Semiconducting polymer thin films by surface-confined stepwise click polymerization

Article abstract of DOI:10.1039/c1cc14479k

Surface-confined stepwise click polymerization was used to prepare surface-attached thin films of semiconducting polymers. These highly uniform films showed extended UV/vis absorption characteristics and a remarkable degree of molecular organization with a unidirectional alignment of the polymer chains normal to the surface.

Full text of DOI:10.1039/c1cc14479k

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COMMUNICATION
Semiconducting polymer thin films by surface-confined stepwise click
polymerizationw
Euiyong Hwang,a Kathie L. Lusker,a Jayne C. Garno,a Yaroslav Losovyjb and
Evgueni E. Nesterov*a
Received 22nd July 2011, Accepted 19th September 2011
DOI: 10.1039/c1cc14479k
Surface-confined stepwise click polymerization was used to prepare
surface-attached thin films of semiconducting polymers. These
highly uniform films showed extended UV/vis absorption character-
istics and a remarkable degree of molecular organization with a
unidirectional alignment of the polymer chains normal to the surface.
molecular layer deposition).7 In this method, a surface-attached
polymer brush can be grown by a repetitive series of discrete
steps of monomer additions to the growing polymer chain end.
Building a polymer by incorporating one monomeric unit per
reaction step may seem to be laborious and time-consuming.
Nevertheless, it provides complete control over polymer structure
and composition and can produce high-density surface-attached
polymer thin films with a low PDI and uniform surface mor-
phology, especially when an efficient chemistry is available so
that each individual step takes only a short time. For example, a
recent report described deposition of a 115 nm thick polyaramide
film via 1000 sequential reaction steps.8 In general, labor inten-
siveness and total preparation time in stepwise polymerization
could be significantly reduced through an easy-to-implement
automation of repeated immersing–washing steps. In the case
of semiconducting polymers, the possibility for preparing polymer
brushes with a complex molecular structure (for example,
polymers with an internal electric field gradient to enhance
charge transport in photovoltaic applications)9 not achievable
by other means would increase the practical value of this
method. Previously, stepwise polymerization was used to grow
surface-attached films of azomethine p-conjugated oligomers,
although, due to the long reaction time required for each step,
only short oligomers could be prepared.10 In this Communication,
we report our initial results for a simple stepwise method to
prepare molecularly-ordered thin films of semiconducting polymers
via click chemistry, and evaluation of the properties of such films.
For a stepwise polymerization to be practical, the avail-
ability of efficient reactions for the polymerization process is
essential for uniform deposition of the surface-attached polymer
films. We chose the Cu(I)-catalyzed acetylene–azide click
reaction (CuAAC)11 as a method to build a polymer chain
by stepwise addition of the monomers (Scheme 1). Owing to
robustness and good regioselectivity, the click reaction is
widely used to prepare complex polymer architectures and
for the surface attachment of polymers in the ‘‘grafting to’’
method.12 Since the click reaction furnishes a 1,2,3-triazole
aromatic connecting unit, it is a suitable way to build semi-
conducting polymers. Indeed, the first semiconducting polymer
poly(triazole fluorene) prepared via solution CuAAC was
reported in 2005.13 Although the electronic structure of the
triazole unit would preclude click polymers from having an
extended p-electron delocalization, the polymers’ potential in
Surface-immobilized polymer brushes, i.e. thin films of surface-
confined polymers, have been extensively researched, especially for
preparation of stimuli responsive materials and ‘‘smart’’ surfaces,1
as well as in organic electronics.2 Such films possess exceptional
mechanical stability and durability and can be easily modified with
various functional groups. Two main approaches for preparation
can be broadly classified as ‘‘grafting to’’ and ‘‘grafting from’’
methods. The ‘‘grafting to’’ method is accomplished by reacting a
separately prepared end-group functionalized polymer with a
complementary functional group on a solid surface. The advan-
tages of this method stem from using appropriately functionalized
pre-synthesized polymers with a low polydispersity index (PDI),
however, it often results in insufficient coverage leading to low
surface density of the polymer films.3 On the other hand, the
‘‘grafting from’’ method, such as surface-immobilized monolayer
initiated in situ chain-growth polymerization, enables fabrication of
thin films with a higher surface coverage as well as increased
thickness. The ‘‘grafting from’’ method is considered particularly
promising for the preparation of thin films of organic semiconducting
polymers.4 This can eliminate the need for bulky solubilizing
side groups on the polymer which prevent close packing (which is
required for enhanced charge carrier mobility), but also act as
sites where photo- and thermal degradation begin.5 However, the
‘‘grafting from’’ method is limited to living chain-growth poly-
merization or electrochemical polymerization and often causes a
problem of ‘‘needle growth’’6 resulting in a high PDI as well as
relatively non-uniform surface morphology of the thin films.
In our search for an alternative synthetic strategy, we focused
on a stepwise surface-confined polymerization (also called
a Department of Chemistry, Louisiana State University, 232 Choppin
Hall, Baton Rouge, LA 70803, USA. E-mail: een@lsu.edu;
Fax: +1-225-578-3458; Tel: +1-225-578-3236
b Center for Advanced Microstructures and Devices, Louisiana State
University, 6980 Jefferson Highway, Baton Rouge, LA 70806, USA
w Electronic supplementary information (ESI) available: Synthetic
procedures, experimental details, and additional figures. See DOI:
10.1039/c1cc14479k
c
11990 Chem. Commun., 2011, 47, 11990–11992
This journal is The Royal Society of Chemistry 2011

