Effects of N-Oxidation on Heteroaromatic Macrocycles: Synthesis, Electronic Structures, Spectral Properties, and Reactivities of Tetraazaporphyrin meso-N-Oxides

Article abstract of DOI:10.1002/chem.201701300

Heteroaromatic N-oxides such as pyridine and quinoline N-oxides are well studied in organic chemistry, and N-oxide formation has long been utilized for tuning the reactivities of heteroaromatics. However, the scope of aromatic N-oxidation is still restric

Full text of DOI:10.1002/chem.201701300

A Journal of
Accepted Article
Title: Effects of N-Oxidation on Heteroaromatic Macrocycles:
Synthesis, Electronic Structures, and Near-IR Properties/
Reactivities of Tetraazaporphyrin meso-N-Oxides
Authors: Naoyuki Toriumi, Shunsuke Yanagi, Atsuya Muranaka,
Daisuke Hashizume, and Masanobu Uchiyama
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To be cited as: Chem. Eur. J. 10.1002/chem.201701300
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Effects of N-Oxidation on Heteroaromatic Macrocycles: Synthesis,
Electronic Structures, Spectral Properties, and Reactivities of
Tetraazaporphyrin meso-N-Oxides
Naoyuki Toriumi,*[a] Shunsuke Yanagi,[a] Atsuya Muranaka,*[b] Daisuke Hashizume,[c] and Masanobu
Uchiyama*[a, b]
Abstract: Heteroaromatic N-oxides such as pyridine and quinoline
N-oxides are well studied in organic chemistry, and N-oxide
formation has long been utilized for tuning the reactivities of
heteroaromatics. However, the scope of aromatic N-oxidation is still
restricted to relatively small azine or azole skeletons, and there has
been little investigation of the photophysical/chemical effects of N-
oxidation on larger heteroaromatic systems. Here, we report
synthesis and unique properties of novel macrocyclic heteroaromatic
N-oxides, tetraazaporphyrin (TAP) meso-N-oxides. N-Oxidation of
TAP reduced the 18π-aromaticity of the TAP ring compared with that
of the parent TAP due to the cross-conjugated resonance structure.
The optical properties of TAPs were significantly changed by N-
oxidation: the N-oxides did not exhibit azaporphyrin-like optical
heterocyclic chemistry, and are utilized in pharmaceuticals,[1]
receptors and ligands,[2] and oxidants.[3] In general, N-oxidation
of heteroaromatics increases dipole moment, decreases basicity,
and facilitates both electrophilicity and nucleophilicity at the
highly polarized C=N bond, due to quaternary nitrogen cation
formation and mesomeric release of N-oxide oxygen,
respectively.[4] Many methods are available for N-oxidation of
heteroaromatics and for removal of oxygen from N-oxides.[5]
Thus, N-oxidation is well established as a strategy to modify the
reactivities of nitrogen-containing heteroaromatics. On the other
hand, utilization of N-oxidation for tuning photophysical/chemical
properties is not common, though it is known that the introduced
oxygen atom often results in red-shifted absorption bands due to
properties, but instead exhibited porphyrin-like optical properties, i.e., intramolecular charge transfer.[4] One reason for this situation is
weak Q absorption bands, strong Soret absorption bands, and weak
fluorescence. These features can be explained by the near-
degenerate frontier molecular orbitals resulting from N-oxide
formation. Singlet oxygen quantum yields were greatly increased to
almost quantitative levels by N-oxidation. The N-oxides showed
near-IR-responsive photoredox properties, and worked as both
oxidants and sensitizers for oxidation reactions. Protonation of the
N-oxides restored TAP-like intense Q bands and red fluorescence,
offering a potential design strategy for fluorescence switches.
the fact that availability of N-oxides is generally limited to rather
small heteroaromatics consisting of 6π-aromatic azine or azole
skeletons, which usually absorb high-energy UV light. In contrast,
the chemistry of heteroaromatic macrocyclic N-oxides has not
been investigated in detail, despite its potential for contributing
to the development of functional visible/near-IR dyes.
