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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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noncovalent bonds. We will restrict our discussion to discrete molecular
species and thus we will not include micelles, membranes or films. The
types of interaction described here are diverse and include hydrophobic
interaction, hydrogen bonding and coordination bonds (metal-ligand
interaction) in particular.
What is the benefit of using noncovalent interactions to build
porphyrinic multicomponent edifices? The most obvious answer is that a
modular approach can be used. Relatively small and easy-to-prepare
fragments can be made separately. In a second step, these "modules" must
be gathered by some means and arranged in space at will, to afford a
complex system. The final multicomponent assembly would be more difficult
to elaborate using a classical linear synthetic strategy but, here, the
main difficulty is concentrated on the assembling reaction. This key step
will have to afford a structurally well-controlled ensemble, using
relatively weak and nonrigid bonds most of
4
Chambron et al.
the time, although this is less true for transition metal-Iigand
interactions.
II. Multiporphyrin Edifices Built via H Bonds and Other Host-Guest
Interactions
This section describes defined multiporphyrin systems in which the
components are assembled by weak interactions such as hydrogen bonds,
salt bridges and van der Waals contacts. As discussed in the preceding
section, natural systems widely use hydrogen bonds and weak interactions
to make highly organized structures. This is a key tool, and it is a
challenge for the chemist to build high-order arrays of components in
this way. Nevertheless, the design of such systems is difficult because
the cohesion and geometry of the final edifice depend on weak
interactions. These two aspects are related to the directionality,
selectivity and energy of hydrogen bonds. The directionality of hydrogen
bonds gives information about the distance between the associated
molecules and the knowledge of the probable spatial orientations of
hydrogen bonds permits a prediction of the geometry of the final system.
The selectivity is linked to the complement between hydrogen-donor and -
acceptor sites which defines the self-recognition process and also to the
strength of the association. Since the energy of a hydrogen bond lies in
the range of 10 to 65 kJmol~ 1 for neutral molecules and rises to 40 to
190 kJmol - 1 between ionic components, multiple hydrogen bonds, which
ensure higher binding constants for self-assembled systems, are
preferable.
To mimic energy transfer processes in light-harvesting antenna
complexes, Sessler and coworkers developed in 1991 the idea of assembling
by noncovalent bonds a zinc porphyrin acting as an energy donor and a
free-base
porphyrin as an energy acceptor.23 They used nucleobase pairing
interactions to self-assemble several porphyrins together. One or two
free-base or zinc porphyrins were covalently linked to a cytosine unit
through a tertiary aliphatic amine function. The dynamics of the
association of two cytosine-bearing porphyrin units leading to the
formation of di-, tri- or tetra-porphyrin ensembles was studied by 'H-NMR
spectroscopy and time-resolved fluorescence measurements. The average
self-association constant was found to be 45 mol ~~ *L. A better
complement between the nucleobases is achieved by assembling a cytosine
bearing one or two zinc porphyrins with a guanine bearing a free-base
porphyrin unit.24 The base pairing interactions in the corresponding
dimers 1 and 2 of Figure 4, involving three H bonds, result in an
increased association constant of 200 mol ~ 1 L.
The aim of these multiporphyrin systems was to study the energy
transfer reaction from the singlet excited state of the zinc(II)
porphyrin to the free-base porphyrin within a hydrogen-bond assembly.
Nevertheless, it was difficult to explain unambiguously the luminescence
quenching of the zinc porphyrin. Due to the flexibility around the amino
group, intracomplex diffusional quenching of the zinc porphyrin excited
state by the free-base porphyrin could not be ruled out.
To circumvent this problem, more rigid hydrogen-bonded systems were
developed by the same group25 and two of them are represented in Figure
5. System 3 is a dimeric ensemble of a zinc porphyrin and a free-base
porphyrin, both linked via a phenyl group to a guanosine and cytidine
recognition unit, respectively. The two porphyrins are arranged side by
side with an interplane angle of about 90 and with a center-to-center
distance of
22.5 A according to CPK models. Each porphyrin possesses about 45 of
rotational freedom around the carbon-carbon bond connecting the phenyl
and nucleobase entities.
Figure 4. Porphyrin dimers 1 and 2 formed via H bonds between cytosine-
guanine base pairs.
40 / Noncovalent Multiporphyrin Assemblies
5


Figure 5. Dimer 3 or trimer 4 of porphyrins formed via H bonds between
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