Observation of co-existence of 'face-on' and 'edge-on' stacking styles in a porphyrin monolayer

Article abstract of DOI:10.1016/j.cplett.2005.01.006

We report a direct observation adopting 'face-on' and 'edge-on' stacking styles in monolayer of a novel porphyrin derivative on highly ordered pyrolytic graphite (HOPG) using scanning tunneling microscope (STM). The porphyrin derivative, 5-hydroxyphenyl-10, 15, 20-trikis(4-dodecyloxyphenyl) porphyrinatozinc(II) (ZnDPPOH) is synthesized and assembled on HOPG. In the STM images, we observed two kinds of ordered domains. One consists of ZnDPPOH dimmers, adopting 'face-on' stacking style, the other exhibits a segmented columnar structure, like piled-up 'armchairs', lying down on the substrate, adopting an 'edge-on' stacking style. In the latter case, the conjugated and highly delocalized π-systems play key roles in stabilizing the monolayer.

Full text of DOI:10.1016/j.cplett.2005.01.006

Chemical Physics Letters 403 (2005) 140–145
www.elsevier.com/locate/cplett
Observation of co-existence of Ôface-onÕ and Ôedge-onÕ stacking
styles in a porphyrin monolayer
*
Yunshen Zhou, Bing Wang, Manzhou Zhu, J.G. Hou
Hefei National Laboratory for Physical Sciences at Microscale, Structure Research Laboratory, University of Science and Technology
of China, 96 Jinzhai Road, Hefei City, Anhui Province 230026, PR China
Received 23 August 2004; in final form 20 December 2004
Available online 12 January 2005
Abstract
We report a direct observation adopting Ôface-onÕ and Ôedge-onÕ stacking styles in monolayer of a novel porphyrin derivative on
highly ordered pyrolytic graphite (HOPG) using scanning tunneling microscope (STM). The porphyrin derivative, 5-hydroxyphenyl-
10, 15, 20-trikis(4-dodecyloxyphenyl)porphyrinatozinc(II) (ZnDPPOH) is synthesized and assembled on HOPG. In the STM
images, we observed two kinds of ordered domains. One consists of ZnDPPOH dimmers, adopting Ôface-onÕ stacking style, the other
exhibits a segmented columnar structure, like piled-up ÔarmchairsÕ, lying down on the substrate, adopting an Ôedge-onÕ stacking style.
In the latter case, the conjugated and highly delocalized p-systems play key roles in stabilizing the monolayer.
ꢀ 2005 Published by Elsevier B.V.
1. Introduction
the porphyrin molecules lie on the surface by the
molecular planes, while in the case of Ôedge-onÕ, the
planes of the porphyrin molecules are tilted from or
perpendicular to the substrate. The Ôedge-onÕ stacking
style optimizes the p-stacking interactions between
the conjugated porphyrin rings and reaches an extend-
edly conjugated and highly delocalized p-system, which
may provide new sight in the design of functional
structures based on porphyrins [14]. The porphyrin
films adopting Ôface-onÕ stacking style have been widely
observed using scanning tunneling microscopy (STM)
[15,16], while the direct observation for porphyrin
monolayers adopting Ôedge-onÕ stacking style is seldom
reported.
Due to the conjugated and highly delocalized p-
bonds, porphyrins demonstrate various chemical and
physical novelties, which make porphyrins important
materials for fabricating functional structures and de-
vices [1–4]. The thin films of the porphyrin derivates
with desired structures and novel properties play impor-
tant roles in molecular architecture and molecular engi-
neering at the surfaces (or the interfaces) [5–7]. It is of
great interest to investigate the spatial stacking and ori-
entation of porphyrin thin films since their structures on
substrates may significantly influence the performance
of the films [2,8,9].
The porphyrin molecules may adopt two possible
stacking styles of Ôface-onÕ and Ôedge-onÕ on a surface
or in an interface [10–13]. In the case of Ôface-onÕ,
Here, we report the synthesis of a novel porphyrin
derivative ZnDPPOH and its assembly on HOPG sur-
faces. Two distinctive types of ordered structures
in ZnDPPOH monolayers adopting Ôface-onÕ and
Ôedge-onÕ stacking styles separately are observed using
STM. The models for the different stacking styles are
illustrated based on the STM observations.
*
Corresponding author. Fax: +86 551 360 2803.
E-mail addresses: bwang@ustc.edu.cn (B. Wang), jghou@ustc.
edu.cn (J.G. Hou).
0009-2614/$ - see front matter ꢀ 2005 Published by Elsevier B.V.
doi:10.1016/j.cplett.2005.01.006

