Visualizing the product of a formal cycloaddition of 7,7,8,8-tetracyano-p- quinodimethane (TCNQ) to an acetylene-appended porphyrin by scanning tunneling microscopy on Au(111)

Article abstract of DOI:10.1002/chem.201100733

Click it: The applicability of a formal [2+2] cycloaddition between electron-rich alkynes and electron-deficient TCNQ on an atomically clean Au(111) surface was demonstrated by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). At low coverage, monomeric and self-assembled dimeric species of the initial compounds as well as of the reaction product, a TCNQ-conjugated porphyrin, could be visualized.

Full text of DOI:10.1002/chem.201100733

DOI: 10.1002/chem.201100733
Visualizing the Product of a Formal Cycloaddition of 7,7,8,8-Tetracyano-p-
quinodimethane (TCNQ) to an Acetylene-Appended Porphyrin by Scanning
Tunneling Microscopy on AuACTHNUTRGNE(UNG 111)
Petra Fesser,[a] Cristian Iacovita,[b] Christian Wꢀckerlin,[c] Saranyan Vijayaraghavan,[b]
Nirmalya Ballav,[c] Kara Howes,[a] Jean-Paul Gisselbrecht,[e] Maura Crobu,[f]
Corinne Boudon,[e] Meike Stçhr,[d] Thomas A. Jung,*[c] and FranÅois Diederich*[a]
The formation of covalently interlinked two-dimensional
structures on surfaces is highly desirable since, in compari-
son to their well-investigated self-assembled analogues,[1]
they feature higher thermal stability and robustness, which
benefit future applications in nanodevices. Furthermore,
these structures should be equipped with functionalities to
provide technologically interesting properties, such as specif-
ic electronic conductivity[2] or high third-order optical non-
occurring on single-crystal metal supports. Until now, only a
few reactions have been observed by STM on metal sur-
AHCTUNGTRENNUNG
faces, mainly in closed-packed arrangements.[4] Of these re-
AHCTUNGTREGaNNUN ctions only radical coupling reactions have been studied by
single-molecule analytical techniques, so that both the isolat-
ed starting material and the reaction product illustrate the
modification of the molecule.[5] [2+2] Cycloadditions have
been used to modify Au nanoparticles[6] and to covalently
attach organic molecules to semiconducting surfaces such as
ACHTUNGTRENNUNG
SiACHTUNGTRNE(NNUG 001), GeAHCTUNRTGENNUGN CAHTNUGTRENNGUN
(001),[7] and diamond(001),[8] shown by STM.
both visualize and study the outcome of chemical reactions
Suitable surface reactions are those that do not require sol-
vent assistance to lower the energy of polar transition states.
Herein, we describe a click-type on-surface reaction, with
the reaction product as obtained in a formal [2+2] cycload-
dition, followed by retro-electrocyclization between elec-
tron-deficient 7,7,8,8-tetracyano-p-quinodimethane (TCNQ)
[a] P. Fesser, Dr. K. Howes, Prof. F. Diederich
Laboratorium fꢀr Organische Chemie
ETH Zꢀrich
Hçnggerberg, HCI
8093 Zꢀrich (Switzerland)
Fax : (+41)44-632-1109
and acetylene-appended porphyrin
1
(Scheme 1).[9] By
means of ultrahigh vacuum (UHV) STM, we were able to
identify single molecules both before and after the transfor-
mation.
E-mail: diederich@org.chem.ethz.ch
[b] Dr. C. Iacovita, S. Vijayaraghavan
Institute of Physics
Preliminary studies demonstrated the applicability of this
transformation both in solvents of different polarity and in
the absence of solvent. Porphyrin 2 was synthesized by stan-
dard protocols[10] and fully characterized, including electro-
chemical investigations (see the Supporting Information).
The cycloaddition with TCNQ occurs in quantitative yield,
and polar solvents (CHCl3/MeOH 1:1 vs. toluene) were
shown to accelerate the reaction by stabilization of postulat-
ed zwitterionic transition states.[11] Formal cycloaddition
may principally occur by the attack of either of the acetylen-
ic carbon atoms C(a) or C(b) (Scheme 1) to the exocyclic
dicyanovinyl group of TCNQ, leading to two regioisomeric
products 2 and 2b, respectively. The ZnII porphyrin and the
anilino moiety in compound 1 are readily oxidized, as
shown by cyclic voltammetry (CV) and rotating disk voltam-
metry (RDV) measurements (see the Supporting Informa-
tion), accordingly both could provide the electron-donating
activation for a or b attack. However, the dimethylanilino
(DMA) moiety is the superior electron donor and activator
University of Basel
Klingelbergstrasse 82, 4056 Basel (Switzerland)
Fax : (+41)61-267-3784
[c] C. Wꢁckerlin, Dr. N. Ballav, Prof. T. A. Jung
Laboratory for Micro- and Nanotechnology
Paul Scherrer Institute
5232 Villingen PSI (Switzerland)
Fax : (+41)56-310-2646
E-mail: thomas.jung@psi.ch
[d] Prof. M. Stçhr
Zernike Institute for Advanced Materials
University of Groningen
Nijenborgh 4, 9747 AG Groningen (The Netherlands)
[e] Dr. J.-P. Gisselbrecht, Prof. C. Boudon
Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide
Institute de Chimie-UMR 7177, C.N.R.S.
