the simplest porphyrin, and the common generic names of the
positions of the pyrrole rings. These macrocycles have important
absorption bands in the red and near infrared, with the
wavelength of the maximum absorption of the lowest energy
absorption band (lmax) indicated in the figure.
The photochemistry of porphyrins and their derivatives has
been particularly well studied for their relevance in biological
processes, as evidenced from the examples above, and also for
their use in many other processessuch as dye-sensitized solar
cells, photocatalysis, molecular electronics, etc. The subject is
vast and has been addressed by many excellent books, reviews,
and specific journals(1,2). More recently, expanded porphyrins,
with additional pyrrole rings conjugated to each other through
carbon groups( 3 ), have contributed to enrich the chemistry of
these molecules. A comprehensive view of this subject is beyond
the scope of this work. Our work focuses on the photochemistry
of themeso-tetraphenylporphyrin (TPP) because it is the most
easily synthesized porphyrin unit, and on the applications of
tetrahydroporphyrins to photodynamic therapy (PDT). This
focus is motivated by the need for designed synthetic products
obtained from abundant sources at affordable prices in
photomedicine, by the increasingly important role of PDT in
photomedicine, and by the perception that the versatility and
photochemical properties of TPPs have the potential to meet
needs of PDT. We emphasize the quest for photostable
tetrahydroporphyrins derivatives because of their strong
absorptions in the red and infrared as a critical step toward a
better use of the phototherapeutic window (720–900 nm, the
most penetrating and least harmful radiation to human tissues
that can generate reactive oxygen species (ROS)).
Porphyrin derivatives have been extensively tested as
photosensitizers for the PDT of cancer for two sets of reasons.
First, their strong absorption of light in the phototherapeutic
window and efficient photoinduced reactions with molecular
oxygen offer a photochemical tool to induce localized cytotoxic-
ity in targeted tissues. Second, porphyrin derivatives have an
intrinsic affinity for tumors( 4 – 6 ). Whereas the spectroscopy
and photochemistry of porphyrin derivatives are very well
understood, the same is not (yet) true for the mechanisms that
contribute to their preferential localization and accumulation
in tumors. This latter subject is outside the scope of this work,
and it will only be briefly mentioned in the context ofin vivo
studies with porphyrin derivatives.
The structure of this work is as follows. First, we offer an
overview of simple models that can give a semiquantitative
DESIGN OF PORPHYRIN-BASED PHOTOSENSITIZERS 189