A glycyl-substituted porphyrin as a starting compound for the synthesis of a π-π-stacked porphyrin-fullerene dyad with a frozen geometry

Article abstract of DOI:10.1039/b820501a

A synthetic route for obtaining a porphyrin-fullerene dyad with a constrained geometry forcing the [60]fullerene sphere to lie on the porphyrin plane is reported, revealing a strong interaction in the ground state between the two chromophores. The Royal S

Full text of DOI:10.1039/b820501a

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A glycyl-substituted porphyrin as a starting compound for the synthesis of a
p–p-stacked porphyrin–fullerene dyad with a frozen geometry†
Angelo Lembo, Pietro Tagliatesta,* Daniel Cicero, Alessandro Leoni and Alisa Salvatori
Received 18th November 2008, Accepted 24th December 2008
First published as an Advance Article on the web 5th February 2009
DOI: 10.1039/b820501a
A synthetic route for obtaining a porphyrin–fullerene dyad with a constrained geometry forcing the
[
60]fullerene sphere to lie on the porphyrin plane is reported, revealing a strong interaction in the
ground state between the two chromophores.
Introduction
face-to-face geometry. Although the pac-man porphyrin–fullerene
systems are much more rigid than the porphyrin–fullerene
parachute and cyclophane systems, they still have conformational
freedom.
The peculiar photochemical and electrochemical properties of
porphyrins and fullerenes have been extensively investigated in
the recent past. The use of these two chromophores for studies
concerning photoinduced energy and electron-transfer reactions
has been reported in several papers. With the aim of finding
the best energy conditions, electronic interactions, and structural
and geometrical effects for obtaining fast charge transfer and a
longer lifetime for the excited radical ion pair, many porphyrin–
fullerene architectures have been assembled either covalently or
Here we wish to report on the synthesis of a new p–p
stacked porphyrin–fullerene dyad with a completely frozen geom-
etry similar to that found in co-crystallized porphyrin–fullerene
systems. The close proximity between the two chromophores
induces a ground state electronic interaction as shown by the
1
13
H and C-NMR and UV-vis studies. Moreover, the synthetic
strategy can allow the introduction of additional photoactive and
electroactive chromophores bearing an aldehyde group, suitable
for the cycloaddition reaction.
1
through non-covalent interactions e.g. hydrogen bonding, p–p
stacking or metal coordination. The huge number of ground state
non-interacting porphyrin–fullerene systems have shed light into
the kinetic processes of energy and/or electron-transfer between
these two chromophores. Fewer studies have been reported on
the photogenerated excited states in porphyrin–fullerene dyads
where a strong electronic interaction is present. It is possible
Results and discussion
The strategy used for obtaining the p-p stacked dyad is similar to
that adopted by Sakata and coworkers who used the ortho position
2
to find in the literature the preparations of many linear or
9
of a phenyl ring to link the fullerene through an amido group.
3
star-shaped multichromophoric systems composed of porphyrin
The synthesis of dyad 6 was achieved by following a synthetic
pathway for obtaining the new tetra-arylporphyrin derivative 4,
bearing in the ortho position of one of the four meso-phenyl rings
a glycyl unit, whose carboxylic group was protected as an ethyl
ester. The starting nitroporphyrin 2 was obtained in a two-step
process involving the synthesis of the 5-phenyldipyrromethane
and a subsequent condensation reaction with benzaldehyde and
photosensitizers and a fullerene electron-acceptor, while only a
few reports have been dedicated to the synthesis of dyads in
which the two chromophores present an interaction between
their respective p-electron systems. Although the stacking be-
tween the two entities is a spontaneous process under co-
4
5
crystallization conditions, it is necessary to maintain the two
units in close contact through covalent bonds in order to have
the possibility to study the kinetic aspects of the excited states in
solution.
Different geometries have been developed to give stacked
porphyrin–fullerene systems such as the porphyrin–fullerene
cyclophane systems synthesized by Diederich and Hirsch, the
porphyrin–fullerene parachute systems synthesized by Schuster
and coworkers, and more recently the porphyrin–fullerene pac-
man systems reported by D’Souza, Barbe and Fukuzumi. In
the first two systems cited above, the p-electronic interaction is
allowed thanks to the conformational flexibility of the aliphatic
branches that brings together the two subunits, while in the
last one a lower conformational freedom induces a constrained
10
2
-nitrobenzaldehyde under Lindsey’s reaction conditions. This
pathway gave the desired mononitroporphyrin in a good yield
22%). The subsequent reduction of the nitro group into the
(
amino group was carried out in a mixture of 1,4-dioxane and
hydrochloric acid in the presence of an excess (12 equivalents)
6
of SnCl
light. The final aminoporphyrin 3 was obtained in 80% yield
see Scheme 1).