Y. Zhou et al. / Chemical Physics Letters 403 (2005) 140–145
141
2. Experimental
ZnDPPOH solids were dissolved in acetone (spectro-
photometric grade 99+%, ACROS, used as received)
with a concentration of less than 10ꢀ4 M and kept in
dark bottle at room temperature. The monolayers were
prepared by depositing a droplet of the solution (10 ll)
onto fresh cleaved HOPG surface and incubated in
water bath (about 40 ꢁC) for 5 min allowing the solvent
to evaporate. The samples were cooled down slowly to
room temperature. The STM characterization was per-
formed in the constant current mode using a Nano-
scopeIIIa system (Digital Instruments, Santa Barbara,
CA) under ambient conditions. The STM tips are
mechanically cut Pt/Ir (90/10) wires. The scales of the
images have been calibrated according to the atomic re-
solved image of the HOPG.
5-Hydroxyphenyl-10, 15, 20-trikis(4-dodecyloxyphe-
nyl)porphyrinatozinc(II) (ZnDPPOH) was synthesized
following a similar method as reported in literature
[17]. The route of synthesis is depicted in Scheme 1.
Freshly distilled pyrrole (6.70 g, 100 mol) was added
to a solution of 4-(dodecyloxy)benzaldehyde (21.75 g,
75 mmol), 4-hydroxybenzaldehyde (3.05 g, 25 mmol) in
propionic acid (200 ml), and the mixture was refluxed
about 3 h. After most of propanoic acid was removed
by distillation under vacuum, methanol (100 ml) was
added. The black solids were collected by filtration then
dissolved in chloroform–methanol (7:3 v/v 200 ml). Zinc
acetate (15 g, 82 mmol) in methanol (100 ml) was added.
The solution was refluxed about 3 h. After evaporation
of the solvent, the residue was extracted by chloroform.
The organic layer was concentrated and purified by
chromatography on silica gel column (eluent:chloro-
form). The second fraction was collected and concen-
trated, and the purification was repeated several times
using chloroform as eluent to get 0.84 g purple solid
ZnDPPOH with yield 2.7%.
3. Results and discussion
Fig. 1 shows the molecular model of ZnDPPOH.
Each ZnDPPOH molecule contains a central core, phe-
nylporphyrinatozinc(II) (briefed as porphyrin core in
the following text), three peripherally connected dode-
cyloxy chains and one hydroxy group. The dimension
of porphyrin core is estimated to be 1.1 nm, while the
chain length of dodecyloxy chain is estimated to be
about 1.6 nm based on carbon backbones [18], as de-
picted in Fig. 1.
In the STM characterization, two different types of
ordered structures are observed in a same sample, as
shown in Figs. 2 and 3. The typical domain size of each
phase is in the order of 200 nm. Statistically, in the total
number of about 100 checked areas of about 1 · 1 lm2,
the phase shown in Fig. 2, entitled as phase a, is ob-
served in 15 different areas, while the phase shown in
Fig. 3, entitled as phase b, is observed only in three dif-
ferent areas. The two different phases are quite sepa-
rated by disordered areas.
The proton nuclear magnetic resonance (1H NMR)
spectra were recorded in CDCl3 on a Bruker DMX-
400 MHz spectrometer with tetramethylsilane (TMS)
as an external standard. The Fourier transform infrared
(FT-IR) spectra were measured with a Bruker Vector
220 Infrared Spectrometer. Elemental analysis was car-
ried out on a Perkin–Elmer 240C analysis instrument.
1
The H NMR(CDCl3) spectra show peaks of: 8.99–
8.96 (8H, m), 8.12–8.06 (8H, m), 7.28–7.25 (6H, m),
7.20 (2H, d), 5.03 (1H, s), 4.24 (6H, m), 1.99 (6H, m),
1.65–1.27 (54H, m), 0.91 (9H, t). The IR (KBr) measure-
ments are (cmꢀ1) 3438, 2921, 2851, 1605, 1508, 1244,
1171, 998, 797. Elements analysis shows C: 77.24, H:
8.16, N: 4.43, consistent with estimation of
C80H100N4O4Zn for C: 77.05, H: 8.08, N: 4.49.
Scheme 1. Synthesis of ZnDPPOH.

