Porphyrin-based systems containing polyaromatic fragments: Decoupling the synergistic effects in aromatic-porphyrin-fullerene systems

Article abstract of DOI:10.1039/d0ra07407a

In this work, we report a two-step synthesis that allows the introduction of four pyrene or corannulene fragments at the para position of meso-tetraarylporphyrins using a microwave-assisted quadruple Suzuki-Miyaura reaction. Placing the PAHs at this position, further from the porphyrin core, avoids the participation of the porphyrin core in binding with fullerenes. The fullerene hosting ability of the four new molecular receptors was investigated by NMR titrations and DFT studies. Despite having two potential binding sites, the pyrene derivatives did not associate with C60 or C70. In contrast, the tetracorannulene derivatives bound C60 and C70, although with modest binding constants. In these novel para-substituted systems, the porphyrin core acts as a simple linker that does not participate in the binding process, which allows the system to be considered as two independent molecular tweezers; i.e., the first binding event is not transmitted to the second binding site. This behavior can be considered a direct consequence of the decoupling of the porphyrin core from the binding event.

Full text of DOI:10.1039/d0ra07407a

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PAPER
Porphyrin-based systems containing polyaromatic
fragments: decoupling the synergistic effects in
aromatic-porphyrin-fullerene systems†
Cite this: RSC Adv., 2020, 10, 36164
Sergio Ferrero, Hector Barbero, Daniel Miguel,
Raul Garcıa-Rodrıguez
*
´
´
´
´
and Celedonio M. A´ lvarez
*
In this work, we report a two-step synthesis that allows the introduction of four pyrene or corannulene
fragments at the para position of meso-tetraarylporphyrins using a microwave-assisted quadruple
Suzuki–Miyaura reaction. Placing the PAHs at this position, further from the porphyrin core, avoids the
participation of the porphyrin core in binding with fullerenes. The fullerene hosting ability of the four
new molecular receptors was investigated by NMR titrations and DFT studies. Despite having two
potential binding sites, the pyrene derivatives did not associate with C60 or C70. In contrast, the
tetracorannulene derivatives bound C60 and C70, although with modest binding constants. In these novel
para-substituted systems, the porphyrin core acts as a simple linker that does not participate in the
binding process, which allows the system to be considered as two independent molecular tweezers; i.e.,
the first binding event is not transmitted to the second binding site. This behavior can be considered
a direct consequence of the decoupling of the porphyrin core from the binding event.
Received 24th July 2020
Accepted 22nd September 2020
DOI: 10.1039/d0ra07407a
rsc.li/rsc-advances
corannulene has played a central role in fullerene recognition
due to its excellent structural complementarity. However,
Introduction
8
The rational design and development of functional complex pristine corannulene does not associate with C60 strongly
9
architectures capable of interacting with nanometer-sized enough for the adduct to be observed in solution. Therefore,
1
carbon allotropes, especially fullerenes, in a supramolecular the incorporation of corannulenes in different supramolecular
2
10
fashion is a challenging current topic. However, the rational frameworks is critical to achieve interactions with fullerene.
design of efficient systems of this type is not always straight- Sygula's buckycatcher I, in which a cyclooctatetraene tether
forward, since it depends on a delicate balance of favorable and links two corannulene moieties, represented a breakthrough in
11
unfavorable interactions, and requires an understanding of the the area. This molecular pincer was able to establish strong
3
different subtle factors at play.
interactions with C60, sparking a search for other corannulene-
Among the fullerenes, buckminsterfullerene (C60), and to based supramolecular architectures able to interact with C60
.
a lesser extent, C70, have attracted the most attention. The One obvious way to improve the interaction of a host with C60 is
iconic C60 fullerene is constructed of 12 pentagonal rings and 20 the introduction of more corannulene units. However, the
hexagonal ones, which give rise to its unique hollow spherical seminal work by Sygula demonstrated that the introduction of
4
5
symmetry and electronic and magnetic proprieties. However, more corannulene units might not necessarily improve the
10a,12
as a result of its non-planar p-surface, engineering supramo- interaction with C60
.
This and subsequent studies high-
lecular interaction of C60 with typical planar benzenoid rings is lighted the importance of not only the preorganization of the
difficult. Therefore, in order to achieve efficient supramolecular molecular pincer, but also more subtle factors such as the
interactions with C60, a host must exhibit both effective pre- exibility of the scaffold and the interplay of different binding
1
3
organization and a wide complementary surface in order to events.
3a,3f,6
maximise the area of the convex–concave interaction.
We have turned our attention to porphyrin-based systems as
ideal scaffolds for the formation of corannulene-based recep-
tors for fullerenes. Porphyrins and fullerenes are known to
interact; however, the interactions between a single porphyrin
6
a,7
Among the curved polyaromatic hydrocarbons (PAHs),
GIR MIOMeT, IU CINQUIMA/Qu ´ı mica Inorg a´ nica, Facultad de Ciencias, Universidad core and fullerenes are usually too weak to be detected in
de Valladolid, E-47011 Valladolid, Spain. E-mail: celedonio.alvarez@uva.es; raul.
garcia.rodriguez@uva.es
14
solution. We have shown that the introduction of four cor-
annulene arms at the meta positions of a tetraaryl meso-
substituted porphyrin leads to a “picket fence” molecular clip
able to bind C60 (Fig. 1a). However, the formation of four
†
Electronic supplementary information (ESI) available: Methods and details,
synthetic procedures and spectra of compounds are provided in PDF format.
See DOI: 10.1039/d0ra07407a
15
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idea of preparing a double “classical”-tweezer-based supra-
molecular system with two potential binding sites (Fig. 1c).
Introduction of the PAHs at the more remote para position in
this new supramolecular scaffold should prevent cooperativity
between the porphyrin core and the PAH pincers, and therefore
provide a means to decouple the effect of porphyrin core in
fullerene hosting.
