3,3′- and 4,4′-biphenylene-bridged subporphyrin dimers

Article abstract of DOI:10.1021/ol8023924

(Figure Presented) 3,3′- and 4,4′-biphenylene-bridged subporphyrin dimers were synthesized by means of a Ni-catalyzed homocoupling reaction. Distinctly red-shifted absorption bands and enhanced fluorescence were observed only for the latter, indicating effective through-bond interactions through a 4,4′-biphenylene bridge, which is not possible for porphyrins.

Full text of DOI:10.1021/ol8023924

ORGANIC
LETTERS
2008
Vol. 10, No. 24
5561-5564
3,3′- and 4,4′-Biphenylene-Bridged
Subporphyrin Dimers
Yasuhide Inokuma and Atsuhiro Osuka*
Department of Chemistry, Graduate School of Science, Kyoto UniVersity Sakyo-ku,
Kyoto 606-8502, Japan
osuka@kuchem.kyoto-u.ac.jp
Received October 16, 2008
ABSTRACT
3,3′- and 4,4′-biphenylene-bridged subporphyrin dimers were synthesized by means of a Ni-catalyzed homocoupling reaction. Distinctly red-
shifted absorption bands and enhanced fluorescence were observed only for the latter, indicating effective through-bond interactions through
a 4,4′-biphenylene bridge, which is not possible for porphyrins.
Subporphyrin is a genuine contracted porphyrin that pos-
sesses a curved 14π-aromatic circuit along a bowl-shaped
structure. The chemistry of subporphyrin was triggered by
our first synthesis of tribenzosubporphine in 2006.1 Since
then, a variety of periphery-substituted2 and core-modified3
analogues have been explored which display fascinating
properties such as intense green fluorescence and large meso-
aryl-substituent effects on the electrochemical and optical
properties. A promising next step will be to explore co-
valently linked subporphyrin oligomers, but only two ex-
amples have been reported so far, which are both constructed
by axial coordination at the boron atom.2b,4 This present
situation of subporphyrins is clearly different from that of
subphthalocyanines5 in that various covalently linked sub-
phthalocyanine dimers and trimers have been synthesized
and shown to exhibit interesting electronic properties.6 In
this context, the interchromophore interactions in covalently
linked subporphyrin oligomers still remain virtually unex-
plored. Herein, we wish to report the synthesis of 3,3′- and
4,4′-biphenylene-bridged subporphyrin dimers as the first
examples of aromatic-bridged subporphyrin oligomers.
Scheme 1 outlines the synthetic route of biphenylene
dimers 1 and 2. Pyridine-tri-N-pyrrolylborane, benzalde-
hyde, and 4-bromobenzaldehyde were condensed in the
presence of trifluoroacetic acid followed by aerobic refluxing
in 1,2-dichlorobenzene to give a mixture of four possible
subporphyrins. This mixture was dissolved in DMF and
heated to 80 °C in the presence of NiCl2(dppp), Zn powder,
and KI to afford 4,4′-biphenylene-linked dimer 1 in 1.3%
(1) Inokuma, Y.; Kwon, J. H.; Ahn, T. K.; Yoon, M.-C.; Kim, D.; Osuka,
A. Angew. Chem., Int. Ed. 2006, 45, 961–964.
(2) (a) Kobayashi, N.; Takeuchi, Y.; Matsuda, A. Angew. Chem., Int.
Ed. 2007, 46, 758–760. (b) Takeuchi, Y.; Matsuda, A.; Kobayashi, N. J. Am.
Chem. Soc. 2007, 129, 8271–8281. (c) Inokuma, Y.; Yoon, Z. S.; Kim, D.;
Osuka, A. J. Am. Chem. Soc. 2007, 129, 4747–4761. (c) Makarova, E. A.;
Shimizu, S.; Matsuda, A.; Luk’yanets, E. A.; Kobayashi, N. Chem. Commun.
2008, 2109–2111. (d) Tsurumaki, E.; Inokuma, Y.; Easwaramoorthi, S.;
Lim, J. M.; Kim, D.; Osuka, A. Chem.sEur. J., in press (DOI: 10.1002/
chem.200801802).
(5) (a) Meller, A.; Ossko, A. Monatsh. Chem. 1972, 103, 150–155. (b)
Claessens, C. G.; Gonzalez-Rodriguez, D.; Torres, T. Chem. ReV. 2002,
102, 835–853. (c) Torres, T. Angew. Chem., Int. Ed. 2006, 45, 2834–2837.
(6) (a) Claessens, C. G.; Torres, T. Angew. Chem., Int. Ed. 2002, 41,
2561–2565. (b) Fukuda, T.; Stork, J. R.; Potucek, R. J.; Olmstead, M. M.;
Noll, B. C.; Kobayashi, N.; Durfee, W. S. Angew. Chem., Int. Ed. 2002,
41, 2565–2568. (c) Iglesias, R. S.; Claessens, C. G.; Torres, T.; Herranz,
M. A.; Ferro, V. R.; Garcı´a de la Vega, J. M. J. Org. Chem. 2007, 72,
2967–2977. (d) Igleslas, R. S.; Claessens, C. G.; Herranz, M. A.; Torres,
T. Org. Lett. 2007, 9, 5381–5384.
(3) Mys´liborski, R.; Latos-Graz˙yn´ski, L.; Szterenberg, L.; Lis, T. Angew.
Chem., Int. Ed. 2006, 45, 3670–3674.
(4) Inokuma, Y.; Osuka, A. Chem. Commun. 2007, 2930–2940
.
10.1021/ol8023924 CCC: $40.75
Published on Web 11/19/2008
2008 American Chemical Society

