Subazaphenalenephthalocyanine: A subphthalocyanine analogue bearing a six-membered ring unit

Article abstract of DOI:10.1002/anie.201003929

At the core: A novel core-modified subphthalocyanine analogue has been synthesized (see structure; Cl green, red O, blue N). This molecule comprises a six-membered ring unit in place of a five-membered ring unit and exhibits unique spectroscopic (see plot) and electrochemical properties originating from the azaphenalene-containing 14π-electron conjugation system.

Full text of DOI:10.1002/anie.201003929

Communications
DOI: 10.1002/anie.201003929
Subphthalocyanines
Subazaphenalenephthalocyanine: A Subphthalocyanine Analogue
Bearing a Six-Membered Ring Unit**
Hua Zhu, Soji Shimizu, and Nagao Kobayashi*
Subphthalocyanine (SubPc), a contracted congener of phtha-
locyanine (Pc) comprising three isoindole units bridged by
imino-nitrogen atoms, has attracted much attention since its
first synthesis by Meller and Ossko in 1972.[1] The interest lies
in its unique properties, such as intense fluorescence and
nonlinear optical properties stemming from its bowl-shaped
structure and 14p-electron aromatic conjugation system.[2]
Considerable effort has been devoted towards investigating
not only SubPc and its analogues[3] but also their carbon
congeners, subporphyrins, owing to their potential application
in a variety of fields.[4] In 2006, Osuka and co-workers
reported the first synthesis of the subporphyrin called
tribenzosubporphine.[5] Subsequently, Kobayashi and co-
workers reported a facile synthesis of meso-aryl-substituted
subporphyrins using tri-N-pyrrolylborane as a synthetic
precursor,[6] which was further developed by the Osuka
research group.[7] With basic subfamily structures of both
phthalocyanines and porphyrins and knowledge about the
properties of these contracted analogues in hand,[5–9] research
interest is now shifting to their modified analogues, as
exemplified by subpyriporphyrin[10] and [14]triphyrin-
(2.1.1),[11] because of the expectation that such species will
exhibit different absorption, emission, and coordination
properties and lead to an in-depth understanding of con-
tracted conjugation systems. This kind of information should
be indispensable for tuning their properties in respective
applications.
tailored by core-modification. Recently, our research group
has found that core-modified Pc analogues bearing azaphe-
nalene units in place of isoindole units can be synthesized
using 1,8-naphthalenedicarbonitrile as a key precursor. These
azaphenalenephthalocyanines (APPcs)[12] basically retained
the 18p-electron aromatic characteristics of Pc, but the
spectroscopic and electrochemical properties were signifi-
cantly altered depending on the number and positions of the
azaphenalene units. This motivated us to investigate a novel
core-modified SubPc analogue with a macrocyclic 14p-
electron aromatic conjugation system according to the same
synthetic protocol.
Subazaphenalenephthalocyanine (SubAPPc)
2
was
obtained in an overall yield of 1.6% from a mixed-condensa-
tion reaction of 1,8-naphthalenedicarbonitrile[13] and 4,5-di-p-
tert-butylphenyloxyphthalonitrile[14] in the presence of boron
trichloride (1.0m solution in n-heptane), followed by axial
ligand replacement from chloride to phenoxide (Scheme 1).
This replacement step was performed to prevent adsorption
of axially chlorine- or hydroxy-substituted species formed
during separation by column chromatography on silica gel.