Results and discussion
The preparation of 2H-PTetraCor requires a porphyrin core that
allows the easy introduction of four corannulene units at the
para position of the tetraaryl meso-substituted porphyrin. We
reasoned that the formation of such novel tetra-substituted
systems could be accomplished via a tetra Suzuki reaction. In
order to explore the feasibility of this synthetic scheme, we
initially explored the formation of an analogous tetrapyrene-
based scaffold (2H-PTetraPyr). Pyrene (Pyr) is less synthetically
costly than corannulene and therefore offered a good starting
point to test and optimize the synthetic route towards this new
family of para-tetrasubstituted porphyrins. More importantly,
2H-PTetraPyr itself represents an additional framework-scaffold
for supramolecular studies on fullerene hosting.
A porphyrin with suitable reactive points in the para position
Fig. 1 Corannulene tetraaryl porphyrin-based systems. (a and b)
Previously reported “picket fence” and double “picket fence” systems is required for the construction of a more complex system via
resulting from the introduction of four or eight corannulene arms at Suzuki reactions. Two alternatives were evaluated, based on the
the meta positions. (c) Tetracorannulene-based double tweezer
system via substitution at the para position described in the present
involves the preparation of 2H-PTetraBpin with 1-bromopyrene
work. Note: the blue circle represents an idealized interaction of the
cross-coupling partners chosen for the Suzuki reaction. Route A
as the cross-coupling partner, while route B utilises 2H-PTe-
guest with the porphyrin core in the meta substituted systems.
traBr and 1-pinacol pyreneboronate (Pyr-Bpin) (Scheme 1). The
synthesis of both porphyrin precursors was easily achieved by
inseparable atropoisomers diminished the binding ability of
the system and resulted in a difficult to analyse system. None-
theless, this study highlighted the importance of the partici-
pation of the porphyrin core in the recognition process and
inspired us to conduct subsequent studies based on this meta-
substituted tetraaryl porphyrin scaffold.
In order to prevent the formation of atropisomers, we rst
16
introduced eight pyrene units (i.e., in all meta positions), and,
17
very recently eight corannulene units, at the meta positions of
the tetraaryl porphyrin. Fig. 1b shows the resulting octapodal
corannulene-based system, which features two binding sites as
a result of the 4 + 4 arrangement of the corannulene arms.
Although the compound was obtained in very low yield, this new
supramolecular platform displayed very high affinity for C60 due
to the involvement of the four corannulene units, and, impor-
tantly, the participation of the porphyrin core. However, the
active participation of the porphyrin core led to the expansion of
the second active site, which in turn led to its deactivation, as
indicated by DFT calculations. In other words, while benecial
in maximizing the interaction with the rst C60, the active
participation of the porphyrin core led to a deactivating
conformational change in the second binding site.
Scheme 1 Overview of the two synthetic routes evaluated for 2H-
PTetraPyr and Zn-PTetraPyr. Reagents and conditions: (a) propionic
ꢀ
acid, nitrobenzene, MW, 200 C; (b) Zn(OAc)
2 2 3
$2H O, CHCl , MW,
ꢀ
t
Herein, we explore the introduction of corannulene units at 120 C; (c) 1-bromopyrene, [PdCl (dppf)], BuONa, toluene, MW,
the para-position of a meso-tetraarylporphyrin with the simple 130 C; (d) 1-pinacol pyreneboronate (Pyr-Bpin), [PdCl
2
ꢀ
2
(dppf)],
t
ꢀ
3 2 3
BuONa, toluene, MW, 130 C; (e) CF CO H, CHCl , RT.
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Fig. 2 Optimized structure of compound 2H-PTetraCor. a angles are
ꢀ
ꢀ
close to 90 (a ¼ 90.85 ). Note: a is defined as the angle formed by the
corannulene-derived meso substituents with the centroid of the
porphyrin as the vertex (see later discussion and Fig. 4).
and HR-MS. The latter conrmed the formation of these
+
compounds and showed the expected [M] peak at m/z
1606.4994 (calcd 1606.4969) and 1668.4085 (calcd 1668.4104),
respectively, for 2H-PTetraCor and Zn-PTetraCor (see ESI
1
Scheme
2 Optimized synthetic route for the tetracorannulene
Fig. S62 and S64†). In both cases, the H NMR spectra in CDCl
3
porphyrin systems 2H-PTetraCor and Zn-PTetraCor. Reagents and
showed the equivalence of the four corannulene units and the
presence of only one b-pyrrole proton, as expected for the
effective D4h symmetry in solution. In the case of 2H-PTetraCor,
a characteristic upeld-shied broad signal was observed at
t
ꢀ
conditions: (a) [PdCl
CF CO H, CHCl , RT.
2
(dppf)], BuONa, toluene, MW, 130 C; (b)
3
2
3
ꢁ
2.52 ppm for the NH groups. Remarkably, 2H-PTetraCor and
a microwave-assisted method starting from commercially
available reagents. 2H-PTetraBr and 2H-PTetraBpin precipi-
tated out from the reaction mixture and were obtained in 36%
and 18% yield, respectively. Initial attempts to introduce four
pyrene substituents into the porphyrin framework using both
routes A and B led to a mixture of products that was complex to
analyse and separate, and included homocoupling by-products
and products with less than four Pyr substituents. However,
Zn-PTetraCor exhibited sharp signals over a range of concen-
trations. This is in contrast to 2H-Octa(II), for which extensive
broadening was observed due to intramolecular p–p interac-
tions between pairs of corannulene units (leading to the
potential formation of conformers), and thus suggests that
these interactions are not present in 2H-PTetraCor and Zn-
PTetraCor.
