A square cyclic porphyrin dodecamer: Synthesis and single-molecule characterization

Article abstract of DOI:10.1246/cl.2004.578

We prepared a square cyclic porphyrin dodecamer via the tetramerization of a trimer shaped like a right angle. The molecule was visualized by scanning tunneling microscopy to be square.

Full text of DOI:10.1246/cl.2004.578

578
Chemistry Letters Vol.33, No.5 (2004)
A Square Cyclic Porphyrin Dodecamer:
Synthesis and Single-Molecule Characterization
1
1
2
1
3
4
4
Aiko Kato,y Ken-ichi Sugiura,ꢀy ;y Hitoshi Miyasaka,y ;y Hiroyuki Tanaka,y Tomoji Kawai,y
5
1
2
Manabu Sugimoto,y and Masahiro Yamashitaꢀy ;y
1 Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University,
1-1 Minami-Ohsawa, Hachi-Oji, Tokyo 192-0397
y
2 CREST, Japan Science and Technology Corporation, Kawaguchi Center Building, 4-1-8 Honcho,
Kawaguchi, Saitama 332-0012
y
3 PRESTO, Japan Science and Technology Corporation, Kawaguchi Center Building, 4-1-8 Honcho,
Kawaguchi, Saitama 332-0012
y
4 The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047
y
5 Faculty of Engineering, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555
y
(Received February 23, 2004; CL-040199)
reaction of 12 using THF, because the nickel complexes of 3 and
4 have low solubilities, whereas nickel is required to decrease
the aggregation of 1. An important intermediate, porphyrin trim-
er 2 having a right angle, was prepared by the coupling reaction
of 3 with 4. Porphyrin 3 having different halogens at the meso-
positions, bromine and iodine, was prepared by a revised meth-
od.4 Porphyrin 4 functionalized at its 5- and 10-positions was ob-
tained by conventional functional group transformation of the
corresponding diarylporphyrin 8.2a The coupling reaction of a
2:1 mixture of 3 and 4 required strictly controlled conditions us-
ing a combination of AsPh3 and Pd2(dba)3ꢁCHCl3 in a weakly
coordinative solvent, THF, with 16 mM 4 to give 12 in an ac-
ceptable yield. After the replacement of the metals and the intro-
duction of acetylenic groups, the protecting groups were re-
moved to afford 2. The final reaction involved oxidative
coupling. Structurally undefined oligomeric and incomplete di-
meric and trimeric products were separated by gel permeation
chromatography to give dark solid 1 (9%, total yield: 0.007%
in 22 steps).
The results of laser desorption time-of-flight mass spectros-
copy supported the structure of 1, Mþ ¼ 10725:7, calcd
Mþ ¼ 10725:2 for C656H696N48Ni12O48. The absence of the ace-
tylenic C-H stretching mode of 1, e.g., the absorption at
3310 cmꢂ1 observed for 2, also supported the cyclic structure
of 1. However, the most reliable evidence supporting the struc-
ture of 1 was provided by the direct observation of its molecular
shape by ultrahigh-vacuum scanning tunneling microscopy
(UHV-STM) (Figure 1). UHV-STM images of 1 were taken in
its adsorbed state on a Cu (111) surface, prepared using our pulse
injection technique.5 The molecule was recognized as a square-
shaped doughnut with a hole. Each side of the square was ap-
proximately 5-nm long, consistent with the molecular size of 1
estimated by molecular geometry calculation (MM3), 5.1 nm.
The UV–Vis spectrum of 1 is composed of two bands: the
Q-band-like Band I (13500 cmꢂ1, 743 nm, log " ¼ 5:60) and
the Soret-band-like Band II (20500 cmꢂ1, 488 nm, log " ¼
5:77) (in CHCl3). The fact that the relative intensity of Band I
to Band II of 1 is higher than those of structurally similar oligo-
mers may indicate the large oscillator strength of Band I as de-
rived from preliminary theoretical considerations. The excitation
We prepared a square cyclic porphyrin dodecamer via the
tetramerization of a trimer shaped like a right angle. The mole-
cule was visualized by scanning tunneling microscopy to be
square.
