Synthesis and network-like self-assembly of porphyrin-polyselenophene complexes

Article abstract of DOI:10.1002/cphc.201200682

A hexagonal network structure fabricated by self-assembly of a branched conjugated polymer with a porphyrin core and P3HT or P3HS arms is presented (see picture). Polymer symmetry is very important to the network structure formation probably due to the different viscosities in linear and branched polymers. Copyright

Full text of DOI:10.1002/cphc.201200682

DOI: 10.1002/cphc.201200682
Synthesis and Network-Like Self-Assembly of Porphyrin-Polyselenophene
Complexes
[
a]
Lianshan Li, Jon Hollinger, Gerald Guerin, and Dwight S. Seferos*
The molecular orientation of p-conjugated systems has a signif-
icant impact on their solid-state properties. The control of their
ordering has long been the central issue in the molecular
design of new materials for organic electronics, including or-
ganic light-emitting diodes, field-effect transistors, and organic
We characterize the self-assembled structures of these poly-
mers and compare them with the linear polymer, porphyrin
alone, or a linear polymer that is terminated with one porphy-
rin. The cooperation of porphyrin and polymer self-assembly
leads to distinct solid-state structures that are not observed in
any of the control experiments. This new porphyrin-based
strategy prepares hexagonal networks that are quite different
from the 1D fibers that are typically obtained with P3HT poly-
mers.
[
1]
solar cells. As an intensely studied electronic material, the
self-assembly behavior of poly(3-hexylthiophene) (P3HT) has
[2]
been investigated by adjusting the molecular weight, the
[
3]
side-chain length and structure, and controlling phase sepa-
[
4]
[15]
ration in block copolymers. To date these compounds and
their self-assembled structure can be broadly classified as
Externally initiated polymerization was used to synthesize
all of the polymers used in this study (Figure 1). In order to
control macromolecular shape, tetra-(4-bromophenyl)porphyrin
[
3a,5]
[4b,6]
linear, which includes nanowires,
fibers,
or belts and
[
7]
helices. Synthetic strategies to prepare networks of conjugat-
ed polymers and the study of their self-assembled structures
remain largely unexplored.
(Br TPP) was chosen as the initiator. The porphyrin also intro-
4
duces a second p–p stacking motif into the complex. The syn-
thesis begins with the preparation of Br TPP by following liter-
4
[10]
Due to their unique flat and conjugated central tetrapyrrole
macrocycle, porphyrin derivatives have been widely investigat-
ature methods.
Briefly, 4-bromobenzaldehyde (1.85 g,
10 mmol) and pyrrole (0.7 mL, 10 mmol) were dissolved in
15 mL of propionic acid and 5 mL of nitrobenzene. The mixture
was heated at 1208C for 3 h and allowed to settle at room
temperature for three days. The product was filtered and
washed with methanol, hexane, and isopropanol to afford the
[
8]
ed as model molecules in self-assembly. For 2D systems, ad-
sorption and self-assembly of porphyrins and their derivatives
[
9]
on surfaces have been extensively studied. It is well-estab-
lished that porphyrin molecules adsorb in a flat-lying confor-
mation with the macrocycle backbones parallel to surfaces.
The adlayer structures are determined by intermolecular and
molecule-to-substrate interactions. In most cases, porphyrin
product as a dark purple solid. The sample was then recrystal-
1
lized from CHCl /methanol and characterized by H NMR spec-
3
troscopy (Figure S1, Supporting Information). We next added
[
9a]
molecules form 2D close-packed structures.