N
N
N
N
N
18π
N
M
N
6π
N
Introduction
N
O
N
O
N
O
O
Heteroaromatic N-oxides such as pyridine N-oxides and
quinoline N-oxides form an important domain of aromatic
Pyridine N-Oxide
Tetraazaporphyrin (TAP) N-Oxide
We focused on porphyrins, especially azaporphyrins, as
representative heteroaromatic macrocycles suitable for N-
oxidation. Azaporphyrins, composed of four pyrrole subunits
interconnected at the meso positions via nitrogen bridges, are
representative 18π-heteroaromatic macrocycles. They have
attracted considerable attention due to their aromatic/highly
conjugated properties, as well as their absorption/emission in
the visible/near-IR region, and have been extensively employed
as biomedical tools and optoelectronic materials.[6] However,
meso-N-oxides of azaporphyrins, which can be considered as
[a]
[b]
N. Toriumi, S. Yanagi, Prof. Dr. M. Uchiyama
Graduate School of Pharmaceutical Sciences
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
E-mail: naoyuki-toriumi@g.ecc.u-tokyo.ac.jp
uchiyama@mol.f.u-tokyo.ac.jp
Dr. A. Muranaka, Prof. Dr. M. Uchiyama
Elements Chemistry Laboratory and Advanced Elements Chemistry
Research Team, RIKEN Center for Sustainable Resource Science
(CSRS)
RIKEN
extended
18π-heteroaromatic
N-oxides,
are
almost
2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan
E-mail: atsuya-muranaka@riken.jp
Dr. D. Hashizume
Materials Characterization Support Unit
RIKEN Center for Emergent Matter Science (CEMS)
Wako-shi, Saitama 351-0198, Japan
unexplored.[7] To our knowledge, the only example is
diazaporphyrin meso-N-oxide, obtained as a side product during
the synthesis of a diazaporphyrin.[8] Therefore, the chemistry,
generality, and properties of macrocyclic azaporphyrin N-oxides
are still unclear.
[c]
Herein, we report the first N-oxidation of meso-nitrogens of
Supporting information for this article is given via a link at the end of
the document.
azaporphyrins.
Tetraazaporphyrin
(TAP),
the
simplest
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azaporphyrin, which has four nitrogens at the meso positions,[9]
was selected as a model compound. The TAP meso-nitrogens
were easily converted to the corresponding N-oxides with
mCPBA. We found that the N-oxidation significantly alters the
electronic structures and photophysical properties of 18π-
aromatic TAPs. Intriguingly, TAP N-oxide exhibited near-IR
photo-oxidative properties, releasing the oxygen atom and
sensitizing molecular oxygen. Protonation of the N-oxides
restored absorption and fluorescence emission in the red region.
Our findings indicate that N-oxidation is a simple but robust
method for controlling the photophysical/chemical properties of
heteroaromatic macrocycles.
desired N-oxides at all. The D2h isomers 1H2 and 1Zn reacted at
the less hindered meso-nitrogens to give mono-N-oxides 2H2
and 2Zn in moderate yields, respectively. 1Zn additionally
afforded 3Zn, but 1H2 did not yield the corresponding di-N-oxide.
The Cs+C2v isomers 1’H2 and 1’Zn only gave mono-N-oxides
2’H2 and 2’Zn, presumably because the other three meso-
t
nitrogens are sterically hindered by the bulky Bu groups. The
C4h isomer of H2TAP(tBu)4 did not react with mCPBA at all.
These oxidation reactions afforded TAP N-oxides and unreacted
TAPs as isolable compounds, and no major byproducts were
detected in the reaction mixtures (See the SI). It should be noted
that N-oxidation of the TAPs did not occur at the pyrrolic
nitrogen atoms, in contrast to the reported reaction between
octaethylporphyrin and mCPBA.[7c] These meso-N-oxides are
stable under ambient conditions, and were characterized by ESI-
O
N
M
N
N
N
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
1
MS, H and 13C NMR, and UV-vis spectroscopic methods. We
N
N
N
N
N
N
N
N
N
N
N
N
confirmed that the positions of the tBu groups have little effect on
the physical properties of these compounds.
