Coordination-Driven Folding in Multi-ZnII-Porphyrin Arrays Constructed on a Pillar[5]arene Scaffold

Article abstract of DOI:10.1002/chem.201701622

Pillar[5]arene derivatives bearing peripheral porphyrin subunits have been efficiently prepared from a deca-azide pillar[5]arene building block (17) and Zn^II-porphyrin derivatives bearing a terminal alkyne function (9 and 16). For the resulting deca-Zn^II-porphyrin arrays (18 and 20), variable temperature NMR studies revealed an intramolecular complexation of the peripheral Zn^II-porphyrin moieties by 1,2,3-triazole subunits. As a result, the molecules adopt a folded conformation. This was further confirmed by UV/Vis spectroscopy and cyclic voltammetry. In addition, we have also demonstrated that the coordination-driven unfolding of 18 and 20 can be controlled by an external chemical stimulus. Specifically, addition of an imidazole derivative (22) to solution of 18 or 20 breaks the intramolecular coordination at the origin of the folding. The resulting molecular motions triggered by the addition of the imidazole ligand mimic the blooming of a flower.

Full text of DOI:10.1002/chem.201701622

A Journal of
Accepted Article
Title: Coordination-driven folding in multi-Zn(II)porphyrin arrays
constructed on a pillar[5]arene scaffold
Authors: Jean-Francois Nierengarten, Trinh Thi Minh Nguyet, Iwona
Nierengarten, Haifa Ben Aziza, Eric Meichsner, Michel
Holler, matthieu Chessé, Rym Abidi, Christian Bijani, Yannick
Coppel, Emmanuel Maisonhaute, and Béatrice Delavaux-
Nicot
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To be cited as: Chem. Eur. J. 10.1002/chem.201701622
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Coordination-driven folding in multi-Zn(II)porphyrin arrays
constructed on a pillar[5]arene scaffold
Thi Minh Nguyet Trinh,[a] Iwona Nierengarten,[a] Haifa Ben Aziza,[a,b] Eric Meichsner,[a] Michel Holler,[a]
Matthieu Chessé,[a] Rym Abidi,[b] Christian Bijani,[d] Yannick Coppel, [d] Emmanuel Maisonhaute,*[c]
Béatrice Delavaux-Nicot,*[d] and Jean-François Nierengarten*[a]
Dedicated to Prof. Vincenzo Balzani on the occasion of his 80th birthday
Abstract: Pillar[5]arene derivatives bearing peripheral porphyrin
subunits have been efficiently prepared from decaazide
synthesis. In their seminal paper, they have shown that
cyclopentameric macrocyclic compounds can be prepared in
a
pillar[5]arene building block (17) and Zn(II)-porphyrin derivatives
bearing a terminal alkyne function (9 and 16). For the resulting deca-
Zn(II)-porphyrin arrays (18 and 20), variable temperature NMR
studies revealed an intramolecular complexation of the peripheral
Zn(II)-porphyrin moieties by 1,2,3-triazole subunits. As a result, the
molecules adopt a folded conformation. This was further confirmed
by UV/vis spectroscopy and cyclic voltammetry. In addition, we have
also demonstrated that the coordination-driven unfolding of 18 and
20 can be controlled by an external chemical stimulus. Specifically,
addition of an imidazole derivative (22) to solution of 18 or 20 breaks
the intramolecular coordination at the origin of the folding. The
resulting molecular motions triggered by the addition of the imidazole
ligand mimics the blooming of a flower.
excellent
yields
from
1,4-dialkoxybenzene
and
paraformaldehyde in the presence of BF3.Et2O.[3] They also
proposed the name pillararene for this class of macrocycles
because of their tubular shape. Following this report, the field of
pillar[n]arene has generated significant research efforts focused
on both the improvements of their synthetic methods[4] and their
incorporation in supramolecular structures.[1] Owing to the
presence of ten peripheral substituents, pillar[5]arenes are also
attractive compact scaffolds for the preparation of multivalent
compounds for biological applications.[5-6] Examples include
glycopillar[5]arenes for the specific binding to bacterial lectins[5]
and polycationic compounds for gene delivery.[6] On the other
hand, nanomaterials with a controlled distribution of functional
groups on the macrocyclic framework have been also reported.[7]
As part of this research, we now report the preparation of
pillar[5]arene derivatives bearing ten peripheral Zn(II)-porphyrin
subunits. These compounds have been efficiently prepared from
a clickable pillar[5]arene building block and appropriate Zn(II)-
porphyrin derivatives under copper-catalyzed alkyne-azide
cycloaddition (CuAAC) conditions. An unprecedented folding
due to intramolecular coordination of the 1,2,3-triazole linkers to
the peripheral Zn(II)-porphyrin moieties has been evidenced for
the resulting decatriazoles. Interestingly, this coordination-driven
folding can be controlled by an external chemical stimulus,
namely addition of an imidazole ligand. As a result, unfolding of
the molecules occurs and the resulting molecular motions mimic
the blooming of a flower.
