Surface patterning with two-dimensional porphyrin supramolecular arrays

Article abstract of DOI:10.1021/ja0531778

Monolayer arrays of a series of meso-tetra-substituted porphyrins containing octadecyloxy and carboxyl (or pyridyl) groups were prepared on the highly oriented pyrolytic graphite surface at the liquid/solid interface. It was found by means of scanning tun

Full text of DOI:10.1021/ja0531778

Published on Web 06/30/2005
Surface Patterning with Two-Dimensional Porphyrin
Supramolecular Arrays
Joe Otsuki,*,† Emi Nagamine,† Tomohide Kondo,† Kosyo Iwasaki,†
Masumi Asakawa,‡ and Koji Miyake§
Contribution from the College of Science and Technology, Nihon UniVersity, 1-8-14 Kanda
Surugadai, Chiyoda-ku, Tokyo 101-8308, Japan, Nanoarchitectonics Research Center (NARC),
National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba Central 5,
1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, and AdVanced Manufacturing Research
Institute (AMRI), AIST, 1-2-1 Namiki, Tsukuba, Ibaraki 305-8564, Japan
Received May 16, 2005; E-mail: otsuki@chem.cst.nihon-u.ac.jp
Abstract: Monolayer arrays of a series of meso-tetra-substituted porphyrins containing octadecyloxy and
carboxyl (or pyridyl) groups were prepared on the highly oriented pyrolytic graphite surface at the liquid/
solid interface. It was found by means of scanning tunneling microscopy that some porphyrins from this
family assemble into various patterns. Specifically, slightly undulated rows are obtained from 5,10,15-tris-
(4-octadecyloxyphenyl)-20-(4-pyridyl)porphyrin. Meanwhile, rows with more pronounced kinks result from
5-(4-carboxyphenyl)-10,15,20-tris(4-octadecyloxyphenyl)porphyrin. The occurrence of the kinks is dependent
on the arrangement of surrounding porphyrin molecules and is determined by intricate interplay between
directional hydrogen-bonding interactions and packing forces, including molecule-molecule and molecule-
substrate interactions. A double-layer structure is obtained from 5,10-bis(4-carboxyphenyl)-15,20-bis(4-
octadecyloxyphenyl)porphyrin, probably through cyclic hydrogen bond formation. This work proves the
concept that programmed surface patterning is possible by using porphyrins incorporating directional
intermolecular interaction sites.
Introduction
few. In recent years, various porphyrin assemblies have been
investigated on the atomically flat surfaces of highly oriented
Supramolecular chemistry has produced a number of increas-
ingly complex, but well-defined, molecule-based assemblies by
exploiting programmed information embedded in molecules for
higher organization.1 The concept of programmed intermolecular
noncovalent forces has been applied mostly in solution and in
crystals. Constructing supramolecules on surfaces, on the other
hand, is an important and challenging theme in view of
connecting molecular assemblies to the macroscopic world for
potential applications in the field of molecular devices. Scanning
tunneling microscopy (STM) provides chemists a unique
opportunity to observe individual molecules in real space, once
the molecules are immobilized on a surface.2-7 Thus, there is
plenty of room8,9 at the surface as well, with the powerful aid
of STM.
pyrolytic graphite (HOPG)10-19 and metals.20-26 The approach
using HOPG is especially attractive for the study of two-
dimensional assemblies, in that the experiments can be done
under ambient conditions, and hence are less costly, and are
amenable to all kinds of molecules if only soluble.2-4 The
assembly can be subjected to STM observations both at a liquid/
solid interface and at an air/solid interface. Porphyrins applied
on HOPG so far include alkylated derivatives10-14 and elaborate
multiporphyrin compounds,15,16 among others.17-19 Introducing
long alkyl chains into a molecule is a versatile approach to
(6) Hecht, S. Angew. Chem., Int. Ed. 2003, 42, 24-26.
(7) Ruben, M. Angew. Chem., Int. Ed. 2005, 44, 1594-1596.
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Society, Caltech, 1959; http://www.its.caltech.edu/∼feynman/
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Langmuir 2004, 20, 5454-5459.
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1418-1419.
Surface assemblies from porphyrins are of special interest
for their ample electronic and photonic properties, to name a
† Nihon University.
‡ NARC, AIST.
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Interface Anal. 2001, 32, 266-270. (b) Qiu, X.; Wang, C.; Zeng, Q.; Xu,
B.; Yin, S.; Wang, H.; Xu, S.; Bai, C. J. Am. Chem. Soc. 2000, 122, 5550-
5556. (c) Lei, S. B.; Wang, J.; Dong, Y. H.; Wang, C.; Wan, L. J.; Bai, C.
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§ AMRI, AIST.