Universitꢂ de Strasbourg
4, rue Blaise Pascal, 67000 Strasbourg (France)
[f] M. Crobu
Department of Materials (D-MATL)
ETH Zꢀrich
Hçnggerberg, HCI E463.2
8093 Zꢀrich (Switzerland)
(Hammett constant sp
ACHTUNGTRENNUNG
porphyrin, which is also spatially more separated from the
acetylene moiety, therefore only regioisomer 2 is isolated as
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100733.
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Chem. Eur. J. 2011, 17, 5246 – 5250

COMMUNICATION
Scheme 1. Synthesis of push–pull substituted porphyrin 2 obtained by a [2+2] cycloaddition of acetylene-appended porphyrin 1 and TCNQ. Different re-
action conditions can be applied: a) toluene, 808C, 10 h, 100%; b) CHCl3/MeOH 1:1, 228C, 3 h, 100%; c) vibration mill, solvent-free, 30 Hz, 100 min,
100%.
verified by 2D HMBC (heteronuclear multiple quantum co-
herence) NMR spectroscopy (see the Supporting Informa-
tion). No side products are formed in the solution reactions
ported.[15] General STM conditions are described in the Sup-
porting Information. Figure 1a shows a high-resolution STM
image of an individual porphyrin 1 molecule adsorbed on
1
as evidenced by monitoring the transformation by H NMR
AuACHTNGUTERNNU(G 111) before TCNQ deposition. The tert-butyl groups
spectroscopy (see the Supporting Information).
Remarkably, the transformation also proceeds in quantita-
tive yield in complete absence of solvent. In vibration mill
experiments at frequencies of 30 Hz, the reaction of the
mixed two starting materials is complete in 100 min, again
in a regioselective fashion. Even mere crushing of the two
starting materials with a spatula induces the transformation,
as shown by thin layer chromatography and mass spectrom-
etry. These findings led us to explore the transformation on
a AuACHTUNGTRENNUNG(111) substrate under UHV conditions. Sublimation
studies (3308C, 10À5 mbar)[13] gratifyingly revealed that 1 can
be sublimed without decomposition as evidenced by subse-
quent NMR and mass spectral analysis. Porphyrins are
viable candidates for surface investigations; since they pro-
vide low mobility on metal surfaces, they are easily assigna-
ble and sublime in a controllable fashion.
We were delighted to see that the on-surface reaction was
spontaneously initialized by consecutive deposition of por-
Figure 1. STM images of a single porphyrin a) before and b) after TCNQ
addition on Au
(111) (scan size: 3ꢄ3.5 nm2, tunneling parameters: U=
1.0 V, I=20 pA). c) Side and top view of the tentative model for adsorp-
tion of product 2 on Au
(111), generated with Spartan.[14] Two cyano
groups interact with the gold surface, leading to a conformation where
the DMA group points away from the surface.
phyrin 1 and TCNQ onto an AuACTHNUTRGNEUGN(111) support at room tem-
AHCTUNGTRENNUNG
perature under UHV conditions. Nevertheless, it has to be
noted that mechanistic considerations from solution studies
can only provide hints to the mechanism on the surface.
Heterogeneous Au catalysis for cycloadditions has been re-
AHCTUNGTRENNUNG
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T. A. Jung, F. Diederich et al.
appear as bright protrusions,[16] while the DMA unit can be
identified as an oblong tail. It should be considered that in
STM the apparent height is not equivalent to the topograph-
ic height, but contributions from electronic states need also
to be taken into account. After TCNQ deposition, a new
protrusion in the vicinity of the tail and an amplification of
the DMA protrusion both indicate spontaneous TCNQ ad-
dition (Figure 1b). Furthermore, the appearance of a black
line through the porphyrin core illustrates a saddle-shape
conformation,[17] which leads to different apparent heights of
the tert-butyl groups and thereby creates conformational
chirality.[18] In our proposed adsorption model (Figure 1c)
two cyano groups of the expanded TCNQ interact with the
AuACHTUNGTRENNUNG(111) surface, consequently the p-quinodimethano moiety
points laterally, giving rise to the new protrusion next to the
former acetylene tail. The DMA moiety orientates away
from the surface, leading to an increase of its apparent
height (Figure 1b).
