singlet oxygen quantum yields measured by PAC. The heavy-atom
effect expected for the halogen substituents is nicely confirmed by
PAC, and the effect of four Cl atoms is more important than that
of eight F atoms. The values ofFTandFDapproach unity when
eight Cl atoms are included in the phenyl rings of TPP. This shows
that the triplet states of halogenated free-base porphyrins such as
H 2 TCl 2 PP are also sufficiently long-lived (tT> 15 ms in N 2 -
saturated toluene ( 67 )) to transfer energy quantitatively to singlet
oxygen.
It should be emphasized that the measurement of the singlet
oxygen phosphorescence intensity at 1270 nm confirmed the
values FD for the porphyrins but gave systematically lower
values for the bacteriochlorins. PAC measures the amount of
heat released in the formation of transient species, whereas
phosphorescence at 1270 nm measures the amount of singlet
oxygen species. The apparent discrepancy between the two tec-
hniques may be related to the nature of the species that are
formed from the triplet state of bacteriochlorins in the presence
of oxygen and will be discussed in the next section.
Closed-shell metal ions (Mg^2 þ, Zn^2 þ) or diamagnetic metal ions
with low-lying d orbitals (Pd^2 þ, In^3 þ) accelerate the S 1 !T 1
intersystems crossing and increase triplet quantum yields. The
T 1 !S 0 intersystems crossing rate is also increased, but in the
case of PdIITPP, the triplet state lifetime is still sufficiently long
(tT¼ 380 ms in outgassed methylcyclohexane( 64 )) to transfer its
energy quantitatively to singlet oxygen. However, the triplet
state lifetime of ClInIIITPP in N 2 -saturated DMSO is only
1.6ms, and this may be at the origin of the modestFD¼0.72 in
O 2 -saturated DMSO ( 68 ). The spin–orbit coupling constantzof
Pd is larger than that of In, and the decrease of the triplet life-
time in ClInIIITPP is probably associated with the concomitant
presence of Clas axial ligand.
The macrocycles of conformationally strained porphyrins,
and, in particular, 2,3,7,8,12,13,17,18-octaalkyl-5,10,15,20-
tetraarylporphrins, tend to adopt nonplanar conformations. This
has dramatic consequences in singlet and triplet lifetimes.
Increasing the macrocycle distortion with the introduction of
four or more ethyl groups in adjacent b positions decreases
triplet lifetimes to submicrosecond values and compromises
FD. This factor adds to the decrease ofFT due to decrease of
the singlet state lifetime, and FD¼0.07 was reported for
2,3,7,8,12,13-hexaethyl-5,10,15,20-TPP ( 61 ). Porphyrins with
substituents in both b- andmeso-positions tend to have fast
T 1 !S 0 intersystems crossing and relatively short-lived triplet
states ( 69 ), which are inappropriate for PDT.
DESIGN OF PORPHYRIN-BASED PHOTOSENSITIZERS 211