Recently Grill
Zn(Ac) to protect the amine groups from Ni that is introduced
2
et al. reported on open porphyrin networks formed through
in the next step. NMR spectroscopy confirmed the formation
[
10]
covalent polymerization. For 3D packing structures, hollow
hexagonal nanoprisms of ZnTPyP with a tunable length and
aspect ratio were produced by self-assemblies of zinc mesote-
of tetrabromoporphyrin zinc (Br TPPZn) (Figure S2). In order to
4
synthesize the desired initiating complexes, Ni(PPh3)4 was
added to Br TPPZn at room temperature and allowed to react
4
[
11]
tra (4-pyridyl) porphyrin (ZnTPyP). Qiu reported that porphy-
rin can be incorporated into an inorganic framework, such as
overnight. Next, triphenyl phosphine ligands were exchanged
with 1,3-Bis(diphenylphosphino)propane (dppp), as it has been
established that dppp leads to the greatest degree of polymer-
[
12]
silica, to produce hybrid materials. Most recently, a helical
superstructure was constructed based on an achiral porphyrin-
incorporated alkoxysilane (PIA) in which four triethoxysilane
[15b,16]
ization control in a typical P3HT synthesis.
Polymerization
was initiated by the addition of a solution of 2-bromo-5-
chloromagnesio-3-hexylselenophene to the complex in THF at
408C followed by stirring at room temperature for 16 h. Finally,
the reaction mixture was quenched with 2m HCl and precipi-
tated into methanol, after which the precipitated solids were
purified by Soxhlet extraction using methanol, hexanes, and
chloroform.
[
13]
groups are grafted to the central porphyrin core. These re-
sults are important achievements in the research of porphyrins
and their structural construction and utilization.
Herein, we report a strategy to synthesize conjugated poly-
mers using a porphyrin core with four initiation sites. The por-
phyrin initiates the polymerization of both P3HT, and the sele-
[14]
nophene analog, P3HS, by the Grignard metathesis method.
Optical absorption spectroscopy and gel permeation chro-
matography (GPC) were used to investigate the optical proper-
ties and hydrodynamic signatures of the products. Because of
the complexity of this system, it was often necessary to com-
pare spectra with linear polymers and porphyrins of known
[a] Dr. L. Li, J. Hollinger, Dr. G. Guerin, Prof. D. S. Seferos
Department of Chemistry
University of Toronto
8
0 St. George Street
composition. Accordingly, the absorption spectra of Br TPP has
4
Toronto, Ontario, M5S 3H6 (Canada)
E-mail: dseferos@chem.utoronto.ca
a main peak at 420 nm and four peaks at 516, 554, 597 and
6
64 nm corresponding to the Q bands of porphyrin (Figure 2a,
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cphc.201200682.
black solid line). This absorption signature is consistent with
4
110
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2012, 13, 4110 – 4115

4 4
Figure 1. Synthesis of the tetrabromoporphyrin initiated polyhexylthiophene [TPPZn-(P3HT) ] or polyhexylselenophene [TPPZn-(P3HS) ].
[
17]
typical porphyrin complexes. After Zn(Ac) was added, the
(Figure 2a, dotted line). This confirms successful metallization
2
[17b]
absorption peak at 420 nm shifts to 425 nm and the four Q
bands are replaced by two new resonances at 554 and 590 nm
and is also consistent with previous results.
We next investigated the optical properties of the TPPZn-
(
P3HS) complex. The complex has absorption bands that arise
4
from both the porphyrin and P3HS, appearing at 435 nm and
82 nm, respectively (Figure 2a, gray solid line). The presence
4
of the band at 482 nm confirms that the complex contains
P3HS with an optical signature that is in the chain-length de-
pendent regime. Indeed, the absorption bands in the complex
that arise from P3HS are spectroscopically identical to pristine
P3HS, which confirms that a highly p-delocalized material has
been formed. Interestingly, compared with the main absorp-
tion band of Br TPPZn (425 nm), the porphyrin band in TPPZn-
4
(
P3HS) shifts to 435 nm, indicating that the conjugation of the
4
porphyrin has been extended through the P3HS ’arms’. In the
solid state, both absorption bands further shift to 444 and
575 nm (Figure 2a, dashed line), respectively. This latter point
suggests that the materials form a self-assembled structure
with further electronic delocalization.
