Triazatrinaphthylene, a three-fold symmetry planar conjugated system with two-dimensional self-assembly properties

Article abstract of DOI:10.1039/b504704h

A novel threefold symmetry planar system based on a re-conjugated triazatrinaphthylene core has been synthesized and deposited on atomically flat surfaces of highly oriented pyrolytic graphite (HOPG). Scanning tunnelling microscopy (STM) of 2,8,14-trimeth

Full text of DOI:10.1039/b504704h

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Triazatrinaphthylene, a three-fold symmetry planar conjugated system with
two-dimensional self-assembly properties{
N. Saettel,a N. Katsonis,b A. Marchenko,b M.-P. Teulade-Fichou*a and D. Fichou*b
Received 5th April 2005, Accepted 27th May 2005
First published as an Advance Article on the web 30th June 2005
DOI: 10.1039/b504704h
A novel threefold symmetry planar system based on a p-conjugated triazatrinaphthylene core has
been synthesized and deposited on atomically flat surfaces of highly oriented pyrolytic graphite
(HOPG). Scanning tunnelling microscopy (STM) of 2,8,14-trimethyl-5,11,17-triazatrinaphthylene
(denoted TrisK by analogy with the Triskele Celtic symbol) deposited at the n-tetradecane/HOPG
interface reveals the spontaneous formation of self-assembled monolayers with a hexagonal
close-packed arrangement extending over several tens of nanometers. Surprisingly, sub-molecular
STM resolution reveals an extremely bright contrast spot on one of the three symmetrical
branches of the triangular-shaped molecule TrisK. This non-equivalent adsorption site can be
interpreted as originating from a local specific molecule–HOPG interaction. Besides, ab-initio
calculations show that a purely molecular contribution may also be involved.
counter-clockwise respectively) make TrisK a prochiral mole-
Introduction
cule exhibiting two enantiotopic faces. As pointed out above,
Extended planar aromatic systems represent attractive targets
as active materials in electronic devices.1 Some of them are
potentially interesting as neutral precursors of nucleic acid
the large aromatic surface of triazatrinaphthylenes makes them
binders11 as well as liquid crystals by substituting the methyl
also commonly used in biology to achieve structure-specific
recognition of DNA through p–p overlapping of nucleic
bases.2 This class of compounds thus represents key motifs at
group by longer alkyl chains. Following the description of its
chemical synthesis, we show here that compound TrisK forms
the crossroads between materials and bioorganic chemistry
long range self-assembled monolayers (SAMs) with hexagonal
and this duplicity is illustrated by numerous recent studies
packing on HOPG and that high resolution STM reveals the
making use of polynuclear aromatics for studying the ability of
presence of a high contrast adsorption site on one of its three
DNA to transport electrical charges.3 In order to achieve their
branches. This intramolecular non-equivalent adsorption site
self-organization in solution or the solid state it is therefore
is very surprising for a symmetrical molecule and is interpreted
essential to tailor them properly.4 From this viewpoint, planar
in terms of a strong local electronic interaction between certain
conjugated molecules having threefold symmetry and bearing
atoms of the molecule and those of graphite.
long alkyl chains such as hexa-peri-hexabenzocoronenes5 or
hexaazatrinaphthylenes6 are of particular interest because they
behave as liquid crystals. It has also been reported recently
that alkoxy-triphenylenes7–9 and conjugated {C}3-oligothio-
phenes10 form two-dimensional (2D) self-assemblies on highly
oriented pyrolytic graphite (HOPG) as revealed by scanning
tunnelling microscopy (STM).
We report here on the synthesis and 2D self-assembly
on graphite of 2,8,14-trimethyl-5,11,17-triazatrinaphthylene, a
novel C3h-symmetry conjugated compound denoted TrisK by
analogy with the Celtic symbol Triskele meaning ‘‘three-
legged’’. The molecule of TrisK consists of a central benzene
ring with three peripheral branches made of quinoline moieties,
each of them terminated by a methyl group. The relative
positions of the three nitrogen atoms and of the three methyl
groups on the same side of each branch (clockwise and
Results and discussion
aLaboratoire de Chimie des Interactions Mole´culaires, UPR CNRS,
Colle`ge de France, 4, place Marcelin Berthelot, 75005 Paris, France.
