Versatile Micropatterns of N-Heterocyclic Carbenes on Gold Surfaces: Increased Thermal and Pattern Stability with Enhanced Conductivity

Article abstract of DOI:10.1002/anie.201807197

Patterned monolayers of N-heterocyclic carbenes (NHCs) on gold surfaces were obtained by microcontact printing of NHC–CO₂ adducts and NHC(H)[HCO₃] salts. The NHC-modified areas showed an increased conductivity compared to unmodified gold surface areas. Furthermore, the remaining surface areas could be modified with a second, azide-functionalized carbene, facilitating further applications and post-printing modifications. Thorough elucidation by a variety of analytical methods offers comprehensive evidence for the viability of the methodology reported here. The protocol enables facile access to versatile, microstructured NHC-modified gold surfaces with highly stable patterns, enhanced conductivity, and the option for further modification.

Full text of DOI:10.1002/anie.201807197

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Accepted Article
Title: Versatile Micropatterns of N-Heterocyclic Carbenes on Gold
Surfaces: Increased Thermal and Pattern Stability with Enhanced
Conductivity
Authors: D. Thao Nguyen, Matthias Freitag, Martin Körsgen, Sebastian
Lamping, Andreas Rühling, Andreas Schäfer, Martin
Siekman, Heinrich Arlinghaus, Wilfred Van der Wiel, Frank
Glorius, and Bart Jan Ravoo
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201807197
Angew. Chem. 10.1002/ange.201807197
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10.1002/anie.201807197
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Versatile Micropatterns of N-Heterocyclic Carbenes on Gold
Surfaces: Increased Thermal and Pattern Stability with Enhanced
Conductivity
D. Thao Nguyen,‡ Matthias Freitag,‡ Martin Körsgen, Sebastian Lamping, Andreas Rühling, Andreas H.
Schäfer, Martin H. Siekman, Heinrich F. Arlinghaus, Wilfred G. van der Wiel, Frank Glorius,* and
Bart Jan Ravoo*
Abstract: Patterned monolayers of N-heterocyclic carbenes on gold
al. demonstrated self-assembled monolayers (SAMs) of NHCs to
be exceptionally stable under chemical, oxidative and physical
stress.[7] Recently, Glorius, Fuchs and coworkers contributed to a
deeper understanding of NHC SAMs and NHC mobility on gold.[8]
In spite of these insightful findings, to date, metal surfaces
were only modified by immersion techniques or physical-vapor-
deposition of carbene-CO2 adducts in ultrahigh vacuum, where
the entire gold surface was covered by the NHCs.[2,5-10b]
Considering the versatile nature of NHCs, it would be of broad
interest to functionalize gold surfaces with multiple NHCs located
in well-defined patterns.[1,2,10] Microcontact printing (µCP) is a soft
lithography technique to decorate substrates with patterned
monolayers.[11,13a,b,d] The transfer of molecules is commonly
ensured by polymeric stamps with a relief pattern, i.e. highly
ordered micro- or nanostructures.
surfaces were realized by microcontact printing of NHC-CO2 adducts
-
and NHC+-HCO3 salts. The NHC modified areas showed an
increased conductivity compared to unmodified gold surface areas.
Furthermore, the non-covered surface could be modified with a
second, azide-functionalized carbene, facilitating further applications
and post-printing modification. Thorough elucidation by a variety of
analytical methods offers comprehensive evidence for the viability of
the methodology reported here. The protocol enables facile access to
versatile, microstructured NHC modified gold surfaces with highly
stable patterns, enhanced conductivity and the option for further
modification.
N-Heterocyclic carbenes (NHC) have unique σ-electron-donating
and π-accepting properties and are commonly used as electron-
rich ligands in transition-metal catalysis.[1] Within the last years,
NHCs have proven to be highly valuable ligands on nanoparticles
and surfaces.[1-10] In 2011, Weidner et al. discovered that NHCs
could serve as ligands on gold surfaces.[5] However, gold surfaces
with NHCs as ligands are still rarely explored, though a promising
future can be envisioned, in which NHCs play a profound role for
novel sensors, electronics and other nanomaterials, many of
which utilize gold as a crucial component.[2] Typically, thiols are
employed as ligands for gold surfaces.[2c,11,12] However, the
widespread application of thiols is limited due to a lack of long
term stability even under ambient conditions.[2c,12] Therefore, thiol-
functionalized surfaces frequently do not match the properties
required for modern devices.[2c,12] The Johnson group
demonstrated that alkene substituted NHCs deposited on a gold
surface could be further chemically modified with a transition
metal catalyst for a ring-opening metathesis polymerization and
can be a potential substitution for thiols.[6] Moreover, Crudden et
Herein we report the first successful patterning of NHCs on
gold surfaces by µCP with the additional ability to fill the
unmodified areas with a second NHC that can be further
functionalized (Figure 1).