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Y. Zhou et al. / Chemical Physics Letters 403 (2005) 140–145
Fig. 1. Molecular structure and CPK model (Corey–Pauling–Koltun space-filling model) of ZnDPPOH.
Fig. 2. (a) Typical STM image of ZnDPPOH monolayer on HOPG adopting Ôface-onÕ stacking style, taking at a set point condition of the microscope:
900 mV and 50 pA; area: 75 · 75 nm2. (b) Magnified STM image of image-a, area: 24 · 24 nm2. (c) and (d) Line profiles indicated in (b). (e) Illustration
of ZnDPPOH dimmers connected by hydrogen-bonds. (f) Top-view model of the monolayer and corresponding unit cell to the one in (b).
Fig. 2a displays a typical periodic structure of the a
phase. In the magnified image (Fig. 2b), we observe that
the bright spots show a shape of parallelogram with size
of 2.8 0.1 nm · 1.4 0.1 nm, in contrast to the results
reported by other groups that a single porphyrin center
shaped as a square [15,16,19,20]. The shorter lateral of

Y. Zhou et al. / Chemical Physics Letters 403 (2005) 140–145
143
Fig. 3. (a) Typical STM image of ZnDPPOH monolayer on HOPG adopting Ôedge onÕ stacking style, taking at a set point condition of the
microscope: 600 mV, 483 pA, area: 40 · 40 nm2. The lower inset is the Fourier transform spectrum of the image, and the upper inset is the line profile.
(b) Magnified STM image, area: 10 · 10 nm2. (c) Side-view model of the monolayer adopting Ôedge onÕ stacking style, each of the discs representing a
porphyrin core, which tilts from the substrate by an angle of / and the spacing of d. s is the observed spacing from the top view. (d) Suggested model
structure for the porphyrin monolayer of Ôedge-onÕ stacking style. (e) Illustration of Ôoffset stackedÕ arrangement of porphyrin cores and
corresponding electrostatic interactions between p-charge distribution.
1.4 nm of the bright spots accords well with the esti-
mated line length of ZnDPPOH core, as indicated in
Fig. 1, while the longer lateral is double of the shorter
one. So, we suggest that each bright spot observed in
Fig. 2b represents close packed two ZnDPPOH cores.
A unit cell of the ordered structure is plotted in Fig.
2b with lattice constant of a = 3.8 0.1 nm,
b = 3.8 0.1 nm, and h = 106ꢁ 1ꢁ, exhibiting in rhom-
bus. Fig. 2c and d show the line profiles indicated in Fig.
2b. The corrugations along both the directions are about
0.3 nm. Based on the STM images, the structure of the
monolayer shown in Fig. 2b is modeled as depicted in
Fig. 2e and f. Two porphyrin cores are connected by
hydrogen-bonds between peripherally substituted hy-
droxyl group and oxygen atoms in dodecyloxy chains,
forming ZnDPPOH molecule dimmers and appearing
as parallelogram bright spots in STM images (Fig. 2e).
Then, as modeled in Fig. 2f, the dimmers are assembled
into two-dimensionally ordered structure via the van der
Waals interactions between linear alkyl substituents. A
unit cell is also plotted in the model, where the lattice
constants of a0 = 3.8 nm, b0 = 3.8, and h0 = 106ꢁ are as-
sumed, which is in accordance to the spatial stacking
of the molecules.
The images displayed in Fig. 3 exhibit a different
striped structure of the b phase. The lower inset in
Fig. 3a shows the Fourier transform pattern of the
image, which exhibits a periodic structure of the

