1,4-Phenylene-bridged subporphyrin-porphyrin dyad, triad, and tetrad

Article abstract of DOI:10.1246/cl.2009.206

1,4-Phenylene-bridged subporphyrin-porphyrin hybrid molecules were prepared by the Suzuki-Miyaura cross coupling reaction of meso-(4-bromophenyl)- substituted subporphyrin 2 and 5-pinacolateboryl porphyrin 3. In all hybrid models examined, intramolecular

Full text of DOI:10.1246/cl.2009.206

206
Chemistry Letters Vol.38, No.3 (2009)
1,4-Phenylene-bridged Subporphyrin–Porphyrin Dyad, Triad, and Tetrad
Yasuhide Inokuma, Shin-ya Hayashi, and Atsuhiro Osukaꢀ
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502
(Received December 15, 2008; CL-081164; E-mail: osuka@kuchem.kyoto-u.ac.jp)
Ph
1,4-Phenylene-bridged subporphyrin–porphyrin hybrid
OMe
molecules were prepared by the Suzuki–Miyaura cross coupling
reaction of meso-(4-bromophenyl)-substituted subporphyrin 2
and 5-pinacolateboryl porphyrin 3. In all hybrid models exam-
ined, intramolecular energy transfer from the subporphyrin moi-
ety to the porphyrin moieties proceeds efficiently.
Ar
Ar
N
B
N
N
N
N
1 (Ar = Ph)
2 (Ar = 4-BrPh)
O
O
Zn
B
3
N
N
Ar
Ph
Ph
Ar1
4 (Ar1 = Ar2 = Ph)
5 (Ar1 = Por, Ar2 = Ph)
6 (Ar1 = Ar2 = Por)
Ph
MeO
N
N
N
N
N
Since our discovery of tribenzosubporphine in 2006,1 sub-
porphyrin, a genuine contracted porphyrin with a bowl-shaped
14ꢀ-aromatic circuit, has gathered much attention as a new class
of visible-fluorescent chromophore, unique triangular bowl-
shaped molecule, and boron-chelating supramolecular assem-
bly.2,3 One of the most significant characteristics of subporphy-
rin is the large meso substituent effects on the electronic proper-
ties of subporphyrin core, which is attributed to the low rotation-
al energy barrier of meso-aryl groups.3c In this regard, a variety
of meso-aryl-substituted subporphyrins have been explored so
far. For example, intramolecular charge-transfer interactions in
meso-(4-nitrophenyl)- or (4-aminophenyl)-substituted subpor-
phyrins cause drastic alterations of absorption and fluorescence
properties.3c,4 Oligo(1,4-phenyleneethynylene)-substituted sub-
porphyrins exhibit progressive increases in the absorption coef-
ficients, fluorescence quantum yields, and two-photon absorp-
tion cross sections.5 Furthermore, significant interchromophore
interactions between two subporphyrin cores have been dis-
closed for a 4,40-biphenylene-linked subporphyrin dimer.6 The
fluorescence of meso-phenyl-subporphyrin 1 is observed in a
range of 500–600 nm and nicely matches with the Q-band ab-
sorption of porphyrin. This situation will allow an efficient ener-
gy transfer from subporphyrin to porphyrin in covalently-linked
models. In this context, phenylene-bridged subporphyrin–por-
phyrin hybrids 4–7 were synthesized and their optical and elec-
trochemical properties were investigated.
N
B
Zn
Ph
N
Ar2
N
N
N
N
Por =
Zn
Ph
Ph
Ph
N
N
B
MeO
N
Zn
N
N
7
N
N
Ph
Ph
Chart 1.
Figure 1. X-ray crystal structure of dyad 4.
A single crystal of dyad 4 suitable for X-ray diffraction anal-
ysis was grown from a mixture of CH2Cl2/MeOH. The solid-
state structure of 4 shows a rather coplanar conformation with
a small dihedral angle (16ꢁ) between the mean planes of porphy-
rin and subporphyrin, which are respectively tilted by 51.0 and
62.7ꢁ towards the 1,4-phenylene bridge (Figure 1).8 The distance
˚
between the boron and zinc ions is 12.2 A. The central Zn ion
meso-Tri(4-bromophenyl)subporphyrin 2 and 5-pinacolate-
borylporphyrin 37 were coupled in the presence of a Pd-catalyst
under standard Suzuki–Miyaura cross coupling conditions. Sub-
sequent reduction of unreacted bromo groups afforded 1,4-phen-
ylene-bridged subporphyrin–porphyrin dyad 4, triad 5, and
tetrad 6 in 18, 19, and 7% yields, respectively, for two steps
(Chart 1). The 1H NMR spectrum of 4 shows Cs symmetric fea-
tures by showing four doublets at 9.43, 9.26, 9.13, and 9.10 ppm
due to the porphyrin ꢁ-protons, and a couple of doublets and a
singlet at 8.52, 8.30, and 8.19 ppm due to the subporphyrin ꢁ-
protons. Observation of the protons of the 1,4-phenylene bridge
as a pair of doublets indicates its free rotation with respect to the
II
in the porphyrin unit is apically coordinated by a methanol
molecule.
