Dynamic cis/trans isomerisation in a porphyrin-fullerene conjugate

Article abstract of DOI:10.1039/a803434f

A porphyrin with two fullerene substituents is prepared by condensation of a C₆₀ aldehyde derivative and dipyrrol-2-yl methane and by reaction of a preconstructed porphyrin with C₆₀ itself; it is obtained as a mixture of two conforme

Full text of DOI:10.1039/a803434f

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Dynamic cis/trans isomerisation in a porphyrin–fullerene conjugate
Jean-François Nierengarten,*† Laurence Oswald and Jean-François Nicoud
Groupe des Mate´riaux Organiques, Institut de Physique et Chimie des Mate´riaux de Strasbourg, Universite´ Louis Pasteur and
CNRS, 23 rue du Loess, F-67037 Strasbourg, France
A porphyrin with two fullerene substituents is prepared by
condensation of a C60 aldehyde derivative and dipyrrol-2-yl
methane and by reaction of a preconstructed porphyrin with
C60 itself; it is obtained as a mixture of two conformers in
slow equilibrium, as shown by a variable temperature NMR
study.
carbon sphere with bismalonate derivatives developed by
Diederich and co-workers.6 Treatment of C60 with 8, I2 and
DBU in toluene at room temperature afforded a mixture of
compounds from which the desired ZnII–porphyrin 2 could be
subsequently isolated in 2% yield by tedious chromatographic
separations. This poor yield could be easily explained by a low
selectivity of the intramolecular addition to C60 due to
competition between the different available reactive malonic
units on the porphyrin core after its first intermolecular reaction
with C60.
All of the spectroscopic studies and elemental analysis results
were consistent with the proposed molecular structures.‡ The
1H NMR spectrum of 1 at room temperature showed the
presence of two conformers in a 1:1 ratio (Fig. 1). Molecular
modelling studies on compound 1 revealed that each fullerene
group is located to one side of the plane of its bridging phenyl
ring. Therefore due to the high barrier to rotation of the phenyl
substituents on the porphyrin, two conformers are possible for
1. The two carbon spheres in 1 can be in either a cis or trans
relative orientation (Fig. 1). Whereas the two porphyrin meso-
Since the first reported preparation of a C60-linked porphyrin by
Gust,1 several other fullerene–porphyrin hybrids have been
described.2 Intramolecular processes such as electron and
energy transfer have been observed in some of these compounds
and C60 appears as a particularly interesting electron acceptor in
photochemical molecular devices because of its symmetrical
shape, large size and the properties of its p-electron system.3
Concerning their synthesis, all the previously reported por-
phyrin–C60 hybrids have been prepared by reaction of a
preconstructed porphyrin with C60 itself or a C60 acid
derivative.2 We have recently shown that a porphyrin could be
constructed starting from a C60 aldehyde derivative and
pyrrole.4 As a part of this research, we now report the synthesis
of the novel porphyrin 1 with two C60 groups directly attached
to the core using our methodology. Furthermore, the corre-
sponding ZnII–porphyrin 2 has been prepared by metalation of
1, but also by reaction of a preconstructed ZnII–porphyrin with
C60 itself. Interestingly, both 1 and 2 have been obtained as a
mixture of two conformers and an NMR study reveals a
previously unreported type of dynamic cis/trans isomerisa-
tion.
CHO
O
O
O
O
RO
O
O
O
+
OR
The synthesis of porphyrin 1 and its ZnII complex 2 is
depicted in Scheme 1. Aldehyde 3,4 compound 44 and dipyrrol-
2-yl methane 55 were prepared according to previously reported
methods. The condensation of 3 and 5 was performed in CHCl3
at room temperature in the presence of BF3·Et2O. After 12 h,
p-chloranil was added to irreversibly convert the porphyrinogen
to the porphyrin. The desired porphyrin 1 was isolated in 38%
yield and subsequent metalation with Zn(OAc)2 gave 2 in 97%
yield. Compound 2 could also be prepared starting from 4:
deprotection (TFA, CH2Cl2, H2O), subsequent condensation of
the resulting aldehyde 6 with 5 in CHCl3 with BF3·Et2O as
catalyst and p-chloranil oxidation yielded porphyrin 7, which
was metalated with Zn(OAc)2 to give 8. Fullerene incorporation
was based on the regioselective macrocyclisation reaction of the
O
Z
N
H
N
H
5
i
RO
3 R = C12H25
OR
O
ii
OR
1
2
RO
O
iii
O
O
RO
OR
O
O
O
O
O
4 Z =
, R = C12H25
O
6 Z = CHO, R = C12H25
iv
RO
O
O
OR
O
O
O
O
O
O
O
OR
N
N
N
N
RO
RO
OR
OR
M
RO
O
O
O
O
OR
O
O
O
O
RO
O
O
OR
O
7 M = 2H, R = C12H25
8 M = Zn, R = C12H25
O
v
O
OR
O
O
N
N
N
N
vi
M
O
O
2
RO
O
O
O
O
Scheme 1 Reagents and conditions: i, CHCl3, BF3·Et2O, room temp., 12 h,
then p-chloranil, reflux, 2 h, 38%; ii, Zn(OAc)2, CHCl3, MeOH, reflux, 2 h,
97%; iii, TFA, CH2Cl2, H2O, room temp., 5 h, 80%; iv, 5, CHCl3, BF3·Et2O,
room temp., 12 h, then p-chloranil, reflux, 2 h, 47%; v, Zn(OAc)2, CHCl3,
MeOH, reflux, 3 h, 92%; vi, C60, DBU, I2, toluene, room temp., 12 h,
2%
O
O
RO
OR
1 M = 2H, R = C12H25
2 M = Zn, R = C12H25
RO
Chem. Commun., 1998
1545

