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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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red by more than 20 nm. The same was true for the fluorescence maxima.
The splitting of the Soret band is usually explained by excitonic
interactions. In this case, the interactions between the porphyrins are
important, since the amount of splitting observed corresponds to an
interaction energy of 1035 cm - 1.
When the biomimetic metal Mg2 + , which can accommodate a sixth
ligand, is used instead of Zn2 +, the transisomer allows for the
formation of a significant amount of trimer species (34%).40 However,
higher-order oligomers are not observed. In the same vein, Schugar and
coworkers studied the dimerization of a Zn-bacteriochlorin bearing a 2-
pyridyl substituent at one meso position.41 Whereas dimerization in the
case of the SP induces a red shift of the Qy band, that is, the SP is a
better energy acceptor than the separated chlorophylls, the electronic
spectra of Schugar's dimer and the monomer are nearly superimpo-sable. As
shown by 'H NMR spectroscopy studies (NOE effects) and X-ray crystal
structure analysis, the bacterio-chlorin units stack such that the
pyrroline (reduced pyrrole) portion of one bacteriochlorin overlaps the
pyrrole portion of the other, thus minimizing n overlap between the
monomer n systems. Schugar et al. also studied how the anchoring position
of the 2-pyridyl ligand to the Zn-w&sotetraaryl or ws.votetraalkyl
porphyrins would affect the
17
18
Figure 14. Dimerization of Zn(ll) porphyrin 17. The porphyrin -
substituents in the dimer 18 have been omitted for simplicity.
19
20 22
Figure 15. Dimerization of Zn(ll) porphyrins 19 and 21. In the resulting
dimeric structures (respectively 20 and 22), the nonpyridyl me so
substituents have been omitted for clarity.
40/Noncovalent Multiporphyrin Assemblies
11
aggregation process (Figure 15).42 When the 2-pyridyl substituent was
anchored to a meso position, as in 19, a stacking dimer (20) was obtained
and structurally characterized as having a 3.30 A distance between the
porphyrin mean planes and a lateral offset of 5.5 A between the porphyrin
centers. These are structural features close to those of the SP. When the
2-pyridyl substituent was anchored to a /5-pyrrole atom, as in 21, the
dimer 22 was quantitatively formed only when the temperature was lowered
to - 40 C, because the mesoa\ky\ electron donating groups reduce the
affinity of Zn2+ for apical ligands, as compared to mesodsy\ groups.
Using related systems, Shinkai and coworkers found association
constants larger than 104 mol 1L for the dimerization process.43 The
system of Martensson and coworkers is somewhat more complicated than the
previous ones.44 As shown in Figure 16, it is made from a central diaza
crownether subunit which bridges two porphyrins. When Zn(II) is inserted
into one of the porphyrins, as in
23, dimerization can occur, affording 24 at least at low temperature in
solution, the distal nitrogen atom of the crown playing the role of the
apical ligand for the Zn2 + cation of the other porphyrin. The pendent,
free-base porphyrin does not play any role in the dimerization process.
Burrell et al.45 anchored a (4-pyridyl)vinyl substituent to a /^-
pyrrole atom of a TPP (5,10,15,20-tetraphenylpor-phyrin) analogue using
a Wittig reaction. Both trans 25-(62%) and cis 26- (20%) isomers were
obtained and were separated by chromatography. As shown by a
radiocrystallography analysis, upon insertion of Zn(II) the ?ra"i-isomer
gave a product that crystallized as an infinite solid-state polymer (27),
each pyridyl arm being coordinated, as an axial ligand, to the Zn2 +
cation of an adjacent molecule. In contrast, Zn(II)-complexation by the
-isomer produced a dimeric species (28), as shown also by an X-ray
crystal structure analysis. Interestingly, upon
Ph
irradiation with near UV light, the polymeric structure can be switched
to a dimeric one, as represented in Figure 17.
This m-/;ra".s-isomerization is of particular interest, because it
involves pendant pyridyl ligands having different orientations with
respect to the porphyrin plane, and it shows that these orientations
determine the nature of the aggregates. As in the case of the 2-pyridyl
substituent, the m-(4-pyridyl) vinyl substituent naturally favors a
coplanar arrangement of the two porphyrins because of its more or less
orthogonal orientation. The /raw.v-(4-pyridyl )vinyl substituent led to
the formation of a coordination polymer, with a zig-zag structure.
Fleischer and Shachter46 also observed that a Zn(TPP) analogue bearing a
4-pyridyl substituent (29), forms coordination polymers (30) with the
same structure, as shown in Figure 18. Therefore, the formation of a
square tetrameric arrangement of porphyrins, as shown in Figure 11, does
not seem to be an obvious issue contrary to the principles of self-
assembly.
The effect of the position of the coordinating nitrogen atom of the
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