Synthesis
E-mail: mp.teulade-fichou@college-de-france.fr; Tel: +33 1 44 27 13 74
bCEA-Saclay, LRC Nanostructures et Semi-Conducteurs Organiques
(CNRS-CEA-UPMC), DSM/DRECAM/SPCSI, 91191 Gif-sur-Yvette,
In general, fused aromatic systems containing heteroatoms, in
particular the aza-series, are more easily accessible than their
France. E-mail: fichou@drecam.cea.fr; Tel: +33 1 69 08 43 74
{ Electronic supplementary information (ESI) available: orbital
coefficients. See http://dx.doi.org/10.1039/b504704h
all-carbon counterparts. The synthesis of polyazaaromatic
compounds mainly proceeds via three well-established
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procedures, namely the condensation of an aromatic
o-aminoaldehyde with an enolizable ketone (Friedla¨nder
method),12,13 the condensation of o-phenylene diamines
with quinones (Skraup reaction)14,15 and the amination of
aryl halides (Ullmann–Goldberg type)16 followed by an intra-
molecular cyclization. However, the two former methods are
often limited by the availability and the stability of the starting
materials.17
Accordingly, we successfully synthesized compound TrisK
via a three-step process based on N-arylation of 1,3,5-tribro-
mobenzene. N-Arylation allows in principle the use of a variety
of substrates, but it often leads to poor yields and low repro-
ducibilities when copper catalysis is carried out.16 However,
during the past decade, considerable improvements of this
synthetic process have been performed through the develop-
ment of novel palladium catalytic systems.18 In respect of these
advances and to our previous works in the synthesis of various
acridine and quinacridine series,19 we selected Pd-catalyzed
N-arylation as the key step of our synthetic approach to TrisK.
As shown in Scheme 1, the condensation of 1,3,5-tribro-
mobenzene with the 5-methyl derivative of anthranilic acid
catalyzed by Pd(OAc)/BINAP affords the tris-condensation
product 1 in good yield under mild conditions. This inter-
mediate is then submitted to ring closure by treatment in
P(O)Cl3,20 which leads to the tris-chloro derivative. Finally,
compound TrisK is obtained from the reduction of the tris-
chloro intermediate 2 by LiAlH4. These conditions allow the
tris-amination to go to completion in good conversion yield
(.60%), whereas copper catalysis (Cu/CuI) only affords a
mixture of mono- and di-substituted products in low yield
(,15%).
Fig. 1 STM images of a self-assembled monolayer of TrisK recorded
at the interface between graphite HOPG and n-tetradecane. (a) 61.1 6
61.1 nm2, Ut 5 138 mV, It 5 15 pA; (b) 20.2 6 20.2 nm2, Ut 5 177 mV,
It 5 12 pA; (c) high resolution image (4.4 6 4.4 nm2, Ut 5 177 mV,
It 5 15 pA) revealing a hexagonal arrangement of high contrast spots;
(d) one possible arrangement of molecule TrisK on HOPG as deduced
from (c).
interface. The monolayer consists of highly ordered domains
extending over a few tens of nanometers and delineated by
linear dislocations generated by the misfit between the graphite
and organic 2D lattices. These dislocations form a triangular
pattern with angles of 60u which reflect the underlying geo-
metry of the graphite substrate. Higher resolution (Fig. 1b)
reveals that molecules of TrisK self-assemble as a hexagonal
Scanning tunnelling microscopy
Fig. 1a is a large scale STM image of a self-assembled
monolayer of TrisK physisorbed at the n-tetradecane/HOPG
Scheme 1 Synthetic pathway to compound TrisK. Reagents and conditions: i) Pd(OAc)2/BINAP/Cs2CO3/dioxane; ii) P(O)Cl3, reflux; iii) LiAlH4,
THF, reflux, then FeCl3/NH4OH.
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in STM.22–24 The same concept also applies to large polycyclic
aromatic hydrocarbons as recently reported by Samori
et al.25,26 Given this, if we position all carbon atoms of mole-
cule TrisK as if it were a graphite layer (Fig. 2a), adsorption
sites of each of the three branches of TrisK (denoted a, b and c
in Fig. 2) are equivalent. The identical positions of the three
methyl groups with respect to HOPG (large black circles)
provide a clear example of such a symmetrical situation. This
model graphite-like position of TrisK on HOPG cannot in
itself account for the observed non-equivalent STM contrast
feature. Since molecule TrisK contains a number of non-
carbon atoms (in particular three nitrogen atoms), it is very
likely that its position on a graphite surface is slightly shifted
compared to the model position.
