On-surface reductive coupling of aldehydes on Au(111)

Article abstract of DOI:10.1039/c4cc09634g

On-surface synthesis of a polyphenylene vinylene oligomer via reductive coupling of a terephthalaldehyde derivative on Au(111) is reported. Scanning tunneling microscopy and photoelectron spectroscopy experiments confirmed oxygen dissociation and its deso

Full text of DOI:10.1039/c4cc09634g

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On-Surface Reductive Coupling of Aldehydes on Au(111) DOI: 10.1039/C4CC09634G
Oscar D´ıaz Arado,a,b Harry Mo¨nig,∗,a,b, Jo¨rn-Holger Franke,c, Alexander Timmer,a,b, Philipp
∗,d
∗,a,b
Alexander Held,d, Armido Studer, and Harald Fuchs
Received Xth XXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
First published on the web Xth XXXXXXXXXX 200X
DOI: 10.1039/b000000x
On-surface synthesis of a polyphenylene vinylene oligomer
via reductive coupling of a terephthalaldehyde derivative
on Au(111) is reported. Scanning tunneling microscopy
and photoelectron spectroscopy experiments confirmed
oxygen dissociation and its desorption from the surface.
Density functional theory calculations provided a reason-
Fig. 1 Investigated on-surface reaction.
able reaction mechanism involving reactive sites on the
substrate.
(DTA 1) monomers on the Au(111) surface under UHV con-
ditions (Fig. 1). We will show that successful C-C coupling of
DTA proceeds directly on Au(111) by temperature induced
C-H activation of the aldehyde moiety, resulting in a para-
polyphenylene vinylene derivative (p-PPV) 2 adsorbed on the
substrate. Given their great potential for the design of elec-
On-surface synthesis of covalently interlinked organic nanos-
tructures under ultrahigh vacuum (UHV) conditions has ob-
tained extensive attention in recent years1. Several reactions,
such as the Ullmann coupling2, formation of Schiff bases3,
condensation of boronic acids4, acylation reactions5, homo-
coupling of alkanes6 and alkynes7 and more recently, azide-
alkyne cycloadditions8 have been successfully conducted on
surfaces. Each approach aims to the same objective: to
develop well-ordered functional nanostructures on surfaces,
which sometimes are difficult or even impossible to synthesize
via classical solution phase chemistry6. Despite the versatility
of these on-surface chemical processes for such a purpose, the
investigation of new reaction types that could allow the prepa-
ration of more complex nanostructures with tunable properties
remains of great interest in this field.
One suitable candidate for on-surface synthesis could be
the equivalent of the reductive coupling of carbonyls in so-
lution (McMurry reaction)9, which is known to produce ma-
terials with attractive electronic properties like polyphenylene
vinylene (PPV). In particular, aldehydes have been shown to
form covalent bonds after reductive coupling, which is cat-
alyzed by Ti and V9,10. Resembling the McMurry-type chem-
istry, in the present study we use a combination of scan-
ning tunneling microscopy (STM), X-ray photoelectron spec-
troscopy (XPS) and density functional theory (DFT) to investi-
gate the on-surface coupling of 2,5-dihexylterephthalaldehyde
tronic devices and particularly organic solar cells research11
the bottom-up growth of PPV-based polymers directly on sub-
strates opens a new possibility for the development and fabri-
cation of functional organic nanomaterials.
,
The design of the DTA molecules allows them to be ther-
mally deposited on the surface at a moderate sublimation
temperature. Moreover, the reactants are charged with two
aldehyde moieties, which allow this particular compound to
undergo oligomerization. DTA 1 was next sublimated onto
the Au(111) surface kept at room temperature. STM exper-
iments were performed to study their adsorption and self-
assembly behavior, revealing a single densely packed mono-
layer of monomers 1 on the substrate (Fig. 2). Within the self-
assembled monolayer (DTA-SAM), the aldehyde moieties are
oriented along the ~a direction (inset in Fig. 2a), following a
periodicity of a = (0.81 0.04) nm [0.92 nm, calculated value
for the gas phase]. When comparing the calculated center-
to-center distance with a, a reasonable quantitative agree-
ment is found, where the small difference can be ascribed to
the absence of the Au(111) surface in the gas phase calcula-
tions, which restrains out-of-plane movements of the molecu-
lar components.
