[18]/[20]π hemiporphyrazine: A redox-switchable near-infrared dye

Article abstract of DOI:10.1021/ja210410c

An aromatic hemiporphyrazine with an 18π-electron structure has been synthesized by oxidizing 20π-electron 8,10,21,23-tetrahydroxy-28,30-dicarba- 27H,29H-hemiporphyrazine with bulky aryl ether substituents. The aromatic nature of the oxidized form was cha

Full text of DOI:10.1021/ja210410c

Communication
pubs.acs.org/JACS
[18]/[20]π Hemiporphyrazine: A Redox-Switchable Near-Infrared Dye
Atsuya Muranaka,*,†,‡ Shino Ohira,† Daisuke Hashizume,§ Hiroyuki Koshino,∥ Fumiko Kyotani,†
Machiko Hirayama,† and Masanobu Uchiyama*,†,⊥
†Advanced Elements Chemistry Research Team, RIKEN-ASI, Wako-shi, Saitama 351-0198, Japan
‡PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
§Advanced Technology Support Division, RIKEN-ASI, Wako-shi, Saitama 351-0198, Japan
∥Molecular Characterization Team, RIKEN-ASI, Wako-shi, Saitama 351-0198, Japan
⊥Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
S
* Supporting Information
Chart 1. Structures of Phthalocyanine and
Hemiporphyrazines
ABSTRACT: An aromatic hemiporphyrazine with an
18π-electron structure has been synthesized by oxidizing
20π-electron 8,10,21,23-tetrahydroxy-28,30-dicarba-
27H,29H-hemiporphyrazine with bulky aryl ether sub-
stituents. The aromatic nature of the oxidized form was
characterized by means of various spectroscopic methods
and single-crystal X-ray analysis, with the help of quantum-
chemical calculations. The oxidized hemiporphyrazine
exhibited an intense absorption at ∼850 nm. The redox
process was found to be reversible.
orphyrins and their analogues, such as phthalocyanines,
P
usually have an 18π-electron aromatic system. Hemi-
porphyrazines are a broad class of phthalocyanine analogues in
which two opposing pyrrole units are replaced by other
moieties such as pyridine, triazole, or benzene.1 These
compounds do not display aromatic behavior. The bonding
in these macrocycles is not delocalized, and they act as Schiff
base ligands. The macrocycle has been considered to be
formally a 20π-electron system,2 but little is known about the
electronic structures of these compounds. We anticipated that
18π-electron hemiporphyrazines would be formed by two-
electron oxidation of the hemiporphyrazine skeleton if
hemiporphyrazines do indeed have a 20π-electron system.3
Modification of hemiporphyrazine π-electron systems, however,
has not been reported to date. Torres and co-workers
attempted to oxidize triazolehemiporphyrazines to the
corresponding aromatic compounds, but aromatic compounds
were not obtained by either chemical or electrochemical
methods.1c,d In this communication, we report the synthesis
and characterization of the first aromatic hemiporphyrazine.
Intriguingly, it was found that the hemiporphyrazine is a redox-
switchable near-IR chromophore.4
Scheme 1 shows the synthesis of the aromatic hemi-
porphyrazine 2. Diiminoisoindoline was prepared from 4,5-
bis(2,6-dimethylphenoxy)phthalonitrile.7 The reaction of the
diiminoisoindoline with diaminoresorcinol dihydrochloride
gave dicarbahemiporphyrazine 1 in 79% yield. Upon the
addition of 1 equiv of 2,3-dichloro-5,6-dicyano-1,4-benzo-
quinone (DDQ) to a chloroform solution of 1 at room
temperature, the solution showed a marked color change from
orange to dark-green. The oxidized product 2 was successfully
isolated in 70% yield by silica-gel column chromatography and
characterized by NMR spectroscopy, high-resolution mass
spectrometry, and elemental analysis [see the Supporting
Information (SI)]. Both of the macrocycles 1 and 2 are soluble
in common organic solvents such as chloroform, THF,
pyridine, and DMF.
In the design of an aromatic hemiporphyrazine, we selected
the resorcinol unit as the redox-active moiety of the
hemiporphyrazine framework.5 We designed 1, a tetrahydroxy-
dicarbahemiporphyrazine6 with aryl ether substituents. The
bulky groups were introduced to increase the solubility and
allow easier chromatographic separation. A two-electron-
oxidized form of the phenol form 1 can be formally drawn as
a quinoidal form, 2 (Chart 1).
Received: November 5, 2011
Published: December 13, 2011
© 2011 American Chemical Society
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Journal of the American Chemical Society
Communication
a
Scheme 1. Synthesis of Aromatic Hemiporphyrazine 2
a
(i) NEt3 (2 equiv), EtOH, 80 °C, 24 h. (ii) DDQ (1 equiv), CHCl3,
rt, 25 min.
