Supramolecule-regulated photophysics of oligo(p-phenyleneethynylene)-based rod-coil block copolymers: Effect of molecular architecture

Article abstract of DOI:10.1002/chem.200700305

Three new topology-varied rod-coil block copolymers, comprising the same oligo(p-phenyleneethynylene) (OPE) rod components and the same coil components, were synthesized by atom-transfer radical polymerization. Their photophysical properties were systematically studied and compared in consideration of their solid-state structures and self-assembly abilities. These copolymers have similar intrinsic photophysical properties to the OPE rods, as reflected in dilute solution. However, their photophysical properties in the solid state are manipulated to be dissimilar by supramolecular organization. Wide-angle X-ray diffraction (WAXD) and atomic force microscopy (AFM) data demonstrate that these copolymers possess different self-assembly abilities due to the molecular-architecture-dependent π-π interactions of the rods. Hence, the aggregates in the solid state are formed with a different mechanism for these copolymers, bringing about the discrepancy in the solid-state luminescent properties.

Full text of DOI:10.1002/chem.200700305

FULL PAPER
DOI: 10.1002/chem.200700305
Supramolecule-Regulated Photophysics of Oligo(p-phenyleneethynylene)-
Based Rod–Coil Block Copolymers: Effect of Molecular Architecture
[a, b]
Kan-YiPu,
Xiao-Ying Qi,[b] Yan-Lian Yang,[c] Xiao-Mei Lu,[d] Ting-Cheng Li,[b]
[a]
[a]
Qu-LiFan,* Chen Wang,[c] Bin Liu,[e] Hardy Sze On Chan,[d] and WeiHuang*
Abstract: Three new topology-varied
rod–coil block copolymers, comprising
the same oligo(p-phenyleneethynylene)
(OPE) rod components and the same
coil components, were synthesized by
atom-transfer radical polymerization.
Their photophysical properties were
systematically studied and compared in
consideration of their solid-state struc-
tures and self-assembly abilities. These
copolymers have similar intrinsic pho-
tophysical properties to the OPE rods,
as reflected in dilute solution. Howev-
er, their photophysical properties in
the solid state are manipulated to be
dissimilar by supramolecular organiza-
tion. Wide-angle X-ray diffraction
(WAXD) and atomic force microscopy
(AFM) data demonstrate that these co-
polymers possess different self-assem-
bly abilities due to the molecular-archi-
tecture-dependent p–p interactions of
the rods. Hence, the aggregates in the
solid state are formed with a different
mechanism for these copolymers,
bringing about the discrepancy in the
solid-state luminescent properties.
Keywords:
block copolymers
·
luminescence · self-assembly
Introduction
performance of optoelectronic devices is determined mainly
by the interchain behaviors, rather than by the intrinsic
properties of the conjugated polymers.[5] To control and take
advantage of their interchain behavior, conjugated polymers
or oligomers have been introduced into a rod–coil block co-
polymer architecture as the rod components.[6] As these
emissive rod–coil block copolymers can form a rich variety
of nanoscopic organizations, ranging from lamellar, spheri-
cal, cylindrical, to vesicular morphologies,[7] it is generally
considered that they are providing a new method of con-
structing supramolecular optoelectronic devices.[8] However,
to date, most research has focused on the self-assembly
properties of these block copolymers, while their underlying
supramolecule-related photophysics is little known.[7,8] In
this context, the relationship between the molecular archi-
tecture of the rod–coil block copolymer and the supramole-
cule-regulated photophysical properties of the rod compo-
nent will be investigated.
During the past decades, conjugated polymers have been in-
vestigated extensively for their applications in optoelectron-
ic devices, such as light-emitting diodes,[1] photovoltaic
cells,[2] organic field-effect transistors,[3] and sensors.[4] The
[a] K.-Y. Pu, Dr. Q.-L. Fan, Prof. W. Huang
Institute of Advanced Materials (IAM)
Nanjing University of Posts and Telecommunications (NUPT)
66 XinMoFan Road, Nanjing 210003 (China)
Fax : (+86)25-8349-2333
E-mail: iamqlfan@njupt.edu.cn
wei-huang@njupt.edu.cn
[b] K.-Y. Pu, X.-Y. Qi, Dr. T.-C. Li
Institute of Advanced Materials
Fudan University
Shanghai 200433 (China)
[c] Dr. Y.-L. Yang, Prof. C. Wang
National Center for Nanoscience and Technology
Beijing, 100080 (China)
Among conjugated polymers, poly(p-phenyleneethynyl-
AHCTREeUNG ne)s (PPEs) and their derivatives, in which aryl groups are
[d] X.-M. Lu, Prof. H. S. O. Chan
linked by alkyne units, are of considerable interest.[9] One of
the most fascinating properties of PPEs is their sensitive
chromic behavior upon environmental variation.[10] For in-
stance, when turning from dilute solutions into thin solid
films, the absorption and emission spectra of PPEs always
have a larger bathochromic shift relative to other conjugat-
ed polymers, such as polyfluorenes and poly(phenylene vi-
Department of Chemistry, National University of Singapore
Singapore 117543 (Singapore)
[e] Prof. B. Liu
Department of Chemical and Biomolecular Engineering
National University of Singapore
Singapore 117576 (Singapore)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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nylene)s.[11–12] In previous studies, the bathochromic shifts of
PPEs were attributed either to the aggregate or to the back-
bone planarization.[9–10] The aggregate is an interchain be-
havior, which involves intimate p–p interaction of two or
more chromophores in the ground state by extending the
delocalization of p-electrons over those chromophores.[13] It
usually leads to bathochromic shifts or new peaks in the
spectra. In contrast, the backbone planarization is an intra-
chain behavior, which is associated with the conformational
transformation of a single chain.[14] The first interpretation
of the spectral shifts of PPEs in terms of aryl twisting and
planarization was offered by Bunz et al.[15] In actual fact, for
most conjugated polymers, when the conformation of a po-
lymer chain changes from a twisted one into a relatively
planar one, the p-electron delocalization along the backbone
would become easier, and hence the spectra would red-shift
to a certain extent.[16] However, it has been found that this
intrachain effect is extremely strong for PPEs, owing to the
relatively low rotational barrier of the aryl–alkyne single
bonds along the backbone, which is estimated at less than
1 kcalmolÀ1.[17] A recent study with 9,10-bis(phenylethynyl)-
emission spectra.[21e–f] However, the photophysical properties
of these copolymers were not studied in detail, and satisfac-
tory explanations for these spectral changes were not given
in the literature. We envisage that this discrepancy of spec-
tral changes for these grafted PPEs should be ascribed to
the different molecular architecture, which plays an impor-
tant role in determining the intra- and interchain behaviors.
