Azepiphthalocyanine-an unprecedented large twist of a π-conjugation system upon core-modification with a seven-membered ring unit

Article abstract of DOI:10.1039/c0cc05276k

Aromatic dicarbonitrile bearing cyano groups in a 1,4-relationship was utilized for a phthalocyanine synthesis to give azepiphthalocyanine having a seven-membered ring unit instead of a five-membered ring unit. This molecule exhibits a significantly twist

Full text of DOI:10.1039/c0cc05276k

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Azepiphthalocyanine—an unprecedented large twist of a p-conjugation
system upon core-modification with a seven-membered ring unitw
Soji Shimizu,* Hua Zhu and Nagao Kobayashi*
Received 30th November 2010, Accepted 20th January 2011
DOI: 10.1039/c0cc05276k
Aromatic dicarbonitrile bearing cyano groups in a 1,4-relationship
was utilized for a phthalocyanine synthesis to give azepiphthalo-
cyanine having a seven-membered ring unit instead of a five-
membered ring unit. This molecule exhibits a significantly
twisted structure and large splitting of the Q band absorption,
indicative of its azachlorin-like conjugation system.
the main absorption bands, were altered depending on the
positions and numbers of azaphenalene units because the
six-membered ring units mediated the interaction between
the inner Pc-like 18p annulenic system and the outer aromatic
ring units. The variation in the frontier molecular orbitals
is similar to the case of benzene-fused low symmetry Pc
molecules.7 Further motivation to reveal the role of the ring
unit in the core led us to a synthesis of a novel core-modified
Pc analogue with a larger seven-membered ring unit. Based on
the spectroscopic analyses and theoretical calculations, this
molecule, which can be referred to as azepiphthalocyanine
(AZPPc),8 is found to possess an azachlorin-like electronic
structure4 due to the significant twist of the embedded seven-
membered ring unit.
Phthalocyanine (Pc) has been a potential candidate as a
functional molecule in various fields such as photovoltaic
cells, photosensitizers, molecular photonics, and molecular
electronics because of its unique optical and electrochemical
properties stemming from the aromatic 18p-electron conjugation
system.1 Modification of this conjugation system by contraction,2
expansion,3 skeletal hydrogenation,4 and core-modification5 is
an important research direction towards novel functional
molecules suitable for respective applications. Among Pc
analogues, core-modified analogues have been rarely investigated
mainly due to the synthetic difficulty, but they are an important
class of molecules because introduction of hetero-aromatic
rings in the core in place of isoindole units will cause variation
not only in a conjugation system but also in coordination
modes.1,5 Furthermore, core-modification is also beneficial for
achieving fine tuning of the absorption in the visible and near
infrared (vis/NIR) region which is a culminating concern in
the fields of photovoltaic cells and photosensitizers. It should
be mentioned, however, that core-modification without any
loss of a Pc-like aromatic conjugation system has remained a
challenge because a conjugation system of an introduced
hetero-aromatic subunit and a Pc-like 18p annulenic conjugation
system often compete. This can be easily understood from the
fact that most hemiporphyrazine molecules are non-aromatic.5
Recently we succeeded in the synthesis of core-modified Pc
analogues having six-membered ring units in the core by
introducing azaphenalene units in place of isoindole units
and revealed that Pc-like electronic structures were basically
retained in these molecules.6 Meanwhile, the absorption
spectra and the energy level of molecular orbitals, which relate
A mixed-condensation reaction of 4,5-di-p-tert-butylphenyl-
oxyphthalonitrile (1a)9 and 2,20-biphenyldicarbonitrile (2)10 in
the presence of anhydrous nickel acetate and ammonium
molybdate in quinoline at 330 1C provided a mixture of Pc
(3a) and AZPPc (4a, Scheme 1). 4b was similarly obtained
from a reaction using 4,5-di-p-tert-butylphenylphthalonitrile.11
The 1H NMR spectra of 4a and 4b in CDCl3 at room
temperature exhibit three signals due to the a-benzo protons
at 8.55, 8.53, and 8.37 ppm and at 9.06, 9.04, and 8.81 ppm,
respectively, while the tert-butyl protons are observed at 1.34,
1.33, and 1.31 ppm and at 1.37, 1.35, and 1.31 ppm. These
signal patterns suggest C2 symmetric molecular structures. The
1H–1H COSY experiments on 4a and 4b enable assignment of
the biphenyl proton signals at 7.86, 7.58, 7.29, and 6.58 ppm
and at 7.91, 7.60, 7.36, and 6.68 ppm, respectively (see ESIw).
