A quadruply azulene-fused porphyrin with intense near-IR absorption and a large two-photon absorption cross section

Article abstract of DOI:10.1002/anie.200600892

(Chemical Equation Presented) Extending conjugation: Azulene-fused porphyrins (see example) are synthesized through the oxidation of meso-(4-azulenyl)porphyrins with FeCl₃. The azulene-fused strategy allowed highly π-conjugated porphyrinic elec

Full text of DOI:10.1002/anie.200600892

Communications
conjugated porphyrins have been shown to exhibit large two-
photon absorption (TPA) cross sections as a promising
attribute.[4] The great promise of conjugated porphyrins
relies on their flexible electronic systems, which are suscep-
tible to peripheral modifications. An effective synthetic
strategy is to fuse an aromatic segment directly onto the
periphery of the porphyrin unit to expand a conjugated
electronic systemthat is forced to take a planar conformation
favorable for p conjugation.[5] Among such aromatic-fused
porphyrins, Scott and co-workers reported an anti-bisnaph-
thoazulene-8-one-fused–CuII porphyrin that exhibited a Q-
band-like absorption band at 1204 nmand a very small
electrochemical HOMO–LUMO gap (1.17 V) as the most p-
expanded example of a monomeric porphyrin.[5d] In this
fusion strategy, azulene may be an effective aromatic segment
because of the unique dipolar and polarizable electronic
property that arises fromits nonalternant hydrocarbon
nature.[6] Previously, we prepared a series of directly linked
meso-azulenylporphyrins, except for one bearing a 4-azulenyl
group, and found that 1- and 6-azulenyl groups actually served
as electron-donating and electron-withdrawing substituents,
respectively, toward a ZnII porphyrin, but the influence of
such singly linked azulenyl groups upon the electronic system
of the porphyrin are only small.[7]
Herein, we employed 4-azulenylporphyrin as a precursor
of azulene-fused porphyrin targets, as the electron-rich 3-
position of the azulene, which is directed toward the
porphyrin, may be used for an oxidative ring-closing reaction.
It has been reported that 1-methoxycarbonyl-4-formylazu-
lene (3a) was prepared from1-methoxycarbonyl-4-methyl-
azulene (1a) by selective dibromination at the 4-methyl group
followed by oxidation,[8] but treatment of 1a with N-
bromosuccinimide (NBS) led to predominant bromination
at the 3-position in our hands. Therefore, aldehyde 3a was
prepared via enamine 2a from 1a.[9] Treatment of 1a with
N,N-dimethylformamide (DMF) dimethylacetal in DMF at
1408C gave 2a, which was oxidized to 3a with NaIO4 in 81%
yield in two steps. Dipyrromethane 4a was prepared by the
condensation reaction of 3a with excess pyrrole in 94% yield
(Scheme 1).
Porphyrinoids
DOI: 10.1002/anie.200600892
A Quadruply Azulene-Fused Porphyrin with
Intense Near-IR Absorption and a Large Two-
Photon Absorption Cross Section**
Kei Kurotobi, Kil Suk Kim, Su Bum Noh,
Dongho Kim,* and Atsuhiro Osuka*
Considerable efforts have been continuously devoted to the
exploration of extensively p-conjugated porphyrins in light of
their potential applications in optoelectronic devices, sensors,
photovoltaic devices, and so forth.[1–3] Recently, some of these
[*] K. S. Kim, S. B. Noh, Prof. Dr. D. Kim
Center for Ultrafast Optical Characteristics Control and
Department of Chemistry
Yonsei University, Seoul 120-749 (Korea)
Fax: (+82)2-2123-2434
E-mail: dongho@yonsei.ac.kr
Dr. K. Kurotobi, Prof. Dr. A. Osuka
Department of Chemistry
Graduate School of Science
Kyoto University
and
Core Research for Evolutional Science and Technology (CREST)
Japan Science and Technology Agency
Sakyo-ku, Kyoto 606-8502 (Japan)
Fax: (+81)75-753-3970
Scheme 1. Synthesis of 3 (R=Me (a), octyl (b), 2,4,6-tri-tert-butyl-
phenyl (c)). a) Me2NCH(OMe)2, DMF, 1408C; b) NaIO4, THF/H2O.