Thereby, as aforesaid, it is highly desirable to address the
issue of the effect of molecular architecture on the photo-
physics of emissive block copolymers.
In this contribution, to investigate the effect of molecular
architecture on the photophysics of emissive rod–coil block
copolymers, we designed and synthesized three new rod–coil
block copolymers as model systems containing the same
oligo(p-phenyleneethynylene) (OPE) chromophores as the
rod components and the same coils, but with different mo-
lecular architectures as shown in Scheme 2. We will present
ACHTREUNGanthracene as a model compound showed that the planariza-
tion and conformational dynamics of the three aryl rings
result in drastic changes in the spectra, which are manifested
in the formation of the aggregates.[18] On the other hand, the
backbone planarization will give rise to a closer interchain
contact, which in turn facilitates the formation of the aggre-
gates in most circumstances.[19]
To control the interchain behavior of PPEs, they have
been recently incorporated into linear rod–coil block co-
polymers as the rod components,[20] or grafted by poly-
mers.[21] It was hoped that the p–p interactions in these
grafted PPEs would be restrained by the coil segments. Con-
sequently, the solid-state absorption and emission spectra
should retain the features seen in dilute solution. In fact, the
results were not as expected. When dividing these grafted
PPEs into two kinds, as depicted in Scheme 1 based on the
Scheme 2. Chemical structures of the block copolymers and OPE.
Scheme 1. Schematic representation of the chemical structures of the
grafted PPEs.
the first systematic comparative study on the photophysical
properties of OPE and these OPE-based topology-varied
block copolymers. OPE was chosen instead of the polymer
so that the chemical defects and polydispersities formed
during Sonogashira coupling polymerization could be avoid-
ed. Atom-transfer radical polymerization (ATRP) was ap-
plied to endow these block copolymers with the same coils
at different positions on the OPE molecule. Meanwhile, on
molecular architecture, it is interesting to find that upon
going from dilute solution to thin films, the grafted PPEs
with the structure GP1 (Scheme 1) showed slightly red-shift-
ed absorption and emission spectra,[21a–d] while the grafted
PPEs with the structure GP2 (Scheme 1) exhibited signifi-
cantly red-shifted and broadened spectra, especially for the
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Photophysics of Rod–Coil Block Copolymers
FULL PAPER
Table 1. ATRP conditions and results of the block copolymers.[a]
account of the potential application of these copolymers,
poly(2-(carbazol-9-yl)ethyl
methacrylate)
(PCzEMA),
Entry M/S
t
Conv.
Mn
Mn
Mn
Mw/
[e]
AHCTREUNG
(theor)[d] (NMR) (GPC)[e] Mn
rather than other flexible polymers such as polystyrene and
poly(methyl methacrylate), was selected as the coil compo-
nent, owing to its unique electronic properties as demon-
strated in our previous studies.[22] Three rod–coil block co-
polymers were synthesized: the linear block copolymer (PL),
the cross-shaped block copolymer (PC), and the T-shaped
block copolymer (PT). Among these copolymers, PC and PT
are the model compounds of GP1 and GP2, respectively.
PC
PL
PT
2:5
2:5
2:5
150
150
93
92
11800
11700
10200
10700
1.14
9800
10400
1.16
1.18
12000 94646600
5500
[a] [dNBpy]/ACHTER[UNG CuCl]/[I]=4:2:1. All the ATRP reactions were conducted
at 908C. [b] Ratio of monomer versus solvent [gmLÀ1]. [c] The conver-
sion was determined by comparing the relative integrals of the isolated
peaks for monomer and polymer. [d] The theoretical molecular weight
was calculated by the conversion rate of monomer and the monomer-ini-
tiator ratio. [e] Mn and Mw/Mn were determined by GPC using polystyr-
ene standards.