18
2H-PTetraCor was investigated using DFT calculations, and
2 2
metallation of the porphyrins with Zn(OAc) $2H O to give Zn-
Fig. 2 shows the most stable conformer (see ESI† for details). In
this structure, the four tetraaryl moieties are approximately
perpendicular to the planar porphyrin core, which is consistent
with previously reported X-ray structures for similar para-
substituted tetraaryl porphyrin systems. No p–p intra-
molecular interactions between corannulene moieties are
observed. This is in contrast to 2H-Octa(II), in which the cor-
annulene units could establish intramolecular interactions that
had to be overcome prior to the inclusion of C60 and that were
possible due to the bending of the porphyrin core, revealing its
relatively exible nature. In the case of 2H-PTetraCor, the two
corannulene moieties on each side of the porphyrin are preor-
ganized for C60 interaction and adopt an approximately
concave–concave conformation with two potential binding
sites, as shown in Fig. 2. The cle formed by the two cor-
annulene units is large enough (ca. 16 A) to accommodate C60
PTetraBr and Zn-PTetraBpin (step b in Scheme 1, which can be
easily accomplished in excellent yields, 90%, see Experimental
part) dramatically improved the introduction of the Pyr groups.
In this fashion, the pyrene-tetrasubstituted porphyrin Zn-PTe-
traPyr was easily accessible in remarkably good yield via route B
19
(80%; 66% via route A); this compound can be subsequently
demetallated in virtually quantitative yield to produce 2H-PTe-
traPyr, thus providing two pyrene-based scaffolds.
Having established and optimized the synthetic route for 2H-
PTetraPyr, we turned our attention to introducing four cor-
annulene units at the para positions of the porphyrin system
using a similar microwave-assisted methodology. A method
analogous to route B employing Cor-Bpin and Zn-PTetraBr was
therefore selected. Zn-PTetraCor was obtained via this
quadruple Suzuki reaction in 70% yield (Scheme 2). Quantita-
tive demetallation furnished 2H-PTetraCor, thus providing an
additional supramolecular scaffold. This yield is particularly
good within the context of such systems and bearing in mind
that four corannulene arms are introduced (cf. with 2H-Octa(II)
17
ꢀ
,
suggesting that only a minor deformation of the system would
be necessary to maximize its interaction with C60 (see later). It is
also clear from the optimized structure that as expected, the
porphyrin core is now far from the corannulene units, and
therefore would not be expected to participate in the
(Fig. 1b), which was obtained in 15%). The new tetracor-
annulene derivatives were characterized using UV-vis, NMR,
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recognition process. Our new system is therefore predicted to these para systems. In fact, in our previous work, meta-pyrene-
15,16
behave like a “classic molecular tweezer”, in which the substituted porphyrins
showed host–guest formation,
porphyrin is a simple linker, in contrast to our previous re- which was partially attributed to the participation of the
ported meta-substituted “picket fence” systems, in which the porphyrin core, resulting in a cooperative effect with the PAH
active participation of the porphyrin resulted in large binding units.
constants due to its synergetic effect (Fig. 1).
Moving to the tetracorannulene systems, Zn-PTetraCor and
In order to test these ideas and evaluate the decoupling of 2H-PTetraCor exhibited changes in their chemical shis upon
the porphyrin from the binding event, we next moved to study the addition of C60 or C70. This was indicative of supramolecular
whether the new para-tetrasubstituted porphyrins could serve association with the fullerene, which was now favoured by the
as supramolecular receptors for fullerenes, as suggested from presence of corannulene units instead of pyrene, since the
DFT (Fig. 2). The pyrene-based systems Zn-PTetraPyr and 2H- corannulene moieties are ideally placed for effective comple-
PTetraPyr did not show any evidence of interaction with C or mentary concave–convex p–p interactions with the C60 surface.
6
0
1
C
70. Indeed, no change in their H NMR chemical shis was
To further explore the supramolecular behaviour of these
observed in toluene-d
8
aer the addition of an excess of either new molecular receptors, titration experiments were carried out
8
of the fullerenes (10 equivalents). Although at rst glance, this between Zn-PTetraCor or 2H-PTetraCor and C60 in toluene-d .
result is not unexpected, since planar PAHs are known to not In both cases, the addition of aliquots of C60 resulted in a clear
1
exhibit supramolecular association with C60 due to their poor change in their H NMR chemical shis, and several signals
shape complementarity, it also suggests that the porphyrin core could be followed during the course of the titration, as shown in
is not involved in the supramolecular recognition process in Fig. 3. The obtained nonlinear binding isotherms in the titra-
tions of both receptors could be tted to a 1 : 1 model (see insets
2
in Fig. 3), yielding binding constants of K
1
¼ 2.73 ꢂ 0.02 ꢃ 10
1
and K
1
¼ 8.73 ꢂ 0.05 ꢃ 10 , respectively, for Zn-PTetraCor and
20
2
H-PTetraCor. The results of titrations with C70 paralleled
those obtained with C60, and Table 1 summarizes the estimated
binding constants obtained by tting to a 1 : 1 system (however,
the actual binding behaviour of these systems is believed to be
more complex; see later discussion and ESI† for details). While
both Zn-PTetraCor and 2H-PTetraCor exhibited association
with C70, the association constants were very similar to those
obtained for C60 (i.e., these para-tetraaryl-porphyrins did not
show discrimination between C70 or C60). Importantly, these
constants are substantially smaller than those obtained for our
2H-Tetra(I) or 2H-Octa(II), in which the porphyrin core actively
participated in the recognition process. In particular, they are
approximately two orders of magnitude smaller in the case of
1
binding to C (cf. 8.73 ꢂ 0.05 ꢃ 10 for 2H-PTetraCor vs. 5.4 ꢂ
6
0
3
0
2
.2 ꢃ 10 (averaged value from atropisomers) for 2H-tetra(I) and
4
15,17
.71 ꢂ 0.08 ꢃ 10 for 2H-Octa(II)).
The relative difference is
1
even greater for the association of C70 (cf. 5.24 ꢂ 0.02 ꢃ 10 for
5
17
2
H-PTetraCor vs. 2.13 ꢂ 0.10 ꢃ 10 for 2H-Octa(II)).