NiII–5-(4-azulenyl)porphyrin 6 was prepared fromthe
cross-condensation of dipyrromethanes 4a and 5 with 3,5-
di(tert-butyl)benzaldehyde followed by NiII-ion insertion in
16% yield, and NiII–5,15-bis(4-azulenyl)porphyrin 7 was
prepared from 4a and 3,5-di(tert-butyl)benzaldehyde in
16% yield. 1H NMR spectrumof 7 displays two sets of
signals in a 1:1 ratio, thus indicating the presence of two
atropisomers. NiII–5,10,15,20-tetrakis(4-azulenyl)porphyrin
E-mail: osuka@kuchem.kyoto-u.ac.jp
[**] This work was partly supported by a Grant-in-Aid (B)
(No. 17350017) from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan (A.O.) and the Star Faculty Program
of the Ministry of Education, Korea for Human Resources (D.K.). We
thank Shinetsu Igarashi and Kenji Yoza (Bruker, Japan) for the X-ray
analysis.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
8a was prepared from 3a under Lindsey conditions[10] in 12%
under similar conditions to regioselectively provide only the
yield as a mixture of atropisomers. Structural determination
of these azulenylporphyrins were mainly based on H NMR
spectroscopic, MALDI-TOF mass-spectrometric, and UV/
Vis absorption spectroscopic analysis. The structure of the
CuII complex of 6 [Cu(6)] was confirmed by X-ray diffraction
analysis (Figure 1),[11] which showed that the 4-azulenyl group
anti isomer 10 in 84% yield. Curiously, the syn isomer was not
detected at all. This result suggested that the regiochemistry
of the second ring-closing reaction is strongly influenced by
the preformed azulene-fused structure at the opposite side.
The symmetric anti structure of 10 is indicated by its simple
1H NMR spectrum, which exhibits a single set of signals for
the meso aryl protons, para protons (d = 7.83 ppm), and
ortho protons (d = 7.91 ppm). A fourfold ring-closing reac-
tion of 8a was attempted with FeCl3 under similar conditions;
however, a complicated mixture was yielded. Separation of
this mixture turned out to be quite difficult as a result of its
extremely low solubility. We thus changed substrates to more
soluble porphyrins 8b and 8c,[14] which were prepared from
3b and 3c, respectively, through the same synthetic route used
for 8a. Although the similar treatment of 8b with FeCl3 also
led to an intractable mixture of low solubility, porphyrin 8c
was cleanly transformed into quadruply azulene-fused por-
phyrin 11 as a major product in 60% yield. The product 11
showed its parent-ion peak at m/z 2017.0052, calculated for
1
À
is linked to the porphyrin ring with a C C bond length of
1.505 Š and a dihedral angle of 818, similar to other meso-
azulenylporphyrins.[7]
Figure 1. X-ray crystal structure of [Cu(6)]. a) Oblique view, b) side
view. The tert-butyl groups and hydrogen atoms have been omitted for
clarity.
C
136H140N4O8Ni m/z 2017.0057 in the high-resolution electro-
spray ionization TOF mass spectrum. In line with the
1
symmetric structure, the H NMR spectrumof 11 was quite
Attempted oxidative ring-closing reaction of 6 with
simple and featured a single set of signals for the azulene
protons (d = 8.74, 8.64, 8.45, 7.93, and 9.89 ppmfor H 2, H5, H6,
H7, and H8, respectively; Scheme 2) and a singlet at d =
9.12 ppmfor the pyrrolic b-protons. Furthermore, X-ray
diffraction analysis confirmed the structure, in which the four
azulene moieties are fused to the central porphyrin ring in a
symmetric manner to constitute an overall coplanar structure,
which exhibits strong ruffling with a mean deviation of 0.81 Š
[12]
AgPF6 did not cause any conversion, but the use of 2,3-
dichloro-5,6-dicyano-p-benzoquinone (DDQ)/[Sc(OTf)3][2d]
(Tf = trifluoromethanesulfonyl) as a stronger oxidant gave
rise to a complicated mixture. In the meantime, we found that
[13]
treatment of 6 with FeCl3 under mild conditions (5 min,
roomtemperature) afforded azulene-fused porphyrin
9 in
63% yield. A double ring-closing reaction of 7 was performed
Scheme 2. Compounds studied herein. R=Me (8a), octyl (8b), 2,4,6-tert-butylphenyl (8c, 11).
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Communications
with respect to the porphyrin macrocyclic plane of 24 atoms
comparable to the largest one reported by Scott and co-
workers.[5d] Interestingly, the spectral shapes of the Q-band
shape are considerably distorted for symmetry-broken fused
porphyrins 9 and 10, whereas that of the C2-symmetric 11
restores a typical vibrational structure common to normal
porphyrins.
(Figure 2).[15] The Ni N bond lengths are 1.863, 1.877, 1.893,
À
and 1.894 Š between the Ni center and N(1)–N(4), respec-
tively. The opposite halves are bent with 120.9 and 111.68
dihedral angles.
As the very low-energy absorption band for 11 suggested a
very small HOMO–LUMO gap, cyclic voltammetric studies
were performed in benzonitrile (Table 1). The first oxidation
Table 1: Redox potentials in benzonitrile (versus ferrocene/ferrocenium;
in V).
Compound
Eox,1
Eox,2
Ered,1
Ered,2
Eox,1ÀEred,1
6
9
10
11
0.65
0.38
0.25
0.13
0.61
À1.62
À1.20
À0.98
À0.88
À1.73
2.27
1.58
1.23
1.01
2.34
0.55
0.47
0.33
À1.58
À1.27
À1.05
Figure 2. X-ray crystal structure of 11. a) Top view, b) side view. The
ester groups and hydrogen atoms have been omitted for clarity.