Results and Discussion
Synthesis and characterization: ATRP, as one of the most
popular controlled radical polymerizations, provides an ef-
fective strategy to construct well-defined polymers with
varying composition, functionality, and also architecture,
which is difficult or sometimes even impossible for the tradi-
tional polymerization technologies.[23] By using a convergent
method with a series of well-known organic reactions, such
as the Sonogashira coupling reaction and the Williamson
ether reaction, the hydroxyl groups were integrated into the
different positions of OPE and the macroinitiator precursors
(compounds 1–8, see Scheme S1 in the Supporting Informa-
tion for their structures), with the aim to construct linear, T-
shaped, and cross-shaped block copolymers. Such precursors
were successfully synthesized in satisfactory yields, shown in
Scheme S1 in the Supporting Information as 4, 7a, and 7b,
respectively. Meanwhile, the model compound 3, oligo(2,5-
dihexyloxy-1,4-phenyleneethynylene), to be used for com-
parison, was also prepared by a similar procedure. Finally,
these macroinitiator precursors were esterified with 2-bro-
moisobutyryl bromide to obtain the final OPE macroinitia-
tors, 8a, 8b, and 8c, with high yields close to 100%. The
chemical structures of both the precursors and the macroini-
Figure 1. GPC profiles of 8a and the block copolymers.
poly
ACHTREmUNG er components with low polydispersities were ach-
ieved.
The chemical structures of these polymers were deter-
1
mined by H and 13C NMR spectroscopy. As a representa-
tive example, the 1H and 13C NMR spectra of PL coupled
with its related macroinitiator (8a) are represented in Fig-
ures 2 and 3, respectively, which all follow linear superposi-
1
tiators were confirmed by H and 13C NMR spectroscopy. In
1
addition, the macroinitiators and 3 (OPE) were further veri-
fied by their matrix-assisted laser desorption ionization-
time-of-flight mass spectra (MALDI-TOF MS) (see Fig-
ure S1 in the Supporting Information).
tion of 8a and PCzEMA. In the H NMR spectrum of PL,
the existence of the peak at d=1.84ppm due to the alkoxyl
In our previous work, we studied ATRP of CzEMA, and
well-controlled PCzEMA was gained.[23] In this study, similar
ATRP conditions were used to synthesize block copolymers
with different molecular architectures, but containing the
same components, shown in Scheme 2: PL, PC, and PT. To
the best of our knowledge, this is the first report of the syn-
thesis of an OPE-based T-shaped rod–coil diblock copoly-
mer. The degree of polymerization of each coil block of
these copolymers was pre-designed to be 20. The ATRP
conditions are summarized in Table 1. The polydispersities
and molecular weights of these copolymers were determined
by gel permeation chromatography (GPC). Figure 1 illus-
trates the GPC profiles of the copolymers, and also the
linear macroinitiator (8a) as a representative of the macroi-
nitiators for comparison, indicating that ATRP was per-
formed well upon these macroinitiators and the desired
Figure 2. 1H NMR spectra of 8a and the related block copolymer (PL) in
CDCl3. (Symbols * and # represent peaks from CDCl3 and H2O, respec-
tively).
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W. Huang, Q.-L. Fan et al.
ried out (Figure 5). These
WAXD samples were prepared
in the same way as those for
the solid-state photophysical
studies, so that the connection
between the solid-state struc-
tures and photophysics could be
rationalized. It is known that
the solid-state structures of
PPEs can, depending on their
side-chain type and concentra-
tion, display different solid-
state supramolecular organiza-
tion structures, including a cy-
lindrical morphology, a lamellar
one, and an interdigitated
Figure 3. 13C NMR spectra of 8a and the related block copolymer (PL) in CDCl3.
one.[25] Compound 3 shows a set
of clearly resolved reflection
À
À
sides of the OPE segment ( CH2 ) shows the successful in-
corporation of OPE into the block copolymer, while the dis-
appearance of the peak at d=2.08 ppm due to the reso-
À
peaks, indicating the highly ordered solid-state structure.
Among these peaks, the sharp first-order reflection peak at
nance of
CACHTREUNG(CH3)Br indicates that 8a is fully initiated in
ATRP. This conclusion could also be drawn from the
13C NMR spectrum of PL, that is, the peaks at d=30.68 and
55.51 ppm (shown as peaks 1 and 2 in Figure 3), assigned to
À
À
the C(=O)CACHTREUNG(CH3)2Br and C(=O)CACHTREU(GN CH3)2Br of 8a, re-
spectively, are absent in the 13C NMR spectrum of PL due to
the local changes in the chemical environment after poly-
1
merization. Similar results could be obtained from the H
and 13C NMR spectra of PT and PC (see Figure S2 in the
Supporting Information). Furthermore, the polymer compo-
sition can be estimated according to the ratio between the
relative integrals of the isolated peak from the carbazyl
proton of the PCzEMA block at d=7.92 ppm (shown as
peak c’ in Figure 2) and the isolated peak from the alkyl
À
À
sides of the OPE segment ( CH2 ) at 1.84ppm (shown as
peak 1’ in Figure 2). The results are summarized in Table 1,
and are in line with the data from GPC.
Figure 4. a) DSC profiles of 3, for the first cooling scan and second heat-
ing scan, both at a rate of 108CminÀ1. b) DSC profiles of the block co-
Solid-state structure: The solid-state photophysical proper-
ties of conjugated molecules depend on the solid-state pack-
ing structure, rather than being intrinsic properties.[5] There-
fore investigating the solid-state structures of these com-
pounds would provide useful information for better under-
standing their photophysical properties.
polymers for the second heating scan at a rate of 108CminÀ1
.
