diagrams for several metal porphyrins ( 22 ). The 3d orbitals of
ZnTPP are particularly low in energy, and the HOMO is a por-
phyrinporbital. The first oxidation of ZnTPP removes an elec-
tron from the HOMO a2u and leads to the p-cation radical
[ZnIITPP]þ, whereas in the first four reduction stages, the suc-
cessive electrons are added to the LUMO 2eg(p*). The 3d orbitals
of CuTPP are also low in energy but the odd electron now
occupies the b1g(dx (^2) – y 2 ) HOMO. Somewhat surprisingly, the first
oxidation removes an electron from the porphyrin a2uorbital and
not from the HOMO, and the first reduction adds an electron to
FIG. 4. Calculated energy levels for the frontier molecular orbitals of
metalmeso-tetraphenyl porphyrins and for TPP (with no H atoms in
the porphyrin cage). Electron occupancies are indicated for frontier
molecular orbitals of the metal. Adapted from Ref. (22) with neglect of
various nonactive orbitals.
DESIGN OF PORPHYRIN-BASED PHOTOSENSITIZERS 195