A doubly 2,6-pyridylene-bridged porphyrin-perylene-porphyrin triad

Article abstract of DOI:10.1039/c2cc30966a

A doubly 2,6-pyridylene-bridged porphyrin-perylene-porphyrin triad was synthesized via Suzuki-Miyaura cross coupling reaction, which captures a tetrakis(3-pyridyl)porphyrin guest in a 2:1 manner to form a supramolecular complex that undergoes photo-induced electron transfer. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc30966a

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COMMUNICATION
A doubly 2,6-pyridylene-bridged porphyrin–perylene–porphyrin triadwz
Shin Ikeda,a Naoki Aratani*ab and Atsuhiro Osuka*a
Received 10th February 2012, Accepted 8th March 2012
DOI: 10.1039/c2cc30966a
A doubly 2,6-pyridylene-bridged porphyrin–perylene–porphyrin
triad was synthesized via Suzuki–Miyaura cross coupling reaction,
which captures a tetrakis(3-pyridyl)porphyrin guest in a 2 : 1
manner to form a supramolecular complex that undergoes
photo-induced electron transfer.
2,6-pyridylene-bridged bent porphyrin belts7 and porphyrin
barrels,8 which exhibited C60-encapsulating ability. As an
extension of this strategy, we envisioned that insertion of a
large aromatic spacer between a 2-pyridyl spacer would lead to
modulation of molecular shape of porphyrin arrays in curvature
and ring diameter. With this idea, we tried the synthesis of a
doubly 2,6-pyridylene-bridged porphyrin–perylene–porphyrin
triad as a prototype. Perylene is an important and potential
functional organic molecule that has been used in various
applications such as field effect transistors (FETs), electrical
conductors, and organic electroluminescence (EL) devices.9
Interestingly, we have found that a triad zinc(II) complex can
bind a tetrakis(3-pyridyl)porphyrin guest in a 2 : 1 manner, in
which the efficient electron transfer takes place from the
zinc(II) porphyrin host to the free base porphyrin guest.
The synthetic scheme of porphyrin–perylene–porphyrin
triad 5 is shown in Scheme 1. 2,5,8,11-Tetraborylperylene 210
was coupled with an excess amount of 2,6-dibromopyridine
under Suzuki–Miyaura cross-coupling conditions to afford
2,5,8,11-tetrakis(6-bromopyridin-2-yl)perylene 3 in 50% yield,
which is practically insoluble in common organic solvents.
5,10,15-Triaryl-2,18-diborylporphyrin nickel(II) complex 42
was coupled with suspended 3. Separation by gel permeation
chromatography (GPC) and silica gel chromatography afforded
the triad 5Ni in 14% yield. The high-resolution electrospray-
ionization time-of-flight (HR-ESI-TOF) mass spectrum of
the hybrid porphyrin dimer displayed the parent ion peak at
m/z = 1209.0845 (calcd for C164H160N12Ni2 = 1209.0868 [M]2+).
Covalently linked multiporphyrin arrays have been extensively
explored for their applications in photosynthetic models, sensors,
molecular wires, nonlinear optical (NLO) materials, and so on.1
Among these, we have recently explored iridium(I)-catalysed
b-borylation reaction of porphyrins that provides b-borylated
porphyrins selectively.2 Using these b-borylated porphyrins we
have developed various functional oligomeric porphyrin arrays
including doubly b-to-b bridged porphyrin arrays 1a, 1b, 1c–d
and 1e that have butadiyne-,3 2,5-thienylene-,4 2,6-pyridylene-5
and 5,15-porphyrinylene-spacer,6 respectively. Interestingly,
these porphyrin arrays (1a–d) displayed large two-photon
absorption (TPA) cross-section values owing to the effective
p-conjugation between two porphyrins.7
1
The H-NMR spectrum of 5Ni in CDCl3 at room temperature
was very broad but became sharpened at 60 1C to exhibit one
singlet peak and two doublet peaks for the b-protons at 9.78,
8.92 and 8.69 ppm, a singlet peak at 10.71 ppm for the meso-
protons, two singlet peaks for the perylene protons at 8.69 and
8.41 ppm, and two doublets and one triplet for the pyridine
protons at 8.09, 7.98 and 7.78 ppm. The structural assignment
We have also developed three-dimensional porphyrin ladders
via combined reaction sequences of Ag(I)-promoted meso–
meso coupling of porphyrins and Suzuki–Miyaura coupling.6
Furthermore, a similar strategy allowed us to synthesize doubly
1
of perylene has been proved by a H–1H NOESY technique
(see Fig. S4 in ESIz).
a Department of Chemistry, Graduate School of Science,
Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.
The single crystals of 5Ni suitable for X-ray diffraction
analysis were grown by slow vapour diffusion of acetonitrile
into its chloroform solution (Fig. 1).y The crystallographic
asymmetric unit of 5Ni contains two molecules (triads A and B)
of different structures, both of which took belt-like syn confor-
mations. The centre-to-centre distances between the nickel ions
are 16.68 and 18.98 A for triads A and B, respectively. The
porphyrin moieties are tilted by 50.171, 59.431, 68.731 and 74.911
relative to the perylene mean planes. These facts indicate certain
E-mail: aratani@kuchem.kyoto-u.ac.jp, osuka@kuchem.kyoto-u.ac.jp;
Fax: +81 75 753 3970; Tel: +81 75 753 4008
b PRESTO, Japan Science and Technology Agency, Japan
w This article is part of the ChemComm ‘Porphyrins and phthalocyanines’
web themed issue.
z Electronic supplementary information (ESI) available: Experimental
details of the synthesis and spectroscopic analytical data of new
compounds. CCDC 865051. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2cc30966a
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4317–4319 4317