The thermal properties of 3, PL, PT, and PC were exam-
ined by differential scanning calorimetry (DSC) and their
thermograms are depicted in Figure 4. The traces of 3 are
similar to those reported previously,[24] featuring a melting
peak at 1188C for the second heating scan and a crystalliza-
tion peak at 1038C for the first cooling scan. After introduc-
tion of PCzEMA as coils, all the copolymers show only a
glass-transition temperature at around 1258C for the second
heating scan instead of any melting peaks, which is consis-
tent with the PCzEMA homopolymer.[22]
To further identify the solid-state structures of the copoly-
mers and 3, wide-angle X-ray diffraction (WAXD) was car-
Figure 5. WAXD profiles of 3 and the block copolymers.
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Photophysics of Rod–Coil Block Copolymers
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2q=4.98 corresponding to a d-spacing of 18 Š, and the high
wide-angle reflection peak at 2q=14.48 corresponding to a
d-spacing of 6.19 Š, are also present in the polymer counter-
part of 3. This has been attributed to the layered packing
structure in which the stiff main chains are coplanar.[25a] It is
worth noting that the reflection peaks at 2q=23.88 corre-
sponding to a d-spacing of 3.8 Š stands for the distance be-
tween the two parallel packed chains. This short distance
usually facilitates the formation of aggregates.[26] In contrast,
these OPE-based block copolymers do not show these char-
acteristic reflection peaks of OPE (3). However, the reflec-
tion peak at 4.18 corresponding to a d-spacing of 21.5 Š is
weakly visible for PT and PL, which was often observed for
the conjugated polymers with hexyloxy side groups.[27] The
residue of this peak in PT and PL reflects the small extent of
side chain alignment of the OPE rods. Hence, there should
be a few ordered domains in the thin films of PT and
PL.[27c–d] However, the invisibility of this peak for PC indi-
cates that PC is less ordered than PT and PL.
nent-dependence similar to our previous study,[22] reflecting
that the desired component ratios of these copolymers were
achieved. The emission spectra of these copolymers in dilute
solution obtained by excitation at 405 nm are nearly identi-
cal to that of 3, exemplifying the maximal emission peaks at
446 nm from the OPE chromophores.
The formation of low-energy sites is usually a diffusion-
controlled process. Hence, it is often observed in conjugated
oligomers and polymers that owing to the enhanced inter-
chain interactions as well as the occurrence of a sufficient
number of collisions at the concentrated solution, low-
energy sites including excimers or aggregates are always
formed, and this in turn leads to a change of emission spec-
trum.[29–30] To examine the influence of the solution concen-
tration on the emission spectra of these compounds, the
emission spectra with the concentrations of 10À6 and
1 mgmLÀ1 are shown in Figure 7, all normalized to the maxi-
The crystalline property and the layered solid-state pack-
ing structure of 3 are evidence of strong p–p interactions,
whereas the lack of melting peak and evident characteristic
reflection peaks regarding the OPE rods for these copoly-
mers is indicative of the greatly reduced p–p interaction of
the rods.
Solution-state photophysics: The optical properties of 3 and
the macroinitiators in dilute solution are shown in Figure S3
in the Supporting Information. The optical properties of 3
and the macroinitiators in the dilute solutions are nearly
identical and in good agreement with their counterparts.[28]
For convenience and clarity, 3, representing all the macroini-
tiators, will be used for the discussion and comparison of the
initiators and their related block copolymers in the follow-
ing sections.
Figure 7. Normalized PL emission spectra of 3 and the block copolymers
in dilute and concentrated solutions. The excitation wavelength of the PL
emission spectra is 405 nm.
The optical properties of these copolymers in dilute solu-
tions are shown in Figure 6. In the absorption spectra of the
copolymers, two types of absorption bands are present. One
ranging from 300 to 350 nm corresponds to the PCzEMA
blocks, the other between 350 to around 450 nm is due to
the OPE blocks. The absorption spectra also show a compo-
mal emission peak from the OPE chromophores. The emis-
sion spectrum of 3 in the concentrated solution changes
drastically in contrast to that in dilute solution. The maximal
emission peak located at 446 nm in dilute solution is red-
shifted by 6 nm to 452 nm in the concentrated solution, indi-
cating the adoption of a planar conformation. Furthermore,
the relative intensity at the long-wavelength side increases
sharply, which is consistent with the similar OPE compounds
in the previous report and could be attributed to the forma-
tion of low energy sites.[31] For PL and PT, the maximal emis-
sion peak is also red-shifted by 3 nm and 5 nm, respectively,
and the relative intensity at the long-wavelength side is
slightly increased to a different extent. However, for PC, the
emission band is extremely stable, witnessed by the nearly
identical emission bands at these two different concentra-
tions. It is important to point out that the same phenomenon
could also be observed from a solution using a different sol-
vent, such as toluene or chloroform. Thus the solvent-in-
duced dissimilarity in photophysical properties is not in-
Figure 6. Normalized UV/Vis absorption and PL emission spectra of 3
and the block copolymers in tetrahydrofuran (10À6 mgmLÀ1). The excita-
tion wavelength is 405 nm.