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10.1021/ja0531778 CCC: $30.25 © 2005 American Chemical Society

Two-Dimensional Porphyrin Supramolecular Arrays
A R T I C L E S
rangement in a 4-fold symmetry on HOPG.17 Another porphyrin
derivative with one hydroxyl group, 5-(4-hydroxyphenyl)-10,-
15,20-tris(4-dedecyloxyphenyl)porphyrinatozinc(II), adsorbs on
HOPG, adopting a face-on or an edge-on style, the involvement
of the hydroxyl groups in hydrogen bonding being implied in
both of the assemblies.13 Systematic investigation into the effect
of the positions and numbers of carboxyl groups on the
porphyrin core on the supramolecular structures has only been
done on the Au(111) surface under ultra-high-vacuum environ-
ments at a low temperature of 63 K, using carboxyphenyl- and
3,5-di-tert-butylphenyl-substituted porphyrins.20
Chart 1. Porphyrin Building Blocks for Two-Dimensional Arrays
We have investigated surface assemblies at the liquid/solid
interface on HOPG from meso-tetrasubstituted porphyrins
having both octadecyloxyphenyl and carboxyphenyl groups in
all possible configurations. Also included in the study are
porphyrins with octadecyloxyphenyl and pyridyl groups in all
possible combinations (see Chart 1). We have obtained new
two-dimensional patterns from three of the less symmetrical
porphyrins, i.e., P1R3, C1R3, and c-C2R2, proving the concept
that programmed surface patterning is possible by using
porphyrins incorporating directional intermolecular interaction
sites, as we describe herein.
immobilize the alkylated molecule, due to the tendency of alkyl
moieties to form closely packed arrays on HOPG.2 Thus, 5,-
10,15,20-tetrakis(alkoxyphenyl)porphyrin (e.g. R4, see Chart 1)
forms a linear lamellar array, the alkyl chains from neighboring
rows being interdigitated.10-12
Porphyrins may also be suitable as models in examining the
relationship between the molecular structures and the supramo-
lecular structures, since diverse substituents for intermolecular
interactions can be synthetically introduced to the porphyrin
scaffold in different symmetries in a systematic way. We
reasoned that introducing directional forces such as hydrogen
bonding, especially in less than 4-fold symmetrical ways, might
produce novel patterns on surfaces. Hydrogen bonding has been
used in non-porphyrinoid compounds and found to have a great
impact on the surface assembly structures.3,5
Experimental Procedures
Synthesis. Reactions were carried out in the dark by wrapping the
flask with aluminum foil to protect it from ambient light. Column
chromatography was performed using Kanto Chemical silica gel (SiO2)
60N. Recycle GPC was conducted using CHCl3 as eluent on a Japan
Analytical Industry LC-9201 system equipped with Jaigel-2H and
Jaigel-1H columns in tandem, with exclusion limits of 5000 and 1000,
respectively. 1H NMR spectra were recorded on a JEOL GX400
spectrometer. High-resolution mass spectra (HRMS) of sample solutions
in CHCl3 were taken on an Agilent G1969A system with an atmospheric
pressure chemical ionization (CI) chamber and a time-of-flight mass
spectrometer. The purity of the prepared samples was carefully
1
confirmed with TLC, GPC, and H NMR data.
Methyl Esters of Porphyrins Containing Carboxyphenyl and
Octadecyloxyphenyl Groups. The reaction conditions were taken from
a reported procedure.27 Pyrrole (2.73 g, 40.8 mmol) was added dropwise
to a mixture of methyl 4-formylbenzoate (1.74 g, 10.6 mmol),
4-octadecyloxybenzaldehyde (11.3 g, 30.2 mmol), and Zn(OAc)2‚H2O
(1.89 g, 9.58 mmol) in propionic acid (200 mL), and the solution was
stirred at room temperature for 1 h and then refluxed for 4 h. The solvent
was removed to yield a black solid, which was passed through a SiO2
column. Fractions that emit red fluorescence were collected to afford
a purple solid. This solid, together with pyridine (0.8 mL) and 2,3-
dichloro-5,6-dicyanobenzoquinone (0.81 g, 3.6 mmol), was dissolved
in CH2Cl2 (100 mL), which was refluxed for 1 h. This solution was
washed with 18% hydrochloric acid (40 mL × 3), saturated aqueous
NaHCO3 (40 mL × 3), and then brine (40 mL). Drying on Na2SO4
and removal of solvent afforded an isomeric mixture of porphyrins.
Repeated chromatography (SiO2, CHCl3 to CH2Cl2) afforded pure
isomers.
Trimethyl Ester of C3R1. Rf (SiO2, CHCl3) ) 0.14. 1H NMR
(CDCl3): δ ) -2.80 (2H, s, NH), 0.87 (3H, t, J ) 7 Hz, RCH3), 1.2-
1.5 (28H, m, CH2), 1.63 (2H, quintet, J ) 7 Hz, OCCCH2), 1.99 (2H,
quintet, J ) 7 Hz, OCCH2), 4.11 (9H, s, OCH3), 4.25 (2H, t, J ) 7
Hz, OCH2), 7.28 (2H, d, J ) 8 Hz, ROPhH), 8.10 (2H, d, J ) 8 Hz,
ROPhH), 8.30 (6H, d, J ) 8 Hz, CO2PhH), 8.44 (6H, d, J ) 8 Hz,
CO2PhH), 8.79 (2H, d, J ) 4 Hz, â), 8.81 (4H, s, â), 8.92 (2H, d, J )
4 Hz, â). CI-HRMS: m/z calcd for C68H73N4O7 ([M + H]+),
1057.5473; found, 1057.5440.
