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Cite this: Chem. Commun., 2015,

51,4283

Microporous polymer network films covalently

bound to gold electrodes†

Daniel Becker,‡

Nina Heidary,‡

Marius Horch,

Ulrich Gernert,

Ingo Zebger,

Johannes Schmidt,

Anna Fischer*

and Arne Thomas*

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 homo-

geneous and robust coatings. Covalent binding of the films to the

electrode is confirmed by SEIRAS measurements.

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.

The introduction of various functional groups into the

backbone of MPNs has led to applications in catalysis,

energy

conversion and storage

or gas sorption

and separation.

Recently also the promising properties of MPNs in organic

electronic devices were described.

The first microporous poly-

mers prepared using the concept of rigid, contorted tectons to

avoid dense packing of polymer chains (polymers of intrinsic

microporosity, PIMs),

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 applica-

tions. This is especially true for possible applications in organic

electronic devices for which MPNs must be processed as thin films

on electrodes. Other possible applications of MPNs requiring

film morphology comprise gas separation membranes and

catalytically active coatings.

Consequently, several groups have attempted the synthesis

of films or other morphologies of MPNs. The PIM approach was

further extended to microporous, conjugated dendrimers,

or MPN

nanoparticles

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.

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.

Naturally, this approach is restricted to monomers

which can be electropolymerized (such as carbazole or thiophene

functionalized tectons).

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.

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.

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.

Asimilar

approach has been used to produce ultrathin films of molecular

networks on amine terminated SAMs.

In another relevant

approach, organic films and later free standing membranes of

polymer carpets were fabricated by cross-linking of SAMs.

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 (Scheme 1). A gold surface was chosen as being a superior

Department of Chemistry, Functional Materials, Technische Universita

¨tBerlin,

Hardenbergstr. 40, 10623 Berlin, Germany. E-mail: arne.thomas@tu-berlin.de

Department of Chemistry, Technische Universita

¨t Berlin, Strasse des 17. Juni 135,

10623 Berlin, Germany

Universita

¨t Freiburg, Institut fu

¨r Anorganische und Analytische Chemie,

Albertstrasse 21, 79104 Freiburg, Germany. E-mail: anna.fischer@ac.uni-freiburg.de

Center for Electron Microscopy (ZELMI), Technische Universita

¨tBerlin,

Strasse des 17. Juni 135, 10623 Berlin, Germany

†Electronic supplementary information (ESI) available: Synthetic details, analy-

tical data as well as results from DFT calculations. See DOI: 10.1039/c4cc09637a

‡These authors contributed equally to this work.

Received 2nd December 2014,

Accepted 5th February 2015

DOI: 10.1039/c4cc09637a

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electrode and amenable to modification by SAMs. The gold

surface was functionalized with 4-bromothiophenol since halo-

genated 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,

Yamamoto

or Suzuki

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). Furthermore the covalent

attachment via the SAM to the gold surface can be proven by

surface enhanced infrared absorption (SEIRA) spectroscopic

measurements (see ESI,†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.

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 m

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 polymeriza-

tion, 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 appearance was observed

after the polymerization reaction, on SAM-functionalized gold

surfaces thin coatings have formed (Fig. S1, ESI†). 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

C solid-state NMR

and ATR-FTIR measurements (Fig. S2 and S3, ESI†). Nitrogen

sorption (Fig. S4, ESI†) revealed an apparent surface area of

2977 m

1

which is a remarkable high value even though lower

than the value of 4023 m

1

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

C-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 ESI,†Fig. S5).

To investigate the formed MPN films, SEM analysis was

carried out. Fig. 1(a) 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. 1(b)–(f)).

It can be assumed that the homogeneous 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

Scheme 1 Illustration of the synthesis of PPN-6 films on 4-bromophenyl

functionalized gold substrates.

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).

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formed, which consists of agglomerates of polymer spheres. It

can be reasoned that this layer consists of polymer particles

firstly formed in solution and subsequently bound to the

homogeneous primary polymer layer via terminal bromine groups.

