Microporous polymer network films covalently bound to gold electrodes

Article abstract of DOI:10.1039/c4cc09637a

Covalent attachment of a microporous polymer network (MPN) on a gold surface is presented. A functional bromophenyl-based self-assembled monolayer (SAM) formed on the gold surface acts as co-monomer in the polymerisation of the MPN yielding homogeneous and robust coatings. Covalent binding of the films to the electrode is confirmed by SEIRAS measurements.

Full text of DOI:10.1039/c4cc09637a

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Microporous polymer network films covalently bound
to gold electrodes
Cite this: DOI: 10.1039/x0xx00000x
Daniel Becker,†a Nina Heidary,†b,d Marius Horch,b Ulrich Gernert,c Ingo Zebger,b
Johannes Schmidt,a Anna Fischer*b,d and Arne Thomas*a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
Covalent attachment of a microporous polymer network processed as thin films on electrodes. Other possible applications
(MPN) on gold surface is presented. functional of MPNs requiring film morphology comprise gas separation
bromophenyl-based self-assembled monolayer (SAM) formed membranes and catalytically active coatings.
a
A
Consequently, several groups have attempted the synthesis of
on the gold surface acts as co-monomer in the polymerisation
of the MPN yielding homogeneous and robust coatings.
Covalent binding of the films to the electrode is confirmed by
SEIRAS measurements.
films or other morphologies of MPNs. The PIM approach was
further extended to microporous, conjugated dendrimers,8 or MNP
nanoparticles9 which could be as well processed from solutions. A
further improvement was made by the design of novel organic cages,
soluble in many solvents and advantageously self-aggregating into
porous crystals after solvent removal.10 Some examples are reported
in which MPNs are deposited on surfaces present during
synthesis.11,6b Also electropolymerizable films have been grown on
surfaces, for example using carbazole functionalized tectons.12
Naturally, this approach is restricted to monomers which can be
electropolymerized (such as carbazole or thiophene functionalized
tectons).13 However it has been reported that conjugated polymer
films made by electropolymerization are not effective for organic
electronic applications as some of the electrolyte used during
synthesis inevitably stays in the polymer films, thus strongly
influencing their electronic properties.14
In all these approaches the polymer/electrode interface is not
well controlled and it can be assumed that films are attached to the
surfaces just by dispersion forces, thus potentially prone to electrode
/ surface detachment. This would certainly be advantageous in case
free-standing films or membranes are targeted. However, for other
e.g. electrocatalytic or organic electronic applications formation of
stable films on electrodes is an essential requirement.
The possibility to combine the advantages of porous materials with
the functionalities found in organic molecules has made microporous
polymer networks (MPNs) an emergent research field.1 The
introduction of various functional groups into the backbone of MPNs
has led to applications in catalysis,2 energy conversion and storage3
or gas sorption4 and separation.5 Recently also the promising
properties of MPNs in organic electronic devices were described.6
The first microporous polymers prepared using the concept of rigid,
contorted tectons to avoid dense packing of polymer chains
(polymers of intrinsic microporosity, PIMs),7 were soluble and thus
processable from solutions. However, it had been later recognized
that very high surface areas and structural stability can be reached
when a higher degree of crosslinking is realized, yielding the
development of MPNs. Unfortunately, due to their cross-linked
structure MPNs are insoluble and inmeltable, which make any
processing nearly impossible, a serious drawback for a range of
envisaged applications. This is especially true for possible
applications in organic electronic devices for which MPNs must be
a
Department of Chemistry, Functional Materials, Technische Universität
A more controlled attachment of films to electrode surfaces was
developed in the field of metal organic frameworks (MOFs). Surface
bonded MOFs (SurMOFs) can be grown from functional self-
assembled monolayer by a layer-by-layer process.15 A similar
approach has been used to produce ultrathin films of molecular
networks on amine terminated SAMs.16 In another relevant
approach, organic films and later free standing membranes of
polymer carpets were fabricated by cross-linking of SAMs.17
b
Berlin, Hardenbergstr. 40, 10623 Berlin. Department of Chemistry,
Technische Universität Berlin, Strasse des 17. Juni 135, 10623 Berlin
c
Center for Electron Microscopy (ZELMI), Technische Universität
d
Berlin, Strasse des 17. Juni 135, 10623 Berlin. Universität Freiburg,
Institut für Anorganische und Analytische Chemie, Albertstrasse 21,
79104 Freiburg.
† These authors contributed equally to this work
Electronic Supplementary Information (ESI) available: Synthetic details,
analytical data as well as results from DFT calculations. See
DOI: 10.1039/c000000x/
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appearance was observed after the polymerization reaction, on
SAM-functionalized gold surfaces thin coatings have formed (Fig.
S1). Before this coating was further analyzed, the reaction protocol
was verified by analyzing the bulk polymer powder (PPN-6), which
has additionally precipitated from solution by 13C solid-state NMR
and ATR-FTIR measurements (Fig. S2,3). Nitrogen sorption (Fig.