There is some precedent for surface assemblies of porphyrins
with hydrogen-bonding capability. The porphyrin with four
carboxyl moieties in a 4-fold symmetry, 5,10,15,20-tetrakis(4-
carboxyphenyl)porphyrin (C4), when coadsorbed with stearic
acid, forms a hydrogen-bonded network, resulting in an ar-
(16) Elemans, J. A. A. W.; Lensen, M. C.; Gerritsen, J. W.; van Kempen, H.;
Speller, S.; Nolte, R. J. M.; Rowan, A. E. AdV. Mater. 2003, 15, 2070-
2073.
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Wan, L. J.; Bai, C. L. J. Phys. Chem. B 2001, 105, 10838-10841.
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Bai, C. J. Phys. Chem. B 2002, 106, 12569-12574.
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S.; Mashiko, S. Nano Lett. 2002, 2, 601-604.
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9
J. AM. CHEM. SOC. VOL. 127, NO. 29, 2005 10401

A R T I C L E S
Otsuki et al.
Dimethyl Ester of c-C2R2. Rf (SiO2, CHCl3) ) 0.48. 1H NMR
(CDCl3): δ ) -2.77 (2H, s, NH), 0.87 (6H, t, J ) 7 Hz, RCH3), 1.2-
1.6 (56H, m, CH2), 1.63 (4H, quintet, J ) 7 Hz, OCCCH2), 1.99 (4H,
quintet, J ) 7 Hz, OCCH2), 4.11 (6H, s, OCH3), 4.25 (4H, t, J ) 7
Hz, OCH2), 7.28 (4H, d, J ) 8 Hz, ROPhH), 8.10 (4H, d, J ) 8 Hz,
ROPhH), 8.30 (4H, d, J ) 8 Hz, CO2PhH), 8.44 (4H, d, J ) 8 Hz,
CO2PhH), 8.77 (2H, d, J ) 5 Hz, â), 8.79 (2H, s, â), 8.89 (2H, s, â),
8.91 (2H, d, J ) 5 Hz, â). CI-HRMS: m/z calcd for C84H107N4O6
([M + H]+), 1267.8185; found, 1267.8172.
Dimethyl Ester of t-C2R2. Rf (SiO2, CHCl3) ) 0.60. 1H NMR
(CDCl3): δ ) -2.78 (2H, s, NH), 0.87 (6H, t, J ) 8 Hz, RCH3), 1.2-
1.5 (56H, m, CH2), 1.62 (4H, quintet, J ) 7 Hz, OCCCH2), 1.98 (4H,
quintet, J ) 7 Hz, OCCH2), 4.11 (6H, s, OCH3), 4.25 (4H, t, J ) 7
Hz, OCH2), 7.28 (4H, d, J ) 8 Hz, ROPhH), 8.09 (4H, d, J ) 8 Hz,
ROPhH), 8.30 (4H, d, J ) 8 Hz, CO2PhH), 8.44 (4H, d, J ) 8 Hz,
CO2PhH), 8.78 (4H, d, J ) 5 Hz, â), 8.94 (4H, d, J ) 5 Hz, â). CI-
HRMS: m/z calcd for C84H107N4O6 ([M + H]+), 1267.8185; found,
1267.8172.
J ) 5 Hz, â). CI-HRMS: m/z calcd for C99H138N4O5 ([M]+),
1465.0773; found, 1465.0777.
Porphyrins Containing Pyridyl and Octadecyloxyphenyl Groups.
The reaction conditions were taken from a reported procedure.29
A
solution of 4-octadecyloxybenzaldehyde (6.18 g, 16.5 mmol) and
4-pyridinaldehyde (5.42 g, 50 mmol) in propionic acid (250 mL) was
stirred at room temperature for 1 h and then heated to reflux. Once the
solution was boiling, pyrrole (4.65 mL, 67 mmol) was added dropwise.
The reaction was stirred at reflux for 1 h and then at room temperature
for 20 h. The residue obtained upon evaporation of the solvent was
partitioned between CHCl3 and H2O, and the organic layer was put
aside. The aqueous layer, which was neutralized with NaHCO3, was
extracted with CHCl3. The residue obtained from the combined organics,
after being dried over Na2SO4, was chromatographed. Each isomer was
isolated through repeated SiO2 column chromatography (CHCl3/MeOH)
and recycle GPC. Final materials were reprecipitated from CH2Cl2 and
MeOH, if necessary, to remove contaminants from SiO2 or solvents.
1
P3R1. Rf (SiO2, CHCl3/MeOH (9/1)) ) 0.54. H NMR (CDCl3): δ
Methyl Ester of C1R3. Rf (SiO2, CHCl3) ) 0.77. 1H NMR
(CDCl3): δ ) -2.76 (2H, s, NH), 0.88 (9H, t, J ) 7 Hz, RCH3), 1.2-
1.5 (84H, m, CH2), 1.63 (6H, quintet, J ) 7 Hz, OCCCH2), 1.99 (6H,
quintet, J ) 8 Hz, OCCH2), 4.11 (6H, s, OCH3), 4.25 (6H, t, J ) 7
Hz, OCH2), 7.28 (6H, d, J ) 8 Hz, ROPhH), 8.10 (6H, d, J ) 8 Hz,
ROPhH), 8.30 (2H, d, J ) 8 Hz, CO2PhH), 8.43 (2H, d, J ) 8 Hz,
CO2PhH), 8.76 (2H, d, J ) 4 Hz, â), 8.88 (4H, s, â), 8.89 (2H, d, J )
4 Hz, â). CI-HRMS: m/z calcd for C100H141N4O5 ([M + H]+),
1479.0929; found, 1479.0933.