N
N
N
M
N
+
N
M
N
N
O
N
O
a)
b)
1H2: M = H2
1Zn: M = Zn
2H2: M = H2 (46%)
2Zn: M = Zn (50%)
3Zn: M = Zn (6%)
tBu
tBu
NH
N
tBu
tBu
N
N
M
N
N
N
N
N
N
N
N
N
N
N
M
N
N
N
tBu
tBu
tBu
tBu
N
O
NH
N
1'H2: M = H2
1'Zn: M = Zn
2'H2: M = H2 (45%)
2'Zn: M = Zn (61%)
N
O
Scheme 1. Synthesis of TAP meso-N-oxides. Reagents and conditions:
mCPBA (1.1 eq.), CH2Cl2 (for 1H2, 1’H2), THF (for 1Zn, 1’Zn), rt, 24 h.
c)
N
N
HN
N
HN
N
N
Results and Discussion
N
N
N
N
NH
NH
N
Synthesis and Characterization
N
O
N
O
In the reaction between mCPBA and H2TAP with no peripheral
substituents, generation of the desired N-oxide was indicated by
MALDI-MS and UV-vis measurements, but its poor solubility
hampered further purification and analysis. In addition, gradual
decomposition was observed during the reaction due to
increased electrophilicity of the macrocycle at pyrrole rings,
arising from installation of the N-oxide functionality. In order to
improve the solubility and suppress oxidative/nucleophilic
degradation, we focused on the introduction of t-butyl (tBu)
groups onto the TAP pyrrole rings. H2TAP(tBu)4 was synthesized
as a mixture of four regioisomers and separated into three
fractions (D2h, Cs+C2v, and C4h structures) by standard column
chromatography on silica gel (see the Supporting Information
(SI)). ZnTAP(tBu)4 was prepared by insertion of a Zn ion into
H2TAP(tBu)4. Scheme 1 shows the synthesis of TAP N-oxides.
All the reactions were performed using mCPBA at room
temperature. Other typical oxidants used for synthesis of
pyridine N-oxides,[5] such as peracetic acid, trifluoroperacetic
acid, H2O2, Oxone, and dimethyldioxirane, did not afford the
Figure 1. a) X-ray structure of 2H2. The thermal ellipsoids are drawn at the
50% probability level. All the hydrogens and one of the two disordered oxygen
atoms are omitted for clarity. b) Selected bond lengths (Å) of H2TAP (top) and
H2TAP mono-N-oxide (bottom) calculated at the B3LYP/6-31G** level. c) Two
possible resonance structures of H2TAP mono-N-oxide.
The introduction of an oxygen atom at the less-hindered
meso-nitrogen was unambiguously confirmed by X-ray
diffraction analysis of crystals obtained by slow diffusion of
methanol into a toluene solution of 2H2 (Figure 1a). The N-oxide
molecule has
a crystallographic-inversion center, and the
symmetry brings about a disordering of the entire molecule by
superimposing a pair of mirror images. This prohibits our
discussing the bond lengths of the skeleton in detail. In order to
obtain geometrical information, we calculated the structures of
H2TAP and its N-oxide (Figure 1b). The calculated N–O distance
of 1.28 Å in H2TAP N-oxide is similar to the N–O distances
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10.1002/chem.201701300
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observed in pyridine N-oxides.[10] The C–N(O) bond lengths
were estimated to be 1.38 and 1.39 Å, longer than the C–
N(meso) bonds of H2TAP (1.32 and 1.34 Å). This increased
single-bond character of the C–N bonds can be attributed to a
cross-conjugated resonance structure with an N=O double bond
(Figure 1c), in which the 18π-aromatic circuit of the TAP
skeleton is disrupted by the oxygen atom. The IR spectrum of
2’H2 displays an intense absorption band at 1234 cm–1, assigned
to the N–O stretching vibration, as commonly seen in pyridine N-
oxides (Figure S4-1).[11]
a)
b)
c)
10
8
1’H2
2’H2
6
4
2
0
300
400 500 600 700 800 900
10
8
1’Zn
2’Zn
a)
1H2
H
6
H
tBu-H
(TMS)
N
tBu
tBu
tBu
4
β-H
N
N
N
H
N
N
N
2
H
N–H
–2.5
tBu
0
N
H
300 400 500 600 700 800 900
H
9.0
8.5 2.5
2.0 0.5
–1.0
9.5
10
b)
2H2
H
(TMS)
3Zn
H
tBu-H
8
6
4
2
0
N
tBu
tBu
tBu
N
N
N
H
β-H
N
N
N
H
β-H
tBu
N
N–H
H
H
O
9.5
9.0
8.5 2.5
2.0 0.5
–1.0
–2.5
300 400 500 600 700 800 900
δ [ppm]
Wavelength [nm]
Figure 2. 1H NMR spectra of a) 1H2 and b) 2H2 in CDCl3 at room temperature.