We conducted GPC experiments on the Br TPPZn, TPPZn-
4
(
(
P3HS) complex and a physical mixture of Br TPPZn and P3HS
4 4
Figure 2b). The pure Br TPPZn sample has a strong elution
4
peak at 32.5 mL retention volume (Figure 2b, black solid line).
The mixture of Br TPPZn and P3HS has two peaks at 32.5 and
4
2
7.8 mL, which we assign to Br TPPZn and P3HS, respectively.
4
Importantly, this confirms that the mixture can be separated
and detected by GPC (Figure 2b, gray solid line). We next car-
ried out GPC measurements on the TPPZn-(P3HS) complex.
4
Here, only one peak at a retention volume around 28 mL was
observed. This is important because it confirms that there are
no free porphyrins in the sample. It also shows that a complex
with large hydrodynamic radii has been formed (Figure 2b
dashed line). It is however, difficult to confirm that all sites on
Br TPPZn initiator have been appended with P3HS.
4
Figure 2. Optical absorption spectra and GPC data for the TPPZn-(P3HS)
4
complex compared with various control experiments.
ChemPhysChem 2012, 13, 4110 – 4115
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4111

works are not observed, suggesting that the substrate also
plays a role in the formation of the assemblies.
Transmission electron microscopy (TEM) images also show
the network structure of the film with hexagonal holes (Figures
S7a,b). Confocal fluorescence images of the film formed by
TPPZn-(P3HS) are only weakly emissive, which is probably due
4
[4b]
to the weak fluorescence of P3HS (Figure S7c). We also in-
vestigated the self-assembly of TPPZn-(P3HT) (the sulfur ana-
4
logue), however, and found that TPPZn-(P3HT) networks are
4
very emissive (Figure S7d), likely because they do not contain
[4b]
heavy Se atoms . Similar structures are obtained with TPPZn-
P3HT)4 although the networks are not as regular as those
(
formed by the TPPZn-(P3HS) (Figure S8). The solution optical
4
spectra of TPPZn-(P3HT) contains only the absorption peaks
4
from P3HT (Figure S9), which is likely due to the strong absorp-
tion of P3HT relative to TPPZn and the overlapping absorption
bands of these two species. Elemental line scans confirm the
presence of sulfur and nitrogen in the TPPZn-(P3HT) compo-
4
site network (Figure S8c).
Because of the uncertainty of the number and length of the
“
arms” in the TPPZn-(P3HS) complex, we performed numerous
4
control experiments to confirm that only this complex will lead
to the network structures. In one instance, we investigated the
self-assembly structures of Br TPPZn alone. Br TPPZn was dis-
Figure 3. STEM images (a–d) and elemental line scan (e–g) of the networks
formed by the TPPZn-(P3HS) complex.
4
4
4
solved in chloroform at various concentrations, which were
then spin coated on copper grids. STEM images show that
nanorods that are 200 nm wide and 1 mm long are obtained
The self-assembly behavior of the TPPZn-(P3HS)4 complex
was examined next. In these experiments, the polymer was dis-
solved in chloroform and spin-coated onto a copper grid.
Scanning transmission electron microscopy (STEM) images
show that large networks are formed (Figure 3a), which is
quite different from the commonly obtained fiber structures
À1
from a 0.5 mgmL solution (Figure S10a). Magnified images
show that these nanorods are not continuous and appear
broken (Figure S10b). This self-assembled structure is consis-
[19]
tent with typical porphyrins reported previously. When the
À1
concentration was decreased to 0.25 mgmL , irregular net-
[
18]
formed by P3HS polymers. Magnified images show that the
holes in the network have regular hexagonal symmetry with
works are obtained (Figure S10c). Magnified images show that
the edges of the networks are rough and the holes are less
than 500 nm in diameter (Figure S10d), which is much smaller
than those formed by the TPPZn-(P3HS)4 or TPPZn-(P3HT)4
complexes. When the concentration is further decreased to
1.5–2 mm diameter holes that are about 200–500 nm thick at
their edges (Figure 3b). In order to investigate the elemental
composition of the networks, elemental line scans were carried
out. This experiment reveals strong selenium peaks from P3HS
as well as a less intense nitrogen peak from porphyrin core
À1
0.1 mgmL , networks are also obtained and the width be-
comes thinner (Figures S10e,f). Overall, these control experi-
(Figure 3e–g). Overall, elemental mapping shows that P3HS
ments show that Br TPPZn cannot form the networks formed
4
and porphyrin coexist in the networks, as would be expected
by the TPPZn-(P3HS) or TPPZn-(P3HT) complex.