Introduction
Pillar[5]arene are macrocyclic compounds at the forefront of
modern
supramolecular
chemistry.[1]
Whereas
their
paracyclophane skeleton was already reported in the 80s,[2] the
synthesis of such compounds was difficult thus limiting both their
availability and applicability.[2] This problem was solved in 2008
when Ogoshi and co-workers reported an efficient one-step
[a]
Dr. T. M. N. Trinh, Dr. I. Nierengarten, Dr. H. Ben Aziza, E.
Meichsner, Dr. M. Holler, M. Chessé, Dr. J.-F. Nierengarten
Laboratoire de Chimie des Matériaux Moléculaires, Université de
Strasbourg et CNRS (UMR 7509), Ecole Européenne de Chimie,
Polymères et Matériaux
Results and Discussion
25 rue Becquerel, 67087 Strasbourg Cedex 2, France.
E-mail: nierengarten@unistra.fr
Synthesis. The preparation of the first clickable porphyrin
building block is depicted in Scheme 1. Treatment of alcohol 1
with p-toluenesulfonyl chloride (p-TsCl) gave tosylate 2 in 77%
yield. Reaction of p-hydroxybenzaldehyde (3) with tosylate 2
under classical Williamson etherification conditions (K2CO3, DMF,
80°C) afforded 4 in 85% yield. Porphyrin 7 was obtained by
using the reaction conditions developed by Lindsey for the
synthesis of sterically hindered porphyrins.[8] A key feature of
these conditions involves BF3-ethanol co-catalysis. The
condensation of 4 (1 equiv.), mesitaldehyde (5, 3 equiv.) and
pyrrole (6, 4 equiv.) was performed in CHCl3 (commercial CHCl3
containing 0.75% ethanol as stabilizer) at room temperature in
[b]
Dr. H. Ben Aziza, Prof. R. Abidi
Laboratoire d’Applications de la Chimie aux Ressources et
Substances Naturelles et l’Environnement, Faculté des Sciences de
Bizerte, Université de Carthage
7021 Zarzouna Bizerte, Tunisia.
Prof. E. Maisonhaute
Sorbonne Universités, UPMC Univ Paris 06, UMR 8235, Laboratoire
Interfaces et Systèmes Electrochimiques, F-75005 Paris, France.
E-mail: emmanuel.maisonhaute@upmc.fr
C. Bijani, Dr. Y. Coppel, Dr. B. Delavaux-Nicot
Laboratoire de Chimie de Coordination du CNRS, Université de
Toulouse, UPS, INPT,205 route de Narbonne, BP 44099, F-31077
Toulouse Cedex 4, France.
[c]
[d]
E-mail: beatrice.delavaux-nicot@lcc-toulouse.fr
the presence of BF3.Et2O. After
3
h, p-chloranil
Supporting information for this article is given via a link at the end of
the document.
(tetrachlorobenzoquinone) was added to irreversibly convert the
porphyrinogen to the porphyrin. Free base porphyrin 7 was
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subsequently isolated in 15% yield by tedious chromatographic
separations. Metalation of 7 with Zn(OAc)2 gave 8 in 83% yield.
The metalation is conveniently followed by the changes in the
absorption spectra. Actually, four bands (Q bands), responsible
for the red-purple color in solution, are observed in the visible
region (515, 549, 593 and 648 nm) of the absorption spectrum of
free base porphyrin 7. A very sharp and intense band (B band
also called Soret band) is also present in the near UV region
(419 nm). Upon metalation, the four Q bands collapse into two
bands (550 and 587 nm) due to the higher symmetry of the
porphyrin core when metallated (D4h vs. D2h), but the Soret band
(422 nm) is barely affected.[9] The changes in the visible region
result also in a clear color change when going from 7 to 8.
Effectively solutions of free base porphyrin 7 are red-purple
while solutions of Zn(II)-porphyrin 8 are pink-purple.
The preparation of a second clickable Zn(II)-porphyrin
building block having a slightly longer spacer between the
porphyrin core and the terminal alkyne function was also
achieved (Scheme 2). Aldehyde 11 was prepared from 3 and
ethyl bromoacetate according to
a
previously reported
method.[10] A3B porphyrin 12 was then obtained in 14% yield by
reaction of 11 (1 equiv.) and 5 (3 equiv.) with pyrrole (6, 4
equiv.) under typical Lindsey conditions. Subsequent treatment
with Zn(OAc)2 afforded the corresponding Zn(II)-porphyrin (13)
in 99% yield.
CHO
N
N
N
N
(a)
M
O
CO2Et
SiMe3
CHO
O
SiMe3
SiMe3
CO2Et
12 M = H2
13 M = Zn
(a)
11
3
HO
(b)
O
(b)
OH
OTs
1
2
CHO
4
R
N
N
N
(c)
Zn
O
CO2
N
nBu4N
CHO
14
7 R = SiMe3, M = H2
O
5
(d)
8 R = SiMe3, M = Zn
N
H
6
(e)
N
N
N
N
(c)
M
H
9 R = H, M = Zn
HO
(f)
H
N
15
N
O
O
Zn
O
10 R = H, M = H2
(d)
N
N
16
Scheme 1. Reagents and conditions: (a) p-TsCl, pyr, CH2Cl2, rt (77%); (b)
K2CO3, DMF, 80°C (85%); (c) BF3.Et2O, CHCl3, rt, then chloranil (15%); (d)
Zn(OAc)2.2H2O, CHCl3/MeOH,  (83%); (e) TBAF, THF, 0°C (91%); (f) TFA,
CH2Cl2, rt (96%).