Three main points can be extracted from these studies. (a)
Although metal coordination is known to greatly affect the
conformation of the porphyrin ring, the binding constants for
2H-PTetraCor and Zn-PTetraCor are similar (Table 1), and,
more importantly, relatively small compared to those of our
previously reported meta-substituted porphyrin systems. These
facts suggest that the porphyrin core has little-to-no involve-
ment in the recognition process. (b) Connected to this, the lack
1
ꢁ1
Fig. 3 H NMR spectra (500 MHz, toluene-d
) showing the H , H
8 6 7
, and Table 1 Summary of apparent binding constants Ka (M ) of 2H-
H
(
10 chemical shifts (see ESI† for labelling scheme) of Zn-PTetraCor PTetraCor and Zn-PTetraCor for different fullerenes obtained by
top) and 2H-PTetraCor (bottom) upon the addition of aliquots of C60
fitting data from NMR titration in toluene-d at 298 K to a 1 : 1 model
Insets: plots of the changes in chemical shift against [G]/[H], where G
guest) is C60 and H (host) is Zn-PTetraCor or 2H-PTetraCor. The red
line corresponds to the nonlinear fitting of Dd for H to a 1 : 1 binding
.
8
(
C60
C70
6
ꢁ4
1
1
isotherm. [H] ¼ 10 M. The titration was carried out by adding known 2H-PTetraCor
8.73 ꢂ 0.05 ꢃ 10
2.73 ꢂ 0.02 ꢃ 10
5.24 ꢂ 0.02 ꢃ 10
2.16 ꢂ 0.01 ꢃ 10
ꢁ
3
2
2
portions of a stock solution of C60 or C70 ([G] ¼ 10 M). Note: the Zn-PTetraCor
dotted red line over the spectrum is a guide for the eye.
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ꢁ
1
of discrimination between C70 and C60 in the present PTetraCor between these energies (0.50 kcal mol ) indicates that 2H-PTe-
systems could also be traced, at least in part, to the lack of traCor interacts almost identically with both fullerenes, which is
participation of the porphyrin core in the association event. (c) in agreement with the experimentally observed lack of selectivity
Despite the presence of two potential binding sites in these between C60 and C70 (see Table 1). In order to form the inclusion
tetracorannulene systems (see Fig. 2), the titration experiments complexes, the two corannulene arms of one side of the molecule
indicate 1 : 1 association and provide little evidence of 1 : 2 must reorganize slightly via C–C bond rotation to align with the
adducts (i.e., both 2H-PTetraCor and Zn-PTetraCor apparently convex outer surface of the guest, and the fullerene must come
behave as single-tweezer molecular receptors for fullerenes).
into close enough proximity with the host to establish signicant
To obtain further insight into the details of the recognition attractive interactions in a tweezer-like fashion. A slightly more
process of these novel para-substituted molecular receptors, we open tweezer would be expected in the case of C70 to account for
carried out computational DFT studies at the B97D3/6-31+G(d,p) its eccentricity (Fig. 4c). However, little deformation of the system
21
level of theory. The optimized geometries of C @2H-PTetraCor is required to accommodate either C60 or C . More specically,
6
0
70
and C70@2H-PTetraCor are depicted in Fig. 4. The average the angle (a) formed by the corannulene-derived meso substitu-
distances between the inner concave surface of the corannulene ents with the centroid of the porphyrin as the vertex (see Fig. 4a)
ꢀ
19
fragments and outer convex surface of the fullerenes in the is 90.85 for 2H-PTetraCor. In order to interact with the
computed structures of the adducts C60@2H-PTetraCor and fullerene, the two corannulene units move toward one another in
ꢀ
C
70@2H-PTetraCor fell within the usual range for favourable a pincer fashion, resulting in a slight decrease in a (76.50 and
ꢀ
22
ꢀ
dispersion interactions (#3.5 A). The calculated interaction 80.41 for C @2H-PTetraCor and C @2H-PTetraCor, Fig. 4b
energies are 43.75 kcal mol and 44.25 kcal mol , respectively and c, respectively), whereas the angle a formed by the two cor-
6
0
70
ꢁ
1
ꢁ1
(
see ESI† for details of the calculations). The very small difference annulene units not engaged in supramolecular binding remains
ꢀ
ꢀ
almost unaltered (90.71 and 90.68 , respectively, for C60@2H-
PTetraCor and C70@2H-PTetraCor). That is, the rst binding
event (i.e., the small compression of one pincer) is not con-
formationally transmitted to the second binding site of the
molecule. The deformation energy, i.e., the energy required for
the host to adapt its conformation for fullerene binding, was very
ꢁ
1
accessible and was estimated to be 5.55 kcal mol
and
ꢁ1
3
.78 kcal mol for the C @2H-PTetraCor and C @2H-PTe-
6
0
70
traCor assemblies, respectively (see ESI† for further information).
We also studied the effect of the interaction with a second
2
fullerene and optimized the adducts (C60) @2H-PTetraCor and
70 2
(C ) @2H-PTetraCor, which were not observed experimentally, in
order to estimate the deformation energies for the second binding
event (see ESI† for details). The calculated values of
ꢁ
1
ꢁ1
9
.04 kcal mol and 7.41 kcal mol are only slightly larger than
those estimated for the association of one C60 or one C , respec-
70
tively. In other words, the association of the rst fullerene does not
substantially affect the second binding site, and they can be
considered as two approximately independent binding sites. The
small increase in deformation energy for the second event, along
1
with the very modest binding constants (see Table 1 for K , and
note that on purely statistical grounds, K would be 4 times lower
2
than K1 for two completely independent sites) could help to
rationalize the observed “apparent” 1 : 1 binding association by
NMR titrations despite the fact that the two binding sites act
23
approximately independently (i.e., not in an inhibitory fashion).
