A directly fused tetrameric porphyrin sheet and its anomalous electronic properties that arise from the planar cyclooctatetraene core

Article abstract of DOI:10.1021/ja057812l

Oxidation of a directly meso-meso linked cyclic porphyrin tetramer 2 gave a porphyrin sheet 3. The symmetric square structure of 3 is indicated by its simple ¹H NMR spectrum that exhibits only two signals for the porphyrin β-protons. The absorp

Full text of DOI:10.1021/ja057812l

Published on Web 03/08/2006
A Directly Fused Tetrameric Porphyrin Sheet and Its
Anomalous Electronic Properties That Arise from the Planar
Cyclooctatetraene Core
Yasuyuki Nakamura,† Naoki Aratani,† Hiroshi Shinokubo,† Akihiko Takagi,‡
Tomoji Kawai,‡ Takuya Matsumoto,‡ Zin Seok Yoon,| Deok Yun Kim,|
Tae Kyu Ahn,| Dongho Kim,*,| Atsuya Muranaka,§ Nagao Kobayashi,*,§ and
Atsuhiro Osuka*,†
Contribution from the Department of Chemistry, Graduate School of Science, Kyoto UniVersity,
and CREST, Japan Science and Technology Agency, Sakyo-ku, Kyoto 606-8502, Japan,
The Institute of Scientific and Industrial Research, Osaka UniVersity, and CREST, Japan Science
and Technology Agency, Mihogaoka, Ibaragi, Osaka 567-0047, Japan, Department of Chemistry,
Graduate School of Science, Tohoku UniVersity, Aoba-ku, Sendai 980-8578, Japan, and
Center for Ultrafast Optical Characteristics Control and Department of Chemistry,
Yonsei UniVersity, Seoul 120-749, Korea
Received November 30, 2005; E-mail: osuka@kuchem.kyoto-u.ac.jp; dongho@yonsei.ac.kr; nagaok@mail.tains.tohoku.ac.jp
Abstract: Oxidation of a directly meso-meso linked cyclic porphyrin tetramer 2 gave a porphyrin sheet 3.
1
The symmetric square structure of 3 is indicated by its simple H NMR spectrum that exhibits only two
signals for the porphyrin â-protons. The absorption spectrum of 3 displays characteristic Soret-like broad
bands and weak Q-bands, and its magnetic circular dichroism (MCD) spectrum exhibits a negative Faraday
A term at the 762 nm band as a rare case, indicating the absorption as a transition from a nondegenerate
level to a degenerate level. A slightly longer S1-state (1.1 ps) and smaller TPA cross section (2750 GM)
than a tetrameric linear porphyrin tape also indicate its unique electronic properties. The porphyrin sheet
3 forms stable 1:2 complexes with guest molecules G1 and G2, whose 1H NMR spectra exhibit remarkable
downfield shifts for the guest protons that are located just above the cyclooctatetraene (COT) core of 3,
whereas the imidazolyl protons bound to the zinc(II) porphyrin local cores are observed at slightly upfield
positions. These results have been qualitatively accounted for in terms of the presence of a strong paratropic
ring current around the COT core that propagates through the whole π-electronic network of 3, hence
competing with and canceling the weak diatropic ring currents of the local zinc(II) porphyrins. This explanation
was supported by DFT calculation performed at the GIAO-B3LYP/6-31G* level, which indicated large positive
NICS values within the COT core and small NICS values within the local zinc(II) porphyrins.
Introduction
In the past decade, we have explored various directly meso-
meso linked porphyrin arrays on the basis of silver(I)-promoted
coupling of a 5,15-diaryl substituted zinc(II) porphyrin.2,3 These
porphyrin arrays are promising as an optical molecular wire by
transmitting captured excitation energy over a long distance.4
Porphyrin is one of the most extensively studied macrocyclic
systems, displaying rich coordination chemistry with various
metal ions. Its 18π-conjugated electronic network in a square-
planar circuit causes a strong aromatic diatropic ring current
that is quite useful in host-guest chemistry, particularly for the
identification of organic molecules ligated on the central metal
of porphyrin. It is also known that the electronic nature of
porphyrin is susceptible to peripheral functional modifications,
which have, through introduction of π-conjugated segments at
the porphyrinic periphery, been used for the creation of a variety
of conjugated porphyrins and porphyrin oligomers that exhibit
unique optical and electrochemical properties.1
(2) (a) Osuka, A.; Shimidzu, H. Angew. Chem., Int. Ed. Engl. 1997, 36, 135.
(b) Aratani, N.; Osuka, A.; Kim, Y. H.; Jeong, D. H.; Kim, D. Angew.
Chem., Int. Ed. 2000, 39, 1458. (c) Nakano, A.; Osuka, A.; Yamazaki, I.;
Yamazaki, T.; Nishimura, Y. Angew. Chem., Int. Ed. 1998, 37, 3023. (d)
Nakano, A.; Yamazaki, T.; Nishimura, Y.; Yamazaki, I.; Osuka, A. Chem.-
Eur. J. 2000, 6, 3254. (e) Kim, Y. H.; Jeong, D. H.; Kim, D.; Jeoung, S.
C.; Cho, H. S.; Kim, S. K.; Aratani, N.; Osuka, A. J. Am. Chem. Soc.
2001, 123, 76. (f) Kim, D.; Osuka, A. J. Phys. Chem. A 2003, 107, 8791.
(3) (a) Susumu, K.; Shimidzu, T.; Tanaka, K.; Segawa, H. Tetrahedron Lett.
1996, 37, 8399. (b) Khoury, R. G.; Jaquinod, L.; Smith, K. M. Chem.
Commun. 1997, 1057. (c) Senge, M. O.; Feng, X. Tetrahedron Lett. 1999,
40, 4165. (d) Wojaczynski, J.; Latos-Grazynski, L.; Chmielewski, P. J.;
Van Calcar, P.; Balch, A. L. Inorg. Chem. 1999, 38, 3040. (e) Shi, X.;
Liebeskind, L. S. J. Org. Chem. 2000, 65, 1665. (f) Ogawa, K.; Kobuke,
Y. Angew. Chem., Int. Ed. 2000, 39, 4070. (g) Imahori, H.; Tamaki, K.;
Araki, Y.; Sekiguchi, Y.; Ito, O.; Sakata, Y.; Fukuzumi, S. J. Am. Chem.