Figure 1. Procedure for well-defined pattern of NHCs by microcontact printing
(µCP).
[*]
D. T. Nguyen, ‡ M. Freitag, ‡ S. Lamping, Dr. A. Rühling,
Prof. F. Glorius, Prof. B. J. Ravoo
Organisch-Chemisches Institut, Westfälische Wilhelms-Universität
Münster, Corrensstraße 40, 48149 Münster, Germany
E-mail: b.j.ravoo@uni-muenster.de; glorius@uni-muenster.de
M. Körsgen, Prof. H. F. Arlinghaus
Physikalisches Institut, Westfälische Wilhelms-Universität Münster,
Wilhelm-Klemm-Straße 10, 48149 Münster, Germany
Andreas H. Schäfer
nanoAnalytics GmbH, Heisenbergstraße 11, 48149 Münster,
Germany
Martin H. Siekman, Prof. Wilfred G. van der Wiel
NanoElectronics Group, MESA+ Institute for Nanotechnology,
University of Twente, P.O. Box 217, 7500 AE Enschede, The
Netherlands
Crudden et al. have reported that BIiPr, an analogue of BIMe,
forms ultra-stable, highly ordered SAMs.[7a] Bench stable NHC-
CO2 adducts of benzo[d]imidazol-3-ium (BIMe) or the hydrogen-
carbonate salt precursor of BIiPr were applied as a clean source
of free carbenes instead of imidazolium salt-based NHC
precursors.[14] Thus, an addition of activating agents (i.e. bases)
concomitant with inherent impurities could be avoided.[7b,8] In
addition, these NHC precursors were found to be compatible with
both polar-protic solvents and elastomeric stamps used for µCP,
in contrast to free NHCs. An ozonized PDMS stamp with the
desired relief pattern is wetted with an ink solution of BIMe-CO2 in
ethanol. The solution is allowed to soak into the elastomeric
[‡]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
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Figure 2. Thermal stress test conducted with BIMe@Au samples (100 µm lines
spaced by 50 µm, applied by µCP). a, b) ToF-SIMS analysis (Sum of
[M+H]+,[M+Au]+ and [2M+Au]+, field of view = 500 x 500 µm2); a) Stored under
argon; b) Functionalized surface was subjected to 120 °C for 16 h; c) High-
resolution XPS of the N 1s region: before stress test (black), after stress test
(red).
stamp. Subsequently, the stamp is brought into contact with a
gold surface. Heating at 55 °C in vacuo for 30 min generates the
free BIMe under liberation of CO2.[14] The free NHC is attached to
the surface via a covalent bond exclusively at the interface
between the bare gold and stamp surface. As a proof for the
generality of our printing methodology, µCP was also performed
-
with BIiPr-HCO3 under optimized printing conditions (55 °C in
High-resolution XPS of the N 1s region revealed a retention of
BIMe@Au of 94% (Figure 2c, see the Supporting Information for
further details).
vacuo for 30 min). The viability of the herein developed
methodology was elucidated by X-ray photoelectron
spectroscopy (XPS), time-of-flight secondary ion mass
spectrometry (ToF-SIMS), atomic force microscopy (AFM) and
scanning electron microscopy (SEM).
The NHC-gold bond is known to be conductive and it was
shown that NHCs can reduce the work function of gold by up to
2 eV due to strong electron donation to the metal surface and
therefore an increased surface dipole.[6,10b] We conducted
conductive probe AFM (CP-AFM) measurements in PeakForce
tapping mode (Figure 3a and 3b) and observed that areas
modified with BIMe and BIiPr@Au (5 µm stripes, current-to-
distance diagram in Figure 3b and 3d) showed an increased
current when compared to undecorated areas (3 µm stripes),
coincident with an increased conductivity of the NHC decorated
surface areas. Although the observed phenomenon could be due
to formation of NHC2Au0 species and subsequent oxidation to
NHC2Au+, induced by the applied voltage from the CP-AFM tip,
we note that such charged species were so far only
experimentally observed on nanoparticles.[2b,3a-b] More likely, the
contact resistance of the modified gold surface is reduced as
recently reported for gold electrodes of OFET transistors. [7c] This
would then lead to a reduced contact resistance to the AFM tip
and therefore locally resolved higher current at the point where
the electric circuit is closed by the AFM tip.
The transfer of the NHC to gold was traced by XPS. The
patterning was only successful at temperatures of at least 55 °C,
whereas at temperatures of 50 °C or lower still two bands for the
CO2-adduct could be detected in the C 1s region of the XPS high
resolution spectra (Figure S2). In addition, the emission energy
found for the carbene C-N* linkage (400.8 eV) in the N 1s region
of the XPS high resolution spectra (Figure S3) is in excellent
agreement with binding energies for NHCs on gold surfaces
reported in the literature.[4e,7,8] Additionally, no traces of C*-O were
-
found in the XPS spectra of printed BIiPr-HCO3 (Figure S4).