The structures of 1 and 2 were determined by single-crystal
X-ray diffraction analysis (Figure 1). The macrocycle of 1
adopts a nonplanar saddle conformation. This type of saddle
conformation is often observed in hemiporphyrazines, although
tetrahydroxycarbahemiporphyrazine without peripheral sub-
stituents has a planar conformation in the X-ray structure.6
Interestingly, the oxidized form 2 was essentially planar and
adopted a zwitterionic form. Thus, the internal pyrrole protons
migrated to the meso nitrogen atoms. The O−H···N and N−
H···O distances are 2.57 and 2.59 Å, respectively, supporting
the presence of intramolecular hydrogen bonds in 2.
Selected bond distances in the X-ray structures of 1 and 2 are
shown in Figure 2. The bond lengths of the phenol form 1 are
similar to those of nonaromatic dicarbahemiporphyrazines.8
Thus, the C(resorcinol)−N(meso) bonds have single-bond
character (ca. 1.42 Å), while double-bond character is seen for
the C(pyrrole)−N(meso) bond (ca. 1.29 Å). The bond length
difference disappeared upon oxidation [1.342(8) and
1.327(8) Å for the nonprotonated C−N bonds]. The bond
lengths are consistent with those of regular phthalocyanines.9
As expected, one C−O distance in each of the resorcinol units
was shortened upon oxidation [1.241(8) Å]. The bond
distances in the resorcinol moiety of 2 were very similar to
those in the X-ray structure of protonated quinonemonoi-
mine.10
NMR spectra of 1 and 2 were measured to elucidate the
solution structures (see the SI). Broad H NMR signals were
observed for the phenol form 1, probably as a result of its
Figure 1. X-ray structures of (a) 1 and (b) 2. The thermal ellipsoids
are scaled to the 50% probability level. In both cases, solvent molecules
have been omitted for clarity.
1
flexible skeleton at room temperature. In contrast, a well-
1
resolved H NMR spectrum of the quinoidal form 2 was
obtained (Figure 3a). The spectrum indicates an apparently
symmetrical nature of the structure, so we consider the
interconversion between the two tautomers (Figure 3b) to be
fast under the experimental conditions used. The broad signal
at 11.61 ppm can therefore be attributed to NH and OH
protons (Ha). Sharpening of the peak was observed when the
temperature was lowered (see the SI). The chemical shift of the
internal CH protons (He) was observed in an upfield region at
−0.49 ppm, indicating that a diatropic ring current is present in
2. The isoindoline protons (Hb) appeared at 7.66 ppm. Since
the signals of the isoindoline protons of octakis(2,6-
dimethylphenyl)phthalocyanine are seen at 8.18 ppm,7 the
ring current effect of the present system appears to be
somewhat weaker than that in regular phthalocyanines.
Geometry-optimization calculations on 1 and 2 were
performed at the B3LYP/6-31G(d,p) level to gain insight
into the structures (see the SI). The lengths of the
C(resorcinol)−N(meso) and C(pyrrole)−N(meso) bonds of
the phenol form 1 were calculated to be 1.40 and 1.28 Å,
respectively, whereas those of the nonprotonated C−N bonds
Figure 2. Selected bond distances (Å) in the X-ray structures of 1 and
2.
of the oxidized form 2 were 1.35 and 1.33 Å, respectively.
These features are in good agreement with the X-ray data.
Nucleus-independent chemical shift (NICS) indices11 and H
1
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Journal of the American Chemical Society
Communication
Figure 5. Frontier molecular orbitals and energy levels of the
optimized structures of 1 and 2 calculated at the B3LYP/6-31G(d,p)
level. Isosurface plots of molecular orbitals of the 18π-electron
perimeter model are also shown.
1
Figure 3. (a) Partial H NMR spectrum of the quinoidal form 2 in
HOMO and LUMO of the phenol form 1 are similar to those
of a pair of degenerate LUMOs of the 18π-electron perimeter
model. It is noteworthy that the LUMO and LUMO+1 orbitals
of the quinoidal form 2 are derived from the HOMO and
LUMO of the phenol form 1, respectively. In addition, the
nodal pattern of the HOMO of 2 is analogous to that of one of
the degenerate HOMOs of the perimeter model. These features
are consistent with those of phthalocyanines having an 18π-
electron structure. From these results, it is now clear that the
phenol and quinoidal forms of the hemiporphyrazine are 20-
and 18π-electron systems, respectively.
The large change in the optical properties of 1 and 2 can be
interpreted in terms of the difference in the frontier orbitals.
