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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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face conformation of two porphyrins: the dimer bridged with relatively
flexible linkers at multi-points, and that with a rigid linker at a
single point. The
quadruple bridged dimer, 160, and double bridged one, 161, are typical
examples of the multilinked dimers.188184 The dicobalt complexes of the
latter were examined as four-electron reduction catalysts. The anthracene
pillared dimer, 162, is an example of a porphyrin dimer with a single
linker.140 All of these face-to-face dimers show a broadened and/or blue-
shifted Soret band due to exciton interaction between the two porphyrin
rings.
Because the introduction of aromatic rings at the meso positions of
porphyrins is relatively easy to accomplish, it provides an excellent
systematic method with which to prepare various types of porphyrin
dimers. The first example of a non-face-to-face dimer linked with the
rigid aromatic moiety is a gable-type porphyrin, 29, which mimics the
heme orientation in cytochrome c.v
45.191.192
Porphyrin
161
162a
162b
46/Porphyrins and Metalloporphyrins as Receptor Models
-" "X" Ar >-NH N
Ar =
163a rQ 163c fjQ !63e
163g
163b
163d jLJQ i63f
f" X X . Jl
>-N HN-/ . N-T.
< ; -Ar-/
^-NH N--\ ,/"N N"
Af - 4 // 164a
dimers linked with naphthyl moieties, 163, show split Soret bands.11'3
Their dipole-exciton-splitting energies show good agreement with those
predicted from their orientations and distances between two porphyrin
moieties.Various types of porphyrin dimers have been synthesized by a
similar aryl-linking method. Some half-metal complexes such as 164 and
165 were examined as models exhibiting efficient intramolecular electron
and/or energy-transfer reactions between two porphyrin chromophores.
l4,'-|l,K Extension of this strategy to larger scale systems using aryl
moieties as the linkers gives a variety of multiple porphyrins, including
linear, branched and stacked multi-porphyrins, and their
combinations.144--03 These multi-porphyrin systems have been drawing
considerable interest as the mimics of light-harvesting complexes and the
prototypes of synthetic molecular devices such as molecular
166
wires. For the phenyl-linked trimeric porphyrin array 166, the photon can
transfer between distal and central porphyrins in a coherent manner until
it is trapped by the proximal porphyrin.204 The pentameric array
containing a central tetraarylporphyrin linked to four other
tetraarylpor-phyrins via ethyne moieties, 167, shows efficient energy
transfer from the zinc porphyrin to the free-base porphyrin, though
electronic interactions between these chromophores are relatively weak
because of their relatively long center-to-center distance of ~ 20 A.205-
2I)(' The analogous linear porphyrin tetramer having a boron-
dipyrromethene dye as an optical input acts as a molecular photonic wire
or an optoelectronic T gate which supports excited-state energy transfer
over the range of about 90 A.2(I7-20X The mechanism of a highly efficient
energy transfer predominantly involves through-bond communication via the
diarylethyne linker. It should be noted that competitive electron
transfer is not observed in these porphyrin arrays.204 210 Because the
porphyrin derivatives having the phenyl acetylene moieties at the meso
positions are convenient building blocks for syntheses of large
multiporphyrin systems as shown in these examples, they have been
advantageously utilized in the synthesis of nano-scale substances such as
tripodaphyrins 168.211 Interestingly, no intramolecular interaction
between the chromophores is observed for these systems even in the
presence of a paramagnetic Cu(Il) chelating ion.
The cyclic oligomers are another interesting class of synthetic
multiporphyrin systems. These systems may not only mimic the highly
concentrated porphyrin clusters found in biological structures containing
closely coupled tetrapyrroles (such as the photosynthetic reaction center
and the cytochrome families of various types),212 but also show an
interesting manner of molecular recognition and sell-
327
328
Ogoshi et al.
168
assembling. For example, the cyclic porphyrin trimer, 169, recognizes
2,4,6-tri-(4-pyridyl)-s-triazine with a large association constant
(>109M~'), and the open-chain tetrameric analogue reveals unique template
effects in the coupling reactions of the phenylacetylene moieties.36-213
B. SELF-ASSEMBLING MULTIPORPHYRIN SYSTEM
Most of the large-scale multiporphyrin systems found in nature are
situated within peptide frameworks, and their relative configurations and
orientations are held fast with noncovalent interactions between protein
residual groups and the chromophores. The advantages of the use of
noncovalent interactions is twofold: (1) a variety of noncovalent
interactions having different characteristics is available for assembling
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