Synthesis, Characterization and Properties of Expanded Pyriporphyrins: A New Family of Alkylidenyl Porphyrin Homologues Bearing meso-Exocyclic Double Bonds

Article abstract of DOI:10.1002/chem.201602136

We present the synthesis and characterization of a new family of expanded meso-alkylidenyl (2,6-pyri)porphyrinoids bearing multiple exocyclic double bonds at the meso-positions. The synthesis was accomplished by using mixed pyrrole condensation. Similar to meso-alkylidenyl porphyrinoids, this study revealed that pyriporphyrinoids do not possess a porphyrin-like, global-aromatic character. The synthesized 2,6-pyripentaphyrin 1 displays selective ratiometric sensing of pyrophosphate anion in organic solvent.

Full text of DOI:10.1002/chem.201602136

DOI: 10.1002/chem.201602136
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Supramolecular Chemistry
Synthesis, Characterization and Properties of Expanded
Pyriporphyrins: A New Family of Alkylidenyl Porphyrin
Homologues Bearing meso-Exocyclic Double Bonds
Abeje Abebayehu, Ranjan Dutta, and Chang-Hee Lee*[a]
Chem. Eur. J. 2016, 22, 1 – 8
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Abstract: We present the synthesis and characterization of
a new family of expanded meso-alkylidenyl (2,6-pyri)porphyr-
inoids bearing multiple exocyclic double bonds at the meso-
positions. The synthesis was accomplished by using mixed
pyrrole condensation. Similar to meso-alkylidenyl porphyri-
noids, this study revealed that pyriporphyrinoids do not pos-
sess a porphyrin-like, global-aromatic character. The synthe-
sized 2,6-pyripentaphyrin 1 displays selective ratiometric
sensing of pyrophosphate anion in organic solvent.
Introduction
tinuing efforts for the development of new porphyrin ana-
logues bearing exocyclic double bonds at multiple meso-posi-
tions, we here report the syntheses and chemical properties of
new meso-diethylmalonylidene-2,6-pyriporphyrins 1 and 2. The
observations made in these studies showed that the pyripor-
phyrinoids do not possess porphyrin-like, global-aromatic char-
acter. These unusual properties based on our previous report
have inspired us to further explore the anion-sensing proper-
ties of the pyriporphyrinoid 1.[16]
The chemistry of many structural proxies of porphyrin ana-
logues and their congeners including isomeric porphyrins, aro-
matic and nonaromatic analogues, and expanded porphyrins
have drawn immense attention owing to their unique photo-
physical properties. The unique structural and spectroscopic
features of these macrocycles enable impending applications
in various areas such as anion recognition,[1] photodynamic
therapy[2] and material sciences.[3] One of the enticements has
been the understanding of the electronic features of the relat-
ed macrocycles. Various core-modified porphyrinoids, including
carbaporphyrinoids[4–7] and their metal complexes,[8,9] pyridine-
containing porphyrinoids,[10] and thia- or oxaporphyrinoids,[11]
have been synthesized and studied. As a result of these efforts,
it was found that the benziporphyrins and pyriporphyrins[12,13]
bearing a benzene or pyridine subunit as a part of the core
structure did not exhibit porphyrin-like macroaromatic charac-
teristics in most cases, presumably reflecting the fact that the
full conjugation pathway is broken. In fact, it was empirically
found that the presence of tautomerizable functional groups is
essential and required to maintain aromaticity.[14] Porphyrins
bearing bulky meso-alkylidenyl substituents do not possess
porphyrin-like, global-aromatic character as a consequence of
a sterically dictated, variable disruption of extended cyclic p-
conjugation (Scheme 1).
Results and Discussion
Synthesis of the key building block, dipyrropyridine derivative
6 was carried out as shown Scheme 2. Pyridine-2,6-dimethanol
(3) was oxidized with selenium dioxide to give pyridine-2,6-di-
carbaldehyde (4),[17] which was then condensed with diethyl
malonate to afford the corresponding bis-vinyl derivative 5 in
26% yield. The acid-catalyzed condensation of pyrrole with 5
afforded desired building block 6 in 62% yield. Many different
conditions were tested in an effort to optimize this condensa-
tion, and it was found that the highest yield of 6 was obtained
when TFA was used as a catalyst.[12b,c] Although these reactions
1
afforded a single regioisomer, the H NMR spectrum of 6 indi-
cated the formation of diastereomers. Compound 6 has been
previously utilized in the synthesis of meso-alkylidenylthiapyri-
porphyrin, oxapyriporphyrin and their respective expanded an-
alogues.[12b,c] We speculated that if pyrrole is incorporated into
the system instead of thiophene or furan, the system would be
geometrically more organized and expected to give a single
conformational isomer, and thus facilitate studying the spec-
troscopic properties of the compounds.