Along with the previous discussion, DFT calculations
revealed the effects of the lack of participation of the porphyrin
core in the binding to fullerene. Decoupling the inuence of the
porphyrin tether from the molecular pincer formed by the
corannulenes has profound implications for the current para-
tetrasubstituted porphyrin system. (1) The porphyrin core now
acts merely as a linker and is not involved in the recognition
process, resulting in lower binding energy and the loss of
cooperativity between the porphyrin and the polyaromatic
fragment. This is reected in the comparatively lower binding
constants observed experimentally. (2) A secondary effect
Fig. 4 (a) Depiction of the angle (a) between the two meso substit-
uents in the computed structures, where the green dot corresponds to
the centroid of the porphyrin core and acts as the vertex. Computa-
tionally modelled structures of C60@2H-PTetraCor (b) and C70@2H-
PTetraCor (c). Fullerenes are represented in spacefill style and color-
ized in purple and orange for C60 and C70, respectively.
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connected to this is that the porphyrin does not transmit the light and/or by immersion in anisaldehyde. NMR spectra were
information of the rst binding event to the second binding recorded on a 400 MHz MR Agilent or 500 MHz Agilent DD2
site, and the system can be considered to consist of two inde- instrument equipped with a OneNMR probe. NMR titrations
pendent pincers. This is in contrast to the large inhibitory effect were recorded on 500 MHz Agilent DD2 instruments equipped
observed for 2H-Octa(II), in which the active participation of the with a cold probe in the Laboratory of Instrumental Techniques
1
13
porphyrin in C60 or C70 binding, although benecial for (LTI) Research Facilities, University of Valladolid. H and
C
attaining high affinities towards C60 and C70, strongly inhibited NMR chemical shis (d) are reported in parts per million (ppm)
the binding of a second fullerene. In the 2H-Octa(II) system, the and are referenced to tetramethylsilane (TMS) using the solvent
involvement of the porphyrin resulted in the compression of residual peak as an internal reference. Coupling constants (J) are
one face of the porphyrin concomitant with the expansion of the reported in Hz. Standard abbreviations are used to indicate
second binding site (Fig. 1b). Finally, the lack of discrimination multiplicity: br ¼ broad, s ¼ singlet, d ¼ doublet, t ¼ triplet, m ¼
1
13
between C60 and C70 in the present systems most likely stems, at multiplet. H and C peak assignments were performed using
1
1
1
1
least in part, from the lack of participation of the porphyrin core 2D NMR methods ( H– H COSY, H– H DQFCOSY, band-
1
1
1
13
in the binding, thus revealing more subtle implications than selective H– H-ROESY, band-selective H– C HSQC, band
1
13
would be anticipated for the simple decoupling of the porphyrin selective H– C HMBC). Due to low solubility, some carbon
1
13
from the binding event.
signals were detected indirectly via H– C-HSQC/HMBC experi-
ments; in these cases, the abbreviation in is used. High-
resolution mass spectra were recorded at the mass spectrom-
etry service of the Laboratory of Instrumental Techniques of the
Conclusions
By using a synthetic strategy based on a quadruple Suzuki– University of Valladolid using a Bruker Autoex Speed MALDI-
Miyaura reaction, we were able to access meso-tetraaryl TOF system (N laser: 337 nm, pulse energy 100 mJ, 1 ns; accel-
substituted porphyrins in which four pyrene or corannulene eration voltage: 19 kV, reector positive mode). Trans-2-[3-(4-tert-
fragments were installed relatively distant from the porphyrin butylphenyl)-2-methyl-2-propenylidene]malonitrile (DCTB),
2
core in the para positions. Both NMR titration experiments and 1,8,9-anthra-cenetriol, and 1,8-dihydroxy-9(10H)-anthracenone
density functional theory (DFT) calculations indicate that the (dithranol) were used as matrixes. UV/vis spectra were recorded
porphyrin core is not involved in the binding event. While the using a Shimadzu UV-1603 with spectrophotometric grade
tetrapyrene-porphyrin-based systems do not host fullerenes solvents. UV/vis absorption spectral wavelengths (l) are reported
(
either C60 or C ), the corannulene-based ones 2H-PTetraCor in nanometers (nm), and molar absorption coefficients (3) are
70
ꢁ1
ꢁ1
and Zn-PTetraCor can act as “classical molecular tweezers” with reported in M
cm . Corannulene, 1-pinacol corannulene-
two approximately independent pincers, as supported by DFT boronate (Cor-Bpin) and 1-pinacol pyreneboronate (Pyr-Bpin)
16,17,25
studies. The fact that this system exhibits apparent 1 : 1 asso- were obtained following literature procedures.
ciation is attributed mainly to the very low binding constants
rather than a strong inhibitory effect, in contrast to our previ-
ously reported meta-substituted “picket fence” system. There-
fore, although detrimental in terms of fullerene binding ability,
this “decoupling” of the porphyrin core can also present bene-
2
H-PTetraBr
Pyrrole (278 mL, 4 mmol), p-bromobenzaldehyde (740 mg, 4
mmol), propionic acid (13 mL, 173 mmol), and nitrobenzene (7
mL, 68 mmol) were mixed in a sealed vessel specically
designed for microwave irradiation. The mixture was stirred
inside a microwave reactor at 200 C for 15 min. The resulting
dark crude was stored in a refrigerator for two days, and then

ts, such as the prevention of large structural changes that
inhibit the potential second binding event. These ndings
highlight the important role of the tether in the construction of
complex supramolecular systems, along with the interplay of
different binding events in the fullerene recognition process.
We hope these results will lead to a better understanding of the
complexity in porphyrin-based supramolecular systems and
help to pave the way for the rational design of new and func-
tional complex supramolecular platforms.
ꢀ

ltered using a B u¨ chner funnel and washed with MeOH. The
obtained solid was placed in an oven under reduced pressure
ꢀ
(190 C, 2 h, 50 mbar) to remove residual nitrobenzene. Finally,
the dark purple solid 2H-PTetraBr was obtained (339 mg, 36%
yield). The spectroscopic data agreed with those reported in the
literature.
