photocatalytic mechanism shown in Fig. 10 was proposed to
account for the formation of^1 O 2 ,O 2 ,H 2 O 2 , and OHobserved
in aqueous solutions.
To the best of our knowledge, the first“stable”bacteriochlorins
were published in 2002( 66 ), but the concept of“stable” that
needs qualification. We classify 5,10,15,20-(2,6-difluorophenyl)
bacteriochlorin, H 2 TF 2 PB, as“stable” because its half-live for
degradation in aerated toluene solution under ambient light
and at 120C exceeds 2 weeks. The preparation of these
halogenated bacteriochlorins was criticized for being limited to
the synthesis of bacteriochlorins containing inert functionalities
( 88 ). This criticism was proved unsound ( 89 ), and more recently,
an environmentally friendly method was developed to synthesize
99% pure halogenated and sulfonated bacteriochlorins in gram
batches ( 90 ).
The remarkable stability of fluorinated and chlorinated TPBs
is assigned to the stabilization of their HOMO and was inspired
by the increase in oxidation potentials remarked for MgII(TF 2 PP)
or ZnII(TF 2 PP) ( 20 ). The other bacteriochlorins presented in
Fig. 8 were also developed with the concern of producing more
stable photosensitizers than bacteriochlorophylla. Their synthe-
sis was guided by three observations: (i) appropriate metals in
the tetrapyrrolic ring increase its oxidation potential and stabi-
lize the bacteriochlorins against oxidation (Tookad), as in
metallo-bacteriochlorophylls (24,91); (ii) geminal dialkyl groups
in each reduced pyrroline ring lock-in the reduction level of the
bacteriochlorins (H 2 DOH 2 PTMB), as in Tolyporphin A ( 92 ); and
(iii) exocyclic rings impart stability toward oxidation
FIG. 10. Mechanism proposed for the interaction between molecular
oxygen and halogenated and sulfonated bacteriochlorins ( 65 ).
DESIGN OF PORPHYRIN-BASED PHOTOSENSITIZERS 219