Synthesis of direct β-to-β linked porphyrin arrays with large electronic interactions: Branched and cyclic oligomers

Article abstract of DOI:10.1002/anie.201407032

Direct β-to-β linked branched and cyclic porphyrin trimers and pentamers have been synthesized by the Suzuki-Miyaura coupling of β-borylporphyrins and β-bromoporphyrins. The cyclic porphyrin trimer, the smallest directly linked cyclic porphyrin wheel to date, and its twined pentamer, exhibit small electrochemical HOMO-LUMO gaps, broad nonsplit Soret bands, and red-shifted Q-bands, thus indicating large electronic interactions between the constituent porphyrin units.

Full text of DOI:10.1002/anie.201407032

Angewandte
Chemie
DOI: 10.1002/anie.201407032
Cross-Coupling
Synthesis of Direct b-to-b Linked Porphyrin Arrays with Large
Electronic Interactions: Branched and Cyclic Oligomers**
Hao Cai, Keisuke Fujimoto, Jong Min Lim, Chaojie Wang, Weiming Huang, Yutao Rao,
Senmiao Zhang, Hui Shi, Bangshao Yin, Bo Chen, Ming Ma, Jianxin Song,* Dongho Kim,* and
Atsuhiro Osuka*
Abstract: Direct b-to-b linked branched and cyclic porphyrin
trimers and pentamers have been synthesized by the Suzuki–
Miyaura coupling of b-borylporphyrins and b-bromoporphyr-
ins. The cyclic porphyrin trimer, the smallest directly linked
cyclic porphyrin wheel to date, and its twined pentamer, exhibit
small electrochemical HOMO–LUMO gaps, broad nonsplit
Soret bands, and red-shifted Q-bands, thus indicating large
electronic interactions between the constituent porphyrin units.
We found that treatment of the b-borylated porphyrins 2
and 3 (Figure 1), which were prepared by iridium-catalyzed b-
selective borylation of the 5,10,15-triaryl nickel(II) porphyrin
1,[7] with CuBr2 in THF at 1058C overnight[8] gave the b-
bromoporphyrins 4 and 5 in 80 and 85% yield, respectively.
Suzuki–Miyaura coupling of 2 with 4 in the presence of
a [Pd2(dba)3]/PPh3 catalyst and cesium bases gave the direct b-
to-b linked dimer 6 in 90% yield. High yield of 6 can be
ascribed partly to small steric constraints stemming from the
absence of a meso substituent in 2 and 4. This beneficial
structural motif has been amply utilized in the synthesis of the
U-shaped trimer 10(Ni), from 2 and the dibromide 5, in 50%
yield and the branched star-shaped pentamer 11(Ni) from 4
and 7 in 35%. The structures of 10(Ni) and 11(Ni) are fully
consistent with their spectroscopic data (see the Supporting
Information).
We also found that iridium-catalyzed borylation of 6
under standard reaction conditions furnished the diborylated
linear nickel(II) porphyrin dimer 8 almost quantitatively in
a highly regioselective manner. Then, we examined the
Suzuki–Miyaura coupling of 8 and 5 with an expectation for
the formation of cyclic porphyrin oligomers owing to the
meso-free structural motif. To our delight, this was indeed
true, in that cyclization proceeded smoothly to produce
a directly linked cyclic porphyrin trimer, 12(Ni), in 42% yield,
despite the predicted ring strain. High-resolution MALDI-
TOF mass measurement detected the parent ion peak of
12(Ni) at m/z = 2789.49 (calcd for (C186H210N12Ni3)+ = 2789.49
([M]+). The 1H NMR spectrum of 12(Ni) is quite simple, thus
exhibiting only a single set of signals for the porphyrin and is
in line with its symmetric structure. A singlet resulting from
Cyclic porphyrin arrays have been extensively explored as
synthetic models of photosynthetic antennae and functional
hosts possessing convergent multidentate coordination
sites.[1–4] Cyclic porphyrin structures are usually ensured by
employing appropriately bent bridges or assisted by a tilting
distortion of the porphyrins and/or bridges. Efficient excita-
tion-energy “hopping” along the cyclic array is enhanced by
the close proximity of porphyrins or conjugated spacers, both
of which increase the electronic communication between
constituent porphyrins. As a rare and extreme case, directly
meso-to-meso linked cyclic porphyrin arrays (without
a spacer) have been explored as photosynthetic models
using a 5,10-diaryl zinc(II) porphyrin building block.[5] While
these porphyrin rings display efficient excitation-energy
transfer, they are rather nonconjugative owing to the tilted
conformations of the porphyrin constituents, a feature inher-
ent to meso-to-meso linked porphyrin arrays.[6] Herein, we
report the synthesis of a direct b-to-b linked porphyrin trimer,
the smallest directly linked porphyrin wheel known to date,
and its twined pentamer. These porphyrin oligomers display
large electronic interactions among the constitutional por-
phyrins owing to less tilted structures.
