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The porphyrin handbook - Kadish K.M.

Kadish K.M. The porphyrin handbook - Academic press, 2000. - 368 p.
Download (direct link): kadishsmishgulilard2000.djvu
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the driving force for the selective formation of the dimer, starting from
a monomeric porphyrin atropisomer.
This self-organized system has interesting features. The cavity formed
within the zinc porphyrin dimer has a length of 8-9 A and is of
sufficient size to coordinate axially bidendate ligands such as pyrazine
or its derivatives. In particular, a 2-substituted pyrazine bearing a
side methylene chain ended with a benzoyl group led to the formation of a
2:1 complex with an equilibrium constant having the same order of
magnitude (107 mol 'L) as that for binding to it a nonsubstituted
pyrazine unit. Spectroscopic titration and 'H NMR studies showed evidence
for the axial coordination
Zn /
i
\ i >
/ Zn /
"
!
R = (CH2)9CH3 ,
R' = (CH2)7CH3 or (CH2)9CH3
Figure 7. Supramolecular cage-like compound 6 formed through 12 H bonds
between two porphyrins covalently linked to 5-alkyluracil recognition
groups, and two alkyltriaminopyrimidine units.
8
Figure 8. Representation of the xacta atropisomer of the zinc-complexed
meso-tetrakis (2-carboxy-4-nonylphenyl) porphyrin 7 and the dimer 8
formed thanks to a hydrogen-bond network between eight carboxylic acid
functions.
40 / Noncovalent Multiporphyrin Assemblies
7
of this pyrazine derivative inside the cavity of the zinc pophyrin dimer.
In this three-component complex, the side chain of the pyrazine lies
outside the pocket formed by the zinc porphyrin dimer but does not
disrupt the hydrogen-bonding network.
Another category of noncovalent systems has been recently developed,
in which interactions between the components of the systems are due to
electrostatic forces within a salt bridge. This bridge permits the
modeling of proton-coupled electron transfer in biology and with that
aim, several donor-acceptor systems have been studied.30-31 Nevertheless,
only one edifice bearing two porphyrins or their derivatives has been
described by Sessler and coworkers.32 System 9, represented in Figure 9,
is unique in that a carboxylate function linked to a porphyrin interacts
with the central core of a protonated sapphyrin. The sapphyrin has the
dual function of being a carboxylate-binding receptor and an energy
acceptor towards the porphyrin energy donor.
'H NMR binding titration studies confirmed the 1:1 stoichiometry of
association and gave a binding constant of 2.6 xlO3 mol 1L in deuterated
dichloromethane. To characterize the intraensemble energy transfer
dynamics of this system, time-resolved fluorescence measurements were
made. Singlet energy transfer from the porphyrin to the protonated
sapphyrin within the anion-chelation complex could be established with a
rate constant of 1.8 x 10 4 s 1 and a quantum yield of 0.96. The rate
constant was consistent with the theoretical rate calculated according to
a dipole-dipole Forster mechanism. The absence of a deuterium isotope
effect on the rate of energy transfer confirmed the nonparticipation of
the bridge in the energy transfer process and was also an argument in
favor of a dipole-dipole mechanism.
A particular way of assembling two water-soluble porphyrins within a
common system is to use the highly
R
R = (CH2)3CH3
9
Figure 9. A salt-bridge interaction is responsible for the dimeric
structure 9 formed between a porphyrin bearing a carboxylate group and a
protonated sapphyrin.
hydrophobic and rigid cavity provided by cyclodextrins. Nolte and
coworkers have, in this aim, described the synthesis of a cyclodextrin
dimer 10, formed by linking covalently two cyclodextrins to a bipyridine
bridge.33 Titration curves obtained by monitoring the fluorescence
intensity changes of 5,10,15,20-tetrakis(p-sulfonate phenyl)-porphyrin 11
in the presence of this cyclodextrin dimer 10 and the metallic cation Zn2
1 were consistent with the formation of a 2:2 complex 12 (Figure 10) and
also in agreement with *H NMR spectroscopic data. The estimated value of
the binding constant is > 5 x 107 mol ~ 'L.
The 2:2 complex 12 is formed from two cyclodextrin dimers that cross
each other and encapsulate two porphyrins. Each porphyrin binds to two
cyclodextrins not belonging to the same dimer. The formation of the 2:2
complex is facilitated by the presence of 0.5 equivalent of Zn2 f ions,
as shown in 12, which coordinate the bipyridine units in a tetrahedral
fashion.
III. Assembly of Porphyrins via Metal-Ligand Bonds
This section will deal with the formation and properties of discrete
molecular assemblies of porphyrins based on metal-ligand bonds and will
therefore not cover the area of coordination polymers based on
metalloporphyrins. These assemblies can be classified according to
whether the metal involved in the assembly process is incorporated or not
into the porphyrin, and upon consideration of the nuclearity, that is,
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