Fig. 1 X-Ray crystal structures of 5Ni. (a) Side and (b) top view of
triad A and (c) side and (d) top view of triad B. Substituents on phenyl
groups are omitted for clarity. The ellipsoids are scaled to the 20%
probability.
Scheme 1 Synthesis of porphyrin–perylene–porphyrin hybrid triads
and host–guest complexation of 5Zn with mP.
conformational flexibility for 5Ni. In the solid state, triads A
and B are alternatively stacked with their facing perylene parts
closer at a distance of ca. 3.6 A.z
A nickel complex was demetalated with H2SO4/TFA to give
free base porphyrin dimer 5H, which was then converted into
the corresponding zinc(II) complex 5Zn upon treatment with
Zn(OAc)2. The UV/vis absorption spectrum of 5Zn in CH2Cl2
is shown in Fig. 2. The 5Zn exhibits a blue-shifted Soret band
at 429 nm and Q bands at 554 and 596 nm as compared with
those of reference 5,10,15-triaryl-2,18-dipyridylporphyrin zinc(II)
complex 6Zn (Soret band; 433 nm, Q bands; 557 and 598 nm).z
These phenomena suggest the weak but distinct excitonic inter-
action between the porphyrin and perylene units.11
The bent structure of 5Ni and its conformational flexibility
inspired us to examine the supramolecular interaction with
tetrakis(3-pyridyl)porphyrin (mP).12,13 Photometric titration
experiments were conducted to determine the stoichiometry
between 5Zn and mP in CH2Cl2. Upon the addition of mP to
the 5Zn solution, the absorption intensities at 429 and 554 nm
were found to decrease and that at 515 nm was seen to increase
with isosbestic points at 437 and 562 nm. As shown in Fig. S13
(ESIz), a plot of the absorbance at 429 nm versus [mP] has an
inflection point at [mP]/[5Zn] = 0.5. The Job’s plot clearly
supported a 2 : 1 stoichiometry (Fig. S12, ESIz). The ion peak
of the complex was successfully detected by HR-ESI-MS in a
Fig. 2 UV/vis absorption spectrum of 5Zn in CH2Cl2. Inset: fluores-
cence spectra of 5Zn (black) and 5Zn2–mP (red) excited at 437 nm in
CH2Cl2.
positive mode at m/z = 2740.2639 for [5Zn2–mP + 2H]2+
(calcd for [C368H346N32Zn4]2+ = 2740.2713) (Fig. S11, ESIz).
These phenomena explain that 5Zn and mP form a 2 : 1
1
complex. The H NMR spectrum of the 2 : 1 mixture of 5Zn
and mP shows large complex-induced changes in chemical
c
4318 Chem. Commun., 2012, 48, 4317–4319
This journal is The Royal Society of Chemistry 2012