˚
lattice having a unit cell with a 5 b 5 16.1 ¡ 0.2 A. The
triangular shape of TrisK is already visible but becomes
even more apparent in Fig. 1c where intramolecular details
also appear. The corresponding molecular 2D-arrangement of
TrisK is given in Fig. 1d as a superposition of its carbon
backbone onto the STM image. However, at this point it
remains impossible to locate precisely either the nitrogen
atoms or the terminal methyl groups which can be on either
side of the three branches of the molecule because of its
prochiral nature. Note that the dark contrast regions appear-
ing between molecules of TrisK in Fig. 1c,d correspond to
graphite areas only covered by n-tetradecane.
Surprisingly, a hexagonal arrangement of extremely bright
spots is clearly visible in Fig. 1c, each of them being located on
the side-end of one of the three branches of TrisK. Such bright
spots correspond to high tunnelling currents and are a priori
not compatible with the ternary symmetry of TrisK where
the three branches are supposed to be equivalent towards
adsorption. This non-equivalent adsorption site is dominated
either by molecule–substrate interactions or by a hypothetical
asymmetry of molecular orbitals (MOs).
Starting from the graphite-like position and by sliding
molecule TrisK only by a few tenths of angstro¨ms, it appears
that adsorption sites remain equivalent in two branches while
that of the third branch becomes non-equivalent. One example
of such a situation is represented in Fig. 2b where TrisK has
been shifted along the (110) direction of graphite. In this new
geometry, methyl groups of branches a and b have similar
graphite environments (black circles) while the methyl group
of branch c becomes non-equivalent due to a different graphite
environment (red circle). This is also clearly the case for the
nitrogen atoms (represented by blue spots in Fig. 2) which
adopt equivalent graphite environments in branches a and c
and a non-equivalent one in branch b (Fig. 2b). Note that
various similar molecule–substrate positions leading to one
non-equivalent intramolecular adsorption site can be obtained
by shifting TrisK along other directions starting from the
graphite-like position of Fig. 2a. The geometrical situation
shown in Fig. 2b illustrates our interpretation and may indeed
explain the non-equivalent STM contrast observed on one
branch of the TrisK molecule. It is well known that a sub-
angstro¨m scale displacement of a molecular adlayer may
induce a pronounced tunnelling contrast change.
A first way to address this question consists of building a
model packing of the adlayer of TrisK on graphite. It is well
known that the graphite surface is constituted of two types of
carbons marked as small black and white dots in Fig. 2. Two
adjacent graphite layers are shifted so that their respective
carbon atoms are located either at the apex of each other
(white dots) or at the apex of the center of a hexagon (black
dots). Therefore, prominent tunnelling sites of bare graphite
(carbons in white) correspond to each second carbon atom of
the hexagonal lattice.21 The main consequence is that atoms
(or groups of atoms) of each individual molecule of an organic
adlayer will have different graphite environments and different
STM contrasts. For example, a number of studies have shown
that consecutive CH2 groups of long rod-like n-alkanes self-
assembled on graphite are alternately close to either of the two
types of carbon and only one CH2 group of every two is visible
An alternative origin for the dissymmetrical intramolecular
contrast of TrisK when physisorbed on graphite could be the
loss of planarity of the molecule. Such an explanation has
been recently proposed for phenylene-alkyl substituted hexa-
benzocoronenes to account for different tunnelling contrasts of
the single aromatic cores within a monolayer.25 In this case,
the loss of planarity was attributed to steric interactions
between the aromatic core and the phenylene substituents and
was homogeneously distributed over the entire molecule. In
contrast, here compound TrisK does not contain any bulky
substituent susceptible to distortion of its planar conforma-
tion. Furthermore, non-planarity should be inhomogeneous
from one molecular branch to the other resulting in an
intramolecular contrast peculiarity.
Ab-initio calculations
Fig. 2 Two geometrical model positions of molecule TrisK with
respect to the HOPG substrate. (a) Carbon atoms of TrisK are posi-
tioned as if it were a graphite layer. All adsorption sites are equivalent
such as for example that of the terminal methyl groups inside the black
circles. (b) Position obtained by shifting TrisK by a few tenths of
angstro¨ms along the (110) direction of graphite. Adsorption sites of
two of the methyl groups remain equivalent (black circles) while the
third one becomes non-equivalent (red circle).
However, a purely molecular contribution cannot be excluded
given the particular ternary symmetry of molecule TrisK. In
STM images, the contrast is mainly due to the electronic
interaction between the tip and the adlayer–substrate system,
more specifically through the HOMO and LUMO levels.