yield for two steps along with triphenylsubporphyrin 3
(2.4%). 3,3′-Biphenylene-bridged subporphyrin dimer 2 was
also prepared by the same procedure from 3-bromobenz-
aldehyde in 1.0% yield. The parent ion peak was observed
at m/z ) 969.3696 for 1 (calcd for C67H43N6B2O1 ) 969.3699
[M - OMe]+), while two peaks were detected for 2 at m/z
) 969.3694 and 1023.3789 (calcd for C68H46N6B2O2Na )
1023.3781 [M + Na]+) by ESI-TOF mass analysis. The 1H
NMR spectrum of 1 exhibits a single set of subporphyrin
signals featuring a singlet at 8.15 ppm and a couple of
doublets at 8.26, 8.18 ppm due to ꢀ-protons and a singlet at
0.89 ppm due to the axial methoxyl protons. Observation of
the protons in the 4,4′-biphenylene bridge as a pair of
doublets at 8.27 and 8.14 ppm indicates its free rotation at
the subporphyrin mean plane. These structural features are
favorable for the effective through-bond interaction of two
subporphyrin chromophores.
1
room temperature. A similar H NMR spectral pattern was
recorded for 3,3′-linked dimer 2, thus suggesting that two
subporphyrin subunits are equivalent in both 1 and 2.
Scheme 1. Synthesis of Biphenylene-Linked Dimers 1 and 2
Figure 1. ORTEP representations of the crystal structure of 1 drawn
at the 50% probability level (black, carbon; red, oxygen; blue,
nitrogen; green, boron).
Figure 2. UV-vis absorption (solid line) and fluorescence (dashed
line) spectra of subporphyrins in CH2Cl2.
Single-crystal X-ray diffraction analysis revealed the solid-
state structure of 1 (Figure 1)7 in which the two axial
methoxy groups are pointed at the opposite direction, as can
be seen in the side view, and hence takes Ci molecular
symmetry. The B-B′ distance is 15.9 Å, and the C5-C5′
(two meso-carbons at the linkage) distance is 10.1 Å. The
two bowl-shaped cores are arranged in a wavelike form.
Interestingly, the 4,4′-biphenylene subunit displays a rather
coplanar conformation with a dihedral angle of 50.9° toward
Figure 2 shows UV-vis absorption and fluorescence
spectra of 1-3 in CH2Cl2. The absorption spectrum of 1
differs substantially from those of 2 and 3. The dimer 2
shows a Soret-like band at 374 nm and Q-like bands at 461
and 486 nm, which is similar to that of the parent meso-
triphenyl-substituted subporphyrin 3. Thus, the absorption
spectrum of 2 can be roughly reproduced by the summation
of that of 3, which indicates merely weak electronic
interaction between the two subporphyrin units in the ground
state. In contrast, the dimer 1 shows a definitely red-shifted
Soret-like band at 379 nm and Q-like bands at 463 and 494
nm along with slight but distinct broadening. In addition,
the Q(0,0) band of 1 is enhanced compared to Q(1,0)
(7) Crystall data for 1: C68H48B2N6O2·CH2Cl2, M ) 1085.65, monoclinic,
space group P21/c (no. 14), a ) 11.866(4) Å, b ) 19.05(7) Å, c ) 12.211(5)
Å, ꢀ ) 103.446(15)°, V ) 2748.9(17) Å3, Z ) 2, Dcalc ) 1.312 g cm-3, R1
) 0.0822 for I > 2σ(I), wR ) 0.2784 for all data, CCDC 705219.
5562
Org. Lett., Vol. 10, No. 24, 2008