Scheme 2. Reagents and conditions: (a) 5, 6, BF3.Et2O, CHCl3, rt, then
chloranil (14%); (b) Zn(OAc)2.2H2O, CHCl3/MeOH,  (99%); (c) K2CO3, H2O,
THF,  then nBu4NOH (85%); (d) DCC, DMAP, HOBt, CH2Cl2, 0 to 25°C
(95%).
Reaction of trimethylsilyl (TMS)-protected alkyne 8 with tetra-n-
butylammonium fluoride (TBAF) in THF afforded Zn(II)-porphyrin
1
9 bearing a terminal alkyne function. Inspection of the H NMR
spectrum of 9 indicates the disappearance of the diagnostic
TMS signal observed at  = 0.20 ppm for 8 and the concomitant
apparition of a triplet (J = 3 Hz) at  = 2.04 ppm corresponding to
the resonance of the terminal alkyne proton. Moreover, in the IR
spectrum of 9, the C-H stretching band characteristic of the
terminal alkyne function is observed at 3302 cm-1. Finally, Zn(II)-
Hydrolysis of the ester function of 13 was achieved by treatment
with K2CO3 in
carboxylate is poorly soluble in CH2Cl2 as its potassium salt. In
order to obtain good solubility in organic solvents,
a mixture THF and water. The resulting
a
tetrabutylammonium hydroxide was added to the reaction
mixture and carboxylate 14 was conveniently extracted with
CH2Cl2 as its tetrabutylammonium salt. Finally, reaction of 14
with alcohol 15 under esterification conditions using N,N’-
porphyrin
9
was also demetallated by treatment with
trifluoroacetic acid (TFA) to afford the corresponding free-base
porphyrin (10) that was used as a reference compound for the
photophysical studies.
dicyclohexylcarbodiimide (DCC),
4-(dimethylamino)pyridine
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(DMAP) and 1-hydroxybenzotriazole (HOBt) afforded clickable
Zn(II)-porphyrin 16 in 95% yield.
reaction by simple thin layer chromatography (TLC) analysis.
Indeed, IR spectroscopy was a precious complementary tool to
indicate the total consumption of the azide residues by
monitoring the disappearance of the diagnostic azide band at
2089 cm-1. In this particular case, completion of the reaction was
clearly observed after 48 h. After work-up and purification,
compound 18 was obtained in 96% yield. The ten peripheral
Zn(II)-porphyrin moieties were then demetallated by treatment of
18 with TFA to afford 19.
The grafting of the Zn(II)-porphyrin building blocks (9 and
16) onto the pillar[5]arene scaffold was achieved under CuAAC
conditions (Scheme 3). Pillar[5]arene building block 17 bearing
ten peripheral azide functions was prepared according to a
previously reported procedure.[7a] The reaction of 17 with
terminal alkyne 9 was achieved under the typical CuAAC
conditions developed for the functionalization of multi-azide
cores[11-12] (CuSO4⋅ 5H2O, sodium ascorbate, CH2Cl2/H2O). It
can be noted that it was difficult to follow the progress of the
R
R
R
N
R
R
N
N
N
N
N
N
N
N
N3
N
N
N
N
N3
N3
N
O
N3
N
N3
O
O
O
O
O
O
O
O
O
(a)
O
O
O
O
O
O
O
O
O
O
N3
N
N
N3
N
N
N3
N
N
N
N
N3
N
N
17
N
N3
N
N
R
N
R
N
R
R
R
18 / 20
N
N
N
N
N
N
N
N
O
R
R
=
18
20
=
Zn
Zn
O
O
O
(b)
(b)
NH
N
NH
N
N
N
O
R
R
=
19
21
=
O
O
O
HN
HN
Scheme 3. Preparation of porphyrin-substituted pillar[5]arene derivatives. Reagents and conditions: (a) 9 or 16, CuSO4⋅ 5H2O, sodium ascorbate, CH2Cl2, H2O,
25°C [from 9: 18 (96%); from 16: 20 (66%)]; (b) TFA, CH2Cl2, rt [from 18: 19 (92%); from 20: 21 (97%)].
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The reaction conditions used for the
preparation of 18 were then applied to the
second clickable Zn(II)-porphyrin derivative
(16). A mixture of 17 (1 equiv.), 16 (12 equiv.),
CuSO4⋅ 5H2O (0.4 equiv.), sodium ascorbate
(0.3 equiv.) in CH2Cl2/H2O was vigorously
stirred at room temperature for 6 days. After
work-up
and
purification
by
column
chromatography on SiO2 followed by gel-
permeation chromatography (Biobeads SX-1,
CH2Cl2), pillar[5]arene derivative 20 was
obtained in 66% yield. Finally, treatment of 20
with TFA afforded the corresponding free base
prophyrin-substituted pillar[5]arene derivative
(21) in 97% yield.
Characterization
of
the
porphyrin-
substituted pillar[5]arene derivatives.
Compounds 18-21 were characterized by 1H
and 13C NMR, IR and UV/vis spectroscopies.