In order to check this assumption, we performed ab initio
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calculations of charge density distribution in TrisK. Ab-initio
molecular orbital (MO) calculations have been performed on
an isolated molecule TrisK using the Gaussian 03W package,27
with geometry optimization at the HF/STO-3G level. Further
optimization is obtained with the local density approximation
(LDA) using the spin-density functional of Vosko et al.28
(SVWN) and the 6-31G** basis set, which is comparable to the
double numerical basis sets with polarization functions used in
other studies.10 MOs are visualized with the program Molden
4.029 and a contour value of 0.01.
Conclusion
In summary, we have synthesized 2,8,14-trimethyl-5,11,17-
triazatrinaphthylene TrisK, the first member of a new class
of planar conjugated molecules with ternary symmetry.
Compound TrisK physisorbs on the basal plane of graphite
HOPG to form self-assembled domains extending over several
tens of nanometers. Sub-molecular STM resolution reveals a
high contrast spot located on one of the three branches of
TrisK. We interpret this peculiar local contrast in terms of a
non-equivalent intramolecular adsorption site due to a strong
local interaction with HOPG. If the precise identification of
the responsible atoms is premature, the terminal part of one
of the three branches of TrisK plays an important role due
to its coincidence with the bright spot on the STM images.
As a first argument, we show that a slight shift of TrisK on
the graphite lattice away from a model graphite-like position
may induce non-equivalent intramolecular substrate environ-
ments. Furthermore, ab-initio MO calculations lead to two
degenerate HOMOs with slightly different contributions of the
methyl-2pz coefficients in the overall molecular orbital, thus
suggesting that it may also contribute to the non-equivalent
adsorption sites of TrisK on graphite. We are presently modify-
ing the chemical structure of molecule TrisK, in particular by
substituting the terminal methyl group by a longer alkyl chain
in view of investigating potential liquid crystal properties
and tuning the possibility of a stereochemical morphology of
the monolayers.30
Our results show that TrisK is a planar molecule belonging
to the C3h symmetry point group. It has a two-fold degenerate
HOMO level (two components denoted HOMO 1 and HOMO
2) but only one LUMO. The separation between the HOMO
and the LUMO levels is found to be 2.14 eV. Interestingly,
Fig. 3 clearly shows that the three branches of molecule TrisK
have different MO contours in both HOMOs whereas they
are perfectly identical in the LUMO. Such a HOMO picture
has already been reported for other conjugated molecules
with ternary symmetry.10 In addition, a prominent feature
in the HOMO is the difference in the coefficients on the
terminal methyl groups. For both HOMO 1 and HOMO 2, the
contribution of the carbon 2pz orbital is predominantly
located on two methyl groups (see values of coefficients in
the ESI{). Although these calculations are performed on a
non-interacting molecule, they suggest that the HOMO of
TrisK may contribute to the non-equivalent adsorption
sites observed on HOPG. They also suggest that the methyl
group could play a role in the intense contrast spot of the
STM images.
Experimental
Synthesis
5,59,50-Trimethyl-2,29,20-(1,3,5-phenylenetriamino)tribenzoic
acid (1). 1.79 g (5.7 mmol) of 1,3,5-tribromobenzene, 3 g
(19.8 mmol, 3.5 eq.) of 2-amino-5-methylbenzoic acid, 0.32 g
(0.5 mmol, 0.09 eq.) of BINAP, 0.08 g (0.3 mmol, 0.06 eq.) of
Pd(OAc)2 and 9.22 g (28.3 mmol, 5 eq.) of Cs2CO3 are
introduced into an argon-purged round-bottom flask with
20 mL freshly distilled 1,4-dioxane on sodium and refluxed
during 24 h under an argon stream. After cooling, the dioxane
is evaporated and the residue is dissolved in 80 mL methanol–
water 1 : 1. The solution is acidified with concentrated HCl
until the pH drops significantly (pH y 4.5). The green
precipitate is filtered and extracted with hot ether. The filtrate
is evaporated and methanol is added; the resulting precipitate
is filtered and dried, yielding 1 as a pale yellow solid (1.08 g,
1
isolated yield 36%). H NMR (DMSO): 9.52 (br s, 3 COOH),
7.82 (s, 3H), 7.41 (m, 6H), 6.74 (s, 3H), 2.35 (s, 9H). 13C NMR
(DMSO): 170.1, 144.3, 143.7, 135.2, 132.0, 127.3, 116.2,
114.1, 105.4, 20.4. ESI-MS (H2O–MeOH) m/z: 526.28
(MH+); mp .260 uC.