Soc. 2002, 124, 5165. (h) Bonifazi, D.; Diederich, F. Chem. Commun. 2002,
2178. (i) Miller, M. A.; Lammi, R. K.; Prathapan, S.; Holten, D.; Lindsey,
J. S. J. Org. Chem. 2000, 65, 6634. (j) Ottonelli, M. Synth. Met. 2003,
138, 173.
† Kyoto University and CREST.
‡ Osaka University and CREST.
§ Tohoku University.
| Yonsei University.
(1) Jaquinod, L. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M.,
Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 1, pp 201-
237.
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J. AM. CHEM. SOC. 2006, 128, 4119-4127
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A R T I C L E S
Nakamura et al.
Scheme 1. Synthesis of 3a
a Conditions: (a) DDQ, Sc(OTf)3, toluene, 55 °C, 77%. Ar ) 3,5-di-tert-butylphenyl.
Actually, the efficient excitation energy transfer (EET) is
achieved over a long distance through meso-meso linked zinc-
(II) porphyrin arrays.4 This function relies on their excitonically
coupled but not fully π-conjugated electronic character. Further
oxidation of singly meso-meso linked porphyrin arrays under
stronger conditions (DDQ and Sc(OTf)3) provided meso-meso,
Figure 1. 1H NMR spectrum of 3 in CDCl3 containing 1% butylamine.
â-â, â-â triply linked porphyrin arrays (porphyrin tapes),5,6
whose Q-like bands, the lowest electronic absorption bands,
exhibit continuous red-shifts, reaching deeply into the infrared
region due to the extensive π-conjugation over the molecules.
One-dimensionally extended π-conjugation causes a significant
perturbation for the molecular orbital of porphyrins that breaks
down the degeneracy of LUMOs. As a consequence, the lowest
energy bands that correspond to formally forbidden Q-bands
transition in the D4h-symmetry zinc(II) porphyrin monomer are
increasingly intensified upon the extension of the arrays.
In this paper, we report the synthesis and characterizations
of a square-planar porphyrin sheet 3, which is, to the best of
our knowledge, the first example of two-dimensionally π-ex-
tended porphyrinic systems caused by symmetric direct fusion
of four porphyrins. The molecule 3 is of great interest, because
it allows for direct comparison of molecular morphology effects,
linear versus square, upon the overall electronic properties of
fused-conjugation of porphyrins. The electronic network of 3
consists of only porphyrins and thus is also interesting from a
viewpoint of its analogy with those of polyacene and graphene
analogues.7 Anomalous electronic properties of 3 that differ
significantly from those of usual porphyrins particularly in
respect of aromaticity will be discussed in relation to the
structural motif of its enforced planar cyclooctatetraene (COT)
core.
pure form from this reaction mixture. In the next attempt, as a
starting substrate, we employed directly meso-meso linked
cyclic porphyrin tetramer 2 that was prepared from 1 through
a stepwise coupling reaction sequence.8 In toluene, 2 was treated
with 30 equiv of DDQ and Sc(OTf)3 at 55 °C for 5 h followed
by separation over a short alumina column to provide 3 as black
solids in 77% yield (Scheme 1). Inherent poor solubility of 3
can be improved by addition of a small amount of butylamine,
which was quite important for its separation, final purification,
1
and H NMR measurements. MALDI-TOF-MS revealed the
parent ion peak of 3 at m/z ) 2977.4 [M+] (calcd for
[C192H184N16Zn4]+ ) 2977.2). In accord with the highly
1
symmetric structure, the H NMR spectrum of 3 in CDCl3
containing 1% butylamine exhibited two singlets at 5.98 and
6.14 ppm for the porphyrin â-protons and a doublet at 7.01
ppm and a triplet at 7.29 ppm for the ortho- and para-protons
of the meso-aryl substituents, respectively (Figure 1). The
peripheral pyrrolic â-protons are observed at higher field as
compared to those of the corresponding singly meso-meso
linked porphyrin arrays2 or linear porphyrin tapes,9 which can
be attributed to an attenuated diatropic ring current in 3.
STM Measurements. Recently, we have succeeded in the
STM detection of the dodecameric porphyrin wheel as discrete
ring images.10 By using the same pulse injection method, a dilute
solution of 3 in CHCl3 was sprayed onto clean flat Cu(100)
surfaces that were obtained by cycles of annealing and Ar-ion
sputtering.11 In-situ STM measurements were performed at room
temperature in ultrahigh vacuum (<10-10 mbar) in a constant
current mode. The STM images of 3 taken at a sample bias
voltage of 1 V and a tunneling current of 3 pA revealed four
spots arranged in a square manner with 3.2 × 3.2 nm2 area, a
peak-to-peak distance of 1.2 nm, and height of 0.21 nm (Figure
2).12 Molecular size estimated from the STM image is roughly
Results and Discussion
Synthesis. Initially, we attempted to prepare porphyrin
tetramer 3 by direct oxidative tetramerization of 5,10-diaryl
zinc(II) porphyrin 1 with DDQ and Sc(OTf)3. Analysis of this
reaction mixture by matrix assisted laser desorption ionization
time-of-flight mass spectroscopy (MALDI-TOF-MS) indicated
the formation of 3, which, however, could not be isolated in a
(4) Aratani, N.; Cho, H. S.; Ahn, T. K.; Cho, S.; Kim, D.; Sumi, H.; Osuka,
A. J. Am. Chem. Soc. 2003, 125, 9668.
(5) (a) Tsuda, A.; Furuta, H.; Osuka, A. Angew. Chem., Int. Ed. 2000, 39,
2549. (b) Tsuda, A.; Furuta, H.; Osuka, A. J. Am. Chem. Soc. 2001, 123,
10304. (c) Tsuda, A.; Osuka, A. Science 2001, 293, 79. (d) Kamo, M.;
Tsuda, A.; Nakamura, Y.; Aratani, N.; Furukawa, K.; Kato, T.; Osuka, A.