As a consecutive step, the DTA-SAM was annealed at tem-
peratures ranging from 100 °C to 250 °C. STM experiments
after the annealing at T = 250 °C (activation temperature) re-
vealed disordered oligomers on the Au(111) substrate (see
ESI†), indicating that a possible activation of the aldehyde
moiety and subsequent C-C coupling could have occurred.
Given the large dissociation energy for the oxygen of the alde-
hyde moiety12, the requirement of such high activation tem-
†
Electronic Supplementary Information (ESI) available: See DOI:
10.1039/b000000x/
a
Physikalisches Institut, Westfa¨lische Wilhelms-Universita¨t Mu¨nster,
Wilhelm-Klemm-Strasse 10, 48149 Mu¨nster, Germany
b
Center for Nanotechnology (CeNTech), Heisenbergstrasse 11, 48149
Mu¨nster, Germany
c
Department of Physics, Universite´ Libre de Bruxelles, Campus Plaine-CP
231, 1050 Brussels, Belgium
d
Organisch-Chemisches Institut, Westfa¨lische Wilhelms-Universita¨t Mu¨nster,
Corrensstrasse 40, 48149 Mu¨nster, Germany
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DOI: 10.1039/C4CC09634G
Fig. 2 Deposition and on-surface reaction of DTA monomers on Au(111). a) STM image (12×12 nm2; U = -1.50 V; I = 400 pA) after the
deposition onto the surface kept at room temperature. The inset shows the intermolecular distance between adjacent monomers along the
aldehyde moiety direction. b) STM image (30×30 nm2; U = -0.65 V; I = 7 pA) after the deposition onto the surface kept at 250 °C. The
targeted coupling reaction is achieved, producing p-PPV oligomers (white arrows) on the surface. c) Oligomers 2 chain length distribution. d)
High resolution STM image (5.7×8.5 nm2; U = 0.65 V; I = 7 pA) of the p-PPV products 2. The inset schematically represents the chemical
structure of the reacted specie.
perature is not surprising. However, we found that the pro-
nounced desorption the reactants undergo at such temperature
is a limiting factor for the on-surface coupling. To circum-
vent this limitation, DTA monomers were deposited onto the
Au(111) surface kept at 250 °C. STM imaging after the depo-
sition revealed that oligomerization was accomplished (white
arrows in Fig. 2b), with about 6 % of the reactants forming
the targeted p-PPV products (details in the ESI†). Moreover,
the products 2 are surrounded, in a ratio ∼1:8, by a disordered
phase which is difficult to characterize with STM imaging. An
improvement of these factors was not observed in experiments
on other Au substrates (see ESI†). However, XPS experiments
on Au(111) allowed to exclude the presence of oxygen on the
surface after the on-surface reaction (see below). Therefore,
the disordered phase could consist of entangled oligomers 2
and/or oxygen-free side-products.
A representative sample of clearly identifiable 562 re-
acted monomers from different deposition experiments al-
lowed the estimation of the oligomers 2 chain length distri-
bution (Fig. 2c). Oligomers with a length of n = 3 − 8 units
(reacted monomers) are obtained more frequently, whereas the
maximum length observed corresponded to n = 32 units. It
is interesting to note that the oligomers 2 do not follow the
Au(111) herringbone reconstruction, as it has been observed
for other on-surface reactions on this substrate2. Their relative
disorder could be due to the substrate reconstruction providing
insufficient confinement during the reaction. The enhanced
diffusion of the reactants 1 and products 2 on the substrate
at the activation temperature could additionally contribute to
such disorder. Furthermore, the absence of monomers 1 in
the STM images indicates that in this case reactants also
desorb, similar as with the DTA-SAM annealing at 250 °C.
Moreover, STM tip manipulations were carried out to inves-
tigate the intra-molecular and molecule-substrate interaction
for the oligomers 2 on Au(111) (see ESI†). These experiments
showed the p-PPV products to weakly (non covalently) inter-
act with the metal substrate, while the intra-molecular units
are strongly (covalently) interlinked.
High resolution STM images (Fig. 2d) allowed the struc-
tural characterization of the newly formed oligomers. As it
can be observed, the aliphatic chains follow a distinct zig-
zag arrangement, likely induced by van der Waals interactions.
More importantly, the products 2 are on a trans configuration,
given that the cis alternative would not follow the linear ar-
rangement observed on the surface. The existence of cis iso-
mers in the disordered phase, however, cannot be excluded.