Significant up-field shifts of biphenyl protons at 6.58 ppm for
4a and at 6.68 ppm for 4b infer the presence of diatropic ring
current effects arising from macrocyclic aromatic conjugation
systems. Based on the variable temperature 1H NMR analyses,
4a exhibits broadening of the signals upon decreasing the
measurement temperature, while all the signals sharpened
Department of Chemistry, Graduate School of Science,
Tohoku University, Sendai 980-8578, Japan.
E-mail: ssoji@m.tohoku.ac.jp, nagaok@m.tohoku.ac.jp
w Electronic supplementary information (ESI) available: Syntheses,
spectroscopic data, and theoretical calculations. CCDC 791481. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c0cc05276k
Scheme 1 Synthesis of dibenzoazepiphthalocyanines 4a and 4b.
c
3072 Chem. Commun., 2011, 47, 3072–3074
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upon heating (see ESIw). These results imply structural
flexibility of this molecule.
The structure of 4b was unambiguously elucidated by X-ray
crystallographic analysis on a crystal which was obtained by
slow diffusion of 1-propanol into a toluene solution of 4b
(Fig. 1).z The biphenyl moiety is nearly orthogonal from
the mean plane, which is determined by four coordinating
nitrogen atoms, mainly due to the strain caused around the
seven-membered ring and the steric congestion of the biphenyl
moiety, while the rest of the molecule takes a slightly distorted
ruffle conformation. The central nickel ion does not have a
significant effect on this deformation, considering that the
structural optimization on model compounds of a free-base
and a zinc complex suggests similar distorted structures (see
ESIw). Hydrogen atoms at C3 and C12 positions lie above and
below the molecular plane, which results in the observed
Fig. 2 Absorption (bottom) and MCD (top) spectra of 3a (blue
dashed line) and 4a (black solid line) in CHCl3.
1
up-field shift of these proton signals in the H NMR spectra.
The single bond nature of the C1–C2 and C13–C14 bonds,
1.514(5) and 1.501(5) A, respectively, infers a small contribution
of an azepin-like conjugation system in this molecule. On the
other hand, the double bond nature of the C1–N1 and
N1–C14 bonds, 1.366(5) and 1.372(5) A, respectively, and
the small bond length alternation between these bonds are
indicative of the major contribution of these bonds for the
[18]annulene-like conjugation system (see ESIw).
The absorption spectra of 4a and 4b in CHCl3 exhibit
significantly split Q-bands in both the higher and lower energy
regions relative to the Q bands of 3a and 3b at 675 and 691 nm
in CHCl3, respectively (Fig. 2 and see ESIw). The tail of the
absorption extends to 1000 nm, and 4a and 4b cover a broad
range in the vis/NIR region, in which conventional Pcs do not
exhibit absorption. These absorption spectra are also clearly
different from those of periphery expanded low-symmetry Pc
analogues, which exhibit similar split Q band absorptions in
the lower energy region than Pcs.7,12 The magnetic circular
dichroism (MCD) spectrum of 4a shows negative and positive
envelopes in ascending energy corresponding to the split Q
bands in the absorption spectrum (Fig. 2). This signal pattern
is assigned as Faraday B terms indicating non-degenerate
excited states due to the absence of a 3-fold or higher
symmetry axis in this molecule.1b,13 The peak position of the
negative envelope deviates to the red by 387 cmꢀ1 relative to
the lmax of the lower energy Q band absorption. This result
can be tentatively illustrated based on contribution of several
vibrational bands due to the structural flexibility as inferred by
variable temperature 1H NMR measurements (see ESIw). This
can also be assumed by the broad nature of this Q band in the
absorption spectra. TDDFT calculations at the B3LYP/
6-31G(d) level reproduce the observed absorption spectra well
and predict two energetically different bands in the Q band
region at 729 and 647 nm with the oscillator strengths of 0.33
and 0.23, respectively. These two theoretical bands mainly
consist of transitions from the HOMO to LUMO and from
the HOMO to LUMO+1, respectively (see ESIw).