Org. Lett. 2003, 5, 2079. (e) Nakamura, Y.; Aratani, N.; Tsuda, A.; Osuka,
A.; Furukawa, K.; Kato, T. J. Porphyrins Phthalocyanines 2003, 7, 264.
(f) Tsuda, A.; Nakamura, Y.; Osuka, A. Chem. Commun. 2003, 1096.
(6) (a) Blake, I. M.; Krivokapic, A.; Katterle, M.; Anderson, H. L. Chem.
Commun. 2002, 1662. (b) Bonifazi, D.; Scholl, M.; Song, F.; Echegoyen,
L.; Accorsi, G.; Armaroli, N.; Diederich, F. Angew. Chem., Int. Ed. 2003,
42, 4966.
(8) Nakamura, Y.; Hwang, I.-W.; Aratani, N.; Ahn, T. K.; Ko, D. M.; Takagi,
A.; Kawai, T.; Matsumoto, T.; Kim, D.; Osuka, A. J. Am. Chem. Soc. 2005,
127, 236.
(9) Ikeue, T.; Aratani, N.; Osuka, A. Isr. J. Chem. 2005, 45, 293.
(10) Peng, X.; Aratani, N.; Takagi, A.; Matsumoto, T.; Kawai, T.; Hwang, I.-
W.; Ahn, T. K.; Kim, D.; Osuka, A. J. Am. Chem. Soc. 2004, 126, 4468.
(11) (a) Tanaka, H.; Nakagawa, T.; Kawai, T. Surf. Sci. 1996, 364, L575. (b)
Takagi, A.; Yanagawa, Y.; Tsuda, A.; Aratani, N.; Matsumoto, T.; Osuka,
A.; Kawai, T. Chem. Commun. 2003, 2986.
(12) The estimated molecular width based on the calculated structure of meso-
phenyl substituted 3 was ca. 2.7 nm.
(7) Watson, M. D.; Fechtenko¨tter, A.; Mu¨llen, K. Chem. ReV. 2001, 101, 1267.
9
4120 J. AM. CHEM. SOC. VOL. 128, NO. 12, 2006

A Directly Fused Tetrameric Porphyrin Sheet
A R T I C L E S
Figure 2. (a) STM images of 3. (b) Height profile along the bar in (a). (c) An enlargement spot and molecular model.
consistent with the calculated structure, and observed bright
spots have been assigned to two neighboring meso-di-tert-
butylphenyl substituents as shown in Figure 2c. This assignment
is in agreement with the previous STM studies on meso-(3,5-
di-tert-butyl)phenyl porphyrins.13
UV-Vis Absorption and Magnetic Circular Dichroism
Spectra. The porphyrin sheet 3 exhibits considerably broadened
absorption bands in the wide range from UV-visible to near-
IR (Figure 3a, bottom), which can be roughly divided into three
distinct spectral regions, Band I (300-600 nm), Band II (600-
1000 nm), and Band III (1000-1500 nm). Band III is consider-
ably weaker as compared to that of one-dimensional tetrameric
porphyrin tape. The magnetic circular dichroism (MCD) spec-
trum of 3 shows weak and intense signals for Bands I and II,
respectively (Figure 3a, top). Because these spectral features
have first derivative shapes, these signals can be assigned as
Faraday A terms arising from transitions from a nondegenerate
level to a degenerate level.14 Notably, a distinct negative Faraday
A term is observed for Band II, that is, positive/negative signs
in ascending energy as a rare case,15 which is in contrast to
positive Faraday A terms that are usually observed for porphyrin
skeletons. The MCD spectral pattern for the Band III region
has been assigned as Faraday B terms, because the spectrum is
similar to those of the absorption spectra, which indicates that
the bands can be assigned as transitions between nondegenerate
states. The absorption spectrum has been simulated using time-
dependent Hartree-Fock theory based on the ZINDO/S Hamil-
tonian (TDHF-ZINDO/S) on meso-phenyl-substituted porphyrin
sheet 3Ph as shown in Figure 3b.16 Two intense absorption bands
are predicted at 479 nm (oscillator strength f ) 2.930) and 640
Figure 3. (a) Experimental MCD (top) and absorption (bottom) spectra of
3 recorded in pyridine-d5 at room temperature. (b) Stick absorption spectrum
of 3Ph calculated by TDHF-ZINDO/S method. (c) Calculated structure of
(13) (a) Gimzewski, J. K.; Jung, T. A.; Cuberes, M. T.; Schlittler, R. R. Surf.
Sci. 1997, 386, 101. (b) Yokoyama, T.; Yokoyama, S.; Kamikado, T.;
Mashiko, S. J. Chem. Phys. 2001, 115, 3814. (c) Moresco, F.; Meyer, G.;
Rieder, K.-H. Phys. ReV. Lett. 2001, 86, 672.
3Ph; top view (left) and side view (right).
(14) (a) Michl, J. J. Am. Chem. Soc. 1978, 100, 6801. (b) Ho¨weler, U.; Downing,
J. W.; Fleischhauer, J.; Michl, J. J. Chem. Soc., Perkin Trans. 2 1998,
1101. (c) Muranaka, A.; Yokoyama, M.; Matsumoto, Y.; Uchiyama, M.;
Tsuda, A.; Osuka, A.; Kobayashi, N. ChemPhysChem 2005, 6, 171.