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W. Huang, Q.-L. Fan et al.
volved here, because these compounds have the same good
solubility due to the very similar components.[32]
Solid-state photophysics: The solid-state absorption and
emission spectra of 3 and these copolymers are illustrated in
Figure 8. As expected by their solid-state structures, these
Figure 9. Normalized PLE spectra of 3 and the block copolymers in thin
films, monitored at 493 nm and 458 nm, respectively.
peak at around 450 nm. Excimers are dimers of the same
chromophores and exist only under excitation, but are disso-
ciative in the ground state. Therefore, the excimers do not
have any absorption peak. On the other side, the aggregates
are the new ground-state species formed by extending the
delocalization of p-electrons over those chromophores.
Hence the aggregates have an absorption peak.[13] Thus the
emission species with the absorption peak at around 450 nm
could result from the aggregates but not from the excimers.
Moreover, the decay dynamics of the emission peak at
492 nm is well fitted by a double-exponential with lifetimes
of 0.614ns (28%) and 1.96 ns (72%) (Table 2). This shows
Table 2. Time-resolved PL decay-fitting parameters of 3 and the block
copolymers.[a]
Entry
lprobe [nm]
t1 [ns][b]
t2 [ns][b]
c2
3
493
500
458
500
460
500
461
500
0.590 (0.28)
0.587 (0.20)
0.621 (1.00)
0.723 (0.84)
0.606 (1.00)
0.714(0.78)
0.599 (1.00)
0.703 (0.65)
1.96 (0.72)
2.24(0.80)
1.085
1.119
1.229
1.051
1.131
1.273
1.056
1.211
Figure 8. a) Normalized UV/Vis absorption spectra and b) PL emission
spectra of 3 and the block copolymers in thin films. The excitation wave-
length is 405 nm.
PC
PL
PT
2.97 (0.16)
2.86 (0.22)
2.94(0.35)
copolymers exhibit clearly different solid-state photophysi-
cal properties in contrast to 3. To probe the origin of this
discrepancy, photoluminescence excitation (PLE) and time-
resolved photoluminescence (PL) were carried out at the
same time.
[a] Films are excited at 371 nm, thereby, excitation of the PCzEMA
blocks of the block copolymers is avoided. [b] Values given in percent-
AHCTREaUNG ges.
As for 3, both the solid-state absorption spectrum and the
emission spectrum are largely red-shifted relative to those in
the solution state, implying that aggregates may be formed
owing to the layered packing structure as detected by
WAXD. The solid-state absorption spectrum of 3 is compli-
cated, featuring two adjacent peaks at 412 and 424 nm and
also a relatively weak and red-shifted peak at 450 nm
(Figure 8). The solid-state emission spectrum of 3 shows a
maximal peak located at 492 nm with a 45 nm red-shift com-
pared to the dilute solution state. The PLE spectrum of 3
monitored at 492 nm is different from the absorption spec-
trum. It shows a maximal peak at 450 nm (Figure 9). This
shows that the maximal solid-state emission peak mainly
roots from the emission species with the maximal absorption
that two emission species are responsible for the emission at
492 nm. The double-exponential decay dynamics with the
domination of the longer lifetime safely affirms the forma-
tion of the aggregates.[33] Consequently, the emission species
with the absorption peak at 450 nm could be due to the ag-
gregates. It is known that PPE systems always exhibit signifi-
cant conformation-dependent photophysical properties, as
mentioned in the Introduction section.[14] Thereby, the other
absorption peaks at 412 and 424 nm in the solid-state ab-
sorption spectrum of 3 could be naturally attributed to the
unimolecular OPE with a different extent of backbone pla-
narization.[34] The absence of the emission from the unimo-
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Photophysics of Rod–Coil Block Copolymers
FULL PAPER
lecular OPE molecules could be explained by the efficient
energy transfer from them to the aggregates in the solid
state, in which the energy transfer is an efficient three-di-
mensional process.[16] The designation of these peaks coin-
cides with the literature in which the aggregation behaviors
of oligo(2,5-dibutoxy-1,4-phenyleneethynylene), the ana-
logue of 3 in this study, were studied by Pang et al.[28b]
tophysical properties in the dilute solution state, because the
similar twisted conformations are assumed at this approxi-
mately unimolecular level at which molecules are separated.
Upon increasing the solution concentration, intermolecular
behavior begins to occur. As a result, the emission bands of
these compounds change to the different profiles, insinuat-
ing the different extent of the p–p interactions. In the solid
state, molecules contact intensively, and the intermolecular
behavior is maximized. Introducing OPE into the rod–coil
block copolymers greatly destroys the p–p interactions of
the OPE rods due to the shielding effect of the coils.[37]
Therefore, differing from the aggregate-occupied solid-state
spectra of 3, all the copolymers exhibit the solid-state spec-
tra dominated by the signals from unimolecular OPE chro-
mophores with planar conformations. In addition, it can be
confidently concluded that in contrast to the unimolecular
OPE with the twisted conformations, the planar conformers
of the OPE chromophores exhibit maximal absorption and
emission peaks slightly red-shifted by 10–20 nm, while the
aggregates of OPE chromophores have their maximal ab-
sorption and emission peaks greatly red-shifted by 30–
50 nm. This observation is in perfect agreement with the
studies of the effects of chromophore planarization and ag-
gregation on photophysics with 1,4-bis(phenylethynyl)ben-
zene and 1,4-bis(2-hydroxy-2-methyl-3-butynyl)-2-fluoroben-
zene as the model compounds.[17,19]
As for these copolymers, in accordance with the WAXD
data, the peak of the aggregates does not show up markedly
in the solid-state absorption spectra due to the evident ab-
sence of strong p–p interactions (Figure 8). Instead, all
these copolymers display two kinds of absorption bands, the
band ranging from 300 to 350 nm and the band between 350
to around 460 nm, corresponding to the PCzEMA and the
OPE blocks, respectively. In contrast to the dilute solution
state, the maximal absorption peaks from the OPE chromo-
phores of these copolymers in the solid state was slightly
red-shifted by 8 nm from 404 to 412 nm. On the basis of the
assignment of the similar solid-state absorption peak at
412 nm of 3, the slightly red-shifted absorption bands stem
from the unimolecular OPE chromophores adopting planar
conformations with respect to the twisted conformations in
the dilute solution. In the solid-state emission spectra, all
these copolymers show a maximal peak slightly red-shifted
by 13 nm relative to that in the solution state (Figure 8).