) -2.87 (2H, s, NH), 0.87 (3H, t, J ) 7 Hz, CH3), 1.2-1.5 (28H, m,
CH2), 1.63 (2H, quintet, J ) 7 Hz, OCCCH2), 1.99 (2H, quintet, J )
7 Hz, OCCH2), 4.26 (2H, t, J ) 7 Hz, OCH2), 7.30 (2H, d, J ) 8 Hz,
Ph), 8.10 (2H, d, J ) 8 Hz, Ph), 8.16 (6H, dd, J ) 4 Hz, 1.5 Hz, py),
8.82 (2H, d, J ) 4 Hz, â), 8.85 (4H, s, â), 8.97 (2H, d, J ) 4 Hz, â),
9.05 (6H, dd, J ) 4 Hz, 1.5 Hz, py). CI-HRMS: m/z calcd for
C59H64N7O1 ([M + H]+), 886.5094; found, 886.5161.
1
c-P2R2. Rf (SiO2, CHCl3/MeOH (9/1)) ) 0.69. H NMR (CDCl3):
δ ) -2.82 (2H, s, NH), 0.87 (6H, t, J ) 7 Hz, CH3), 1.2-1.5 (56H,
m, CH2), 1.63 (4H, quintet, J ) 7 Hz, OCCCH2), 1.99 (4H, quintet, J
) 7 Hz, OCCH2), 4.26 (4H, t, J ) 7 Hz, OCH2), 7.29 (4H, d, J ) 8
Hz, Ph), 8.10 (4H, d, J ) 8 Hz, Ph), 8.16 (4H, dd, J ) 4 Hz, 1.5 Hz,
py), 8.79 (2H, d, J ) 5 Hz, â), 8.83 (2H, s, â), 8.91 (2H, s, â), 8.94
(2H, d, J ) 5 Hz, â), 9.04 (4H, dd, J ) 4 Hz, 1.5 Hz, py). CI-HRMS:
m/z calcd for C78H101N6O2 ([M + H]+), 1153.7980; found, 1153.7987.
t-P2R2. Rf (SiO2, CHCl3/MeOH (9/1)) ) 0.91. 1H NMR (CDCl3): δ
) -2.83 (2H, s, NH), 0.87 (6H, t, J ) 7 Hz, CH3), 1.2-1.5 (56H, m,
CH2), 1.63 (4H, quintet, J ) 7 Hz, OCCCH2), 1.99 (4H, quintet, J )
7 Hz, OCCH2), 4.26 (4H, t, J ) 7 Hz, OCH2), 7.29 (4H, d, J ) 8 Hz,
Ph), 8.10 (4H, d, J ) 8 Hz, Ph), 8.17 (4H, dd, J ) 4 Hz, 1.5 Hz, py),
8.80 (4H, d, J ) 5 Hz, â), 8.94 (4H, d, J ) 5 Hz, â), 9.04 (4H, dd, J
) 4 Hz, 1.5 Hz, py). CI-HRMS: m/z calcd for C78H101N6O2 ([M +
H]+), 1153.7980; found, 1153.7982.
Porphyrins Containing Carboxyphenyl and Octadecyloxyphenyl
Groups: General Procedure. The hydrolysis followed a reported
procedure.28 The purified methyl ester was suspended in a mixture of
2-propanol and 2 M aqueous KOH, which was refluxed for 6 h. The
reaction, after being allowed to cool to room temperature, was acidified
with concentrated HCl to pH ∼3, and extracted with CHCl3. The organic
layer was washed with saturated aqueous NaHCO3 and dried over Na2-
SO4, and the solvent was evaporated to afford a residue.
C3R1. The residue was reprecipitated from MeOH and H2O. Rf (SiO2,
1
CHCl3/MeOH (9/1)) ) 0.10. H NMR (DMSO-d6): δ ) -2.94 (2H,
s, NH), 0.78 (3H, t, J ) 7 Hz, CH3), 1.2-1.5 (28H, m, CH2), 1.52
(2H, quintet, J ) 7 Hz, OCCCH2), 1.87 (2H, quintet, J ) 7 Hz,
OCCH2), 4.35 (2H, t, J ) 7 Hz, OCH2), 7.34 (2H, d, J ) 8 Hz,
ROPhH), 8.09 (2H, d, J ) 8 Hz, ROPhH), 8.33 (6H, d, J ) 8 Hz,
CO2PhH), 3.38 (6H, d, J ) 8 Hz, CO2PhH), 8.8-8.9 (8H, m, â). CI-
HRMS: m/z calcd for C65H67N4O7 ([M + H]+), 1015.5004; found,
1015.4982.