(15) Recently, negative Faraday A terms for the cyclo[n]pyrrole system have
been reported. Gorski, A.; Ko¨hler, T.; Seidel, D.; Lee, J. T.; Orzanowska,
G.; Sessler, J. L.; Waluk, J. Chem.-Eur. J. 2005, 11, 4179.
nm (f ) 2.377), which can be assigned as the main electronic
transitions of Bands I and II, respectively. The predicted excited
states (Eu) are doubly degenerate, which leads to a prediction
of Faraday A terms in the MCD spectrum. Two dipole-forbidden
transitions (1177 nm (A2g), f ) 0 and 1056 nm (B1g), f ) 0)
and an allowed transition with weak oscillator strength (965
(16) Nguyen, K. A.; Day, P. N.; Pachter, R.; Tretiak, S.; Chernyak, V.; Mukamel,
S. J. Phys. Chem. A 2002, 106, 10285.
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Figure 5. Excited-state dynamics of 3 in toluene containing 1% butylamine.
LUMO (329, b1u) of 3Ph are nondegenerate. The low-energy
Faraday B terms in the Band III region can be readily accounted
for on this basis. The calculated 640 nm band is predicted to
arise from a transition linking a degenerate orbital with a
nondegenerate one (such as 322(323)f329 and 325(326)f330),
whereas the 479 nm band is predicted to arise primarily from
a transition linking a nondegenerate orbital and a degenerate
one (i.e., electron-dominating transition). Negative Faraday A
terms tend to arise from transitions in which orbital angular
momentum (OAM) is greater in the ground state than in the
excited state, while positive Faraday A terms are associated with
transitions where OAM is greater in the excited state. Because
the OAM associated with electron circulation would normally
be anticipated to be greater within a degenerate orbital, the
results obtained from the TDHF-ZINDO/S calculation are
consistent with the positive and negative Faraday A terms
observed for Bands I and II, respectively.
Excited-State Dynamics. With an increase in the number
of porphyrins, linear porphyrin tapes exhibit a gradual red-shift
of the lowest energy absorption band due to a decrease of the
HOMO-LUMO energy gap, leading to a systematic decrease
in the S1-state lifetime from 4.5 ps (dimer) to 0.72 ps
(tetramer).18,19 The transient absorption decay profile of por-
phyrin sheet 3 (λpu ) 605 nm, λpr ) 430 nm) revealed a single-
exponential decay of τ ) 1.1 ps (Figure 5), indicating that the
S1-state lifetime of 3 is remarkably reduced as compared to zinc-
(II) porphyrin monomer (τ ) 2.4 ns) but is slightly longer than
that of the tetrameric porphyrin tape (τ ) 0.72 ps). In addition,
we could not observe any transients in the nanosecond flash
photolysis measurement, indicating inefficient triplet state
formation. Therefore, the major relaxation pathway of the
S1-state of the porphyrin sheet 3 is believed to be an internal
conversion process mainly due to a much reduced HOMO-
LUMO energy gap as compared to zinc(II) porphyrin monomer.
Two-Photon Absorption Cross Section. The two-photon
absorption (TPA) cross section (σ) of 3 was also measured,
because the TPA values of organic molecules reflect the degree
of the extent of π-conjugation of chromophores. Actually, TPA
cross-section values have been determined for the porphyrin
tapes, 11 900 GM for the dimer, 33 100 GM for the trimer, and
93 600 GM for the tetramer at 1200 nm.19 The TPA cross section
of the porphyrin sheet 3 was determined by using the open-
Figure 4. Frontier molecular orbital diagram and contour plots of occupied
and virtual orbitals of 3Ph (TDHF-ZINDO/S). The dotted lines indicate four
frontier molecular orbitals of D4h zinc tetraphenylporphyrin (Zn-TPP).
nm (Eu), f ) 0.016) are predicted in the Band III region. The
broad and weak Faraday B terms observed for Band III can be
associated with these three transitions, because absorption
intensity could arise either from vibronic couplings or from a
reduction of the ideal D4h symmetry.
The origin of the unusual spectral features can be readily
deduced from the orbital degeneracies of the frontier molecular
orbitals (MOs) of the π-system (Figure 4). In the case of
metalloporphyrins, the LUMO is typically orbitally degenerate
(eg) and the HOMOs are typically accidentally near degenerate
(a1u and a2u).17 In contrast, both the HOMO (328, a1u) and the
(17) Gouterman, M. J. Mol. Spectrosc. 1961, 6, 138.
(18) (a) Cho, H. S.; Jeong, D. H.; Cho, S.; Kim, D.; Matsuzaki, Y.; Tanaka, K.;
Tsuda, A.; Osuka, A. J. Am. Chem. Soc. 2002, 124, 14642. (b) Kim, D.
Y.; Ahn, T. K.; Kwon, J. H.; Kim, D.; Ikeue, T.; Aratani, N.; Osuka, A.;
Shigeiwa, M.; Maeda, S. J. Phys. Chem. A 2005, 109, 2996.
(19) Ahn, T. K.; Kim, K. S.; Kim, D. Y.; Noh, S. B.; Aratani, N.; Ikeda, C.;
Osuka, A.; Kim, D. J. Am. Chem. Soc. 2006, 128, 1700.
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A R T I C L E S
diagonally bound on 3 as shown in Scheme 2. The distances
between the coordinating two nitrogen atoms are estimated as
ca. 12.4 and 11.6 Å for G1 and G2, respectively, being adjusted
to the calculated distance between the diagonal zinc(II) ions in
3 (11.9 Å).
Synthetic routes to G1 and G2 are outlined in Scheme 3.