Ni–TDBPP
The absorption spectra of 6–8 exhibit slightly red-shifted,
broadened Soret and Q bands. Such trends become larger
upon an increase in the number of azulenyl groups; the Soret
bands are observed at 422, 426, and 437 nmfor 6–8,
respectively, and Q(1,0) bands are intensified in the order of
6 < 7 < 8 (Figure 3a). In sharp contrast, the azulene-fused
structure causes more profound impacts in the electronic
systemof the porphyrin. The absorption spectrumof 9 shows
a broad Soret band at 467 nmwith a shoulder at 526 nmand a
quite broad Q band that reaches around 1000 nm; the
absorption spectrumof 10 shows a split Soret band at 487
and 545 nmand red-shifted, intense Q bands at 763, 898, and
1014 nmwith roughly equal intensities; and the quardruply
azulene-fused porphyrin 11 exhibits a drastically perturbed
absorption spectrumwith bands at 684 and 1136 nm, which
cover the whole visible and near-IR regions up to approx-
imately 1200 nm (Figure 3b). To the best of our knowledge,
this large bathochromic shift is, as a monomeric porphyrin,
potential (Eox,1) and the first reduction potential (Ered,1) of 6
were positively shifted by introduction of an electron-with-
drawing 4-azulenyl group relative to those of NiII–5,10,15,20-
tetrakis(3,5-di-tert-butylphenyl)porphyrin (Ni–TDBPP). The
separation of Eox,1 and Ered,1 was decreased dramatically for 9
(1.58 V) relative to 6 (2.27 V), and such a decrease was more
significant for 10 (1.23 V) and 11 (1.01 V), thus reflecting the
increased expanded p-electronic systems.
We investigated the excited-state dynamics by transient
absorption measurements (see the Supporting Information).
The time-resolved spectroscopic investigations revealed that
the photoexcited states of 9–11 decay biexponentially with
fast decay components of 1–2-ps time constants and relatively
slow components. The contribution of a long-lived ( ꢀ 1.4 ns)
component to the decay of photoexcited 11 is significant, thus
1
indicating that the porphyrin (p,p*) state is largely respon-
sible for the deactivation process. On the other hand, the
photoexcited dynamics of 9 are very fast, mainly governed by
an approximately 1-ps decay component. This feature is
attributable to the decay of the (d,d) state, formed by
unoccupied d orbitals of the central NiII in the porphyrin
ring, as it has been well established that the (d,d) state acts as
a quenching state in the energy relaxation dynamics of
photoexcited NiII porphyrins.[16] The decay of photoexcited 10
shows an intermediate behavior between 9 and 11, thus
suggesting a competition between the 1(p,p*) and (d,d) states.
In other words, while the low-lying (d,d) state remains at the
relatively same energy, the position of the porphyrin-ring
1(p,p*) state in energy changes dramatically as the p-
conjugation pathway increases from 9 to 11.
The TPA cross-section values were measured by an open-
aperture Z-scan method[17] with wavelength-tunable 130-fs
pulses at a 5-kHz repetition rate generated froma femto-
second Ti:sapphire regenerative amplifier system. Pulses of
sufficiently high power readily afford the power density
enough for the TPA process without tight focusing, which
eliminates unwanted nonlinear effects, such as self-focusing
and white-light continuum generation. The maximum TPA
Figure 3. a) UV/Vis absorption spectra of 6–8. b) UV/Vis–near IR
absorption spectra of 9–11.
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Angewandte
Chemie
cross section s(2) values are 340 GM at 1200 nmfor 1a,
2050 GM at 1200 nmfor 10, and 7170 GM at 1380 nmfor 11,
respectively. In addition, the TPA spectra of 11 in the 1300–
1450-nmregion indicate that two-photon allowed states
should exist near 680–700 nm(see the Supporting Informa-
tion). In the case of 11, a highly notable enhancement of the
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fused structures is the overriding factor in increasing the
s
(2) value. The enlargement of azulene-fused structures gives
rise to a very effective p-electron conjugation pathway
throughout the porphyrin moiety.[4f]
In summary, the azulene-fused porphyrins, 9, 10, and 11
were synthesized through the oxidation of meso-(4-azulenyl)-
porphyrins 6, 7, and 8c, respectively, with FeCl3. The azulene-
fused strategy allowed the realization of highly p-conjugated
porphyrinic electronic systems, which are promising TPA
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Received: March 8, 2006
Published online: May 9, 2006
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1358.69, monoclinic, space group P21/n, Z = 4, a = 19.2311(15),
Keywords: azulenes · fused-ring systems · optical properties ·
porphyrinoids · two-photon absorption
.
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