Since the HOMO and LUMO of 1 are derived from a pair of
perimeters with equal numbers of angular nodes, the HOMO
→ LUMO transition is predicted to be electric-dipole-
forbidden.14 The weak absorption band observed in the 450−
550 nm region can be assigned to the HOMO → LUMO
transition of a [4n]π-electron ring system. In the case of the
oxidized form 2, the number of nodes of the HOMO is
different from that of the LUMO, so the HOMO → LUMO
transition is allowed theoretically. The HOMO−LUMO gap of
the quinoidal form (1.70 eV) was calculated to be smaller than
that of the phenol form (2.73 eV). Time-dependent density
functional theory (TDDFT) calculations support the above
interpretation. Thus, the lowest-energy transition of the phenol
form 1 (594.9 nm) is associated with the HOMO → LUMO
transition, and the oscillator strength was estimated to be zero.
In the case of the quinoidal form 2, two allowed transitions
were predicted in the longer-wavelength region (714.7 and
609.5 nm).
CDCl3. Solvent signals were overlapped with signals c. (b) Possible
tautomeric forms of 2. The π-electron conjugation pathway is shown
by thick lines.
NMR chemical shifts were calculated for the optimized
structures (see the SI).
Figure 4 shows the electronic absorption and magnetic
circular dichroism (MCD) spectra of the two forms of the
carbahemiporphyrazine in chloroform. The phenol form 1
exhibited an absorption band at ∼400 nm and a weak band at
longer wavelengths. The spectral features of the quinoidal form
2 are significantly different from those of the phenol form. The
intense absorption bands observed at 852 and 653 nm are
attributable to two split Q bands, since coupled Faraday B
terms were observed for these absorption bands.12 The MCD
pattern of the oxidized form 2 is characteristic of low-symmetry
phthalocyanine derivatives.13We measured the fluorescence
spectra of the macrocycles in chloroform (see the SI). The
quinoidal form 2 exhibited a fluorescence peak at 871 nm (ΦF
= 0.001), while no detectable fluorescence was observed for the
phenol form 1.
Frontier molecular orbitals of the hemiporphyrazines are
shown in Figure 5. Obviously, the nodal patterns of the
Finally, we investigated the reversibility of the redox process
between 1 and 2. The oxidized form 2 was easily reduced with
NaBH4 in chloroform at room temperature to give the original
phenol form 1. The oxidized form was recycled at least seven
times under our experimental conditions during the redox
process (see the SI).
In conclusion, the newly designed carbahemiporphyrazine 2
is the first example of an aromatic hemiporphyrazine with an
18π-electron structure. The hemiporphyrazine was easily
prepared in high yield by making use of the resorcinol
functionality in the skeleton of dicarbahemiporphyrazine 1. It
Figure 4. (top) MCD and (bottom) electronic absorption spectra of 1
(orange) and 2 (green) in CHCl3 at room temperature.
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Journal of the American Chemical Society
Communication
(9) McKeown, N. B. Phthalocyanine Materials: Synthesis, Structure and
Function; Cambridge University Press: Cambridge, U.K., 1998.
was found that the hemiporphyrazine is a redox-switchable
near-IR chromophore. We believe that such aromatic hemi-
porphyrazines offer a new perspective for the design of near-IR-
absorbing organic materials for optoelectronic applications.
(10) Braunstein, P.; Siri, O.; Taquet, J.; Rohmer, M.-M.; Ben
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Welter, R. J. Am. Chem. Soc. 2003, 125, 12246.
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(12) (a) Mack, J.; Stillman, M. J.; Kobayashi, N. Coord. Chem. Rev.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, spectral data, crystallographic data
(CIF), and computational details. This material is available free
of charge via the Internet at http://pubs.acs.org.
(13) Miwa, H.; Ishii, K.; Kobayashi, N. Chem.Eur. J. 2004, 10,
4422.
(14) (a) Fleishhauer, J.; Howeler, U.; Michl, J. J. Phys. Chem. A 2000,
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104, 7762. (b) Muranaka, A.; Matsushita, O.; Yoshida, K.; Mori, S.;
Suzuki, M.; Furuyama, T.; Uchiyama, M.; Osuka, A.; Kobayashi, N.
Chem.Eur. J 2009, 15, 3744. (c) Cho, S.; Yoon, Z. S.; Kim, K. S.;
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AUTHOR INFORMATION
■
Corresponding Author
atsuya-muranaka@riken.jp; uchiyama@mol.f.u-tokyo.ac.jp
ACKNOWLEDGMENTS
■
This research was partly supported by the JST PRESTO
Program. A.M. is indebted to the SPR Program at RIKEN. We
also gratefully acknowledge financial support from the Takeda
Science Foundation, Mitsubishi Foundation, and Uehara
Memorial Foundation (to M.U.). We are grateful to RIKEN
Integrated Cluster of Clusters (RICC) for the computer
resources used for the DFT calculations and to the RIKEN
chemical analysis team for elemental analyses.
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