction Times, and Monomer
Conversions of all Synthesized Polymers
Mn, SEC
Reaction
time (h)
Conversion
(%)
Sample
(g/mol)
PDI
M/CBDB
P1a
17,000a
24,000a
33,100a
8,800a
16,400a
13,600a
14,400a
11,100a
9,300a
1.29
1.37
1.85
1.25
1.29
1.20
1.24
1.20
1.16
1.24
1.17
1.30
60
60
16
20
23
16
14
25
17
16
18
8.5
7
64
P1b
85
P1c
60
93
P1d
20
74
P1e
60
77
P2a
60
44
P2b
20
83
P3
20
87
P2-stat-P3
20
–
P2-stat-PMMA
P3-stat-PMMA
P2-stat-PMMA-stat-P3
14,600b
10,500b
10,800b
100
100
150
64 (MMA), 79 (M2)
66 (MMA), 91 (M3)
29 (MMA)
16
a
b
CHCl3, PS standard.
CHCl3, PMMA standard.
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polymerization40 process, whereas they are stable during the
RAFT polymerization.41,42 To prove that the triple bonds stay
unaffected during the RAFT polymerization, 13C NMR spectra
were recorded. The typical shifts of tertiary carbon signals
that belong to alkyne functions are located at around
90 ppm (see Supporting Information for 13C NMR spectrum
of P1d). Additionally, the solubility did not decrease signifi-
cantly after the polymerization (bad solubility or swelling is
an indication for cross-linking). A further proof for the stabil-
ity of triple bonds throughout the RAFT process is given in
the ‘‘optical properties’’ section below.
Table 1 summarizes the molar masses obtained by SEC, the
according polydispersity index (PDI) values, and the monomer
conversions of each polymerization. It is possible to obtain
well-defined polymers with conjugated side chains which have
PDI values between 1.15 and 1.30, as typically obtained for
RAFT polymerizations. A control over the reaction could be
obtained up to conversions of approximately 90%. For higher
conversions, the PDI values increased (P1c), which was al-
ready described, e.g., for polymerizations of N-isopropylacryla-
mide.43 This is a well-known phenomenon for CRP due to pos-
sible chain coupling reactions that are more likely to occur at
higher conversions.31 The conversions were calculated using
1H NMR spectroscopy. The molar masses, which were obtained
by SEC measurements, are based on polystyrene (PS) stand-
ards (see Fig. 2 for SEC traces for all P1 samples; for P1c, the
exclusion limit of the column was reached). Therefore, the
question arises if the molar mass is also overestimated, as in
the case of conjugated linear PPEs.44
FIGURE 1 1H NMR spectrum of P1a (CDCl3, 300 MHz).
the aromatic proton signals due to their similar surrounding
at the polymerizable function; as a consequence, no conver-
sion over time plot could be measured. In this case, M3 seems
to be incorporated slightly faster in the polymeric backbone.
Under the same reaction conditions for the polymerizations as
already described above, monomer M3 was also incorporated
faster than the MMA in the PMMA-containing copolymer P3-
stat-PMMA. For the polymer P2-stat-PMMA, an [oligo-
mer]:[MMA] ratio of 1:9 was applied for the polymerization
resulting in a donor trimer content of 14%. Having the
increased polymerization rate of M3 in mind, the ratio
between of [MMA]:[M3] was set to 97:3 for the copolymeriza-
tion. The obtained content of acceptor trimer in P3-stat-
PMMA was 8%. For the PMMA-containing donor–acceptor co-
polymer P2-stat-PMMA-stat-P3, a reaction mixture with a
[M2]:[M3]:[MMA] ratio of 8:16:76 was used to obtain a com-
parable donor–acceptor ratio to P2-stat-P3. The final compo-
sition of the polymer reveals 12% donor trimer, 24% acceptor
trimer, and 64% PMMA. A potential explanation for the
increased oligomer content could be the purging process
before the polymerization, due to the evaporation of MMA.
In order to investigate this issue, three selected samples (P1b,
P1d, and P1e all with a narrow molar mass distributions)
were studied in toluene by analytical ultracentrifugation
(AUC) – an absolute molar mass characterization method – to
obtain more information about the ‘‘real’’ molar masses of the
polymers. The calculation of the average Mn and Mw values by
the Svedberg equation is sophisticated and depends on the
relation between the sedimentation velocity coefficient and
the molar mass. The obtained value MsD : Msf is situated
Structural Characterization and Molar Masses
1
The obtained polymers were characterized by H NMR spec-
troscopy and SEC. The aromatic bands, the alkoxy chains,
and the backbone protons can be distinguished for all sam-
ples. Fig. 1 provides a representative 1H NMR spectrum of
P1a (see Supporting Information for P2, P3, P2-stat-PMMA,
P3-stat-PMMA as well as P2-stat-P3 and P2-stat-PMMA-
stat-P3). The 1H NMR spectrum depicts that the splitting of
the monomer signals (for the 1H NMR spectra of the mono-
mers, see the Supporting Information) disappeared after the
polymerization and turned into the typical broad polymer
signals. The vinylic protons of the methacrylate moiety of
the monomer also vanished. Alkyne functions are known to
polymerize by free RP19 and during the nitroxide-mediated
FIGURE 2 SEC traces of P1a to P1e (CHCl3/isopropyl alcohol/
triethylamine).
