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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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wire consists of a boron-dipyrromethene dye as an input chromophore at
one end, a linear array of three zinc porphyrins as a signal-transmission
element, and a free-base porphyrin as an optical output element at the
other end (Figure 44). Absorption of a photon by the input chromophore at
one end causes the emission of a photon as the optical output at the
other end. The overall yield of energy transmission from input to output
is 76%.
Crossley and Bum also reported a molecular wire based on a more
conjugated backbone.96 The tetrakis(por-phyrin) is about 65 A in length
and possesses a number of terf-butylporphyrin groups along the backbone
that provide the molecule with good solubility in a number of organic
solvent systems. Individual porphyrins are either directly fused or are
linked by a coplanar aromatic system and are expected to be reasonably
conductive (Figure 45).
The synthesis of this extended porphyrin system involved a seven-step
procedure. The final porphyrin tetramer was obtained by the condensation
of porphyrin-a-dione with bis-a-diamine. "Molecular optoelectronic
gates" were also prepared by the same research group to see if the
emission signal at the output chromophore in the photonic wire could be
turned on and off in a controlled manner.97 In order to achieve this goal
a different metalloporphyrin-used as a redox switch-was inserted into the
molecular wire. This metalloporphyrin must meet four design conditions:
(1) it must have a higher absorption energy in the neutral state, so that
the energy transfer does not occur between the free base and the
metalloporphyrin; (2) when oxidized, the metalloporphyrin should not
fluoresce upon excitation; (3) it must have reversible redox properties;
(4) it should possess the highest HOMO of all pigments in the assembly to
ensure that oxidation takes place at the designated site. Mg-porphyrins
seem to meet all the criteria. A schematic of the
two gates designed is shown in Figure 46. In the linear design, the
switch is attached to the end of the wire but the T-shaped gate has the
redox switch attached to the Zn-porphynn. Oxidation was achieved by
either chemical method or electrochemical method. In both cases the
fluorescence of the free-base porphyrin decreased significantly. Addition
of triethylamine causes reduction of the Mg-porphyrin radical cation and
recovery of the fluorescence.
Shimidsu et al. have obtained both one-dimensional and two-dimensional
porphyrin polymers with oligothienyl molecular wires.98-101 The electron
conductivity of the 1-D polymers with conjugating oligothiophene was
strongly enhanced by irradiation of light and was dependent on the light
intensity. The 1-D porphyrin array with insulating wire was prepared by
condensing the dichloro-P-porphyrin and the corresponding diol. The 2-D
porphyrin network was synthesized by electropolymerization of the
corresponding phosphorus-porphyrin with four oligothienyl groups at the
meso positions. The 2-D porphyrin arrays showed similar functions as the
1-D porphyrin arrays with conjugating wires (Figure 47).
3. Synthetic Light-Harvesting Antennae
Photosynthetic microorganisms use light-harvesting antenna to trap solar
energy and funnel it to the reaction centers.102-105 The energy
conversion and migration can involve hundreds of pigments. A light-
harvesting antenna is a multicomponent assembly that uses chromophores
which can absorb strongly in the blue and red regions of the solar
spectrum, and then transfer the energy to a single, terminal energy-
acceptor component. Synthetic porphyrin arrays are obvious candidates for
the design of artificial antennas since chlorophylls are the main natural
chromophores. Such exploration, however, has been limited by a lack of
appropriate synthetic methodology for the preparation of large porphyrin
arrays. Conventional synthetic strategies frequently involve a large
number of sequential steps and extensive chromatographic purification.
Consequently, the majority of the research done in this area focuses on
the synthesis of multiporphyrin networks. Viable syntheses of suitable
large arrays have only recently become available with the development of
building-block methodologies.
68
Chou et al.
ft
Input
?
output
redox switched site
redox switched site
Figure 46. Molecular gates (A) linear, (B) T-shape.
41 / Porphyrin
Materials Chemistry
69

Figure 47. Porphyrin assemblies. (A) Porphyrin monomer with two
oligothiophene, (B) 1-D porphyrin arrays connected with molecular
conjugated wires, (C) insulating wires and (D) 2-D porphyrin arrays
connected with molecular conjugated wires. Reprinted with permission from
Segawa, H.; Nakayama, N.; Shimidzu, T. J. Chem. Soc., Chem. Commun. 1992,
7B4.
70
Chou et al.
Figure 47. (continued)
In this section, we concentrate on synthetic methods since the
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