The syntheses of the desired pyriporphyrinoids were accom-
plished in a straightforward manner by using mixed pyrrole
condensation, as shown in Scheme 3. The pyridine tripyrrane
analogue 6,[12b,c] pyrrole and pentafluorobenzaldeyhde were
dissolved in dichloromethane and treated with a catalytic
amount of trifluoroacetic acid. Subsequent oxidation with DDQ
and purification by column chromatography, followed by re-
crystallization from chloroform/methanol, afforded the desired
pentaphyrin 1 in 8% yield.[16] The process has also resulted in
the formation of partially oxidized form of dipyrihexaphyrin in
2% yield which is further oxidized by DDQ in refluxing toluene
to afford 2. Both the synthesized compounds are not expected
to exhibit macroaromatic properties. Similar to m-benzipor-
phyrins bearing meso-alkylidene, substituent 2,6-pyriporphyrins
1 and 2 also exists in a single tautomeric form. The presence
of exocyclic double bond at the meso-position is expected to
be distorted due to the steric congestion between the malonyl
Scheme 1. Structural features of meso-alkylinenyl porphyrin.
The bulky meso-substituents prevent the prototropic shift
and planarity of the macrocycles resulting in the suppression
of the tautomeric equilibrium. These in a way suggest that
proper steric hindrance and nonplanar conformation are essen-
tial to stabilize the less stable conformer.[15] As part of our con-
[a] A. Abebayehu, Dr. R. Dutta, Prof. C.-H. Lee
Department of Chemistry, Kangwon National University
Chuncheon 200-701 (Korea)
E-mail: chhlee@kangwon.ac.kr
Supporting information for this article can be found under http://
dx.doi.org/10.1002/chem.201602136.
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Scheme 2. Synthesis of building block 6.
group and the adjacent pyridine ring. In order to reduce such
steric hindrances, either the ester functionalities must be dis-
torted or the pyridine ring must be out of plane.
in the 1H NMR spectrum are assigned as b-pyrrole-H atoms.
These pyrrole rings are inverted from the core and pyrrolic-NH
groups are hydrogen bonded to carbonyl groups. This was fur-
The 1H NMR spectrum of compound 1 showed a symmetrical
character and no other tautomeric forms were detected. The
exceptionally low field signals of three pyrrole NÀH groups ap-
peared at 11.10 ppm (two protons) and 10.31 ppm (one
proton), clearly indicating the presence of intramolecular hy-
drogen bonding. These observations suggest that the two pyr-
role NÀH groups hydrogen bond with the adjacent carbonyl
groups and the other core NÀH also hydrogen bonds with the
adjacent nitrogen to from a stable dipyrromethene structure.
Comparative 1H NMR and UV/Vis spectra of pyripentaphyrin
1 and analogous m-benzipentaphyrin[16] are provided in the
Supporting Information (Figures S18 and S19 in the Supporting
Information). Moreover, a dilution experiment also supports
the presence of this intramolecular hydrogen bond in 1 (Fig-
ure S20).
ther confirmed from the H-1H COSY and H-13C HMBC spectra
which displayed a cross-peak correlation between b-pyrrole-H
with inverted NÀH and b-pyrrole-C with inverted NÀH, respec-
tively. The two doublets pyrrolic b-H atoms appearing at d=
6.40 ppm (J=5.10 Hz, 2H (14, 20)) and d=6.65 ppm (J=
1
1
1
5.05 Hz, 2H (15, 19)) in the H NMR spectrum also displayed
1
cross-peak correlation in the H-1H COSY spectrum.
The expanded porphyrin 2 was also isolated in low yield.
1
The H NMR spectrum of 2 showed two broad signals for pyr-
rolic NÀH groups (Figure S22 in the Supporting Information).