1
P1R3. Rf (SiO2, CHCl3/MeOH (9/1)) ) 0.97. H NMR (CDCl3): δ
) -2.78 (2H, s, NH), 0.88 (9H, t, J ) 7 Hz, CH3), 1.2-1.5 (84H, m,
CH2), 1.63 (6H, quintet, J ) 7 Hz, OCCCH2), 1.99 (6H, quintet, J )
7 Hz, OCCH2), 4.25 (6H, t, J ) 7 Hz, OCH2), 7.28 (6H, d, J ) 8 Hz,
Ph), 8.10 (6H, d, J ) 8 Hz, Ph), 8.16 (2H, dd, J ) 4 Hz, 1.5 Hz, py),
8.77 (2H, d, J ) 5 Hz, â), 8.89 (4H, s, â), 8.92 (2H, d, J ) 5 Hz, â),
9.02 (2H, dd, J ) 4 Hz, 1.5 Hz, py). CI-HRMS: m/z calcd for
C97H138N5O3 ([M + H]+), 1422.0827; found, 1422.0867.
STM Measurements. o-Dichlorobenzene was purchased from Kanto
Chemical and distilled before use. STM measurements were carried
out with an SII SPI3800N-SPA400 microscope under ambient condi-
tions. STM tips were made mechanically with Pt/Ir (9:1) wire. STM-1
grade HOPG was purchased from GE Advanced Ceramics. The
uppermost layers of HOPG were peeled off with adhesive tape
immediately before use. After the freshly cleaved HOPG was observed
to confirm the atomic resolution of the tip, a droplet of a porphyrin
solution in o-dichlorobenzene was applied on the surface just below
the tip using a syringe. Usually, images could be taken for up to ∼30
min before the solvent evaporated. The concentration values in the text
refer to the initial concentrations. The bias voltage refers to the substrate
voltage with respect to the tip. Estimated errors in unit cell parameters
a and b are (0.2 nm, and that in θ is (3°.
c-C2R2. The residue was chromatographed (SiO2, CHCl3/MeOH (9/
1)) and then reprecipitated from CHCl3 and MeOH. Yield: 45%. Rf
1
(SiO2, CHCl3/MeOH (9/1)) ) 0.32. H NMR (CDCl3): δ ) -2.75
(2H, s, NH), 0.88 (6H, t, J ) 7 Hz, CH3), 1.2-1.7 (60H, m, CH2),
2.00 (4H, quintet, J ) 7 Hz, OCCH2), 4.26 (4H, t, J ) 7 Hz, OCH2),
7.30 (4H, d, J ) 8 Hz, ROPhH), 8.12 (4H, d, J ) 8 Hz, ROPhH), 8.35
(4H, d, J ) 8 Hz, CO2PhH), 8.51 (4H, d, J ) 8 Hz, CO2PhH), 8.80
(2H, s, â), 8.81 (2H, d, J ) 5 Hz, â), 8.91 (2H, s, â), 8.93 (2H, d, J
) 5 Hz, â). CI-HRMS: m/z calcd for C82H103N4O5 ([M + H]+),
1239.7872; found, 1239.7876.
t-C2R2. The residue exhibited too low a solubility in usual solvents
to be purified and characterized properly.
C1R3. The residue was chromatographed (SiO2, CHCl3/MeOH (10/0
to 9/1)). Yield: 95%. Rf (SiO2, CHCl3/MeOH (9/1)) ) 0.63. 1H NMR
(CDCl3): δ ) -2.76 (2H, s, NH), 0.88 (9H, t, J ) 7 Hz, CH3), 1.2-
1.7 (90H, m, CH2), 1.99 (6H, quintet, J ) 7 Hz, OCCH2), 4.26 (6H, t,
J ) 7 Hz, OCH2), 7.28 (6H, d, J ) 8 Hz, ROPhH), 8.10 (6H, d, J )
8 Hz, ROPhH), 8.34 (2H, d, J ) 8 Hz, CO2PhH), 8.49 (2H, d, J ) 8
Hz, CO2PhH), 8.78 (2H, d, J ) 5 Hz, â), 8.89 (4H, s, â), 8.90 (2H, d,
(28) Luo, C.; Guldi, D. M.; Imahori, H.; Tamaki, K.; Sakata, Y. J. Am. Chem.
Soc. 2000, 122, 6535-6551.
(29) Avlasevich, Y. S.; Chevtchouk, T. A.; Knyukshto, V. N.; Kulinkovich, O.
G.; Solovyov, K. N. J. Porphyrins Phthalocyanines 2000, 4, 579-587.
9
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Two-Dimensional Porphyrin Supramolecular Arrays
A R T I C L E S
Figure 1. STM images for P1R3 in o-dichlorobenzene (0.1 mM). (Left)
50 × 50 nm2; tunneling conditions: I ) 98 pA, V ) -0.80 V. (Right) 10
× 10 nm2; tunneling conditions: I ) 98 pA, V ) -1.00 V; a ) 4.2 nm,
b ) 3.9 nm, θ ) 69°, Z (the number of molecules in a unit cell) ) 2, A
(the area of a unit cell) ) 15.3 nm2, Z/A ) 7.6 nm2. The blue dots indicate
pyridyl groups.
Figure 2. STM images for C1R3 in o-dichlorobenzene (0.1 mM). (Left)
50 × 50 nm2; tunneling conditions: I ) 49 pA, V ) -1.15 V. The arrow
indicates a defect. (Right) 0.1 mM; 10 × 10 nm2; tunneling conditions: I
) 20 pA, V ) -1.20 V; a ) 4.6 nm, b ) 4.7 nm, θ ) 82°, Z ) 2, A )
21.4 nm2, Z/A ) 10.7 nm2. Only pattern A (see text) is shown. The yellow
dots indicate carboxyl groups.
