Metal Phthalocyanines and Porphyrazines Modified Gold Single-Crystal Electrodes

1
Metal Phthalocyanines and Porphyrazines Modified
Gold Single-Crystal Electrodes Mohammad F.
Khanfar1, Susan H. Zheng1, Hong Zong2, Brian M.
Hoffman2 and Sylvie Morin1 1Department of
Chemistry, York University, Toronto, Ontario, M3J
1P3 Canada 2Department of Chemistry, Northwestern
University, Evanston, Illinois 60208-3113
STM imaging of MPcs such as FePc, CoPc, and CuPc
have been carefully characterized by Hipps and
co-workers 5. In their work some MPc molecules
appear with dark (holes) or bright regions in
their centers. They have proposed that STM
imaging of adsorbed phthalocyanines monolayers
occurs through orbital mediated tunneling
mechanism, where the tunneling current flows
between the STM tip and the organic layer beneath
via empty or half filled molecular orbital of
suitable energy. Fig. 12 is a schematic
presentation of the proposed mechanism 9,10. In
the three MPcs mentioned above, the orbital of
the metal ion involved in the tunneling process
is the dz2. That orbital is half filled in both
CoPc and FePc, therefore, the metal ionic center
appears as a bright. In CuPc, the dz2 is full
filled, hence no tunneling takes place through
such an orbital and the metal ionic center
appears as a hole. Similar theoretical treatment
could apply to explain the features seen in the
STM image for C24H24N8S8Mg. In this case the Mg2
center has a full filled pz molecular orbital, as
a result no tunneling flows through that orbital
and the metallic center appears as a hole in the
center of the MPz.
Abstract Gold single-crystal electrodes were
modified by three different phthalocyanines and
porphyrazines. These compounds are hexadecafluro
ruthenium phthalocyanine, F16RuPc,
octapropylporphyrazine, C40H58N8, and magnesium
octakis(methylthio) porphyrazine, C24H24N8S8Mg.
The modified surfaces were characterized by
in-situ scanning tunneling microscopy. Our
preliminary results point to the formation of
ordered monolayers of the mentioned compounds
adsorbed on the gold single-crystal surfaces.
3.Results and Discussion Modification of the Au
(111) surface by F16RuPc is presented in Fig 6.
The adsorbed phthalocyanine molecules are seen as
bright spots that are decorating all six Au (111)
terraces visible in the figure.
Figure 12 Electron tunneling through
porphyrazines modified gold single crystal
surfaces (adapted from Ref. 10).
1.Introduction Phthalocyanines are
macrocyclic molecules composed of four fused
iminoisoindoline units. They have central
cavities large enough to accommodate a wide range
of metal ions. In addition, functionalization of
the four rings with many different substituents
has been reported in a large number of articles.
Nature of metal ion as well as substituents
strongly affects the physical and chemical
behavior of a metal phthalocyanine (MPc)1.
Figure 1 shows structure of an unsubstituted
phthalocyanine. Porphyrazines, Pzs, are
structural analogues of phthalocyanines. Pzs are
simply porphyrin compounds with four nitrogen
atoms at the meso positions. A significant
attention has been devoted to Pzs and Metal Pzs
(MPzs), mainly due to convenience of preparation
of these macrocycles with a wide range of metal
central ions and/or organic peripheral
substituents. Figure 2 shows structure of the
unsubstituted free base porphyrazine 2. In
general there are two main categories of MPcs
modified surfaces. The first involves
polycrystalline surfaces, such as gold and
carbon, which are covered by thin films of MPcs
3. In the second category, MPcs exist as
ordered monolayers adsorbed on single-crystal
electrodes, such as Au (111) and Cu(100) 4.
MPcs adsorbed molecules can be easily recognized
when monolayers of 25250 nm2 are imaged. STM
imaging of MPcs adlayers has been performed under
ultra high vacuum conditions 5, in air 6, and
in solutions 7. It has been found that STM
images can provide valuable information about
orientation the molecules in the adlayers and
dimensions of the adsorbed molecules 8. To
our knowledge, STM imaging of MPzs adlayers have
not been reported. We are also interested in the
investigation of the electrochemical behavior of
Pzs monolayers. As a first step in this direction
we have modified gold single-crystal surfaces
with two Pzs, octapropylporphyrazine, C40H58N8,
and magnesium octakis(methylthio) porphyrazine,
C24H24N8S8Mg. Structures of these two Pzs are
shown in Fig. 3 and 4, respectively. In this work
STM imaging of hexadeca fluoro ruthenium (II)
phthalocyanine, F16RuPc was also performed.
Structure of F16RuPc is shown in Fig. 5.
Figure 6 (a) STM image of F16RuPc adlayer on Au
(111) at 0.15 VSCE. The image was acquired in 0.1
M HClO4. Set point current and tip bias of the W
tip were 1 nA and -0.050 V, respectively. (b)
constant current contour along the white line
indicated in part (a).
