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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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absorb the photon energy. The Cu(I) complex is suggested to participate
in these electron-transfer processes via both direct (redox) and indirect
(superexchange) mechanisms. The complex formation of 1,10-phenanthroline
derivatives with Cu(I) is also utilized for the self-assembling system
using the independent interface molecule as shown in type III. The
bisphenanthroline interface connected with the biphenylether spacer
assembles two macrocyclic compounds bearing an appended porphyrin to give
the porphyrin dimer 181.228 Mixed-metal porphyrin complexes of this type
do not detectably "scramble" when kept in CDiCb solution at room
temperature for 24 hours. The similar assembly using combination of a
terpyridine moiety and Ru(II) is also reported.229
It is one of the great advantages of coordination interactions as
interfaces for assemblies that the mutual orientation of porphyrin
components in the assemblies may be regulated by the coordination
geometry of the interface metal. Interesting square tetrameric
assemblies, 182 and 183, are designed by taking such advantage of the
metal coordination.230 These assemblies are prepared simply by titrations
of fra/j.s-PdC^NCPhb or cw-Pt(NCPh)2Cl2 with the solution of 5,10- or
5,15-dipyridylporphyrin, respectively. Fluorescence polarization
experiments on the free-base tetramers of 182 and 183 yield Pmax = 0.32
in agreement with square structures, which is in contrast to almost
complete depolarization in the excitation spectrum of the corresponding
polymeric compounds. The similar tetrameric porphyrin assemblies are
prepared by using
Pd(II) and Pt(II) bisphosphine complex as auxiliaries.231 Because of the
greater Pd-N, Pt-N bond strengths, the assemblies are found to be stable
in a wide concentration range from 10~l) to at least 10 2 M. The Re(I)
ion is also useful as the comer element for these porphyrin assemblies.
The tetraporphyrin assembly obtained from Re(CO)sCI and
bis(pyridyl)porphyrin strongly binds tetrapyridylporphyrin to form
pentaporphyrin assembly 184 with an association constant of 4 x 107 M~
l.105 The complete substitution of
46 / Porphyrins and Metalloporphyrins as Receptor Models
184
Ph
the metal coordination sites of Pt(II) or Pd(Il) with porphyrins results
in a propeller-type tetrakis-porphyrin assembly 185 where the four
chromophores are assembled on the single central metal.232 The 'H NMR
spectra suggest that all four porphyrins around the metal ion are
identical on the NMR time scale and that the Pt and Pd ions have square
planar environments in solution.
One of the most versatile interactions used for molecular recognition
in nature is the hydrogen-bonding interaction and, therefore, it also
provides the most promising methodology for the construction of
biomimetic multiporphyrin self-assemblies. Because the energy for the
187
hydrogen bond lies in the range of 0.5-5 kcal/mole, substantially weaker
than that for the metal coordination, this type of porphyrin assembly is
usually constructed based on the multipoint hydrogen-bonding sites for
molecular recognition. For example, the self-assembling porphyrin dimer
186 is designed based on the hexa-hydrogen-bonding complementarity
between the barbiturate derivative and two
2,6-diaminopyridine units linked through an isophthalate spacer for the
one molecular recognition site.|( Similar hexa-hydrogen-bonding
recognition is observed for the selfassembling face-to-face dimer 187
which is directly detected by electrospray mass spectrometry.233 The 'H
NMR spectra of the mixture of the triaminopyrimidine derivative and two
rotamers of the porphyrin component indicate that the self-assembly of
the dimer 187 through twelve hydrogen bonds shifted the syn-anti
equilibrium of the atropisomerization towards the syn rotamer. The
combination triaminotriazine and barbituric derivatives generate the more
condensed porphyrin hexamer 188.234 Vapor-phase osmometry measurements in
toluene show a molecular weight of 5298 276 that is consistent with the
assembly 188 of three porphyrin dimer molecules and three
5,5-di(butyl)barbituric acid units (MW =5156). This type of assembly,
bearing three sets of metalloporphyrin dimers, shows a striking
resemblance to the ring of P850 chlorophylls found in one of the two sets
of porphyrin arrays present in the light-harvesting complex in
photosynthetic bacteria, where the chromophores also play an essential
role in the assembly of the entire protein-ring assembly.
The application of Watson-Crick nucleobase-pairing interactions
provides another fruitful example of the selfassembling multiporphyrin
systems.163 The self-assembling systems consist of guanosine- and
cytidine-bearing porphyrins 148-149 providing interesting model systems
for both singlet and triplet energy transfer between the donor-acceptor
porphyrins as discussed in the preceding section.
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