Sonogashira coupling of 1-methyl-2-iodoimidazole with tri-
methylsilylacetylene gave 4 in 75% yield, which was then
deprotected with KOH in methanol to afford 2-ethynyl-1-
methylimidazole (5) in 85% yield. Sonogashira coupling of 5
with 1,4-diiodobenzene provided G1 in 29% yield. Acid-
catalyzed condensation of 2-formyl-1-methylimidazole with
meso-hexyldipyrromethane followed by oxidation with p-
chloranil gave G2 in 8.3% yield.20
Stable complexes of 3 with G1 and G2 were prepared by
addition of excess amounts of the guest molecules to a CHCl3
solution of 3, and the resulting solutions were diluted with
acetonitrile and were carefully evaporated to induce precipitation
of the corresponding pure complexes. The 1H NMR spectra of
these complexes revealed that the stoichiometry of 3 to guest
was 1:2, thus indicating the formation of 3-(G1)2 and 3-(G2)2
complexes (Scheme 2).21 The parent ion peaks were successfully
detected by high-resolution electrospray ionization mass spec-
troscopy (HR-ESI-MS) in a positive mode at m/z ) 1774.7307
for [3-(G1)2]2+ (calcd for [C228H212N24Zn4]2+ ) 1774.7245) and
2126.9865 for [3-(G2)2]2+ (calcd for [C272H276N32Zn4]2+
)
2126.9881), respectively (Figure 7). Curiously, the complex
1
3-(G1)2 displays a clear H NMR spectrum, in which the 1,4-
phenylene protons (Hd) appear at 10.95 ppm, being 3.78 ppm
downfield shifted from its original chemical shift, whereas the
imidazolyl protons (Ha, Hb, and Hc) exhibit reverse upfield shifts
(Figure 8). The complexation-induced shifts (CIS), which are
defined as the difference of chemical shifts of signals of bound
guests and free guests (CIS ) δbound - δfree), are summarized
Figure 6. The open aperture Z-scan traces (circle) of (a) 3 and (b) fused
dimer in toluene containing 1% butylamine. The sample concentrations for
(a) 3 and (b) fused dimer are 0.039 and 0.165 mM, respectively. The peak
irradiance at the focal point is 30 GW/cm2. The solid lines are the best
fitted curves of experimental data. (c) Molecular structure of the fused dimer
(Ar1 ) p-nonylphenyl, Ar2 ) phenyl).
1
in Chart 1. The H NMR spectrum of 3-(G2)2 is relatively
simple, exhibiting sharp signals, but some of the peaks (He and
the aryl protons of 3, and the â-protons of 3 and solvent protons)
are severely overlapped. To distinguish each proton signal in
the complex 3-(G2)2, the 1H NMR spectrum has been examined
at various temperatures (Supporting Information). The assign-
aperture Z-scan method with 130 fs pulses at 1600 nm where
its linear absorption is almost negligible. Despite its tetrameric
porphyrin constitution, the porphyrin sheet 3 exhibits a small
TPA value of 2750 GM (Figure 6a), which is only ca. one-
fourth of the value of the corresponding dimeric porphyrin tape
(Figure 6b,c), suggesting that the electronic delocalization in 3
is quite unique, being different from those of normal porphyrins.
1H NMR Studies on Supramolecular Assemblies. As
described above, the conjugated π-electronic system of 3 is
significantly altered from those of normal porphyrins, as high-
lighted by the nondegenerate HOMO and LUMO, the negative
Faraday A terms for Band II, the slightly longer S1-state lifetime,
and the relatively small TPA value. An important structural
feature of 3 is the presence of the formal planar π-conjugated
COT core, which is a typical antiaromatic segment. Therefore,
the influences of the planar COT moiety on the overall electronic
properties of 3 are of great interest. Key questions are (1) does
the COT segment really cause any paratropic ring current in
3 and (2) is the diatropic ring current in each constitutional
zinc(II) porphyrin preserved or significantly perturbed by the
planar COT segment? We thus examined the magnetic properties
of 3 through host-guest interactions with 1,4-bis(1-methylimi-
dazol-2-ylethynyl)benzene (G1) and 5,15-bis(1-methylimidazol-
2-yl)-10,20-dihexylporphyrin (G2), which were designed to be
1
ment of the H NMR signals of the complexes relies on the
comprehensive 2D-COSY and 2D-NOESY spectra as well as
proton-deuterium exchange experiments by addition of D2O.
The latter experiments allowed for the assignment of the
porphyrin inner NH protons of G2. Interestingly, the porphyrin
inner NH protons show the largest downfield shift (CIS ) +5.29
ppm), whereas the imidazolyl protons (Ha, Hb, and Hc) and the
side-chain hexyl protons (Hg and Hh) exhibit more or less upfield
shifts (Chart 1). These observations revealed a strong deshielding
magnetic effect of the central COT core but a weak diatropic
ring current for the local zinc(II) porphyrin subunits.
DFT Calculation. To understand the anomalous shielding
and deshielding magnetic effects of 3, theoretical calculations
were performed for the meso-unsubstituted porphyrin sheet 3H
and its free-base form 3HFB, which gave respective energy
minimized structures and nucleus-independent chemical shifts
(NICS).22 Structures were optimized with Becke’s three-
(20) Ikeda, C.; Fujiwara, E.; Satake, A.; Kobuke, Y. Chem. Commun. 2003,
616.
(21) Screen, T. E. O.; Thorne, J. R. G.; Denning, R. G.; Bucknall, D. G.;
Anderson, H. L. J. Am. Chem. Soc. 2002, 124, 9712.
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A R T I C L E S
Nakamura et al.
Scheme 2. Schematic Representation of Complexation of 3 with Two Bisimidazolyl Guest Molecules
Scheme 3. Syntheses of G1 and G2a
Chart 1. Molecular Structures and CIS (ppm) of Proton Signals of
(a) G1 and (b) G2 at Room Temperature, and Expected Structures
of (c) 3-(G1)2 Complex, and (d) 3-(G2)2 Complexa
a Conditions: (a) Pd(PPh3)2Cl2, CuI, Et3N, THF, 75%; (b) KOH, MeOH,
85%; (c) Pd(PPh3)2Cl2, CuI, Et3N, THF, 29%; (d) (i) TFA, CHCl3, (ii)
p-chloranil, then Et3N, 8.3%.
a
In (c) and (d), bold line drawings are on 3 and gray line drawings are
back of 3.
Figure 7. Observed and calculated ESI mass spectra of (a) 3-(G1)2 and
(b) 3-(G2)2.
Figure 8. 1H NMR spectra of 3-(G1)2 in toluene-d8. (a) Low-field region
and (b) high-field region. Open and filled circles indicate the signals of 3
and G1 in the complex, respectively.
parameter hybrid exchange functional and the Lee-Yang-Parr
correlation functional (B3LYP), employing the LANL2DZ basis
set for zinc and 6-31G* for the rest of the atoms (denoted as
631LAN). The optimized structures have revealed that both 3H
and 3HFB take perfectly square-planar conformations, in which
the bond length alternation (∆R) in the COT moiety is small,
(22) (a) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; Hommes, N. J.
