A convenient synthesis of pentaporphyrins and supramolecular complexes with a fulleropyrrolidine

Article abstract of DOI:10.3390/molecules24173177

A simple and straightforward synthesis of diporphyrins and pentaporphyrins is reported here. The supramolecular interactions of the new porphyrin derivatives with C₆₀ and PyC₆₀ (a pyridyl [60]fulleropyrrolidine) were evaluated by absorption and fluorescence titrations in toluene. While no measurable modifications of the absorption and fluorescence spectra were observed upon addition of C₆₀ to the porphyrin derivatives, the addition of PyC₆₀ to the corresponding mono-Zn(II) porphyrins resulted in the formation of Zn(porphyrin)–PyC₆₀ coordination complexes and the binding constants were calculated. Results show that the four free-base porphyrin units in pentaporphyrin 6 have a significant contribution in the stabilization of the 6–PyC₆₀ complex. The crystal and molecular features of the pentaporphyrin Zn5 were unveiled using single-crystal X-ray diffraction studies.

Full text of DOI:10.3390/molecules24173177

molecules
Article
A Convenient Synthesis of Pentaporphyrins and
Supramolecular Complexes with a Fulleropyrrolidine
Joana I. T. Costa , Andreia S. F. Farinha 2 , Filipe A. Almeida Paz 3 and Augusto C. Tomé 1,*
1
1
QOPNA and LAQV-REQUIMTE, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
King Abdullah University of Science and Technology (KAUST), Water Desalination and Reuse
2
Center (WDRC), Division of Biological and Environmental Sciences (BESE), Thuwal, Saudi Arabia
Department of Chemistry, CICECO—Aveiro Institute of Materials, University of Aveiro, 3810-193 Aveiro,
3
Portugal
*
Correspondence: actome@ua.pt; Tel.: +351-234-370-712
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editors: M. Graça P. M. S. Neves, M. Amparo F. Faustino and Nuno M. M. Moura
Received: 30 July 2019; Accepted: 31 August 2019; Published: 1 September 2019
ꢀ
ꢁꢂꢃꢄꢅꢆ
Abstract: A simple and straightforward synthesis of diporphyrins and pentaporphyrins is reported
here. The supramolecular interactions of the new porphyrin derivatives with C60 and PyC60 (a pyridyl
[60]fulleropyrrolidine) were evaluated by absorption and fluorescence titrations in toluene. While no
measurable modifications of the absorption and fluorescence spectra were observed upon addition of
C60 to the porphyrin derivatives, the addition of PyC60 to the corresponding mono-Zn(II) porphyrins
resulted in the formation of Zn(porphyrin)–PyC60 coordination complexes and the binding constants
were calculated. Results show that the four free-base porphyrin units in pentaporphyrin
6 have
a significant contribution in the stabilization of the –PyC60 complex. The crystal and molecular
6
features of the pentaporphyrin Zn5 were unveiled using single-crystal X-ray diffraction studies.
Keywords: multiporphyrin; fullerene; supramolecular chemistry
1
. Introduction
Porphyrins are extremely versatile compounds from structural and chemical viewpoints.
They display a rigid planar geometry, high stability, intense electronic absorption and emission,
a small HOMO–LUMO energy gap, and flexible tunability of their optical and redox properties by
changing the metal center [
meso-(hetero)aryl substituents. Consequently, these compounds are one of the most attractive building
blocks for the formation of covalent or supramolecular assemblies [ ]. The design and synthesis of
multiporphyrin arrays is an important research topic due to their potential application as artificial
light-harvesting systems [ 13], functional materials (near infrared dyes, electron-conducting molecular
wires, nonlinear optics, molecular recognition and sensing) [14 17], and also as agents for medical
imaging and photodynamic therapy [18]. Several synthetic strategies have been developed to produce
multiporphyrin oligomers, including directly meso–meso-, meso– - and -linked multiporphyrin
systems, oligoporphyrins with fused -systems and arrays bearing rigid or flexible spacers. In this
context, different types of covalent or supramolecular porphyrin arrays with a large structural
diversity [19 22], including linear [23 25], zig-zag [26], and dendritic arrays [27 28], tapes [29 31],
belts [32], barrels [33], rings [34–44] and boxes [45] and balls [46], have been synthesized.
1]. Porphyrins can be efficiently modified at the meso or beta positions, or at the
2–6
7–
–
β
β–β
π
–
–
,
–
Fullerenes are another group of attractive building blocks, frequently used in association with
porphyrins. In fact, architectures consisting of porphyrin derivatives (donors) and fullerenes (acceptors)
are of particular interest, either as models for natural photosynthesis or for the conversion of
light into electricity. Because of the unique three-dimensional structure of fullerenes, their low
reduction potentials and small reorganization energy, providing the formation of long-lived charge
Molecules 2019, 24, 3177; doi:10.3390/molecules24173177
www.mdpi.com/journal/molecules