The high-resolution ESI-FT-ICR mass spectrometry
experiment revealed an intense peak at m/z = 1187.4768
(calcd for C74H66BClN6O5Na [M++Na]: 1187.4774), and is
indicative of chlorination at one of the peripheral or non-
peripheral positions, which is often induced during the
formation of subphthalocyanines using BCl3 as a template
reagent.[15] The formation of this chlorinated species was also
´
Subpyriporphyrin, which was reported by Latos-Graz˙yn-
1
ski and co-workers, is the first core-modified subporphyrin in
which one pyrrole ring is replaced by a pyridine ring, and this
molecule exhibits non-aromatic character when nonmeta-
lated as a result of the localization of conjugation on the
pyridine ring.[10] Core-modification with no loss of the 14p-
electron macrocyclic conjugation has thus remained a chal-
lenge, and a core-modified analogue of SubPc has not yet
been realized despite the expectation that absorption and
emission properties in the visible and near IR region can be
confirmed by H NMR spectroscopy. Signals corresponding
to the protons of the axial phenoxy ligand are observed at d =
6.80, 6.66, and 5.74 ppm. The up-field shift of protons at
ortho positions of the axial phenoxy ligand suggests the
presence of a diatropic ring current effect arising from the
14p-electron aromatic conjugation system. The aromaticity of
2 is, however, rather less significant than that of 1, which has a
signal at d = 5.30 ppm that corresponds to the ortho protons
of the axial phenoxy ligand. Signals corresponding to the
naphthalene moiety are observed at d = 8.77 (1H), 8.63 (1H),
8.09 (1H), and 7.59 ppm (2H), the correlations of which
suggest that the 4- or 5-position of the naphthalene moiety is
chlorinated. To avoid chlorination, reaction parameters were
varied including reaction temperature, amount of BCl3, and
solvents employed for the reaction. It was found that
reactions performed at various temperature ranging from
1608C to 2308C did not afford the nonchlorinated species, and
SubAPPc was not formed below 1608C owing to the low
reactivity of 1,8-naphthalenedicarbonitrile. The amount of
BCl3 influenced the yield of SubAPPc, but the nonchlorinated
species was not obtained even though a minimal amount of
BCl3 was used for the reaction. It is well known that electron-
[*] H. Zhu, Dr. S. Shimizu, Prof. Dr. N. Kobayashi
Department of Chemistry, Graduate School of Science
Tohoku University
Aoba-ku, Sendai 980-8578 (Japan)
Fax: (+81)22-795-7719
E-mail: nagaok@m.tohoku.ac.jp
[**] This work was supported by a Grant-in-Aid for Scientific Research on
Innovative Areas (no. 20108001, “pi-Space”) from MEXT, and a
Grant-in-Aid for Young Scientists B (no. 20750025) from JSPS. We
thank Prof. T. Iwamoto and Dr. S. Ishida, Tohoku University for X-ray
analysis.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003929.
8000
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Scheme 1. Synthesis of subazaphenalenephthalocyanine 2. Reaction conditions: a) BCl3, 1-chloronaphthalene, 1908C, 5 min; b) phenol, 1208C,
2 h.
rich aromatic solvents such as p-xylene are efficient for
preventing chlorination of SubPc during BCl3-template
syntheses,[16] but the use of a 1.0m solution of BCl3 in p-
xylene did not provide the nonchlorinated species. These
results can be attributed to the high reactivity of SubAPPc
towards electrophilic aromatic substitution reaction.
from the mean plane defined by three meso-nitrogen atoms,
whereas two isoindole units are tilted by about 238 from the
À
À
mean plane. Slightly longer bond lengths of C1 C2 and C10
C12 of 1.467(8) and 1.475(8) ꢀ, respectively, indicate small
contribution of the outer naphthalene moiety to the macro-
cyclic 14p-electron conjugation system.
Single crystals suitable for X-ray analysis were obtained
by slow diffusion of 1-propanol into a solution of 2 in
toluene.[17] Crystallographic analysis unambiguously revealed
its bowl-shaped structure with a 0.58 ꢀ deviation of the boron
atom from the mean plane defined by three coordinating
nitrogen atoms (Figure 1). This structural feature us compa-
In the absorption spectrum, 1 exhibits one intense Q-band
absorption at 573 nm, whereas 2 shows red-shifted and rather
complex Q-band absorption at 708, 659, 616, and 600 nm,
which is indicative of alteration of the electronic structure
(Figure 2). The color of a solution of 2 is blue rather than pink
[2]
rable to those of general SubPcs. A chloride substituent is
Figure 1. ORTEP plot of 2, top view (left) and side view (right). The
thermal ellipsoids are drawn at the 50% probability level. para-tert-
Butylphenyloxy substituents and hydrogen atoms in the side view are
omitted for clarity.
Figure 2. Absorption (bottom) and MCD (top) spectra of 1 (a) and
2 (c) in CHCl3, and fluorescence spectrum of 2 (b) in CHCl3.
placed at one of the peripheral positions of the naphthalene
moiety as is expected by the mass spectrometry and 1H NMR
spectroscopy. Because the unsymmetrically substituted SubPc
species is inherently chiral as previously reported by Claes-
sens and Torres,[18] a pair of enantiomers, which possess the
chloride substituent at the 4- or 5-position of the naphthalene
moiety, respectively, are observed in the crystal structure of 2.
