Meso,meso′-bis(5-azaindol-2-yl)-appended meso meso-linked Zn(II) diporphyrin: A discrete fluorescent assembly

Article abstract of DOI:10.1021/ol902294r

Cu(II)-catalyzed intramolecular cyclization of meso-(4-aminopyrid-3-yl) ethynyl Zn(II) porphyrin provided meso-(5-azaindol-2-yl)-substituted Zn(II) porphyrin. meso,meso′-Bis(5-azaindol-2-yl)-substituted diporphyrin 7 was similarly prepared and was found t

Full text of DOI:10.1021/ol902294r

ORGANIC
LETTERS
2009
Vol. 11, No. 22
5322-5325
meso,meso′-Bis(5-azaindol-2-yl)-Appended
meso-meso-Linked Zn(II) Diporphyrin:
A Discrete Fluorescent Assembly
Chihiro Maeda,† Hiroshi Shinokubo,*,‡ and Atsuhiro Osuka*,†
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo-ku,
Kyoto 606-8502, Japan, and Department of Applied Chemistry, Graduate School of
Engineering, Nagoya UniVersity, Chikusa-ku, Nagoya 464-8603, Japan
osuka@kuchem.kyoto-u.ac.jp; hshino@apchem.nagoya-u.ac.jp
Received October 4, 2009
ABSTRACT
Cu(II)-catalyzed intramolecular cyclization of meso-(4-aminopyrid-3-yl)ethynyl Zn(II) porphyrin provided meso-(5-azaindol-2-yl)-substituted Zn(II)
porphyrin. meso,meso′-Bis(5-azaindol-2-yl)-substituted diporphyrin 7 was similarly prepared and was found to form a fluorescent trimeric
prismatic assembly consisting of single atropisomer 7in-in
.
Since the structure of the light-harvesting complex LH2 of
the purple bacterium was determined to be a circular
chromophoric architecture, cyclic porphyrin arrays have
attracted considerable interest in view of photosynthetic
antenna models.1,2 Toward the construction of such arrays,
both covalent3 and noncovalent approaches4 have been
attempted. Most of covalent approaches need time-consuming
and many tedious synthetic steps,5,6 while in some cases
template synthesis facilitates the formation of cyclic arrays.7
As a promising noncovalent method, metalloporphyrins
having coordinating substituents have been often used for
the construction of cyclic porphyrin arrays via metal-ligand
coordination interactions. Among these, we have explored
† Kyoto University.
‡ Nagoya University.
(1) (a) McDermott, G.; Prince, S. M.; Freer, A. A.; Hawthornthwaite-
Lawless, A. M.; Papiz, M. Z.; Cogdell, R. J.; Isaacs, N. W. Nature 1995,
374, 517–521. (b) Roszak, A. M.; Howard, T. D.; Southall, J.; Gardiner,
A. T.; Law, C. J.; Isaacs, N. W.; Cogdell, T. J. Science 2003, 302, 1969–
(5) (a) Sugiura, K.; Tanaka, H.; Matsumoto, T.; Kawai, T.; Sakata, Y.
Chem. Lett. 1999, 1193–1194. (b) Sugiura, K.; Fujimoto, Y.; Sakata, Y.
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10.1021/ol902294r CCC: $40.75
Published on Web 10/16/2009
 2009 American Chemical Society