H
18b
1
H NMR (500 MHz, chloroform-d): d 8.84 (s, 8H,
), 8.07 (d, J ¼ 8.3 Hz, 8H, H ), 7.91 (d, J ¼ 8.3 Hz, 8H, H ),
2.87 (br, 2H, H ). C{ H} NMR (101 MHz, chloroform-d):
2
6
7
1
3
1
Experimental
General methods
ꢁ
1
d 140.79 (C ), 135.78 (C ), 131.14 (C ), 129.95 (C ), 122.61 (C ),
5
6
2
7
8
+
118.95 (C ). HR-MS (MALDI-TOF): m/z for C H Br N [M]
4 44 26 4 4
All reagents were purchased from commercial sources and used
calcd: 929.8883, found: 929.8851 (3.2 ppm error). UV/vis
toluene): l 401 (3 ¼ 76 900), 421 (3 ¼ 384 900), 515 (3 ¼
0 100), 550 (3 ¼ 9300), 592 (3 ¼ 6300), 648 (3 ¼ 4200).
without further purication. Solvents were either used as
(
2
24
received or dried according to procedures described elsewhere.
Microwave reactions were carried out using an Anton Paar
Monowave 300 Reactor. Column chromatography was carried out
using silica gel 60 (particle size 0.040–0.063 mm; 230–400 mesh)
Zn-PTetraBr
as the stationary phase, and TLC was performed on precoated 2H-PTetraBr (114 mg, 0.12 mmol) and Zn(AcO)
2 2
$2H O (39 mg,
silica gel plates (0.25 mm thick, 60 F254) and visualized under UV 0.18 mmol) were mixed in a sealed vessel specically designed
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for microwave irradiation, and 5.5 mL of CHCl
The mixture was stirred inside the microwave reactor at 120 C
for 150 min. Subsequently, the crude was dissolved in 40 mL of
3
was added. Zn-PTetraPyr
ꢀ
Zn-PTetraBr (13 mg, 0.013 mmol), 1-pinacol pyreneboronate
(
2
Pyr-Bpin) (19 mg, 0.058 mmol), [PdCl (dppf)] (7.6 mg, 0.010
CHCl , placed in a separatory funnel, and washed with 20 mL
t
3
mmol), and BuONa (15 mg, 0.15 mmol) were placed in a sealed
vessel specically designed for microwave irradiation, which
was placed inside a two-necked round-bottom ask in order to
keep the system under an inert atmosphere. 1.4 mL of dry
toluene and 36 mL of pyridine were added. The mixture was
2
of H O. The organic layer was removed under low pressure to
give the purple solid Zn-PTetraBr (105 mg, 87% yield). The
spectroscopic data agreed with those reported in the liter-
1
8b
1
ature.
.07 (d, J ¼ 8.3 Hz, 8H, H
NMR (101 MHz, chloroform-d): d 149.99 (C ), 141.43 (C ),
2
H NMR (500 MHz, chloroform-d): d 8.94 (s, 8H, H ),
1
3
1
8
6
), 7.90 (d, J ¼ 8.3 Hz, 8H, H
7
). C{ H}
ꢀ
stirred inside the microwave reactor at 130 C for 60 min. Aer
cooling, the solvent was removed under vacuum, and the crude
3
5
1
35.66 (C ), 132.04 (C ), 129.80 (C ), 122.35 (C ), 119.93 (C ).
6 2 7 8 4
was puried by column chromatography on silica gel using
+
HR-MS (MALDI-TOF): m/z for C H Br N Zn [M] calcd:
4
4
24
4 4
3
hexane/AcOEt gradient elution (3 : 1–2 : 1–1 : 1) and CHCl to
9
4
5
93.7964, found: 993.7985 (2.1 ppm error). UV/vis (toluene): l
01 (3 ¼ 32 500), 426 (3 ¼ 162 600), 551 (3 ¼ 21 500), 591 (3 ¼
300).
1

nally give Zn-PTetraPyr as a purple solid (15 mg, 80% yield). H
NMR (500 MHz, chloroform-d): d 9.33 (s, 8H, H
2
), 8.70 (d, J ¼
9
4
H
.2 Hz, 4H, H10), 8.54 (d, J ¼ 7.7 Hz, 8H, H ), 8.44 (d, J ¼ 7.7 Hz,
6
H, H ), 8.42 (d, J ¼ 7.7 Hz, 4H, H ), 8.318.26 (m, 12H, H
+
11
1
7
18
+ H ), 8.24 (d, J ¼ 8.9 Hz, 4H, H ), 8.20 (d, J ¼ 8.9 Hz, 4H,
1
2
14
16
2H-PTetraBpin
13 1
H ), 8.14–8.08 (m, 12H, H + H ). C{ H} NMR (126 MHz,
1
5
7
13
Pyrrole (173 mL, 2.5 mmol), 4-formylphenylboronic acid-pinacol
ester (580 mg, 2.5 mmol), propionic acid (7 mL, 93 mmol), and
nitrobenzene (13 mL, 126 mmol) were mixed in a sealed vessel
specically designed for microwave irradiation. The mixture
chloroform-d): d 150.36 (C
3
), 142.12 (C
), 131.56 (C21), 131.43 (C
), 128.05 (C19), 127.98 (C18), 127.75
11), 127.52 (C15 + C16), 126.07 (C13), 125.46 (C10), 125.20 (C12 or
).
HR-MS (MALDI-TOF): m/z for C108H N Zn [M] calcd:
5
), 140.22 (C
8
), 137.61
), 131.08
(C
(C
(C
9
), 134.70 (C
20), 130.79 (C22), 128.76 (C
6
), 132.19 (C
2
q
7
ꢀ
was stirred inside a microwave reactor at 200 C for 15 min. The
C
14), 125.03 (C23), 124.93 (C12 or C14), 124.88 (C17), 120.90 (C
4
dark crude was stored in a refrigerator for two days, and then
+
60
4

ltered using a B u¨ chner funnel and washed with hexane. The
1476.4115, found: 1476.4116 (0.1 ppm error). UV/vis (toluene): l
345 (3 ¼ 72 400), 404 (3 ¼ 28 100), 430 (3 ¼ 379 300), 552 (3 ¼
17 600), 592 (3 ¼ 6400).
obtained solid was placed in an oven under reduced pressure
ꢀ
(
180 C, 2 h, 50 mbar) to remove residual nitrobenzene. Finally,
the dark purple solid 2H-PTetraBpin was obtained (127 mg,
1
8% yield). The spectroscopic data agreed with those reported
18a
1
in the literature.
