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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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monomers. The absorption properties of these face-to-face porphyrin
aggregates are understood as excitonic interactions between the porphyrin
rings (Kasha model71). In the case of the trimer 67, the Soret bands are
not simple overlaps of the corresponding bands of the Ru(II) porphyrin
subunits and the axial porphyrin subunits; there is a decrease in band
intensity, accompanied by a broadening of the band. Although splitting of
the Soret bands was not observed, these results suggest the occurrence of
excitonic interactions between the two axial porphyrin ligands.
Alessio and coworkers72 assembled two and four [Ru(TPP)CO] complex
fragments, using bidentate porphyrins 58 and 59, and tetradentate
porphyrin 61. In the trimers, the attached porphyrins were either in cis
or trans orientations with respect to each other. The pentamer (68)
obtained (Figure 34) was very stable in solution, even at concentrations
as low as 3x10-4molL-1. Only the central porphyrin undergoes chemical
shift changes upon complexation of the four Ru porphyrins; the a protons
of the pyridyl ligand move upfield by 7 ppm and the NH protons are also
shielded, but to a lesser extent, by about 2 ppm.
Chambron et al.
Figure 30. Templated cyclotrimerization of the Zn(ll) complex of the
dioxo porphyrin 56. The /^-substituents have been omitted in precursor 55
and covalent trimer 57.
Figure 31. Multiporphyrin assemblies based on porphyrins bearing one,
two, three or four coordinating subunits, (a) dimer, (b)-(d) trimers, (e)
tetramer (f) pentamer. The metalloporphyrins are represented
schematically by a thick line and the free-base porphyrins bearing
coordinating subunits (arrows) are represented by empty squares.
Therefore, the shielding field of the peripheral porphyrins influences
even the central part of the template porphyrin. 'H NMR studies also show
that the phenyl substituents are fixed in space; they are perpendicular
both to the Ru(II) porphyrin mean planes and the central, assembling,
free-base porphyrin, forming the rims of a molecular box. In fact, the
pentamer really plays the role of a molecular box when the central
porphyrin is complexed with Zn(II) and is able to selectively recognize
S-bonded Ru(II) complexes of Me2SO.
Imamura and coworkers73 used the same principle to assemble, in a
systematic manner, one, two, three and four [Os(OEP)CO] porphyrin
fragments on tetraaryl porphyrin templates containing one, two, three and
four 4-pyridyl meso substituents, respectively, yielding dimer 69,
trimers 70 and 71, tetramer 72 and pentamer 73, respectively (Figures 35
and 36). Assembling two [Os(OEP)CO] subunits could be done in two
different manners as shown by compounds 70 and 71, depending on whether
cis (58) or trans (59) bis(4-pyridyl)diphenylporphyrins were used as
assembling units. Spectroscopic and electrochemical studies showed that
there was no electronic interaction between the orthogonal arrays of
Os(II) porphyrins. The protons of the central, assembling free-base
tetraarylporphyrin were sensitive to ring current effects of the
coordinated, peripheral
40 / Noncovalent Multiporphyrin Assemblies

Figure 33. Dimer (65) and trimers (66 and 67) made from Ru(OEP)CO
porphyrins axially bound to porphyrins
45 and 59.
Chambron et al.
Figure 34. A tetramer of RuTPP(CO) axially held by porphyrin 61. The
phenyl substituents of the Ru(ll) porphyrins have been omitted for
porphyrins, as observed by Alessio and coworkers.72 The NH protons,
located in the center of the arrays, were sensitive to the number of
peripheral porphyrins, whereas the 2,6-pyridyl protons were sensitive
only to the porphyrin to which the pyridyl fragment was coordinated. Even
for the simple dimer, upfield shifts as large as 7.5 ppm were observed,
and the same was true for the pentamer of Alessio and coworkers.72-74
The magnetic anisotropy created by the peripheral porphyrins in the
region of space of the assembling, free-base porphyrin was used to
observe porphyrin tautomerism (Figure 37). For example, in the case of
the dimer 69, one signal was observed at room temperature due to fast
exchange, while at low temperature (193K), two sharp singlets were
observed. The higher field signal was assigned to NH on the pyrrole ring
of ring I or , the lowest field signal to that on ring III or IV. The
rate constant for exchange was evaluated to 2 = 7.2 x 103 s 1. For the
cis-trimer 70, even more remarkable features were observed. The singlet
observed at room temperature was separated into three singlets at 193K,
integrating 1:2:1. Considering the effects of the porphyrin ring
currents, the signals at
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