Results
responding porphyrin-porphyrin separation in the crystal of 5,-
10,15,20-tetrakis(4-carboxyphenyl)porphyrinatozinc(II) (2.24
nm), in which the porphyrin units are connected with the
hydrogen-bonded carboxyl dimers.32 The close-packed structure
has to be compromised by the hydrogen bond formation, since
the carboxyl dimer formation would result in the appearance
of void space on the surface; the surface occupancy per molecule
increases from 7.6 nm2 for P1R3 to 10.7 nm2 for C1R3. The
decrease in the surface density results in the reduction of
interactions between alkyl chains as well as molecule-substrate
interactions, as will be discussed later.
STM images for a 0.1 mM P1R3 solution in o-dichloroben-
zene on HOPG are shown in Figure 1. The images show rows
similar to those of R4, but with a slight undulation in every
other molecule. A schematic diagram of the most likely
molecular arrangement is superimposed on the image in the right
panel. The squares represent porphyrins, the sides being 13 Å
in length, corresponding to the distance between the cis oxygen
atoms in meso-tetrakis(4-methoxyphenyl)porphyrin, which is
based on the crystal structure.30,31 The rounded bars represent
extended octadecyl chains, 22 Å long and 4.5 Å wide.2 The
dotted corners indicate pyridyl groups. In applying the structural
model, it is assumed that alkyl chains adopt the fully extended
conformation. The pyridyl group cannot participate in hydrogen
bonding without a hydrogen-bond donor. Water molecules,
present under ambient conditions, could participate in hydrogen
bonding with the pyridyl groups. However, there is no evidence
implying the involvement of the pyridyl groups in P1R3 in
hydrogen bonding in any form. The undulation may reduce the
vacant space produced by the removal of alkyl chains from the
array of R4.
The domain size of the array of P1R3 is estimated to be much
larger than 50 × 50 nm2, since a domain boundary was rarely
observed on repeated observation at different positions. We
could obtain images for 0.5 and 1 mM samples, but the
porphyrin images were more diffuse and the reproducibility was
poor. A possible explanation is that more than one layer of
porphyrins are deposited on the surface at the higher concentra-
tions.
C1R3 is similar to P1R3 in terms of shape but with a carboxyl
group that is capable of forming a complementary hydrogen
bond. STM images for C1R3 are shown in Figure 2. There are
kinks every even number, i.e., 2 or 4, or rarely 6, of molecules
on the row of molecules. A modeling on the image reveals that
the carboxyl dimers form at these kinks. The porphyrin
arrangement is shown by the schematics on the image in the
right panel, and the carboxyl dimers are indicated with the pairs
of dots. The center-to-center distance between the pair of C1R3
molecules across the kink measures 2.5 nm. This is in
agreement, within the experimental uncertainty, with the cor-
The competition is apparent from the observation that the
kinks appear every other molecule in some places (pattern A),
where hydrogen bonds dominate, whereas four or more mol-
ecules align linearly in other places (pattern B), where packing
forces outweigh. In the rows of pattern B, there must be dangling
carboxyl groups or at least carboxyl groups not participating in
the optimal complementary hydrogen bond. There is com-
munication between neighboring rows through alkyl-alkyl
interactions; the number of molecules between kinks is the same
as in the adjacent positions in the neighboring rows related by
the a axis, in most of the cases. Interestingly, it was occasionally
observed that patterns A and B coexist side by side, as indicated
by the arrow in the left panel of Figure 2; pattern A is to the
left and pattern B to the right of the arrow, resulting in a point
defect. It is also the case that the size of a domain for the array
of C1R3 is much larger than 50 × 50 nm2.
c-C2R2 assembles into a double-layer structure, as shown in
Figure 3. Each row consists of a pair of porphyrins in a head-
to-head configuration. The same motif of interdigitation of alkyl
10-12
chains as in the case of R4
fits nicely to the arrangement,
as shown in the right panel. The cell parameters of the pseudo-
unit-cell composed of the inner porphyrins (crossed in the right
panel of Figure 3) across the alkyl layer (4.0 nm × 2.1 nm,
82°) are nearly identical with previously reported values for R4
(4.0 nm × 2.0 nm, 77°).10 The value of b (2.1 nm) corresponds
to the porphyrin-porphyrin separation along the row. The
separation between the porphyrins making the head-to-head
association across the row also measures 2.1 nm. This value is
close to the porphyrin-porphyrin separation in C4 (1.9 nm),
for which cyclic hydrogen bond formation is proposed, sup-
ported by molecular mechanics calculations.17 Therefore, it is
(30) Hamor, M. J.; Hamor, T. A.; Hoard, J. L. J. Am. Chem. Soc. 1964, 86,
1938-1942.
(31) Mink, L. M.; Polam, J. R.; Christensen, K. A.; Bruck, M. A.; Walker, F.
A. J. Am. Chem. Soc. 1995, 117, 9329-9339.
(32) Diskin-Posner, Y.; Goldberg, I. Chem. Commun. 1999, 1961-1962.