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this method (no wasted monomers or polymers, in contrast to
procedures relying on solution spin-casting) and its suitability for
automation as the routine technical procedures (repetitive
immersing into two solutions with intermediate rinsing) can be
performed by a simple robotic ‘‘arm’’. It is likely that the actual
time required to complete one click reaction step can be even less
than 1 h, and future optimization of the reaction conditions will
enable running this process rapidly and efficiently.
A gradual increase in UV/vis absorbance of the polymer film
was consistent with the stepwise polymerization (Fig. 1a). Unlike
typical conjugated polymers, triazole-incorporating click
polymers did not show extended p-electron delocalization, and
therefore the absorbance increase was not accompanied by an
expected bathochromic shift (except for the few very first click
steps). Interestingly, the wavelength of the absorption maximum
of the thin films during the polymer growth was found to
alternate between the two distinct values of 330 nm (after
reaction with monomer 2) and 360 nm (after reaction with
monomer 3) (Fig. S1 in ESIw). Such behavior was consistent
with the relatively ‘‘localized’’ electronic nature of the polymer
P1 and could be due to alternation of the relative fraction of the
two kinds of monomeric units in the thin film, although more
studies are required to explain this behavior. The lower extent of
the intramolecular p-electron delocalization could be partially
compensated by significant intermolecular electronic interactions
in the closely packed molecularly orderedthin film. To evaluate
the effect of the intermolecular interactions, a soluble analogue of
P1—polymer P3 bearing dodecyl solubilizing groups—was
prepared by click polymerization in solution and compared with
the P1 thin film (Fig. S2, ESIw). Whereas the spin-cast film of
polymer P3 displayed a relatively narrow absorption band (with
absorption onset at B450 nm), the P1 thin film was characterized
by a much broader band spanning to almost 700 nm. This
indicated a significant extent of p-electron delocalization
through intermolecular interactions in the densely packed film of P1.
The polymer P1 films were found to be remarkably stable and
unaffected by prolonged ultrasonication in organic solvents. The
P1 film also demonstrated high electrochemical stability in cyclic
voltammetry (CV) experiments where it showed reversible redox
behavior unaffected by multiple CV scanning cycles (Fig. S3, ESIw).
Scheme 1 Preparation of semiconducting polymer thin films via
surface-confined stepwise click polymerization.
organic electronics was demonstrated by using them as active
materials in dye-sensitized solar cells,14 or for the construction
of conducting molecular wires to bridge nanogaps.15
In an initial study, we prepared surface-immobilized thin films
of poly(bithiophene triazole fluorene) semiconducting polymer
P1 through 34 CuAAC polymerization steps using the bis-azide
monomer 2 and the bis-acetylene monomer 3 (Scheme 1).
A quartz substrate with a covalently attached monolayer of the
TMS-acetylene initiator 1a was first immersed into a solution of
the monomer 2 and CuI in DMSO for 1 h at 40 1C, followed by
rinsing and ultrasonicating in CHCl3 for 10 min. Then it was
placed in a DMSO solution of the monomer 3 and CuI for 1 h at
40 1C, followed by ultrasonication in CHCl3, etc. The polymer
growth was monitored by UV/vis absorption spectroscopy
(Fig. 1a). Since in each polymerization step only the click
reaction between a functional group of the monomer and not
yet reacted complementary functional group at the thin-film
surface can occur, no consumption of the monomers to form
bulk polymers in solution was observed. Therefore, the initially
prepared monomer solutions could last for the entire duration of
polymerization (34 steps) without replacement (we did observe
some decomposition of the monomer solutions when they were
used for a large number of steps, but this did not affect their
performance). This illustrates both the economy of materials for
From the CV data, the energy level of the HOMO (EHOMO
)
was estimated at À5.28 eV. The optical gap energy (Eg) of P1
(2.52 eV) was determined from the onset of the thin film
absorption spectrum; this together with EHOMO gave the
LUMO energy level (ELUMO) at À2.76 eV. These energy levels
were in line with the values found for common semiconducting
polymers. The surface morphology and film thickness were
evaluated using AFM (Fig. 2). The film exhibited uniformly-sized
cylindrical domains of nearly 90 nm in diameter with tight
packing density. The domains were oriented normally to the
substrate protruding through the entire film. Each domain
likely had a uniform and well ordered packing of the polymer
P1 molecules. The film thickness was B28 nm, measured
locally by ‘‘nanoshaving’’ a hole in the film (Fig. S4, ESIw).
Polarization-dependent ultraviolet photoemission spectro-
scopy (UPS) utilizing linearly polarized light from a synchrotron
source was used to provide further insights into the molecular
organization and alignment in the P1 film.16 For UPS studies,
a thin film of P1 was prepared in 15 click steps starting with
Fig. 1 (a) UV/vis monitoring of the growing film of P1 (total 34 CuAAC
steps); (b) comparison of UV/vis spectra of thin films of P1 and P2.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11990–11992 11991