For all the compounds, the proposed
structures were also confirmed by MALDI-TOF
mass spectrometry. For both free-base
porphyrin derivatives (19 and 21), the 1H NMR
spectra recorded at room temperature
revealed a single set of signals for all the
peripheral porphyrin moieties thus showing
that they are all equivalent. This is in perfect
agreement with the 5-fold symmetrical
structures of compounds 19 and 21. In
addition to the signals corresponding to the 10
equivalent
peripheral
moieties,
the
resonances arising from the pillar[5]arene core
are clearly observed for both 19 and 21.
Actually, the ten aromatic protons of the
macrocyclic core are equivalent and give rise
to a singlet at  = 6.81 for 19 and 6.69 ppm for
21. Similarly, a singlet is observed for the
methylene bridging moieties of the pillar[5]arene core ( = 3.45
and 3.34 ppm for 19 and 21, respectively). Whereas well
Figure 1. 1H NMR spectra (400 MHz, CDCl2CDCl2) of compounds 18 (A) and
19 (B) recorded at different temperatures (: CH2Cl2).
1
resolved signals are observed in the H NMR spectrum of the
free base porphyrin derivatives (19 and 21) recorded at room
temperature, the spectra of the corresponding Zn(II)-porphyrin
analogues (18 and 20) are broad under the same conditions.
This is shown in Figure 1A for compound 18. Variable NMR
studies revealed however a perfectly reversible narrowing of the
signals by increasing the temperature. Indeed, the 1H NMR
spectra of 18 and 20 recorded at high temperature show the
expected set of signals (Figures 1A and S13b). These
In this particular case, limited molecular motions of the
peripheral groups at low temperature results in
a slight
broadening of the signals corresponding to the protons located
close to the core moiety of the molecule. At higher temperature,
the increased thermal agitation of the peripheral substituents
contributes to sharpen these signals (Ha-b and H12, see Figure
1B). Moreover, agitation also contributes to simplify the pattern
observed for the signals of some specific protons. For example,
the two CH2 groups of the ethylene linker are both diastereotopic
(H-13/13’ and H-14/14’, see Figure 1) and accordingly give rise
to four sets of signals at 30°C. In contrast, only two signals are
observed at 105°C for the same protons (H-13 and H-14, see
Figure 1B) and both methylene subunits appear as enantiotopic
observations suggest
a fast dynamic exchange between
different conformers on the NMR timescale at high temperatures.
Importantly the dynamic motions at the origin of the broadening
of the spectra of 18 and 20 at room temperature are specific to
the presence of the Zn(II) centers. Effectively, as shown in
Figure 1B, the 1H NMR spectra of free-base porphyrin 19
reveals only minor changes as a function of the temperature.
under
these
conditions.
As
already
observed
for
glycopillar[5]arenes,[5a] the thermal agitation of the peripheral
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substituents prevents the transfer of the chiral information
between the peripheral subunits and the central core.
coordination of Zn(II) centers with 1,2,3-triazole groups occurs.
1H NMR spectra of 18 were also recorded a low temperature
(from room temperature to -70°C). A continuous broadening was
observed thus showing that the coordination/decoordination
process remains faster than the NMR timescale even at -70°C
and/or that compound 18 does not adopt a preferential folded
conformation at -70°C. A similar behavior has been also
observed for compound 20 with longer spacer subunits. In both
cases, this interpretation was further supported by the self-
diffusion coefficients (D) of compounds 18-21 deduced from
DOSY experiments (see Table S1). Comparison of the free-base
porphyrin derivatives with their Zn(II) coordinated analogues
revealed an increase in their D value (13(±5)% for 19 and
9(±6)% for 21) corresponding to a respective volume reduction
of 32(±12)% (18 vs. 19) and 22(±15)% (20 vs. 21). These
observations showing a reduced average volume for the Zn(II)-
porphyrin derivatives is fully consistent with a dynamic intra-
molecular coordination process leading to a folded conformation.
The occurrence of intramolecular interactions of 1,2,3-
triazole groups with neighboring Zn(II)-porphyrin subunits in 18
and 20 was also supported by their absorption spectra recorded
in CH2Cl2 at room temperature (Figures 3, S11g and S13g).
In the case of 18 and 20, the dramatic changes observed in the
1H NMR spectra as a function of the temperature cannot be
simply explained by an increased thermal agitation of the
peripheral substituents. The Zn(II) centers play obviously a key
role in the dynamic motions contributing to the broadening of the
spectra at low temperature. Actually, the presence of 1,2,3-
triazole moieties in 18 or 20 could result in intramolecular
interactions with neighboring Zn(II)-porphyrin subunits as
schematically depicted in Figure 2. Aromatic compounds with a
donating nitrogen moiety are well known ligands of Zn(II)-
porphyrins.[13-15] Pyridine and imidazole derivatives are typical
examples. Even if 1,2,3-triazoles are weaker ligands owing to
their lower basicity, their binding to Zn(II)-porphyrins has been
already documented in the literature.[14]
Ar
N
Ar
N
Zn
N
Ar
O
N
Ar
N
N
N
Zn
N
O
N
R
N
N
N
N
Ar
Ar
N
R
O
N
R
N
N
O
R
N
N N
N
N
N
O
N
O
N
N
O
R
R
N
N
N
N
N
O
N
O
R
O
N
N
O
R
N
N
O
N
O
N
N
O
N
O
N
N
N
O
O
R
R
N
N
N
N
R
N
N
N
O
R
O
N N
N
N
O
N
N
N
O
N
N
N
O
N
R
R
R
N
R
N N
N
N
N
N
R
R
Figure 2. Schematic representation of the dynamic conformational changes
resulting from the intramolecular coordination of a 1,2,3-triazole moiety to a
Zn(II)-porphyrin subunit of compound 18.