Figure 8 STM image of C40H58N8 adlayer on Au
(111) at 0.15 VSCE. The image was acquired in 0.1
M HClO4. Set point current and tip bias of the W
tip were 1 nA and -0.150 V, respectively.
Molecules appear to be laying flat on the gold
surface. Height of the Pz molecules and distances
between molecules were estimated from Fig. 8.
Fig. 9 shows a height profile that allows
molecular height of three Pz molecules to be
estimated. Molecular height of C40H58N8 is
approximately 0.09 nm above the gold surface and
molecules are separated by ca. 1 nm.
All of the demonstrated STM images show that the
Pz molecules are oriented parallel to the surface
rather than vertically. The intermolecular
distance between Pz molecules is smaller than
what is expected for a end-to-end packing of the
alkyl side chains. That observation points to
interdigitation of adjacent alkyl side chains, as
proposed in Fig. 13.
Modification of gold (111) single crystal
surface by C40H58N8 has been performed
successfully in this work. Fig. 7 shows STM image
of the porphyrazine as a monolayer decorating the
single-crystal surface. The figure shows how
porhyrazines molecular domains are distributed on
the gold terraces. Drift in the image prevents
all domains to be visible. It also appears that
the gold reconstruction is lifted (at least
partially) under these conditions (gold islands
are visible in Fig. 7).
Figure 9 Molecular height of C40H58N8 molecules
above the gold surface
STM image of the modified gold (111) surface
with C24H24N8S8Mg is shown in Fig. 10. In this
image, each MgPz molecule appears as a doughnut
with a central hole. Similarly to the other Pz
the gold reconstruction appears lifted by the
presence of the molecules. This contrast with
layers formed with Pc molecules where no gold
islands were observed (see Fig. 6 for
comparison). A cross section analysis of Fig. 10
allows estimation of molecular distances and
height to be about 1.5 nm and 0.05 nm,
respectively (Fig. 11).
2. Experimental The adsorbed monolayers were
formed by immersing the gold surfaces in the MPc
saturated benzene solution for almost 1 hour, or
the MPzs 0.1 mM methylene chloride solutions were
used and the immersion time was 24 hours. The
electrochemical measurements were performed in a
three compartment Teflon cell under a blanket of
N2. Platinum wire and saturated calomel
electrodes were employed as the counter and the
reference electrodes, respectively. The
electrochemical STM measurements were carried out
at room temperature, with W tips etched in 2 M
NaOH. The tips were coated with Apiezon wax to
minimize residual faradaic currents. A Molecular
Imaging Picoscan equipped with a bi-potentiostat
is used for the in-situ STM experiments.
Figure 13 Interdigitation of adjacent MPz
molecules on gold (111) surface
4.Conclusions Our preliminary results show
interesting assembly of the Pz, MPz and MPc
molecules on Au(111). In the future, improvement
of the STM imaging conditions should allow more
accurate evaluation of molecular dimensions,
domain size/orientation and 2D unit cell of the
adsorbed molecules. Another important aspect of
this work is the investigation of the effects of
electrode potential on molecular packing, domain
size and on orientation of the adsorbed layers as
well as on the redox properties of the adsorbed
layers.
Acknowledgments This work was financially
supported by the Natural Sciences and Engineering
Research Council (NSERC) of Canada, the Canadian
Foundation for Innovation, the Ontario Innovation
Trust and York University. S. M. also
acknowledges the financial support from the
Canada Research Chair Program.
Figure 7 STM image of C40H58N8 adlayer on Au
(111) at 0.15 VSCE. The image was acquired in 0.1
M HClO4. Set point current and tip bias of the W
tip were 1 nA and -0.150 V, respectively.

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Figure 1 phthalocyanine
Figure 2 porphyrazine
Figure 3 octapropylporphyrazine, C40H58N8
Figure 10 40 x 40 nm2 STM image of C24H24N8S8Mg
in 0.1 M HClO4. Tip bias and set point are -0.135
V and 1.450 nA, respectively.
In an attempt to estimate dimensions of the
porphyrazine molecules, STM image was zoomed in
and 10 x 10 nm2 image analyzed (see Fig. 8). Each
Pz molecule can be recognized as a fuzzy spot on
the gold surface. Ideally, we believe that each
Pz molecule should appear with a hole at its
center surrounded by four bright spots
corresponding the aromatic residues. The poor
quality of the image prevent us at this point to
see such details.
Figure 11 Molecular height of C24H24N8S8Mg
molecules above the gold surface.
Figure 5 hexahalo ruthenium(II) phthalocyanine
Figure 4 Magnesium octakis(methylthio)- Porphyraz
ine, C24H24S8N8Mg