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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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ligation points.221-222 Among these self-assemblies, the trimer 174
exhibits notable exciton splitting of the Soret band, which is consistent
with the porphyrin plane angle of 60. The pyridyl moiety in 173a is also
used for photoinduced energy and/or electron-transfer systems.158 The
free-base porphyrin bearing two pyridyl linkers strongly ligates the zinc
porphyrin dimer which has a naphthalenediimide or terephthaloamide group
as the spacer. In contrast with the assembly 142-143 containing the
naphthalenediimide spacer which shows electron transfer activity, the
assembly 175, which has no electron-acceptor group, shows enhanced
fluorescent emission of the free-base porphyrin and reduced emission for
the zinc porphyrin. The observation indicates energy transfer from zinc
porphyrin to the free-base porphyrin. In both cases, the shortest bonded
connection between the chromophores is 24 bonds or roughly 35 A, whereas
the separation of the chromophores in space is 10 A at most. Therefore,
it is probable that the pathway for the energy transfer is also through
space or through the solvent occupying the space between the two
chromophores. The metalloporphyrin molecules having a complementary
metal-ligand configuration spontaneously form very tight face-to-face
dimers via mutual coordination interactions. The Rh(III) complex of 5,15-
bis(2-pyridyl)-octaethylporphyrin exists as the dimeric form 176 in
CDCI3, where steric hindrance, even at the CH2 groups, is so large that
rotation around the ethyl C-C bonds is strongly restricted.221 The
interaction between the two porphyrin molecules may be released by the
addition of excess amounts of pyridine. The strong exciton interaction is
observed for the similar slipped cofacial self-assembling dimer 177. The
NMR data indicate the predominant formation of a dimer and negligible n-
aggregate formation.224

As described previously, the coordination dimers of porphyrins are also
utilized as the active component for the
176
177
46/Porphyrins and Metalloporphyrins as Receptor Models
331
intra-assembly ET reaction through phospholipid membranes.174-17'' The
assembly 154-155 consists of a trianionic zinc porphyrin which binds
cytochrome at the membrane surface and is anchored to the membrane-
spanning manganese porphyrin in the membrane interior via a terminal
imidazole coordination interaction.
Another interesting application of the coordination interaction to the
self-assembling system is conformational regulation of macrocyclic
porphyrin oligomers. The atropisomer distribution of the flexible cyclic
trimer of porphyrin 178 prepared by hydrogenation of 169 is drastically
shifted by addition of the bidentate or tridentate ligand, that is, s-
tri(4-pyridyl)triazine induces the optimum geometry for three
simultaneous pyridine-Zn interactions, and DABCO is recognized by
cofacial binding of two poiphyrin units, forcing the third unit into a
perpendicular geometry as shown in Figure 18.225 The cyclic tetramer 179
strongly recognizes tetrapyridylporphyrin to give a very stable complex.
The absorption spectrum of the complex is essentially the sum of the
absorption spectra of the two component parts, indicating negligible
ground-state interactions between these chromophores. However, the fluor-
escence-emission spectrum of the complex is around 1000 times less
intense than that of either component. This
observation suggests that photoinduced electron transfer occurs, as in
natural photosynthetic reaction centers, followed by nonradiative decay
of the charge-separated state.226
Introduction of independent recognition sites into the multiporphyrin
systems as shown in type II makes it possible to more freely design the
mutual configuration between poiphyrin components in the assembly.
Another advantage of the independent recognition sites is that, in
principle, assembly formation does not modify the structures and/or
properties of the component porphyrins. These characteristics potentially
mean that the reaction and assembling behavior of this type of
multiporphyrin system can be independently controlled by using the
function of molecular recognition. Among the multiporphyrin assemblies
having independent recognition sites, the most intensively investigated
systems are those using metal ions as the adhesive interface to
congregate the porphyrin components at their second ligation points. For
example, the porphyrin dimer 165 can be self-assembled to form a
tetrameric porphyrin assembly 180 in the presence of Cu(l) ion.227 The
mixed multicomponent assembly, comprising Au(III) and Zn(II)
bisporphyrins, undergoes a variety of electron-transfer reactions
according to which porphyrins
0 Zn
179
Figure 18. Geometry regulation of cyclic porphyrin oligomer.
332
i
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Ogoshi et al.
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