The position of the chloride atom, however, could not be
resolved owing to its disorder at the 4- and 5-positions in one
enantiomer or at the 5- and 4-positions in the other
enantiomer with 7:3 occupancy. In the packing diagram, two
molecules form a p–p stacking dimer using the naphthalene
moieties (see the Supporting Information). The strain caused
by incorporation of the six-memebered ring unit in the core is
released by planar arrangement of the azaphenalene moiety
which is the typical color of SubPc (see the Supporting
Information). The tail of the lower-energy Q band extends to
750 nm, which is longer than that of subnaphthalocyanine.[3a,b]
These results indicate that our synthetic protocol towards
SubPc analogues bearing unique spectroscopic properties is
effective. The magnetic circular dichroism (MCD) spectrum
and band deconvolution analysis reveal the split Q00 bands at
708 and 659 nm, and the complex Q-band absorption is
assigned tentatively based on the presence of overlapping
pairs of Q00 and Q01 bands followed by a more complex set of
vibrational bands at higher energy (see the Supporting
Information). The MCD signals in this region can be regarded
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as Faraday B terms, which are indicative of the absence of a
To further support the absorption spectra and MO
calculations, electrochemical measurements were performed
on these compounds. A quasireversible one-electron oxida-
tion process and a reversible one-electron reduction process
are observed at 0.53 and À1.54 V (vs. Fc+/Fc), respectively, for
1 (see the Supporting Information). The first reduction
potential of 2 at À1.61 V is comparable with that of 1,
whereas the first oxidation potential at 0.15 V is shifted
negatively, and is indicative of a high-lying HOMO as
predicted by DFT calculations. This results in a decrease of
the potential energy difference (1.76 V for 2 and 2.06 V for 1),
which is in agreement with the large red shift of the Q band.
In summary, a novel subphthalocyanine analogue com-
prising an azaphenalene moiety was prepared according to
the same synthetic protocol employed for the synthesis of
APPcs.[12] This new species retained a 14p-electron aromatic
conjugation system despite the presence of a six-membered
ring unit in its conjugated system, and the electronic structure
was significantly altered as a result of the incorporation of the
azaphenanlene moiety. SubAPPc can absorb a wide range of
light energy, which is a useful property for functional dyes
employed in various fields. The introduction of azaphenalene
units is a promising method for modifying electronic struc-
tures and properties of Pc, SubPc, and their related com-
pounds. Further investigation using this approach is currently
underway.
threefold or higher symmetry axis, and these kinds of SubPc
molecules generally possess nondegenerated excited states.[19]
Although the molar absorption coefficient of 2 is about half of
that of 1, it should be emphasized that the absorption
spectrum of 2 covers a wide range in which neither general
Pc nor SubPc have absorption. This observation is particularly
favorable for photoenergy conversion processes because dyes
which effectively absorb a wide range of light energy are
strongly desired. Compound 2 also shows fluorescence at
718 nm with a Stokes shift of 197 cmÀ1, and the fluorescence
quantum yield (FF) of 0.18[20] and fluorescence lifetime (tF) of
3.6 ns are comparable with those of the general SubPcs.[2]
Molecular orbital calculations were performed using DFT
methods at the B3LYP/6-31G(d) level on model compounds
of 1 and 2 in which the peripheral substituents and axial ligand
were replaced with hydrogen atoms and a hydroxy group for
simplicity (Figure 3). The frontier orbitals of 2 show similar
Experimental Section
Preparation of 1 and 2: A suspension of 1,8-naphthalenedicarbonitrile
(90 mg, 0.5 mmol) and 4,5-di-p-tert-butylphenyloxyphthalonitrile
(1 equiv, 212 mg, 0.5 mmol) in 1-chloronaphthalene (2 mL) was
heated at 1908C for approximately 10 seconds. Then, a solution of
boron trichloride in n-heptane (1.0m, 1 mL, 1 mmol) was added by a
syringe under a nitrogen atmosphere and the mixture was heated at
1908C for another 5 min to give a blue/purple solution. Preparative
thin-layer chromatography (silica gel, F254, 60, eluent: CHCl3) was
employed to remove impurity and the purple and the blue fractions
were collected. These fractions were further purified by GPC-HPLC.