The new porphyrin 4 was obtained in 56% yield by the
2
·2H
2
O, while protecting the reaction mixture from the
11
7
(
8
nucleophilic substitution of the amino group of the porphyrin 3 on
the iodine of the ethyl iodoacetate, carried out in anhydrous DMF.
The glycyl porphyrin ethyl ester derivative 4 was characterized by
FAB mass spectrometry, which shows a molecular peak at m/z
1
7
15, and H-NMR analysis, in which the signals due to the ethyl
Dipartimento di Scienze e Tecnologie Chimiche, University of Rome-Tor
Vergata, Via della Ricerca Scientifica, 00133, Rome, Italy. E-mail: pietro.
tagliatesta@uniroma2.it; Fax: +39-06-72594754
ester glycine moiety are clearly visible: a 3H triplet at 1.11 ppm
and a 2H quartet at 4.03 ppm due to the methyl and methylene of
the ester group respectively, and a (not well defined) 2H doublet
at 3.84 ppm arising from the methylene group in the position a
†
Electronic supplementary information (ESI) available: Experimental
conditions and characterisations. See DOI: 10.1039/b820501a
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Scheme 1 Synthesis of aminoporphyrin 3.
Scheme 2 Synthetic pathway to dyad 6.
to the amino group of the glycine. The saponification of the ester
group (KOH in THF/H O) affords, almost quantitatively, the final
porphyrin 5, which was directly used for the Prato condensation
fullerene sphere was covalently forced to lie on the porphyrin plane,
2
the signals of b-pyrrole protons are strongly perturbed. In fact, it
1
is evident that in the H-NMR spectrum of dyad 6, these signals
12
reaction with benzaldehyde and fullerene to give the dyad 6 in
increased in number, becoming much more broadened and shifted
towards lower and higher fields, clear evidence of the electronic
perturbation induced by the fullerene electron density (see Fig. 1).
The protons trapped between the porphyrin macrocycle and the
curved surface of the fullerene show the signals of the pyrrolidine
shifted both down-field and up-field. In particular, the singlet
arising from the proton of the phenyl-substituted methyne group
of the pyrrolidine ring is shifted down-field by about 0.5 ppm
compared to the same signal in the reference compound 7 (N-
methyl-2-phenyl-3,4-fulleropyrrolidine), while the two doublets
are shifted up-field with an average shift of 1 ppm, as is clearly
visible in Fig. 2.
1
2% yield (see Scheme 2).
Although an excess of benzaldehyde was used in order to force
the complete conversion of porphyrin 5 into the final porphyrin–
fullerene dyad, the yield was poor. This is probably due to the steric
hindrance of the porphyrin macrocycle, which partially prevents
the cycloaddition of the incoming ylide to the fullerene cage.
However, it is common to obtain quite low yields when hindered
porphyrins are used to synthesize porphyrin–fullerene dyads with
4f,8,13
+
a constrained geometry.
The FAB mass analysis of dyad 6
reveals a cluster peak at m/z 1451 corresponding to the molecular
mass of the dyad and a base peak at m/z 731 due to the loss of the
entire fullerene sphere arising from the cleavage of the carbon–
Moreover, the electronic perturbation induced by the fullerene
also affects the resonance of the porphyrin NH protons, which lie
at at 2.65 ppm in the dyad 6 and at 2.59 ppm in the porphyrin 4.
1
carbon bonds of the pyrrolidine ring. Moreover, the H-NMR
spectrum of compound 6 shows the presence of the proton signals
due to the pyrrolidine ring: a 1H singlet at 5.48 ppm due to the
phenyl-substituted methyne group of pyrrolidine ring, and two 1H
doublets at 3.79 and 3.40 ppm due to the methylene group of the
pyrrolidine ring.
1
In order to verify the degrees of freedom of the dyad, a second H-
◦
NMR spectrum was recorded at 45 C. The only result was that
the peaks were much more broadened, with a further loss of signal
definition, an additional confirmation of the frozen geometry of
1
The electronic interaction between the p-electron system of
the dyad 6 (see ESI†). The H-NMR analysis provides evidence
1
13
porphyrin and fullerene is clearly evident from H and C-
for how the p-electron density of C60 induces a considerable
alteration of all the porphyrin proton resonances, including the
meso-phenyl protons, due to the close distance between the two
1
NMR and UV-vis measurements. In the H-NMR pattern of
the b-pyrrole protons of porphyrin 4, it is possible to see a
simple system composed of one singlet at 8.94 ppm and two
doublets at 9.00 and 8.95 ppm, one of them coalescing with
the singlet signal. This pattern provides evidence for a pseudo-
1
3
chromophores. Comparing the C NMR spectrum of N-methyl-2-
phenyl-3,4-fulleropyrrolidine with that of the porphyrin–fullerene
dyad (see ESI†), it is possible to see two very different patterns.