We determined the center-to-center distance between adjacent
phenyl rings experimentally (0.5 0.2 nm) and by DFT cal-
culations for the gas phase (0.67 nm). When compared with
the unit cell direction a, corresponding to the aldehyde moi-
ety orientation in the DTA-SAM (Fig. 2a), a significant reduc-
tion of the intermolecular distance (∼0.3 nm) is observed. The
reduced center-to-center distance between the phenyl rings
clearly indicates that oligomers 2 (schematically represented
in Fig. 2d) were formed as a result of a reductive coupling
process on the Au(111) surface.
To provide detailed chemical information about the ob-
served on-surface coupling of the DTA monomers, XPS ex-
periments were carried out. After depositing the monomers
onto the Au(111) surface kept at room temperature, C1s and
O1s spectra were recorded (Fig. 3a,b). In this case, the C1s
region shows two distinct peaks which correspond well with
contributions from three carbon species at binding energies
of 284.7 eV (aromatic), 285.0 eV (hydrocarbon) and 287.6 eV
(carbonyl). In addition, a single O1s peak is detected at a bind-
ing energy of 531.9 eV, corresponding well with physisorbed
aldehyde oxygen. Quantitative analysis reproduced the com-
position of the DTA reactants (carbon = 91.8 %; oxygen =
8.2 %). Spectra collected after depositing DTA monomers
onto the Au(111) surface kept at 250 °C show an absence
of the oxygen-related signals. In this case, the previously
observed O1s peak at 531.9 eV (Fig. 3c) and C1s peak at
287.6 eV (Fig. 3d) are not detected. Motivated by the con-
figuration proposed for the p-PPV oligomers (Figure 2d), the
peak fitting of the C1s region in this case includes a new car-
bon specie consistent with the vinylene moiety of the products.
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action mechanism must be considered.
To elucidate the mechanism driving the coupling of alde-
hydes on the Au(111) surface, we performed systematic
DFT calculations. Hereinafter, the most energetically fa-
vored mechanism is described in detail, while less favored
alternatives are presented in the ESI†. Two simplified DTA
monomers adsorbed on the substrate with the carbonyl groups
facing each other (Fig. 4; config. 1) are defined as the state
with 0 eV. Taking into consideration the large dissociation en-
ergy for the aldehyde oxygen12, we investigated the C-H acti-
vation of the carbonyl group via H abstraction by the substrate.
We found that a surface adsorbed α-benzoyloxy benzyl radi-
cal is generated while the H-atom remains bound to the surface
(Fig. 4; config. 2), increasing the total energy to 0.84 eV. This
radical formally derives from the acyl radical addition to the
carbonyl O-atom of the second reactant. This process can oc-
cur at a relatively low transition state energy of 1.13 eV. At this
point, a transfer of the hydrogen atom back from the surface
to the O-atom of the ester carbonyl group of the α-benzoyloxy
benzyl radical leads to a 1,3-biradical (1.27 eV transition state
energy), increasing the total energy to 0.90 eV (Fig. 4; con-
fig. 3). Following from the 1,3-biradical configuration, the
reaction can progress more energetically favored following a
two-step process: (i) the 1,3-biradical first cyclizes to a hy-
droxyoxirane (Fig. 4; config. 4) that is fairly stable at 0.72 eV,
with a transition state energy of 2.22 eV; (ii) subsequent oxy-
gen scission from the oxirane ring results in the dissociated
oxygen atom adsorbed on the Au(111) surface and an enol
(Fig. 4; config. 5) now at a total energy of 0.88 eV, with a tran-
sition state energy barrier of 1.81 eV.
Fig. 3 XPS analysis of the on-surface reaction. a-b) XPS data of
the C1s and O1s regions after deposition onto the substrate kept at
room temperature. Peak fitting analysis reproduced the stoichiometry
of intact monomers. c-d) XPS data collected after the deposition
onto the substrate kept at 250 °C show a complete absence of oxygen
species. The C1s signal is consistent with the formation of p-PPV 2,
but does not exclude contributions from oxygen-free side products in
the disordered phase.
This strongly evidences a complete desorption of oxygen from
the surface, in agreement with previously reported studies for
Au(111)13. Furthermore, this fact suggests the oxygen scis-
sion from the aldehyde as a prerequisite for double bond for-
mation. Moreover, the formation of oxygenated side-products
after the on-surface reaction can hereby be excluded.