The PLE spectra of these copolymers monitored at this
maximal solid-state emission peak are almost identical
(Figure 9), showing the broad bands with the maximal peak
at around 418 nm almost equal to those of the solid-state ab-
sorption spectra. This confirms that the emission at around
458 nm comes from the unimolecular OPE chromophores
with the planar conformations rather than from the aggre-
gates. Additionally, time-resolved PL experiments exhibit
single-exponential decay dynamics of these maximal emis-
sion peaks with a lifetime around 0.6 ns, comparable to
around 0.7 ns in the dilute solution (Table 2), which further
proves this conclusion. Meanwhile, it is known that in the
solution state the rotationally disordered chromophores
have to planarize before they emit, while in the solid state
the chromophores are already planarized and no significant
geometric reorganization between ground and excited state
is necessary.[35] Thus, the relatively shorter decay time in the
solid state compared to the dilute solution is also indicative
of the adoption of the planar conformations. A similar phe-
nomenon is also observed for PPEs.[12b] However, it should
be noted that the homogeneous and broad feature of these
absorption bands from 350 to around 460 nm implies that
the OPE rods of these copolymers exist in the solid state
with a variety of conformations, ranging from the less planar
one to the more planar one. Also, because the absorption
peak of the aggregates is liable to be hidden under the inho-
mogeneous broadened absorption spectrum,[36] the possibili-
ty that the aggregates may exist in a very small amount in
these copolymers merits serious consideration, which will be
discussed in the next sections.
Effect of molecular architecture on forming the aggregates:
As illustrated by the WAXD data, a few ordered domains
are present in the thin films of PL and PT, inferred from the
weakly visible reflection peak at 4.18. Taking into account
the inhomogeneous broadening in the solid-state absorption
spectra of these copolymers, the possibility of forming the
aggregates of the OPE rods in these small, ordered domains
emerges. Apparently, the solid-state emission spectra of
these copolymers extend to the long-wavelength side, and
overlap the aggregate dominated emission spectrum of 3, as
shown in Figure 8. The relative intensity at 500 nm in the
emission spectra (Figure 8), and the absolute quantum
yields of these copolymers in these thin films, are calculated
and summarized in Table 3. The relative intensities at
Table 3. Photophysical data for comparison.
Entry
PC
PL
PT
intensity ratio[a]
relative intensity[b]
F[c]
1.00
0.33
2.70
1.30
0.41
2.10
1.70
0.51
1.40
[a] The intensity ratios at 500 nm for the solution concentration of
1 mgmLÀ1 to that of 10À6 mgmLÀ1 are calculated from the emission spec-
tra of Figure 7. [b] Relative intensity is according to the emission spectra
of Figure 8b. [c] The fluorescence quantum yields in thin films relative to
3 assuming 1.00 for comparison were obtained from the same samples
used for photophysical measurements.
500 nm are in the order of PC <PL <PT, while the quantum
yields are in the reversal sequence. It has been widely ob-
served that forming the aggregates is always concomitant
From these data, one can see that compound 3 and the
OPE segments of the copolymers have nearly identical pho-
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1211

W. Huang, Q.-L. Fan et al.
with an increase in relative intensity at the long-wavelength
side as well as a decrease in quantum yields,[36b,38] thus one
can foresee that not only are the aggregates of the OPE
chromophores formed for these copolymers, but also the
populations of the aggregates should be different from each
other. It is important to point out that the thin films studied
here were prepared from the same concentration of solu-
tions of the sample in tetrahydrofuran (10 mgmLÀ1) under
the exactly same spin-casting conditions. Furthermore, the
same phenomenon could also be observed by the thin films
casting from the solutions by using different solvents such as
toluene and chloroform.
To further confirm the existence of the aggregates, PLE
and time-resolved PL spectra were performed at 500 nm.
The PLE spectra of these copolymers monitored at 500 nm
show broad bands extending to the long-wavelength side,
with the maximal peak near 439 nm (Figure 10), which is
Figure 11. PL decay curves of 3 and the block copolymers in thin films
monitored at 500 nm (excitation at 371 nm).
are different from each other with the sequence of PC <PL <
PT. These different ratios verify that the population of aggre-
gate is different,[33,36] which is the cause of the different rela-
tive emission intensities at the long-wavelength side and dif-
ferent quantum yields of the solid state for these copoly-
mers.
In addition, as mentioned in the previous section, when
increasing the concentration, the emission spectra of these
copolymers turn into different profiles, and the emission in-
tensity at 500 nm increases to a different extent for these co-
polymers as shown in Figure 7. This implies that aggregates
with different amounts for these copolymers are also
formed in the concentrated solutions. The ratio of emission
intensity at 500 nm for the concentration of 1 mgmLÀ1 to
10À6 mgmLÀ1 is calculated and summarized in Table 3.