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Y. Zhou et al. / Chemical Physics Letters 403 (2005) 140–145
monolayer. The upper inset in Fig. 3a shows the line
profile indicated by the line, which gives a corrugation
of about 0.05 nm. The period of the bright stripes is
4.0 0.1 nm. A magnified high-resolution STM image
is shown in Fig. 3b, which shows periodic segments with
a period s = 0.7 0.1 nm along the bright stripes, which
is utterly different from what have been observed in Fig.
2b. The quite small period of 0.7 nm suggests that the
structure of the monolayer observed in Fig. 3 might
adopt Ôedge-onÕ stacking style [21,22], in which the plane
of porphyrin molecule is tilted from the substrate sur-
face. The estimated thickness of porphyrin molecule is
d = 0.34 nm in the case of a typical p–p stacking interac-
tion [21,22]. The difference between the observed period
of s = 0.7 nm and the thickness of porphyrin molecules
of d = 0.34 nm is attributed to the planes of the porphy-
rin cores tilt from the substrate surface normal by angle
/ = 61ꢁ as illustrated in Fig. 3c, where the ZnDPPOH
molecules is suggested to be piled up like armchairs by
the porphyrin cores lying down on the HOPG and form
the so-called Ôsegmented columnar structureÕ [21,22]. A
top view model is depicted in Fig. 3d. The alkyl chains
tilt form the planes of the porphyrin cores by an angle
of c = 107ꢁ 1ꢁ, as depicted in Fig. 3b. In the lying
down ZnDPPOH columns the porphyrin cores are not
completely overlapped but adopt an Ôoffset stackedÕ
arrangement as displayed in Fig. 3e, which effectively
stabilize the p–p stacking arrangements by minimizing
the p-electron repulsion and maximizing the attraction
between the r-framework of one porphyrin core with
its neighboring moleculesÕ p-electrons [23]. In this sys-
tem several major interactions might contribute to the
formation and stability of the film. The first and most
important interaction is p–p stacking interaction be-
tween the conjugated porphyrin cores, which makes par-
allel porphyrin cores an extendedly conjugated and
highly delocalized p-system [14,23]. The second is the
van der Waals forces, which is optimized by Ôedge-onÕ
stacking style for close van der Waals contacts [23].
The third is hydrogen-bonding interactions. Hydrogen-
bonds cause ZnDPPOH molecules to be connected with
each other, which make ZnDPPOH columns more easily
being constructed and effectively stabilized. It has been
reported that in solution phase porphyrins molecules
could aggregate into porphyrin columnar structures sta-
bilized by p–p interactions between large aromatic rings
[21,22,24,25]. So, we propose that ZnDPPOH molecules
are pre-organized in solution phase by hydrogen-bonds
and p–p interaction and produce the columnar oligo-
mers of ZnDPPOH molecules, which make the Ôedge-onÕ
stacking style favorable when assembled on solid sub-
strates. The columnar oligomers would act as seeds for
the growth of corresponding lamellar arrays adopting
Ôedge-onÕ stacking style on the HOPG surface.
For the a phase, due to the relatively strong interactions
between the molecules and the substrate, the ZnDPPOH
molecules prefer the energy comfortable conformation
of the Ôface-onÕ stacking when the dimmers of the por-
phyrin cores form, and then assembled into two-dimen-
sionally ordered structure via the van der Waals
interactions between linear alkyl substituents, similar
to the mechanism described in [15] and [16]. While, for
the b phase, the factors of van der Waals interactions
of alky chains, hydrogen-bonding and p–p stacking
interactions interplay, causing ZnDPPOH molecules tilt
from the HOPG surface and adopt a piled-up-armchair-
like configuration. We suggest that the competition of
the molecule-substrate interactions and the p–p stacking
interactions plays a key role in determination of the dif-
ferent phases. Since the ordered structure of the a phase
is dominant, we think that the molecule-substrate inter-
action is larger than the p–p stacking interaction of mol-
ecules, and thus, the b phase is a metastable phase. In
addition, the hydrogen-bonds and the van der Waals
interactions between linear alkyl substituents presented
in the system provide additional stabilizing effects.
4. Conclusions
We have synthesized a novel porphyrin derivative,
ZnDPPOH. Well-ordered and highly stable monolayers
are constructed by using ZnDPPOH as building units.
Besides the frequently observed structure of Ôface-onÕ
stacking style, a structure of Ôedge-onÕ stacking style is
also observed by STM. In the latter case the ZnDPPOH
molecules adopt a piled-up-armchair-like configuration
and offset stacked packing arrangement. The van der
Waals forces and the hydrogen-bonds, as well as p–p
interactions between the conjugated porphyrin cores,
contribute to the stability of the monolayer. The compe-
tition of the molecule-substrate interactions and the p–p
stacking interactions may play a key role in determina-
tion of the different phases.
Acknowledgements
This work is partially supported by the National Pro-
ject for the Development of Key Fundamental Science
in China (G2001CB3095), by the Natural Science Foun-
dation of China (50121202, 50132030, 10374083).
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