Fig. 3 Ab-initio calculation of the charge density distribution of
the frontier orbitals of 2,8,14-trimethyl-5,11,17-triazatrinaphthylene
TrisK. (a) The two components of its degenerate HOMO orbital
(symmetry E0, eigenvalue 20.215690 a.u.) and (b) the non-degenerate
LUMO orbital (symmetry A0, eigenvalue 20.119220 a.u.). Note
that the three branches of TrisK have different MO contours in both
components of the HOMO whereas they are perfectly identical in
the LUMO.
6,12,18-Trichloro-2,8,14-trimethyl-5,11,17-triazatrinaphthyl-
ene (2). 0.51 g (0.97 mmol) of 1 and 8 mL of POCl3 are
introduced into an argon-purged round-bottom flask and
refluxed for 3 h with a CaCl2 guard. After evaporation of
POCl3, the residue is dissolved in 50 mL of chloroform and
slowly poured onto 15 mL of ice-cooled 15% NH4OH. The
organic layer is decanted and the aqueous layer is further
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extracted with chloroform. The organic phase is dried over
Na2SO4, and concentrated almost to precipitation. Methanol
is added to form a precipitate, which after filtration gives 2
as a brown solid (0.18 g, 35%).1H NMR (CDCl3): 8.475
5
3
(s, 3H), 8.26 (d, 3H, J 5 8.7 Hz), 7.75 (dd, 3H, J 5 1.8 Hz,
5J 5 8.7 Hz), 2.72 (s, 9H). (CI-MS) m/z: 526 (MH+). UV-vis
(CHCl3) lmax/nm (M21 cm21) 5 294 (40526) 307(48421),
323(37526); Mp m260 uC.
2,8,14-Trimethyl-5,11,17-triazatrinaphthylene (TrisK). To a
suspension of 0.10 g LiAlH4 (2.6 mmol.) in 8 mL freshly
distilled THF under argon is slowly added 0.18 g of 2
(0.32 mmol.) in 8 mL THF. After refluxing overnight , the
mixture is cooled to 0 uC and 0.2 mL water, 0.2 mL 15% NaOH
and 0.6 mL water are added successively. After stirring for
20 min the mixture is filtered and the solid residue washed with
CHCl3 until decoloration. The filtrate is decanted and the
aqueous phase is extracted with CHCl3. The organic phase,
containing TrisK along with hemireduced products, is treated
as follows: after evaporation, the residue is taken up in 12 mL
EtOH and a solution of 0.44 g of FeCl3?6H2O (1.62 mmol.) in
2 mL water is added and refluxed overnight. After cooling to
0 uC, 0.6 mL of 15% NH4OH is added and the mixture is
filtered. The precipitate is washed with CH2Cl2 and the filtrate
is decanted and further extracted with CH2Cl2. After drying
over Na2SO4 and evaporation, the residue is taken up in a 1 : 3
mixture of CH2Cl2–MeOH. The formed precipitate is filtered
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1
and dried, yielding TrisK as an orange solid (30 mg, 22%). H
5
NMR (CDCl3): 9.96 (s, 3H), 8.25 (d, 3H, J 5 8.4 Hz), 7.952
5
(s, 3H), 7.71 (d, 3H, J 5 8.4 Hz), 2.67 (s, 9H). (CI-MS) m/z:
424 (MH+); UV-vis (CHCl3) lmax/nm (M21 cm21) 5 288
(62457) 298 (73898), 311 (53237), 368 (14237); 386 (14151);
fluorescence (CHCl3, lexc 5 298 nm) lem/nm 5 390, 413, 437;
mp .260 uC.
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STM is performed using a PicoSPM (Molecular Imaging)
equipped with a home-built liquid cell. Samples (5 6 5 mm2)
of highly oriented pyrolytic graphite (HOPG) are cleaved just
prior to sample deposition. A small quantity of purified 2,8,14-
trimethyl-5,11,17-triazatrinaphthylene TrisK is solubilized
in n-tetradecane C14H30 (99+% purity, Aldrich). The STM
tips are mechanically cut from a 250 mm Pt–Ir (80 : 20)
wire and tested on cleaved HOPG. STM images are recorded
at room temperature at the HOPG/n-tetradecane interface
in the constant-current mode. Typical imaging conditions
are 100–300 mV for the tip voltage and 10–20 pA for the
tunnelling current.
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