These data are in good accordance with the relative emis-
sion intensity at 500 nm in the solid state, presenting a grad-
ual increase with the same order of PC <PL <PT.
Figure 10. Normalized PLE spectra of the block copolymers in thin films
monitored at 500 nm coupled with the absorption spectrum of PC for
comparison.
On the basis of this evidence, it can be concluded that the
tendency to form the aggregates is different for these co-
polymers, and determined by the molecular architecture de-
pendent p–p interactions, producing the discrepancy in lu-
minescent properties both in the concentrated solutions and
the thin films. The tendency to form the aggregates of the
OPE chromophores in these copolymers and 3 follows the
order of PC <PL <PT <3.
different from the PLE spectra monitored at 458 nm with
the maximal peak at 418 nm (Figure 9). Relative the absorp-
tion spectra, we observe that the maximal peaks are largely
red-shifted by around 30 nm, implying that the emission at
500 nm does not come just from the unimolecular species.
To further substantiate the point, time-resolved PL experi-
ments were performed upon these copolymers probing at
500 nm, as shown in Figure 11. As expected, all the decays
are best described by the double-exponential dynamic mech-
anism, and the longer lifetime is comparable to that of the
aggregates of 3, attesting that the aggregates are indeed in
existence. However, differing from the decay of 3 at 500 nm,
all these decays of the copolymers are prevailed by the
shorter lifetime species rather than the longer lifetime spe-
cies of 3. Considering that there is no evident aggregate
peak in the absorption spectra and no reflection peaks at
3.8 nm in WAXD profiles for these copolymers, it is reason-
able to believe that although the aggregates are formed,
they are in an infinitesimal portion. When comparing the
decay dynamics of these copolymers, one finds that the
amount ratios of longer life species related to the aggregates
Origin of the aggregates: It is known that the self-assembly
ability of a conjugated rod–coil block copolymer is governed
not only by the Flory–Huggins interaction and the volume
fraction of the two blocks, but also more importantly by the
p–p interactions between the rods.[39] As a consequence,
concomitant formation of these ordered nanoscopic domains
with the appearance of the aggregates is observed.[6a,b,37a,40]
Thereby, studying the self-assembly ability of these materials
can give a better understanding of the extent of the p–p in-
teractions between the OPE rods, which further helps us un-
derstand the origin of the aggregates.
In this respect, tapping-mode atomic force microscopy
(AFM) was performed. AFM is known to provide a straight-
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Chem. Eur. J. 2008, 14, 1205 – 1215

Photophysics of Rod–Coil Block Copolymers
FULL PAPER
forward morphological characterization of surfaces with a
lateral resolution in the order of a few nanometers, and a
vertical resolution in the order of 1 Š.[41] Figure 12 shows
studies. Consequently, it further confirms the conclusion
that the tendency to form aggregates of the OPE rods in
these copolymers follows the order of PC <PL <PT. More-
over, one can conclude that the residue of ordered nano-
structures in the thin films, in which the p–p interactions of
the OPE rods are present, is the possible origin of the for-
mation of the aggregates for these copolymers.
Conclusion
In summary, we have demonstrated a facile synthetic way to
desirably control the molecular architecture of a conjugated
rod–coil block copolymer through ATRP. To this end, three
new rod–coil block copolymers, consisting of the same OPE
as rod components and the same PCzEMA as the coil com-
ponents, but with different molecular architectures, were
prepared. The underlying supramolecule-regulated photo-
physics of these block copolymers and OPE were systemati-
cally studied in detail by combining the spectroscopic, solid-
state structural, and self-assembly analyses. The solid-state
structures of these compounds were examined by DSC and
WAXD. The results illustrate that OPE has a crystalline, or-
dered packing structure with strong p–p interactions, where-
as the copolymers are nearly amorphous, with greatly re-
duced p–p interactions, especially for PC. In dilute solution,
the copolymers and OPE have almost identical absorption
and emission spectra to the OPE chromophores, verifying
the similar intrinsic photophysical properties. In the solid
state, the absorption and emission spectra of OPE are domi-
nated by the aggregates, while the maximal absorption and
emission peaks of the copolymers are dominated by the uni-
Figure 12. AFM phase images (2”2 mm) of a) PT, b) PL, and c) PC.