R. v. E. J. Am. Chem. Soc. 1996, 118, 6317. (b) Chen, Z.; Wannere, C. S.;
Corminboeuf, C.; Puchta, R.; Schleyer, P. v. R. Chem. ReV. 2005, 105,
3842.
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A Directly Fused Tetrameric Porphyrin Sheet
A R T I C L E S
Figure 9. NICS values (ppm) of (a) 3HFB, (b) 3H, (c) porphine, and (d) magnesium(II) porphine. Point A was defined as the center of the porphyrin internal
cross, and points B and C were defined as the mean points of two adjacent CN bonds.23
∆R ) 0.052 and 0.039 Å for 3H and 3HFB, respectively, because
the bond length of C-C bonds between the pyrroles and the
bond length of C-C bonds in the pyrroles are similar, being
1.392 and 1.444 Å for 3H and 1.401 and 1.440 Å for 3HFB,
respectively. On the basis of these energy-minimized structures,
NICS values have been calculated at the GIAO-B3LYP/631LAN
level for 3HFB and 3H as shown in Figure 9. Inside the
porphyrin, the NICS values have been calculated to be + 1.7,
+1.7, and +2.1 for points A, B, and C in Figure 9a, suggesting
the aromatic character of porphyrin is substantially weakened
and almost lost in 3HFB.23 Because the NICS calculation in the
center of porphyrin units in 3H is impossible due to the presence
of zinc(II) ion, the values at points B and C have been calculated
to be -1.1 and +2.8, respectively (Figure 9b). These values
are exceptionally small as compared to those calculated for
porphine and magnesium(II) porphine (Figure 9c,d),23 also
suggesting the disappearance of the aromatic character for the
porphyrin ring in 3H. On the other hand, the NICS values of
the center of the COT core have been calculated to be +21.7
for 3HFB and +61.7 for 3H, which are comparable to or larger
than a NICS value (+29.2) calculated for a hypothetical D4h
cyclooctatetraene that is a typical antiaromatic species.22a,24,25
These calculations indicate that there is a strong paratropic ring
current along the COT core that propagates to the whole
π-network of 3, affecting the local 18π aromatic porphyrin
systems, whereas the four zinc(II) porphyrin subunits still
preserve weak diatropic ring currents. As a consequence of the
two opposite effects, small upfield shifts are observed for the
imidazolyl protons coordinating to the zinc(II) centers.
currents are preserved at each local zinc(II) porphyrin. These
anomalous properties are likely arising from the COT seg-
ment, which is enforced to be planar by the direct fusion of
four zinc(II) porphyrins. As such, conjugation of a strongly
antiaromatic segment with porphyrin may be a general and
effective way to alter the electronic properties of porphyrin,
which will lead to creation of novel porphyrinoid. Along this
line, our efforts are in progress toward exploration of novel
porphyrinic molecules.
Experimental Section
General Information. All reagents and solvents were of commercial
reagent grade and were used without further purification except where
noted. Dry toluene and Et3N were obtained by distilling over CaH2.
Dry THF was obtained by distilling over sodium benzophenone ketyl.
1H NMR spectra were recorded on a JEOL ECA-delta-600 spectrometer,
and chemical shifts were reported as the delta scale in ppm relative to
CHCl3 (δ ) 7.26 ppm) or methyl protons of toluene (δ ) 2.10 ppm).
Mass spectra were recorded on a Shimadzu/KRATOS KOMPACT
MALDI 4 spectrometer, using the positive-MALDI ionization method
with 9-nitroanthracene (9NA) matrix, or on a BRUCKER microTOF
using positive-ion mode, and CH3CN as a solvent.
Synthesis. Porphyrin Sheet 3. To a solution of 2 (1.53 mg, 0.51
µmol) in dry toluene (3 mL) were added DDQ (3.5 mg, 1.5 µmol) and
Sc(OTf)3 (10.3 mg, 1.5 µmol), and the resulting mixture was stirred
for 5 h at 55 °C with shielding from light under Ar atmosphere. The
reaction mixture was diluted with THF and was passed through an
alumina column with THF. After evaporation of the solvent, the residue
was dissolved in THF, poured on alumina column, and washed with
THF to remove impurities. A purple fraction that still remained on the
alumina column was then eluted with toluene and butylamine alterna-
tively. This procedure was repeated twice or three times. The combined
eluant was evaporated, and the residue was recrystallized from
CH2Cl2/CH3CN to afford black solids of 3‚4(butylamine) (1.28 mg,
0.39 µmol, 77%). 1H NMR (CDCl3) δ 7.29 (t, J ) 1.6 Hz, 8H, Ar-p),
7.01 (d, J ) 1.9 Hz, 16H, Ar-o), 6.14 (s, 8H, â), 5.98 (s, 8H, â), and
Summary
The directly fused tetrameric porphyrin sheet 3 that was
synthesized from the directly linked porphyrin ring 2 exhibits
unusual properties including the broad absorption spectrum,
the negative Faraday A terms for Band II, the slightly longer
S1-state lifetime, and the relatively small TPA value. The
calculations on TDHF-ZINDO/S revealed their nondegenerate
HOMO and LUMO orbitals. A strong paratropic ring current
was detected above the COT core, while weak diatropic ring
t
1.22 (s, 144H, Bu) ppm; MALDI-TOF MS m/z 2977.4, calcd for
C192H184N16Zn4 2977.2; UV-vis (CHCl3/5% n-BuNH2) λmax(ꢀ) ) 521
(96 000), 639 (42 000), 756 (77 000), and 1256 (9700) nm.