The binding constant for the intramolecular equilibrium between
coordinated and uncoordinated Zn(II)-porphyrins in 18 is
temperature dependent. At high temperature, the dynamic
equilibrium is displaced in favor of uncomplexed species and the
conformation of 18 must be similar to that of its analogous free-
1
base analogue (19). Indeed, as shown in Figure 1, the H NMR
spectra recorded at 105°C for 18 and 19 are very similar. In
contrast, at low temperature, association is favored thus
generating important conformational changes in 18. The
coordination of the Zn(II)-porphyrin moieties implies a folding of
the molecule and aromatic porphyrin rings are located closer to
the core moiety of the molecule. This view is clearly supported
by the dramatic shielding observed for the signals of some
protons by decreasing the temperature (H-a, H-b and H-12, see
Figure 1A). Indeed, these chemical shift changes result from the
ring current effect of the aromatic porphyrin moieties that are
spatially close to H-a, H-b or H-12 when intramolecular
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Figure 3. (A) Absorption spectra of compounds 18 (black) and 9 (red)
recorded in CH2Cl2 at 25°C. (B) Absorption spectra of compounds 18 recorded
in different solvents at 25°C (blue: CHCl3, black: CH2Cl2, red: toluene).
These observations further support our interpretation of the
UV/vis data of the Zn(II) analogues 18 and 20. The splitting of
the Soret band and the overall broadening of the absorption
spectra observed for 18 and 20 are not related to electronic
interactions between their peripheral porphyrin subunits but
result from an intramolecular coordination-driven conformational
folding of the molecules. Indeed, the molecular motions resulting
from the coordination/decoordination phenomena observed for
compounds 18 and 20 mimic the blooming of a flower. As
deduced from the variable temperature NMR studies, the folded
conformation is favored at low temperature. By increasing the
temperature, the intramolecular coordination is disfavored and
the molecules adopt an open conformation like a flower that
bloom.
In both cases, the absorption bands of the Zn(II)-porphyrin
moieties are effectively broadened and red-shifted when
compared to those of appropriate model compounds. This is a
clear signature[13-15] for the apical coordination of Zn(II)-porphyrin
moieties in 18 and 20 by a nitrogen ligand. Moreover, no
evidence of molecular aggregation (e.g. intermolecular
coordination) could be detected within the range of
concentrations used for the optical measurements (10-5 to 10-7
M) as the absorbance follows the Beer-Lambert law for both 18
and 20. Therefore, the apical coordination of the Zn(II)-porphyrin
moieties by 1,2,3-triazoles in 18 and 20 occurs in an
intramolecular manner. It can be also noted that the shape of the
absorption spectra of both 18 and 20 is significantly affected by
the nature of the solvent as shown in Figure 3B for
dichloromethane, chloroform and toluene. Close inspection of
the shape of the Soret band suggests a higher degree of
association in the less polar solvent (toluene). In this solvent, the
red-shifted absorption maxima of the Soret band that is typical
for coordinated Zn(II)-porpyrins becomes effectively more
pronounced. This observation is consistent with the solvent-
dependence of the binding constant (KA) reported in the
literature for complexes resulting from the intermolecular
association of N-ligands and Zn(II)-porphyrin derivatives.[14] The
KA values are in general higher in less polar solvents.
Blooming of the molecular flowers with a chemical stimulus
(input). In order to gain more control on the conformational
changes observed for 18 and 20, we became interested in using
an external input to trigger the blooming of molecular flowers 18
and 20. For this purpose, 1-phenylimidazole (22) was selected
as an external chemical input. Indeed, compound 22 is a
stronger ligand for Zn(II)-porphyrins[13-15] when compared to
1,2,3-triazoles and one may anticipate that a preferential
coordination of the Zn(II)-porphyrin groups in 18 or 20 will occur
in the presence of an excess of 22. As shown in Figure 5,
dramatic changes were evidenced in the UV/vis spectrum of 20
upon addition of an excess of imidazole derivative 22 (14 equiv.).
The absorption spectra of the free-base porphyrin-
substituted pillar[5]arene derivatives 19 and 21 are depicted in
Figures 4. In contrast to what was observed for their Zn(II)
analogues, the absorption spectra of 19 and 21 are only very
slightly broadened when compared to model free-base porphyrin
10. Indeed, their absorption spectra corresponds well to the sum
of their constitutive subunits (10 x 10) thus showing rather
limited electronic interactions (if any) among the free-base
porphyrin moieties in 19 and 21.