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of heating, however the observed reversible narrowing of all the
peaks unambiguously shows that a dynamic effect does occur.
This cis/trans isomerism has also been seen for compound 2 and
can be related to the diastereoisomerism observed for some
terphenyl systems.8 Whereas atropisomerism in bis(o-substi-
tuted phenyl) porphyrins is well known, the conformational
isomerism observed for 1 and 2 is, to the best of our knowledge,
the first example of a bis(m-substituted phenyl) porphyrin for
which the barrier to free rotation is high enough at room
temperature to be able to distinguish the cis and trans
conformers.
Preliminary luminescence measurements show a strong
quenching of the porphyrin emission in 1 and 2 and their
photophysical properties are currently under investigation.
We thank the CNRS for financial support, C. Bourgogne for
his help with the molecular modelling, Hoechst AG for samples
of C60 and Professor F. Diederich (Zu¨rich, Swiss) for his interest
in our work and his support.
H-Ar
ArOCH2
+
Porph-CH2
Solvent
ArCH2O
ArH
Notes and References
† E-mail: niereng@michelangelo.u-strasbg.fr
‡ Selected data for 1: lmax(CH2Cl2)/nm (e/dm3 mol21 cm21) 259
(275 580), 317 (96 370), 382 (sh, 133 130), 408 (284 740), 506 (27 000),
539 (10 580), 573 (11 050), 624 (2680); nmax(CHCl3)/cm21 1748 (CNO);
m/z (ESI) 4360 (M+) (Calc. for C304H270N4O24: C, 83.68; H, 6.24; N, 1.28.
Found: C, 83.79; H, 6.29; N, 1.27%).
(a)
Porph-Me
meso H
1 P. A. Liddell, J. P. Sumida, A. N. Macpherson, L. Noss, G. R. Seely,
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(b)
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10
9
8
7
6
5
4
3
dH
1
Fig. 1 H NMR spectra of 1 (CDCl2CDCl2, 400 MHz) at (a) 125 and (b)
25 °C, and molecular models of the two conformers of compound 1 (the
four didodecyloxyphenyl groups have been omitted for clarity)
3 H. Imahori and Y. Sakata, Adv. Mater., 1997, 9, 537.
4 J.-F. Nierengarten, C. Schall and J.-F. Nicoud, Angew. Chem., in the
press.
Hs are equivalent in the trans conformer, they are non-
equivalent in the cis one. As expected, three singlets in a 1 :2:1
ratio are observed at d 10.20, 10.23 and 10.27, respectively, for
the meso-Hs in the 1:1 mixture of conformers. Furthermore, the
four methyl groups on the porphyrin form equivalent pairs in
both conformers and the expected four singlets are clearly
observed (d 2.60, 2.63, 2.64 and 2.67) in the 1H NMR spectrum.
A variable-temperature NMR study showed a clear coalescence
at 125 °C (Fig. 1). By monitoring the coalescence of the
porphyrin meso-Hs, the free energy barrier for the conforma-
tional equilibrium was calculated as DG‡ = 85 kJ mol21.7 A
sharp symmetric spectrum could not be obtained below the limit
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Compounds, Wiley, New York, 1994.
Received in Cambridge, UK, 7th May 1998; 8/03434F
1546
Chem. Commun., 1998