Figure 3. Electronic absorption (solid) and fluorescence (broken) spectra of
(a) 1’H2 (gray) and 2’H2 (blue) in CHCl3, (b) 1’Zn (gray) and 2’Zn (blue) in THF,
and (c) 3Zn (blue) in THF.
1
Figure 2 shows H NMR spectra of 1H2 and 2H2 in CDCl3 at
t
room temperature. The internal N–H protons, Bu protons, and
external pyrrole β protons of 1H2 were observed at –2.40, 2.23,
and 8.95 ppm, respectively, clearly indicating the existence of a
strong diatropic ring current typical of 18π-aromatic
porphyrinoids. As for 2H2, the pyrrole β protons gave two split
signals at 8.55 and 9.05 ppm, reflecting their different
environments due to the oxygen atom. Judging from the
chemical shifts of the internal N–H protons (–0.15 ppm) and the
tBu protons (2.10 ppm), the ring current strength of 2H2 is slightly
decreased compared to that of 1H2. The nucleus-independent
chemical shift (NICS)[12] values of H2TAP and H2TAP mono-N-
oxide at 1 Å above the center of the skeleton were estimated to
be –13.49 and –10.52 ppm, respectively (Figure S6-1), which
are consistent with the slightly diminished diatropic properties of
2H2. Calculated 1H chemical shifts also agreed with the
observed weaker ring current effects upon N-oxidation (Figure
S6-2). This reduced 18π-aromatic ring current of 2H2 is likely
associated with the contribution of the cross-conjugated
resonance structure discussed above. A similar tendency was
reported4 and computed for pyridine and its N-oxide (NICS(1): –
11.08 and –8.43 ppm, respectively) (Figure S6-1).
Photophysical Properties
To investigate the effects of the N-oxide functionality on the
optical properties, we measured absorption and fluorescence
spectra of TAPs and their N-oxides (Figure 3). TAPs displayed
intense Q bands (1’H2: 622, 553 nm; 1’Zn: 591 nm) and Soret
bands (1’H2: 336 nm; 1’Zn: 336 nm), which are characteristic of
azaporphyrinoids.[13] Interestingly, TAP N-oxides showed
significantly altered electronic properties. 2’H2 and 2’Zn
exhibited weaker and red-shifted Q bands in the 720-520 nm
region, while the red-shifted Soret bands in the 500-400 nm
region remained strong compared with those of the parent TAPs.
These drastic spectral changes are in marked contrast to the
optical properties of meso-N-protonated TAPs, which usually
retain their intense Q and Soret bands.[14] The absorption
spectrum of di-N-oxide 3Zn gave even more red-shifted and
weaker bands in the near-IR region up to 829 nm. These
behaviors of N-oxides are reminiscent of standard porphyrin
systems with meso-carbons, which generally show weak Q
bands and intense Soret bands. The magnetic circular dichroism
(MCD) spectra of N-oxides 2’H2 and 2’Zn both show coupled
Faraday B terms in the Q and Soret band regions, similarly to
porphyrins (Figure S4-2).[15] 2’H2 and 2’Zn displayed
fluorescence at 705 nm and 695 nm, respectively, but the
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LUMO+1
LUMO+1
E [eV]
quantum yields (2’H2: ΦF = 0.007; 2’Zn: ΦF = 0.005) were much
smaller than those of TAPs (1’H2: ΦF = 0.244; 1’Zn: ΦF = 0.224)
(Table 1), probably due to the decreased extinction coefficients
of the red-shifted Q bands. No fluorescence was detectable for
3Zn.