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TABLE 2 Comparison of the Molar Masses Obtained by SEC
groups by the polymeric backbone is expected to cause a
stronger intermolecular and intramolecular packing (p-stack-
ing) as well as ordering of the conjugated oligomers, com-
pared to single oligomers. In contrast to the homopolymers,
for the polymers which are not dye-functionalized through-
out the whole polymer backbone, this effect is not expected.
and AUC
Mn, SECa
(Mn þ Mw)/2b
MAUC
Sample
(g/mol)
(g/mol)
(g/mol)
P1b
P1d
P1e
a
24,000
8,800
28,400
9,900
58,000
14,000
27,000
Table 3 summarizes selected optical properties of the syn-
thesized monomers and polymers, in particular, absorption
and emission maxima and the according extinction coeffi-
cients, quantum yields, and lifetimes.
16,400
18,800
CHCl3, PS calibration.
b
Mn and Mw determined by SEC.
First, the absorption and emission behavior of the monomers
and of the according polymers (solutions and films) were
compared (Fig. 3 for M2/P2). It is illustrated that in every
case, the polymers showed, as expected, the same absorption
in solution compared with their corresponding monomers.
The identical absorption and emission behavior of the mono-
mers and polymers are the final proof that the conjugated
system of the monomers stays unaffected during the RAFT
polymerization process. In the case of alkyne polymerization,
a cross-linking would occur which would, as a consequence,
lead to an enlargement of the conjugated system. Therefore,
additional bands in the UV–visible (UV–vis) absorption and
emission spectra would necessarily have to appear.
between Mn and Mw, in the case of an ideal solvent system
MsD : Msf ¼ (Mn þ Mw)/2. In this case, the situation is close
to the ‘‘ideal’’ one (i.e., the thermodynamically ideal solution).
Table 2 compares the molar masses obtained by the two dif-
ferent analytical methods. The problem for the SEC measure-
ments is that there is no comparable standard for the synthe-
sized polymers. In all cases, the PS standard was considered
as the best suitable one. The values between SEC and AUC dif-
fer by the factor 1.4 up to more than 2. In contrast to linear
PPEs, where the molar masses are overestimated by SEC due
to the rigid structure,44 the molar mass of the investigated
polymers is underestimated in the SEC measurements. This is
supposed to be a result of the dense comb-like structure of
the polymers that hinders the formation of loose polymer
coils (e.g., as in the case of PS) in solution.14
All polymer films revealed a slight bathochromic shift of 5–
10 nm of the absorption maxima. This represents a first indi-
cation for interactions of the conjugated side chains in the
film.45 In contrast, the oligomers featured worse or no film
formation. A red-shift of the emission maxima of the polymer
solutions compared to the monomer solutions as well as a
red-shift of the emission maxima of the polymer film com-
pared with polymer solutions was observed in each case. For
P1 and P2, this effect is much stronger for the polymer films
than for the polymer solutions whereas the formation of the
film has no influence on the emission for P3 (see Supporting
Information for selected UV–vis spectra). This means that for
P1 and P2, a film formation induces a further planarization
and interaction of the oligomers, whereas in the case of P3,
the side chains might be already aligned planar in solution
due to the lower steric hindrance of less alkoxy side
chains.46,47
In contrast to the homopolymers P1–P3 and P2-stat-P3, the
PMMA-containing copolymer’s molar masses were deter-
mined only by SEC (Table 1). The accordance between the
theoretical molar masses and the measured ones leads to
the conclusion that SEC provides reliable values. Further-
more, the low content of oligomers incorporated is expected
not to cause significant changes in the hydrodynamic volume
of the polymers compared to the PMMA standards.
Optical Properties
The target of this work was the design of new comb poly-
mers with conjugated, monodisperse side chains, and to
study the optical behavior of the resulting structures. For
that purpose, five different structures have been chosen: the
donor–dimer polymers (P1), the donor–trimer polymers
(P2/P2-stat-PMMA), and in contrast to the others, the two
electron withdrawing moiety containing acceptor–trimer
polymers (P3/P3-stat-PMMA). The connection of the side
Another strong evidence for increased side chain interactions
in the polymers is depicted in Fig. 4 for P2 (see Supporting
Information for the spectra of P1, P3 as well as P2-stat-P3).
The monomer and the according polymer absorption
TABLE 3 Summary of Selected Optical Properties of the Monomers
Sample
kabs (nm)
kem (nm)
mstokes (cmꢀ1
)
U (%)
s1 (ns)
s2 (ns)
Egap (eV)
M2
318, 369
313, 369
317, 369
312, 431
308, 427
310, 430
402
406
406
529
536
529
2, 224
2, 469
2, 469
4, 298
4, 762
4, 352
47
22
67
82
6
0.66
1.17 (0.33)a
3.08
3.08
3.08a
2.58
2.58
2.58
P2b
0.21 (0.31)a
P2-stat-PMMA
0.91
M3
5.43
1.48 (0.40)a
P3
0.26 (0.35)a
P3-stat-PMMA
61
4.11
a
In brackets preexponential factors of the double exponential fit are given.
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FIGURE 3 UV–vis absorption (left) and emission spectra (right) of M2 and P2 in CHCl3 (c ¼ 10ꢀ5 M).
behavior were studied in CHCl3 solution under increasing
MeOH content in solution (vol %). The lower solubility of
the polymer in MeOH should force the polymer chains to col-
lapse as well as the side chains to planarize and to interact
with increasing MeOH concentration.