Since the XPS data showed that a complete desorption of
all oxygen species occurred after the on-surface reaction, we
can assume that the dissociated oxygen atom desorbed from
the substrate at this point, probably as a molecular specie. It
is important to note that the large entropy increase upon des-
orption of a molecular specie decreases the free energy of the
system by 2.55 eV per O2 molecule at the experimental condi-
tions, which provide the thermodynamic driving force for this
desorption process15. Another important aspect to be consid-
ered is the existence of many reactive sites on the Au(111)
substrate due to surface reconstruction and/or surface diffu-
sion of single Au atoms at elevated temperatures. Therefore,
the next step of the reaction is calculated in a system where the
oxygen atom is already removed from the surface and an ex-
tra Au atom mimicking a reactive site (see ESI†) is introduced
(Fig. 4; config. 6), increasing the total energy to 1.60 eV and
decreasing the free energy at the experimental conditions to
0.32 eV accordingly. From that point on, H donation to the
substrate can occur after overcoming a transition state energy
of 1.69 eV forming a Au-enolate (Fig. 4; config. 7) and leaving
the system at a free energy of 1.26 eV. Hydrogen recombina-
tion with the Au-enolate results in the formation of an epoxide
(Fig. 4; config. 8) now at a free energy of 0.79 eV after a transi-
tion state energy of 2.05 eV. Subsequent deoxygenation results
In previous studies of the McMurry-type chemistry, the
coupling mechanism is explained by the formation of vicinal
diolate intermediates between a metal atom from the catalyst
and the oxygen atom from the aldehyde moiety at room tem-
perature. The diolates were shown to induce the dissociation
of the oxygen atom and the C-C coupling after subsequent
thermal treatment10. However, our XPS experiments demon-
strate that on Au(111) at room temperature, the O1s binding
energy for the DTA-SAM (Fig. 3b) corresponds to an aldehyde
specie14. This is also in agreement with our DFT calculations
(see ESI†), which are consistent with a weak physisorption of
monomers 1 on the substrate, strongly suggesting that a co-
valently bound diolate is not formed. Furthermore, if diolates
were formed directly after the deposition onto the Au(111)
surface kept at 250 °C, chemisorbed DTA monomers and/or
coadsorbed oxygen from the aldehyde moiety should be ex-
pected to remain on the metal substrate10. In contrast, our
XPS data show a complete absence of oxygen related signals,
evidencing the desorption of unreacted monomers 1 and the
dissociation of oxygen during the on-surface reaction form-
ing the p-PPV products. Therefore, taking also the absence of
well-established catalysts (titanium, zinc, vanadium or cop-
per) in our experiments into consideration9, an alternative re-
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Fig. 4 Energetics of the proposed reaction mechanism. Purple lines indicate free energy estimates after oxygen desorption at the experimental
conditions. The rate limiting step appears highlighted in red.
in the dissociated oxygen adsorbed on the substrate (not bound
to the extra Au atom) and the formation of the final product
(Fig. 4; config. 9) at a free energy of 1.40 eV after a transi-
tion state energy of 1.93 eV. Given the poor interaction of the
dissociated oxygen with the extra Au atom and that our XPS
results demonstrate the absence of oxygenated species on the
surface after the reaction, in the final configuration the oxygen
and the extra Au atoms are removed from the system simulta-
neously (Fig. 4; config. 10). This results in an increase of the
total energy of the system up to 3.22 eV and a reduction of its
free energy down to 0.67 eV at the experimental conditions.
The rate-limiting step of the proposed reaction mechanism is
the 1.37 eV that the system must overcome for the formation
of the Au-enolate in config. 7. It is important to note that such
a barrier can be overcome at our experimental conditions.
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In conclusion, we have shown the successful reductive cou-
pling of an aldehyde on a Au(111) surface leading to the for-
mation of a PPV derivative. The obtained oligomers 2 were
found to be covalently interlinked and to remain weakly ad-
sorbed on the substrate while oxygen completely desorbs dur-
ing the reaction. Based on our results, the so far accepted
reaction pathway for reductive aldehyde coupling involving
a vicinal diolate intermediate is not dominating the observed
on-surface reaction. Different alternatives were explored, and
an energetically favored C-C coupling mechanism proceeding
via initial C-H activation of the aldehydes followed by a de-
oxygenation process favored by gold adatoms on the surface
is proposed. With this on-surface synthesis approach, nanos-
tructures with tunable optoelectronic properties on substrates
can be developed, thus increasing the existing pool of suitable
reactions for bottom-up growth of organic nanostructures on
surfaces.
Financial support by the Deutsche Forschungsgemeinschaft
(DFG) through the SFB 858 (project B2) and the transregional
collaborative research center TRR 061 (projects B3 and B7) is
gratefully acknowledged.
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