Macromolecular surface design: Photopatterning of functional stable nitrile oxides

Article abstract of DOI:10.1002/anie.201500485

The efficient trapping of photogenerated thioaldehydes with functional shelf-stable nitrile oxides in a 1,3-dipolar cycloaddition is a novel and versatile photochemical strategy for polymer end-group functionalization and surface modification under mild a

Full text of DOI:10.1002/anie.201500485

Angewandte
Chemie
DOI: 10.1002/anie.201500485
Surface Chemistry
Hot Paper
Macromolecular Surface Design: Photopatterning of Functional Stable
Nitrile Oxides**
Ozcan Altintas, Mathias Glassner, Cesar Rodriguez-Emmenegger, Alexander Welle,
Vanessa Trouillet, and Christopher Barner-Kowollik*
Abstract: The efficient trapping of photogenerated thioalde-
hydes with functional shelf-stable nitrile oxides in a 1,3-dipolar
cycloaddition is a novel and versatile photochemical strategy
for polymer end-group functionalization and surface modifi-
cation under mild and equimolar conditions. The modular
ligation in solution was followed in detail by electrospray
ionization mass spectrometry (ESI-MS). X-ray photoelectron
spectroscopy (XPS) was employed to analyze the functional-
ized surfaces, whereas time-of-flight secondary-ion mass
spectrometry (ToF-SIMS) confirmed the spatial control of
the surface functionalization using a micropatterned shadow
mask. Polymer brushes were grown from the surface in
a spatially confined regime by surface-initiated atom transfer
radical polymerization (SI-ATRP) as confirmed by TOF-
SIMS, XPS as well as ellipsometry.
polymer brushes has recently attracted increasing attention
owing to potential applications in areas such as biosensors,[2]
biocompatible materials,[3] and protein immobilization.[4]
Surfaces can be functionalized in a spatially controlled
fashion by applying photo-induced reactions. In recent
years, light-induced modular ligation reactions[5] that provide
spatial control over the surface grafting and light-controlled/
living radical polymerization techniques have been devel-
oped.[6] Several photo-induced reactions, including Diels–
Alder,[7,8] thiol–ene,[9] thiol–quinone,[10] and thiol–yne[11] liga-
tions, have been applied to attach functional groups and
biomolecules to surfaces and to control patterning. In this
line, the photogeneration of cyclooctynes,[12] nitrile imines,[13]
oximes,[14] and o-naphthoquinone methides,[15] which can be
efficiently trapped by dienophiles, has recently been reported
by us and others. Furthermore, in 2013, our research group
introduced a light-triggered ligation method based on the
Diels–Alder reaction of photogenerated thioaldehydes as an
efficient method for spatially controlled surface design.[16] The
photofragmentation of phenacyl sulfides (i.e., photocaged
thioaldehydes) is a mild and efficient method for the
preparation of highly reactive thioaldehydes, which can be
trapped in situ by dienes, nucleophiles,[17] or in a 1,3-dipolar
cycloaddition with nitronate esters.[18,19]
Nitrile oxides are among the most reactive species in
organic chemistry and are usually trapped by 1,3-dipolar
cycloaddition reactions.[20,21] The cycloaddition between ni-
trile oxides and alkynes represents a metal-free ligation
reaction that has been employed for polymer end-group
modifications.[22] However, nitrile oxides are prepared in situ
from the oxime precursors owing to their rapid dimerization
to furoxans, which strongly limits their applicability in
modular ligation strategies. The spontaneous dimerization
to furoxans can be prevented by substituents on the aromatic
ring that is attached to the nitrile oxide group. However,
T
he controlled immobilization of biomolecules, such as
proteins and peptides, onto the surface of a solid substrate
with a certain size and dimension is of fundamental impor-
tance in biotechnology.[1] Microscale surface patterning with
[*] Dr. O. Altintas, Dr. M. Glassner, Dr. A. Welle,
Prof. Dr. C. Barner-Kowollik
Preparative Macromolecular Chemistry
Institut fꢀr Technische Chemie und Polymerchemie
Karlsruhe Institute of Technology (KIT)
Engesserstrasse 18, 76128 Karlsruhe (Germany)
E-mail: christopher.barner-kowollik@kit.edu
Homepage: http://www.macroarc.de
Dr. C. Rodriguez-Emmenegger
Institute of Macromolecular Chemistry
Academy of Sciences of the Czech Republic v.v.i.