In order to investigate a potential energy transfer from a do-
nor to an acceptor unit within the comb polymes, a statisti-
cal copolymer P2-stat-P3 was prepared. For a more facile
comparison, an analogous mixture of both homopolymers
(30:70 P2:P3) was investigated via UV–vis spectroscopy in
CHCl3 solution with increasing MeOH content (Supporting In-
formation). In both cases, a similar behavior was observed
concerning the lowered absorbances as well as the batho-
chromic shifts of the maxima. In order to investigate the
influence of the distance between the donors and acceptors,
a PMMA-containing D–A copolymer (P2-stat-PMMA-stat-P3)
was prepared. In this case, the PMMA-containing analogous
donor and acceptor polymers were used as references. For
this purpose, a mixture of P2-stat-PMMA and P3-stat-
PMMA was prepared with the same donor:acceptor ratio
(1:2) as in P2-stat-PMMA-stat-P3 and diluted to the same
absolute absorbance in solution to ensure a comparable
chromophore concentration.
In fact, the spectral behavior of all three homopolymers is
altered upon MeOH addition, whereas the monomer absorp-
tion behavior is independent from the CHCl3:MeOH ratio. For
all PMMA-containing polymers, a dependency on the sol-
vent:non-solvent ratio could not be observed. The absorption
of the homopolymers was red-shifted and comparable to the
polymer film. This effect is stronger for the longer conju-
gated side chains, due to the lower flexibility and the
increased p-conjugated system. First changes occur at MeOH
contents between 40 and 50% due to an increasing aggrega-
tion.48 P2 as well as P3 could not be measured in solutions
with MeOH contents higher than 80 and 60%, respectively,
due to the onset of precipitation.
FIGURE 4 UV–vis absorption spectrum of M2 (left) and P2b (right) in CHCl3 at different MeOH concentrations. The inset shows the
factor loadings of the factor analysis, for two species, contributing to the absorption spectra.
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FIGURE 5 Steady-state extinction of M3 and emission spectra of M3 and P3 (left), time-correlated single photon counting mea-
surement in dichloromethane at 296 K (right). Solid lines represent the fitting curves.
The influence of the polymerization of M2 and M3 on the
emission properties was additionally investigated by time-
resolved measurements and measurements of the fluorescence
quantum yields of polymerized and unpolymerized species.
The results of these measurements are summarized in Table
3: comparing the quantum yields of M3, P3-stat-PMMA, and
P3, one finds that the quantum yield is reduced by 20% when
M3 is polymerized to P3-stat-PMMA and is further reduced
when M3 is polymerized to homopolymer P3.
setup was chosen in order to avoid distortion of the inten-
sity ratio between perpendicular and parallel intensities
through the polarization sensitive imaging spectrograph.
Before recording the fluorescence intensities with perpendic-
ular and parallel polarized laser pulses, the sensitivity of the
setup for perpendicular and parallel polarized light was
tested (see Fig. 6, left). Therefore, scattered photons from a
diluted scatterer irradiated with the used excitation pulses
were detected with perpendicular and parallel orientation
between the laser polarization and the trans axis of the
Glan–Thompson polarizer placed in the emission channel.
For perpendicular orientation, only 40 counts in the maxi-
mum were detected, in contrast, to more than 5,000 counts
in the maximum for parallel orientation. Thus, the setup
used for time-resolved fluorescence anisotropy measure-
ments is highly sensitive for the polarization direction.
Accordingly, a shortening of the fluorescence lifetimes is
observed after polymerization of M3 (see Fig. 5, right). This
indicates that polymerization of M3 causes quenching of the
fluorescence. As described by Farinha and Martinho,49
quenching of emission in RAFT polymers can be caused by
the RAFT end group. Furthermore, in polymers processes,
such as singlet energy migration, chromophore diffusion, seg-
mental rotation, and energy trapping by excimer-forming
sites might play important roles in fluorescence quenching.
For P3, the fluorescence quenching is stronger than that for
P3-stat-PMMA because of the higher chromophore content
leading to an enhanced interaction of the adjacent chromo-
phores. These interactions result in an increased trapping
efficiency. Furthermore, for P3, the emission decay is not
single-exponential anymore (Fig. 5, right). This is often
observed in polymers with high chromophore fractions.50,51
In order to decipher the reason for the changed fluorescence
decay times after polymerization, time-resolved anisotropy
measurements were applied and used to test for energy
migration in the polymers P3-stat-PMMA and P3. Generally,
energy migration accelerates fluorescence depolarization.52
Thus, a shortening in the anisotropy decay of P3-stat-PMMA
and P3 compared with M3 could provide indications for
energy migration within the polymers.
The anisotropy r(t) was calculated using the measured inten-
sities Imm and Ihm: r(t) ¼ (Ivv ꢀ Ihv)/(Ivv þ 2Ihv).
The experimental time-resolved anisotropies are shown in
Figure 6, right: the decays were fitted by the single-exponen-
tial decay r(t) ¼ r0 exp[(t ꢀ t0)/h] þ r , where r0 is the ini-
1
tial anisotropy, t0 is the excitation point in time, H is the ani-
sotropy relaxation time, and r is the residual anisotropy.