At a first sight the spectrum of the dyad 6 has a higher number
C
2
symmetry of the porphyrin macrocycle. However, when the
1
094 | Org. Biomol. Chem., 2009, 7, 1093–1096
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the porphyrin electron density exerted on the few carbons of the
fullerene surface stacked on the porphyrin plane. In particular the
fullerene signals at 156.98, 154.80 and 154.12 ppm in the spectrum
of 7 disappear in the spectrum of 6, moving towards higher field.
A similar trend is evident also for the aliphatic carbons of the
pyrrolidine ring and the functionalized six-membered ring of the
fullerene, which reside between 70 and 85 ppm for 7 and between 65
and 75 ppm for 6. The UV-vis spectra recorded in toluene solution
further underline the interaction between the two units. In Fig. 3
a comparison of the absorption spectra of compounds 6 and 7
with that of the tetraphenylporphyrin is reported. The Soret band
of the porphyrin in the dyad 6 is red-shifted by 8 nm compared
to that of the tetraphenylporphyrin, and a similar trend is evident
also for the Q band, red-shifted by about 6 nm. Furthermore
the molar extinction coefficient of the Soret band in the dyad is
-
1
-1
lower (265000 M cm ) than that of the tetraphenylporphyrin
-
1
-1
(
383000 M cm ), providing evidence for a partial migration of
the electron density from the tetrapyrrole ring toward the fullerene
sphere.
1
Fig. 1 H-NMR b-pyrrole proton pattern of (top) porphyrin 4 and
(
bottom) dyad 6.
Fig. 3 Absorption spectra in toluene solution of compounds 6, 7 and
tetraphenylporphyrin. All the compounds have a concentration of 5.5 ¥
-6
1
0 M. In the inset the Q-band pattern is shown.
The close proximity of the two chromophores results in a
considerable electronic interaction in the ground state, producing a
partial charge-transfer electronic state with an absorption band at
7
16 nm, red-shifted by about 12 nm with respect to the absorption
band of the fullerene reference compound 7 located at 704 nm
(
see Fig. 4). These findings are in line with those reported for
4
b,c,f,8
other p–p-stacked porphyrin–fullerene dyads.
The absorption
1
Fig. 2 H-NMR pyrrolidine proton pattern of (top) the N-methyl-
2
band of the charge transfer state was obtained by subtracting the
absorption of the tetraphenylporphyrin and that of 7 from the
absorption of the dyad 6. All the data reported here confirm
the existence of a strong electronic interaction in the electronic
ground state between the porphyrin and fullerene due to the
short distance between their respective p-electron systems. This
interaction is induced by the almost completely frozen geometry of
the dyad, as also revealed by the geometric optimization performed
at the MM+ level. The optimized geometry reported in Fig. 5
shows the fullerene sphere restricted on the tetrapyrrole ring at
-phenyl-3,4-fulleropyrrolidine 7 and (bottom) dyad 6.
of signals, some of them not well resolved, while in the spectrum
of the reference compound 7 the signals arising from the fullerene
sphere, lying between 135 and 160 ppm, are clearly visible. The
signals around 130 ppm are ascribable to the phenyl group.
There is no resemblance between the two spectra, and this fact
further underlines the strong electronic interaction occurring
between porphyrin and fullerene. Such strong interaction modifies
completely the resonances of the fullerene carbons in the dyad
˚
an average distance of 3 A, corresponding to that found in the
5a
6
. Nevertheless it is possible to discern the shielding effect of
co-crystallites. .
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5
6
Fig. 5 Optimized geometry of dyad 6 represented using the van der Waals
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In conclusion, in this paper we have presented the use of a new
glycyl-substituted porphyrin for the synthesis of a porphyrin–
fullerene dyad with an almost completely frozen geometry,
similar to that found in the co-crystallite systems. In order to
favour electronic interaction in the ground state between the two
chromophores, they were linked so as a result in a short distance
between them. The introduction of the glycine moiety on the
tetrapyrrole ring offers the opportunity to link different electro-
and/or photoactive groups such as ferrocene, tetrathiafulvalene
and many others bearing an aldehyde group, useful for cycload-
dition reactions, with the possibility of constructing a compact
triad in order to evaluate multiple electronic interactions in the
electronic ground state.
7
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