Heyrovsky sq. 2 162 06 Prague (Czech Republic)
Dr. O. Altintas, Dr. A. Welle, Prof. Dr. C. Barner-Kowollik
Institut fꢀr Biologische Grenzflꢁchen (IBG)
Karlsruhe Institute of Technology (KIT)
Hermann-von-Helmholtz-Platz 1
76344 Eggenstein-Leopoldshafen (Germany)
=
employing such stable nitrile oxides in cycloadditions to C C
ꢀ
or C N multiple bonds for polymer modifications requires
elevated temperatures.[23,24] Only one example of a cyclo-
addition of a thermally generated thioaldehyde with a stable
nitrile oxide has been reported to date.[25]
V. Trouillet
Institute for Applied Materials (IAM) and Karlsruhe Nano Micro
Facility (KNMF), KIT
Hermann-von-Helmholtz-Platz 1
76344 Eggenstein-Leopoldshafen (Germany)
To address these significant barriers, we herein report, for
the first time, the catalyst-free 1,3-dipolar cycloaddition of
shelf-stable and functional nitrile oxides with photochemi-
cally generated thioaldehydes for end-group modifications of
linear polymers as well as between small molecules in solution
at ambient temperature (Scheme 1). The modular ligation
reaction was subsequently adopted for the spatially resolved
(dual) surface encoding of functional nitrile oxides. Further-
more, poly(oligoethylene glycol methyl ether methacrylate)
[**] C.B.-K. acknowledges funding from the Karlsruhe Institute of
Technology (KIT) in the context of the BioInterfaces program of the
Helmholtz association. C.R-E. thanks the Alexander von Humboldt
Foundation for financial support through a Humboldt Research
Fellowship for Postdoctoral Researchers and the Grant Agency of
the Czech Republic (GACR; P20512G118).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500485.
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[poly(MeOEGMA)] brushes
were grafted from silicon sur-
faces by surface-initiated
atom-transfer radical poly-
merization
(SI-ATRP)[26]
using an ATRP initiator
immobilized on the surface
by the 1,3-dipolar cycloaddi-
tion as depicted in Scheme 1
(bottom).
The key idea of the current
study is to confirm the trap-
ping of photogenerated thio-
aldehydes with stable nitrile
oxides and subsequently uti-
lize the reaction for light-trig-
gered surface encoding at
ambient temperature. To
establish the suitability of the
concept, we performed reac-
tions on a model system in
solution to assess the reaction
kinetics in detail as well as to
analyze the reaction products
by electrospray ionization
mass spectrometry (ESI-MS)
and 1H and 13C NMR spec-
troscopy. For these purposes,
polymer 1 was prepared by
coupling of commercially
available
monohydroxy-
capped poly(ethylene glycol)
methyl ether (mPEG-OH,
2000 gmolÀ1) with (phena-
cylthio)acetic acid as previ-
ously described.[16] 2,4,6-Tri-
methylbenzaldehyde oxime
(2) and 4-bromo-2,6-dimethyl-
benzaldehyde oxime (3) were
prepared according to a previ-
ously reported procedure.[27]
The oxime moieties were
converted into the corre-
sponding stable nitrile oxides
in the presence of triethyl-
amine and N-bromosuccini-
mide (NBS; for details of the
synthesis and characteriza-
tion, see the Supporting Infor-
mation). A 36 W compact flu-
orescent lamp (lmax = 355 nm;
for the emission spectrum, see
the Supporting Information,
Figure S36) was employed as
the UV source for the photo-
fragmentation of phenacyl sul-
fide end-capped polymer
1 (see Scheme 1). The latter
was irradiated for two hours in
Scheme 1. Top: The photolytic cleavage of phenacyl sulfides yields acetophenone and thioaldehydes, which
can be trapped with shelf-stable nitrile oxides. Middle: Surface patterning of silicon wafers: Various
functional stable nitrile oxides are patterned employing a 1,3-dipolar cycloaddition reaction by irradiation
through a shadow mask, also in a dual fashion. Bottom: The ATRP initiator is subsequently utilized for
growing poly(MeOEGMA) on the pattern areas.