9
J. AM. CHEM. SOC. VOL. 127, NO. 29, 2005 10403

A R T I C L E S
Otsuki et al.
porphyrins forms on the surface. Incidentally, concentrations
around 0.1 mM were found to be suitable for these porphyrins
to give STM images under the present conditions, that is, at
the o-dichlorobenzene/HOPG interface, in air, at room temper-
ature, and under the atmospheric pressure. It should be noted,
however, that the concentration is being changed under the
microscope as the solvent evaporates.
Second, the adsorbed molecules must have a lateral mobility.
Otherwise, scattered isolated molecules would be observed, since
the molecules will not move from the position of first encounter
with the surface. However, the molecules must be immobilized;
that is, lateral diffusion must be frozen to give an STM image.
This may be accomplished by forming a close-packed structure
covering a large enough area. In practice, the class of porphyrin
compounds in the present study have enough mobility, since
we have never observed a clear image of isolated molecules in
this and related10,11 studies. Therefore, the key to obtain an STM
image is the immobilization.
Figure 3. STM images for c-C2R2 in o-dichlorobenzene (0.06 mM). (Left)
50 × 50 nm2; tunneling conditions: I ) 63 pA, V ) -1.10 V. (Right) 10
× 10 nm2; tunneling conditions: I ) 63 pA, V ) -1.10 V; a ) 5.8 nm,
b ) 2.1 nm, θ ) 82°, Z ) 2, A ) 12.1 nm2, Z/A ) 6.0 nm2. The yellow
dots indicate carboxyl groups. Inner porphyrins are crossed, see the text.
For the pyridyl derivatives studied in this study, we observed
a surface pattern only of P1R3. It seems that at least three
octadecyl chains are required to afford a stable adlayer for STM
observation under the present conditions. The fact that a surface
pattern is observed for c-C2R2 but not for c-P2R2 points to the
importance of hydrogen bonding in stabilizing two-dimensional
structures.
Close-packing of alkyl chains on HOPG is driven primarily
by molecule-substrate interactions (adsorption energy) and
secondarily by molecule-molecule interactions (two-dimen-
sional crystallization energy). According to a molecular me-
chanics calculation by Bai et al.,34 both of the energies are
dominated by the van der Waals forces. The adsorption energies
per CH2 unit were estimated to be ca. -12 and -10 kJ mol-1
for the flat and vertical orientations of the alkyl chain,
respectively, while the crystallization energies were estimated
to be -5 and -6 kJ mol-1, respectively. Another molecular
mechanics calculation carried out by Rabe et al. indicated
somewhat modest values of -7.7 to -8.0 kJ mol-1 for the
adsorption potential per CH2 unit, depending on the orientation
of adsorbed alkane molecules.35
Figure 4. Domain boundary in the array of c-C2R2 in o-dichlorobenzene
(0.06 mM). 25 × 25 nm2; tunneling conditions: I ) 63 pA, V ) -1.10 V.
likely that similar cyclic hydrogen bonds are formed in the spine
of the rows, as represented by the associated four yellow dots
in the right panel of Figure 3.
The domain sizes for the array of c-C2R2 are not much larger
than 50 × 50 nm2, since we occasionally observed domain
boundaries, such as the one shown in Figure 4. It would be
anticipated that adjacent crystalline domains would orient at an
angle of a multiple of 60°, as shown incidentally in the figure,
if the underlying graphite surface determined the orientation of
the molecular array.2 However, there seems to be no definite
correlation between the orientations of the adjacent crystalline
domains, since we have sometimes encountered angles other
than a multiple of 60° as well.
On the other hand, the binding energy in the optimal
hydrogen-bonded dimer of carboxylic acid was estimated as -81
kJ mol-1 20
Let us estimate a gain or loss of energy on going
.
Discussion
from the closer-packed non-hydrogen-bonded P1R3 to the looser-
packed hydrogen-bonded C1R3. These porphyrins have three
octadecyl chains, the adsorption energy amounting to -594 kJ
mol-1, using a value of -11 kJ mol-1 as the adsorption energy
per CH2 unit. The crystallization energies for both porphyrins
are assumed to be the same, since bundles of six alkyl groups
separated by void spaces are formed in both of them, as depicted
in Figures 1 and 2. Thus, the energy loss associated with the
looser packing for C1R3 per unit area (1 nm2) is 23 () 594/7.6
- 594/10.7) kJ mol-1 nm-2. If we use a value of -7.9 kJ mol-1
as the adsorption energy per CH2 unit, the energy loss is 16 kJ
mol-1. Including the adsorption energy for the porphyrin core
would further increase this energy loss. On the other hand, the
To obtain STM images of molecules at the liquid/solid
interface, some mutually conflicting requirements have to be
met. First, an amount of molecules sufficient to cover the whole
area must be adsorbed onto the surface from the solution, under
the equilibrium between solution-phase molecules and adsorbed
molecules. Hence, the concentration in the solution must be high
enough. On the other hand, the concentration must be low to
prevent undefined aggregate formation. In the case of porphyrins
used in the present study, an aggregate may result through ππ
stacking and hydrogen-bonding interactions at high concentra-
tions. Also, there are some reports that show the concentration
affects the morphology of surface assemblies not only at the
molecular scale but at larger scale as well.15,33 Hence, we have
to find a concentration “window” in which a single layer of
(34) Yin, S.; Wang, C.; Qiu, X.; Xu, B.; Bai, C. Surf. Interface Anal. 2001, 32,
248-252.