Most modern electronic devices operate at elevated
temperatures, e.g. processors, solar cells. Therefore, organic
molecules deposited on surfaces must ensure a high thermal
stability.[10b,15] Glorius, Fuchs and coworkers reported recently
that isolated NHCs could perform ballbot-type motions in the
nanometer regime.[8] Hence, a temperature stress test was
applied to assess the overall endurance of the patterns and the
persistence of the NHC-gold bond. For this reason, a sample of
printed BIMe@Au was subjected to 120 °C for 16 h. To our delight,
the stripes of NHC remained almost unaltered as analyzed by
ToF-SIMS, since this method offers high resolution and sensitivity
towards all molecular fragments on a surface (Figure 2a and b).[16]
This confirms the observation by Glorius, Fuchs and coworkers
that NHCs grouped on a surface remain stationary.[8]
Figure 3. Conducting probe AFM (CP-AFM) measurement of BIMe@Au and
BIiPr@Au (stripes, applied by µCP). a, c) CP-AFM image of the peak current
with BIMe and BIiPr on the broader 5 µm stripes, respectively. The white line
represents a cross section of the current depending on the location; b) The
current profile is shown based on the cross section in a, c).
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To examine the surface quality after µCP, SEM
measurements were performed. A highly surface-sensitive upper
secondary electron detector was used. BIMe-CO2 was applied by
µCP (100 µm broad lines spaced by 50 µm) and interspaces were
filled with 1-octadecanethiol from solution. Bright areas represent
the NHC-modified surface. The increased brightness can be
attributed to a higher secondary electron yield due to a more
electron-rich gold surface compared to the thiol-modified area
(Figure S32).
at 215.9 ppm in the 13C spectra can be attributed to the carbene
carbon, while the peak for the C2 proton of the imidazole ring at
10.94 ppm is vanished. Moreover, the 1H signals for the C5 proton
of the imidazole ring and the OCH2 protons are shifted upfield in
case of the carbene, while the N3CH2 peak is not shifted. After
quenching of the solution, no mass peaks could be attributed to
coupling products of 1, but protonated 1 can be clearly detected
(see the Supporting Information for further details and discussion).
Both techniques gave no evidence for triazene formation in
solution. We further analyzed a gold surface modified with a drop
of a 10 mM solution of 1 by XPS. The peaks at 399.8 eV and
404.7 eV in the N 1s region can be attributed to the azide group
and a peak at 400.9 eV indicates the formation of a covalent
carbene-gold bond (Figure S9).
With this knowledge in hand, the gold substrates patterned
with BIMe@Au were immersed into a solution of in situ generated
free carbenes 1 or 2. The resulting XPS spectra showed bands
for the azide head group in the N 1s region for both NHC 1 and
NHC of 2 (Figure S14, S17 and S20). Due to the low intensity of
the azide band, we additionally conducted Attenuated Total
Reflection Infrared Spectroscopy (ATR-IR) of the surface
modified with NHC 1 for further support of the XPS data and a
band corresponding to the azide group was found at 2086 cm-1
(Figure S33).
Finally, mannose (MAN) or biotin (BIO) with a terminal
alkyne moiety were taken as substrates for CuAAC, as these
molecules are well-known and established for protein
recognition.[13b,c] The results of the CuAAC are 1-MAN@Au, 2-
MAN@Au and 1-BIO@Au. A XPS spectrum was then recorded
after each of the various chemical modification steps (Figures S13
to S21). Furthermore, ToF-SIMS was applied to visualize the
patterning of the NHC SAMs. As an example, Figures 5a and b
depicts the high-resolution XPS spectra of BIMe@Au and 1-
MAN@Au. The finalizing azide-alkyne cycloaddition resulting in
1-MAN@Au resulted in peaks characteristic for triazole in the N
1s spectra and a vanished azide band (Figure 5a).[13e]
IMes-CO2 was also used for µCP to compare it directly to its
HBr salt counterpart. The transfer of IMes-CO2 was confirmed by
XPS and ToF-SIMS (Figures S5 and S23).[7,8] µCP was then
repeated with IMes • HBr to show that the methodology would not
be applicable to the salt analogue. IMes • HBr could not be
detected by XPS after the obligatory washing step (Figure S8). It
can be concluded that a covalent bond between NHC and Au was
-
only obtained by using the CO2-adducts or the HCO3 -salts.