Molecules 2019, 24, 3177
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separated states [47], porphyrin–fullerene photoactive systems have provided promising materials for
photovoltaic applications [48 54]. In many cases, the photoactive system is a supramolecular complex
formed by a multiporphyrin receptor and a fullerene guest [55 63]. Combining interactions with
–
–
π–π
hydrogen bonding [64] or axial coordination [65] is another strategy that results in the assembly of
stable and robust architectures.
5,10,15,20-Tetrakis(pentafluorophenyl)porphyrin (2) reacts with a range of nucleophiles, namely
amines, thiols, alcohols and phenols, leading to the formation of nucleophilic aromatic substitution
products under mild conditions [66]. The para-fluorine atoms are selectively substituted by the
nucleophile and mono- to tetra-substituted porphyrin derivatives may be obtained. We have reported
the synthesis of multiporphyrin compounds via nucleophilic aromatic substitution of fluorine atoms in
porphyrin
procedure for the preparation of diporphyrins and pentaporphyrins. The interaction of pentaporphyrins
and Zn with C60 and pentaporphyrin with 1-methyl-2-(4-pyridyl)[60]fullero[c]pyrrolidine (PyC60)
is also discussed.
2 [20] or hexafluorobenzene [67,68]. Using a similar approach, herein we present a simple
5
5
6
2
. Results and Discussion
2
.1. Synthesis and Characterization
We have shown that 5-(4-hydroxyphenyl)-10,15,20-triphenylporphyrin (1) reacts as a nucleophile
with hexafluorobenzene or pentafluorophenyl groups to give multiporphyrin derivatives under very
mild conditions [67]. As indicated above, porphyrin 2 reacts with nucleophiles to yield nucleophilic
aromatic substitution products. Thus, it seemed obvious to us that we could access multiporphyrin
compounds by reacting porphyrins
1
and
2
3
(Scheme 1 and Figure 1). In fact, the reaction of porphyrins
in 55% yield; minor amounts of triporphyrin derivatives
1
and (in excess) afforded diporphyrin
2
were also formed. Similarly, the reaction of porphyrin
by metalation of with zinc acetate, produced diporphyrin
treated with 5 equivalents of porphyrin , the pentaporphyrin
as the main product. Similarly, pentaporphyrin (Figure 1) was obtained in 64% yield using
Zn as the starting porphyrins. Metalation of pentaporphyrin with zinc acetate afforded Zn
quantitative yield.
1
with the Zn
in 32% yield. When porphyrin
(Figure 1) was obtained in 76% yield
and
in
2
complex, previously prepared
2
4
2
was
1
5
6
1
2
5
5
The structures of the new multiporphyrin compounds
UV–Vis, and mass spectrometry. Comparing the H NMR spectra of diporphyrin
3
–
6
and Zn
5
were confirmed by NMR,
and pentaporphyrin
-pyrrolic and NH protons.
-pyrrolic protons of the fluorinated porphyrin appear as a
8.93 ppm), and two doublets. However, due to the symmetry of pentaporphyrin , only
one singlet is observed (at 9.16 ppm) for the eight -pyrrolic protons of the central porphyrin unit. Both
spectra show two singlets assigned to the NH protons: 2.87 (2H) and 2.73 ppm (2H) for diporphyrin
, and 2.78 (8H) and 2.69 (2H) for pentaporphyrin
1
3
5
, the main differences are in the signals due to the resonances of the
For diporphyrin , the resonances of the
singlet (at
β
3
β
δ
5
β
−
−
3
−
−
5
. The integration of those signals (1:1 or 1:4)
1
agrees with the presence of two or five porphyrin units, for
of diporphyrin and pentaporphyrin
2.74 ppm and
2.78 ppm, respectively). The 19F NMR spectra of the diporphyrins
the pentaporphyrins and further confirm the proposed structures, particularly the number of the
para-fluorine atoms substituted by porphyrin units. The F NMR spectra of diporphyrins
a multiplet between δ −174.82 and 174.65 ppm for diporphyrin , and between δ −175.03 and
ppm for diporphyrin , assigned to the three para-fluorine atoms. On the other hand, the absence of
signals due to the para-fluorine atoms in the 19F NMR spectra of pentaporphyrins
and confirm
the tetra-substitution. The mass spectra of diporphyrins and show the expected [M + H] ions
m/z = 1585.3 for diporphyrin and m/z = 1647.3 for diporphyrin ). The HRMS (ESI) of diporphyrin
also shows a peak at m/z = 793.2 corresponding to the [M + 2H] ion, which results from multiple
protonation of the molecule (Figure S4). The MALDI-TOF mass spectra of pentaporphyrins and
3
and
5, respectively. The H NMR spectra
4
6
exhibit only one singlet at high field assigned to the NH protons
and and
(−
−
3
4
5
6
1
9
3
and
4
show
−
3
−174.85
4
5
6
+
3
4
4
(
3
3
2
+
5
6

Molecules 2019, 24, 3177
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+
reveal the peaks corresponding to the monoisotopic masses of the [M + H] ions (m/z = 3416.0 for
pentaporphyrin and m/z = 3477.9 for pentaporphyrin ). However, the HRMS (ESI) of pentaporphyrin
shows only the peaks corresponding to the [M + 2H] and [M + 3H]
5
6
2
+
3+
5
.
Scheme 1. Synthesis of diporpyrins 3 and 4.