transition, which also contrasts to the usual spectral feature
of meso-aryl substituted subporphyrins as seen in 2 and 3.
The enhanced electronic interactions of 1 can be seen also
in the fluorescence. The dimer 2 emits green fluorescence at
516 nm with its quantum yield (ΦF) of 0.15, which is
comparable to that of 3 (ΦF ) 0.13). On the other hand, the
dimer 1 exhibits yellowish-green fluorescence at 531 nm
tailing over 650 nm as a good mirror image of Q(0,0) band.
Interestingly, the fluorescence quantum yield of 1 (ΦF )
0.28) is ca. 2-fold of 3.
between the biphenylene linker and porphyrin mean plane
is roughly perpendicular. This leads to small through-bond
interactions between two porphyrins in the ground state. On
the other hand, 4,4′-biphenylene bridge in 1 can serve as a
conjugated linker which enables strong orbital interactions
between two subporphyrins. Consequently, a 4,4′-biphenyl-
ene subunit can provide remarkable through-bond interaction
for the subporphyrin case.
To confirm the enhanced electronic interactions in 1, the
electrochemical properties of 1-3 were examined by cyclic
voltammetry, which also revealed a distinctly different
property of 1 from those of 2 and 3 (Table 1 and Supporting
Information). The first oxidation and reduction waves of 2
were observed at 0.81 and -1.91 V (vs ferrocene/ferroce-
nium ion couple) both as reversible processes. On the basis
of these results, the electrochemical HOMO-LUMO gap
of 2 has been estimated to be 2.72 eV, which is similar to
that of monomer 3 (2.68 eV). On the other hand, the dimer
1 shows two reversible oxidation waves at 0.61 and 0.75 V.
The first oxidation potential of 1 is remarkably lower than
those of 2 and 3, although the reduction potential of 1 is
similar to 2 and 3. As a consequence, the HOMO-LUMO
gap of 1 is ca. 0.15 V smaller than 2 and 3, which is in
good agreement with optical HOMO-LUMO gaps (2.51,
2.55, and 2.56 eV for 1, 2, and 3, respectively).
Figure 3. Frontier molecular orbitals of 1 and 2 calculated at the
B3LYP/6-31* level.
Table 1. Redox Potentialsa and Electrochemical
HOMO-LUMO Gaps of 1-3
While the B-B′ distance in 1 is fixed, that of 2 can change
upon bond rotation. Since axially coordinated boron ligand
can be changed reversibly by carboxyl groups,2,4 we
examined axial ligand exchange reaction with biphenyl-3,3′-
dicarboxylic acid (4) to form a 1:1 cyclic complex, 5, in
which the distance and arrangement of two subporphyrins
are fixed. A 1:1 mixture of 2 and 4 was refluxed in toluene
under diluted conditions (∼125 µM). After removal of the
solvent, complex 5 was obtained quantitatively. The ESI-
TOF mass spectrum of 5 exhibits its parent cation peak at
m/z ) 1179.4028, which is calculated for C80H49N6B2O4 )
1179.4019 [2 - 2 × MeOH + 4]+. The 1H NMR spectrum
also supports the cyclic structure of 5, featuring C2 symmetric
signal pattern and shielded signals at 6.7-7.0 ppm due to
axial ligand. The highly deshielded singlet at 9.28 ppm is
assigned to the biphenylene bridge proton (Ha in Scheme
2), and this characteristic shift is attributed to the diatropic
ring current of the two subporphyrins forced in a face-to-
face conformation.
E2
E1
E1
HOMO-LUMO gap
ox,1/2
ox,1/2
red,1/2
1
2
3
0.75
0.61
0.81
0.71
-1.93
-1.91
-1.97
2.54
2.72
2.68
a Values are reported in V vs ferrocene/ferrocenium ion couple.
Measurement conditions: solvent, CH2Cl2 with 0.10 M Bu4NPF6 as a
supporting electrolyte; working electrode, glassy carbon; counter electrode,
platinum; reference electrode, Ag/AgClO4.
Molecular orbital calculations of 1 and 2 were performed
with the Gaussian 03 package8 at the B3LYP/6-31G* level
on the basis of the crystal structure and the optimized
structure, respectively. In the optimized structure, the dimer
2 takes a relatively planar conformation with a B-B′ distance
of 14.5 Å. Interestingly, HOMO and LUMO of 1 show large
orbital coefficients on the biphenylene bridge, whereas those
of 2 are vanishingly small at the bridge, suggesting efficient
and inefficient elongation of π-conjugation for 4,4′- and 3,3′-
biphenylene linkages, respectively. The HOMO-LUMO
gaps of 1, 2, and 3 were calculated to be 2.94, 3.17, and
3.18 eV, respectively, showing a certain decrease for 1 in
accord with the experimental results. In the case of bi-
phenylene-bridged porphyrin dimers,9 the dihedral angle
The dihedral angle of two subporphyrins’ mean plane is
88° for 5, and the boron-boron distance is kept distinctly
short (10.3 Å) (Supporting Information). UV-vis absorption
spectrum of 5 recorded in CH2Cl2 was, however, almost
identical to that of 2, exhibiting absorption maxima at 371,
458, and 481 nm. Although the center-to-center interchro-
mophore distances of 2 and 5 are considerably shorter than
1, perturbations observed in their UV-vis absorption spectra
(8) For the full citation, see the Supporting Information.
(9) Cho, S.; Yoon, M.-C.; Kim, C. H.; Aratani, N.; Mori, G.; Joo, T.;
Osuka, A.; Kim, D. J. Phys. Chem. C 2007, 111, 14881–14888.
Org. Lett., Vol. 10, No. 24, 2008
5563