[*] H. Cai, C. Wang, W. Huang, Y. Rao, S. Zhang, H. Shi, Dr. B. Yin,
Prof. Dr. B. Chen, Prof. Dr. M. Ma, Prof. Dr. J. Song
Key Laboratory of Chemical Biology and Traditional Chinese
Medicine Research (Ministry of Education of China)
Hunan Normal University, Changsha 410081 (China)
E-mail: jxsong@hotmail.com
the determination and analysis of the X-ray Structure. The work at
Hunan Normal University was supported by the National Natural
Science Foundation of China. (Grant No. 21272065), the Aid
Program for Science Innovative Research Team in Higher Educa-
tional institutions of Hunan Province, the Scientific Research
Foundation for the Returned Overseas Chinese Scholars, State
Education Ministry, and Scientific Research Fund of Hunan
Provincial Education Department (Grant No.13k027). The work at
Kyoto was supported by Grant-in-Aid from MEXT (Nos.: 25220802
(Scientific Research (S)). The work at Yonsei was supported by
Global Research Laboratory (2013K1A1A2A02050183) and Global
Frontier R&D Program on Center for Multiscale Energy System
(2012-8-2081) funded by the National Research Foundation under
the Ministry of Science, ICT & Future, Korea.
K. Fujimoto, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University, Sakyo-ku, Kyoto 606-8502 (Japan)
E-mail: osuka@kuchem.kyoto-u.ac.jp
Dr. J. M. Lim, Prof. Dr. D. Kim
Spectroscopy Laboratory for Functional p-Electronic Systems
and Department of Chemistry, Yonsei University, Seoul 120-749
(Korea)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201407032.
E-mail: dongho@yonsei.ac.kr
[**] We thank Prof. Xiaoyi Yi, Central South University (China) and Prof.
Naoki Aratani, Nara Institute of Science and Technology (Japan), for
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Figure 2. X-ray structure. a) Top view and b) side view of 13(Ni). The
thermal ellipsoids are 50% probability level. tert-Butyl groups and
hydrogen atoms are omitted for clarity.
are 1.289 ꢀ for the central porphyrin and 0.744 ꢀ on average
for the peripheral porphyrins. The central porphyrin plane is
highly tilted relative to the peripheral porphyrins with
dihedral angles ranging from 668 to 718, while the two
peripheral porphyrins within the same cyclic trimer are more
coplanar with small dihedral angles of 118 and 348. This
conformation means that the porphyrins in the cyclic trimeric
ring are in a coplanar arrangement as a result of the strain
associated with the ring structure.
Figure 1. Porphyrin compounds studied in this paper. Ar=3,5-di-tert-
butylphenyl, Bpin=pinacolatoboryl, M=metal.
the meso proton was observed at d = 12.41 ppm. Then, the
dimer 8 was converted into the b,b’-dibrominated porphyrin
dimer 9 in 80% yield under the above-mentioned bromina-
tion conditions. This dimer was coupled with 3 through the
Suzuki–Miyaura reaction to give the same trimer 12(Ni) in
a comparable yield of 40%. Finally, the coupling of 7 with
2 equivalents of 9 provided the porphyrin pentamer 13(Ni),
which possesses a twined cyclic trimeric structure, in 20%
yield. The parent ion peak of 13(Ni) was observed at m/z =
4458.32 (calcd for (C296H328N20Ni5)+ = 4458.31 ([M]+) in the
high-resolution MALDI-TOF mass spectrum, and the
1H NMR spectrum displayed two meso-proton singlets at
d = 12.28 and 12.02 ppm in a ratio of 2:1, which is consistent
with the twined structure (see the Supporting Information).