2-Trimethylsilylethynyl-1-methylimidazole (4). To a solution of
2-iodo-1-methylimidazole (1.0 g, 4.8 mmol), Pd(PPh3)2Cl2 (336 mg,
0.48 mmol), and CuI (91 mg, 0.48 mmol) in dry THF (3 mL) and
Et3N (13 mL) was added a solution of trimethylsilylacetylene (2.4 g,
24 mmol) in dry THF (2 mL). The solution was deoxygenated via
freeze-pump-thaw cycles and was stirred at 35 °C under Ar
atmosphere. After 9 h, the resulting mixture was filtrated and
evaporated. The residue was extracted with ethyl acetate and washed
with water. The combined organic layer was dried over Na2SO4 and
evaporated. Silica gel column chromatography (ethyl acetate:hexane,
(23) Cyran˜ski, M. K.; Krygowski, T. M.; Wisiorowski, M.; Hommes, N. J. R.
v. E.; Schleyer, P. v. R. Angew. Chem., Int. Ed. 1998, 37, 177.
(24) About planar COT and its derivatives: (a) Kla¨rner, F.-G. Angew. Chem.,
Int. Ed. 2001, 40, 3977 and references therein. (b) Matsuura, A.; Komatsu,
K. J. Am. Chem. Soc. 2001, 123, 1768. (c) Fowler, P. W.; Havenith, R. W.
A.; Jenneskens, L. W.; Soncini, A.; Steiner, E. Angew. Chem., Int. Ed.
2002, 41, 1558.
(25) A large difference between the calculated NICS vales for the COT center
of 3H and 3HFB is difficult to explain but may be ascribed due to symmetry
difference.
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A R T I C L E S
Nakamura et al.
2:1) of the residue gave 4 as a pale yellow oil (640 mg, 3.6 mmol,
75%). 1H NMR (CDCl3) δ 7.00 (s, 1H, Im), 6.86 (s, 1H, Im), 3.70 (s,
3H, Me), and 0.25 (s, 9H, TMS) ppm.
was pumped by a Q-switched Nd:YAG laser (Clark MXR model ORC-
1000), a pulse stretcher/compressor, an optical parametric amplifier
(Clark MXR OPA), and an optical detection system. A femtosecond
Ti:sapphire oscillator pumped by a cw Nd:YVO4 laser (Coherent, Verdi)
produces a train of ∼80-fs mode-locked pulses with an averaged power
of 650 mW at 800 nm. The amplified output beam regenerated by
chirped pulse amplification (CPA) had a pulse width of ca. 150 fs and
a power of ca. 1 W at a repetition rate of 1 kHz, which was divided
into two parts by a 1:1 beam splitter. One part was color-tuned for the
pump beam by an optical parametric generation and amplification
(OPG-OPA). The resulting laser pulse had a temporal width of ∼150
fs in the vis/IR range. The pump beam was focused to a spot diameter
of ∼1 mm, and the laser fluence was adjusted, using a variable neutral-
density filter. The other part was focused onto a flowing water cell to
generate a white-light continuum, which was again split into two parts.
One part of the white-light continuum was overlapped with the pump
beam at the sample to probe the transient, while the other part of the
white-light continuum was passed through the sample without overlap-
ping the pump beam. The time delay between pump and probe beams
was controlled by making the pump beam travel along a variable optical
delay line. The white-light continuum beams after the sample were
sent through an appropriate interference filter and then were detected
by two photodiodes. The outputs from the two photodiodes at the
selected wavelength were processed by a combination of a boxcar
averager and a lock-in amplifier, to calculate the absorption difference
at the desired time delay between pump and probe pulses. Two-photon
absorption cross sections were measured by an open-aperture Z-scan
method using the setup reported elsewhere.18b
2-Ethynyl-1-methylimidazole (5). To a solution of 2-trimethyl-
silylethynyl-1-methylimidazole (4) (300 mg, 1.7 mmol) in MeOH (15
mL) was added a small portion of aqueous KOH solution. After being
stirred for 30 min at room temperature, the solution was diluted with
water and was extracted with ethyl acetate. The combined organic layer
was dried over Na2SO4 and evaporated. Silica gel column chromatog-
raphy (ethyl acetate:hexane, 3:1) gave 7 as a pale yellow oil (152 mg,
1
1.4 mmol, 85%). H NMR (CDCl3) δ 7.04 (s, 1H, Im), 6.90 (s, 1H,
Im), 3,74 (s, 3H, Me), and 3.31 (s, 1H, CtCH) ppm.
1,4-Bis(1-methylimidazol-2-ylethynyl)benzene (G1). To a mixture
of 1,4-diiodobenzene (40 mg, 0.12 mmol), Pd(PPh3)2Cl2 (25 mg, 0.031
mmol), and CuI (6.9 mg, 0.031 mmol) was added a solution of 5 (32
mg, 0.31 mmol) in dry Et3N (1 mL) and dry THF (1 mL). The solution
was deoxygenated via freeze-pump-thaw cycles and was stirred at
room temperature under Ar atmosphere. After 1 day, the resulting
mixture was filtrated and evaporated. The residue was extracted with
ethyl acetate and washed with water. The combined organic layer was
dried over Na2SO4 and evaporated. Silica gel column chromatography
(ethyl acetate) of the residue gave G1 as yellow brown solids (10.1
mg, 0.035 mmol, 29%). 1H NMR (CDCl3) δ 7.55 (s, 4H, Ph), 7.11 (s,
2H, Im), 6.96 (s, 2H, Im), and 3.80 (s, 6H, Me) ppm; (toluene-d8) δ
7.17 (s, 4H, Ph), 7.12 (s, 2H, Im), 6.23 (s, 2H, Im), and 2.91 (s, 6H,
Me) ppm. ESI-MS m/z 287.120 [M + H]+; calcd for C18H15N4+, m/z
287.129.
5,15-Bis(1-methylimidozol-2-yl)-10,20-dihexylporphyrin (G2).
2-Formyl-1-methylimidazole (200 mg, 1.8 mmol), meso-hexyldipyr-
romethane (418 mg, 1.8 mmol), and NaCl (53 mg, 0.9 mmol) were
dissolved in CHCl3 (189 mL). After the solution was purged with N2
for 15 min, TFA (0.2 mL, 2.7 mmol) was added, and the resulting
solution was stirred at room temperature with shielding from light under
nitrogen atmosphere. After 6 h, p-chloranil (664 mg, 2.7 mmol) was
added to the reaction mixture, and the solution was stirred for 12 h.