are negligible. These facts indicate the importance of the
through-bond interaction in 1.
reaction as the first example of an aromatic-bridged subpor-
phyrin dimer. Cyclic dimer 5 was also synthesized from 2
and dicarboxylic acid 4 as a distance- and orientation-fixed
model. 4,4′-Biphenylene-bridged dimer 1 exhibited a batho-
chromically shifted absorption spectrum, intensified fluores-
cence emission, and a distinctly reduced HOMO-LUMO
gap as revealed by the electrochemical and computational
studies, whereas such perturbations were only marginal for
2 and 5 in spite of their shorter interchromophore distances
compared to 1. Therefore, the distinctly perturbed charac-
teristics of 1 can be ascribed to the effective π-conjugation
through a 4,4′-biphenylene unit which is difficult for por-
phyrins. It can be concluded that through-bond interaction
assisted by the conjugation bridge is particularly important
for interchromophore communication between two subpor-
phyrins.
1
Scheme 2
.
Axial Ligand Exchange Reaction of 2 and H NMR
Spectra of 2 and 5 in CDCl3 (*: Solvent Peak)
Acknowledgment. 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.
Supporting Information Available: Experimental pro-
cedures and spectroscopic data for new compounds and
crystal data of 1 (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
In summary, we have prepared biphenylene-bridged sub-
porphyrin dimers 1 and 2 by a Ni-catalyzed homocoupling
OL8023924
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Org. Lett., Vol. 10, No. 24, 2008