the AFM images of these copolymers obtained from the
semiconcentrated solutions of the samples in tetrahydrofur-
an (0.01 mgmLÀ1). The solutions were deposited on a mica
substrate in a solvent-saturated atmosphere. These copoly-
mers form different morphologies, which is a reflection of
the different self-assembly abilities. Interestingly, PT forms a
rectangular bilayer packing nanostructure with an average
height of 3 nm as shown in Figure 12 a. Glotzer et al. recent-
ly reported the simulation of the self-assembly of a T-
shaped rod–coil block copolymer. They demonstrated that a
rectangular bilayer packing structure of the ribbons rather
than a hexagonal packing structure is most favorable for this
copolymer due to the competition between maximizing the
rod–rod interaction and the entropy contributed by the
coils.[42] Thereby, the very similar nanostructure for PT re-
flects that the p–p interactions of the OPE rods are still in
existence and play an important role in the self-assembly
into this extremely ordered nanostructure. The PL copoly-
mer exhibits a typical nanoribbon structure of a linear rod–
coil block copolymer, with an average width and height of
100 and 20 nm, respectively (Figure 12b).[43] A similar nano-
ribbon structure was also observed for PPE-based linear
rod–coil block copolymers reported by Lazzaroniꢁs group,
which was indicative of the present p stacking of the rod as
one of the important driving forces.[44] However, PC only
shows nanoscale grains with an average diameter and height
of 25 and 4nm, respectively (Figure 12c). This confirms that
stemming from the shielding of coils at both sides of the
OPE rods, the p–p stacking of the OPE rods of PC are fur-
ther destroyed with respect to PT and PL, which in turn gives
PC a relatively poor self-assembly ability and low suscepti-
bility to form aggregates. The poorer self-assembling ability
of PC relative to PT and PL can also be seen from the
WAXD data as shown in the previous section, that is, the re-
flection peak at 4.18 is absent for PC but present for PT and
PL.
AHCTREmUNG olecular OPE chromophores with planar conformations, in
contrast to those in the dilute solution. These observations
are consistent with the conclusion drawn from the solid-
state structures.
More importantly, the effect of molecular architecture on
the photophysical properties of emissive rod–coil block co-
polymers has been revealed for the first time. As confirmed
by AFM and WAXD, these copolymers have dissimilar p–p
interactions of the OPE rods, dependent on the molecular
architecture, leading to the discrepant tendencies toward ag-
gregates. Thereby, these copolymers have different solid-
state emission spectra and quantum yields. Among three
block copolymers, the block copolymer with the cross-
shaped molecular architecture (PC), which is the model com-
pound of GP1, has the weakest p–p interactions of the OPE
rods and thereby prevents most strongly the formation of
aggregates. The block copolymer with the T-shaped molecu-
lar architecture (PT), which is the model compound of GP2,
is the most susceptible towards aggregates with the strongest
p–p interactions and self-assembly ability. On the basis of
these data, the different spectral changes of the aforemen-
tioned PPE-based graft copolymers upon going from dilute
solutions into thin solid films can be attributed to the dis-
crepant tendency to form aggregates. These findings provide
profound guidelines for engineering supramolecular opto-
These results manifest that the p–p interaction of the
OPE rods for these copolymers, resulting in these nano-
structures, are different and are modulated by the molecular
architecture. Besides, the resulting different self-assembly
abilities of these copolymers are consistent with the tenden-
cy to form aggregates, as deduced from the photophysical
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1213

W. Huang, Q.-L. Fan et al.
alumina to remove the catalysts. The polymers were precipitated into an
excess of methanol and dried in vacuum at 408C. Light or primrose
yellow powdery products were obtained.
ACHTREUNG
Experimental Section
Acknowledgements
Characterization: NMR spectra were collected on a Varian Mercury Plus
400 spectrometer with tetramethylsilane as the internal standard.
This work was financially supported by the National Natural Science
Foundation of China under Grants 60325412, 90406021, 20504007,
50428303, and NY207011. K.Y.P. also thanks Prof. Chi-Fei Wu, School of
Materials Science and Engineering, East China University of Science and
Technology, for DSC experiments.
MALDI experiments were carried out using
a Shimadzu AXIMA-
CFRTM plus time-of-flight mass spectrometer (Kratos Analytical, Man-
chester, U.K.). The instrument was equipped with a nitrogen laser emit-
ting at 377 nm, a 2 GHz sampling rate digitizer, a pulsed ion extraction
source, and an electrostatic reflectron. Spectra were acquired in the posi-
tive-ion mode using the reflectron. Elemental analyses were performed
on a Vario EL III O-Element Analyzer system. GPC analysis was con-
ducted with a HP1100 HPLC system equipped with 7911GP-502 and GP
NXC columns by using polystyrene as the standard and THF as the
eluent at a flow rate of 1.0 mLminÀ1 at 358C. DSC measurements were
performed under a nitrogen atmosphere at heating and cooling rates
both of 108CminÀ1, with NETZSCH DSC 200PC apparatus. WAXD data
was obtained on a Bruker D8 Discover diffractometer with GADDS as a
2D detector. Calibration was conducted using silicon powder and silver
behenate. AFM experiments were performed under ambient conditions
and room temperature on a Nanoscope IIIa microscope (Digital Instru-
ments, Santa Barbara, CA) operating in a tapping-mode. UV/Vis spectra
were recorded on a Shimadzu 3150 PC spectrophotometer. Fluorescence
measurements were carried out on a Shimadzu RF-5301 PC spectrofluor-
ophotometer with a xenon lamp as the light source. Time-correlated
single photon fluorescence studies were performed using an Edinburgh
Instruments LifeSpec-PS spectrometer. The LifeSpec-PS comprises a
371 nm picosecond laser (PicoQuant PDL 800B) operated at 2.5 MHz
and a Peltier cooled Hammamatsu microchannel plate photomultiplier
(R3809U-50). Lifetimes were determined from the data by using the Ed-
inburgh Instruments software package. Measurement of the absolute PL
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reaction mixture was cooled in liquid N2 in order to quench the polymeri-
zation and diluted with THF, then passed through a column of neutral
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Received: February 23, 2007
Revised: August 27, 2007
Published online: November 14, 2007
Chem. Eur. J. 2008, 14, 1205 – 1215
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1215