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pendant pyridyl substituent was, in fact, systematically investigated by
Hunter and coworkers47-4S who designed and synthesized a series of
porphyrins bearing a pyridyl group connected via arylamide spacers to one
meso position of the macrocycle, such that the orientation of the pendant
ligand with respect to the plane of the porphyrin was perfectly
controlled. As shown in Figure 11, a 90° angle will favor the dimer, a
120° angle the trimer, and a 180° angle the tetramer arrangement. The
corresponding molecules, respectively 31, 32 and 33 are shown in Figure
19. The orientations of the pendant pyridyl groups are controlled by atom
connectivity and intramolecular hydro-gen-bonding interactions. The
cyclic aggregates formed selectively, with high association constants
owing to the high preorganization of the systems: 108mol-lL for the dimer
(34), 5 x 10l2mol-2L2 for the trimer (35), and nearly 1013 mol - 3L3 for
the tetramer (36). In the case of the
Figure 16. Schematic representation (24) of the
dimerization in solution of the bis-porphyrin conjugate 23.
Chambron et al.
Figure 17. Zn(ll) porphyrin 25 forms zig-zag polymeric aggregates 27,
which upon light irradiation are converted into discrete dimers (28) of
Zn(ll) porphyrin 26. For the sake of clarity, the porphyrin meso
substituents have been omitted in the aggregate structures.
Figure 18. Zn(ll) porphyrin 29 forms polymeric zig-zag aggregates (30) in
the solid state. For clarity, phenyl meso substituents have been omitted
in the polymer structure.
trimer (35), the Soret band is split by 9 nm, with attenuation of the
molar extinction coefficient by a factor of two. As mentioned above, this
is characteristic of an exciton-coupling interaction. However, in this
case, the porphyrins, although close in space, are not coplanar. Another
remarkable property of these aggregates is the fact that they
encompass large H-bonding cavities. The dimer (34) was shown to be able
to "host" terephthalamide with a binding constant of 1,400 mol - 'L.47
This host-guest compound is the counterpart of what was observed for some
hydrogen-bonded assemblies, where the host was built on hydrogen bonds
and the guest held by metal-ligand interactions.
It is remarkable that no polymeric systems were observed for 33. In
discussing this point, Hunter and coworkers48 suggested that in the case
of the 4-pyridyl-me.vo-substituted porphyrin 29 of Fleischer et al.,46
there was an equilibrium between a cyclic tetramer and a polymeric
species, the latter being predominant at room temperature or higher. A
definitive answer to this issue will be provided below.
Metals other than Zn(II) or Mg(II) have been used to induce porphyrin
self-aggregation. In an early work, directly related to what has been
discussed above, Ogoshi and coworkers49 prepared porphyrins bearing 8-
quinolyl and 2-pyridyl ligands anchored at the opposite 5- and 15-meso
positions, in order to maximize preorganization of the monomers.
Dimerization was observed to take place for [Rh(III)Cl] metalloporphyrin
complexes, but not for Zn(II) or [Fe(III)Cl] derivatives. Dimers of
porphyrins containing Fe(III) were described by Goff and Scheidt, who
made and characterized Fe(III)50 or Mn(III) TPP51 analogues in which one
wesoaryl substituent was an ortho phenol. As shown in Figure 20 for
Mn(III), in basic solution the phenolate generated on one porphyrin (37)
is able to coordinate the central metal cation of another porphyrin,
producing the dimer 38. The X-ray crystal structure obtained for the
Fe(III) complex shows that there is a poor overlap between the
porphyrins, which lie 4.09 A apart. Favorable stability of the double
bridge and steric constraints apparently preclude formation of polymeric
units, or /(-oxî bridges. The dimers can be reversibly broken into
monomeric units by addition of a protic acid.
40 / Noncovalent Multiporphyrin Assemblies
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