Electron transfer between mechanically linked porphyrins in a [2]rotaxane

Full text of DOI:10.1021/ja972413e

J. Am. Chem. Soc. 1997, 119, 11329-11330
11329
Electron Transfer between Mechanically Linked
Porphyrins in a [2]Rotaxane
Myriam Linke, Jean-Claude Chambron, Vale´rie Heitz, and
Jean-Pierre Sauvage*
Laboratoire de Chimie Organo-Mine´rale
URA 422 au C.N.R.S., Institut Le Bel
UniVersite´ Louis Pasteur, 4, rue Blaise Pascal
67070 Strasbourg, France
Figure 1. Donor-acceptor multiporphyrin conjugates. The donor
porphyrin (white diamond) and the acceptor porphyrin (hatched
diamond) are mechanically linked in either rotaxane (a) or catenane
structure (b).
ReceiVed July 18, 1997
Charge separation is a key process of natural photosynthe-
sis,1,2 and many synthetic systems have been proposed as
functional mimics of the photosynthetic reaction center based
on transition metal complexes,3-5 organic chromophores,6,7 or
porphyrins.8-12 An interesting question is related to the intimate
mechanism of electron transfer (ET), with the now classical
distinction between “through-bond” and “through-space” ET
steps.13-15 If connection between the various electron donors
(D) and acceptors (A) have almost exclusively been realized
using covalent bonds, a few examples of so-called “self-
assembled” systems, mostly based on hydrogen-bonding but also
on ionic interactions (salt-bridge-mediated systems), have
recently been proposed.16-19 In order to suppress any classical
bond and, in particular, to make impossible the recognition of
an electron transfer pathway involving a bond sequence, we
designed multicomponent systems in which D and A are
maintained in the same molecule by mechanical (or topological)
bonds only20-22 (Figure 1).
We now report the synthesis of a [2]rotaxane (Figure 1a)
whose ring is attached to a pendent porphyrin (A) and whose
stoppering groups are electron donor porphyrins (D). Prelimi-
nary luminescence measurements show that ET between D
(singlet excited state) and A is likely to take place and is a fast
process (<1 ns).
(1) Deisenhofer, J.; Epp, O.; Miki, K.; Huber, R.; Michel, H. J. Mol.
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C.; Balzani, V.; Barigelletti, F.; De Cola, L.; Flamigni, L. Chem. ReV. 1994,
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Chem. Soc. 1993, 115, 6109-6114.
Figure 2. Sequence of reactions leading to [2]rotaxane 7+ and the
various metal-complexed [2]rotaxanes [M‚7]2+. The black disk is Cu+
in [Cu‚6]2+ and in [Cu‚7]2+, Ag+ in [Ag‚7]2+, and Li+ in [Li‚7]+. Also
indicated are protons 5 and 6 of the phenanthroline nucleus incorporated
into the macrocycle.
(11) Harriman, A.; Odobel, F.; Sauvage, J.-P. J. Am. Chem. Soc. 1995,
117, 9461-9472.
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(13) Winkler, J. R.; Gray, H. B. Chem. ReV. 1992, 92, 369-379.
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J. H.; Chang, I.-J.; Winkler, J. R.; Gray, H. B. J. Am. Chem. Soc. 1993,
115, 1485-1489.
Rotaxane 7+ (Figure 2) is composed of the following two
components: a macrocycle incorporating a 2,9-diphenyl-1,10-
phenanthroline (dpp) chelate and bearing a pendent gold(III)
porphyrin, and a dumbbell-shaped molecular thread incorporat-
ing the same dpp chelate and stoppered by two identical bulky
zinc(II) porphyrins. It was synthesized by using an approach
derived from the transition metal template strategy developed
for making catenanes, rotaxanes, and knots in our laboratory.22-24
The precursors are shown in Chart 1.
(16) Sessler, J. L.; Wang, B.; Harriman, A. J. Am. Chem. Soc. 1993,
115, 10418-10419.
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Soc. 1997, 119, 4929-4933.
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117, 8051-8052.
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(21) Amabilino, D.; Sauvage, J.-P. Chem.Commun. 1996, 2441-2442.
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prehensiVe Supramolecular Chemistry; Pergamon: Oxford, 1996; Vol. 9,
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115, 12378-12384.
S0002-7863(97)02413-X CCC: $14.00 © 1997 American Chemical Society