Presumably, the signals appearing at d=11.66 and 11.51 ppm
are the inverted pyrrole NÀH groups that form intramolecular
hydrogen bonds with carbonyl groups. All the pyrrolic b-pro-
tons were in the range of d=5.93–7.53 ppm. These observa-
tions clearly indicate an asymmetric structural character of 2
(Figure 2). The multiplets in the range d=5.93–6.07 ppm could
be assigned to the b-H atoms of the pyrrole containing ÀNH
groups. While the other b-H atoms appeared as doublets in
the range d=6.66–7.53 ppm with same coupling constant. The
double doublet and triplet signals in the range d=7.29–
7.36 ppm and d=7.51–7.53 ppm, respectively, could be as-
The unambiguous confirmation of the structural characteri-
1
zation was obtained by 2D NMR spectroscopy including H-1H
1
1
COSY, H-13C HSQC and H-13C HMBC experiments (Figure 1 and
Figures S4–S9 in the Supporting Information). For instance, in
compound 1, two multiplets appearing at d=5.73 ppm (J=
1.87 Hz, 2H (10, 24)) and d=6.62 ppm (J=1.66 Hz, 2H (9, 25))
Scheme 3. Synthesis of the expanded pyriporphyrins 1 and 2.
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Figure 3. UV/Vis absorption spectral changes of 1 observed upon addition
of TFA (0–30 equiv) [1]=26 mm in CH3CN.
mine), the original free-base spectrum was fully restored, indi-
cating that the protonation is reversible.
Pyrrole-encompassing compounds can be used to form hy-
drogen bonds with anions under varying conditions.[18] Howev-
er, rigid and planar porphyrin cavities having hidden NH
groups lack accessibility and are unable to bind anions effi-
ciently.[19–24] Since the compounds contain inverted multiple
core NÀH groups that could form hydrogen bonds with anion-
ic guests,[25] we examined the anion-binding properties of
compound 1. Preliminary solution-phase anion-binding behav-
Figure 1. Selected region of 1H-1H COSY spectrum of 2,6-pyripentaphyrin 1.
ior of 1 was studied by absorption spectral changes upon ad-
3À
dition of various anions (FÀ, ClÀ, BrÀ, HSO4À, H2PO4À, HP2O7
,
CH3CO2À, C6H5CO2 and OHÀ as their tetrabutyl ammonium
salts) in CH3CN. Among all the tested anions, acetate, hydrox-
ide and hydrogen pyrophosphate displayed concentration-de-
pendent, sequential spectral and color changes (see the Sup-
porting Information). Sensing of hydrogen pyrophosphate and
dihydrogen phosphate is important due to their role in real-
time DNA sequencing and replication.[26]
À
1
Figure 2. Partial H NMR spectrum of compound 2 in CDCl3.
signed to the pyridyl group. The a-protons of the malonyl
group appeared as a doublet at 4.04 ppm, which suggest no
exocyclic bonds are present. The MALDI-TOF mass spectrum
revealed the molecular ion peak at m/z 1456.06 (Figure S24),
corresponding to an endocyclic structure. Thus, the 1H NMR
and MALDI-TOF spectral features clearly confirm no exocyclic
double bond is present in 2.
In order to check the protonation properties, compounds
1 and 2 were treated with acid (trifluoroacetic acid, TFA) in
acetonitrile. The changes in UV/Vis absorption spectra associat-
ed with protonation are shown in Figure 3. The free base
1 originally showed two absorption maxima, consisting of
a Soret-like band at 467 nm and Q-band at 759 nm, which
were red-shifted upon addition of TFA. For instance, the Soret-
like band observed at 467 nm gradually decreased in intensity
and the Q-band originally appearing at 759 nm was shifted to
918 nm with clear isosbestic points.
Two distinctive spectral changes are noticed depending on
the added hydrogen pyrophosphate concentration. The origi-
nal Soret-like band observed at 467 nm shifted to 490 nm
whereas the Q-band shown at 759 nm shifted to 828 nm upon
addition of approximately 5 equiv of hydrogen pyrophosphate
anion. Further addition of hydrogen pyrophosphate resulted in
an additional shift of the absorption bands (Figure 4). For ex-
ample, the Soret-like band was shifted to 529 nm and the Q-
band to 920 nm upon addition of 40 equiv of hydrogen pyro-
phosphate anion. The original green colored solution turned
pink-red upon addition of hydrogen pyrophosphate. Acetate
and hydroxide anions show similar spectral changes as hydro-
gen pyrophosphate at lower equivalents of anion (Figures S12
and S13 in the Supporting Information). However, the second
spectral changes were not observed even after addition of
excess acetate or hydroxide.