1
This decay function can successfully describe the anisotropy
of the monomer and the polymers. Although the fit function
is not rooted in a molecular modeling of the physical proc-
esses leading to fluorescence anisotropy decay, it delivers
values for the anisotropy correlation time, which can be dis-
cussed qualitatively and used for mutual comparison of the
different compounds (Table 4).
Comparing the values of the anisotropy fit constants in Table
4, it can be noted that for all compounds, the sum of r0 and
r
is smaller than 0.4, which is the maximum value of ani-
For time-resolved fluorescence anisotropy measurements,
the trans axis of the polarizer on the emission side was set
to vertical orientation while the polarization of the excitation
light was set by a zero-order half-wave-plate perpendicular
and parallel to the trans axis of the emission polarizer. This
1
sotropy for randomly orientated fluorophores with parallel
absorption and emission transition moments. When assum-
ing randomly orientated fluorophores in solution, the initial
anisotropy r0 þ r is expected to be 0.4. Values below 0.4
1
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FIGURE 6 Sensitivity test of the setup for perpendicular and parallel polarized light (left). The inset shows a zoom-in into the
region of interest. Fluorescence anisotropy decay in dichloromethane at RT (right). The anisotropy was calculated from r(t) ¼ (Ivv
ꢀ Ihv)/(Ivv þ 2Ihv). Solid lines represent the fit.
point to depolarizing effects, which are not fully time-
resolved experimentally. For the polymer P3, the initial ani-
the polymer chain of P3, but further studies with improved
temporal resolution need to detail the kinetics of the
process.
sotropy, r0 þ r is smaller than that of the monomer M3,
1
indicating a fast depolarization process, which is not cap-
tured within the time resolution of the experiment. This pro-
cess is apparently not observable for P3-stat-PMMA. The an-
isotropy relaxation time of M3 (0.26 ns) is the time in which
rotational diffusion takes place. For M3, rotational diffusion
is the reason why the fluorescence becomes depolarized. In
polymers like P3 and P3-stat-PMMA, the fluorescence depo-
larization can be caused by energy transfer between individ-
ual fluorophores (homo energy transfer). In cooperation with
rotational diffusion, one would hence expect a faster loss of
the fluorescence anisotropy compared to the monomer
M3.52,53 This is the case, when considering the initial value
A different photophysical behavior after polymerization was
found for M2, P2-stat-PMMA, and P2: the emission quantum
yield is increased when M2 is polymerized to P2-stat-
PMMA. For P2, the quantum yield is again reduced (Table
3). The fluorescence lifetimes of the oligomers in the poly-
mers indicate the same type of behavior as the quantum
yields. This means that in P2-stat-PMMA nonradiative decay
rates are reduced most likely by environmental and struc-
tural changes upon polymerization. This was also observed
for another PMMA polymer with fluorophores in the side
chains.28 Similar to P2-stat-PMMA and P3, the fluorescence
decay changes from a mono-exponential decay for P2-stat-
PMMA to a more complex decay for P2, the latter likely
caused by increased interactions of adjacent chromophores
because of the high molar fraction of chromophores (100%)
in the polymer side chain.
r0 þ r of the anisotropy for P3. The value of the initial ani-
1
sotropy indicates fast fluorescence depolarization for the
polymer P3 which is not time resolved with the experimen-
tal setup in addition to the measurable anisotropy decay
time of 0.19 ns.
As already discussed for P3, the non-mono-exponentially
decay of the homopolymer emission and the weak fluores-
cence quantum yield of P2 and P3 indicate homo-energy
transfer within the polymer, which becomes feasible by the
close spatial arrangement of chromophores in the polymers.
In addition to the rapidly decaying anisotropy components,
the polymer anisotropies exhibit long-lived non-zero compo-
nents r . These are most likely due to slower overall rota-
1
tional motion of the polymer chains. The reduced relaxation
time for P3 compared to P3-stat-PMMA stems from confor-
mational differences between the polymers, which lead to
different volumes of the rotating molecule, and thus, differ-
ent rotation correlation times. The number of repeating units
is estimated to be 21 and 78 for P3 and P3-stat-PMMA,
respectively. In the latter polymer, 8 mol % of the repetiting
units exhibit a chromophor in the side chain. These struc-
tural differences are one reason for the differences in the
measured correlation times.
In order to design a system capable of hetero-energy trans-
fer, the statistical donor–acceptor copolymer P2-stat-P3 was
TABLE 4 Fitting Parameter for the Anisotropy Decay:
r(t) 5 r‘ 1 r0 exp (-t/H)
Sample
r0
H (ns)
r
1
Finally, it can be stated that polymerization of M3 reduces
the lifetime and the emission quantum yield of the polymers
compared to the single oligomer. The experimental data for
the fluorescence anisotropy indicate energy migration within
M3
0.32
0.22
0.17
0.26
0.48
0.19
ꢀ0.03
0.07
P3-stat-PMMA
P3
0.06
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FIGURE 7 Comparison of UV–vis absorption (left) and emission spectra (right) of the homopolymers P2a (kexc ¼ 375 nm) and P3
(kexc ¼ 429 nm), the copolymer P2-stat-P3 (kexc ¼ 375 nm), and its corresponding mixture of the homopolymers (kexc ¼ 375 nm) in
solution.
