Thesis submitted 23-03-2012.pdf - University of Limpopo ...

pure single compounds that exhibit photosensitization properties and they
include a number of porphyrins and phthalocyanines derivatives.
1.9.6.2 Porphyrins
Porphyrins are a group of organic compounds many of which occur in
nature. One of the best known is heme, the pigment in red blood cells.
Heme is a cofactor of the protein hemoglobin.They are aromatic
heterocyclic macrocycles composed of four modified pyrrole subunits
interconnected at their α carbon atoms via methine bridges (═CH─).
Porphyrins are aromatic. That is, they obey Hückel's rule for aromaticity,
possessing 4n+2 π-electrons (n=4 for the shortest cyclic path around the
macrocycle) that are delocalized over the macrocycle. The macrocycles,
therefore, are highly-conjugated systems. Consequently, they typically
have very intense absorption bands in the visible region and may be deeply
colored. The name porphyrin comes from a Greek word for purple. The
macrocycle has 26 pi electrons in total. The parent porphyrin is porphine,
and substituted porphines are called porphyrins.
One of the more common synthetic routes for porphyrins is based on work
by Paul Rothemund. 37a His techniques underpin more modern syntheses
such as those described by Adler and Longo. 37b The synthesis of simple
porphyrins such as meso-tetraphenylporphyrin (H2TPP) is also commonly
carried out in university teaching labs. 37c It can be done by heating an
equimolar mixture of pyrrole and benzaldehyde in refluxing propionic acid
for about an hour.
Using this method, porphyrins are assembled from pyrrole and substituted
aldehydes. Acidic conditions are essential for porphyrin formation; formic
acid, acetic acid, and propionic acid are typical reaction solvents, or p-
toluenesulfonic acid can be used with a non-acidic solvent. Lewis acids
such as boron trifluoride etherate and ytterbium triflate have also been
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