Definitive structural confirmation of 13(Ni) was provided
by X-ray diffraction analysis, which revealed a twined cyclic
trimeric structure (Figure 2).[9] All of the porphyrin subunits
are highly twisted and none of the porphyrin cores are
coplanar with respect to each other. The maximum displace-
ments of the b-carbon atoms from the porphyrin mean plane
The electrochemical properties of 1, 6, and 10(Ni)–13(Ni)
were investigated by cyclic voltammetry and differential pulse
voltammetry (Table 1). The monomer 1 exhibits two oxida-
tion potentials at 0.48 and 0.87 V and a reduction potential at
À1.83 V, which leads to an electrochemical HOMO–LUMO
gap (DEHL) of 2.31 eV. The dimer 6 shows oxidation
potentials at 0.47 and 0.62 V, which have been interpreted
as split first potentials with DVO = 0.15 V owing to the
electronic interaction of the two nickel(II) porphyrins.[10]
The DEHL value for 6 is 2.35 eV. The branched oligomers
10(Ni) and 11(Ni) display more complicated electrochemical
responses, that is, they show five oxidation potentials and two
reduction potentials. Since these arrays contain different
nickel(II) porphyrin units, comparison of DVO is not easy but
the DEHL values of 10(Ni) and 11(Ni) are both 2.16 eV, and
slightly smaller than those of 1 and 6. Importantly, the
symmetric cyclic trimer 12(Ni) displays oxidation potentials at
0.34, 0.57, and 0.68 V and reduction potentials at À1.54,
À1.69, and À1.90 V, which results in DEHL = 1.88 eV. The first
two oxidation potentials of 12(Ni) have been also interpreted
2
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Table 1: Electrochemical properties of 1, 6, and 10(Ni)–13(Ni) in CH2Cl2 with 0.1m Bu4NPF6.[a]
Compound
Eox5
Eox4
Eox3
Eox2
Eox1
Ered1
Ered2
Ered3
Ered4
DEHL [eV]
1
6
–
–
0.99
1.06
–
–
–
0.85
0.86
–
–
0.87
0.62
0.56
0.49
0.57
0.52
0.48
0.47
0.42
0.45
0.34
0.29
À1.83
À1.88
À1.74
À1.71
À1.54
À1.46
–
–
–
–
–
–
–
–
–
–
2.31
2.35
2.16
2.16
1.88
1.75
1.08
0.66
0.60
0.68
0.62
10(Ni)
11(Ni)
12(Ni)
13(Ni)
À1.85
À1.81
À1.69
À1.59
À1.90
–
0.92
0.83
À1.78
À1.94
[a] Potentials (V) were determined vs ferrocene/ferrocenium ion by differential pulse voltammetry. Working electrode: glassy carbon. Counter
electrode: Pt wire. Reference electrode: Ag/AgClO4. Cyclic voltammetry experiments indicated that most of the redox processes were reversible (for
details, see the Supporting Information).
as split potentials, thus leading to evaluation of DVO = 0.23 V.
These data indicate that the electronic interaction in 12(Ni) is
larger than that of 10(Ni) and 11(Ni) because of the enforced
cyclic structure. The cyclic pentamer 13(Ni) exhibits electro-
chemical properties similar to those of 12(Ni), again indicat-
ing larger electronic interactions.
In the next step, demetalation of 10(Ni)–13(Ni) gave the
corresponding 10(H2)–13(H2) and subsequent zinc(II) metal-
ation provided 10(Zn)–13(Zn) almost quantitatively. All
these zinc(II) complexes were fully characterized by spectro-
scopic methods (see the Supporting Information). Figure 3
displays the UV/Vis absorption and fluorescence spectra of
10(Zn)–13(Zn) in dichloromethane. The linear oligomers
10(Zn) and 11(Zn) exhibit split Soret bands at l = 416 and
435 nm, and at l = 416 and 470 nm, respectively, thus reflect-
ing the presence of two different transition dipole moments
along the long molecular axis and short perpendicular axis. In
contrast, the cyclic oligomers 12(Zn) and 13(Zn) both exhibit
broader Soret and Q-bands which are presumably affected by
large electronic interactions between the porphyrin units.[5,11]
The fluorescence spectra of 10(Zn) and 11(Zn) exhibit typical
porphyrin-like vibronic structures with fluorescence quantum
yields (FF) of 0.033 and 0.056, respectively, while the
fluorescence emission bands of 12(Zn) and 13(Zn) are red-
shifted and the fluorescence quantum yields are considerably
small, that is FF = 0.013 and 0.004, respectively. The fluores-
cence lifetimes of the noncyclic oligomers 10(Zn) and 11(Zn)
were determined to be 2.1 nanoseconds, which is comparable
to that of a zinc(II) porphyrin monomer (2.0 ns). However,
the cyclic oligomers 12(Zn) and 13(Zn) revealed short
fluorescence lifetimes of 800 and 560 picoseconds, respec-
tively. While the fluorescence spectra of noncyclic oligomers
are attributed to the monomeric porphyrin unit, the reduced
fluorescence quantum yields and lifetimes of 12(Zn) and
13(Zn) may arise from large electronic interactions and
structural strain associated with formation of the cyclic
structure. Moreover, it should be noted that the previous
direct b-to-b linked zinc(II) porphyrin dimer does not show
the absorption features extending into NIR spectral region,
and the fluorescence lifetime and quantum yield are
2.36 nanoseconds and 0.077, respectively.[11] In this sense,
the broad and structureless absorption spectra of the cyclic
porphyrin oligomers, 12(Zn) and 13(Zn), indicate large
electronic interactions between the porphyrin units and may
arise from not only their proximities but also coplanarity in
the cyclic conformation.