To expand the versatility of the patterning, non-utilized
surface areas were decorated orthogonally with alternative
surface modifiers, such as a second NHC from solution. Newly
designed NHC precursors 1 (azide in the backbone) or 2 (azide
on the N-substituted side chains) were applied as salts from
solution by in situ deprotonation (Figure 4). The functionalization
with azide functionalized NHCs offered the opportunity for copper
catalyzed azide-alkyne cycloaddition (CuAAC).[13]
Figure 4. a) Synthesis of IMes-derivative 1. a 1) NEt3 (1.1 equiv), CH2Cl2
(0.1 M), 0.5 h, RT; 2) removal of CH2Cl2, EtOAc (0.04 M), Ar; 3)
BrCH2(CH2)4CH2Br (3 equiv), 60 °C, 16 h; b NaN3 (1.2 equiv), DMF (0.04 M),
60 °C, 4 h; c KOtBu (1.2 equiv), THF (0.01 M), RT. b) IMes derivatives 1 and 2
are taken to react with either MAN or BIO in a CuAAC.
The formation of triazene adducts is a serious concern
when discussing azide moieties in conjunction with NHCs.[2a,17]
Accordingly, NHCs modified with azides could suffer from
undesired intermolecular coupling or step-growth polymerization
reaction. To obtain more insight, whether coupling of 1 in solution
occurs, KOtBu (1.2 equiv) in THF-d8 was added to a 10 mM
solution of 1(N3) • HBr in THF-d8. After a reaction time of one
hour, the solution was analyzed by NMR and HRMS. A new peak
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Figure 5. Samples with BIMe@Au and 1-MAN@Au. Final XPS spectra
a) N 1s region with triazole (red, green) and C-N* (blue) peak. b) C 1s region
with C*-C (red), C*-N (blue) and C*-O (green) peak. c) ToF-SIMS analysis of
BIMe@Au (Sum of [M+H]+, [M+Au]+ and [2M+Au]+, field of view = 500 x 500
for IR measurements. We are grateful to Andreas Lerchen for
helpful discussions.
+
µm2); d) ToF-SIMS analysis of 1-MAN@Au (Sum of C4H9+, CHO+, C2H5 and
CH2O+, field of view = 500 x 500 µm2) BIMe-CO2 printed as dots (100 µm spaced
by 50 µm) and 1 was used to fill up all unmodified areas. CuAAC facilitated the
addition of MAN.
Conflict of interest
The authors declare no conflict of interest.
Keywords: Microcontact printing • N-heterocyclic carbenes •
In the C 1s region, a distinct shoulder to a higher binding energy
could be observed, which can be attributed to numerous C-O
bonds in mannose. The ToF-SIMS mapping showed
characteristic molecular fragment signals for BIMe@Au as dots
with the right dimensions and even the non-fragmented molecule
could be observed as gold-adduct (Figure 5c). To our delight, the
sharp edges of the patterns remained unchanged over the whole
process and characteristic mannose fragments could only be
detected in the interspaces, which indicates a predominant
modification of NHC 1 with a homogeneous coverage (Figure 5d).
More analytical data regarding post-printing chemical modification
can be found in the Supporting Information.
self-assembled monolayers • gold surface
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As a proof of functionality, protein recognition tests of mannose
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In summary we introduced a new and straightforward
approach to a well-ordered and micropatterned functionalization
of gold surfaces with NHCs, utilizing the robustness of µCP and
subsequent orthogonal modifications with a second NHC from
solution while retaining an excellent spatial separation of the two
NHCs. Once the NHCs were applied to gold, it was found that the
patterning could withstand exposure to high temperature for at
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Generous funding from the Deutsche Forschungsgemeinschaft
(SFB 858 and Leibniz award) is gratefully acknowledged. We
thank Johannes B. Ernst and Ute Allebrod for experimental
support. We are grateful to the MEET for offering the facility to
sputter the Au surfaces. Martin Bühner (nanoAnalytics GmbH) is
acknowledged for SEM analysis. We thank Deb Kumar Bhowmick
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10.1002/anie.201807197
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
D. Thao Nguyen,‡
Matthias Freitag,‡
Martin Körsgen,
Sebastian Lamping,
Andreas Rühling,
Andreas H. Schäfer,
Martin H. Siekman,
Heinrich F.
Arlinghaus, Wilfred G.
van der Wiel, Frank
Glorius,* and Bart Jan
Ravoo*Page No. –
Page No.
NHC Fine Print: Patterned monolayers of N-heterocyclic carbenes (NHCs) on gold
surfaces were obtained by microcontact printing of NHC-CO2 adducts and NHC+-HCO3
-
salts. The protocol enables facile access to versatile NHC modified gold surfaces with
highly stable patterns, enhanced conductivity and the option for further modification.
Versatile
Micropatterns of N-
Heterocyclic
Carbenes on Gold
Surfaces: Increased
Thermal and Pattern
Stability with
Enhanced
Conductivity
This article is protected by copyright. All rights reserved.