As mentioned before, the singlet state lifetimes of porphyrins
rarely exceed 10 ns, and this implies that the efficiency of porphy-
rin singlet state reactions with molecular oxygen is lower than 8%.
A spin-allowed reaction of singlet states with molecular oxygen
that was observed in MgTPP and ZnTPP is assisted intersystems
crossing to form the triplet state of the photosensitizer while
molecular oxygen remains in the ground state ( 56 ). More efficient
photoreactions can only be obtained with long-lived triplet states
formed with high triplet quantum yields.
C. TRIPLETSTATE
The phosphorescence of free-base porphyrins is extremely
weak. The triplet energy of H 2 TPP was reported as 33.5 kcal/
mol ( 57 ), and Harriman presented a phosphorescence spectrum
in methylcyclohexane at 77 K that is consistent with this value
( 58 ). Moore and coworkers challenged this triplet energy on the
basis of photoacoustic calorimetry (PAC) data and suggested that
it could be 3–9 kcal/mol lower in energy ( 59 ). However, the photo-
acoustic measurements of Moore only give the product of the
triplet energy by the quantum yield of its formation,
FTET¼21.4 kcal/mol, andFTis not known with sufficient accu-
racy to provide a more reliable estimate of ET. Brauer and
coworkers revisited the phosphorescence of H 2 TPP with a sensi-
tive near infrared detector and reported ET¼33.3 kcal/mol in
TABLE IIEMPIRICALCOEFFICIENTSFITTED TO THEINTERSYSTEMSCROSSINGRATES OFFREE-BASE
ANDMETALLOPORPHYRINSa
S 1 !T 1 T 1 !S 0c 1 c 2 c 1 c 2Octaalkylporphyrins X in alkyl 5 10 ^710 ^5
X inbeta 2 10 ^55 10 ^4
X inmeso 10 ^410 ^3
Metalloporphyrins 2 10 ^42 10 ^5
H 2 TPP X in
phenyl
ortho
2 10 ^6ZnTPP X in
phenyl
ortho
2 10 ^62 10 ^45 10 ^72 10 ^5aThe coefficientc 1 refers to the halogen X, andc 2 refers to the metal( 53 ).208 LUIS G. ARNAUT