Figure 2 shows absorption and fluorescence spectra of 4–6.
The absorption spectrum of 4 exhibits porphyrin-based Soret-
and Q-bands at 415 and 543 nm along with those of subporphy-
rin at 373 and 490 nm, which can be almost reproducible by a
linear combination of the absorption spectra of the fragment
components. In the absorption spectra of 5 and 6, the relative in-
tensities of the porphyrin-based bands are increased and the sub-
porphyrin-based Soret-like bands undergo slight but significant
broadening, and the subporphyrin-based Q(0,0) band is progres-
sively red-shifted in the order of 4 (490 nm) < 5 (494 nm) < 6
(496 nm). Fluorescence spectra of 4–7 were recorded by excita-
1
subporphyrin core. A similar H NMR spectral pattern was re-
corded for 5. On the other hand, the 1H NMR spectrum of 6 ex-
hibits C3 symmetric features by displaying a singlet at 8.73 ppm
due to the six ꢁ-protons of subporphyrin. 1,3-Phenylene linked
subporphyrin–porphyrin dyad 7 was also prepared by a similar
method.3d
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.3 (2009)
207
of HOMO of 4 is by chance similar to that of ZnTPP. These
results are in good agreement with the observed oxidation poten-
tials, in that the first and second oxidation waves comparable to
those of ZnTPP are assignable to the oxidation processes of the
porphyrin segment and the third one at 0.88 V is assignable to the
first oxidation of the subporphyrin site. Briefly, the porphyrin
segment in 4 serves as an electron-withdrawing substituent for
the subporphyrin subunit, but the electronic interactions are
weak due to the orthogonal conformation of the 1,4-phenylene
bridge to the porphyrin segment.
In conclusion, we prepared hybrids 4–7 as the first examples
of covalently linked subporphyrin–porphyrin models by the
Suzuki–Miyaura cross coupling reaction. Efficient intramolecu-
lar energy transfer from subporphyrin to porphyrin subunit was
observed for 4–6. Examination of further detailed of these
hybrid systems is actively in progress in our laboratory.
Figure 2. UV–vis absorption and fluorescence (dashed line;
ꢆex ¼ 373 nm) spectra of 4–6 in CH2Cl2.
tion at 373 nm, where the subporphyrin subunit is selectively ex-
cited. Interestingly, the fluorescence emission from the subpor-
phyrin was completely quenched, and the fluorescence from
the porphyrin only was observed at 598 and 636 nm. The fluores-
cence quantum yields are 3.4, 3.3, 3.3, and 2.6% for 4, 5, 6, and
7, respectively. The similar fluorescence spectra and quantum
yields (ꢂF ¼ 3:1, 2.9, 3.0, and 2.3% for 4, 5, 6, and 7; Support-
ing Information; SI)9 were also recorded upon excitation at
415 nm, where the porphyrin units are dominantly excited. The
fluorescence excitation spectra of 4–7 detected at 640 nm were
almost identical to the absorption spectra, indicating the quanti-
tative energy transfer from subporphyrin to porphyrin. In sharp
contrast, a 1:1 mixture of subporphyrin 1 and (5,15-diphenylpor-
phyrinato)zinc(II) (2:0 ꢂ 10ꢃ6 M each) exhibited different fluo-
rescence depending upon excitation wavelength, green fluores-
cence at 514 nm for excitation at 373 nm and red fluorescence
at 630 nm for excitation at 408 nm, indicating inefficient inter-
molecular energy transfer at this concentration.
