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- 4.52, -4.30, and -4.05 ppm were assigned to the NH protons bound to
rings II, I or III, and IV, respectively. Therefore, two tautomers,
existing in the ratio of 1:1 were detected through the NH proton signals
at low temperatures.
An interesting variation of these systems is to substitute the 4-
pyridyl substituent by a 3-pyridyl.75 The perpendicular mode of
coordination of 69 is then switched to an oblique mode, as in compound 74
of Figure 38, which shows noticeable differences as compared to the
former for the 'H NMR spectra. When the pyridyl-containing porphyrin
contains two 3-pyridyl substituents in cis relative positions (63), a
trimer is obtained (75) which can exist as a pair of atropisomers (a/?
and aa forms).
As described in Section III. A.2, Sanders and McCallien67 assembled
four Zn(II) dioxoporphyrins bearing acetylenic groups on the tetrakis(4-
pyridyl) porphyrin template 61. They subsequently submitted the complex
to the Glaser-Hay coupling reaction which produced the cyclic tetramer in
70% yield. Without a template, the same reaction was not so selective; it
afforded the tetramer and trimer species in 40%
Figure 35. Porphyrin dimer (69) and trimers (70 and 71) made from
Os(OEP)CO porphyrins axially bound to porphyrins 45, 58 and 59.
40/Noncovalent Multiporphyrin Assemblies
Figure 36. Porphyrin tetramer (72) and pentamer (73) made from axial
coordination of Os(OEP)CO to porphyrins 60 and 61, respectively.
Figure 37. Proton tautomery in porphyrin dimer 69 and trimer 70, as
observed by 'H NMR at low temperature. Porphyrin substituents are omitted
Chambron et al.
Figure 38. Porphyrin dimer (74) and trimer (75) made from coordination of
Os(OEP)CO to porphyrins 62 and 63, respectively.
yield each. Therefore the D4h symmetry of the porphyrin template was used
to generate, quite selectively, a covalently bound cyclic tetramer.
4. Two-Point Coordination of Porphyrins to Covalently Bound Metal-
a. Coplanar Arrangements of Porphyrins
These systems are based on the principle of porphyrin aggregation seen
in section 1.1, but are more complex than simple dimers. Rempel and
coworkers76-77 used rigid linear Zn(II) bis-porphyrins as platforms for
landing the free-base
5,15-bis(3-pyridyl)porphyrin 64. The association constant of the systems
76 and 77 shown in Figure 39 is high, about 6 x 10f> mol 'L. The authors
showed that energy or electron transfer processes between the Zn(II)
porphyrins of the platform and the free-base porphyrin component of the
assembly could be controlled by the substitution patterns of the Zn(II)
porphyrins. When the latter were meso-substituted, as in 77, energy
transfer was observed, whereas when electron-donating substituents were
introduced on the porphyrin /? positions, as in 76, the photoinduced
intramolecular processes were switched to electron transfer.
ÀÃ = ×0"(ÑÍ2)5ÑÍç
Figure 39. Supramolecular systems 76 and 77 result from the coordination
of porphyrin 64 to p-phenylene-bridged Zn(ll) porphyrin covalent dimers.
Hunter and Hyde34 built a related, but more complex system (Figure
40). As platform, they synthesized a Zn(II) bis-porphyrin bridged by
either a pyromellitimide spacer, which is an electron acceptor, or by a
simple terephthala-mide group, which is not electroactive. Owing to well-
oriented pendant coordinating arms, the free-base porphyrin guest could
bind to the bis-porphyrin platforms with binding constants as high as 3 x
10 8 mol ~ 'L. The resulting systems 78 and 79 showed remarkable
photochemical properties. Light excitation of the free-base component of
78 led to a 70% quenching of its luminescence by electron transfer to the
pyromellitimide acceptor. In the case of the terephtha-lamide-bridged
system 79, the direction and the nature of the intramolecular
photoinduced process were reversed in that energy transfer from the
Zn(II) porphyrins to the free-base porphyrin occurred.
b. Perpendicular Arrangements
The covalently linked bis-Zn(II) bis-porphyrins used by Rempel and
coworkers76-77 as platforms for two-point attachment of 5,15-bis(3-
pyridyl)porphyrin 64, leading to parallel arrangements of porphyrins,
were also used for two-point coordination of 5,10-bis(4-pyridyl)porphyrin
58 or 5,10-bis(3-pyridyl)porphyrin 63, that is, porphyrins in which the
pyridyl arms are in c/s-relative orientations (Figure 41). In the case of
58, a binding constant of 2.4 x 107 mol_ *L was observed for the assembly
40 / Noncovalent Multiporphyrin Assemblies
Figure 40. Supramolecular systems 78 and 79 result from the coordination
of a porphyrin bearing pendant, remote pyridyl groups to pyromellitimide-
(in 78) or p-phenylenediamide- (in 79) bridged Zn(ll) porphyrin covalent