Culture, Sports, Science and Technology, Japan, and by JST
PRESTO program. The authors thank Prof. H. Maeda and
Dr Y. Haketa (Ritsumeikan University) for MALDI-TOF MS
measurement.
Notes and references
y Crystallographic data for 5Ni: C164H160N12Ni2, M = 2416.46,
%
triclinic, space group P1 (#2), a = 20.6104(4), b = 28.4361(5),
c = 32.8677(6) A, a = 113.2820(8)1, b = 98.5654(9)1, g = 95.3241(9)1,
V = 17251.5(6) A3, T = 93(2) K, Z = 4, reflections measured 165 003,
49 820 unique. The final R1 was 0.0971 (>2s(I)), and the final wR on F2
was 0.2069 (all data), GOF = 1.040. CCDC 865051. The contributions to
the scattering arising from the presence of the disordered solvents in the
crystal were removed by use of the utility SQUEEZE in the PLATON
software package.16
Fig. 3 1H NMR spectra of (a) 5Zn, (b) a mixture of 5Zn and mP
(2 : 1), and (c) mP in CDCl3.
1 N. Aratani and A. Osuka, in Handbook of Porphyrin Science,
ed. K. M. Kadish, K. M. Smith and R. Guilard, World Scientific
Publishing, Singapore, 2010, vol. 1, ch. 1, pp. 1–132 and references
therein.
2 H. Hata, H. Shinokubo and A. Osuka, J. Am. Chem. Soc., 2005,
127, 8264.
shifts for the signals corresponding to the pyridyl protons of
mP as shown in Fig. 3. These results clearly demonstrate the
guest-binding ability of 5Zn.
The steady-state fluorescence spectra of 5Zn and 5Zn2–mP
are shown in the inset of Fig. 2. Upon photoexcitation of
5Zn2–mP at 437 nm that corresponded to selective excitation
at the zinc(II) porphyrin moieties, the complex exhibited reduced
fluorescence from the 5Zn part. Importantly, the fluorescence
from the mP was not detected. These data suggested the
intracomplex electron transfer from the photoexcited 5Zn to
mP, while the excitation energy transfer is the most common
process for zinc(II) free-base hybrid porphyrin pairs.14 By cyclic
and differential pulse voltammetry methods in CH2Cl2, the first
reduction potential of mP and the first oxidation potential of
6Zn were measured to be ꢀ1.40 and +0.39 V, respectively, with
respect to the Ag/AgClO4. The free energy calculated by the
Rehm–Weller equation15 indicates that the electron transfer
3 I. Hisaki, S. Hiroto, K. S. Kim, S. B. Noh, D. Kim, H. Shinokubo
and A. Osuka, Angew. Chem., Int. Ed., 2007, 46, 5125.
4 J. Song, S. Y. Jang, S. Yamaguchi, J. Sankar, S. Hiroto, N. Aratani,
J.-Y. Shin, S. Easwaramoorthi, K. S. Kim, D. Kim, H. Shinokubo
and A. Osuka, Angew. Chem., Int. Ed., 2008, 47, 6004.
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Chem.–Eur. J., 2010, 16, 3009; J. Song, N. Aratani, J. H. Heo,
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from (5Zn)* to mP is exothermic by ꢀ0.46 eV, while the free
energy change associated with the excitation energy transfer is
ꢀ0.15 eV.z The coordination interactions between the pyridyl
groups with the Zn(II) centres render the zinc(II) porphyrins
more electron-donating and the pyridyl-appended free base
porphyrin more electron-accepting, which makes the intra-
complex electron transfer more feasible.
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T. B. Marder and R. N. Perutz, Chem. Commun., 2005, 2172.
11 Perfect overlapping of the absorption spectra and large deference
of the molecular extinction coefficients for perylene and porphyrins
make selective photo-excitation of the perylene difficult so that
excitation energy and/or electron transfer from the perylene to the
porphyrin was not clearly observed.
12 R. A. Haycock, A. Yartsev, U. Michelsen, V. Sundstrom and
¨
In conclusion, we have synthesized the doubly 2,6-pyridylene-
bridged porphyrin–perylene–porphyrin triad by Suzuki–Miyaura
cross-coupling reaction. The 1H-NMR, mass, and UV/vis absorp-
tion spectra of this triad revealed the structural characterisation in
solution, and the bent structure was observed in the solid state
of 5Ni. The supramolecular complex 5Zn2–mP leads to novel
electro-photochemical properties. Investigation on the perylene-
inserted porphyrin nanobarrel is currently in progress.
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This work was partly supported by Grants-in-Aid for Scientific
Research (No. 19205006 (A), 23685030 for Young Scientists (A)
and 20108006 ‘‘pi-Space’’) from the Ministry of Education,
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4317–4319 4319