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the presence of five equivalents of the respective stable nitrile
oxide (4 or 5) in degassed dichloromethane (DCM). ESI-MS
analysis (Figure S7) revealed a full shift towards higher mass-
to-charge ratios. Congruent with the complete disappearance
of the quasimolecular ion [1+Na]+ and the appearance of the
cycloaddition products [6+Na]+ or [7+Na]+, these results
demonstrate the successful 1,3-dipolar cycloaddition of the
stable nitrile oxide to the photogenerated thioaldehyde at
ambient temperature. Integration of the mass spectral
abundances after various irradiation times shows that com-
plete conversion was achieved after 30 minutes (Figure 2,
bottom). Further inspection of Figure S7 (bottom) confirms
the characteristic isotopic pattern of bromine-containing
compounds. The experimental and theoretical m/z values of
all species shown in Figure S7 are presented in the Supporting
Information (see Table S1) and are in excellent agreement.
The formation of the 1,3-dipolar cycloaddition adduct was
1
also confirmed by H NMR spectroscopy (Figure S8), which
indicated the disappearance of the proton resonances of the
phenacyl group at d = 7.91, 7.52, and 7.41 ppm and the
appearance of the aromatic proton resonance of the mesityl
group at d = 6.82 ppm as well as of the signal of the 1,4,2-
oxathiazole ring proton at d = 6.43 ppm. Moreover, an addi-
tional model reaction was performed with stable nitrile oxide
4 and compound 8. The formation of the 1,4,2-oxathiazole was
confirmed by 1H NMR (Figure 1) and 13C NMR spectroscopy
Figure 2. Top: ESI-MS spectra of phenacyl sulfide capped PEG (1)
before and after irradiation for various periods of time in the presence
of stable nitrile oxide 4. Bottom: Plot of the conversion versus the
irradiation time derived by integration of the mass spectral abundan-
ces of compounds 1 and 6.
predefined irradiation times demonstrated that complete
conversion was achieved after irradiation for 30 minutes
(Figure 2). To investigate the efficiency of the trapping
reaction, solutions of 1 and various equivalents of stable
nitrile oxide 4 in DCM were degassed for five minutes and
subsequently irradiated for 120 minutes at ambient temper-
ature. The progress of the reactions was monitored by ESI-
MS (Figure S9). One equivalent of nitrile oxide was sufficient
to achieve close to quantitative conversion, placing the
reaction among the most efficient ligation processes.[28]
The model reactions in solution demonstrate that photo-
generated thioaldehydes can be efficiently trapped with
equimolar amounts of shelf-stable nitrile oxides at ambient
temperature. The light-based nature of the conjugation
technique prompted us to translate it to the spatially con-
strained grafting of molecules onto surfaces to obtain
molecular patterns. Surface characterization was performed
by X-ray photoelectron spectroscopy (XPS) and time-of-
flight secondary-ion mass spectrometry (ToF-SIMS). Phe-
nacyl sulfide functionalized silicon wafers were prepared by
employing a functional silane (see the Supporting Informa-
tion and Scheme S1). The surfaces were rinsed extensively
Figure 1. 1H NMR spectrum of ethyl 3-mesityl-1,4,2-oxathiazole-5-car-
boxylate in CDCl3.
(Figure S13), ESI-MS (see the Supporting Information), and
FT-IR spectroscopy (Figure S14). Inspection of Figure 1
clearly indicates the generation of the 1,4,2-oxathiazole ring
in excellent agreement with the 1H NMR spectrum of
polymer 1. Further inspection of the corresponding
13C NMR spectrum (Figure S13) clearly confirms the disap-
pearance of the resonance of the phenacyl carbonyl carbon
atom at d = 194.07 ppm and the appearance of the carbon
resonance of the 1,4,2-oxathiazole ring at d = 86.41 ppm.