Molecules 2019, 24, 3177
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Figure 1. Molecular structures of pentaporphyrins 5, Zn5 and 6.
Comparing the UV–Vis spectra of diporphyrins and pentaporphyrins with those of their porphyrin
precursors (Figure 2), the sole difference is the intensity of the Soret and Q bands, ultimately confirming
that there is no conjugation between the porphyrin units.
1.6
1
A
Zn2
4
1.2
6
0.08
A
0.8
0.04
0
0.4
450
550
λ (nm)
650
0
300
400
500
λ (nm)
600
700
Figure 2. Absorption spectra of
1, Zn2,
4, and
6
in toluene. All compounds were used with a
concentration of 1.0 µM. The inset shows an expansion of the Q bands.

Molecules 2019, 24, 3177
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Pentaporphyrin Zn
5
was isolated as large red single-crystals through recrystallization from a
mixture of chloroform and methanol (among other solvents in minor quantity). Figure 3 depicts
the centrosymmetric molecular unit present in the crystal structure of Zn5·Solvent (side and top
views), clearly reflecting the high level of conformational flexibility of the large molecule. The three
crystallographically independent Zn2 metal centers exhibit identical coordination environments
which strongly resemble a distorted square pyramid: while the basal plane is formed by the four
nitrogen atoms arising from the porphyrinic core [Zn–N distances ranging from 2.040(3) to 2.089(4)
Å (for the three environments)], the apical position is always occupied by a coordinated methanol
molecule with the Zn–Omethanol distance being found in the 2.118(5)–2.197(5) Å range. As also depicted
in Figure 3, the four peripheral porphyrin molecules adopt a conformation so they are close together,
mutually interacting via weak supramolecular contacts most certainly also involving the solvent
molecules of crystallization (not shown).
+
Figure 3. Schematic representation of the pentaporphyrin molecular unit present in the crystal structure
of Zn5·Solvent.
2
.2. Formation of Porphyrin–Fullerene Complexes
The binding capabilities of pentaporphyrins
5 and Zn5 to form complexes with pristine C60
was investigated by absorption and fluorescence titrations in toluene. Surprisingly, the addition of
a toluene solution of C60 to pentaporphyrins and Zn did not induce a quantifiable alteration in
their absorption and fluorescence spectra. This indicates that the eventual interactions established
between the host and C60 are too weak, in solution, to induce spectral modifications. However, the
titration of pentaporphyrin with PyC60 revealed a markedly distinct behavior. The addition of
PyC60 to a solution of in toluene induced a decrease of the Soret band (at 422 nm) along with the
appearance of an isosbestic point at 428 nm (Figure 4). This suggests the formation of a –PyC60
5
5
π-π
6
6
6
complex, presumably via axial coordination of the pyridyl group to the zinc porphyrin (Figure 5), as
reported in similar systems [56,68–72].

Molecules 2019, 24, 3177
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0.16
0.16
A
A
0.12
0.12
0.08
0.08
0.04
0
0.04
400
420
440
460
λ (nm)
0
400
450
500
550
600
650
700
λ (nm)
0.02
-
ΔA
0
.016
.012
.008
.004
0
0
0
0
experimental
data at 422 nm
fit
0
0.2
0.4
0.6
0.8
1
1.2
1.4
5
[
PyC60] x 10 M
−
7
Figure 4. Absorption spectra of
6
(1.0
×
10 M) upon addition of PyC60 (0–113 equiv.) in toluene at 23
◦
C (upper part) and experimental data at 422 nm fitted to a non-linear 1:1 binding model (lower part).
Figure 5. Representation of the coordination complex 6–PyC .
6
0

Molecules 2019, 24, 3177
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To evaluate the contribution of the four free porphyrin units in the stabilization of the
complex, the precursor Zn and the diporphyrin
addition of PyC60 to Zn induced a red shift of the Soret and Q bands with the formation of an isosbestic
point at 423 nm (Figure S14 in the ESI), probably as a result of axial coordination of the pyridyl group
of PyC60 to Zn . D’Souza already observed the complex formation between porphyrin and PyC60 in
dichloromethane [73]. Similar behavior was observed when a solution of diporphyrin in toluene was
titrated with PyC . The incremental addition of PyC caused a decrease and a red shift of the Soret
6–PyC60
2
4 were also titrated with PyC . As expected, the
60
2
2
2
4
60
60
with the formation of an isosbestic point at 425 nm (Figure S16 in the ESI). A red shift was observed in
the Q bands, which is most likely a consequence of coordination complex formation. As discussed
below, the binding constant for the
for the Zn –PyC60 complex, which confirms that the four free-base porphyrin units in pentaporphyrin
have a significant contribution in the stabilization of the –PyC60 complex. The role of the four
peripheral porphyrin units in stabilizing the –PyC60 complex may be compared to the synergic effect
observed in C60 and C70 hosting behavior of corannulene- and pyrene-substituted porphyrins [74 76].
The steady state fluorescence spectrum of pentaporphyrin displays two bands centered at
50 and 715 nm (Figure 6). Upon addition of PyC60 to a solution of in toluene, the fluorescence
intensity decreased significantly when exciting at 428 nm, as depicted in Figure 6. These changes
are fully consistent with strong interactions between the photoexcited and PyC60 in the –PyC60
upon addition
6–PyC60 complex is ca. 5.8 times higher than the binding constant
2
6
6
6
–
6
6
6
6
6
complex [68
,
70
,
73]. The fluorescence quenching of the precursor Zn
2
and diporphyrin 4
of PyC60 was also observed (Figures S15 and S17 in the ESI).
1
I/I
0
0.8
0.6
0.4
0.2
0
580
620
660
λ (nm)
700
740
1.2
experimental data
at 650 nm
I/I
0
1
fit
0.8
0.6
0.4
0.2
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
[
PyC60] x 105
M
−
7
Figure 6. Fluorescence spectra (λexc = 428 nm) of
6
(1.0
×
10 M) upon addition of PyC60 (0–113 equiv.)
◦
in toluene at 23 C (upper part) and experimental data at 650 nm fitted to a non-linear 1:1 binding
model (lower part).

