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do not seem to interact.
The porphyrins were also anchored to chelate ligands like
lerpyridyl*2-*' or phenanthroline.*4 s<' In the first ease, (4-
phenyl)terpyridyl occupied one meso position of a porphyrin. Two such
porphyrins could be assembled at an octahedral Ru(II) center in a bis-
chelate complex. Owing lo the kinetic inertness of Ru(ll), the porphyrin
ligands could be introduced stepwise; this allowed, in particular,
assembly of two different porphyrins, a Au(Ill) porphyrin and a free-base
porphyrin, the latter being subsequently metalated with
Figure 45. Square-planar porphyrin tetramer (84) obtained by coordination
of four porphyrins 45 to a Pd'' metal center.
Chambron et al.
Figure 46. Subunits of porphyrin-containing dendrimers (see 87 and 88 of
Figures 47 and 48) built on Pd-pincer complex metal fragments. 85 is the
core, and 86 is a peripheral unit.
Figure 47. Porphyrin-containing dendrimer (87) based on core 85 and
porphyrin 45 as peripheral unit.
40 / Noncovalent
PhS-Pci - SPh
ê X J
[ hn ó "spi, /
n,.s -1-41 SPil , N.
PUS - P<1 - SPh
w,-/'N "N Võ-
Figure 48. Porphyrin-containing dendrimer (88) based on core 85 and
peripheral unit 86.
Figure 49. A Zn(ll)/Au(lll) bis-porphyrin
assembly (89) held by a Ru(ll) bis-terpyridine complex fragment.
Zn(ll) (Figure 49). The resulting system (89) showed remarkable electron
transfer properties.Excitation of the Zn(ll) porphyrin was followed by
electron transfer to the Ru(ll) center in 50 ps. The reduced |Ru(terpy)^-
| complex fragment was oxidized subsequently by the Au(III) porphyrin
cationic center, a relatively good electron acceptor, in 1.6 ns,
affording a charge-separated slate in which the Zn(fI)-porphyrin w'as a
cation radical and the
Au(IIl) porphyrin a neutral radical. The diradical species had a
relatively long lifetime of 33 ns.
Another system, in which the porphyrins arc assembled via chelation of
a pendant ligand to a transition metal, is shown in Figure 50.s7 Bis-
porphyrin conjugates bridged hy a 2,9-diphenyl-1,10-phenanthroline (dpp)
chelate like 90 were assembled at a Cu(I) center, which forms extremely
stable complexes with dpp-based ligands. The resulting
Chambron et al.
Ar = <i
Figure 50. A covalent bis-porphyrin dimer bridged by a 2,9-diphenyl-1,10-
phenanthroline chelate (90) and the dimer (91) of its Zn(ll)/ Au(lll)
complex formed by coordination to a Cu+ center.
molecular structure is a complex in which two covalent porphyrin dimers
are oriented at right angles to each other in such a way that they
encapsulate the Cu(I) atom (91). The bis-porphyrin conjugate 49 bridged
by a Troger's base derivative,64 which dimerizes around a tetramine
template is reminiscent of the present structure, assembled around a
transition metal. When the bis-porphyrin contained two different metals-
Zn(II) in one porphyrin and Au(III) in the
other-photoinduced electron transfer from the Zn(II) porphyrin fragment
to the Au(III) porphyrin fragment was shown to take place. The rate was
extremely high [(3 ps) - 1 j, and was explained in terms of the
superexchange effect. It was nearly 20 times faster than the rate
observed in the case of the Zn(II)/Au(III) bis-porphyrin conjugate
bridged covalently by a dpp chelate.88
Linear dimers of perpendicularly arranged porphyrins can also be
assembled at a tetrahedral metal center such as Zn(II) or Cu(I), using
ditopic, phenanthroline chelates, as shown by Crossley and coworkers.86
The case of Cu(I) (92), is shown in Figure 51. The phenanthroline ligands
are connected to the tetrapyrrolic macrocycles via fused aromatic rings,
producing ribbon-like molecules.
2. Metallomacrocydes (Closed Structures)
In this section we describe porphyrin aggregates based on