The first fraction (purple) to elute from the GPC was characterized as
a mixture of axially chloro- or hydroxy-substituted subphthalocya-
nines, and the second fraction (blue) was identified as a similar
mixture of subazaphenalenephthalocyanines. Finally the first and the
second fractions were treated with phenol at 1208C for 2 h in the dark,
respectively, and the products were further purified by column
chromatography on silica gel to provide axially phenoxy-substituted
subphthalocyanine (1) and axially phenoxy-substituted subazaphe-
nalenephthalocyanine (2) in 5.6% and 1.6% yields, respectively.
Characterization of subphthalocyanine (SubPc; 1): ESI-FT-ICR-
MS: m/z calcd for C90H89BN6NaO7 [M++Na]: 1399.6783; found:
1399.6778; 1H NMR (CDCl3, 400 mhz, 298 K): d = 8.25 (s, 6H; a-
benzo), 7.42 (d, 12H, J = 8.83 Hz; phenyl), 7.08 (d, 12H, J = 8.81 Hz;
phenyl), 6.74 (dd, 2H, J1 = 7.36 Hz, J2 = 8.35 Hz; axial meta-phenyl),
6.60 (t, 1H, J = 7.34 Hz; axial para-phenyl), 5.31 (d, 2H, J = 8.66 Hz;
axial ortho-phenyl), 1.37 ppm (s, 54H; tert-butyl); UV/Vis (CHCl3):
lmax (nm); e = 357 (16900), 528 (sh, 15600), 553 (sh, 28500), 573
(51400).
Figure 3. Frontier molecular orbital diagrams of 1 (left) and 2 (right).
density distribution patterns to those of 1, and coefficients are
found at the naphthalene moiety in the case of highest
occupied molecular orbital (HOMO) and LUMO + 1
(LUMO = lowest occupied molecular orbital). This results
in significant destabilization of the HOMO and an energy
difference between the LUMO and LUMO + 1 by 0.08 eV.
Furthermore, TDDFT calculations reveal two bands in the Q-
band region which mainly consist of transitions from the
HOMO to LUMO and LUMO + 1. The smaller HOMO–
LUMO energy gap of 2, therefore, explains the red shift of the
lower-energy Q band, and the substantial energy difference
between the LUMO and LUMO + 1 explains the splitting of
the Q bands. These calculation results are in good agreement
with the observed Faraday B terms in the MCD spectrum of 2.
Characterization of subazaphenalenephthalocyanine (SubAPPc;
2): ESI-FT-ICR-MS: m/z calcd for C74H66BClN6O5Na [M++Na]:
1187.4774; found: 1187.4768; 1H NMR (CDCl3, 600 mhz, 298 K): d =
8.77 (d, 1H, J = 7.37 Hz; naphthalene), 8.63 (d, 1H, J = 8.04 Hz;
naphthalene), 8.09 (m, 3H; naphthalene (1H) and a-benzo (2H)),
8.01 (s, 2H; a-benzo), 7.59 (m, 2H; naphthalene), 7.41 (m, 8H;
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Angewandte
Chemie
phenyl), 7.07 (m, 8H; phenyl), 6.80 (dd, 2H, J1 = 7.41 Hz, J2 =
8.37 Hz; axial meta-phenyl), 6.66 (t, 1H, J = 7.32 Hz; axial para-
phenyl), 5.74 (d, 2H, J = 8.62 Hz; axial ortho-phenyl), 1.36 ppm (s,
36H; tert-butyl); UV/Vis (CHCl3): lmax (nm); e = 378 (17400), 600
(24100), 616 (23500), 659 (24100), 708 (25400).
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Received: June 29, 2010
Published online: September 13, 2010
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Keywords: aromaticity · azaphenalenes · fluorescence ·
magnetic circular dichroism · subphthalocyanines
.
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c = 19.764(15) ꢀ,
a = 72.428(10),
b = 86.386(10),
g =
T=
76.354(10)8, V= 3484(5) ꢀ3, Z = 2, 1calcd = 1.199 gcmÀ3
,
À1738C, 16079 measured reflections, 11920 unique reflections
(Rint = 0.0588), R = 0.0859, Rw = 0.2363 (all data), GOF = 0.835.
CCDC 782513 contains the supplementary crystallographic data
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