The reaction mixture was then neutralized with triethylamine, and
the solvent was evaporated. The residue was dissolved in CHCl3 and
passed through a silica gel column with CHCl3 to CHCl3/acetone
(10:3) as eluents. This procedure was repeated three times to give pure
G2 as red-purple solids (48 mg, 0.075 mmol, 8.3%). 1H NMR (toluene-
d8) δ 9.26, 9.23 (each d, J ) 4.8 Hz, 4H, Por-â), 8.83, 8.76 (each d,
J ) 4.6 Hz, 4H, Por-â), 7.74, 7.67 (each s, 2H, Im-H4), 6.94, 6.88
(each s, 2H, Im-H5), 4.77, 4.73 (each m, 4H, Hex-R), 2.83, 2.80 (each
s, 6H, Im-N-Me), 2.48, 2.43 (each m, 4H, Hex-â), 1.68 (m, 4H × 2,
Hex-γ), 1.42 (m, 4H × 2, Hex-δ), 1.36 (m, 4H × 2, Hex-ꢀ), 0.95 (m,
6H × 2, Hex-Me), -2.40, and -2.45 (each br, 2H, Por-NH) ppm;
HR-ESI-MS m/z 639.3924 [M + H]+; calcd for C40H47N8+, m/z
639.3918.
STM Measurements. Clean flat Cu(100) surfaces were obtained
by Ar+-ion sputtering and annealing (580 °C) cycles for a substrate.
The porphyrin sheet molecules dissolved into CHCl3 were deposited
by spraying ca. 0.5 µL of the solution onto the substrate in a vacuum
(10-6 mbar) using a pulse injection method, which is suited for
deposition of large fragile molecules with escaping decomposition often
encountered in sample deposition from the gas phase. In-situ STM
measurements were performed at room temperature in ultrahigh vacuum
(<10-10 mbar) with a home-built STM by using an electrochemical
etched Pt/Ir tip. STM image was obtained in constant height mode.
Steady-State UV-Vis Absorption Spectroscopy. The samples were
prepared in micromolar concentrations in CHCl3 (spectroscopic grade).
Absorption spectra were obtained with a Shimadzu UV-3100PC
spectrometer at room temperature.
Magnetic Circular Dichroism (MCD) Spectroscopy. The MCD
spectrum was recorded in the UV-vis-near-IR region (300-900 nm)
with a JASCO J-725 spectrodichrometer equipped with a JASCO
electromagnet that produces parallel and antiparallel magnetic fields
of up to 1.09 T, and in the near-IR region (700-2000 nm) with a
JASCO J-730 spectrodichrometer equipped with a JASCO electromag-
net that produces magnetic fields of up to 1.5 T. Pyridine-d5 was used
as solvent to eliminate solvent absorption bands in the near-IR region.
The MCD spectrum was combined in a region where there were no
intense absorption bands. The magnitude of the MCD signal is
expressed in terms of molar ellipticity per tesla [θ]M deg mol-1 dm3
cm-1 T-1
.
Computational Details. All calculations were carried out using the
Gaussian 03 program.26 All structures were optimized with Becke’s
three-parameter hybrid exchange functional and the Lee-Yang-Parr
correlation functional (B3LYP)27 without symmetry restriction, employ-
ing the LANL2DZ basis set for zinc atom, and 6-31G* basis set for
the other atoms (denoted as 631LAN). The NICS values were obtained
with the GIAO method at B3LYP/631LAN level. The ring centers were
designated at the nonweighted means of the carbon and nitrogen
coordinates on the conjugate pathways. The ground-state structure of
3 with D4h symmetry was optimized at the level of B3LYP/6-31G*,
where the tert-butyl groups were omitted and replaced with hydrogen
atoms. With this optimized structure, the 40 excitation energies and
oscillator strengths were calculated using time-dependent Hartree-Fock
theory (TDHF) with ZINDO/S Hamiltonian.
Acknowledgment. The work at Kyoto was partly supported
by a Grant-in-Aid for Scientific Research on Priority Area (No.
16033231, Reaction Control of Dynamic Complexes) from the
Ministry of Education, Culture, Sports, Science, and Technol-
ogy, Japan, and 21st Century COE on Kyoto University Alliance
for Chemistry. The work at Tohoku was supported by the COE
project, Giant Molecules and Complex Systems, 2005, and a
Transient Absorption Spectroscopy. The dual-beam femtosecond
time-resolved transient absorption spectrometer consisted of a self-
mode-locked femtosecond Ti:sapphire oscillator (Coherent, MIRA), a
Ti:sapphire regenerative amplifier (Clark MXR model TRA-1000) that
(26) Frisch, M. J.; et al. Gaussian 03, revision B.05; Gaussian, Inc.: Pittsburgh,
PA, 2003.
(27) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3098-3100. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. ReV. B 1988, 37, 785-789.
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4126 J. AM. CHEM. SOC. VOL. 128, NO. 12, 2006

A Directly Fused Tetrameric Porphyrin Sheet
A R T I C L E S
Grant-in-Aid for Scientific Research (B) No. 17350063 from
the Ministry of Education, Culture, Sports, Science, and Tech-
nology, Japan. The work at Yonsei was supported by the
National Creative Research Initiatives Program of the Ministry
of Science and Technology of Korea. Y.N. thanks the JSPS
fellowship for Young Scientists. We thank Prof. Masanobu
Uchiyama and Taniyuki Furuyama for near-IR MCD measure-
ments.
Supporting Information Available: Absorption spectrum,
details of MO calculation, measurement of two-photon absorp-
tion cross section, binding experiments of 3 with G1 and G2,
details of structure optimization calculations, and full citation
for ref 26. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA057812L
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J. AM. CHEM. SOC. VOL. 128, NO. 12, 2006 4127