Strategie synthesis of 2,6-pyridylene-bridged β-to-β porphyrin nanorings through cross-coupling

Article abstract of DOI:10.1002/chem.200903177

(Figure Presented) The bridge determines the size of the loop? Palladium-catalyzed cross-coupling of borylporphyrins led to the construction of β-pyridine-bridged porphyrin nanorings in good yields (around 55-60 % ; see scheme). The photophysical study re

Full text of DOI:10.1002/chem.200903177

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DOI: 10.1002/chem.200903177
Strategic Synthesis of 2,6-Pyridylene-Bridged b-to-b Porphyrin Nanorings
through Cross-Coupling
Jianxin Song,[a] Pyosang Kim,[b] Naoki Aratani,[a, c] Dongho Kim,*[b]
Hiroshi Shinokubo,*[d] and Atsuhiro Osuka*[a]
Since the crystal structure of LH2 was elucidated to be
circularly arranged chromophoric assemblies,[1–3] much effort
has been devoted towards the synthesis of cyclic porphyrin
arrays to study the excitation energy transfer (EET) and
electronic coupling along the wheel.[4] These cyclic porphy-
rin arrays are also interesting in host–guest chemistry,[5]
single-molecule photochemistry,[6] nonlinear optical (NLO)
materials[7] and so on.[8–12] Cyclic porphyrin arrays are con-
structed by means of covalent bonds, noncovalent bonds, or
metal coordination bonds.[8–12] Although the last two are
beneficial to construct cyclic porphyrin arrays, those are
often unstable toward solvent change or addition of other
competing species. The covalently bonded arrays are struc-
turally robust, but often difficult to synthesize. In addition,
the final macrocyclization steps are the most tedious and
generally need assistance from a suitable template.[5] Al-
though there are some reports on covalently bonded cyclic
porphyrin arrays, most of them were constructed through
meso-to-meso bridging methods. As rare examples, we re-
cently reported two b,b’-bridged cyclic porphyrin arrays with
a 1,3-butadiyne or a 2,5-thienyl spacer.[13] Herein, we wish to
report the efficient synthesis of 2,6-pyridylene-bridged b-to-
b porphyrin nanorings by Suzuki–Miyaura coupling, which
is particularly effective for medium to large porphyrin rings.
We have achieved the synthesis of 2,5-thienylene-bridged
cyclic porphyrin dimers and trimers, but it was difficult to
expand the size of the macrocycle beyond trimers. We then
examined the Suzuki–Miyaura coupling of b,b’-diborylated
NiII porphyrin 1 with 2,6-dibromopyridine, which provided
linear oligomers as the main products. However, we noticed
that bromopyridyl-terminated oligomers were selectively ob-
tained with the use of an excess amount of 2,6-dibromopyri-
dine. For instance, b,b’-diborylporphyrin 1–NiII[14] was cross-
coupled with 2,6-dibromopyridine (5 equiv) under standard
conditions to give 1–Br in 80% yield, along with 2–Br
(10%) and 3–Br (2%) (Scheme 1). It is worth noting that
deborylated products were hardly detected in this reaction.
Compound 2–Br was also obtained in 42% yield through
[a] Dr. J. Song, Dr. N. Aratani, Prof. Dr. A. Osuka
Department of Chemistry
Graduate School of Science, Kyoto University
Sakyo-ku, Kyoto 606-8502 (Japan)
Fax : (+81)75-753-3970
the three-component coupling of
1
(1 equiv), 1–Br
(2.5 equiv), and 2,6-dibromopyridine (2.5 equiv). Further
cross-coupling of 1 (1 equiv) with 1–Br (5 equiv) or 2–Br
(5 equiv) afforded 3–Br or 5–Br in 50 or 45% yields, respec-
tively. Compound 7–Br was prepared in 50% yield from 1
and 3–Br.
With the linear precursors 2–Br, 3–Br, 5–Br, and 7–Br in
hand, we examined the cyclization with 1 through a 1:1
Suzuki–Miyaura coupling reaction, which worked very effec-
tively to give porphyrin rings 3–Ni, 4–Ni, 6–Ni, and 8–Ni in
60, 58, 55, and 55% yields, respectively, even without any
template. Treatment of 3–Ni, 4–Ni, 6–Ni, and 8–Ni with sul-
furic acid in chloroform at room temperature induced NiII
demetalation to provide 3–H, 4–H, 6–H, and 8–H quantita-
tively, which were all converted into 3–Zn, 4–Zn, 6–Zn, and
E-mail: osuka@kuchem.kyoto-u.ac.jp
[b] P. Kim, Prof. Dr. D. Kim
Spectroscopy Laboratory for Functional p-Electronic Systems
and Department of Chemistry, Yonsei University
Seoul 120-749 (Korea)
Fax : (+82) 2-2123-2434
E-mail: dongho@yonsei.ac.kr
[c] Dr. N. Aratani
PRESTO
Japan Science and Technology Agency
4-1-8 Honcho Kawaguchi, Saitama 332-0012 (Japan)
[d] Prof. Dr. H. Shinokubo
Department of Applied Chemistry
Graduate School of Engineering, Nagoya University
Chikusa-ku, Nagoya 463-8603 (Japan)
Fax : (+81)52-789-5113
8–Zn upon treatment with ZnACHTNUTRGENNG(U OAc)2 in quantitative yields.
E-mail: hshino@apchem.nagoya-u.ac.jp
These newly synthesized porphyrin rings were fully char-
acterized by high-resolution mass spectrometry and
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903177.
Chem. Eur. J. 2010, 16, 3009 – 3012
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3009

