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Published on the web June 4, 2011
Synthesis and Electronic Properties of an Azulene-fused Tribenzotetraazaporphyrin
Atsuya Muranaka,*1,2 Mitsuhiro Yonehara,1 Machiko Hirayama,1 Asami Saito,1 Nagao Kobayashi,3 and Masanobu Uchiyama*1,4
1Advanced Elements Chemistry Research Team, Advanced Science Institute, RIKEN, Wako, Saitama 351-0198
2PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012
3Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai, Miyagi 980-8578
4Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
(Received April 13, 2011; CL-110315; E-mail: atsuya-muranaka@riken.jp)
A novel tribenzotetraazaporphyrin with a fused azulene
ring was synthesized via mixed condensation reaction of
5,6-dicyanoazulene and 3,6-dibutoxyphthalonitrile. The macro-
cycle exhibited several intense absorption peaks from 600 to
900 nm. The spectroscopic properties were analyzed by DFT
calculations.
temperature, the reaction mixture was poured into methanol and
filtrated. After the filtrate was dissolved in CHCl3, purification
by silica gel column chromatography and size-exclusion column
chromatography (Bio-beads S-X1, Bio-Rad Laboratories Inc.)
using CHCl3 furnished the desired compound as a deep-green
solid (2.6 mg, 1%). Structural determination of 2 was based on
1H NMR, HRMS, and electronic absorption spectroscopy.4
1
Figure 1 shows the H NMR spectrum of 2. The chemical
Much effort has been made to develop various types of near-
infrared organic compounds because of their potential applica-
tions in organic solar cells, photodynamic therapy for cancer,
heat absorbers, and near-infrared imaging.1 Since most near-
infrared dyes are unstable, breakthroughs in molecular design
and synthetic strategy to develop superior dyes are required by
the scientific community. Phthalocyanine derivatives such as
naphthalocyanines are typical near-infrared dyes in terms of
their intense absorption in the Q band region.2 Recently, our
group has reported a new class of near-infrared phthalocyanine
derivatives in which four azulene units were fused to a
tetraazaporphyrin skeleton.3 The azulene-fused macrocycle 1
(Chart 1), which is referred to as azulenocyanine, can be
regarded as a structural isomer of naphthalocyanines. It was
revealed that expanding a tetraazaporphyrin ³-system by fusion
with four azulene units drastically lowered the LUMO level of
the macrocycle. Herein we report the synthesis, spectroscopic
properties, and theoretical calculations of a new unsymmetri-
cally substituted phthalocyanine derivative 2 in which one fused
benzene ring was replaced by an azulene ring. The results were
compared to those of the corresponding phthalocyanine 3 to
enhance our understanding of spectral changes by extended
³-conjugation with azulene.
shifts of azulene protons near the tetraazaporphyrin skeleton (Ha
and Hb) exhibited down-field shifts, which can be interpreted in
terms of the strong ring current of the macrocycle. The extent of
the shifts is similar to that of the azulenocyanine (1).3
Figure 2a shows the electronic absorption and magnetic
circular dichroism (MCD) spectra of 2 in CHCl3, together
with those of 1,4,8,11,15,18,22,25-octabutoxyphthalocyaninato-
nickel(II) (3) as a reference. A single Q band was observed at
745 nm for the symmetric phthalocyanine 3.5 In the case of the
azulene-fused macrocycle, several intense absorption peaks
(746, 816, and 864 nm) were observed in the near-infrared
region. Since spectral red shift of naphthalene-fused tribenzo-
tetraazaporphyrins is much smaller than that of 2,6 it is
confirmed that the azulene unit impacts profoundly the optical
properties of the tetraazaporphyrin, compared with a naph-
thalene unit.
The MCD signal pattern of 2 is characteristic of this class of
low-symmetry phthalocyanine derivatives.7 Thus, the Q band
peak positions in the MCD spectrum were nearly identical to
those in the absorption spectrum, which is in contrast to the
intense derivative-shaped MCD signal of 3. This indicates that
compound 2 has split Q absorption bands.
In order to deepen our interpretation of the spectra of the
azulene-fused tetraazaporphyrin, DFT calculations were carried
The azulene-fused macrocycle 2 was prepared by a cross-
cyclotetramerization reaction. A mixture of 5,6-dicyanoazulene3
(53.5 mg, 0.30 mmol), 3,6-dibutoxyphthalonitrile (245 mg, 0.90
mmol), nickel(II) chloride (194 mg, 1.50 mmol), urea (288 mg,
4.80 mmol), and ammonium molybdate (12.0 mg, 0.01 mmol)
in dry quinoline (3.0 mL) was stirred at 180 °C under argon
atmosphere for 22 h. After the mixture was cooled to room
β
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tBu
e
tBu
RO
N
RO
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OR
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b
d
RO
N
N
N
β
tBu
c
*
*
RO
OR RO
OR
OR
f
HN
N
N
N
N
N
N
N
N
N
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Ni
N
Ni
a
c
e
f
b
d
NH
N
N
N
OR
RO
N
N
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7
11
10
9
8
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OR RO
tBu
tBu
ppm
tBu
2 (R = n-C4H9)
2'
3 (R = n-C4H9)
3'
1
tBu
(R = CH3)
(R = CH3)
1
Figure 1. Partial H NMR spectrum of 2 in CDCl3. Asterisks
indicate solvent peaks.
Chart 1.
Chem. Lett. 2011, 40, 714-716
© 2011 The Chemical Society of Japan