to chlorins( 30 ). Lowering the energy of the HOMOs strongly dis-
favors oxidative destruction of the macrocycle and should con-
tribute to increase stability with respect to oxidation ( 31 ).
Thus, increasing the oxidation potentials provides extra
stability toward oxidative degradation of porphyrin-based
photosensitizers. This can be achieved, for example, replacing
the phenyl rings byo,o^0 -difluorophenyl rings in TPPs. In princi-
ple, it should be possible to synthesize stable fluorinated
tetraphenylbacteriochlorins with intense absorption in the pho-
totherapeutic window. Bacteriochlorins should also be stabilized
by complexation with In^3 þand, to a smaller extent, by complex-
ation with Pd^2 þ.
Another concern specific to the use of porphyrin-based
photosensitizers is the fact that singlet state lifetimes are usu-
ally less than 10 ns and photosensitization occurs mostly from
the triplet state. This requires the selection of systems that effi-
ciently generate triplet states, which is not the case of paramag-
netic metalloporphyrins.
III. Electronic TransitionsA. ABSORPTION ANDEMISSION
The absorption spectra of free-base porphyrins shows the
expected Soret, Qyand Qxbands and two additional bands in
the visible (Fig. 5). These extra bands are due to vibrational cou-
pling effects and derive from HOMO (b1uorbital) to the first
vibrationally excited state of LUMO (b2gorbital) or to LUMOþ 1
(b3gorbital) transitions. Thus, in order of increasing energies, the
optical transitions of H 2 TPP are labeled Qx(0,0), Qx(0,1), Qy(0,0),
and Qy(0,1). Typical free-base porphyrins have moderate fluores-
cence quantum yields (FF¼0.10 for H 2 TPP in deaerated toluene)
( 32 ) and nanosecond fluorescence lifetimes (ts¼14.7 ns for
H 2 TPP) ( 33 ). Most of the singlet states evolve to the lowest
triplets (triplet quantum yieldFT¼0.73 for H 2 TPP in deaerated
toluene) ( 32 ), which have long lifetimes in deaerated solutions
(milliseconds in sufficiently pure solutions).
In general,b-substituents cause steric interactions and, conse-
quently, structural distortion of the porphyrin core, and this is
more accentuated in TPPs than in porphins. Such steric
interactions are evident in the red shift of the Q and B bands of
b-octabromo andb-octatrifluoromethyl porphyrins(12,34). How-
ever, b-octafluoro porphyrins remain nearly planar, and the
absorption bands are blue shifted ( 14 ).
198 LUIS G. ARNAUT