S4) revealed an apparent surface area of 2977 m²/g which is a
remarkable high value even though lower than the value of 4023
m²/g reported by Zhou et al. for PPN-6.21b This might be due to
slight changes in our reaction protocol compared to the reported
procedure. Still the 13C-NMR and nitrogen sorption measurements
confirm the successful synthesis of PPN-6 using the here applied
reaction protocol. Unfortunately, these techniques could not be
applied to the polymer films due to the low amount of sample mass
(see a discussion on Kr-Sorption measurements in the SI, Figure S5).
To investigate the formed MPN films, SEM analysis was carried
out. Fig. 1a) shows a top view of the MPN film grown on the SAM-
functionalized gold electrode. For better visibility a location on the
substrate was chosen, where the polymer film only partially covers
the gold surface. The polymer film is very smooth and
homogeneous. However, some larger particles are also found to be
located on top of the polymer film. This becomes even more
apparent in the cross section SEM picture (Fig. 1b-f).
Scheme 1 Illustration of the synthesis of PPN-6 films on 4-bromophenyl
functionalized gold substrates.
Motivated by these works we attempted to grow a MPN onto an
electrode surface by first covalently attaching a functional monomer
which can take part in the polymerization of the MPN (Scheme1). A
gold surface was chosen as being a superior electrode and amenable
to modification by SAMs. The gold surface was functionalized with
4-bromothiophenol since halogenated aromatic compounds are the
moieties which are applied in the most frequently used C-C-bonding
reactions for the synthesis of MPNs, such as the Sonogashira-
Hagihara,18 Yamamoto19 or Suzuki20 coupling. Provided that the
forming polymer reacts with the exposed bromine atoms of the
SAM, the formed MPN should be tightly connected to the surface by
covalent bonds. Indeed, formation of homogeneous films of the
MPN on the gold surface could be achieved as seen from scanning
electron microscopy (SEM) as well as the covalent attachment via
the SAM to the gold surface, which is proven by surface enhanced
infrared absorption (SEIRA) spectroscopic measurements (see SI for
synthetic details).
For the generation of covalently bound MPN films on gold
electrodes a Yamamoto-type coupling was chosen. This C-C-type
coupling reaction, which is frequently used for the generation of
MPNs, requires just one type of monomer, namely halogen
functionalized aromatic monomers, which can be homo-polymerized
into high surface area networks.19 Tetrakis(4-bromophenyl)methane
was applied as monomer. Yamamoto polymerization of this
monomer in bulk yields
a MPN called “PAF-1” with an
extraordinary high BET surface area of 5600 m²g-1.21a The synthetic
protocol for PAF-1 was further improved by Zhou et al. thereby
renaming PAF-1 to “PPN-6”.21b Since in the present work the latter
protocol was applied the here described polymers are as well entitled
PPN-6. To prepare bromophenyl functionalized gold films, cleaned
gold wafers were immersed in an ethanolic solution of 4-
bromothiophenol for 16 hours. It should be noted that the simplicity
of SAM formation allow for a broad variety of covalently attached
functional groups on gold electrodes, which in principle could be
also used for various other polymerization reactions. The synthesis
of the microporous polymer network PPN-6 was carried out in
presence of these bromophenyl-functionalized gold films. In case the
bromophenyl-SAM molecules on the gold surface act as co-
monomers in the Yamamoto polymerization, it can be expected that
films of PPN-6 are growing from the surface and as a result are
covalently attached to it. All experiments were carried out using an
additional blank, i.e. unfunctionalized gold film, within the same
reaction environment. While for the blank samples no change in
Fig. 1 SEM micrographs of PPN-6 film grown on gold substrate via Yamamoto
cross coupling reaction. Top view (a) and cross section (b-f) micrographs showing
increasing magnification from b to f.
