1.1 Porphyrins - Friedrich-Alexander-Universität Erlangen-Nürnberg

1 Introduction
Certainly, Photofrin® is not the only photosensitizer already approved and applied. But
before we are going into that matter, it should be elucidated how PDT is working and which
role the photosensitizer is playing.
1.2.2 Principles of PDT
As already mentioned, PDT bases on the interaction of light with photoactive dyes unfolding
their therapeutic (i.e. cytotoxic) properties which can best be explained using a JABLONSKI
diagram like is presented in Scheme 13. 48b,50a,61
16
E
1 P, S1
1 P, S0
3 P, T1
Scheme 13. Simplified JABLONSKI diagram with arrows indicating absorption, vibrational
relaxation, fluorescence, internal conversion (IC), intersystem crossing (ISC),
phosphorescence and energy transfer (ET) whereas states assigned with P represent states
of the photosensitizer.
At first, the photosensitizer in its singlet ground state ( 1 P, S0) absorbs adequate light energy
and becomes finally excited into its first exited singlet state ( 1 P, S1). From this state, various
successive processes can occur. The sensitizer can relax back into its singlet ground state in a
radiationless fashion by internal conversion (IC) or by emission of light (fluorescence). The
latter process can be used for tumor imaging in terms of fluorescent diagnosis. If the lifetime
of the first excited singlet state is long enough, the sensitizer can convert into an exited
triplet state ( 3 P, T1) by intersystem crossing (ISC). Although this process is spin-forbidden in
first-order approximation, good sensitizers show high ISC quantum yields. From this state,
the photosensitizer can relax back to a ground state again by emission of light
1 O2
3 O2