Further evidence for the 1,3-dipolar cycloaddition was
obtained by FT-IR spectroscopy (Figure S14): The band at
2293 cmÀ1, which arises from the nitrile oxide, completely
disappeared after photoligation.
The kinetic investigation of the photofragmentation of
1 by integration of the mass spectral abundances after
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with fresh solvents (see the Supporting Information) and
sonicated to remove any physisorbed material from the
surface. To graft stable nitrile oxide 5 to the surface, a silicon
wafer was placed in a quartz flask containing a solution of the
bromine-functionalized stable nitrile oxide 5 in DCM and
irradiated with a suitable light source (36 W, lmax = 355 nm)
for one hour while the reaction flask revolved around the
lamp. XPS was employed to confirm the functionalization of
the silicon wafers. The Br 3d spectrum of the surface after
irradiation in the presence of the bromine-functionalized
nitrile oxide displayed a pronounced bromine signal at
a binding energy of 70.8 eV[29] indicating the successful
surface attachment of the bromine-functionalized nitrile
oxide (Figure 3a). XPS spectra after irradiation without
a stable nitrile oxide as a control experiment are shown in
Figure 3b and c. Importantly, no bromine was detected by
XPS when the phenacyl sulfide functionalized surface was
irradiated in pure DCM (Figure 3b) or in the presence of
4-bromotoluene (Figure 3c), indicating that the bromine
signal observed in the XPS spectrum is exclusively associated
with the covalent attachment of the nitrile oxide.
Whereas XPS can provide information on the binding
situation in the irradiated areas of the substrate, ToF-SIMS
imaging enables the spatially resolved analysis of molecular
patterns on solid substrates. In contrast to traditional
fluorescence imaging, ToF-SIMS data provide detailed infor-
mation on the chemical composition, which is essential for the
analysis of non-fluorescent molecules. For instance, bromi-
nated compounds with their inherent isotopic pattern can be
unambiguously detected by ToF-SIMS. Therefore, bromine-
containing stable nitrile oxide 5 was utilized in the current
study as a molecular marker to spatially map the locally
constrained surface grafting. Photopatterning was achieved
by irradiation of silicon wafers that had been functionalized
with photocaged thioaldehydes and immersed in a solution of
5 utilizing a shadow mask (see Figure S16 and Scheme S1).
The ToF-SIMS image in Figure 3 (bottom) clearly reveals that
BrÀ secondary ions can only be found in the irradiated area.
Therefore, the ToF-SIMS experiments confirmed that the
spatially controlled surface patterning of stable nitrile oxides
is possible.
Figure 3. Top: Comparison of the Br 3d XPS spectra of a phenacyl
sulfide functionalized silicon wafer after irradiation in a) a solution
containing the bromine-functionalized stable nitrile oxide 5, b) pure
DCM, or c) a solution of 4-bromotoluene. Bottom: TOF-SIMS image of
a silicon wafer patterned with 5 detected through its Br isotopes.
acterization (Figure 4). Characteristic mass fragments of the
hydroxy-functionalized stable nitrile oxide 10 (CH2OH+,
Figure 4a), the carboxy-functionalized stable nitrile oxide
12 (COOHÀ, Figure 4b), and functional stable nitrile oxide 13
with a protected amine (CONÀ, Figure 4c) can only be found
in the irradiated area, thus confirming that the incorporation
of the functional groups in the meander pattern accurately
corresponds to the areas irradiated through the shadow mask.