prepared. For a comparison, an analogous mixture of both
homopolymers (30:70 P2:P3) was investigated via UV–vis
spectroscopy. This was done for CHCl3 solutions with
increasing MeOH content (Supporting Information). In both
cases, a similar behavior was observed, i.e., with increasing
content of MeOH, lower absorbances and bathochromic shifts
of the absorption bands are observed.
observed. The residual emission of the acceptor can be
assigned to either weak donor–acceptor energy transfer or –
more likely – to direct excitation of the acceptor units due to
the small non-zero acceptor absorption coefficient at the ex-
citation wavelength. In the case of the copolymer, primarily
the acceptor fluorescence could be observed if the donor is
excited. Hence, donor–acceptor energy transfer is observed
with the donor emission overlapping with the acceptor
A hint towards energy transfer in the donor–acceptor copol-
ymer (P2-stat-P3) is given in Figure 7. The absorption and
emission spectra of both homopolymers, homopolymer mix-
ture, and P2-stat-P3 are displayed. The shape of the absorp-
tion spectra of P2-stat-P3 and the homopolymer mixture is
virtually identical, which leads to the conclusion that the
mixture has the same composition as the statistical copoly-
mer. The emission spectrum of the mixture shows that if the
donor is excited exclusively, mainly donor emission is
€
absorption. Therefore, Forster energy transfer can occur, if
the distance between the donor and acceptor is sufficiently
small54 as in the statistical copolymer (R0 ¼ 43 Å).
The absorption and emission behavior of the films (Fig. 8)
differ from the behavior of the polymers in solution: gener-
ally, the shape of the absorption spectra of the films is virtu-
ally identical to the spectra in solution; however, the absorp-
tion maxima are shifted to longer wavelengths. Thus, the
FIGURE 8 Comparison of UV–vis absorption (left) and emission spectra (right) of the homopolymer films P2a (kexc ¼ 375 nm) and
P3 (kexc ¼ 429 nm), the copolymer film P2-stat-P3 (kexc ¼ 375 nm), and the corresponding film of the homopolymer mixture (kexc
¼ 375 nm).
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FIGURE 9 Emission and excitation spectra of P2-stat-PMMA, P3-stat-PMMA; and excitation spectrum of P2-stat-PMMA-stat-P3
(left); and excitation spectra of P2-stat-PMMA, P3-stat-PMMA, and P2-stat-P3 (right) in dichloromethane at 296 K.
emission of the homopolymer mixture as well as the P2-
stat-P3 shows energy transfer. This effect can be explained
by the shorter distances between different polymer chains
and a closer packing in the films. Presumably, energy trans-
fer in solution only takes place between the oligomers of
one chain due to low concentrations used in the UV–vis
spectroscopy experiments. In contrast, the intermolecular
distances between donor and acceptor units in the optical
active polymer parts are small enough to allow for quantita-
tive energy transfer.
For both donor–acceptor polymers P2-stat-PMMA-stat-P3
and P2-stat-P3, fluorescence excitation spectra could be
measured. This could give rise to the assumption that the
two donor–acceptor systems reveal similar photophysical
properties. However, this is contradicted by the steady-state
emission spectra of the donor–acceptor polymer P2-stat-
PMMA-stat-P3 in Fig. 10. The shape of the donor fluores-
cence from the donor–acceptor polymer P2-stat-PMMA-stat-
P3 differs from the emission shape of P2-stat-P3 (Fig. 10).
The blue donor emission from P2-stat-PMMA-stat-P3 at
400 nm has dropped in intensity compared to the donor
emission at 435 nm (Fig. 10, left).
A further illustration of RET is given in Fig. 9, where fluores-
cence excitation spectra of the donor, acceptor, and donor–
acceptor polymers are depicted together with donor and
acceptor emission spectra: the donor and acceptor fluores-
cence excitation spectra are clearly separated from each
other. However, when the acceptor excitation in the donor–
acceptor polymer is recorded at 550 nm, an additional exci-
tation band is observed, which stems from excited donor
molecules which have transferred their excitation energy to
acceptor molecules.
This behavior is not observed for P2-stat-P3 (Fig. 7, right)
and not for M2 and has to be investigated more in detail:
such a dip in the emission spectra of P2-stat-PMMA-stat-
P3 could indicate reabsorption of emitted photons. But, for
our polymers, reabsorption caused by high concentration is
not detected because after dilution the emission spectra
are nearly unchanged, what is obvious from Figure 10,
right.
FIGURE 10 Emission of P2-stat-PMMA, P2-stat-PMMA-stat-P3, kexc ¼ 375 nm and P3-stat-PMMA, kexc ¼ 430 nm (left). Absorbance
and emission of P2-stat-PMMA-stat-P3 in dichloromethane at 296 K after dilution, kexc ¼ 375 nm (right).