self-sorting assembly of meso-pyridyl-appended Zn(II) di-
porphyrins, where the angle of coordinating direction with
regard to the porphyrin plane controls the size and structures
of assemblies.8 Interestingly, meso-cinchomeronimide-ap-
pended Zn(II) diporphyrins 1 exhibit high fidelity self-sorting
assembly to form cyclic trimer (1in-in)3, tetramer (1in-out)4,
Scheme 2
.
Synthesis of 5-Azaindolyl Zn(II) Porphyrin via
Pd-Catalyzed Annulation Reaction
and pentamer (1out-out)5 through a rigorous self-sorting
process (Scheme 1).8c However, these assemblies have a
serious drawback of the strong fluorescence quenching by
intramolecular electron transfer to the appended electron-
deficient imide moieties.
ethynyl Zn(II) porphyrin and 4-amino-3-iodopyridine (Scheme
2).9 This method allowed for the synthesis of meso-(5-aza-
indol-3-yl)porphyrin but not for a meso-(5-aza-indol-2-yl)-
substituted one. Here we wish to report the synthesis of meso-
(5-azaindol-2-yl)porphyrins via Sonogashira coupling of
meso-bromoporphyrin with (4-aminopyrid-3-yl)acetylene and
subsequent Cu(II)-catalyzed intramolecular cyclization.10
Scheme 1. Homochiral Self-Sorting Assembly of
meso-Cinchomeronimide-Appended Zn(II) meso-meso Linked
Diporphyrinsa
Scheme 3. Synthesis of 4
a The same self-sorting assembling occurs for (S)-enantiomers.
With this background, we embarked on the exploration
of meso,meso-linked Zn(II) diporphyrin bearing a 5-azaindole
ring, which has a shape similar to that of a cinchomeronimide
but does not quench the fluorescence of a Zn(II) porphyrin.
We thus attempted to prepare an azaindole-appended por-
phyrin by the conventional acid-catalyzed reaction using
formyl-substituted azaindoles or the metal-catalyzed cross
coupling using iodinated azaindoles. However, all these
attempts ended in failure. Recently, instead, we developed
the efficient synthesis of meso-5-azaindolyl porphyrins via
Pd-catalyzed intermolecular annulation reaction of meso-
A synthetic route to 5-azaindol-2-ylporphyrin 4 is shown
in Scheme 3. Sonogashira coupling reaction of meso-
brominated porphyrin 2 with 4-ethylamino-3-ethynylpyridine
provided (4-ethylaminopyrid-3-yl)ethynylporphyrin 3 in 65%
yield. Then, we examined the Cu(II)-catalyzed intramolecular
cyclization reaction reported by Hiroya et al.10a for porphyrin
substrate 3. The cyclization reaction was not completed in
1,2-dichloroethane or toluene even after refluxing for 4 days,
but we found that the use of 1,4-dioxane at reflux allowed
full conversion of 3 to provide 4 in 91% yield in 4 days.
The structure of 4 has been confirmed by the spectroscopic
data and single-crystal X-ray diffraction analysis. High-
resolution electrospray ionization time-of-flight mass spec-
trum detected the parent ion peak of 4 at m/z ) 1081.5831
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Ed. 2008, 47, 4993–4996.
1
(calcd for C71H81N6Zn (M + H)+ ) 1081.5809). The H
NMR spectrum of 4 in coordinating pyridine-d5 is fully
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A. Angew. Chem., Int. Ed. 2002, 41, 2817–2821. (b) Hwang, I.-W.; Kamada,
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J. Am. Chem. Soc. 2004, 126, 16187–16198. (c) Kamada, T.; Aratani, N;
Ikeda, T; Shibata, N.; Higuchi, Y.; Wakamiya, A.; Yamaguchi, S.; Kim,
K. S.; Yoon, Z. S.; Kim, D.; Osuka, A. J. Am. Chem. Soc. 2006, 128, 7670–
7678. (d) Maeda, C.; Kamada, T.; Aratani, N.; Sasamori, T.; Tokitoh, N.;
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2496.
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1126–1136. (b) Sakai, N.; Annaka, K.; Fujita, A.; Sato, A.; Konakahara,
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Org. Lett., Vol. 11, No. 22, 2009
5323

Scheme 4. Synthesis of (7in-in)
3
Figure 1. X-ray crystal structure of 4. Hydrogen atoms are omitted
for clarity. The thermal ellipsoids were at 50% probability level.
consistent with its structure (Supporting Information). Slow
vapor diffusion of methanol to a THF solution of 4 provided
nice crystals for X-ray diffraction analysis. The X-ray crystal
structure elucidated the structure of 4 unambiguously,
revealing the meso-appended 5-azaindol-2-yl moiety (Figure
1).11 In line with this structural change, the UV/vis absorption
spectrum changes radically from a strongly perturbed one
with peaks at 450, 583, and 636 nm for the meso-ethynylated
porphyrin 3 to a normal one with peaks at 434, 564, and
607 nm for 4 (Supporting Information). Importantly, the
fluorescence of 4 is observed at 614 and 664 nm with a
certainly high fluorescence quantum yield (ΦF ) 0.055).
With an efficient protocol for azaindole formation, me-
so,meso′-bis(5-azaindol-2-yl)-substituted meso-meso-linked
Zn(II) diporphyrin 7 was prepared as shown in Scheme 4.
Sonogashira coupling of meso,meso′-dibrominated dipor-
phyrin 5 with 4-ethylamino-3-ethynylpyridine provided me-
so,meso′-diethynylated diporphyrin 6 in 62% yield, which
was converted to diporphyrin 7 in 82% yield via the Cu(II)-
Information). The fact that only a single assembling entity
was formed from 7 is different from the assembling behaviors
of cinchomeronimide-appended meso-meso-linked dipor-
phyrins 1. In the latter case, the rotational barrier of the meso-
substituted cinchomeronimide group is rather high, not
allowing free atropisomerism at room temperature and hence
realizing self-sorting assembling to form (1in-in)3, (1in-out)4,
and (1out-out)5 from 1in-in, 1in-out, and 1out-out, respectively.
In the present system, the corresponding rotational barrier
seems smaller, allowing facile atropisomerism to provide
three atropisomers (7in-in, 7in-out, and 7out-out), among which
1
catalyzed intramolecular cyclization. Interestingly, the H
NMR spectrum of 7 in CDCl3 shows only a single set of
peaks with large upfield shifts for the azaindolyl protons (Ha:
6.38, Hb: 3.10, Hc: 2.60, Hd: 5.14 ppm), suggesting the
formation of a single assembling entity, in which the
5-nitrogen atom of the azaindol-2-yl group coordinates to
the Zn(II) center of the other porphyrin in a complementary
manner (Figure 2). By comparing the retention time of this
assembly in the analytical GPC-HPLC with those of cin-
chomeronimide-appended meso-meso-linked diporphyrins,
this assembly has been assigned to a trimer of 7 (Supporting
(11) Crystallographic data for 4: formula: C71H80N6Zn·4C4H8O, Mw
)
1371.22, monoclinic, space group P21/c, a ) 16.5956(13) Å, b ) 34.837(3)
Å, c ) 14.5206(11) Å, ꢀ ) 115.729(2)°, V ) 7562.7(11) Å3, Z ) 4, θ˜calcd
) 1.204 g cm-3, T ) -183 °C, 38986 measured reflections, 13314 unique
reflections (Rint ) 0.0623), R1 ) 0.0937 (I > 2σ(I)), wR2 ) 0.2320 (all
data), GOF ) 1.046.
Figure 2.
1H NMR spectrum of 7in-in in CDCl3.
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Org. Lett., Vol. 11, No. 22, 2009