Furthermore, the characteristic mass fragments of C8H7NOÀ
and CSNÀ are part of the newly formed 1,4,2-oxathiazole ring,
which can only be found in the irradiated region (Figure S35),
thus additionally confirming the formation of the heterocyclic
ring. In a further step, we also confirmed the possibility of
immobilizing species that contain an ATRP initiator. Polymer
brushes with controlled thickness and well-defined composi-
tion and architecture can be grafted under mild conditions
from surfaces containing immobilized initiators for surface-
initiated atom transfer radical polymerization (SI-ATRP, see
below). For this purpose, a new stable nitrile oxide function-
alized with an ATRP initiator, namely 11, was prepared and
immobilized onto the silicon surface by the phototriggered
In the subsequent step, it was mandatory to determine the
tolerance of the photo-induced 1,3-dipolar cycloaddition with
respect to various functional groups attached to the nitrile
oxides to assess its scope for the covalent immobilization of
various functionalities on solid substrates in a spatially
resolved fashion. Thus, the hydroxy-functionalized stable
nitrile oxide 10, carboxy-functionalized stable nitrile oxide 12,
and a functional stable nitrile oxide 13 with a protected amine
were also prepared (see Scheme 1 for their structures) and
1
characterized by H NMR, 13C NMR, and IR spectroscopy
and ESI-MS. Details of the preparation of these nitrile oxides
can be found in the Supporting Information. Photopatterning
of each functional species was achieved by irradiation of three
different silicon wafers that had been functionalized with
photocaged thioaldehydes and immersed in a solution con-
taining functional stable nitrile oxide 10, 12, or 13 utilizing
a shadow mask. The success of the 1,3-dipolar cycloaddition
reactions was unambiguously confirmed by ToF-SIMS char-
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Figure 4. TOF-SIMS images of a silicon wafer patterned with a) hydroxy-functionalized stable nitrile oxide 10 (detected through the CH2OH+
fragment), b) carboxy-functionalized stable nitrile oxide 12 (detected through the COOHÀ fragment), c) functional stable nitrile oxide 13 with
a protected amine (detected through the CONÀ fragment), and d) a stable nitrile oxide functionalized with an ATRP initiator (11, detected through
the Br isotopes).
1,3-dipolar cycloaddition as depicted in Schemes 1 (bottom)
and S2. Details of the preparation of this stable nitrile oxide
can be found in the Supporting Information. The success of
the 1,3-dipolar cycloaddition reaction was unambiguously
confirmed by ToF-SIMS characterization (Figure 4d). Mass
fragments characteristic for bromine-containing species were
solely detected in the irradiated areas.
in the irradiated areas on the silicon surface after the first
irradiation, thus confirming the highly efficient nature of the
1,3-dipolar cycloaddition on the surface. Importantly, the
observation of the fragments discussed above corroborates
that after the first irradiation, the photocaged thioaldehyde
groups are still intact in the non-irradiated areas as they can
The power of the light-induced 1,3-dipolar cycloaddition
reaction for the spatially resolved encoding of functional
nitrile oxides was further investigated in a dual patterning
experiment. Initially, the stable nitrile oxide 13 with a pro-
tected amine was patterned onto the photocaged silicon
surface. After irradiation, the surface was subjected to
a typical washing procedure (see the Supporting Informa-
tion), and the same silicon surface was turned by 908 and
repositioned into the sample holder with the photomask.
Subsequently, the silicon surface was irradiated in the
presence of bromine-functionalized stable nitrile oxide 11.
In Figure 5, ToF-SIMS images of the overlay of the CNOÀ and
BrÀ fragments are depicted. The image unambiguously
confirms the ability of the photofunctional system to pattern
two functional groups onto the same silicon surface. Nitrile
oxide 13 was continuously patterned on the surface during the
first irradiation, whereas 11 was discretely patterned on the
silicon surface in the second irradiation step. The results
indicate that there are no more photoactive phenacyl sulfides
Figure 5. TOF-SIMS images of a silicon wafer patterned with both 13
and 11 (detected through the CONÀ and BrÀ fragments) on the same
surface.
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be activated in a subsequent step to enable additional
functionalization.
