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the number of porphyrin subunits included in the molecular aggregate.
Metal-ligand bonds cover a large range of energies depending on the
nature of the metal and the ligand. Many porphyrin assemblies are based
on the Zn(II)-amine (pyridine, imidazole) interaction, the corresponding
association constant being given by Ê., = 103 mol - 'L.34 The other main
class of multiporphyrin assemblies uses pyridine-transition metal bonds.
The metals involved are principally second- and third-row transition
metals like Re(I), Ru(II), Os(II), Pd(II) or Pt(II). They normally form
thermodynamically very stable and kinetically inert coordination
complexes with nitrogen-containing ligands. However, aggregates based on
first-row transition metals like Fe(IIl) and Mn(lII) or Mn(IV) and
oxygen-containing ligands have also been prepared, and will be described
here as well.
A. METALS INSIDE PORPHYRINS INVOLVED IN ASSEMBLY PROCESSES
1. Ligand Appended to Porphyrins
The principle for construction of aggregates based on these features is
schematically represented in Figure 11. Here true oligomerization
processes such as dimerization, trimerization and so on are considered,
because the metalloporphyrins assemble with themselves; contrary to what
will be described in other sections of this chapter, no additional ligand
or metal is required for the assembly
Chambron et al.
Figure 10. Representation of the molecular components (a bipyridyl-
bridged cyclodextrin dimer 10 and a sulfonated porphyrin 11) from which
the aggregate 12 is obtained. In this supermolecule, two porphyrins are
encapsulated by two cyclodextrin dimers. The stability of the system is
increased by complexation of the bipyridyl bridges with Zn2 .
Figure 11. Porphyrin dimer (a), trimer (b) and tetramer (c) made from
metalloporphyrins (thick line) bearing a pendant-coordinating arm
symbolized by an arrow. The relative orientation of the latter with
respect to the porphyrin plane determines the structure of the assembly.
process to take place. But the metal inside the porphyrin must be at
The dimerization of naturally occurring porphyrinoids was first
evidenced in the 1960s by Katz and coworkers, who proposed structural
models of dimerization, and oligomerization of chlorophylls in
vitro.35'37 For example,
they showed by NMR (1H and l3C) and 1R that in nonpolar solvents, the
keto C=0 function in ring V of one chlorophyll molecule 13 (Figure 12)
acts as a donor to the Mg2 ' cation of another to generate chlorophyll
dimers (like 14) and higher-order oligomers. This model is known as the
Katz model. Many other models of dimerization of natural
40 / Noncovalent Multiporphyrin Assemblies
Figure 13. Stereoview of the X-ray crystal structure of a naturally
occurring dimer, the special pair (16) of the photosynlhetic reaction
center of bacterium Rhodopseudomonas viridis, made from two
bacteriochlorophyll b molecules (15).1'
porphyrinoids have since been proposed because investigation techniques,
such as high liekl NMR, have become more and more powerful. For example.
Smith and coworkers provided direct evidence Ãîã the formation of a
nonsymme-trical aggregate of bacteriophyllide d in solution using 'H NMR
spectroscopy by coordination of the secondary hydroxyl group of one
molecule to (he Mg: f central cation of another molecule.is
Naturally occurring chlorophylls and bacteriochlorophylls aggregate also
in vivo, as observed by X-ray
crystallography. For example, the so-called special pair (SP) of
bacterial reaction centers is a dimer of baclerio-ehlorophylls 15.7 As
shown in Figure 13. in the SP 16, these chromophores exist in a slipped
cofacial orientation, with a 3.2 A separation between their mean planes,
and a lateral offset of 6 A. The resulting intradimer n-n interactions
red shift the lowest energy absorption band of the hacterio-chlorophyll
and make the SP a belter energy acceptor for the light-harvesting
complexes. This simple, naturally occurring aggregate inspired the design
and preparation of many
Chambron et al.
metalloporphyrin dimers. Zn(II) has been used preferentially over the
biomimetic Mg(II) ion, because porphyrin complexes of Zn(II) are more
stable than those of Mg(II).
For example, Kobuke and Miyagi made porphyrins bearing methylimidazole
substituents at the 5- and 15-meso positions on the macrocycle.39 40 Both
the cis- and trans-atropisomers formed dimers in which Zn2 + was
pentacoor-dinated. In particular, the rrawj-isomer (17) formed an
extremely stable dimer (18), even at concentrations used for fluorescence
measurements, that is, 10 9molL 1 (Figure 14). Dimerization had
pronounced effects on the electronic absorption and fluorescence spectra:
the Soret band was split by 18 nm, and the Q bands were shifted to the