The cyclic voltammogram reveals two reversible reductions
at ꢀ1.59 and ꢀ1.23 V (vs. Fc+/Fc) and one reversible and one
quasi-reversible oxidations at 0.22 and 0.64 V for 4a. 4b also
exhibits similar redox potentials to those of 4a (see ESIw).
Relative to the first oxidation and reduction potentials of 3a at
0.42 and ꢀ1.39 V, the reduction potential shifts positively, and
the oxidation potential exhibits a negative shift. Since the first
oxidation and reduction potentials have good correlation with
the energy levels of the HOMO and LUMO, respectively,
these results indicate destabilization of the HOMO and
stabilization of the LUMO. The smaller HOMO–LUMO
gap than that of Pc is in good agreement with the observed
red-shift of the lower energy Q band, which is mainly
composed of transition from the HOMO to LUMO based
on the TDDFT calculations.
The TDDFT calculations indicate that the absorption
spectrum of AZPPc can be basically illustrated using
Gouterman’s four orbital model as a theoretical framework.14
The coefficient of the frontier orbitals is delocalized over the
Fig. 1 Crystal structure of 4b, top view (top) and side view (bottom).
The thermal ellipsoids are scaled to the 50% probability level. tert-
Butylphenyl substituents and hydrogen atoms are omitted for clarity
in the side view.
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Chem. Commun., 2011, 47, 3072–3074 3073

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This work was partly supported by
a
Grant-in-Aid
for Scientific Research on Innovative Areas (No. 20108007,
‘‘pi-Space’’) from MEXT, and a Grant-in-Aid for Young
Scientists B (No. 20750025) from JSPS. H. Z. thanks the
G-COE program of Tohoku University. We thank Dr
E. Kwon, Tohoku University, for X-ray measurement.
Notes and references
z Crystallographic data for 4b: C119H116N8Ni, Mw
=
1716.91,
monoclinic, space group P2/c (no. 13),
a
=
21.907(4),
b =
25.058(5), c = 22.196(4) A, b = 108.6367(5)1, V = 11 545(4) A3,
Z = 4, rcalcd = 0.988 g cmꢀ3, T = ꢀ100(2) 1C, 140 563 measured
reflections, 20 299 unique reflections (Rint = 0.0762), R = 0.0989
(I > 2s(I)), Rw = 0.2695 (all data), GOF = 1.163. CCDC 791481.
1 (a) The Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and
R. Guilard, Academic Press, San Diego, 2003, vol. 15–20;
(b)
Phthalocyanines: Properties and Applications,
ed.
C. C. Leznoff and A. B. P. Lever, Wiley-VCH, New York,
1989–1996, vol. 1–4.
2 (a) A. Meller and A. Ossko, Monatsh. Chem., 1972, 103, 150;
Fig. 3 Frontier molecular orbital diagrams of model compounds of
3a (left) and 4a (right).
(b) C. G. Claessens, D. Gonzalez-Rodrıguez and T. Torres, Chem.
´ ´
Rev., 2002, 102, 835.
3 J. E. Bloor, J. Schlabitz, C. C. Walden and A. Demerdac, Can. J.
Chem., 1964, 42, 2201.
molecule with the exception of the biphenyl moiety, which is
due to the twist of the azepine moiety (Fig. 3). This is
clearly different from Pcs,1 APPcs,6 and periphery expanded
low-symmetry Pc analogues7 in which the frontier molecular
orbitals are delocalized over the whole molecules. A change in
the main conjugation pathway due to the significant twist at
the azepine moiety causes destabilization of the HOMO and
LUMO+1. The HOMO–LUMO gap thus becomes smaller,
while the HOMO–LUMO+1 gap becomes larger. These
results are in good agreement with the observed large splitting
of the Q bands and the Faraday B terms. This variation of the
frontier molecular orbital diagram is essentially similar to
those of azachlorin-type molecules,4,15 indicating that the
electronic structure of AZPPc is rather similar to that of an
azachlorin molecule than that of Pc.