In addition to single molecules, self-assembled dimeric
structures were observed for both the reacted and the un-
reacted species (Figure 2). Unreacted porphyrin 1 forms
open dimers via van der Waals interactions, wherein the por-
phyrin molecules align in an antiparallel fashion with the
two acetylene tails pointing away from each other (Fig-
ure 2a). Upon mere adsorption of TCNQ onto the porphy-
rin-covered AuACHTUNGTRENNUNG(111) substrate, a new closed dimer of the re-
acted species 2 is observed (Figure 2b).[19] The former acety-
lene tails point towards each other and two bright protru-
sions between the two tails can be seen. In the dimeric ar-
rangement, the porphyrin core and its substituents give a
similar intramolecular contrast as observed for the single
molecule 2 (vide supra). The bright protrusion at the tail
can be assigned to the DMA and the p-quinomethano moi-
eties. Based on an STM series at different bias voltages (see
the Supporting Information), a tentative model was devel-
oped for the dimer. The DMA residues undergo electrostat-
ic interactions with the partially negatively charged malono-
nitrile unit of the expanded TCNQ. The reacted tails assem-
ble in an antiparallel fashion to minimize the overall dipole,
with the DMA moiety pointing slightly up and the p-quino-
methano moiety almost sliding underneath.
Several STM manipulation series substantiate the struc-
tural robustness of product 2 and corroborate its formation.
In manipulations of the product in its monomeric form, it
was possible to successively move and rotate the molecule
multiple times without affecting its structural integrity. The
dimeric structure could be rotated as a closed dimer, and
could also be split by moving or rotating one single porphy-
rin (see the Supporting Information).
Figure 2. STM images and model structures, generated with Spartan,[14]
for the self-assembled dimers on the AuACTHUNRGTNE(NUG 111) surface. a) Two unreacted
molecules 1 interact through van der Waals contacts of the di(tert-butyl)-
phenyl groups (scan size: 4ꢄ6 nm2, tunneling parameters: U=1.0 V, I=
10 pA). b) In the dimeric reacted species the partially positively charged
DMA units interact with partially negatively charged malononitrile units
(scan size: 4ꢄ6.4 nm2, tunneling parameters: U=1.0 V, I=25 pA). c) The
side view of the dimer model of the reacted species shows that the DMA
units are pointing slightly up, whereas the p-quinomethano moiety
almost slides underneath the DMA moiety. Di(tert-butyl)phenyl groups
are omitted in this picture for improved visibility.
(Figure 3) since less nitrogen atoms allow for higher reliabil-
ity in the assignment of signals. Additionally and in contrast
to porphyrin 2, which decomposes before sublimation,
TCNQ adduct 4 sublimes at approximately 1508C, allowing
direct comparison of XP spectra obtained from sublimed
TCNQ adduct 4 with the compound prepared on the sur-
face. Indeed, TCNQ adduct 4 formed on the surface exhib-
its identical peak shapes in the N1s spectra (Figure 3c and
Figure 3d) to those of the sublimed adduct 4. This distinct
congruence (overlay of the spectra: see the Supporting In-
formation) provides unambiguous evidence in favor of the
on-surface reaction of TCNQ with aniline 3 and thereby,
contributes to the XPS evidence provided for the on-surface
reaction of TCNQ with porphyrin 1. Here, good qualitative
agreement is observed between the N1s XPS spectra of por-
phyrin adduct 2 formed on the surface and the powder sam-
ples of 2 as obtained by solution synthesis (see the Support-
For X-ray photoelectron spectroscopy (XPS) measure-
ments,[20] the procedure of depositing TCNQ and porphyrin
1 on AuACHTUNGTRENNUNG(111) was replicated to reproduce the STM surface
conditions as precisely as possible with close to monolayer
coverages (general XPS conditions: see the Supporting In-
formation). To deconvolute the N1s core-level signal of por-
phyrin 1 before and after reaction with TCNQ, aniline 3 and
its TCNQ adduct 4 were studied as model compounds
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Cycloadditions on Surfaces
COMMUNICATION
the intensities of the cyano ni-
trogens was 60% of the ideal
stoichiometric ratio, which can
be explained by sub-stoichio-
metric TCNQ exposure. Due to
the low sublimation tempera-
ture of TCNQ (ca. 1008C) it is
very difficult to control the mo-
lecular rate during deposition
and therefore the TCNQ cover-
age. Additional consolidation
for the formation of the desired
product was obtained by ToF-
SIMS analysis of model com-
pounds 3 and 4 (see the Sup-
porting Information), through
identification of their molecular
peaks and fragmentation prod-
ucts. STM data on the model
Figure 3. N1s XP spectra of a) ca. 1 ML TCNQ, b) ca. 1 ML aniline 3, c) TCNQ adduct 4 as obtained by on-
surface reaction, d) ca. 1 ML TCNQ adduct 4 deposited as obtained by solution chemistry, e) porphyrin 1, and
f) porphyrin adduct 2 as obtained by on-surface reaction. Peak fits for cyano groups (red), DMA residues
(green), and nitrogens in porphyrins (blue) are shown. A broad satellite peak at higher binding energy (gray)
is observed in some spectra.