LUMO LUMO+1
–3.0
LUMO+1
–3.20
–3.09
–3.25
LUMO
–3.5
–4.0
–4.5
–5.0
–5.5
–6.0
–6.5
–7.0
LUMO
HOMO
LUMO
N
N
N
N
N
N
N
N
N
N
N
Zn
N
N
N
Zn
N
Table 1. Fluorescence and singlet oxygen quantum yields.
HOMO
N
O
1’H2
1’Zn
2’H2
2’Zn
3Zn
[a]
[d]
HOMO
ΦF
Φ∆
0.244[b]
0.21
0.224 [b]
0.59
0.007 [c]
0.93
0.005 [c]
0.96
n.d. [c]
0.85
–5.77
HOMO
–6.02
–6.03
HOMO–1
HOMO–1
HOMO–1
[a]
[a]
–6.58
–6.97
[a] Fluorescence quantum yields using oxazine 170 as a standard (ΦF = 0.579
in ethanol).[16] [b] Measured in CHCl3. [c] Measured in THF. [d] Singlet oxygen
quantum yields determined from singlet oxygen luminescence measured in
pyridine using ZnPc as a standard (Φ∆ = 0.61 in pyridine).[17]
–6.86
HOMO–1
Figure 4. Frontier π-system molecular orbitals and energy levels of ZnTAP
(left) and ZnTAP mono-N-oxide (right) calculated at the B3LYP/6-31G** level.
[a] σ-Orbitals.
The quite low fluorescence quantum yields of N-oxides imply
the possibility of facilitated intersystem crossing from the S1
state to the T1 state upon N-oxidation, which might lead to
enhanced phosphorescence and singlet oxygen generation.
TAP 1’H2 and 1’Zn exhibited no detectable phosphorescence
and low to moderate singlet oxygen generation (1’H2: Φ∆ = 0.21;
1’Zn: Φ∆ = 0.59) at room temperature (Table 1), as observed for
common azaporphyrins.[18] The increased singlet oxygen
quantum yield upon metalation is explained by heavy atom
effects. In sharp contrast, all the N-oxides showed quite high
singlet oxygen quantum yields (2’H2: Φ∆ = 0.93; 2’Zn: Φ∆ = 0.96;
3Zn: Φ∆ = 0.85), although phosphorescence was not observed.
The Φ∆ value of 2’H2 is quite large for a porphyrinoid with no
heavy atom, and is greater than that of free-base
Molecular Orbital Analysis
In order to gain insight into the altered electronic properties, we
performed molecular orbital (MO) calculations of ZnTAP and
ZnTAP mono-N-oxide (Figure 4). The optical properties of
porphyrins and azaporphyrins are reasonably well explained by
Gouterman’s four-orbital model.[20] The intense Q band of
ZnTAP is derived from the doubly degenerate transition from the
non-degenerate HOMO to the degenerate LUMOs (LUMO and
LUMO+1). N-Oxidation of the meso-nitrogen significantly
destabilizes the HOMO–1 owing to the out-of-phase interaction
between the oxygen and the meso-nitrogen, while the HOMO,
LUMO, and LUMO+1 are slightly stabilized. In other words, the
inductive effect of the electronegative quaternary nitrogen cation
leads to general stabilization of the MOs, while the electron-
donating character of the oxygen anion of the N-oxide
destabilizes the HOMO–1 selectively through the mesomeric
effect. Such phenomena/interactions are well known for pyridine
N-oxide (Figure S6-3). These changes in the energy levels
coincidentally cause near-degeneracy of the HOMOs and
LUMOs, which accounts for the forbidden Q band absorption
and allowed Soret band absorption, as seen in regular porphyrin
systems. H2TAP mono-N-oxide has similar MOs to ZnTAP
mono-N-oxide (Figure S6-4), and time-dependent density
functional theory (TD-DFT) calculations of N-oxides agreed well
with the observed absorption spectra (Figure S6-5). Cyclic
voltammetry (CV) measurements of 2’H2 showed a slight anodic
shift of the first reduction potential compared with a reported
TAP[21] (Figure S4-3), which supports the calculation results on
the stabilized LUMOs of TAP N-oxides.