only 7in-in assembles to form its trimer, since this is the most
stable due to the least entropy cost for the assembling.8d
Evaporation of a CDCl3 solution of 7 left an assembly of
(7in-in)3. When this was dissolved in pyridine-d5, the 1H NMR
spectrum was just broad, not showing particular upfield shifts
for the 5-azaindolyl protons, indicating that 7 exists as a
splitting feature also supports the trimeric assembling of 7
in comparison to the assemblies of 1.12 The fluorescence
spectrum of 7 in pyridine is broad with a peak at 636 nm
and ΦF ) 0.061. On the other hand, the fluorescence of 7 in
CHCl3 shows a clear vibronic structure with peaks at 630
an 685 nm and ΦF ) 0.039.13 The observed vibronic
structure is consistent with the formation of a closely packed,
rigid structure.
In summary, we synthesized meso-(5-azaindol-2-yl)-ap-
pended Zn(II) porphyrins 4 and 7 via Cu(II)-catalyzed
intramolecular cyclization reaction. In noncoordinating
CHCl3, meso-meso-linked diporphyrin 7 predominantly
forms a trimeric fluorescent assembly that consists of three
single atropisomers 7in-in through facile rotation of the
5-azaindol-2-yl substituent and effective trapping via self-
sorting assembling by complementary coordination. A 5-aza-
indol-2-yl substituent at meso-position does not quench the
excited singlet state of Zn(II) porphyrin and allows a unique
self-sorting assembly.
1
monomer in pyridine. In addition, the H NMR spectrum
became slowly sharper to exhibit many peaks due to the
presence of atropisomers by standing at room temperature
for several days (Supporting Information). This change was
accelerated upon heating at 110 °C.
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research from the Ministry of Educa-
tion, Culture, Sports, Science, and Technology, Japan. C.M.
thanks the JSPS fellowship for young scientists.
Figure 3. UV-vis absorption and fluorescence spectra of 7in-in
(red line, in CHCl3; black line, in pyridine).
Supporting Information Available: Experimental pro-
cedures, compound data, and crystallographic data for 4
(CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
The absorption spectrum of 7 in pyridine shows splitting
of the Soret band (1820 cm-1) typical for a meso-meso-
linked diporphyrin, while (7in-in)3 in CHCl3 shows a narrower
splitting width (1500 cm-1), indicating additional exciton
coupling through self-assembling (Figure 3). Namely, transi-
tion dipole moments along with meso-meso linkage are
strongly coupled as J-type interactions in the meso-meso-
linked diporphyrin, while the three dipole moments in
(7in-in)3 in CHCl3 are coupled as additional intermolecular
H-type interaction, giving rise to further blue shift. This
OL902294R
(12) Splitting width of 7in-in in CHCl3 is 1500 cm-1, while those of the
assemblies formed from 1 are 1540 cm-1 for (1in-in)3, 1390 cm-1 for
(1in-out)4, and 1290 cm-1 for (1out-out)5, respectively.8c
(13) This is a substantial improvement in ΦF, since the fluorescence
quantum yields of (1in-in)3, (1in-out)4, and (1out-out
)
in CHCl3 were less
5
than 0.1%.
Org. Lett., Vol. 11, No. 22, 2009
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