system could not be obtained
due to high diffusibility conse-
quent to the low substrate bind-
ing of the small molecules.
In summary, we have intro-
duced formal [2+2] cycloaddi-
ing Information). TCNQ contains four cyano nitrogens char-
acterized by a single N1s peak at 398.7 eV, denoted in red
(Figure 3a),[21] whereas the DMA amine of 3 gives rise to a
peak at 399.2 eV (in green, Figure 3b). Upon addition of
TCNQ to aniline 3, an increase in the binding energy (BE)
for the DMA-N1s to 400.15 eV is observed (in green, Fig-
ure 3c). The cyano 1Ns peak shifts to higher BE and the full
width half maximum increases significantly, which precludes
a single chemical shift for all four nitrogens, but is recog-
nized in the fit by two components (red peaks at a BE of
399.6 and 399.05 eV) corresponding to two nitrogens each.
This splitting is fully in line with the expected threefold in-
equivalence of the nitrogen atoms after cycloaddition,
caused by intramolecular charge transfer. The fact that the
XP spectra can be interpreted without consideration of the
surface molecular interaction provides a hint for the un-
modified, low interaction between N and the Au substrate
for the initial TCNQ (UPS data of TCNQ: see the Support-
ing Information) as well as for the adduct. As for porphyrin
1 (Figure 3e), the N1s spectrum is dominated by the signal
at 397.9 eV given by the four pyrrole nitrogen atoms within
the porphyrin moiety,[22] while the DMA nitrogen signal is
observed as a shoulder at approximately 399.05 eV. On the
basis of the knowledge from aniline 3 and its adduct 4, the
XP spectrum of the on-surface product between TCNQ and
porphyrin 1 can now be deconvoluted into signals of the
DMA nitrogen at 400.05 eV, the cyano nitrogens at 399.5 eV
and 398.95 eV and the pyrrole nitrogens at 397.9 eV. The re-
spective BEs are very close to the values observed for
model compound 4. The stoichiometric ratios obtained from
the fit of the XP spectra are perfectly in line with the struc-
ture of aniline 3 and its TCNQ adduct 4. For porphyrin 2,
tions as a click-type on-surface reaction yielding push–pull
substituted chromophores with notable opto-electronic
properties. High-resolution low-temperature STM images
and manipulations, as well as XPS and ToF-SIMS analysis
demonstrate that the reaction occurs on AuACTHNURGTNE(NUG 111) and XPS
indicates a significant intramolecular charge transfer in por-
phyrin 2. The presented combination of thorough studies of
the TCNQ addition to acetylene-appended porphyrin 1 in
solution, in the solid state, and on AuACTHNUTRGNEUNG(111) contributes to a
methodological and target-oriented approach towards novel
on-surface reactions.
Acknowledgement
This work was supported by the Swiss National Science Foundation
(SNSF), the NCCR “Nanoscale Science”, and the European Union
through the Marie Curie Research Training Network PRAIRIES
(MRTN-CT-2006-035810). We thank the R’Equip program of the SNSF,
the Wolfermann Naegeli Stiftung, the Apparatekredit of the University
of Basel, the G. H. Endress Foundation for their support of our new Low
Temperature STM Laboratory and the Swiss Federal Commission for
Technology and Innovation and Specs Zurich (Nanonis) Inc. for the fruit-
ful collaboration on the data acquisition system. We thank Prof. Nicholas
Spencer for providing access to the ToF-SIMS instrument and Dorota
Chylarecka for her involvement in the XPS experiments and analysis.
Keywords: cycloaddition · porphyrins · scanning tunneling
microscopy
·
surface chemistry
·
X-ray photoelectron
spectroscopy
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