In conclusion, AZPPc was synthesized by using 2,20-
biphenyldicarbonitrile as a key synthetic precursor. The
enhanced distortion of the molecular structure due to the
steric congestion of the seven-membered ring and biphenyl
moiety was revealed by X-ray crystallographic analysis. The
inner seven-membered ring unit has less contribution to merge
the 18p annulenic conjugation system and the peripheral
conjugation system of the biphenyl unit. This results in
variation of the frontier molecular orbital diagram as exemplified
by the high-lying HOMO and LUMO+1. The absorption
spectrum of AZPPc is thus better illustrated by the azachlorin-
like electronic structure. It can be emphasized that by changing
the ring size in the core of Pc molecules, the contribution of
the peripheral aromatic unit can be altered, which leads to
significant changes in optical and electrochemical properties.
Furthermore, investigation along this direction also gives
insight into the nature of aromaticity of porphyrin16 and
phthalocyanine analogues, which is indispensable for a novel
molecular design in this field.
4 (a) R. P. Linstead and M. Whalley, J. Chem. Soc., 1952, 4839;
(b) T. Fukuda and N. Kobayashi, Dalton Trans., 2008, 4685.
5 (a) F. Fernandez-Lazaro, T. Torres, B. Hauschel and M. Hanack,
Chem. Rev., 1998, 98, 563; (b) M. Rodoriguez-Morgade, G. De La
Torre and T. Torres, in The Porphyrin Handbook, ed.
K. M. Kadish, K. M. Smith and R. Guilard, Academic Press,
San Diego, 2003, vol. 15.
6 (a) S. Shimizu, H. Zhu and N. Kobayashi, Chem.–Eur. J., 2010, 16,
11151; (b) N. Kobayashi, N. Sasaki and H. Konami, Inorg. Chem.,
1997, 36, 5674.
7 (a) N. Kobayashi, J. Mack, K. Ishii and M. J. Stillman, Inorg.
Chem., 2002, 41, 5350; (b) N. Kobayashi and T. Fukuda, J. Am.
Chem. Soc., 2002, 124, 8021.
8 Azepiphthalocyanine was named after its porphyrin analogue,
azepiporphyrin. J. C. Petryka, S. T. Chaney and T. D. Lash, Book
of Abstracts for the 213th National Meeting of the American
Chemical Society, San Francisco, CA, 1997, Abstract No. ORGN
298.
9 (a) M. D. Maree and T. Nyokong, J. Porphyrins Phthalocyanines,
2001, 5, 555; (b) S. E. Maree and T. Nyokong, J. Porphyrins
Phthalocyanines, 2001, 5, 782; (c) D. Wohrle, M. Eskes,
K. Shigehara and A. Yamada, Synthesis, 1993, 194.
10 G. Ege and E. Beisiege, Synthesis, 1974, 22.
11 S. Eu, T. Katoh, T. Umeyama, Y. Matano and H. Imahori, Dalton
Trans., 2008, 5476.
12 (a) T. Fukuda, E. A. Makarova, E. A. Luk’yanets and
N. Kobayashi, Chem.–Eur. J., 2004, 10, 117; (b) N. Kobayashi,
H. Miwa and V. N. Nemykin, J. Am. Chem. Soc., 2002, 124, 8007;
(c) H. Miwa, K. Ishii and N. Kobayashi, Chem.–Eur. J., 2004, 10,
4422.
13 J. Mack, M. J. Stillman and N. Kobayashi, Coord. Chem. Rev.,
2007, 251, 429.
14 (a) M. Gouterman, J. Chem. Phys., 1959, 30, 1139;
(b) M. Gouterman, J. Mol. Spectrosc., 1961, 6, 138.
15 Seco-porphyrazines also exhibit similar structural features and
optical properties. (a) N. S. Mani, L. S. Beall, A. J. P. White,
D. J. Williams, A. G. M. Barrett and B. M. Hoffman, J. Chem.
Soc., Chem. Commun., 1994, 1943; (b) A. G. M. Montalban,
S. J. Lange, L. S. Beall, N. S. Mani, D. J. Williams, A. J.
P. White, A. G. M. Barrett and B. M. Hoffman, J. Org. Chem.,
1997, 62, 9284.
16 T. D. Lash, S. A. Jones and G. M. Ferrence, J. Am. Chem. Soc.,
2010, 132, 12786.
c
3074 Chem. Commun., 2011, 47, 3072–3074
This journal is The Royal Society of Chemistry 2011