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EXPERIMENTAL
NTegra Aura (NT-MDT, Moscow, Russia) or using a Nano-
scope IIIa Multimode (Digital Instruments, Veeco, Santa
Barbara, CA) with commercially available TiN-coated canti-
levers (NSC35, MicroMash). Sedimentation velocity experi-
ments were performed using a Beckman XLI analytical
ultracentrifuge (ProteomeLab XLI Protein Characterization
System) with interference optics and Al-double-sector cells
of optical path 12 mm. Rotor speed was 40,000 or 45,000
rpm at 20 ꢁC. The thermogravimetric analysis (TGA) analy-
sis was performed on a Netzsch TG 209 F1 (heating rate:
10 K minꢀ1; Td is given at 5% weight loss), and the dif-
ferential scanning calorimetry (DSC) was measured on a
DSC 204 F1 Phoenix by Netzsch under a nitrogen atmos-
phere in a temperature range from ꢀ100 to 200 ꢁC with
a heating rate of 20 K minꢀ1 (Tg values are reported as
onset values from the second heating run).
Material and General Experimental Details
All chemicals used were purchased from Biosolve, Fluka,
Aldrich, Acros Organics as well as Alfa Aesar and were used
without further purification unless otherwise specified. The
solvents were dried and distilled according to standard pro-
cedures. Preparative size exclusion chromatography was per-
formed on Bio-Rad S-X3 beads (size exclusion limit 2,000 g/
mol), swollen in toluene.
Instrumentation
1D (1H, 13C) and 2D (HSQC) NMR spectra were recorded
on a Bruker AC 300 (300 MHz) and a Bruker AC 250
(250 MHz) at 298 K. Chemical shifts are reported in parts
per million (ppm, d scale) relative to the residual signal of
the deuterated solvent. Coupling constants are given in
hertz. UV–vis absorption and PL emission spectra were
recorded on an Analytik Jena SPECORD 250 and a Jasco
FP-6500 spectrometer, respectively, at 298 K. Absolute PL
quantum yields were evaluated at 298 K using a Hama-
matsu Photonic Multi-Channel Analyzer C 10027. For these
techniques, dilute solutions (10ꢀ6–10ꢀ5 M, 1 cm quartz
cuvette) in chloroform were used. As reference, a quartz
cuvette filled with pristine solvent was used. Matrix-
assisted laser desorption/ionization time-of-flight mass
spectra (MALDI-TOF MS) were obtained using an Ultraflex
III TOF/TOF mass spectrometer with dithranol, trans-2-[3-
(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile
(DCTB), or terthiophene as matrix in reflector as well as
linear mode, respectively. The instrument was calibrated
prior to each measurement with an external PMMA stand-
ard from PSS Polymer Standards Services GmbH (Mainz,
Germany). Elemental analysis was performed on a CHN-
932 Automat Leco instrument. Size exclusion chromato-
grams were recorded on a SEC Shimadzu SCL-10A system
controller, LC-10AD pump, RID-10A refractive index detec-
tor, and PSS SDV pre/lin S column at 50 ꢁC (eluent: chlor-
Steady-state emission measurements were performed as al-
ready described in previous publications.38 Briefly, emission
quantum yields were measured by comparing the corrected
emission intensities of the solvents with the fluorescence in-
tensity of the standard quinine sulfate with a quantum yield
of 55%. The excitation wavelength was set to the most red-
shifted absorption maximum, whereby the extinction maxi-
mum was kept below 0.05.
The fluorescence decay curves were obtained by a Hama-
matsu streakscope C4334 in time-correlated single photon
counting modus under magic angle configuration. Triggering
was performed by the Hamamatsu trigger unit C4792-01. Af-
ter excitation with a frequency-tripled Ti-sapphire laser (Tsu-
nami, Newport Spectra-Physics GmbH), i.e., kex ¼ 290 nm, in
perpendicular direction, the fluorescence emission wave-
length was separated by a Chromex 250IS imaging spectro-
graph. The repetition rate of the laser was adjusted to 0.8
MHz by a pulse selector (model 3980, Newport Spectra-
Physics GmbH). All measurements were performed at con-
centrations below 10ꢀ6 M.
oform:triethylamine:iso-propanol 94:4:2; flow rate of
1
mL/min) using linear PS or PMMA standards. Spin coating
was performed at various spin speeds and accelerations
with a spin coater from Laurell Technologies Corporation
(North Wales). The spin coating time was set to 30 s, and
the experiments were performed at room temperature
(RT). Surface topography was measured by an optical
interferometric profiler Wyko NT9100 (Veeco, Mannheim,
Germany). The profiler was also used to determine the
thicknesses of the films. For this purpose, each film was
scratched with a scalpel in a controlled manner. At five
different positions of the film, the depth of the scratch
was measured with the optical profiler. These positions
are the center of the film and the four edges where the
scratch is located. A UV–vis/fluorescence plate reader from
Analytik Jena (FLASHScan 530, Jena, Germany) was used
to measure the UV–vis absorption spectra of the films. For
the microscope slides, an adapter was used. The measure-
ments were referenced to air. Fluorescence of the films
was measured with a modified Hitachi F-4500. AFM meas-
urements were performed in tapping mode either with a
General Procedure for the RAFT Polymerization
The desired amounts of the monomers (for detailed informa-
tion on the synthesis and characterization, see Supporting
Information) were transferred into the reaction vial (5 mL
reaction vessel) and dissolved in toluene. Thereafter, the cal-
culated volumes of stock solutions of CBDB in toluene and
AIBN in toluene were added. The ratio of [CBDB]:[AIBN] was
always 4:1. Before closing the vial, the reaction solution was
purged with a flow of argon for 30 min. Subsequently, the
reaction was performed in an oil bath at 70 ꢁC overnight
(see Table 1 for the exact reaction times and [M]/[CBDB]
ratios). The obtained polymers were purified by precipitation
into cold ethanol. The polymers were dried under reduced
pressure at RT. In contrast, THF was used as solvent instead
of toluene for the polymerization of M3 or the copolymeriza-
tions, respectively. Before degassing, the solution was heated
to 40 ꢁC and filtered hot to dissolve as much monomer as
possible. Purification by column chromatography using a
BioBeads S-X3 column (solvent toluene, exclusion limit
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2,000 g/mol) and a second precipitation into cold ethanol
was required for these samples.