The color of the solution changes from green to yellow
upon gradual addition of TFA. When the resulting strongly
acidic solution was treated with an excess of base (triethyla-
To investigate the mechanism of binding of hydrogen pyro-
phosphate with 1, we carried out 1H NMR titrations in
[D6]DMSO (Figure 5). Addition of 3 equivalents of HP2O73À indu-
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Figure 5. Partial H NMR titration spectra of compound 1 ([1]=3.29ꢁ10À3 m)
with hydrogen pyrophosphate anion (as its tetrabutylammonium salt) in
3À
[D6]DMSO. i) Free base 1, ii) 1+1.0 equiv HP2O73À, iii) 1+3.0 equiv HP2O7
,
3À
iv) 1+5.0 equiv HP2O73À, and v) 1+10.0 equiv HP2O7
.
Thus, by simply changing the benzipentaphyrin to pyripenta-
phyrin, a switch of sensing behavior from fluoride to hydrogen
pyrophosphate is demonstrated.
The UV/Vis absorption spectral changes of compound 2 is
different from that of compound 1 (Figure S26 in the Support-
ing Information). The UV/Vis absorption spectrum of 2 in ace-
tonitrile shows Soret-like bands at 416 and 526 nm and Q-like
split bands at 750 and 830 nm which are characteristic of non-
aromatic porphyrinoids (Figure S25). The protonation proper-
ties of 2 upon addition of TFA in CH3CN, indicate that approxi-
mately 10 equiv of TFA is necessary for full protonation, al-
though the nature of the protonated form could not be ex-
plained from acid titration. The absorption spectral changes of
compound 2 depending on the concentration of acid exhibit-
ed a drastically different spectral pattern with broad and
strong bands. The UV/Vis titration spectra of 2 display system-
atic changes with clear isosbestic points. For instance Soret-
like bands at 416 and 526 nm red shifted to 420 and 557 nm,
respectively, while the split Q-bands at 750 and 830 nm shifted
to 761 and 979 nm, respectively (Figure S25).
Figure 4. UV/Vis absorption spectral changes of 1 (26 mm in CH3CN) ob-
served upon titration with hydrogen pyrophosphate anion (as its tetrabuty-
lammonium salt): a) 0–5 equiv, and b) 4–40 equiv. Two sequential shifts of
spectra (759–828 nm and 828–920 nm) were clearly observed.
ces no such noticeable changes in the 1H NMR spectrum of
1 in [D6]DMSO (Figures S14 and S15 in the Supporting Informa-
3À
tion). However, further addition of HP2O7 up to 5 equivalents
causes complete disappearance of intramolecular hydrogen
bonded inverted ÀNH protons of 1. Moreover, upfield shift of
core ÀNH proton and b-pyrrolic protons are clearly observed
at this point (Figure S15). This clearly indicates deprotonation
of inverted ÀNH protons at higher equivalents of HP2O73À. Sim-
Conclusions
In summary, we have successfully synthesized a new family of
meso-alkylidenyl expanded pyriporphyrinoids that show inter-
esting chemical properties as well as structural features. Both
the expanded pyriporphyrinoids were well characterized by
different spectroscopic techniques. The structures of the syn-
thesized compounds are consistent with a nonaromatic and
partially conjugated structure that must be severely distorted.
Compound 1 shows distinct spectral and colorimetric changes
with hydrogen pyrophosphate anion. Moreover, appearance of
new bands in the near infrared (NIR) region could enable this
system to be a useful chromogenic sensor for pyrophosphate
anions. We are currently working to elaborate the synthesis of
these types of macrocycles for future applications as ligands
for metal complexes. The chemistry and properties of these
types of nonaromatic macrocycles will be reported in due
course.
1
ilar H NMR and UV/Vis spectral changes are reported for m-
benzipentaphyrin homologue with fluoride anion, where de-
protonation of the one inverted pyrrole ÀNH is observed at
3 equiv of FÀ followed by second deprotonation at higher
equivalents of FÀ in [D6]DMSO.[16] We conclude that the same
3À
binding mechanism is also operating in case of 1 with HP2O7
as the 1H NMR and UV/Vis spectral changes are very similar.
