Face-on and columnar porphyrin assemblies at solid/liquid interface on HOPG

Article abstract of DOI:10.1246/cl.2009.570

Porphyrins having alkyl chains, each of which contains a diacetylene unit, were prepared to examine their self-assembled structures at the liquid/solid interface on graphite. The porphyrins with the longest alkyl chains form a face-on arrangement on the surface, while those with shorter alkyl chains form highly ordered one-dimensional columns. This work demonstrates that introducing a structural perturbation in the alkyl chain is a useful strategy to obtain novel surface assemblies. Copyright

Full text of DOI:10.1246/cl.2009.570

570
Chemistry Letters Vol.38, No.6 (2009)
Face-on and Columnar Porphyrin Assemblies at Solid/Liquid Interface on HOPG
Joe Otsuki,ꢀ Kazuhiko Namiki, Yuki Arai, Makoto Amano, Hirokazu Sawai, Arata Tsukamoto, and Toshiki Hagiwara
College of Science and Technology, Nihon University, 1-8-14 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8308
(Received March 11, 2009; CL-090249; E-mail: otsuki@chem.cst.nihon-u.ac.jp)
Porphyrins having alkyl chains, each of which contains a
Supporting Information10). Image scanning was conducted with
a STM tip, which was mechanically prepared from Pt/Ir wire,
immersed in the solution. The P15 molecules form a surface
pattern with a face-on configuration (Figure 1). The four-leaf
clover-like features appear brightest in the image. Each of these
features is attributed to the tetraphenylporphyrin in the molecule.
The four bright spots may correspond to the region around the
phenyl groups in the molecule. The submolecular resolution sug-
gests that the molecules are tightly immobilized on the surface.
Undulation in contrast is recognized in the intervening area be-
tween the porphyrin moieties. Molecular models superimposed
on the STM image nicely match the four-leaf features and the
undulation in the intervening areas. The kinks introduced in
the alkyl chains by the diacetylene moieties apparently prevent
the alkyl chains from forming a closely packed layer as seen
in the case of porphyrins with simple alkyl chains,5 giving rise
to the new surface pattern with void spaces on the surface.
Unexpectedly, obtained images for P9 were completely
different from those expected for any face-on arrangements
(Figure 2, left). Thin lines with a raised middle align side-by-side
to make a straight striped column. Then the columns align side-
by-side to fill the 2D surface. The stripe pattern is consistent with
stacked porphyrins in an edge-on orientation.8 The interporphy-
rin distance (0.57 nm) is larger than typical ꢀ–ꢀ stacking sepa-
rations (ca. 0.35 nm). This could be either due to steric repulsion
by the phenylene groups, which are twisted from the porphyrin
plane, or by the alkyl chains, each of which occupies a width
of ca. 0.45 nm, or due to some tilt of the porphyrin planes from
the surface normal. The intercolumnar separation of 5.0 nm is
shorter than the length of a stretched-out P9 molecule (ca.
7.0 nm) by 2 nm. There can be two explanations for this observa-
tion. One is that the alkyl chains are in folded conformations.
The other is that the alkyl chains from neighboring columns
may stack vertically, which is allowed because the porphyrin
planes are in an edge-on orientation. The molecular models su-
perimposed on the image show the extent of this overlap, assum-
ing that the alkyl chains are in a fully extended conformation.
diacetylene unit, were prepared to examine their self-assembled
structures at the liquid/solid interface on graphite. The porphy-
rins with the longest alkyl chains form a face-on arrangement on
the surface, while those with shorter alkyl chains form highly
ordered one-dimensional columns. This work demonstrates that
introducing a structural perturbation in the alkyl chain is a useful
strategy to obtain novel surface assemblies.
Liquid/solid interface on highly oriented pyrolytic graphite
(HOPG) provides a unique opportunity that one can observe
molecular arrangements with scanning tunneling microscopy
(STM) at molecular resolution under ambient conditions.1,2 This
in turn gives detailed insights into how molecules assemble
themselves into ordered two-dimensional (2D) patterns. The in-
sights thus obtained will then form the basis of bottom-up fabri-
cation of molecule-based materials and functional surfaces with
controlled molecular arrangements.3 Surface assemblies of por-
phyrins are of particular interest because of their rich optical,
electrical, and catalytic properties. Already, there are a number
of 2D assemblies of porphyrins from simple derivatives to elab-
orate multiporphyrin compounds.4–8 Most of the monomeric
porphyrins studied so far have alkyl chains attached to the por-
phyrin core to gain enthalpy of adsorption onto the HOPG
surface.5–7 These alkylated porphyrins adsorb on HOPG in
close-packed manners to maximize the surface coverage. Thus,
meso-tetrakis(alkoxyphenyl)porphyrins form lamellar arrange-