(35) (a) Hentschke, R.; Schu¨rmann, B. L.; Rabe, J. P. J. Chem. Phys. 1992, 96,
6213-6221. (b) Hentschke, R.; Schu¨rmann, B. L.; Rabe, J. P. J. Chem.
Phys. 1993, 98, 1756-1757.
(33) Milic, T.; Garno, J. C.; Batteas, J. D.; Smeureanu, G.; Drain, C. M.
Langmuir 2004, 20, 3974-3983.
9
10404 J. AM. CHEM. SOC. VOL. 127, NO. 29, 2005

Two-Dimensional Porphyrin Supramolecular Arrays
A R T I C L E S
Conclusions and Prospects
gain in energy associated with the hydrogen bond formation
per the unit area is 3.8 () (81/10.7)/2) kJ mol-1 nm-2. This
estimation, albeit rough, shows that the former outweighs the
latter by a large margin. Nevertheless, the STM observation
showed that C1R3 adopts the hydrogen-bond-directed void-rich,
looser packing structure (pattern A), in competition with the
alkyl-directed closer packing structure (pattern B), suggesting
that these patterns are of comparable stability. This observation
may point to the importance of the role of overlaying solvent
in determining the assembly structure at the liquid/solid
interface. The solvation by the solvent lowers the chemical
potential of the solution-phase molecule, and thus helps the
equilibrium shift to desorption, allowing less dense patterns.
Also, the void space may be filled with the solvent molecules,
although they may be highly mobile. Further, the contribution
by the entropy term has to be taken into account for an exact
depiction of the pattern formation.
In summary, we have demonstrated that specifically substi-
tuted porphyrins can produce a variety of surface patterns. The
structure of the assembly is determined by an intricate interplay
between directional hydrogen-bonding interactions and packing
forces, including molecule-molecule and molecule-substrate
interactions.
To understand fully the principles of surface supramolecular
chemistry, further studies on many factors that influence the
surface structure, including solvent, temperature, molecular
structures, and so on, are needed. From a viewpoint of creating
new structures, there is an immediate interest in what patterns
will result from mixtures of carboxyl and pyridyl porphyrins.
Beyond this, it is envisaged that diverse patterning may be
possible by combining different, specifically programmed
porphyrins.36 Other interactions such as metal coordination5,33,37
may also be utilized. Use of non-porphyrinoid molecules as
structure-directing agents is another possibility. Finally, it may
be possible to extend the assemblies in the direction normal to
the surface by the use of axial coordination to surface-bound
metalloporphyrins.10,38 These projects aiming at controlling
surface assemblies are underway in our laboratories.
The effect of alkyl chains on morphology is evident when
comparing the results in the present work with the adlayers
formed from porphyrins bearing 3,5-di-tert-butyl groups in place
of octadecyloxy groups. Yokoyama et al. reported such struc-
tures of porphyrin assemblies on the surface of Au(111) at 63
K in ultra-high-vacuum STM.20 Their analogue of C1R3, namely,
5-(4-carboxyphenyl)-10,15,20-tris(3,5-di-tert-butylphenyl)por-
phyrin, forms the optimal hydrogen-bonded dimer at low
coverage, where double OH-O hydrogen bonds are involved.
They observed that the integrity of the dimer is maintained even
with increasing coverage. c-C2R2 forms a double-row structure
with a hydrogen-bonding network different from the optimal
hydrogen-bonded dimers, while the aggregation of their ana-
logue of c-C2R2, namely, 5,10-bis(4-carboxyphenyl)-15,20-bis-
(3,5-di-tert-butylphenyl)porphyrin, involves the optimal hydrogen-
bonded dimers to afford a tetramer at a low coverage. At higher
coverages, the tetramers transform into a zigzag pattern due to
a packing effect. However, the optimal hydrogen-bonded dimer
structure is intact also in this case.
Acknowledgment. This paper is dedicated to the 40th
anniversary of Jean-Marie Lehn’s Laboratory. This work was
supported by funding from the Japan Ministry of Education,
Science, Technology, Culture and Sports, Tokyo Ohka Founda-
tion for the Promotion of Science and Technology, Saneyoshi
Scholarship Foundation, and Nihon University.
JA0531778
(36) For three-dimensional crystal engineering of porphyrin, see: Goldberg, I.
Chem. Commun. 2005, 1243-1254.
(37) (a) Drain, C. M.; Nifiatis, F.; Vasenko, A.; Batteas, J. D. Angew. Chem.,
Int. Ed. 1998, 37, 2344-2347. (b) Drain, C. M.; Batteas, J. D.; Flynn, G.
W.; Milic, T.; Chi, N.; Yablon, D. G.; Sommers, H. Proc. Natl. Acad. Sci.
U.S.A. 2002, 99, 6498-6502. (c) Milic, T. N.; Chi, N.; Yablon, D. G.;
Flynn, G. W.; Batteas, J. D.; Drain, C. M. Angew. Chem., Int. Ed. 2002,
41, 2117-2119.
(38) Otsuki, J. Trends Phys. Chem. 2001, 8, 61-72.
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