4
4
from a TPPZn-(P3HS) complex. Importantly, when the sample
As a further control experiment, we synthesized P3HS using
a porphyrin that had only one polymerization initiation site
(BrTPPZn; Figure 4a). After isolation, this complex [TPPZn-
(P3HS)] was also spin coated onto a copper grid under the
4
is annealed at 1208C for 2 h, the superstructure remains intact,
(Figure S5) indicating that it is a thermally stable species. The
addition of small amounts of Br TPPZn (3%, w/w) to the
4
chloroform solution of TPPZn-(P3HT) cause the structure to
same conditions that were used for TPPZn-(P3HS) . STEM char-
4
4
change from networks to flat films with irregular aggregations
acterization shows that small nanofibers with diameters of
20 nm that are 100–200 nm long are formed (Figures 4b,c).
This structure is different from the networks formed by the
(Figure S6a and b). This might be ascribed to interactions be-
tween free Br TPPZn and those in the complex, which destroy
4
the self-assembly of the polymer. This experiment confirms
that free porphyrin is detrimental to the self-assembly process.
Chloroform was found to be the best solvent for the formation
of the networks. When casting the films from chlorobenzene,
the film becomes flat and no networks are formed (Figur-
es S6c,d), which suggests that fast evaporation is required for
the formation of networks. Additionally, when conducting the
same experiments on a hydrophobic silicon substrate, net-
TPPZn-(P3HS) complex. The difference in the self-assembled
4
structures indicates that the degree of branching plays a role
in the formation of the superstructures. The optical properties
of TPPZn-P3HS complex differ from the TPPZn-(P3HS) com-
4
plex. For example, the porphyrin signal appears at 427 nm
(Figure 4d), which is not as red-shifted as TPPZn-(P3HS) . This
4
shows a loss of conjugation at the core as a result of only
having one “arm”. Finally, we note that a physical mixture of
4
112
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ChemPhysChem 2012, 13, 4110 – 4115

packed water droplets that are preserved in the film as air
bubbles. We hypothesize that similar breath figures also form
in our system and lead to the formation of the hexagonal net-
work structures. When a less volatile solvent was used in our
experiments, the networks were also not observed. Polymers
that adopt a spherical shape in solution form regular hexago-
[21]
nal networks. For example, compared with linear PS, a certain
degree of branching density, high molecular weight, and low
viscosity have been the key factors to obtain honeycomb
[22]
membranes.
Recently, Luscombe and coworkers reported
a lower viscosity for branched P3HT than that of linear poly-
[23]
mers. Based on the above discussion, the reason network
structures have not been observed in previous P3HT and P3HS
polymers is likely related to the low branching and high viscos-
ity of these rod-like polymers. The breath figure phenomenon
observed in our system is therefore indicative that branched
polymers were synthesized.
In conclusion, a porphyrin with four polymerization initiation
sites was used to synthesize TPPZn-(P3HS) and TPPZn-(P3HT)
4
4
complexes. Optical absorption spectroscopy shows that the
porphyrin and P3HS signal coexist in the complex. As well, the
porphyrin peak is red-shifted by about 10 nm compared with
the free porphyrin, which indicates an extension of conjuga-
tion through the polymer arms. When allowed to dry from so-
lution on a TEM grid, this complex forms hexagonal networks.