chains), 3.64 (b, OCH3 PMMA), 4.03 (b, OCH2), 6.95, 6.97,
7.21, 7.59, 7.62, 7.70, 7.81, and 7.91 (b, ArH) ppm. EA: C:
64.95%, H: 7.68%, N: 1.37%, S: 1.52%. SEC (CHCl3, PS stand-
ard): Mn ¼ 10,500 g/mol; Mw ¼ 12,300 g/mol, PDI ¼ 1.17.
Homopolymer P1
1H NMR (CDCl3, 300 MHz): d ¼ 0.91 (b, CH3, backbone and
alkoxy chain), 1.1 to 1.6 (b, CH2 backbone and alkoxy chain),
2.30 (b, enclosed H2O), 3.89 (OCH2), 6.74 (b, ArH), 7.01 (b,
ArH), 7.40 (b, ArH) ppm. EA: C: 78.19%, H: 7.15%, N:
0.17%, S: 0.88% (P1a); C: 77.93%, H: 6.91%, N: 0.15%, S:
0% (P1b); C: 78.24%, H: 6.96%, N: 0,14%, S: 0% (P1c); C:
78.52%, H: 6.74%, N: 0.31%, S: 0.43% (P1d); C: 78.59%, H:
6.84%, N: 0.14%, S: 0% (P1e). SEC (CHCl3, PS standard): Mn
¼ 17,000 g/mol (P1a), 24,000 g/mol (P1b), 33,100 g/mol
ꢁ
ꢁ
DSC: Tg ¼ 47 C. TGA: Td ¼ 210 C.
Statistical Donor/Acceptor Containing PMMA P2-stat-
PMMA-stat-P3
1H NMR (CDCl3, 300 MHz): d ¼ 0.89, 1.05, 1.35, 1.47, 1.79,
and 2.07 (b, CH2 and CH3, backbone and alkoxy chains), 3.61
(b, OCH3 PMMA), 3.97 (b, OCH2), 6.87, 6.98, 7.12, 7.44, 7.57,
and 7.67 (b, ArH) ppm. EA: C: 70.49%, H: 6.82%, N: 2.53%,
S: 2.78%. SEC (CHCl3, PS standard): Mn ¼ 10,800 g/mol; Mw
¼ 14,000 g/mol, PDI ¼ 1.30. DSC: Tg ¼ 38 ꢁC. TGA: Td
¼
(P1c), 8,800 g/mol (P1d), 16,400 g/mol (P1e); Mw
¼
ꢁ
21,900 g/mol (P1a), 32,900 g/mol (P1b), 61,200 g/mol
(P1c), 11,000 g/mol (P1d), 21,200 g/mol (P1e); PDI ¼ 1.29
(P1a), 1.37 (P1b), 1.85 (P1c), 1.25 (P1d), 1.29 (P1e). DSC:
Tg ¼ 150 ꢁꢁC (P1a), 170 ꢁC (P1b), ꢁ178 ꢁC (P1c), 123 ꢁC
255 C.
CONCLUSIONS
In summary, the synthesis and characterization of homo- and
statistical copolymers, which contain monodisperse conju-
gated oligomers (OPEs) as side chains were described, using
the RAFT polymerization technique. The polymers combine
the well-defined and adjustable optical properties of the OPE
systems with typical polymer properties (e.g., film formation)
of the polymer backbone; the length and composition of the
polymer can be controlled by the RAFT polymerization pro-
cess. Moreover, the close spatial arrangement of the conju-
gated subunits enhanced the interaction between the OPEs
and changed the optical properties, compared with the non-
polymerized oligomer.
ꢁ
(P1d), 169 C (P1e). TGA: Td ¼ 320 C (P1a), 340 C (P1b),
320 ꢁC (P1c), 290 ꢁC (P1d), 310 ꢁC (P1e). AUC: MAUC
¼
58,000 g/mol (P1b); 14,000 g/mol (P1d).
Homopolymer P2
1H NMR (CDCl3, 300 MHz): d ¼ 0.75, 0.84 (b, CH3, backbone
and alkoxy chain), 1.1 to 1.8 (b, CH2 backbone and alkoxy
chain), 3.86 (b, OCH2), 6.74, 6.84, 7.01, 7.35 (b, ArH) ppm.
