Transcript Slide 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
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.
STM imaging of MPcs such as FePc, CoPc, and CuPc have been carefully characterized by Hipps and coworkers [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.
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.
1.Introduction
STM tip
z
Figure 12: Electron
tunneling through
porphyrazines modified
gold single crystal
surfaces (adapted from
Ref. 10).
STM tip
d x2 - y2
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.
d z2
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.
d xz
d yz
d xy
Hole
Hill
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-toend packing of the alkyl side chains. That observation points to interdigitation of adjacent alkyl side chains, as
proposed in Fig. 13.
10 nm
(b)
x
tunneling
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, Pz’s, are structural analogues of phthalocyanines. Pz’s 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 25–250 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 Pz’s 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.
y
tunneling
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).
(a)
0.0
-0.10
nm
2.0
nm
MeS
SMe
N
MeS
SMe
MeS
N N
2+
N
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 singlecrystal 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).
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.
MeS
N
Mg
SMe
N -
N
N
-
N
MeS
MeS
MeS
SMe
N
MeS
SMe
SMe
MeS
N -
SMe
N
N
Mg
2+
N
2+
N
-
MeS
N
N
N
N
N
SMe
Mg
2+
SMe
N
N
Mg
MeS
SMe SMe
MeS MeS
N -
N
N
SMe
N
SMe
N
N
MeS
N
SMe
-
N -
N
N
-
MeS
N
SMe
N
MeS
Figure 9: Molecular height of C40H58N8 molecules above
the gold surface
MeS
MeS
SMe
Mg
N
SMe
MeS
N
N
SMe
N
SMe
N -
N
SMe
N
N
-
SMe
MeS
SMe
MeS
2+
MeS
MeS
MeS
Mg
2+
N
MeS
MeS
SMe
SMe
N -
N
N
N
-
MeS
N
SMe
N
N
MeS
N
SMe
N
Mg
N
-
N
SMe
N
2+
MeS
N
N -
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).
N
SMe
N
N
Mg
2+
SMe
N
N
N
-
MeS
MeS
SMe
SMe
N
MeS
SMe
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.
H
H
H
H
H
H
HH
HH
N
N
N
H
H
HH
N
H
MeS
Figure 2: porphyrazine
SMe
H
H
H
H
H
H
H
H
Figure 3:
octapropylporphyrazine,
C40H58N8
SMe
NN
N
Mg 2+
N
N
N
-
MeS
SMe
N
MeS
SMe
Figure 4: Magnesium
octakis(methylthio)Porphyrazine, C24H24S8N8Mg
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.
References
nm2
Figure 10: 40 x 40
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.
N
MeS
H
H
H
HH
H
H
Figure 1: phthalocyanine
N
H
H
H
H
HH
H
H
H
H
H
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.
H
N
H
H
H
H
Acknowledgments
HH
H
H
H
N
N
H
N
N
N
H
N
N
N
H
H
H
H
H
H
H
H
N
N
H
H
H
H
H
H
H
Figure 5: hexahalo
ruthenium(II)
phthalocyanine
1 Bekaroglu, O., Journal of Porphyrins and Phthalocyanines 2000, 4, 465-473.
2 Rodriguez-Morgade, M. Salome; S., Pavel A. J. Porphyrins. Phthalocyanines 2004, 8, 1129-1165.
3 Lippel, P. H., Wilson, R. J., Miller, M. D., Woll, C., Chiang, S., Phys. Rev. Lett. 1989, 62, 171-174.
4 Miao, P., Robinson, A. W., Palmer, R. E., Kariuki, B. M., and Harris, K. D. M., J. Phys. Chem. B 2000, 104, 1285 – 1291.
5 Barlow, D; Scudiero, L; and Hipps, K, W; Ultramicroscopy 2003, 97, 47-53.
6 Lie, S. B.; Yin, S. X;Wang, N. H; Xi, F; Lui, H. W; Xu, B; Wan, L. J; and Bai, C. L; J. Phys. Chem. B 2001, 105, 10838-10841.
7 Yashimoto, S; Tada, A; Suto, K; Yau, S-L.; and Itaya, K., Langmuir 2004, 20, 3159 – 3165.
8 Yashimoto, S; Tada, A; Suto, K; and Itaya, K; J. Phys. Chem. B 2003, 107, 5836-5843.
Figure 11: Molecular height of
C24H24N8S8Mg molecules above the
gold surface.
9 Scanning Tunneling Spectroscopy, by K W Hipps; a chapter in "Handbook of Applied Solid State Spectroscopy", Ed: Vij, D. R., Kluer
Scientific (2005).
10 Lu, X.; and Hipps, K. W. J. Phys. Chem. B 1997, 101, 5391-5396.