This diminishes the HOMO–LUMO gap from the porphyrin, to
the chlorin and to the bacteriochlorin, explaining the
bathochromic shift of the Qybands. The oxidation and reduction
potentials of these species follow the energy changes of the HOMO
and LUMO, respectively. Thus, again within a homologous series,
the reduction potentials of the porphyrin, chlorin, and
bacteriochlorin remain essentially the same, whereas the oxida-
tion potentials become increasingly more facile, with
the bacteriochlorin having the lowest oxidation potential and the
porphyrin the highest ( 10 ). These properties are illustrated with
some examples in Table I.
Perfluoroalkyl groups are stronglya-electron withdrawingand do
notfunctionasp-electrondonors.Theelectron-withdrawingeffectof
CF 3 groups in the pyrrolicb-positions ofmeso-TPP distorts the mac-
rocycle and forces it to take a fixed 18p-electron pathway, resem-
bling the electronic structure of bacteriochlorin. This is reflected in
the HOMO–LUMO gap contraction and in the 0.06 V decrease in
the oxidation potential of H 2 TPP-b(CF 3 ) 4 with respect to H 2 TPP
( 12 ). However, this decrease is not as accentuated as the 0.20 V
decrease observed for H 2 TPP-b(CH 3 ) 4 because it is compensated
by the strong electron-withdrawing ability of the trifluoromethyl
group ( 12 ). The electron-withdrawing effect is also responsible for
making the reduction easier.b-Octahalogenation of TPPs with Cl
or Br atoms also produces steric hindrance at the ligand periphery
and leads to highly nonplanar porphyrin cores, butb-octafluoro-
meso-TPPs are nearly planar ( 14 ). This is corroborated by the
hypsochromicshiftoftheabsorptionbandsofbothfree-baseandzinc
complex, and by the remarkable increase of the oxidation potentials
ofb-fluorinated porphyrins ( 13 ).
Fluorinating the phenyl groups of TPP in themeso-positions also
introduces electron-withdrawing substituents that operate exclu-
sively through the inductive effect, as shown in the replacement
of phenyl byo,o^0 -difluorophenyl rings (TF 2 PP), in free-base, Mg,
and Zn porphyrins ( 20 ). Comparison between the oxidation
potentials of MgIITPP and MgII(TF 2 PP) or of ZnIITPP and
ZnII(TF 2 PP) reveals an increase in 0.26–0.27 eV in the oxidation
potential, assigned to a stabilization of both a2uand a1uorbitals.
However, the a2uorbital is expected to be preferentially stabilized
relative to the a1uorbital because the halogenated phenyl rings are
bonded to themeso-carbon atoms, where the electronic density of
the a2uorbital is higher. The 3d metal orbitals of MgIITPP and
ZnIITPP are of lower energy than the HOMO and do not change
the ordering of the active orbitals ( 21 ), hence the changes in
oxidation potentials of fluorinated MgIIand ZnIIporphyrins are
representative of the corresponding changes in free bases.
DESIGN OF PORPHYRIN-BASED PHOTOSENSITIZERS 193