To exemplify the power of the novel ligation technique,
we polymerized poly(MeOEGMA) brushes by SI-ATRP
from initiator-functionalized (11) surfaces (experimental
details can be found in the Supporting Information). A
photopatterned surface was generated for ToF-SIMS imaging,
whereas surface functionalization without a shadow mask was
used for XPS and ellipsometry analysis. After ten hours of
polymerization, 15 nm thick brushes had been generated as
determined by ellipsometry on dry samples. XPS analysis of
the brushes grown from the entire surface (Figure 6, top)
confirmed the successful polymerization. Compared with the
surface carrying the ATRP initiator (Figure 6b), the C 1s
spectrum of the polymer brushes shows a strong increase in
[30]
À
the peak assigned to the C O bonds at 286.4 eV, which is
due to the oligo(ethylene glycol) (OEG) side chains of the
À
brush, and a decrease in the signal corresponding to the C C
bonds at 285.0 eV (Figure 6a), which results from the poly-
(MeOEGMA) growth. A clear signal at 289.0 eV, which was
[30]
À =
attributed to the O C O
moieties present in poly-
(MeOEGMA), could also be observed. As a control experi-
ment, a silicon wafer only carrying the phenacyl sulfides was
subjected to the same polymerization conditions. When no
ATRP initiator has been immobilized on the surface, no
polymer growth from the surface can be detected by XPS
(Figure 6c). Importantly, the grafting kinetics were followed
by measuring the dry thickness of the polymer after various
time intervals by ellipsometry.[31] As confirmed by the linear
increase in thickness with time, the SI-ATRP of poly-
(MeOEGMA) brushes proceeded in a well-controlled fash-
ion (Figure S37).
Figure 6. Top: XPS characterization and comparison of the C 1s
spectra of a) the surface after SI-ATRP of MeOEGMA, and b) the ATRP
initiator surface; c) control experiment. Bottom: TOF-SIMS overlay
image constructed from the images of several intense PEG fragments
(CH3OÀ, CHO2À, and C2H2O2À).
The brush-patterned surfaces were characterized by ToF-
SIMS (Figure 6, bottom). Characteristic mass fragments of
oligo(ethylene glycol) (CH3OÀ, CHO2À, and C2H2O2 ) can
À
only be found in the irradiated area, thus confirming the
growth of poly(MeOEGMA) in the meander pattern corre-
sponding accurately to the shadow mask.
Received: January 18, 2015
Published online: && &&, &&&&
In summary, we have demonstrated that photogenerated
thioaldehydes can be efficiently trapped with readily acces-
sible shelf-stable nitrile oxides at ambient temperature. The
introduced method allows the functionalization of polymers
by 1,3-dipolar cycloadditions of nitrile oxides under mild
conditions, that is, at ambient temperature in the absence of
any additives and, importantly, in equimolar ratios, for the
first time. We confirmed that the spatially controlled surface
patterning of various functional nitrile oxides, such as variants
with hydroxy, bromine, carboxy, or protected-amine substitu-
ents, can be realized—also in a dual co-grafting fashion—
employing surface-attached phenacyl sulfides. The potential
of the strategy was further demonstrated by the immobiliza-
tion of an ATRP initiator and subsequent growth of poly-
(MeOEGMA) brushes exclusively in the previously irradi-
ated area. The living behavior of the SI-ATRP was confirmed
by ellipsometry. We envisage that considering its efficiency,
equimolarity, and mild conditions, this strategy will find
applications in various fields, for example, for the immobili-
zation of biomolecules or the photolithography of conducting
polymers.
Keywords: modular ligation · nitrogen heterocycles ·
polymerization · stable nitrile oxides · surface analysis
.
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Angew. Chem. Int. Ed. 2015, 54, 1 – 8
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7
These are not the final page numbers!

.
Angewandte
Communications
Communications
Surface Chemistry
O. Altintas, M. Glassner,
C. Rodriguez-Emmenegger, A. Welle,
V. Trouillet,
C. Barner-Kowollik*
&&&& — &&&&
Perfect Patterning: The cycloaddition
between a photogenerated thioaldehyde
and stable nitrile oxides functions as
a versatile and easy-to-use photoligation
reaction in solution and on surfaces as
confirmed by ESI-MS, XPS, and TOF-
SIMS analysis. Polymer brushes were
grown from the surface in a spatially
confined regime by surface-initiated
atom-transfer radical polymerization.
Macromolecular Surface Design:
Photopatterning of Functional Stable
Nitrile Oxides
8
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 8
These are not the final page numbers!