H NMR (500 MHz, chloroform-d): d 8.82 (s,
), 8.18 (d, J ¼ 8.0 Hz, 8H, H ),
2
H-PTetraPyr
8
1
H, H
.50 (s, 48H, H10), ꢁ2.81 (br, 2H, H
), 134.03 (C
), 84.07 (C
[M] calcd: 1118.5921, The mixture was stirred at room temperature for 4 h. Subse-
2
), 8.22 (d, J ¼ 8.0 Hz, 8H, H
6
7
1
3
1
1
). C{ H} NMR (101 MHz, Zn-PTetraPyr (12 mg, 0.0081 mmol) was dissolved in 1.5 mL of
), 132.97 (C ), 131.06 CHCl , and 1 mL of CF COOH was then added. The initial
), 25.03 (C10). HR-MS deep red solution turned green aer the addition of the acid.
chloroform-d): d 145.03 (C
5
6
7
3
3
(C
2
), 127.96 (C
8
), 120.06 (C
4
9
+
(MALDI-TOF): m/z for C68
74 4 4 8
H B N O
found: 1118.5958 (3.7 ppm error). UV/vis (toluene): l 401 (3 ¼ quently, it was diluted with 10 mL of CHCl , and a saturated
3
7
¼
4 000), 422 (3 ¼ 426 600), 516 (3 ¼ 20 700), 551 (3 ¼ 9600), 592 (3 solution of Na CO in water was added portion-wise with
2
3
5500), 648 (3 ¼ 3800).
vigorous stirring until the evolution of gas ceased and the
organic layer had become deep red again. This layer was
separated from the aqueous phase and washed three times
with water (10 mL). The organic layer was removed under low
Zn-PTetraBpin
H-PTetraBpin (35 mg, 0.031 mmol) and Zn(AcO)
2
2
$2H
2
O
pressure to give the purple solid 2H-PTetraPyr (11 mg, 96%
1
(55 mg, 0.25 mmol) were mixed in a sealed vessel specically yield). H NMR (500 MHz, chloroform-d): d 9.23 (s, 8H, H
2
),
designed for microwave irradiation, and 2 mL of CHCl
added. The mixture was stirred inside the microwave reactor at J ¼ 7.7 Hz, 4H, H ), 8.41 (d, J ¼ 7.7 Hz, 4H, H ), 8.32–8.25 (m,
3
was 8.68 (d, J ¼ 9.2 Hz, 4H, H10), 8.54 (d, J ¼ 7.7 Hz, 8H, H
6
), 8.44 (d,
1
7
18
ꢀ
1
1
1
37 C for 55 min. Subsequently, the crude was dissolved in 12H, H + H + H ), 8.24 (d, J ¼ 8.8 Hz, 4H, H ), 8.20 (d, J ¼
1
1
12
14
16
2 mL of CHCl , placed in a separatory funnel, and washed with 8.8 Hz, 4H, H ), 8.15–8.08 (m, 12H, H + H ), ꢁ2.48 (br, 2H,
3
15
7
13
1
3
1
0 mL of H
2
O. The organic layer was removed under low pres-
H
1
). C{ H} NMR (126 MHz, chloroform-d): d 141.18 (C
), 137.44 (C ), 134.84 (C ), 131.59 (C21), 131.32 (C
), 8.23 (d, J ¼ 131.11 (C20), 130.88 (C22), 129.03 (C
), 1.50 (s, 48H, H10). (C18), 127.83 (C11), 127.63 (C15), 127.53 (C16), 126.13 (C13),
C{ H} NMR (101 MHz, chloroform-d): d 149.95 (C ), 145.68 125.40 (C10), 125.29 (C12 or C14), 125.23 (C23), 125.05 (C24),
C ), 133.93 (C ), 132.86 (C ), 131.98 (C ), 127.74 (C -in), 121.07 125.01 (C12 or C ), 124.91 (C ), 120.15 (C ). HR-MS (MALDI-
5
),
sure to give the purple solid Zn-PTetraBpin (33 mg, 90% yield). 140.66 (C
8
9
6
q
),
1
H NMR (500 MHz, chloroform-d): d 8.92 (s, 8H, H
.0 Hz, 8H, H
), 8.19 (d, J ¼ 8.0 Hz, 8H, H
2
7
), 128.76 (C19), 127.96
8
6
7
1
3
1
3
(
(
5
6
7
2
8
14
17
4
+
C ), 84.05 (C ), 25.03 (C ). HR-MS (MALDI-TOF): m/z for C - TOF): m/z for C108H N [M] calcd: 1414.4980, found:
4
9
10
68
62 4
+
H B N O Zn [M] calcd: 1180.5058, found: 1180.5036 1414.5022 (4.2 ppm error). UV/vis (toluene): l 349 (3 ¼ 16 900),
7
2 4 4 8
(
3
ꢁ2.2 ppm error). UV/vis (toluene): l 401 (3 ¼ 29 800), 425 (3 ¼ 411 (3 ¼ 16 000), 426 (3 ¼ 53 100), 518 (3 ¼ 8600), 552 (3 ¼
99 100), 551 (3 ¼ 17 400), 591 (3 ¼ 4300).
8000), 595 (3 ¼ 7000), 656 (3 ¼ 6500).
36170 | RSC Adv., 2020, 10, 36164–36173
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RSC Advances
Zn-PTetraCor
404 (3 ¼ 12 200), 426 (3 ¼ 38 500), 518 (3 ¼ 8100), 555 (3 ¼ 7600),
5
95 (3 ¼ 7100), 651 (3 ¼ 7100).
