Pyrrole-bridged porphyrin nanorings

Article abstract of DOI:10.1002/chem.201002219

Bridging the gap: A series of meso-to-meso pyrrole-bridged cyclic porphyrins were prepared by a one-pot Suzuki-Miyaura coupling reaction (see figure). The ¹Ha NMR spectra of these compounds revealed their highly symmetric structures in solution

Full text of DOI:10.1002/chem.201002219

DOI: 10.1002/chem.201002219
Pyrrole-Bridged Porphyrin Nanorings
Jianxin Song,[a] Naoki Aratani,*[a, b] Hiroshi Shinokubo,*[c] and Atsuhiro Osuka*[a]
Covalently linked, cyclic porphyrin arrays have been ex-
plored mainly as artificial photosynthetic antennae and
large host molecules.[1–3] In such arrays, a variety of bridges
that connect porphyrins have been employed to control the
overall molecular shape and to tune the electronic interac-
tion between neighboring porphyrins. Among these, acety-
lenic spacers have been often used, because these are easy
to incorporate into porphyrin arrays and allow for elonga-
tion of conjugation networks.[3] As a different approach, we
have explored various cyclic porphyrin arrays by direct
meso–meso coupling reactions on the basis of AgI-promoted
coupling and Suzuki–Miyaura coupling.[1e,4] In recent years,
we have also explored aromatic heterocycle-bridged cyclic
porphyrin arrays. The b-to-b 2,6-pyridylene-bridged cyclic
porphyrin arrays exhibit relatively large fluorescence quan-
tum yields and long fluorescent lifetimes,[5] and b-to-b 2,5-
thienylene-bridged cyclic porphyrin arrays exhibit large two-
photon absorption (TPA) cross-section values, owing to ef-
fective electronic delocalization through the thienylene link-
ages.[6] Zinc(II) complexes of meso-to-meso 2,2’-bithio-
phene-bridged cyclic porphyrin arrays are suitable for multi-
charge storage and hence are of interest as molecular infor-
mation storage systems.[7]
As an extension of these studies, we attempted to synthe-
size meso-to-meso 2,5-pyrrolylene-bridged cyclic porphyrin
arrays. A pyrrole bridge in cyclic porphyrin nanorings is
quite attractive in view of its electron-rich properties, effec-
tive conjugation, and possible hydrogen-bonding donating
ability. In addition, as indicated for the cyclic porphyrin
trimer in Scheme 1, we can consider a formal hexaphyrin-
Scheme 1. Porphyrin, isophlorin, and a hybrid porphyrinoid.
[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
E-mail: aratani@kuchem.kyoto-u.ac.jp
osuka@kuchem.kyoto-u.ac.jp
like p-electronic network[8] that is spread across the pyrrole
bridges and the parts of porphyrin skeleton. Therefore,
meso-to-meso pyrrolylene-bridged cyclic porphyrin arrays
are expected to lead to new porphyrinic molecules with in-
triguing electronic properties. However, manipulation of
pyrroles is not trivial, because of their instability and high
reactivity. As a consequence, there are only scattered exam-
ples of pyrrole-appended porphyrins in the literature.[9]
Quite recently, we have reported CuI-mediated annulation
of 1,3-butadiyne-bridged diporphyrin with various amines to
provide 2,5-pyrrolylene-bridged diporphyrins in moderate to
good yields.[10] In these diporphyrins, notable electronic in-
[b] Dr. N. Aratani
PRESTO Japan Science and Technology Agency (Japan)
[c] Prof. Dr. H. Shinokubo
Department of Applied Chemistry
Graduate School of Engineering
Nagoya University, Chikusa-ku, Nagoya 464-8603 (Japan)
Fax : (+81)52-789-5113
E-mail: hshino@apchem.nagoya-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002219.
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Chem. Eur. J. 2010, 16, 13320 – 13324