It can be assumed that the homogenous polymer layer is formed
by polymerization of monomers in solution with the bromophenyl
moieties attached to the gold surface. On top of this dense polymer
layer a second open polymer layer has been formed, which consists
of agglomerates of polymer spheres. It can be reasoned that this
layer consists of polymer particles firstly formed in solution and
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subsequently bound to the homogeneous primary polymer layer via changes perpendicular to the surface are enhanced, while vibrations
terminal bromine groups. Fig. 1e clearly shows the primary polymer parallel to the surface remain unamplified. Fig. 2B displays the time
layer and a polymer particle bound to it. This is further supported by evolution of the SAM formation during an incubation period of 16
investigating the films after different reaction times (Fig. S6). hours. Thereby, the most intensive aromatic band at 1466 cm-1 was
Already after 18 min reaction time a thin, homogeneous polymer chosen as marker for the monolayer formation on the gold surface. A
film can be observed on the gold substrate and just a few larger biphasic behavior is observed, with a primary, exponential process
particles can be spotted on top of this film. With ongoing reaction related to a completed SAM adsorption within the first 30 minutes
time both the thickness of the film and the number of particles and an additional, slight but continuous intensity increase over the
increase (Fig. S7) and after 20 h reaction time, a second, open layer remaining 16 hours. The secondary process can be potentially
of agglomerated particles is formed. EDX line and spot scans attributed to rearrangements of thiol molecules. Therefore, a longer
confirm the elemental composition of both the structures that grow incubation time of SAM solution onto the gold surface leads to a
atop the homogeneous film and the film itself (Fig. S8). The polymer more homogeneous monolayer formation. Subsequent removal of
films cannot be detached from the surface when treated with solvents the SAM-containing solution and several rinsing cycles with ethanol
like THF overnight, which further supports the covalent binding of did not affect the overall band intensities, indicating a strong binding
PPN-6 to the gold substrate. However, swelling yield cracks in the of thiol to the gold surface via covalent Au-S bonding. In addition no
polymer films (Fig. S9), which is consistent with a surface attached remaining S-H modes could be detected. Fig. 3 shows the spectra
polymer film that suffers from structural strain caused by swelling before and after the Yamamoto coupling reaction and careful rinsing
and drying. It should be mentioned that even though in most cases of the SEIRA cell. For clarity we plotted the respective difference
the blank gold films are unaffected by the reaction, in rare occasions spectrum obtained by subtracting the spectrum recorded before from
the formation of polymer films was also observed on the blank gold the one taken after the polymerization reaction (Fig. 3C). In such
substrates. So far we were not able to find a systematical explanation way a decrease of the characteristic bands of the aromatic ν(C=C)
to predict when polymer films are formed on unfunctionalized gold stretching at 1562 cm-1 and 1466 cm-1 can be monitored, while new
films. Anyway, these films are easily detached from the gold surface bands at 1602 cm-1 and 1492 cm-1 appear. These significant band
by solvent treatment (Fig. S9), confirming again that the surface shifts of 40 cm-1 and 26 cm-1 to higher wavenumbers are a good
functionalization by SAMs is necessary to yield MPN films indication for the ongoing reaction via the bromine-groups of the
covalently bound to the surface.
SAM, i.e. the formation of new C-C bonds between the aromatic
To further prove the involvement of the SAM in the polymeric ring of the SAM and the phenyl groups of tetrakis(4-
film formation and surface attachment to the gold surface, in-situ bromophenyl)methane under elimination of the bromide end groups
SEIRAS measurements were carried out. In this case the thin of the SAM.
nanostructured gold film acts also as IR signal amplifier to probe
molecules near the gold surface. As the surface enhancement
decreases strongly with distance,22 SEIRAS can be applied as a
surface sensitive method to study mono- und multilayer formation
up to a thickness of about 8 nm. In such way, SEIRAS allows to
monitor the covalent binding of the 4-bromothiophenol monolayer to
the gold surface.
Fig. 3: SEIRA spectra of the SAM recorded in the adsorption equilibrium after 16
hours (black, A), subsequent to the Yamamoto-like coupling reaction (green, B)
and difference spectrum (B minus A i.e. after minus before Yamamoto reaction)
(red, C). Positive bands are related to the emerging polymer matrix, negative
bands are characteristic for the consumed (incorporated) SAM.
Fig. 2 SEIRA spectrum recorded immediately after incubation of the gold film
with SAM solution (A) and time evolution of 4-bromothiophenol monolayer
formation over 16 hours monitored by the aromatic C=C stretching band
1466 cm-1 (B).
Notably, similar shifts to higher wavenumbers of approximately
29 cm-1 and 35 cm-1 are predicted by DFT calculations for the C=C
vibrations of a molecular model system composed of one 4-
Fig. 2 displays the binding of 4-bromothiophenol to the gold
bromothiophenol molecule linked to one monomer species in
surface directly after incubation of the gold film with the SAM
vacuum (Fig. S10-12). Thus, it can be concluded, that the film
solution, as indicated by the characteristic bands of the aromatic
formation is indeed initiated at the –Br terminated surface, resulting
ν(C=C) stretching vibrations at 1562 cm-1 and 1466 cm-1. The
in a covalent attachment of the MPN film to the gold electrode.
“disappearance” of bands compared to the FTIR bulk reference
spectrum (Fig. S10) can be explained considering the SEIRAS
selection rule, i.e. only vibrations with predominant dipole moment
Conclusions
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Films of the microporous polymer network PPN-6 were grown on
SAM-functionalized gold electrodes. SEIRAS measurements
confirm the covalent binding of PPN-6 to the gold surface and prove
that SAM molecules act as co-monomers in the polymerization
reaction. In future this approach will be extended to other cross-
coupling reactions. The here presented method has therefore strong
implications for the deposition of functional microporous polymers
and constitutes an important step to the formation of organic
electronic devices.
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