The electronic interactions between the porphyrin units
were investigated by femtosecond transient absorption (TA)
spectroscopy (see the Supporting Information). Excitation-
energy hopping processes along these cyclic or noncyclic
porphyrin oligomers were investigated by Q-band excitations
at l = 560 nm with variable excitation power to avoid the
involvement of S2-S1 relaxation. Although the overall
arrangement of energy hopping sites in the porphyrin
oligomers may hamper precise estimation of energy hopping
rates with a simple polygon model,[5] it may provide funda-
mental insight into the exciton hopping processes in the
porphyrin oligomers. In the cases of 6(Zn), 10(Zn), and
12(Zn), no distinctive power-dependent decay profiles were
observed, probably because of the small number of porphyrin
units and very fast internal exciton–exciton annihilation
processes. However, the porphyrin pentamers 11(Zn) and
Figure 3. UV/Vis absorption (a) and fluorescence (b) spectra of 10(Zn)
(dotted line), 11(Zn) (dashed line), 12(Zn) (solid line), and 13(Zn)
(bold line) in CH2Cl2.
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
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13(Zn) displayed laser-power-dependent fast TA decays with
time constants of 440 and 400 femtoseconds, respectively,
most probably a result of efficient exciton–exciton annihila-
tion processes (see the Supporting Information). To get more
detailed information on the exciton hopping processes
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(TAA) measurements were carried out (see the Supporting
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In summary, direct b-to-b linked cyclic porphyrin trimers
and its twined pentamer were synthesized by Suzuki–Miyaura
coupling of b-borylporphyrins and b-bromoporphyrins. The
cyclic porphyrin oligomers exhibit optical and electrochem-
ical properties different from those of the corresponding
linear oligomers, properties such as nonsplit and broad Soret
bands, red-shifted Q-bands, small electrochemical HOMO–
LUMO band gaps, and reduced fluorescence quantum yields
and lifetimes. Thus, the cyclic porphyrin trimers display
efficient excitation-energy hopping along the ring. Further
extension of this coupling strategy for the synthesis of more
elaborate porphyrin arrays is actively in progress in our
laboratories.
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Received: July 9, 2014
Published online: && &&, &&&&
[9] Crystal data of 13Ni: C296 H328 N20 Ni5, Mw = 4459.33, triclinic,
ꢀ
space group P1 (No. 2), a = 19.7258(4), b = 22.9851(5), c =
37.9752(8) ꢀ,
a = 79.6520(10),
b = 89.0890(10),
g =
80.7140(10)8, V= 16714.4(6) ꢀ3, Z = 2, Dcalc = 0.886 gcmÀ3, T=
120(2) K, 131231 measured reflections, 49172 unique reflections,
R = 0.1079, Rw = 0.2761 (all data), GOF = 1.062 (I > 2.0s(I)).
CCDC 976052 (13(Ni)) contains the supplementary crystallo-
graphic 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.
Keywords: cross-coupling · cyclization · energy transfer ·
porphyrinoids · structure elucidation
.
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4
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Communications
Cross-Coupling
Molecular twine: The title porphyrin
trimers and pentamers have been syn-
thesized by Suzuki–Miyaura coupling of
b-boryl-porphyrins and b-bromoporphyr-
ins. The cyclic porphyrin trimer, the
smallest directly linked cyclic porphyrin,
and its twined pentamer exhibit small
HOMO–LUMO gaps, broad nonsplit
Soret bands, red-shifted Q-bands, and
efficient excitation-energy hopping.
H. Cai, K. Fujimoto, J. M. Lim, C. Wang,
W. Huang, Y. Rao, S. Zhang, H. Shi, B. Yin,
B. Chen, M. Ma, J. Song,* D. Kim,*
A. Osuka*
&&&& — &&&&
Synthesis of Direct b-to-b Linked
Porphyrin Arrays with Large Electronic
Interactions: Branched and Cyclic
Oligomers
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!