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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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guanosine and cytidine nucleic acids.
The second system 4 is a three-porphyrin array in which a central
free-base porphyrin linked at both sides to a cytidine interacts via two
guanine-cytidine base pairs to two zinc(II) porphyrins. The strength of
the cytidine-to-guanine hydrogen bonding defined from proton NMR
titration analysis gave an association constant of 2.2 x 104mol~ 'L for
system 4. These titration data also confirmed a 1:1 and 2:1 stoichiometry
for systems 3 and 4, respectively. In both systems time-resolved
fluorescence measurements showed that singlet energy transfer occurred
from the zinc(II) porphyrin to the free base porphyrin within the
hydrogen-bonded complex. The rate constant was estimated to be 9xl08s 1
and the efficiency 60%. The most probable mechanism suggested was of
Forster's type and, as expected for a dipole-dipole energy transfer, the
guanine-cytidine hydrogen bonding network only serves to tether together
the chromophores and plays no role in facilitating the singlet energy
transfer process. On the other hand, triplet-triplet energy transfer also
observed in both systems occurred in 1 |is and involved double electron
exchange through the hydrogen-bonded network in agreement with a Dexter
mechanism.
In biological photosystems, electron transfer proceeds through non-
covalently linked protein pathways. In order to understand the electronic
coupling provided by a hydrogen-bond interface, a noncovalent bis-
chromophoric system 5
has been synthesized by Therien and coworkers (Figure 6).26 System 5 is
composed of a zinc(II) and an iron(III) porphyrin both covalently linked
to a benzoic acid derivative. The association of the two chromophores
through two hydrogen bonds has a strength of 440 mol - 1L. Within 5, fast
electron transfer takes place, from the zinc(II)
OCH3 R= OCH3
OCH3
Figure 6. A porphyrin dimer (5) composed of a zinc(ll) and an iron(lll)
porphyrin linked by H bonds between benzoic acid derivatives.
6
Chambron et al.
porphyrin to the iron(III) porphyrin in 8 x 109s~ over an edge-to-edge
distance of 15 A. In a similar system studied by the same group,26 both
chromophores were covalently linked through a [3.3.0] bicyclooctane
bridge, providing the same distance and number of bonds between the two
porphyrins. In this case, the rate of electron transfer was surprisingly
slower, 4.3 x 109s~'. These electron transfer rates showed that a
hydrogen-bond interface provides an equal or even higher electronic
coupling than an analogous interface composed entirely of carbon-carbon a
bonds. This result is highly significant as far as electron transfer
processes in biological systems are concerned.
In the field of self-assembly of molecular components to generate
defined structures which can be used as molecular devices, structural and
functional information must be contained at the molecular level. The
approach described in 1993 in Lehn's group was to use two complementary
components: a meso 5,15-diuracil-substituted porphyrin and an alkyl
triaminopyrimidine.27 The interaction between the hydrogen-bonded sites
of both components led in large part to a bis-porphyrin supramolecular
cage structure 6 as evidenced by proton NMR and fluorescence and
electrospray mass spectroscopy as well as by vapour phase osmometry
(Figure 7).
The cage structure 6 is composed of two face-to-face porphyrins linked
at both sides via six hydrogen bonds to two triaminopyridine entities.
The distance between the cofacial porphyrins is estimated to be 10 A. The
initial meso
5,15-diuracil-substituted porphyrin has two rotameric forms, a syn and an
anti, due to the relative orientations of the two uracil groups with
respect to the porphyrin plane. In the self-assembling process, the syn
rotamer yielded the major component, the cage structure, whereas the anti
conformer yielded a zig-zag strand structure. As for the properties
expected by such systems, metalation of one of the porphyrins allows for
the creation of a self-assembled donor-acceptor system while complexation
of the porphyrins with zinc(II) allows for the cage cavity to be used as
a
host to bind 4,4'-bipyridine derivatives between the two porphyrins.
Another example of a hydrogen-bonded assembly of porphyrins forming a
cage-like structure used as a receptor was described by Kuroda et al.28
The porphyrin used is a zinc-complexed "2e^o-tetrakis(2-carboxy-4-
nonylphenyl)-porphyrin in the form of a mixture of four atropisomers.
Thermal atropisomerization performed at 80 C in a nonpolar solvent
showed an exclusive formation of the aaaa isomer of the zinc porphyrin 7
due to its self-assembly property to form a face-to-face dimer 8 with a
high binding constant of the order of 107 mol - lL (Figure 8).29 The
eight hydrogen bonds formed between four pairs of carboxylic acids are
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