Molecules 2019, 24, 3177
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The binding constants (K) were obtained from the absorption and fluorescence spectral data by
using a non-linear 1:1 binding model (Figures 4 and 6) and are listed in Table 1. The average values
for the binding constants (Kav) increase in the order Zn
2
<
4
<
6
. In particular, the Kav value of the
5
−1
6
–PyC60 complex (Kav = 1.53
Zn –PyC60 complex (Kav = 2.66
the pentaporphyrin arises presumably from the synergic effect of the
fullerene unit and the four free-base porphyrin units.
×
10 M ) was found to be ca. 5.8 times higher than the Kav for the
4
−1
2
×
10 M ). As discussed above, the enhanced binding ability of
interactions between the
6
π–π
Table 1. Binding constants (K, M 1) calculated by absorption and fluorescence measurements for
−
◦
Zn2–PyC , 4–PyC and 6–PyC60 complexes in toluene at 23 C.
60
60
K (M 1)
−
Kav (M ) a
−
1
Complex
Absorption
Fluorescence
4
4
5
4
4
Zn2–PyC60
1.75 × 10
5.04 × 10
1.77 × 10
3.57 × 10
2.66 × 10
4
4
4
6
–PyC60
–PyC60
4.59 × 10
4.82 × 10
5
5
1.28 × 10
1.53 × 10
a
Kav = average of the two calculated K values.
3
. Materials and Methods
3
.1. Chemicals and Instrumentation
1H, 13C and 19F NMR spectra were recorded on Bruker AVANCE 300 or Bruker AVANCE
00 spectrometers. CDCl was use as solvent and tetramethylsilane (TMS) as internal reference.
5
3
The chemical shifts are expressed in
δ (ppm) and the coupling constants (J) in hertz (Hz). UV–Vis
spectra were recorded on a Shimadzu UV-2501PC spectrophotometer using toluene or CHCl as
3
solvent. UV
absorption coefficients (
on a JASCO FP-8300 spectrofluorometer. Mass spectra were recorded using a MALDI TOF/TOF 4800
−
vis absorption spectral wavelengths (λ) are reported in nanometers (nm), and molar
− −1
1
ε
) are reported in M cm . Fluorescence emission spectra were recorded
Applied Biosystems MDS Sciex mass spectrometer, CHCl as solvent and 3-nitrobenzyl alcohol (NBA)
3
as matrix. High-resolution mass spectra (HRMS) were recorded on a Bruker Apex-Qe FTICR mass
spectrometer or on a LTQ Orbitrap XL mass spectrometer using CHCl as solvent. Melting points were
3
measured on a Büchi B-540 apparatus and are uncorrected. Column chromatography was carried out
using silica gel (Merck, 35-70 mesh). Analytical TLC was carried out on precoated sheets with silica gel
(
Merck 60, 0.2 mm thick). Solvents were purified or dried according to the literature procedures [77].
Compounds 1 [78], 2 [78] and PyC60 [79] were prepared according to published procedures.
3
.2. Synthesis
3
.2.1. 5,10,15,20-Tetrakis(pentafluorophenyl)porphyrinatozinc(II) (Zn2)
Prepared as in [73]. Zinc acetate (28.2 mg, 0.154 mmol) was added to a solution of
2 (50.0 mg,
◦
51.3
µmol) in chloroform/methanol (2:1) and the resulting mixture was stirred at 60 C for 15 min.
After cooling down to ambient temperature, the reaction mixture was washed with distilled water.
The organic phase was dried (Na SO ) and the solvent was evaporated under reduced pressure. The
2
4
Zn
2
complex was obtained in quantitative yield. UV–Vis (toluene): λmax (log
ε
) 421 (5.5), 546 (4.2), 581
+
(3.7) nm. MS (MALDI-TOF): m/z 1036.0 [M] .
3
.2.2. Diporphyrin 3
A solution of
1
(20.0 mg, 31.7
µ
mol),
2
(92.7 mg, 95.1
µ
mol) and potassium carbonate (13.1 mg,
◦
9
5.1
µ
mol) in dry DMSO (3 mL) was stirred under a nitrogen atmosphere at 50 C for 3 h. After cooling
to ambient temperature, the porphyrinic compounds were precipitated with an aqueous solution of