Figure 5. (A) Schematic representation of the blooming of molecular flower 20
upon addition of 1-phenylimidazole (22). (B) Absorption spectra of compound
20 before (red) and after (black) addition of 14 equiv. of 22 (CH2Cl2, 25°C, [20]
of the absorption spectra of pillar[5]arene 20
Figure 4. Absorption spectra of compounds 19 (blue), 21 (black) and 10 (red)
recorded in CH2Cl2 at 25°C.
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(black) and model porphyrin 16 (blue) in the presence of 14 equiv. of 22
subunits of 18 is strong enough to prevent the intramolecular
coordination of the 1,2,3-triazole moieties in the multiporphyrin
array. As a result, addition of an excess of 22 promotes the
blooming of molecular flower 18. A similar behavior was also
evidenced for derivative 20 (Figure S16b). In both cases, the
blooming event was also supported by DOSY experiments.
Effectively, significant changes in the D values were observed
upon addition of ligand 22 to CD2Cl2 solutions of 18 and 20 (see
Table S2). In the presence of an excess of ligand 22, the D
values were decreased by 13.0(±5) (18) and 14.0(±6)% (20).
These changes in D values correspond to a coordination-
induced volume expansion of 46(±12) (18) and 49(±15)% (20).
Interestingly, the D values deduced for 18 and 20 in the
presence of an excess of 22 (2.2(±0.1) and 2.1(±0.1) x 10-10
m2.s–1 respectively) are very similar to those recorded for the
corresponding free-base porphyrin derivatives 19 and 21
(2.2(±0.1) and 2.2(±0.1) x 10-10 m2.s–1, respectively) thus
providing good evidence for similar unfolded structures.
(CH2Cl2, 25°C, [20
16
The Soret band is red-shifted and becomes narrow. A similar
effect is also observed for the Q bands. These are diagnostic
signatures for an effective apical coordination of 22 onto the
Zn(II)-porphyrin subunits of 20. Importantly, the absorption
spectrum of 20 upon addition of 22 corresponds well to the one
recorded for model porphyrin 16 in the presence of imidazole 22
(Figure 5B, inset) thus suggesting that the intramolecular
coordination of Zn(II)-porphyrin moieties by 1,2,3-triazole
subunits is not effective anymore. Coordination of the Zn(II)-
porphyrins of compound 20 by imidazole ligand 22 is thus
capable of preventing the coordination-induced folding in the
multi-Zn(II)-porphyrin array. As schematically shown in Figure
5A, addition of ligand 22 to 20 allows for a complete unfolding of
the compound thus mimicking the blooming of a flower.
In order to further evidence this blooming event upon addition of
imidazole 22, 1H NMR binding studies were also carried out. For
both 18 and 20, dramatic changes were observed in the 1H NMR
spectra recorded after addition of imidazole 22. This is shown in
Figure 6 for compound 18.
Electrochemical properties. The electrochemical properties of
pillar[5]arenes 18-21 were determined by cyclic voltammetry
(CV) and Osteryoung Square Wave Voltammetry (OSWV).[16]
For the sake of comparison, electrochemical measurements
have been also carried out with model Zn(II)-porphyrin
derivatives 9 and 16. All the experiments were performed at
room temperature in CH2Cl2 solutions containing tetra n-
butylammonium tetrafluoroborate (0.10 M) as supporting
electrolyte and ferrocene (Fc) as internal reference, with a Pt
wire as the working electrode and a saturated calomel electrode
(SCE) as a reference. Potential data for all of the compounds
are collected in Table 1.
Table 1. Electrochemical data of compounds 9, 16 and 18-21 determined by
OSWV on a Pt working electrode in CH2Cl2 + 0.1M n-Bu4NBF4 at room
temperature. Ferrocene is used as internal reference.[a,b]
ox
ox
ox
Red
Red
Red
Red
E3
E2
E1
E1
E2
E3
E4
19
21
9
+1.66
+1.70
+1.18
+1.18
+0.92
+0.89
+1.06
+1.06
+0.85
+0.85
+0.73
+0.72
-1.23
-1.24
-1.40
-1.41
-1.41
-1.43
-1.60
-1.60
-1.61
-1.83
-1.59
-1.54
-
-1.82
16
18
20
+1.29
+1.24
1.88
-1.65
-1.94
Figure 6. Aromatic region of the 1H NMR spectra of pillar[5]arene 18 (400
MHz, CD2Cl2, 25°C) recorded before (A) and after addition of 5 (B), and 45 (C)
equivalents of imidazole 22 (for the numbering, see Fig. 1).
OSWVs were obtained using a sweep width of 20 mV, a frequency of 20 Hz,
and a step potential of 5 mV. [b] Potential values in oxidation (Eox) and in
reduction (Ered) in Volt vs. SCE (Fc+/Fc is observed at 0.55 V ± 0.01 V vs.
SCE).