This deprotonation property of 1 was confirmed by titrating
with more basic hydroxide anion which also induces deproto-
nation of inverted ÀNH protons at 3 equivalents of TBAOH
(Figures S16 and S17). UV/Vis titration of m-benzipentaphyrin
3À
with HP2O7 anion failed to exhibit such sequential changes
3À
up to 50 equivalents of HP2O7 in acetonitrile (Figure S21).
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7.05 Hz, 4H), 3.98–3.94 (q, J=7.08 Hz, 4H), 1.28–1.25 (t, J=7.14 Hz,
6H), 1.06–1.04 ppm (t, J=7.14 Hz, 6H); 13C NMR (150 MHz, CDCl3):
d=13.80, 13.94, 14.07, 14.19, 61.67, 62.09, 62.44, 108.05, 117.70,
120.37, 124.69, 125.19, 130.04, 132.41, 134.17, 135.19, 135.95,
143.89, 151.36, 157.60, 160.55, 166.19, 167.72 ppm; MALDI-TOF MS
elemental analysis cacld (%) for C58H34F15N5O8 1213.90 [M+], found
1213.23 [M+].
Experimental Section
All reactions were carried out with standard Schlenk techniques
1
under an N2 atmosphere. H NMR spectra (300 MHz, Bruker Avan-
ceTM and 600 MHz, Bruker AvanceTM) were recorded by using TMS
as the internal standard. All NMR data were obtained in CDCl3.
Chemical shifts are reported in parts per million (ppm, d). Spectral
splitting patterns are designated as s: singlet, d: doublet, t: triplet,
q: quartet, m: complex multiplet (chemically non-equivalent Hs), br
s: broad signal. 13C NMR spectra were proton decoupled and re-
corded on a 150 and 100 MHz Bruker spectrometer using TMS as
the internal standard. High resolution mass spectra were obtained
on a Voyager-DE STR MALDI-TOF mass spectrometer. Column chro-
matography was performed over silica gel (Merck, 230–400 mesh).
Pyrrole was distilled at atmospheric pressure over CaH2. All other
reagents were obtained from Aldrich and were used as received
unless noted otherwise. Compounds 4[17] and 5[12b,c] were synthe-
sized according to reported literature procedures.
1
Compound 2: H NMR (400 MHz, CDCl3): d=11.66 (br, s, 1H), 11.51
(br, 1H), 7.72–7.67 (t, J=1.26 Hz, 2H), 7.53–7.51 (d, J=2. 60 Hz,
1H), 7.36–7.34 (d, J=2.00 Hz, 2H), 7.31–7.29 (d, J=2.10 Hz, 2H),
7.21–7.18 (d, J=2.60 Hz, 1H), 6.87–6.85 (d, J=1.50 Hz,1H), 6.68–
6.66 (d, J=1.50 Hz,1H), 6.07–6.04 (m, J=2.80 Hz, 2H), 5.96–6.92
(m, J=2.81 Hz, 2H), 4.34–4.26 (m, J=1.60 Hz, 4H), 4.24–4.19 (m,
J=1.90 Hz, 4H), 4.07–4.04 (d, J=2.60 Hz, 4H), 3.98–3.90 (m, J=
1.60 Hz, 4H), 3.88–3.83 (m, J=1.90 Hz, 4H), 1.21–1.16 (t, J=
2.40 Hz, 6H), 1.12–1.07 (t, J=2.40 Hz, 6H), 0.99–0.94 (t, J=2.40 Hz,
6H), 0.87–0.82 ppm (t, J=2.40 Hz, 6H); 13C NMR (100 MHz, CDCl3):
d=12.80, 13.56, 13.71, 13.87, 13.95, 53.44, 57.06, 61.03, 61.07,
61.79, 61.99, 62.87, 63.31, 109.03, 109.56, 109.88, 116.72, 120.41,
120.85, 121.76, 122.80, 123.11, 126.80, 127.80, 127.87, 128.03,
129.25, 133.04, 133.82, 136.18, 143.84, 156.34, 156.48, 1165.30,
165.79, 166.07, 166.16, 167.12, 167.27 ppm; MALDI-TOF MS ele-
mental analysis cacld (%) for C72H62F10N6O16 1455.26, found 1456.05
[M+H]+.