COMMUNICATION
teractions were observed in the absorption and fluorescence
spectra, which vary upon the pyrrolic N-substituent.
been reactivated in light of its relevance to antiaromatic por-
phyrins.[12–14]
Here we wish to report the efficient synthesis of meso-to-
meso 2,5-pyrrolylene-bridged porphyrin nanorings 3–5 from
porphyrin 1 and pyrrole 2 through Suzuki–Miyaura coupling
(Scheme 2). Interestingly, resonance contributors of these
nanorings can be regarded as ([18]annuleno)3[30]annulene,
Synthesis of pyrrole-bridged porphyrin rings 3H–5H is
straightforward through Suzuki–Miyaura cross-coupling of
5,10-diaryl-15,20-dibromoporphyrin 1[2d] and 2,5-diborylpyr-
role 2[15] (Scheme 2). Separation by gel-permeation chroma-
tography (GPC) and silica-gel chromatography afforded pyr-
role-bridged cyclic porphyrin 3-mer 3H (13%), 4-mer 4H
(5.3%), and 5-mer 5H (2.7%). Notably, the reaction repre-
sents a rare example of one-pot synthesis of such large
cyclic arrays from a monomeric porphyrin precursor without
template molecules.
([18]annuleno)4[40]annulene,
and
([18]annule-
no)5[50]annulene for 3–5, respectively. Annulenoannulenes
that consist of multi-annulated aromatic annulenes have
been extensively investigated in terms of their possible over-
all aromaticity.[11] In these three cases, the central resonance
contributors (shown in gray in Scheme 1) correspond to so-
called expanded isophlorins. Isophlorin (N,N’-dihydropor-
phyrin) is a reduced 20p-electron congener of porphyrin, in
which the macrocyclic conjugation surrounds the carbon pe-
riphery. In recent years, the chemistry of isophlorins has
Matrix-assisted laser desorption ionization time-of-flight
(MALDI-TOF) mass spectra of the cyclic porphyrin arrays
displayed the parent ion peaks at m/z 2250.2995 for 3H
(calcd for C156H166N15 =2250.3413 [M+H]+), 2999.7303 for
4H (calcd for C208H220N20 =2999.7902 [M]+), and 3750.2339
for 5H (calcd for C260H275N25 =3750.2415 [M]+) (see also
1
the Supporting Information). The H NMR spectrum of 3H
in CDCl3 was quite simple, exhibiting two singlet and two
doublet peaks for the b-protons at 10.44, 9.85, 9.12, and
8.88 ppm, a singlet for the bridging-pyrrole NH protons at
10.62 ppm, a singlet for the inner NH protons at À2.25 ppm,
and
a doublet for the bridging-pyrrole b-protons at
8.00 ppm, while no meso-proton signal was observed. Inter-
estingly two ortho-protons of aryl substituents were distin-
guished, because of restricted rotation of the pyrrole bridges
at room temperature. The electronic system is potentially at-
tractive in view of their structural analogy to that of ([18]an-
nuleno)3[30]annulene; however, the 1H NMR exhibited no
ring current arising from 30p circuit. This can be probably
ascribed to strongly localized porphyrin p circuits. The twist
between the planes of the bridging pyrroles and the por-
phyrins (specified later as 42–768) might contribute towards
the localization of the porphyrin p circuits. The 1H NMR
spectra of 4H and 5H displayed similar chemical shifts for
3H, but the one doublet peak of ortho-protons was ob-
served, indicating their more flexible conformations in solu-
1
tion that lead to the simple H NMR spectra (Supporting In-
formation). Consequently, the spectral data were in line
with the highly symmetric cyclic structures of 3H–5H in so-
lution. Again, 4H and 5H hold the formal octaphyrin and
decaphyrin structures in their cores, but exhibit no specific
ring current effects.
Free-base cyclic arrays 3H–5H were converted into the
corresponding zinc(II) and nickel(II) complexes 3Zn–5Zn,
and 3Ni–5Ni upon treatment with ZnACHTNUGTRENNUG(OAc)2 and NiACHTUNGTRENNUNG(OAc)2,
respectively. Single crystals of 3Ni suitable for X-ray diffrac-
tion analysis were grown by vapor slow diffusion of 2-propa-
nol and methanol into a solution of 3Ni in chlorobenzene
(Figure 1).[16] The X-ray crystal analysis revealed a symmet-
ric solid-state structure in which three of the pyrrole units
point into the core. The pyrrole moieties are tilted by 42–
768 relative to the porphyrin mean planes, which consist of
four core nitrogen atoms. The total molecular conformation
is a dome shape, and the center-to-center distances between
the nickel ions are 10.8, 10.9, and 11.4 ꢁ. The bond lengths
Scheme 2. Synthesis of pyrrolylene-bridged porphyrin rings.
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N. Aratani, H. Shinokubo, A. Osuka, and J. Song
Figure 2. X-ray crystal structure of 4Zn. Top view (top) and side view
(bottom). The thermal ellipsoids are 50% probability level. Substituents
on phenyl groups and solvent molecules are omitted for clarity.
Figure 1. X-ray crystal structure of 3Ni. Top view (top) and side view
(bottom). The thermal ellipsoids are 50% probability level. Substituents
on phenyl groups and solvent molecules are omitted for clarity.
(456 nm), while the small absorption shoulder at 416 nm in
3Zn may be considered in terms of H-type dipole–dipole
coupling of parallel components. It is likely that the ob-
served intensity of the split Soret bands reflects the relative
amplitude of the strong interacting (in-line) component to