Molecules 2019, 24, 3177
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citric acid, filtered and washed with water. The solid was dissolved in dichloromethane and then
washed with water. The organic phase was dried (Na SO ) and the solvent was evaporated under
2
4
reduced pressure. The residue was purified by column chromatography on silica gel using a gradient of
dichloromethane/hexane. The first fraction was identified as the unreacted porphyrin . The following
fraction afforded diporphyrin (28 mg, 55% yield) after crystallization from dichloromethane/methanol.
mp > 300 C. H NMR (CDCl , 300 MHz): , ppm 2.87 (s, 2H, NH of moiety ), 2.73 (s, 2H, NH of
), 7.71–7.82 (m, 11H, C H -m-H and Ph-m,p-H), 8.22–8.26 (m, 6H, Ph-o-H), 8.37 (d, J = 8.6 Hz,
2
3
◦
1
δ
−
1
−
3
moiety
2
6
4
2
H, C H -o-H), 8.87 (s, 4H,
β
-H of moiety
1), 8.91–8.97 (3 overlapped d, 6H, β-H), 8.93 (s, 4H, β-H of
6
4
13
moiety 2), 9.09 (d, J = 4.8 Hz, 2H, β-H of moiety 2). C NMR (CDCl , 126 MHz): δ, ppm 103.6, 104.4,
3
19
1
2
14.4, 118.6, 120.3, 120.4, 126.7, 127.8, 131.1 (br s,
82 MHz): , ppm 184.83 (dt, J = 23.2 and 7.6 Hz, 6F, C F -m-F),
C F -m-F), 174.82 to
dd, J = 23.2 and 7.6 Hz, 6F, C F -o-F). UV–Vis (CHCl ): λmax (log
β
-C), 134.6, 135.9, 138.4, 142.1, 157.2. F NMR (CDCl3,
176.86 (dd, J = 23.1 and 9.3 Hz, 2F,
160.40 (dd, J = 23.1 and 9.3 Hz, 2F, C F -o-F), 160.00
) 418 (5.8), 510 (4.3), 548 (4.2), 586
δ
−
−
6
5
−
−
174.65 (m, 3F, C F -p-F),
−
−
6
4
6
5
6 4
(
(
[
ε
6
5
3
+
3.8), 644 (3.7) nm. MS (MALDI-TOF): m/z 1585.3 [M + H] . MS (HRESI): m/z calcd. for C H F N O
M + H] 1585.3016, found 1585.3037, calcd. for C H F N O [M + 2H] 793.1545, found 793.1534.
88
40 19
8
+
2+
88
41 19
8
3
9
.2.3. Diporphyrin 4
A solution of (20.0 mg, 31.7
1
µ
mol), Zn
2
(84.2 mg, 95.1
µ
mol) and potassium carbonate (13.1 mg,
C for 4 h. The workup
. The residue was purified by
column chromatography on silica gel using a gradient of dichloromethane/hexane. The first fraction
5.1 µmol) in dry DMSO (3 mL) was stirred under a nitrogen atmosphere at 50 º
procedures were the same as previously described for diporphyrin
3
was identified as the starting porphyrin Zn
yield) after crystallization from dichloromethane/methanol. mp > 300
2
. The following fraction afforded diporphyrin
4
(23 mg, 32%
1
º
C. H NMR (CDCl , 300 MHz):
3
δ
, ppm
−
2.74 (s, 2H, NH), 7.72–7.83 (m, 11H, C H -m-H and Ph-m,p-H), 8.23–8.27 (m, 6H, Ph-o-H),
6 4
8
.38 (d, J = 8.5 Hz, 2H, C H -o-H), 8.88 (s, 4H,
β
-H of moiety
1
), 8.93 and 8.98 (AB, J = 4.8 Hz,
β-H of
6
4
1
9
moiety
1), 9.01 (s, 4H,
β
-H of moiety Zn
2
), 9.04 and 9.17 (AB, J = 4.7 Hz, 4H,
β-H of moiety Zn
2
).
F
NMR (CDCl , 282 MHz): δ, ppm −184.75 (dt, J = 23.6 and 8.0 Hz, 6F, C F -m-F), −176.77 (dd, J = 23.4
3
6 5
and 9.4 Hz, 2F, C F -m-F),
−
175.03 to
−
174.85 (m, 3F, C F -p-F),
−
160.21 (dd, J = 23.4 and 9.4 Hz, 2F,
C F -o-F), 159.85 (dd, J = 23.6 and 8.0 Hz, 6F, C F -o-F). UV–Vis (toluene): λmax (log ε) 421 (5,8), 513
6 5
6
4
6 5
−
6
4
+
(
4,3), 546 (4,4), 584 (3,9), 646 (3,7) nm. MS (MALDI-TOF): m/z 1647.3 [M + H] . MS (HRESI): m/z calcd.
+
for C H F N OZn [M + H] 1647.2078, found 1647.2110.
88
38 19
8
3
.2.4. Pentaporphyrin 5
A solution of (10.0 mg, 10.2
2
µ
mol),
1
(32.4 mg, 51.3
µ
mol) and potassium carbonate (17.0 mg,
◦
0
.123 mmol) in dry DMSO (1 mL) was stirred under a nitrogen atmosphere at 80 C for 2 h. After
cooling to ambient temperature, the porphyrinic compounds were precipitated with an aqueous
solution of citric acid, filtered and washed with water. The solid was dissolved in dichloromethane
and then washed with water. The organic phase was dried (Na SO ) and the solvent was evaporated
2
4
under reduced pressure. The residue was purified by column chromatography on silica gel using
dichloromethane/hexane (2:1) as the eluent. A major faction was eluted and then the unreacted
porphyrin
1
was recovered. The major fraction afforded pentaporphyrin
5
(27 mg, 76% yield) after
, ppm
2.69 (s, 2H, NH of moiety 2), 7.63–7.78 (m, 44H, C H -m-H and
◦
1
crystallization from dichloromethane/methanol. mp > 300 C. H NMR (CDCl , 300 MHz):
δ
3
−
2.78 (s, 8H, NH of moiety
Ph-m,p-H), 8.15 (dd, J = 7.5 and 1.8 Hz, 16H, 10,20-Ph-o-H), 8.22 (dd, J = 7.3 and 1.7 Hz, 8H, 15-Ph-o-H),
1),
−
6
4
8
.33 (d, J = 8.7 Hz, 8H, C H -o-H), 8.81 (d, J = 6 Hz, 8H,
β
-H of moiety
1
), 8.83–8.86 (m, 16H,
β
-H of
). C NMR (CDCl3,
-C), 134.5,
, ppm -176.92 (dd, J = 23.1 and 9.2 Hz,
160.33 (dd, J = 23.1 and 9.2 Hz, 8F, C F -o-F). UV–Vis (toluene): λmax (log ) 420 (6.2),
13 (5.0), 547 (4.6), 590 (4.5), 648 (4.3) nm. MS (MALDI-TOF): m/z 3416.0 [M + H] . MS (HRESI): m/z
6
4
13
moiety
7
1
), 8.81 (d, J = 6 Hz, 8H,
β-H of moiety
1
), 9.16 (s, 8H,
β-H of moiety 2
5 MHz):
δ, ppm 104.3, 114.3, 116.7, 118.6, 120.2, 120.3, 126.6, 126.7, 127.7, 129.8–132.4 (β
19
1
35.9, 138.3, 142.0, 142.1, 157.1. F NMR (CDCl , 282 MHz):
δ
3
8
F, C F -m-F),
−
ε
6
4
6 4
+
5