(Figure 1).[16] In 3–Ni, the mean plane deviations are 0.37,
0.32, and 0.28 ꢁ for porphyrins A, B, and C, which are con-
nected with center-to-center distances of 10.7 (A–B), 12.39
Figure 1. X-ray structures of the porphyrin wheels. a) Top and side views
of 3–Ni and b) top and side views of 4–Ni. The thermal ellipsoids are
shown at the 50% probability level. meso-Aryl groups and hydrogen
atoms are omitted for clarity.
Scheme 1. Synthesis of porphyrin nanorings. Reagents and conditions:
a) 2,6-dibromopyridine (5.0 equiv) or bis(bromopyridyl)oligoporphyrins
(B–C), and 10.72 (C–A) ꢁ and with dihedral angles of 75.9,
45.7, and 76.38, respectively. Of the three pyridylene bridges,
only py-B takes a different conformation with its nitrogen
atom pointing outward. The para-proton of py-B is located
just above porphyrin A. This location can explain the ob-
served upfield shift of the particular pyridyl proton at d=
4.26 ppm. The tetramer 4–Ni takes a pseudo S4 symmetric
zigzag structure, in which four porphyrins are sorted into
two identical sets: porphyrins A and B and porphyrins C
and D. Porphyrins A and B (hence C and D) are nearly per-
pendicular with a large dihedral angle of 888, whereas por-
phyrins A and C (hence B and D) are nearly parallel with a
small dihedral angle of 21.88. The center-to-center distances
are 8.32 and 11.36 ꢁ for porphyrins A and C and for B and
D. All of the pyridylene bridges are pointing inward.
The UV/Vis absorption and fluorescence spectra of the
ZnII–porphyrins were measured in toluene (Figure 2). All of
the nanorings display substantially perturbed absorption
spectra, in which the Soret bands of 3–Zn, 4–Zn, and 6–Zn
are all split into 2 peaks around 430 and 445 nm, whereas 8–
Zn exhibits only one characteristic broad Soret band at
442 nm. A slight redshift of the Soret band along with
broadening was observed as the ring was enlarged. This
trend can also be seen in the case of its free base and nickel
series (see Figure S18 in the Supporting Information). Inter-
estingly, the shapes of the Soret bands are significantly
changed with the metal ion, and there is no split in the
nickel series. The fluorescence spectra of cyclic arrays all ex-
(5.0 equiv), [Pd
(1.0 equiv), n–Br (1.0 equiv), [Pd
DMF, reflux; c) H2SO4, CHCl3, room temperature; d) ZnAHCTUNGTRENNUNG
(dba)3], PPh3, Cs2CO3, CsF, toluene/DMF, reflux; b) 1
2A(dba)3], PPh3, Cs2CO3, CsF, toluene/
(OAc)2·2H2O,
CTHUNGTRENNUNG
MeOH/CHCl3, room temperature, Ar=3,5-di-tert-butylphenyl, dba=
trans,trans-dibenzylideneacetone; Bpin=3,3,4,4-tetramethyl-2,5-dioxabor-
olanyl.
1H NMR, UV/Vis absorption, and fluorescence spectroscop-
ies (see the Supporting Information). Characteristically, the
1H NMR spectra of the cyclic structures in CDCl3 at room
temperature exhibited singlet peaks for the meso-protons
(Hm), except for the trimers 3–Ni, 3–H, and 3–Zn. In the
case of 3–Ni, the 1H NMR spectrum at À508C in
[D8]toluene exhibited two peaks for the meso protons in a
2:1 ratio, suggesting the presence of two distinguishable por-
phyrins at this temperature.[15] The same feature was also
1
observed for the pyridyl protons. Interestingly, the H NMR
spectrum of 3–Ni at À508C exhibited a high-field-shifted
pyridyl proton at d=4.26 ppm, which merged with other
pyridyl protons in the normal region at higher temperatures.
In contrast, four, six, and eight pyridyl protons appear in the
upfield region in the case of 4–Ni, 6–Ni, and 8–Ni, respec-
tively. These results would indicate that these porphyrin
wheels exhibit flexible conformations, but the conformation-
al changes for the trimers are somewhat slower.
Definitive structural assignments have been accomplished
by X-ray diffraction analysis for 3–Ni and 4–Ni, both of
which reveal seriously ruffled porphyrin structures