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films prepared by stepwise click polymerization can be manipu-
lated by variations of the polymer molecular composition.
In conclusion, we developed a stepwise polymerization method
to prepare thin films of semiconducting polymers via sequential
Cu-catalyzed acetylene–azide click reactions. This method yields
surface-immobilized, stable, and highly ordered anisotropic films
with upright alignment of the polymer molecules. The spectro-
scopic and electronic properties of such polymers can be easily
tuned by changing molecular composition of the polymers.
Currently, we are working to further optimize the conditions of
click polymerization, as well as on the preparation of complex
polymer thin film architectures.
Fig. 2 Surface morphology viewed with contact mode AFM. (a)
Topograph of the film of P1 prepared on a quartz surface (roughness
(RMS) B6 nm); (b) corresponding lateral force image. Center area has
a trench in the film made by ‘‘nanoshaving’’ to show continuity of the
cylindrical domains.
The authors gratefully acknowledge support from the National
Science Foundation (grants DMR-0843962 and DMR-1006336).
the initiator 1b immobilized on a semiconducting indium tin
oxide (ITO) substrate. In these experiments, the incidence
angle of the synchrotron light was changed while the emission
was collected normally to the surface (Fig. 3a).17 The strong
intensity enhancement of the P1 valence band (maximum at
B7 eV) in the p-polarized light was indicative of the close to
normal preferential orientation of the polymer’s transition
dipole (Fig. 3b). Considering the high anisotropy of the
polymer molecule, this reflected the uniformly upright orientation
of the polymer brushes on the surface. Additionally, the
intensity of the P1’s valence band showed strong dependence
on the emission angle a. A plot of the intensity of the valence
band maximum vs. a showed two maxima at 201 and 401
(Fig. S5, ESIw). Considering the significant contribution of the
valence band orbitals to the averaged photoemission intensities,
this indicated that the tilt angle of the P1’s molecules with
respect to the surface normal was ranging between 201 and
401. Such upright polymer chain orientation along with the
high film anisotropy is especially suitable for photovoltaic
applications as it would facilitate charge carriers migration
towards the top and bottom electrodes.
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c
11992 Chem. Commun., 2011, 47, 11990–11992
This journal is The Royal Society of Chemistry 2011