ments with alkyl chains from neighboring rows being interdigi-
tated, leaving no void spaces on the surface.5 One way to obtain
patterns other than close-packed ones on the surface may be
using directional forces such as hydrogen bonds.6 Yet another
way may be introducing a perturbation on part of the alkyl chain,
a strategy that has rarely been attempted. Herein we report on
the surface assemblies made from porphyrins bearing alkyl
chains, each of which contains a diacetylene unit (Chart 1).
The diacetylene moiety imparts kinks in the middle of the alkyl
chains. In addition, the diacetylene units have a potential for
topochemical 2D polymerization, given that they align properly
on the surface.9
A drop of a 1-phenyloctane solution of porphyrin (0.1 mM)
was placed on a freshly cleaved surface of a HOPG block (see
O
O
OC(CH2)8C
C C C(CH2)nCH3
CH3(CH2)nC
C
C
C(CH2)8CO
N
H
P15: n = 15
P9: n = 9
P4: n = 4
N
N
H
N
O
O
Figure 1. STM image of the array of P15 at the interface of 1-
phenyloctane solution (0.1 mM) and HOPG: V ¼ ꢁ1:1 V,
I ¼ 10 pA. a ¼ 7:0 nm, b ¼ 2:9 nm, ꢁ ¼ 49ꢂ.
OC(CH2)8C
C C C(CH2)nCH3
CH3(CH2)nC
C
C
C(CH2)8CO
Chart 1.
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.6 (2009)
571
interface depending on the length of the alkyl chains as revealed
by STM. This work demonstrates that introducing a structural
perturbation into the alkyl chains is a useful strategy to produce
novel surface patterns. The diacetylene units aligned along the
columns could be used for polymerization,9 which forms the
basis for our ongoing study. Furthermore, the highly ordered
porphyrin columns obtained are very interesting as charge and
exciton transport materials.11
This work was supported by Nihon University Joint Re-
search Grant for 2007 and by the MEXT through the programs
of Grant-in-Aid for Scientific Research on Priority Areas,
‘‘Chemistry on Coordination Space’’ and the High-Tech Re-
search Center Project for Private Universities.
Figure 2. STM images of the array of P9 (left) and P4 (right) at
the interface of 1-phenyloctane solution (0.1 mM) and HOPG.
P9: V ¼ ꢁ1:0 V, I ¼ 20 pA, a ¼ 5:0 nm, b ¼ 0:57 nm, ꢁ ¼
87ꢂ. P4: the brightness scales with residual tunnel current under
near constant height scan,10 V ¼ ꢁ1:1 V, I ¼ 10 pA (average),
a ¼ 6:0 nm, b ¼ 0:57 nm, ꢁ ¼ 67ꢂ.
References and Notes
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The porphyrin with the shortest alkyl chains, P4, also gave a
striped pattern reproducibly (Figure 2, right). In this case, there
are two raised portions, which we attribute to the porphyrin cores
and the diacetylene groups, on the basis of a good fit of mole-
cular models, as indicated in the figure. In this configuration,
the intercolumn separation, a ¼ 6:0 nm, nearly matches the ex-
tended molecular size. Therefore, the alkyl chains from neigh-
boring columns are not overlapped as opposed to the case for
P9. The observation that the acetylene units only on one side
of the porphyrin core are visible might be explained on the basis
of different mobilities. It may be possible that strong adsorption
of the acetylene units on one side of the porphyrin core on the
surface prevents acetylene units on the other side from adsorbing
in an optimal fashion onto the surface due to steric reasons.
Some attempts have been made to address the reasons for the
difference in the surface patterns (i.e., face-on vs. column) de-
pending on the alkyl chain length. The UV–vis spectra for these
porphyrins in solutions identical with those used in the STM
measurements (0.1 mM Pn in 1-phenyloctane) exhibit a sharp
Soret absorption peak for each of the porphyrins at 420 nm, val-
ues identical with those in more dilute solutions, which indicates
that every porphyrin molecule exists as monomeric species in
the solution phase under the conditions of the STM measure-
ments regardless of the length of the alkyl chains. This observa-
tion indicates that the 1D column formation is surface-induced.
All porphyrins exhibited sharp melting behavior mostly
within 1 K and showed no sign of a liquid crystalline phase.
The melting points of the Pn series increase as n decreases
(P15: 67.4–68 ꢂC; P9: 75.2–75.8 ꢂC; P4: 81.6–82.2 ꢂC). This trend
indicates that the association of molecules in the solid state is
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cycle and that the alkyl chains serve to alleviate the interaction.
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alkyl chains may prefer surface adsorption in a face-on fashion,
while the porphyrins with shorter alkyl chains may prefer mole-
cule–molecule association over molecule–surface adsorption,
giving rise to the columnar assembles.
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In conclusion, we have studied surface patterns formed
through self-assembly of porphyrins with alkyl chains each con-
taining a diacetylene unit. These porphyrins afforded either face-
on patterns or columnar structures at the 1-phenyloctane/HOPG
Published on the web (Advance View) May 16, 2009; doi:10.1246/cl.2009.570