The broad signals of the pillar[5]arene derivative are sharpened
Free-base porphyrin-substituted pillar[5]arenes 19 and 21
display the diagnostic electrochemical signature of H2-porphyrin
derivatives.[17] The first one-electron transfer is reversible both in
oxidation and in reduction, while the second one is at most only
partially reversible as deduced from their CVs (see ESI). The
by adding increasing amount of ligand 22. This clearly shows
that the dynamic folding at the origin of the broadening of the H
NMR spectrum of 18 is progressively prevented by coordination
of imidazole ligands 22 to the Zn(II) centers of 18. In other words,
the intermolecular coordination of 22 to the Zn(II)-porphyrin
1
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redox potentials are similar for both compounds thus indicating
that the length of the spacer has no significant influence on their
waves of both coordinated and uncoordinated Zn(II)-porphyrins
are simultaneously observed. Saturation is reached upon
addition of 2 equiv. and only the electrochemical signature of
[(16)(22)] is then detected. When compared to 16, the first
oxidation wave is cathodically shifted in [(16)(22)] while the
second one is anodically shifted. This indicates that the radical
cation of the Zn-porphyrin is stabilized upon complexation by 22.
In contrast, while the potential of the first reduction wave is
significantly affected by the presence of 22, the second
reduction is always observed at the same redox potential. This
latter observation suggests a decoordination of 22 upon the first
reduction.
electrochemical
properties.
Moreover
the
observed
electrochemical processes are due to the simultaneous
oxidation or reduction of ten independent free-porphyrin units in
a similar environment. As reported for dendrimers bearing
multiple peripheral redox centers, this indicates that electrostatic
interactions between porphyrinic cations or anions are shielded
by the supporting electrolyte.[18]
The typical electrochemical features of Zn(II)-porphyrin systems
are clearly observed for model compounds 9 and 16. When
compared to 19 and 21, all the redox processes are cathodically
shifted. In contrast, the electrochemical behavior of multi-Zn(II)-
porphyrin arrays 18 and 20 is significantly different (see ESI). As
a typical example, the CVs recorded for pillar[5]arene 20 and the
corresponding model compound (16) are depicted in Figure 7.
Figure 7. Cyclic voltammograms of compounds 16 (pink) and 20 (purple) on a Pt
electrode in CH2Cl2 + 0.1 M [nBu4N][BF4] at a scan rate of 200 mV s-1 (toward anodic
potentials).
Three characteristic oxidation waves are clearly detected for
compounds 18 and 20 in OSWVs and CVs. The first one (E1ox) is
cathodically shifted when compared to model Zn(II)-porphyrins 9
and 16 (E1ox), while the second one observed around 0.90 V
(E2ox) is rather close to the oxidation potentials of the model
compounds. The third one (E3ox) conversely is slightly anodically
shifted. Similarly, the reduction of compounds 18 and 20 also
involves an additional process when compared to the
corresponding model Zn(II)-porphyrins (see ESI). In light of the
NMR and UV-Vis spectroscopic data, it is believed that the origin
of the supplementary oxidation and reduction processes
observed for 18 and 20 must be related to the existence of
intramolecular 1,2,3-triazole-porphyrin interactions in these
compounds. Indeed, these interacting Zn(II)-porphyrin units are
more electron-rich and should be at the same time easier to
oxidize and more difficult to reduce. In order to confirm this
hypothesis, further electrochemical investigations were carried
out in the presence of imidazole ligand 22. The OSWVs
recorded for model compound 16 in the presence of 22 are
shown in Figure 8A (anodic scan, for the cathodic scan, see the
ESI). Upon addition of 1 equiv. of imidazole 22, the oxidation
Figure 8. (A) OSWVs (anodic scan) of compound 16 before (pink) and after
successive additions of 22 [1 (purple) and 2 (green) equiv.] and (B) compound
18 before (purple) and after successive additions of 22 [5 equiv. (green) and
10 equiv. (blue)] on a Pt electrode in CH2Cl2 + 0.1 M [nBu4N][BF4] at room
temperature (frequency 20 Hz, amplitude 20 mV, step potential 5 mV).
All these observations nicely confirm that the peculiar
electrochemical behavior observed for the multiporphyrinic
arrays 18 and 20 results effectively from the partial
intramolecular coordination of Zn(II)-porphyrin moieties with
1,2,3-triazoles. In both cases, the relative intensity of the two
first oxidation waves shows that at least 5 out of the 10
porphyrins are complexed by a triazole unit under these
conditions (CH2Cl2, room temperature), but not all of them. We
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10.1002/chem.201701622
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also noticed that for both 18 and 20, the ratio between the two
first oxidation peaks is unchanged while switching the scan rate
between 0.1 and 1 Vs-1. This indicates that the dynamic
Experimental Section
Synthesis.
interconversion
process
between
coordinated
and
uncoordinated porphyrins does not occur within the voltammetric
timescale.[19] Moreover, as decoordination seems to occur upon
the first reduction, an electrochemical input appears thus
capable of triggering the blooming of molecular flowers 18 and
20. This hypothesis is further reinforced by the only partial
reversibility of the first reduction wave of 18 and 20 in agreement
with a square scheme mechanism.[20]
Finally, CVs and OSWVs of compound 18 and 20 were also
recorded in the presence of imidazole 22. As a typical example,
the OSWVs recorded for 18 upon addition of 22 are depicted in
Figure 8B. The addition of ligand 22 to polytopic receptor 18 is
responsible for a simplification of the voltammogram and only
the typical electrochemical signature of coordinated Zn(II)-
porphyrin subunits is observed. The observed electrochemical
processes are now arising from the simultaneous oxidation of
the ten independent peripheral porphyrin moieties that are all in
a similar state. We also notice that only 5 equiv. are necessary
to switch ensure a total coordination of the ten porphyrin units, in
agreement with the CVs of 18 and 20 without 22.