tetraphenylporphyrin, which is frequently used as
a
photosensitizer (Φ∆ = 0.63).[19] These improved singlet oxygen
yields after N-oxidation can be partly rationalized by the change
from azaporphyrin-like optical properties to porphyrin-like optical
properties, since porphyrins generally sensitize oxygen more
efficiently than azaporphyrins due to their forbidden Q band
absorptions.[19]
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PCl3 (1.4 eq.)
Scheme 4. Methylation of TAP N-oxide 2’H2.
tBu
N
tBu
N
toluene, rt, 5 min
89%
tBu
tBu
N
N
HN
N
HN
N
N
N
Ph3P (2 eq.), NIR (700 nm)
We then examined modification at the N-oxide moiety
(Scheme 4). Meerwein’s salt methylated the oxygen atom of
2’H2 to afford 2’H2-Me+, though the product was difficult to
completely purify and isolate due to its rapid decomposition in
the presence of water. This instability contrasts with the stability
of various isolable N-methoxypyridinium salts.[24] The electronic
absorption spectrum of 2’H2-Me+ showed H2TAP-like
characteristics, i.e., two intense Q bands and a Soret band
(Figure S4-4), since the attached methyl group suppresses
electron donation from the oxygen atom to the macrocycle and
reduces the contribution of the cross-conjugated structure with
an N=O double bond, as confirmed by the calculated bond
lengths (Figure S6-6). The internal N–H protons of 2’H2-Me+
were observed at –1.90 ppm in the 1H NMR spectrum, also
indicating recovery of strong 18π-aromaticity, comparable to that
of TAP 1’H2.
N
N
toluene, rt, 2 h
95%
NH
NH
tBu
tBu
tBu
tBu
N
O
N
Et3N (10 eq.), NIR (700 nm)
toluene, rt, 24 h
70%
2'H2
1'H2
Scheme 2. Reduction of TAP N-oxide 2’H2 to TAP 1’H2.
2'H2 (3 mol%), NIR (700 nm)
Ph3P
Ph3P
O
toluene, O2 (1 atm), rt, 1 h
98%
Ph
Ph
2'H2 (3 mol%), NIR (700 nm)
O
O
toluene, O2 (1 atm), rt, 7.5 h
99%
Ph
Ph
Scheme 3. Catalytic near-IR oxidation with TAP N-oxide.
b)
a)
c)
2’H2
2’Zn
2’Zn
10
8
10
8
Reactivities and Photoreactivities
6
6
We expected that the TAP N-oxides would exhibit unique
reactivities, especially under light irradiation, corresponding to
their physicochemical properties. The metal-free N-oxide 2’H2
was selected for investigation of the reactivities. A strong
reductant, PCl3, immediately reacted with 2’H2 to give TAP 1’H2
in 89% yield (Scheme 2). Interestingly, the reduction to 1’H2 was
also achieved in the presence of weaker reductants, Ph3P and
Et3N, upon near-IR light irradiation under an Ar atmosphere,
generating Ph3PO and acetaldehyde, respectively.[22] During the
photo-reduction with Ph3P, we noticed that 2’H2 oxidized more
4
4
2
2
0
0
300
300
500
700
500
700
ΦF = 0.005
ΦF = 0.245
ΦF = 0.007
ΦF = 0.064
than
1 equivalent of Ph3P under an aerobic condition,
600 700 800
Wavelength [nm]
600 700 800
Wavelength [nm]
suggesting participation of O2 in the reaction. In fact, Ph3P was
completely converted to Ph3PO with a catalytic amount of 2’H2
under an O2 atmosphere upon near-IR irradiation (Scheme 3).