Molecules 2019, 24, 3177
10 of 17
2
+
calcd. for C220H128F N O [M + 2H] 1708.5081, found 1708.5129, calcd. for C220H129F N O
4
[
16
20
4
16 20
M + 3H]3 1139.3411, found 1139.3427.
+
3
.2.5. Pentaporphyrin Zn5
Zinc acetate (16.1 mg, 87.8 mmol) was added to a solution of
chloroform/methanol (2:1) and the resulting mixture was stirred at 60 C for 1 h. After cooling
5 (20.0 mg, 5.85 µmol) in
◦
down to ambient temperature, the reaction mixture was washed with distilled water. The organic
phase was dried (Na SO ) and the solvent was evaporated under reduced pressure. The Zn
5
complex
2
4
1
was obtained in quantitative yield. H NMR (CDCl , 300 MHz):
δ, ppm 7.57–7.74 (m, 44H, C H -m-H
3
6
4
and Ph-m,p-H), 8.09–8.18 (m, 24H, Ph-o-H), 8.28 (d, J = 8.6 Hz, 8H, C H -o-H), 8.80–8.86 (m, 24H,
β-H
6
4
of moiety
1
), 8.92 (d, J = 4.7 Hz, 8H,
β
-H of moiety Zn
1
), 9.13 (s, 8H,
β
-H of moiety Zn
2
). UV–Vis
+
·
(toluene): λmax (log ε) 424 (6.1), 548 (4.9), 588 (4.4) nm. MS (MALDI-TOF): m/z 3734,3 [M] .
3
.2.6. Pentaporphyrin 6
A solution of Zn (10.0 mg, 11.3
2
µmol),
1
(35.6 mg, 56.4
µmol) and potassium carbonate (18.7 mg,
0
.135 mmol) in dry DMSO (1 mL) was stirred under a nitrogen atmosphere at 80
. The residue was purified by column
chromatography on silica gel using dichloromethane/hexane (2:1) as the eluent. A major faction was
ºC for 2 h. The workup
procedures were the same as described for pentaporphyrin
5
eluted and then unreacted porphyrin
1
was recovered. The major fraction afforded pentaporphyrin
6
1
(
25 mg, 64% yield) after crystallization from dichloromethane/hexane. mp > 300
00 MHz):
J = 7.4 and 1.8 Hz, 16H, 10,20-Ph-o-H), 8.22 (dd, J = 7.4 and 1.7 Hz, 8H, 15-Ph-o-H), 8.34 (d, J = 8.5 Hz,
º
C. H NMR (CDCl ,
3
3
δ, ppm -2.78 (s, 8H, NH), 7.60–7.78 (m, 44H, 5-C H -m-H and 10,15,20-Ph-m,p-H), 8.15 (dd,
6 4
8
8
H, 5-C H -o-H), 8.80–8.86 (m, 24H,
β
-H of moiety
1
), 8.93 (d, J = 4.8 Hz, 8H,
, ppm 105.0, 114.3, 117.4, 117.6, 117.7, 118.7,
-C), 132.4, 134.5, 134.6, 135.9, 138.3, 140.6, 140.8,
β-H of moiety 1), 9.25 (s,
6
4
13
H,
β-H of moiety Zn
2). C NMR (CDCl , 126 MHz): δ
3
1
20.28, 120.34, 126.65, 126.73, 127.7, 127.8, 129.8–132.3 (
β
1
9
142.0, 142.1, 142.7, 142.8, 146.0, 148.1, 150.4, 157.2. F NMR (CDCl , 282 MHz): δ, ppm -176.81 (dd,
3
J = 23.4 and 9.2 Hz, 8F, C F -m-F), -160.14 (dd, J = 23.4 and 9.2 Hz, 8F, C F -o-F). UV–Vis (toluene):
6
4
6 4
+
λmax (log ε) 422 (6.1), 513 (4.8), 549 (4.7), 589 (4.4), 645 (4.3) nm. MS (MALDI-TOF): m/z 3477.9 [M + H] .
3
.3. Absorption and Fluorescence Titrations
The UV–Vis and fluorescence titrations were carried out using a stock solution of each compound
in toluene in a quartz cuvette (1 cm path length). A PyC60 stock solution was added in aliquots and
the spectra were recorded after each addition. Absorption spectra were corrected for the contribution
of the added PyC . In the fluorescence measurements the sample was excited at the isosbestic point.
60
Measurements were repeated 2–3 times and found to be reproducible within a margin of error of ca.
◦
1
5%. All measurements were performed at 23 C.
The binding constants (K) were assessed from the UV–Vis and fluorescence titrations. The variation
of the absorbance at the Soret band caused by consecutive additions of PyC60 was used to determine
the binding constants. The spectral changes at the Soret band were fit using non-linear least-squares
procedures using the following equation for a 1:1 binding:
ε
Por[Por] + ε
K[Por][Ful]
Por−Ful
A =
(1)
1
+ K[Ful]
in which A is the absorbance, [Por] is the total concentration of the porphyrin derivative, [Ful] is the
concentration of PyC , and are constants associated with the molar extinction coefficients
ε
ε
60
Por
Por-Ful
of the porphyrin derivative and the complex formed, respectively.