Zn-PTetraBr (9 mg, 0.0090 mmol), 1-pinacol cor-
2
annuleneboronate (Cor-Bpin) (17 mg, 0.045 mmol), [PdCl (dppf)]
t
(
5.8 mg, 0.0079 mmol), and BuONa (11 mg, 0.11 mmol) were Conflicts of interest
mixed in a sealed vessel specically designed for microwave
irradiation, which was placed inside a two-necked round-bottom
There are no conicts to declare.
ask in order to keep the system under an inert atmosphere.
1
.4 mL of dry toluene and 36 mL of pyridine were added. The Acknowledgements
ꢀ
mixture was stirred inside the microwave reactor at 130 C for
6
We thank the Spanish Ministry of Science, Innovation and
Universities (MCIU) for funding (project numbers PGC2018-
0 min. Aer cooling, the solvent was removed under vacuum
and the crude was puried by column chromatography on silica
gel using hexane/AcOEt gradient elution (3 : 1–2 : 1–1 : 1) and
096880-A-I00, MCIU/AEI/FEDER, UE and PGC2018-099470-B-
I00, MCIU/AEI/FEDER, UE). R. G.-R. acknowledges the
Spanish MINECO/AEI and the European Union (ESF) for
a Ramon y Cajal contract (RYC-2015-19035). H. B. acknowledges
the Alfonso Mart ´ı n Escudero Foundation for a postdoctoral
fellowship.
3
CHCl to nally give the purple solid Zn-PTetraCor (10 mg, 70%
1
yield). H NMR (500 MHz, chloroform-d): d 9.26 (s, 8H, H
2
), 8.50
(
d, J ¼ 8.0 Hz, 8H, H ), 8.29 (s, 4H, H10), 8.26 (d, J ¼ 8.9 Hz, 4H,
6
H ), 8.25 (d, J ¼ 8.0 Hz, 8H, H ), 8.01 (d, J ¼ 8.6 Hz, 4H, H ), 7.98
1
8
7
11
(
1
d, J ¼ 8.9 Hz, 4H, H ), 7.95 (d, J ¼ 8.6 Hz, 4H, H ), 7.93–7.88 (m,
1
7
12
1
3
1
6H, H + H + H + H ). C{ H} NMR (126 MHz, chloroform-
13 14 15 16
d) d: 150.35 (C
3
), 142.20 (C
), 136.31 (C ), 136.00 (C
), 130.97 (C ), 129.83 (C19-in), 128.20 (C
27.43 (C12), 127.36 (C ), 127.18 (C + C18), 127.09 (C
27.06 (C ), 126.27 (C10). Being C
q
-in), 141.51 (C
), 135.50 (C26), 134.99 (C
), 127.63 (C17),
+ C11),
5
-in), 138.92 (C
8
-in), Notes and references
1
1
1
1
36.47 (C
q
q
q
6
),
1
2
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7
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x
x
x
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x
¼ C13 or C14 or C15 or C16. HR-
+
MS (MALDI-TOF): m/z for C124H N Zn [M] calcd: 1668.4104,
60
4
found: 1668.4085 (ꢁ1.9 ppm error). UV/vis (toluene): l 404 (3 ¼
13 400), 430 (3 ¼ 89 300), 553 (3 ¼ 9800), 591 (3 ¼ 7700).
Note: Zn-PTetraCor could also be prepared, albeit in lower
yield (56%), using Zn-PTetraBpin and Cor-Br as the coupling
partners (i.e., by a synthetic route similar to route A in
Scheme 1).
2
H-PTetraCor
Zn-PTetraCor (8 mg, 0.0048 mmol) was dissolved in 2.0 mL of
CHCl , and 1 mL of CF COOH was then added. The initial deep
3
3
red solution turned green aer the addition of the acid. The
mixture was stirred at room temperature for 4 h. Subsequently,
it was diluted with 10 mL of CHCl , and a saturated solution of
Na CO in water was added portion-wise with vigorous stirring
2 3
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3
until the evolution of gas ceased and the organic layer had
become deep red again. This layer was separated from the
aqueous phase and washed three times with water (10 mL). The
organic layer was removed under low pressure to give the purple
1
solid 2H-PTetraCor (7 mg, 91% yield). H NMR (500 MHz,
chloroform-d): d 9.15 (s, 8H, H ), 8.50 (d, J ¼ 8.0 Hz, 8H, H ),
2
6
8
4
H
H
.28 (s, 4H, H ), 8.26 (d, J ¼ 8.0 Hz, 8H, H ), 8.25 (d, J ¼ 8.8 Hz,
1
0
7
H, H18), 8.01 (d, J ¼ 8.6 Hz, 4H, H11), 7.98 (d, J ¼ 8.8 Hz, 4H,
17), 7.95 (d, J ¼ 8.6 Hz, 4H, H12), 7.93–7.88 (m, 16H, H13 + H14
+
1
3
1
15 + H16), ꢁ2.52 (br, 2H, H
), 139.2 (C
-in), 135.90 (C -in), 135.53 (C26), 135.16 (C
), 130.88 (C ), 129.75 (C19), 128.35 (C ), 127.67 (C17),
1
). C{ H} NMR (126 MHz, chlo-
-in), 136.50 (C23 or C25), 136.33
),
roform-d): d 141.45 (C
), 136.00 (C
30.97 (C
5
8
(C
q
q
q
6
1
1
1
C
q
q
7
27.53 (C ), 127.40 (C ), 127.20 (C + C ), 127.12 (C + C ),
27.02 (C ), 126.34 (C ), 120.00 (C -in). Being C ¼ C or C or
1
2
x
x
18
x
11
5 (a) R. C. Haddon, L. E. Brus and K. Raghavachari, Chem.
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x
10
4
x
13
14
+
15 or C16. HR-MS (MALDI-TOF): m/z for C124
62 4
H N [M] calcd:
1606.4969, found: 1606.4994 (2.5 ppm error). UV/vis (toluene): l
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RSC Advances
Paper
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