The synthesis of porphyrin oligomers is one of the most at-
tractive topics in contemporary interdisciplinary research.1
Many molecules with various shapes have been reported so
far. Cyclic porphyrin oligomers (CPOs) hold a unique position
in this research field.2 Because the cyclic alignment of porphyr-
inoid compounds is employed in harvesting light energy in pho-
tosynthesis,3 the evaluation of structure-function relationship
has emerged as a new research topic. The CPOs are expected
to be used as model compounds for the elucidation of the light
harvesting phenomenon, and as advanced host molecules having
the capability to incorporate large molecules into their nano-
sized holes.2 In this communication, we report the synthesis
and characterization of cyclic porphyrin dodecamer 1 in which
the porphyrins are connected to each other by acetylenes.
Ar
Ar
M1
Ar
Ar
M1
Ar
Ar
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Ar
M2
M2
Ar
N
N
N
N
N
N
N
N
Ar
M1
Ar
Me
Me
M1
Ar
Ar
O
O
Ar =
Me
Me
N
N
N
N
N
N
N
N
M1
Ar
M1
Ar
Ar
Ar
M1 = M2 = Ni
Ar
M1
Ar
Ar
M1
Ar
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
M2
Ar
M2
Ar
Ar
Ar
1
Formula 1. Molecular structures of 1.
of Band I may be due to the exciton coupling between the a2u
eg excitations of each porphyrin moiety.
In conclusion, novel cyclic porphyrin dodecamer 1 was pre-
!
The synthesis of 1 is summarized in Scheme 1. The selection
of the incorporated metal is vital to our chemistry. Zinc is re-
quired to increase the concentrations of 3 and 4 in the coupling
pared and structurally characterized. The molecule is expected to
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.5 (2004)
579
Ar
R1
M1
N
N
N
N
N
N
N
N
R2
R1
Ar
Ar
M1
G
Ar
Ar
M1
Ar
5 (M2 = 2H, R1 = R2 = H)
6 (M2 = 2H, R1 = Br, R2 = H) (59%)
A
B
C
N
N
N
N
N
N
N
N
7 (M2 = 2H, R1 = Br, R2 = I) (73%)
3 (M2 = Zn, R1 = Br, R2 = I) (79%)
M2
Ar
R1
Ar
R3
12 (M1 = M2 = Zn, R1 = Br) (60%)
(from 3 and 4)
13 (M1 = M2 = 2H, R1 = Br) (95%)
14 (M1 = M2 = Ni, R1 = Br) (87%)
15 (M1 = M2 = Ni, R1 = CC-SiMe3) (83%)
2 (M1 = M2 = Ni, R1 = CC-H) (93%)
N
N
N
N
R3
M2
Ar
H
I
J
F
Ar
8 (M2 = 2H, R3 = H)
D
9 (M2 = 2H, R3 = Br) (quant.)
10 (M2 = Zn, R3 = Br) (quant.)
11 (M2 = Zn, R3 = CC-SiMe3) (quant.)
4 (M2 = Zn, R3 = CC-H) (quant.)
C
E
F
K
1 (9%)
Scheme 1. Condition A: 1 equiv. NBS/CH2Cl2-pyridine. Condition B: I2, PhI(CF3CO2)2/CH2Cl2-pyridine. Condition C: Zn(OAc)2ꢁ4H2O/CHCl3. Condi-
tion D: 2 equiv. NBS/CH2Cl2-pyridine. Conditions E: 9 equiv. Me3CꢃCH, Pd(PPh3)4, CuI/THF-Et3N. Conditions F: (n-BuN)þFꢂ/CH2Cl2. Condition G: 3
and 4 (=2:1), Pd2(dba)3ꢁCHCl3, AsPh3/THF-Et3N. Condition H: CF3CO2H/CHCl3-MeOH. Condition I: Ni(acac)2/toluene. Condition J: 40 equiv.
Me3CꢃCH, Pd(PPh3)4, CuI/THF-Et3N. Condition K: 90 equiv. CuCl, TMEDA, Air/CH2Cl2.
(a)
(b)
(c)
Figure 1. (a) Space filling drawing of 1 obtained by MM3 calculation. Isopentyloxy groups on the phenyl groups were removed for clarity. (b) UHV-STM
image (21 ꢄ 21 nm2) on a Cu (111) surface at liquid nitrogen temperature. The imaging conditions are I ¼ 2 pA and Vs ¼ ꢂ1:5 V. (c) Higher resolution
UHV-STM image (7 ꢄ 7 nm2). The imaging conditions are I ¼ 2 pA and Vs ¼ þ0:7 V.
References
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1
2
3
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teristics and its single-molecule nano-host–guest chemistry us-
ing a ca. 2-nm diameter hole of 1.
This work was supported in part by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology, Japan and Japan Science and
Technology Corporation.
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¨
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A. Nakano, H. Shimidzu, and A. Osuka, Tetrahedron Lett., 39, 9489 (1998).
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Published on the web (Advance View) April 19, 2004; DOI 10.1246/cl.2004.578