EA: C: 76.99%, H: 8.00%, N: 0.17%, S: 0% (P2a); C: 78.61%,
H: 8.14%, N: 0.16%, S: 0% (P2b). SEC (CHCl3, PS standard):
Mn ¼ 13,600 g/mol (P2a), 14,400 g/mol (P2b); Mw
¼
16,300 g/mol (P2a), 17,900 g/mol (P2b); PDI ¼ 1.20 (P2a),
1.24 (P2b). DSC: no Tg or Tm observed. TGA: Td ¼ 370 ꢁC
For this purpose, a series of three different OPEs, con-
taining a polymerizable methacrylate function, were syn-
thesized by Sonogashira cross-coupling reactions, varying
the length of the oligomer (donor dimer and trimer; M1
and M2) as well as the optoelectronic properties (donor
and acceptor trimer; M2 and M3). After the RAFT poly-
merization, the polymers were characterized by 1H NMR
spectroscopy, SEC, and MALDI-TOF mass spectrometry. In
addition, further information about the absolute molar
masses of the polymers (P1c, P1d, and P1e) was
obtained from AUC; the latter results indicate that the
molar masses obtained by SEC are underestimated by a
factor of 1.4–2.
ꢁ
(P2a), 360 C (P2b).
Homopolymer P3
1H NMR (CDCl3, 300 MHz): d ¼ 0.92 (b, CH3, backbone and
alkoxy chain), 1.2 to 1.9 (b, CH2 backbone and alkoxy chain),
3.93 (b, OCH2), 6.80 (b, ArH), 7.10 (b, ArH), 7.51 (b, ArH) ppm.
EA: C: 72.73%, H: 5.08%, N: 5.25%, S: 6.09%. SEC (CHCl3, PS
standard): Mn ¼ 11,100 g/mol; Mw ¼ 13,300 g/mol; PDI ¼
ꢁ
1.20. DSC: no Tg or Tm observed. TGA: Td ¼ 408 C.
Statistical Donor/Acceptor-Copolymer P2-stat-P3
1H NMR (CDCl3, 300 MHz): d ¼ 0.83, 0.92 (b, CH3, backbone
and alkoxy chains), 1.1 to 1.9 (b, CH2, backbone and alkoxy
chains), 3.94 (OCH2), 6.81, 6.92, 7.10, and 7.52 (b, ArH) ppm.
EA: C: 74.95%, H: 6.10%, N: 3.37%, S: 4.03%. SEC (CHCl3, PS
standard): Mn ¼ 9,300 g/mol; Mw ¼ 10,800 g/mol, PDI ¼
Besides these homopolymers (P1–P3), also a random co-
polymer was synthesized using M2 and M3, which contains
‘‘donor’’ as well as ‘‘acceptor’’ units. In addition, the donor
trimer as well as the acceptor trimer content along the
polymer chain was lowered by copolymerization with MMA
(P2-stat-PMMA, P3-stat-PMMA, and P2-stat-PMMA-stat-
P3).
ꢁ
1.16. DSC: no Tg or Tm observed. TGA: Td ¼ 402 C.
Donor Containing PMMA P2-stat-PMMA
1H NMR (CDCl3, 300 MHz): d ¼ 0.89, 1.02, (b, CH3, backbone
and alkoxy chains) 1.35, 1.59, 1.82 (b, CH2, backbone and
alkoxy chains), 3.60 (b, OCH3 PMMA), 4.00 (b, OCH2), 6.85,
6.87, 6.99, 7.09, 7.44, 7.47, 7.52, and 7.55 (b, ArH) ppm. EA:
C: 69.87%, H: 8.24%, N: 0.15%, S: 0.00%. SEC (CHCl3, PS
standard): Mn ¼ 14,600 g/mol; Mw ¼ 18,100 g/mol, PDI ¼
The optical properties of the monomers and polymers were
investigated in solution as well as in thin films. All poly-
mers revealed an identical absorption in solution compared
with the corresponding monomer unit. Furthermore, inter-
actions between the oligomers in the homopolmer chains
were induced by an increasing MeOH content in the solvent,
which could be demonstrated via UV–vis absorption spec-
troscopy. A hint toward homo energy transfer between
ꢁ
ꢁ
1.24. DSC: Tg ¼ 87 C. TGA: Td ¼ 210 C.
Acceptor Containing PMMA P3-stat-PMMA
1H NMR (CD2Cl2, 300 MHz): d ¼ 0.84, 1.02, 1.25, 1.38, 1.50,
1.59, 1.83, and 1.91 (b, CH2 and CH3, backbone and alkoxy
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19 Obi, M.; Morino, S.; Ichimura, K. Chem. Mater. 1999, 11,
1293–1301.
oligomers in the homopolymer P3 is given by time-resolved
fluorescence anisotropy measurements. The increased
oligomer fraction in polymer P3 causes smaller distances
between the oligomers than in P3-stat-PMMA allowing for
higher rates of energy transfer. For the statistical donor–
acceptor copolymers, a dual fluorescence was observed and
the acceptor fluorescence exhibits clearly contributions
from excitation through donor molecules. This suggests the
conclusion that an energy transfer occurred.54 Film forma-
tion, which leads to higher spatial aggregation of the chro-
mophores, caused higher interactions of the p-conjugated
oligomers. Having this information in hand, the described
system can be potentially applied as antenna system to
mimic light-harvesting photosynthetic proteins in plants.
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ACKNOWLEDGMENTS
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The authors thank the Thu¨ ringer Ministerium fu¨ r Bildung,
Wissenschaft und Kultur (Grant No. B 514-09049: Photo-
Mic) as well as the Dutch Polymer Institute (DPI, technology
area HTE). U. S. Schubert, B. Dietzek, and J. Popp acknowl-
edge financial support of the Fonds der Chemischen
Industrie.
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