Elemental line scans that were made across the networks show
both selenium (or sulfur) signal from P3HS (or P3HT) and nitro-
gen signal from porphyrin, while GPC characterization confirms
that free porphyrin is not present in the sample. All characteri-
zation has led us to conclude that the synthesis prepares
a novel polymer-porphyrin complex with multiple arms. While
it is difficult to confirm the number of arms, the complex pre-
pared with the porphyrin with four polymerization initiation
sites shows dramatically different self-assembly behavior com-
pared to that of linear polymers, the porphyrin alone, a physical
mixture of linear polymer and porphyrin, and the complex pre-
pared with the porphyrin with one polymerization initiation
site. Breath figures as well as the polymer structure appear to
be responsible for the formation of the hexagonal networks.
These types of networks have not been observed in P3HT- or
P3HS-type polymers and show the possibility of controlling the
film morphology by changing the branching or symmetry of
the polymer, thereby demonstrating a new method to control
the solid state properties of conjugated polymers.
Figure 4. Molecular structure (a), STEM images (b, c) of TPPZn-P3HS poly-
mers, and absorption spectra (d) of the BrTPP, BrTPPZn, TPPZn-P3HS and
4
TPPZn-(P3HS) complexes.
Br TPPZn and P3HS (or P3HT) does not produce the same su-
4
perstructures and results in flat films with irregular small fibers
(Figure S11).
Here, we provide an important additional structural aspect
of the networks formed by the TPPZn-(P3HS) complex. Specifi-
4
cally, lamellar stacking at the molecular scale is observed in the
hexagonal networks. The lamella stacking of the complex sub-
stituted with a hexyl side-chain are confirmed by a characteris-
tic 15.5 ꢁ X-ray scattering signal (or 16.3 ꢁ for TPPZn-(P3HT)4,
Figure S12). Both of these values are consistent with interlayer-
spacing distances for P3HS and P3HT that were reported previ-
[
18a]
ously (15.5 and 16.0 ꢁ, respectively).
These experiments
show that the network is sufficiently crystalline, despite the
fact that rapid solvent evaporation seems to be required to
obtain this structure. The crystallization lends support to the
hypothesis that this process is at least in part due to self-as-
sembly.
Experimental Section
Materials
All reagents were used as received unless otherwise noted. 4-Bro-
mobenzaldehyde, pyrrole, pyrrol, propionic acid, tetrakis(triphenyl-
It is important to note that network structures have not
been previously observed in P3HS- or P3HT-type polymers. In
phosphine)nickel(0) (Ni(PPh ) ) and dppp were purchased from Al-
[
20]
3 4
one previous report, ordered macroporous arrays of holes
were formed in polystyrene films by the formation and crystal-
lization of “breath figures”. Breath figures form when a cold
solid surface is brought in contact with a volatile liquid. In the
presence of moist air (laboratory humidity levels are typically
drich.
Synthesis
Tetra(4-bromophenyl)porphyrin (Br TPP): This molecule was synthe-
4
[15]
20–50%), evaporative cooling generates a layer of hexagonally
sized following
a
literature method.
4-Bromobenzaldehyde
ChemPhysChem 2012, 13, 4110 – 4115
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org
4113

(
1
1.85 g, 10 mmol) and pyrrole (0.7 mL, 10 mmol) were dissolved in
5 mL of propionic acid and 5 mL of nitrobenzene. The mixture
ture gel permeation chromatography system) with narrow weight
distribution polystyrene standards. Dark-field scanning transmission
electron microscopy (STEM) imaging was carried out at 200 kV
using a Hitachi HD-2000 instrument. Transmission electron micros-
copy (TEM) images were obtained using a Hitachi H-7000 at
a 100 kV accelerating voltage. Laser confocal microscopy was car-
ried out on Leica TCS SP2. The fluorescent images were taken
under the excitation wavelength of 488 nm. Wide-angle X-ray scat-
tering (WAXS) spectra were recorded on a Bruker AXS D8 Discovery
with a GADDS area detector. Absorption spectra were recorded
using a Varian Cary 5000 spectrometer.
was heated at 1208C for 3 h and allowed to stand at room temper-
ature for 3 days. The product was filtered and washed thoroughly
with methanol, hexane, and iso-propanol to afford the product as
a dark purple solid. The sample then was recrystallized from CHCl /
methanol. Spectroscopy is identical to the literature: H NMR
3
1
(
8
CDCl , 400 MHz): d=8.84 (s, 1H), 8.07 (d, J=8 Hz, 1H), 7.91 (d, J=
3
Hz, 1H).