Molecules 2019, 24, 3177
11 of 17
In a similar way, the spectral changes at maximum fluorescence intensity were fit using non-linear
least-squares procedures using the following equation for 1:1 binding:
ΦPor[Por] + Φ
K[Por][Ful]
Por−Ful
I =
(2)
1
+ K[Ful]
where I is the fluorescence intensity, ΦPor and ΦPor-Ful are constants associated with the emission
quantum yields of the porphyrin derivative and of the complex formed, respectively [80].
3
.4. Single-Crystal X-Ray Diffraction Studies of Zn5·Solvent
Single crystals of compound Zn5·Solvent were manually harvested from the crystallization vial and
immersed in highly viscous FOMBLIN Y perfluoropolyether vacuum oil (LVAC 140/13, Sigma-Aldrich)
to avoid degradation caused by the evaporation of the solvent [81]. Crystals were mounted on Hampton
Research CryoLoops with the help of a Stemi 2000 stereomicroscope equipped with Carl Zeiss lenses.
X-ray diffraction data were collected at 179(2) K on a Bruker D8 QUEST equipped with Mo K
α sealed
tube ( = 0.71073 Å), a multilayer TRIUMPH X-ray mirror, a PHOTON 100 CMOS detector, and an
λ
Oxford Instruments Cryostrem 700+ Series low temperature device. Diffraction images were processed
using the software package SAINT+ [82], and data were corrected for absorption by the multiscan
semi-empirical method implemented in SADABS [83]. The structure was solved using the algorithm
implemented in SHELXT-2014 [84], which allowed for the immediate location of almost all of the
heaviest atoms composing the molecular unit. The remaining missing and misplaced non-hydrogen
atoms were located from difference Fourier maps calculated from successive full-matrix least-squares
2
refinement cycles on F using the latest SHELXL from the 2017/1 release [85]. All structural refinements
were performed using the graphical interface ShelXle [86].
The crystal diffracted poorly throughout the entire angular range, this despite a long acquisition
◦
time per frame (ca. 1 min) and the use of a wide frame strategy (0.5
resolution the mean I/
for a resolution of 1.20 Å the Rmerge is already above 10%). The poor quality of the overall diffraction has
important consequences in the modeled structure, giving rise to a number of alerts in PLATON [87 88].
φ scans). Indeed, below 1.0 Å of
σ
drops below 5.0 with the Rmerge value being concomitantly high (for example,
,
Nevertheless, structure solution immediately showed the presence of the most important chemical
moiety in the crystal structure.
Besides the pentaporphyrin Zn
smaller ones (total volume of about 2446 Å as estimated by PLATON) [87
5
derivative, the crystal structure contains one large void and two
3
,88] centred at (0.385 0.3305
0
.000), (0.263 0.819 0.417), and (0.736 0.181 0.583), which most likely contain highly disordered solvent
−
molecules. From difference Fourier maps it was possible to discern the presence of a considerable
smeared-out electron density in these locations. Nevertheless, several attempts to locate and model
solvent molecules proved to be unproductive. The original data set was treated using the SQUEEZE [89
subroutines implemented in PLATON [87 88] in order to remove the contribution of these highly
disordered molecules. It was estimated that the aforementioned cavities would contain a total of ca.
32 electrons. The calculated solvent-free reflection list was used for subsequent structural refinements
]
,
6
that converged to the solvent-free structure reported in this manuscript and having the reliability
factors listed below.
The asymmetric unit is composed of half of the pentaporphyrin in which the metallic centres
are coordinated to a methanol molecule. These three crystallographically independent methanol
molecules were found to be severely affected by positional disorder and were included in the final
structural model with the distances restrained to common (refineable) distances and with isotropic
displacement parameters for the non-hydrogen atoms, so as to ensure chemically reasonable geometries
for these moieties.
Hydrogen atoms bound to carbon were placed at their idealized positions using appropriate HFIX
instructions in SHELXL: 43 (aromatic carbon atoms and coordinated hydroxyl groups) and 137 (for all
methyl groups). These hydrogen atoms were included in subsequent refinement cycles with isotropic