the weak (parallel) one. The UV/Vis absorption spectra of
4Zn and 5Zn also exhibit split Soret bands, but their split-
ting extents are smaller than that of 3Zn. The observed
larger splitting energy of 3Zn may reflect its more rigid con-
formation, while in 4Zn and 5Zn, the flexible conforma-
tions may lead to distribution of many conformers, which
exhibit different exciton coupling. This situation leads to
averaged and thus weaker exciton coupling. The absorption
spectra of 3H–5H are very similar to those of the corre-
sponding ZnII complexes (Figure 3b). The fluorescence
spectra of 3Zn–5Zn exhibit a broad structure (Figure 3c).
The fluorescence quantum yields in CH2Cl2 at room temper-
ature are 0.040, 0.067, and 0.076 for 3Zn–5Zn, and 0.035,
0.065, and 0.070 for 3H–5H, respectively. Interestingly both
series indicate the increase in the fluorescence quantum
yield upon the increase in size.
between the porphyrin and the neighboring pyrrole (1.46–
À
1.49 ꢁ) are slightly shorter than normal C C single bonds.
This structure is consistent with the H NMR spectral pat-
1
tern.
Single crystals of 4Zn were grown by vapor slow diffusion
of ethanol into a solution of 4Zn in toluene (Figure 2).[17]
The X-ray crystal analysis revealed a nonsymmetric solid-
state structure in which two opposite nitrogen atoms of the
bridging pyrrole rings point into the core and the rest out of
the core, thus maintaining the overall planarity of the
system. The center-to-center distances between the zinc
atoms are about 11.2 ꢁ. The pyrrole moieties are tilted by
44–628 relative to the porphyrin mean planes.
The UV/Vis absorption spectra of 3Zn–5Zn in CH2Cl2
are shown in Figure 3a. The UV/Vis absorption spectrum of
3Zn shows a split Soret band at 416 and 456 nm and Q-
bands at 559 and 632 nm. This split Soret bands of 3Zn can
be understood in terms of the exciton coupling theory owing
to the close proximity of center-to-center distance
ACHTUNGTRENNUNG
The electrochemical properties of 3Zn–5Zn were investi-
gated by cyclic voltammetry. Although the first oxidation
potential waves of 3Zn–5Zn could not be identified clearly,
dipole moments of each porphyrin unit. The in-line compo-
nent of transition dipoles leads to a red-shifted Soret band
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Porphyrin Nanorings
COMMUNICATION
the influence of the positive charge of the first-generated
ZnII–porphyrin radical cations.
In conclusion, we prepared a 3-mer, 4-mer, and 5-mer
series of meso-to-meso pyrrolylene-bridged cyclic porphyrins
through one-pot Suzuki–Miyaura coupling reaction from di-
bromoporphyrin monomer. The 1H NMR spectra of these
cyclic compounds revealed their highly symmetric structures
in solution, although the 1,3-alternative structure was ob-
served in the solid state of 4Zn. The UV/Vis absorption
spectra of the 3Zn–5Zn indicate the strong excitonic inter-
actions between porphyrin units. Because the cyclic arrays
have strong excitonic and electronic couplings between por-
phyrin units, the coherent and/or incoherent energy hopping
processes are easily expected. Also, investigation on longer
oligopyrrole bridged cyclic porphyrin arrays is currently in
progress.
Acknowledgements
This work was partially supported by Grants-in-Aid for Scientific Re-
search from MEXT, Japan (22245006 (A), 21685011, and 20108001 “pi-
Space”) and the PRESTO program from JST, Japan. H.S. gratefully ac-
knowledges financial support from the Toray Science Foundation. The
authors thank Prof. H. Maeda, T. Hashimoto and Y. Haketa, Ritsumei-
kan University, for MALDI-TOF MS measurements.
Keywords: aromaticity · cross-coupling · exciton coupling ·
porphyrinoids · pi conjugation
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[16] Crystallographic
data
for
3Ni:
C156H159N15Ni3·ACHTUNGTRENNUNG(PhCl)2.485·
¯
(iPrOH)3.685 A(MeOH)0.358, Mr =2907.98, triclinic, space group P1 (no.
·CHTUNGTRENNUNG
2), a=18.401(3), b=19.428(3), c=25.183(4) ꢁ, a=105.026(3), b=
106.858(3), g=90.738(3)8, V=8283(2) ꢁ3, T=90(2) K, Z=2, reflec-
tions measured 42641, 28619 unique. The final R1 was 0.0807 [I>
2s(I)], and the final wR on F2 was 0.2516 (all data), GOF=1.026.
CCDC-779923 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
[17] Crystallographic data for 4Zn:
C208H212N20O4Zn4, Mr =3317.46,
monoclinic, space group Cc (no. 9), a=61.414(18), b=8.454(3), c=
49.699(15) ꢁ, b=114.579(4)8, V=23464(12) ꢁ3, T=90(2) K, Z=4,
reflections measured 46676, 26113 unique. The final R1 was 0.1301
[I>2s(I)], and the final wR on F2 was 0.3384 (all data), GOF=
1.092. CCDC-779924 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif. The contributions to the scattering arising
from the presence of the disordered solvents in the crystals of 4Zn
were removed by use of the utility SQUEEZE in the PLATON soft-
ware package.[19].
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[19] PLATON, A Multipurpose Crystallographic Tool, Squeeze-Platon,
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Received: August 3, 2010
Published online: November 4, 2010
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Chem. Eur. J. 2010, 16, 13320 – 13324