Fig. 1(e) clearly shows the primary polymer layer and a polymer

particle bound to it. This is further supported by investigating the

films after diﬀerent reaction times (Fig. S6, ESI†). Already after

18 min reaction time a thin, homogeneous polymer film can be

observed on the gold substrate and just a few larger particles can be

spotted on top of this film. With ongoing reaction time both the

thickness of the film and the number of particles increase (Fig. S7,

ESI†) and after 20 h reaction time, a second, open layer of agglom-

erated particles is formed. EDX line and spot scans confirm the

elemental composition of both the structures that grow atop the

homogeneous film and the film itself (Fig. S8, ESI†). The polymer

films cannot be detached from the surface when treated with

solvents like THF overnight, which further supports the covalent

binding of PPN-6 to the gold substrate. However, swelling yield

cracks in the polymer films (Fig. S9, ESI†), which is consistent with a

surface attached polymer film that suﬀers from structural strain

caused by swelling and drying. It should be mentioned that even

though in most cases the blank gold films are unaﬀected by the

reaction, in rare occasions the formation of polymer films was also

observed on the blank gold substrates. So far we were not able to

find a systematical explanation to predict when polymer films are

formed on unfunctionalized gold films. Anyway, these films are

easily detached from the gold surface by solvent treatment (Fig. S9,

ESI†), confirming again that the surface functionalization by SAMs is

necessary to yield MPN films covalently bound to the surface.

To further prove the involvement of the SAM in the polymeric

film formation and surface attachment to the gold surface, in situ

SEIRAS measurements were carried out. In this case the thin

nanostructured gold film acts also as IR signal amplifier to probe

molecules near the gold surface. As the surface enhancement

decreases strongly with distance,

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. 2 displays the binding of 4-bromothiophenol to the gold

surface directly after incubation of the gold film with the SAM

solution, as indicated by the characteristic bands of the aromatic

n(CQC) stretching vibrations at 1562 cm

1

and 1466 cm

1

.The

‘‘disappearance’’ of bands compared to the FTIR bulk reference

spectrum (Fig. S10, ESI†) can be explained considering the

SEIRAS selection rule, i.e. only vibrations with predominant

dipole moment changes perpendicular to the surface are

enhanced, while vibrations parallel to the surface remain unam-

plified. Fig. 2B displays the time evolution of the SAM formation

during an incubation period of 16 hours. Thereby, the most

intensive aromatic band at 1466 cm

1

waschosenasmarkerfor

the monolayer formation on the gold surface. A biphasic beha-

vior is observed, with a primary, exponential process related to a

completed SAM adsorption within the first 30 minutes and an

additional, slight but continuous intensity increase over the

remaining 16 hours. The secondary process can be potentially

attributed to rearrangements of thiol molecules. Therefore, a

longer incubation time of SAM solution onto the gold surface

leads to a more homogeneous monolayer formation. Subsequent

removal of the SAM-containing solution and several rinsing

cycles with ethanol did not aﬀect the overall band intensities,

indicating a strong binding of thiol to the gold surface via

covalent Au–S bonding. In addition no remaining S–H modes

could be detected. Fig. 3 shows the spectra before and after the

Yamamoto coupling reaction and careful rinsing of the SEIRA

cell. For clarity we plotted the respective difference spectrum

obtained by subtracting the spectrum recorded before from the

one taken after the polymerization reaction (Fig. 3C). In such way

a decrease of the characteristic bands of the aromatic n(CQC)

stretching at 1562 cm

1

and 1466 cm

1

can be monitored, while

new bands at 1602 cm

1

and 1492 cm

1

appear. These signifi-

cant band shifts of 40 cm

1

and 26 cm

1

to higher wavenumbers

are a good indication for the ongoing reaction via the bromine-

groups of the SAM, i.e. the formation of new C–C bonds between

the aromatic ring of the SAM and the phenyl groups of tetrakis(4-

bromophenyl)methane under elimination of the bromine end

groups of the SAM.

Notably, similar shifts to higher wavenumbers of approxi-

mately 28 cm

1

and 35 cm

1

are predicted by DFT calculations

for the CQC vibrations of a molecular model system composed

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 CQC

stretching band 1466 cm

1

(B).

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 con-

sumed (incorporated) SAM.

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of one 4-bromothiophenol molecule linked to one monomer

species in vacuum (Fig. S11 and S12, ESI†). Thus, it can be

concluded, that the film formation is indeed initiated at the

bromine terminated surface, resulting in a covalent attachment

of the MPN film to the gold electrode.

Films of the microporous polymer network PPN-6 were

grown on SAM-functionalized gold electrodes. SEIRAS measure-

ments 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.

We thank the European Research Council (ERC) (Grant:

278593ORGZEO) for financial support. Further the DFG and

the Cluster of Excellence UniCat EXC 314/2 and the BMBF

(grant FKZ 03X5524) is gratefully acknowledged for funding as

well as the ZELMI of the TUB for access to the SU8030 FEG

HR-SEM (Hitachi, DFG INST 131/631–1).

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