The mechanism could involve reaction of singlet oxygen with
Ph3P or single electron transfer from Ph3P to excited N-oxide
followed by quenching of Ph3P·+ with triplet oxygen, as proposed
for catalytic oxidation of triarylphosphine under UV/visible light
irradiation.[23] Singlet oxygen generation by near-IR light
irradiation was further demonstrated for endoperoxide formation
on 9,10-diphenylanthracene. The photoreactions did not
proceed in the dark. These results indicate that TAP N-oxides
can be utilized as near-IR responsive photo-oxidizing sensitizers.
Figure 5. a) Electronic absorption (top) and fluorescence (bottom) spectra of
2’H2 in CHCl3 (blue) and in 0.2% TsOH-CHCl3 (red). b) Electronic absorption
(top) and fluorescence (bottom) spectra of 2’Zn in THF (blue) and in 0.2%
H2SO4-THF (red). c) Photographs of 2’Zn in THF (left) and in 0.2% H2SO4-
THF (right) under visible (top) and UV (bottom) light.
Switching Optical Properties Using the N–Oxide
Functionality
The result of methylation prompted us to investigate protonation
of TAP N-oxides. Figure 5 shows absorption and fluorescence
spectra of 2’H2 and 2’Zn upon addition of acid. Both protonated
compounds exhibited much more intense Q bands than the
neutral forms, presumably due to structural changes caused by
protonation of the oxygen atoms. The TAP-like intense Q bands
of the protonated and methylated TAP N-oxides were further
confirmed by TD-DFT calculations (Figure S6-7). Fluorescence
quantum yields were also enhanced by protonation, and,
surprisingly, the red fluorescence of 2’Zn became approximately
tBu
N
tBu
N
tBu
tBu
N
N
HN
N
HN
N
N
N
–
Me3O+BF4
N
N
CH2Cl2, rt, 1 h
NH
NH
tBu
tBu
tBu
tBu
N
O
N
O
BF4
Me
2'H2
2'H2-Me
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10.1002/chem.201701300
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We have prepared TAP meso-N-oxides using mPCBA as an
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Acknowledgements
This work was partly supported by JSPS KAKENHI Scientific
Research (S) (grant No. 24229001 (to M.U.)) as well as the
Research Foundation for Opt-Science and Technology, Naito
Foundation, and The Asahi Glass Foundation (to M.U.). N.T. is
grateful for funding from the Graduate Program for Leaders in
Life Innovation (GPLLI) and JSPS Research Fellowships for
Young Scientists. RIKEN Integrated Cluster of Clusters (RICC)
and HOKUSAI GreatWave (HOKUSAI-GW) provided the
computer resources for the DFT calculations.
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Keywords: conjugation • fluorescence • N-oxides •
porphyrinoids • singlet oxygen
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10.1002/chem.201701300
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FULL PAPER
FULL PAPER
Naoyuki Toriumi,* Shunsuke Yanagi,
Atsuya Muranaka,* Daisuke Hashizume,
and Masanobu Uchiyama*
Page No. – Page No.
Effects of N-Oxidation on
Heteroaromatic Macrocycles:
Synthesis, Electronic Structures,
Spectral Properties, and Reactivities
of Tetraazaporphyrin meso-N-Oxides
Macrocyclic heteroaromatic N-oxides: Tetraazaporphyrin (TAP) meso-N-oxides
were synthesized by direct oxidation of TAPs. The introduction of an oxygen atom
significantly alters the physicochemical properties: the N-oxides exhibited weaker
red/near-IR absorption and fluorescence while singlet oxygen quantum yields were
greatly enhanced. The N-oxides showed near-IR-responsive photoredox properties,
and protonation of the N-oxides restored TAP-like intense red absorption and
fluorescence.
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