Molecules 2019, 24, 3177
12 of 17
thermal displacements parameters (Uiso) fixed at 1.2 (for the former family of hydrogen atoms) or
.5×Ueq (solely for those associated with the methyl groups) of the parent atoms.
The last difference Fourier map synthesis showed the highest peak (1.947 eÅ 3) and the deepest
1
−
−
3
hole (-1.354 eÅ ) located at 0.39 and 0.07 Å from H2M and C2M (associated with the coordinated
methanol molecule), respectively. Structural drawings have been created using the software package
Crystal Impact Diamond [90].
Crystal data for Zn5·Solvent (SQUEEZE Data): C225H136F N O Zn , M = 3894.38, triclinic, space
16
20
9
5
◦
◦
◦
group P
¯
ı, Z = 1, a = 12.995(2) Å, b = 15.034(3) Å, c = 34.607(7) Å,
α
= 78.338(10) ,
β
= 81.133(9) , γ= 79.449(10) ,
3
−1
−3
V = 6461(2) Å ,
µ
(Mo-K
α
) = 0.518 mm , D
c
= 1.001 g cm , red block with crystal size of 0.20
×
0.10
3
×
0.05 mm . Of a total of 127,498 reflections collected, 23,500 were independent (R = 0.0640). Final
int
◦
R1 = 0.0769 [I > 2σ(I)] and wR2 = 0.2353 (all data). Data completeness to theta = 25.24 , 99.5%.
Crystallographic data (including structure factors) for the crystal structure of compound Zn5·Solvent
have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication
No. CCDC-1825479. Copies of the data can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB2 2EZ, UK. FAX: (+44) 1223 336033. E-mail: deposit@ccdc.cam.ac.uk.
4
. Conclusions
The reaction of 5-(4-hydroxyphenyl)-10,15,20-triphenylporphyrin (
pentafluorophenyl)porphyrin ( ), under mild conditions affords a diporphyrin (
) selectively and in good yields. Similarly, the reaction of with Zn leads to the nono-Zn(II)
1
) with 5,10,15,20-tetrakis
(
(
2
3) or a pentaporphyrin
5
1
2
diporphyrin 4 or to the mono-Zn(II) pentaporphyrin 6.
Absorption and fluorescence titrations of the new porphyrin derivatives with C60 in toluene did
not show the formation of supramolecular complexes. However, titrations with PyC60 lead to the
formation of Zn(porphyrin)–PyC60 coordination complexes and the corresponding binding constants
4
−1
)
were calculated. Comparing the binding constants for the Zn
2
–PyC60 complex (Kav = 2.66
10 M ), it is evident that the four free-base porphyrin units in
pentaporphyrin 6 have a significant contribution in the stabilization of the 6–PyC complex.
×
10 M
5
−1
and the
6–PyC60 complex (Kav = 1.53
×
60
The method reported here for the synthesis of pentaporphyrins
5 and 6 may be useful to decorate
a central porphyrin with one to four structural units with specific functions. Considering that
porphyrin–fullerene systems are potentially useful for photovoltaic applications, this method may be
used to decorate a porphyrin with other photoactive units or substituents able to make strong binding
interactions with fullerenes (pyrene of corannulene units, for instance).
Supplementary Materials: The following are available online: H, 13C and 19F NMR spectra and absorption and
1
fluorescence titrations with PyC .
60
Author Contributions: Conceptualization, A.C.T.; synthesis of the porphyrin derivatives, J.I.T.C.; absorption and
fluorescence titrations, A.S.F.F.; single-crystal X-ray diffraction studies, F.A.A.P.
Funding: This research was funded by Fundaç
ão para a Ciência e a Tecnologia (FCT), through project
PTDC/QEQ-QOR/6160/2014 and the research units UID/QUI/00062/2019 and UID/CTM/50011/2019.
Acknowledgments: Thanks are due to the University of Aveiro and Fundaç o para a Ci ncia e a Tecnologia
ã
ê
(
FCT, Portugal) for the financial support to the project PTDC/QEQ-QOR/6160/2014 and the research units
UID/QUI/00062/2019 and UID/CTM/50011/2019 through national funds and where applicable co-financed by the
FEDER, within the PT2020 Partnership Agreement, and also to the Portuguese NMR Network.
Conflicts of Interest: The authors declare no conflict of interest.
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2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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