11330 J. Am. Chem. Soc., Vol. 119, No. 46, 1997
Communications to the Editor
Chart 1
All four rotaxanes described in this paper involve two zinc-
(II) porphyrins, which are good electron donors in their singlet
excited state, and a gold(III) porphyrin, which is the partner of
choice for its electron-accepting properties.33 Remarkably, the
components incorporating these chromophores and electrophores
are either connected by metal-ligand bonds (when M ) Cu+,
Ag+, Li+) or not connected chemically, although held together
by a mechanical link. We had observed earlier by transient
absorption spectroscopy electron transfer from a zinc porphyrin
connected to a gold(III) porphyrin by a dpp spacer. As expected
from thermodynamic data, energy transfer could experimentally
be ruled out unambiguously.34 In addition, the rate of electron
transfer was strongly enhanced by coordination of the spacer
to copper(I).10
The [2]rotaxanes described here now allow for the study of
electron transfer through metal-ligand bonds (in [M‚7]2+ where
M ) Cu+, Ag+ or Li+) or through space (in 7+). Preliminary
results were obtained from steady state luminescence measure-
ments. For all compounds, strong quenching of the lumines-
cence of the zinc porphyrin components is observed. The
quenching mechanism in [Li‚7]2+ is likely to be ET, whereas
for [Cu‚7]2+ and [Ag‚7]2+ the mechanism is less straightforward,
since the central complex fragments could more or less
participate in the quenching processes. For free rotaxane 7+,
which involves only zinc(II) and gold(III) porphyrins as electron
transfer partners, the situation is not as ambiguous as for the
other compounds, owing to the lack of any additional electro-
active component. The fluorescence of the zinc porphyrin
subunits of [2]rotaxane 7+ is decreased by 87%. As supported
by previous studies,10,34 this strong quenching is very likely to
be due to electron transfer from a zinc porphyrin stopper in its
singlet excited state to the gold porphyrin cation attached to
the ring. The electron transfer rate can be roughly estimated
from the present steady state luminescence measurements and
the known singlet excited state lifetime of 5,15-bis[3,5-bis(1,1-
dimethylethyl)phenyl]-2,8,12,18-tetrahexyl-3,7,13,17-tetramethyl-
21H,23H-porphine,35 according to eq 1:
Macrocycle 1+,21 dialdehyde 2,25 and Cu(CH3CN)4PF6 were
mixed in CH2Cl2/CH3CN, forming prerotaxane [Cu‚5]2+ quan-
titatively. This complex was subsequently reacted with 2 equiv
of aldehyde 326 and 4 equiv of dipyrrylmethane 427-30 in
dichloromethane acidified with CF3COOH31 (Figure 2). After
the reaction was stirred overnight, oxidation of the porphyrino-
gen intermediates with chloranil and chromatographic separation
provided copper(I)-complexed [2]rotaxane [Cu‚6]2+ 17% iso-
lated yield. This corresponds to an average yield of single
porphyrin formation of 41%, since two porphyrins are formed
simultaneously in the same molecule. The free-base porphyrins
were metalated with zinc(II) by reaction of [2]rotaxane [Cu‚6]2+
with Zn(OAc)2‚2H2O in a refluxing mixture of CHCl3 and CH3-
OH, and copper(I)-complexed [2]rotaxane [Cu‚7]2+ was ob-
tained in 82% yield after chromatography. The template cation
was finally removed, by treatment of [Cu‚7]2+ with KCN in
CH3CN/CH2Cl2/H2O mixtures. The reaction proved sluggish
and did not go farther than 35%. It was thus necessary to
separate the rotaxane liberated (i.e., 7+) using chromatography
and to subject the starting material left to fresh KCN/CH3CN/
CH2Cl2/H2O mixtures. This process allowed finally to free all
the complexed [2]rotaxane [Cu‚7]2+ from Cu(I). No macrocycle
unthreading was observed, proving the very rotaxane nature of
7+.
In both compounds, complexed [Cu‚7]2+ and metal-free 7+
[2]rotaxanes, the two porphyrinic stoppers of the dumbbell
sandwich the phenanthroline subunit belonging to the macro-
cycle. This is clearly established by 1H NMR spectroscopy. A
0.4 ppm upfield shift of protons 5 and 6 of this subunit in
[Cu‚7]2+ is induced as compared to prerotaxane [Cu‚5]2+. For
[2]rotaxane 7+, the shielding is even stronger: the chemical
shift difference is as large as 3 ppm! This suggests that
following the demetalation process the dumbbell component has
moved toward the gold(III) porphyrin of the macrocycle,
allowing the zinc(II) porphyrins to clamp more tightly the
phenanthrolinic subunit of the macrocycle.
kET ) (1/τ0)(I0/I - 1)
(1)
In this equation, I is the emission intensity of the [2]rotaxane
and I0 is that of the reference porphyrin. With τ0 ) 1.94 ns,
the value found for kET is 3.5 × 109 s-1, which is much slower
than the rate observed for the bis-porphyrin conjugate where
the zinc and the gold porphyrins are connected by a dpp spacer.34
This is consistent with the lack of any chemical bond between
the zinc(II) and the gold(III) porphyrins in [2]rotaxane 7+.
Transient absorption spectroscopy studies are underway and will
be reported in due course.
A rich coordination chemistry could be developed at the bis-
dpp tetrahedral site left by the metal template. [2]Rotaxane 7+
could be metalated with monocations Li+ and Ag+.32 The
insertion of Ag+ (using AgBF4 in CH3CN/CH2Cl2) afforded
[Ag‚7]2+ cleanly and quantitatively. The reaction with LiBF4
in CH2Cl2/MeOH produced [Li‚7]2+ which was not purified by
chromatography, due to its expected poor stability. Neverthe-
Acknowledgment. Dedicated to Professor Dieter Seebach on the
occasion of his 60th birthday. M. L. thanks the Ministe`re de
l’Enseignement Supe´rieur et de la Recherche for a grant. We are grateful
to Jean-Daniel Sauer for high-field NMR experiments and Raymond
Huber for FAB mass spectra. We also thank the European Communities
for financial support.
less, the purity of [Li‚7]2+ could be estimated to 90% by H
NMR.
1
(25) Note: dialdehyde 2 was obtained in 47% yield by reaction of 1,
10-phenanthroline (1 equiv) with the p-lithio-derivative of benzaldehyde
neopentylacetal (2.5 equiv) in THF, hydrolysis, and MnO2 oxidation of the
intermediate.
(26) Chardon-Noblat, S.; Sauvage, J.-P. Tetrahedron 1991, 47, 5123-
5132.
Supporting Information Available: Spectral data and characteriza-
tion for 7+ (5 pages). See any current masthead page for ordering and
Internet access instructions.
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JA972413E
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