The preparation and characterization (Figures S1-14) of all the new
compounds are described in the ESI.
Electrochemistry.
The electrochemical properties of compounds 9, 16, 18-21 were
determined by cyclic voltammetry (CV) and Osteryoung Square Wave
Voltammetry (OSWV) (See experimental selected curves in SI).The
cyclic voltammetric measurements were carried out with a potentiostat
Autolab PGSTAT100. Experiments were performed at room temperature
in a homemade airtight three-electrode cell connected to a vacuum/argon
line. The reference electrode consisted of a saturated calomel electrode
(SCE) separated from the solution by a bridge compartment. The counter
electrode was a platinum wire of ca. 1 cm² apparent surface. The working
electrode was a Pt microdisk (0.5 mm diameter). The supporting
electrolyte [nBu4N][BF4] (Fluka, 99% electrochemical grade) was used as
received and simply degassed under argon. Dichloromethane was
freshly distilled over CaH2 prior to use. The solutions used during the
electrochemical studies were typically 2 × 10-4 or 3 × 10-3 M in compound
and 0.1 M in supporting electrolyte. Before each measurement, the
solutions were degassed by bubbling Ar and the working electrode was
polished with
a polishing machine (Presi P230). Under these
experimental conditions, Fc+/Fc is observed at + 0.55 ± 0.01 V vs. SCE.
OSWVs were obtained using an amplitude of 20 mV, a frequency of 20
Hz, and a step potential of 5 mV. For addition experiments on 16 and 18,
a CH2Cl2 solution of 0.5 mL of compound 22 was prepared under argon,
and the adjusted volume was syringed into the stirred electrochemical
mixture under argon atmosphere.
Conclusions
Pillar[5]arene derivatives bearing peripheral porphyrin subunits
have been efficiently prepared from a clickable pillar[5]arene
building block (17) and Zn(II)-porphyrin derivatives bearing a
terminal alkyne function (9 and 16). The characterization of the
resulting deca-Zn(II)-porphyrin arrays (18 and 20) has been
complicated owing to an intramolecular complexation of the
peripheral Zn(II)-porphyrin moieties by 1,2,3-triazole subunits.
As a result of this intramolecular coordination, the 1H NMR
spectra of 18 and 20 are broad at room temperature. Variable
temperature NMR studies have shown that the conformational
equilibrium resulting from the coordination-driven molecular
folding is dynamic and well-resolved 1H NMR spectra have been
recorded at high temperature. Actually, the thermodynamic
equilibrium is displaced in favor of uncoordinated species at high
temperature and molecules 18 and 20 adopt an open
conformation similar to the one of their free-base porphyrin
analogues (19 and 21) under these conditions. In contrast,
coordination of the Zn(II)-porphyrin moieties by 1,2,3-triazole
groups is favored at low temperature and thus the molecules
adopt a folded conformation. Finally, we have shown that the
coordination-driven folding of 18 and 20 can be controlled by an
external chemical stimulus. Specifically, addition of an imidazole
derivative (22) to solution of 18 or 20 prevents the intramolecular
coordination at the origin of the folding. As a result, unfolding of
molecules 18 and 20 occurs. This view is further supported by a
variation of the diffusion coefficient as measured by DOSY and
by the electrochemical signature. The resulting molecular
motions triggered by the addition of the imidazole ligand mimics
the blooming of a flower.
Acknowledgements
Financial support by the International Center for Frontier Research in
Chemistry, the LabEx “Chimie des Systèmes Complexes”, the CNRS and
the University of Strasbourg is gratefully acknowledged. TMNT thanks
the University of Strasbourg (IdEx 2013) and HBA the Tunisian Ministry
of Higher Education and Scientific Research and Technology for their
fellowships. We thank Dr. A. Saquet (LCC) for electrochemical
measurements, Drs. M. Schmitt and E. Wasielewski (UMR 7509) for
high-field NMR measurements and Dr. J.-M. Strub (ECPM) for the mass
spectra. We further thank Drs. R. Caballero and M. Vartanian for the
initial preparation of compound 16.
Keywords: Pillararene • Porphyrin • CuAAC • Molecular
machine • Molecular folding
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Entry for the Table of Contents
FULL PAPER
Molecular flowers. Deca-Zn(II)-
porphyrin arrays constructed on a
pillar[5]arene scaffold adopt a folded
conformation due to intramolecular
complexation of the Zn(II)-porphyrin
moieties by 1,2,3-triazole subunits. An
external chemical stimulus can trigger
the unfolding of these compounds
thus mimicking the blooming of a
flower.
Thi Minh Nguyet Trinh, Iwona
Nierengarten, Haifa Ben Aziza, Eric
Meichsner, Michel Holler, Matthieu
Chessé, Rym Abidi, Christian Bijani,
Yannick Coppel, Emmanuel
Maisonhaute,* Béatrice Delavaux-Nicot,*
and Jean-François Nierengarten*
Page No. – Page No.
Coordination-driven folding in multi-
Zn(II)porphyrin arrays constructed on
a pillar[5]arene scaffold
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