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2,6-Pyridylene-Bridged b-to-b Porphyrin Nanorings
COMMUNICATION
reduced energy gap between the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbi-
tal (LUMO). However, the lifetimes of singlet excited states
become longer as the number of porphyrin units increases
from 3–Zn and 4–Zn to 6–Zn and 8–Zn (Table 1). Their life-
times are also longer than that of zinc–tetraphenylporphyrin
(Zn–TPP).
Table 1. Optical properties of nanorings.
[c]
Compound
lem [nm][a]
tf [ns][b]
FF
Zn–TPP
3–Zn
4–Zn
6–Zn
8–Zn
644
653
659
653
657
2.13
2.67
2.58
3.07
3.09
0.033
0.032
0.035
0.035
0.033
[a] Fluorescence maximum wavelength. [b] Singlet excited-state lifetime.
[c] Fluorescence quantum yield.
In summary, we have achieved the synthesis of b-2,6-pyri-
dylene-bridged porphyrin nanorings in good yields with
Suzuki–Miyaura cyclization of 6-bromopyridine-terminated
linear oligoporphyrins with diborylated porphyrin 1. These
porphyrin arrays exhibit weak electronic coupling among
porphyrins and longer singlet-excited state lifetimes, which
have an apparent advantage in the process of excitation
energy transfer. Further exploration into their photophysical
properties and interactions with other molecules are cur-
rently underway in our laboratory.
Figure 2. a) UV/Vis absorption and b) emission spectra of 3–Zn (black
line), 4–Zn (red line), 6–Zn (blue line), and 8–Zn (green line) in toluene.
Samples were excited at the absorption maxima.
Acknowledgements
hibit typical vibronic structures with fluorescence quantum
yields of around 0.035. The maximum emission peaks are
redshifted in the order 3–Znꢀ6–Zn<8–Zn<4–Zn, in line
with the absorption peak maximum wavelengths. The band
positions and shapes in the absorption and fluorescence
spectra indicate that there exist unique excitonic and elec-
tronic communications in the cyclic porphyrin units. In par-
ticular, tetramer cases exhibited the largest excitonic inter-
action among the series, probably reflecting its semi-rigid
and extended structure.
This work was supported by Grant-in-Aids for Scientific Research (Nos.
19205006, 20108001 “pi-Space”, and 21685011) from MEXT. H.S. grate-
fully acknowledges financial support from the Nagase Science and Tech-
nology Foundation. J.S. thanks the JSPS for Research Fellowships and
the Global COE Program, “Integrated Materials Science”. The work at
Yonsei University was supported by the Star Faculty program from the
Ministry of Education and Human Resources Development, Korea. P.K.
acknowledge the BK21 fellowships. The authors thank Prof. H. Maeda,
Y. Haketa, and T. Hashimoto, Ritsumeikan University, for MALDI-TOF
MS measurements.
To obtain information on the relative orientation between
the absorption and emission transition dipole moments, we
measured the fluorescence excitation anisotropy (see Fig-
ure S22 in the Supporting Information).[17] The absolute ani-
sotropy values of all the nanorings were smaller than 0.1 in
the whole excitation region, reflecting that the reorientation
of the transition dipole moment occurs in the nanorings.[18]
This indicates that the singlet–singlet excited-state energy
transfer is quite efficient along the ring.[19] The time-resolved
fluorescence decay profiles of nanorings were measured in
toluene (see Figure S23 in the Supporting Information). The
lifetimes of ZnII–porphyrin arrays[20,13b] generally become
shorter as the number of porphyrin moieties increases due
to the acceleration of a nonradiative process caused by the
Keywords: cross-coupling
chemistry · nanorings · porphyrinoids
·
fluorescence
·
host–guest
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Received: November 19, 2009
Published online: February 19, 2010
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Chem. Eur. J. 2010, 16, 3009 – 3012