5
-(4-Bromophenyl)-10,15,20-tri(phenyl)porphyrin (BrTPP): This mole-
[24]
cule was prepared by the Adler and Longo method. Specifically,
a
1
stochiometric mixture of 4-bromobenzaldehyde (2.63 g,
4 mmol), benzaldehyde (4.53 g, 42 mmol), and freshly distilled
Thin Film Preparation
pyrrole (3.82 g, 56 mmol) were added to 250 mL of refluxing pro-
pionic acid. The mixture was refluxed for 30 min and cooled to
room temperature in air. Purification was accomplished by filtering
the solution and by washing the residue three times with metha-
nol, followed by hot water to afford purple crystals (1.4 g; 14%
yield).
À1
Polymer solutions (10 mgmL ) were spin coated on a carbon cov-
ered copper grid for dark-field STEM and laser confocal microscopy
characterization. For WAXS characterization, solutions were drop
coated onto a silicon substrate. For the absorption experiments,
solutions were spin coated onto a quartz microscope slide at 500
rotations/min.
Tetra(4-bromophenyl)porphyrin zinc (Br TPPZn):
A solution of
4
Br TPP (93 mg, 0.1 mmol) in DCM (50 mL) was added to a solution
4
of Zn(OAc) ·H O (75 mg, 0.37 mmol) in methanol (3 mL). The mix-
2
2
Acknowledgements
ture was stirred for 5 h at room temperature. The solvent was re-
moved by evaporation and the product was purified by column
chromatography [silica gel, CH Cl /hexane (1:1)] to afford a purple
We thank the University of Toronto, the Natural Sciences and En-
gineering Research Council of Canada, the Canadian Foundation
for Innovation, MaRS Innovation, and the Ontario Research Fund
for supporting this work.
2
2
1
solid. H NMR (CDCl , 400 MHz): d=8.94 (s, 1H), 8.07 (d, J=8 Hz,
3
1
H), 7.90 (d, J=8 Hz, 1H).
Synthesis of the initiator: Ni(PPh3)4 (34 mg, 0.03 mmol) was dis-
solved in 3 mL toluene. Br TPP-Zn (6.5 mg, 0.0065 mmol) in 5 mL
4
toluene was added. The homogeneous mixture was stirred for
Keywords: conjugated polymers
·
polyselenophene
·
2
0 min and was allowed to stand unperturbed overnight. The origi-
polythiophene · porphyrins · self-assembly
nal deep red color of the reaction mixture gradually changed to
yellow green together with the formation of green precipitates,
which were separated by filtration, washed with dry hexane, and
dried in a vacuum. The precipitates were redispersed in 4 mL tolu-
ene and 40 mg dppp was added. The mixture was stirred for 2 h
and the solvent was removed. In the case of BrTPPZn, the stoichi-
ometry was adjusted and all other conditions remained the same.
[
[
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by Soxhlet extraction using methanol, hexanes, and chloroform.
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1
3
7
0
4%), TPPZn-(P3HS) : H NMR (CDCl , 400 MHz): d=8.92 (br, 1H),
4
3
.97 (br, 1H), 7.67 (br, 1H), 7.12 (s, 3H), 2.73 (5H), 1.26–1.75 (17H),
À1
À1
.91 (6H). M =8.0 kgmol , M =16 kgmol , PDI=2.0; regioregu-
n
w
À1
À1
larity=90%. TPPZn-(P3HT)4: M =19 kgmol , M =38 kgmol
,
n
w
PDI=2.0, regioregularity=98%.
[
[
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Received: August 20, 2012
Published online on November 5, 2012
ChemPhysChem 2012, 13, 4110 – 4115
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org
4115