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40 / Noncovalent
Figure 67. The construction principle. The string consists of two
chelates (thick line) connected by a linker X. The terminus functions Y
have to be small enough to pass through the ring in order for the string
to thread the two porphyrin-appended macrocycles. The gathering and
threading process is governed by coordination of the four chelating units
of the system to the two metal centers (white circle).
Figure 68. The compounds used as the molecular string (123) and rings
(124) and the prototypical homoporphyrin threaded system (125) derived
Chambron et al.
127: Mn = Zn(ll); M2 = Au(III) : 8%
Figure 69. The threading of two different porphyrin rings (118 and 124)
onto molecular string 123 to generate the heteroporphyrinic complex 127,
in addition to the two homoporphyrinic complexes 125 and 126. All three
compounds can be separated by chromatography.
Figure. 70. Donor-acceptor multiporphyrin conjugate. The porphyrins
represented by white diamonds are electron donors in the excited state,
and the acceptor porphyrin is represented by a hatched diamond. Light
excitation of the donor porphyrins leads to intramolecular electron
transfer between mechanically linked molecular components.
W + I
Figure 71. Formation of prerotaxane 129 by transition-metal-directed
threading of macrocycle 118 onto phenanthroline 128.
40 / Noncovalent Multiporphyrin Assemblies
Figure 72. Sequence of reactions leading to -rotaxane 134. Prerotaxane
129 is used as a dialdehyde in the condensation reaction with aldehyde
130 and dipyrrylmelhane 131, which affords Cu(l)-complexed -rotaxane
132. After metallation of the porphyrin stoppers with Zn(ll), to afford
133, the template metal is removed with KCN, affording the free -
intramolecular electron transfer takes place, it will be impossible to
recognize a pathway involving a bond sequence. This principle is
illustrated in Figure 70.
The real molecule which we made according to the scheme of Figure 70,
as well as the sequence of reactions leading to it, are represented in
Figures 71 and 72.
The prerotaxane 129 is formed quantitatively by mixing the two organic
components 118 and 128, with a stoichiometric amount of Cu(CH',CN)4 1 .
Reaction of 129 with
aldehyde 130 and dipyrrylmethane 131 afforded 132 in moderate yield
(17%). Structure 132 could be metalated with zinc(II) and, finally, the
copper(I) template cation was removed by treatment of 133 with KCN.
Preliminary luminescence measurements on 134 indicate that electron
transfer takes place between (he singlet excited state of one of the
zinc(.11) porphyrins and the gold(IIl) porphyrin component. At room
temperature and in dichloromethane, the electron transfer occurs in a few
Chambron et al.
Figure 73. Representation of the self-assembly reaction of two
tetracationic porphyrins 135 with four macrocycles 136 to generate the
supramolecular assembly 137.
picoseconds, which is, as expected, slower than the similar reaction for
the bis-porphyrin conjugate where the D and A porphyrins are connected by
a 2,9-diphenyl-1,10-phenan-throline spacer.88
Another threaded system, namely a -rotaxane-like compound,116 has
been proposed as a model of the special pair (SP) of bacteriochlorophylls
found in the photosynthetic reaction centers of various photosynthetic
bacteria.71213 Two copper(II) porphyrins are brought to close proximity
in an interesting process, as represented in Figure 73.
Porphyrin 135 bears four ammonium groups that are able to interact
with macrocyclic polyethers. The 34-membered ring 136 can accommodate two
ammonium groups which will thread through the ring. The assembly process
leading to the bis-porphyrin utilizes four rings and two tetrafunctio-
nalized copper(II) porphyrins, to afford the 6-component edifice 137
represented in Figure 73. The X-ray structure analysis of 137 revealed an
elegant structure, with the four side arms of each porphyrin being
threaded through the cavities of four 136, each polyether ring
accommodating two dibenzyl ammonium fragments. The two porphyrins
interact via n-n stacking, with an average interplane distance of 3.65 A.
The Cu- • Ñè distance is much longer, due to the slipped situation
adopted by the porphyrinic nuclei.
In the course of the last decade, numerous multiporphyrin assemblies have
been constructed via noncovalent bonds. The main type of interaction used
is based on metal-ligand bonds, although purely organic systems such as
hydrogen bonds or aromatic n-n stacking interactions have also been
utilized. The levels of complexity and structural control have now