Oxidation potentials of 4–6 were measured by cyclic vol-
tammetry in CH2Cl2 containing 0.10 M Bu4NPF6 as a support-
ing electrolyte. Under similar conditions, the reference mole-
cules, 1 and ZnTPP,10 exhibit the oxidation potentials at
0.71 V and at 0.42 and 0.71 V (vs. ferrocene/ferrocenium pair),
respectively. Three reversible oxidation waves were observed at
0.36, 0.70, and 0.88 V for 4 and 0.34, 0.74, and 0.92 V for 5. In
the case of 6, two reversible oxidation waves were at 0.39 and
0.75 V11 (SI). These results indicate that subporphyrin and por-
phyrin segments retain their individual electrochemical charac-
ters but the covalent linkages of these electron-deficient seg-
ments and the generation of cationic sites lead to slight positive
shifts of the oxidation potentials.
This work was supported by a Grant-in-Aid (A) (No.
19205006) for Scientific Research from MEXT. Y. I. thanks
the JSPS Research Fellowship for Young Scientists.
References and Notes
1
2
3
Y. Inokuma, J. H. Kwon, T. K. Ahn, M.-C. Yoo, D. Kim, A.
Osuka, Angew. Chem., Int. Ed. 2006, 45, 961.
a) T. Torres, Angew. Chem., Int. Ed. 2006, 45, 2834. b) Y.
Inokuma, A. Osuka, Dalton Trans. 2008, 2517.
a) N. Kobayashi, Y. Takeuchi, A. Matsuda, Angew. Chem., Int.
Ed. 2007, 46, 758. b) Y. Takeuchi, A. Matsuda, N. Kobayashi,
J. Am. Chem. Soc. 2007, 129, 8271. c) Y. Inokuma, Z. S. Yoon,
D. Kim, A. Osuka, J. Am. Chem. Soc. 2007, 129, 4747. d) Y.
Inokuma, A. Osuka, Chem. Commun. 2007, 2938. e) E.
Tsurumaki, S. Saito, K. S. Kim, J. M. Lim, Y. Inokuma, D.
Kim, A. Osuka, J. Am. Chem. Soc. 2008, 130, 438. f) S. Saito,
K. S. Kim, Z. S. Yoon, D. Kim, A. Osuka, Angew. Chem., Int.
Ed. 2007, 46, 5591.
4
5
Y. Inokuma, S. Easwaramoorthi, Z. S. Yoon, D. Kim, A.
Osuka, J. Am. Chem. Soc. 2008, 130, 12234.
Y. Inokuma, S. Easwaramoorthi, S. Y. Jang, K. S. Kim, D.
Kim, A. Osuka, Angew. Chem., Int. Ed. 2008, 47, 4840.
Y. Inokuma, A. Osuka, Org. Lett. 2008, 10, 5561.
A. G. Hyslop, M. A. Kellett, P. M. Iovine, M. J. Therien, J. Am.
Chem. Soc. 1998, 120, 12676.
6
7
8
Crystallographic data for 4: C67H46BN7O2Zn, Mr ¼ 1057:29,
˚
orthorhombic, space group Pbca, a ¼ 18:861ð3Þ A, b ¼
3
˚
˚
˚
20:114ð3Þ A, c ¼ 26:675ð5Þ A, V ¼ 10120ð3Þ A , T ¼ 123 K,
Z ¼ 8, ꢃ(Mo Kꢄ) = 0.543 mmꢃ1
,
Dcalcd ¼ 1:388 g/cm3,
In order to understand the optical and electrochemical char-
acteristics of these hybrid compounds, DFT calculation of 4 was
performed at the B3LYP/6-31Gꢀ level (SI). The calculation re-
sults show that LUMO and LUMO+1 are almost localized at the
porphyrin segment and LUMO+2 and LUMO+3 are localized
at the subporphyrin segment. Slight but distinct orbital interac-
tions through a 1,4-phenylene bridge are observed for HOMO
and HOMOꢃ1, although the former is dominated at the porphy-
rin and the latter is dominated at the subporphyrin segment,
respectively. As a consequence, HOMOꢃ1 of 4, which corre-
sponds to subporphyrin a1-orbital, undergoes remarkable stabili-
zation compared to that of 1. On the other hand, the energy level
11582 reflections measured, 8240 unique (Rint ¼ 0:109),
R1 ¼ 0:0551 (I > 2ꢅðIÞ), Rw ¼ 0:1497 (all data), CCDC-
711435.
9
Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.
html.
10 N. Armaroli, F. Diederich, L. Echegoyen, T. Habicher, L.
Flamigni, G. Marconi, J.-F. Nierengarten, New J. Chem.
1999, 77.
11 In differential pulse voltammetry (DPV) measurement, the
third oxidation wave of 6 was observed at 0.99 V (see
Figure S2 in SI).
Published on the web (Advance View) January 31, 2009; doi:10.1246/cl.2009.206