UV/Vis spectroscopic analysis was carried out on a Varian Cary 100
conc. spectrophotometer by using slit width of 1.0 nm and
matched quartz cells. The cell holder of the spectrophotometer
was thermostated at 258C for consistency in the recordings. The
studies were performed in HPLC grade CH3CN. Stock solutions of
the receptor 1 being studied were prepared in acetonitrile with
the final concentration being 1ꢁ10À5 m. The anion solutions used
to effect the titration contained the same concentration as the re-
ceptor solutions into which they were being titrated in order to
eliminate dilution effects that arise during the titrations.
Acknowledgements
Synthesis of tetraethyl 2,2’-(pyridine-2,6-diylbis ((1H-pyrrol-2-yl)-
methylene))dimalonate (6): Compound 5 (0.3 g, 1.44 mmol) and
TFA (0.054 mL, 0.72 mmol) were added to pyrrole (0.984 mL,
14.4 mmol). The reaction mixture was allowed to stir at room tem-
perature for 5 h and then quenched with brine (10 mL) and ex-
tracted in a separatory funnel with DCM (20 mLꢁ3). The organic
layers were dried over Na2SO4, filtered and concentrated under re-
duced pressure. The crude product was recrystallized from Et2O
and hexane to give product 6 (0.66 g, 62%). 1H NMR (400 MHz,
CDCl3); d=8.59 (br, s, 2H), 7.31–7.29 (m, 1H), 7.20 (m, 1H), 7.14 (m,
2H), 6.63–6.60 (m, 2H), 6.05–6.02 (m, 2H), 5.92 (m, 2H), 4.79 (d, J=
14 Hz, 2H), 4.15 (d, J=14.2 Hz, 2H), 4.15–4.09 (m, 4H), 3.90–3.86
(m, 4H), 1.19–1.15 (t, J=7.12 Hz, 6H), 0.97–0.93 ppm (t, J=7.12 Hz,
6H); MALDI-TOF m/z elemental analysis cacld (%) for 551.4215
[M+], found 575.3812 [M+H+Na]+. All the spectroscopic data
matched with those reported in the literature.[12b,c]
Support from Global Research Laboratory Program through
the National Research Foundation of Korea funded by the Min-
istry of Science, ICT and Future Planning (2014K1A1A2064569)
and (2015R1A2A1A10052586) and the Central Instrumentation
facility at KNU is acknowledged.
Keywords: anions
· carbaporphyrins · meso-alkylidene ·
pyriporphyrin · sensors
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tri(pentafluorophenyl)-(2,6-pyri)pentaphyrin (1) and expanded
analogue (2): Product 6 (0.51 g, 0.92 mmol) was dissolved in di-
chloromethane (450 mL) and then pyrrole (0.07 mL, 1.02 mmol),
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0.45 mmol) were added and the reaction mixture was allowed to
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again with EtOAc/hexane=1:4 to obtain compound 1 (0.09 g, 8%)
and partially oxidized form of compound 2 (0.02 g, 2%). This was
further treated with DDQ in refluxing toluene to afford 2.
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Received: May 5, 2016
Published online on && &&, 0000
Chem. Eur. J. 2016, 22, 1 – 8
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FULL PAPER
Branch and bind: A new family of ex-
panded meso-alkylidenyl (2,6-pyri)por-
phyrinoids (see figure) bearing multiple
exocyclic double bonds at the meso-po-
sitions were synthesized and explored
for anion-sensing studies.
&
Supramolecular Chemistry
A. Abebayehu, R. Dutta, C.-H. Lee*
&& – &&
Synthesis, Characterization and
Properties of Expanded
Pyriporphyrins: A New Family of
Alkylidenyl Porphyrin Homologues
Bearing meso-Exocyclic Double Bonds
Alkylidenyl Porphyrin Homologues
A new family of meso-alkylidenyl porphyrinoid-bearing
multiple exocyclic double bonds at the meso-positions and
pyridine moiety is reported in the Full Paper by C.-H. Lee
and colleagues on page && ff., which was explored for
anion-sensing studies. This particular family of porphyrins,
bearing bulky meso-alkylidenyl substituents, do not possess
porphyrin-like, global-aromatic character as a consequence
of a sterically dictated, variable disruption of extended cyclic
p-conjugation. The bulky meso-substituents prevent the
prototropic shift and planarity of the macrocycles resulting
in the suppression of the tautomeric equilibrium.
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Chem. Eur. J. 2016, 22, 1 – 8
www.chemeurj.org
8
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!