2.8 ms in methylcyclohexane at 77 K( 64 ). The phosphorescence
quantum yields tend to increase in the opposite order, and for
PdTPP, the phosphorescence becomes so intense that it can be
observed in deaerated solutions at room temperature with a life-
time of a few hundred microseconds. The nature of the emissive
state for all these metalloporphyrins is the macrocycle (p,p*) state.
With the spectroscopic triplet energy of ZnTPP, it is straightfor-
ward to use time-resolved PAC to determine the triplet quantum
yield. The values ofFTpresented in Table III were obtained using
the triplet energies given by phosphorescence. It is very clear that
introducing heavy atoms in theortho-positions of the phenyl rings
further increases the intersystems crossing rate from S 1 to T 1 and
the triplet quantum yields approach unity.
Time-resolved PAC was also employed to measure the triplet
quantum yields of halogenated H 2 TPP, namely,meso-tetraphenyl
(2,6-dichlorophenyl)porphyrin (H 2 TCl 2 PP), meso-tetraphenyl
(2-chlorophenyl)porphyrin (H 2 TClPP), meso-tetraphenyl(2,6-
difluorophenyl)porphyrin (H 2 TF 2 PP), and their chlorin (H 2 TCl 2 PC,
H 2 TClPC, H 2 TF 2 PC), and bacteriochlorin analogues (H 2 TCl 2 PB,
H 2 TClPB, H 2 TF 2 PB)(32,66,67). The emphasis in such halogenated
derivatives is justified on the basis of the discussion of the stability
of porphyrins. Cl and F substituents raise the oxidation potential of
porphyrins and can protect them from oxidative degradation.
Table III summarizes the most relevant data on the triplet and
TABLE III
TRIPLET ANDSINGLETQUANTUMYIELDS OFHALOGENATEDPORPHYRIN OR
BACTERIOCHLORINSENSITIZERSSensitizer FT FD(PAC) FD(phosph.)
H 2 TPPa 0.730.10b 0.670.14 0.71c
ZnTPPa 0.860.17 0.680.19
ZnTF 2 PPa 0.990.04 0.990.05
ZnTCl 2 PPa 1.020.01 0.900.07
H 2 TF 2 PBd 0.780.04 0.48
H 2 TClPBd 0.950.06 0.59
H 2 TCl 2 PBd 1.060.04 0.60
H 2 TCl 2 PPOHc 1.00
H 2 TCl 2 PBOHc 0.85
aFrom Ref. ( 32 ).
bThis value ofFTwas calculatedET1reported by Harriman for H 2 TPP,ET1¼33 kcal/
mol ( 58 ), which is open to discussion; using the value of porphin,ET1¼36 kcal/mol
( 62 ), we obtainFT¼0.670.10.
cFrom Ref. ( 65 ).
dFrom Ref. ( 66 ).210 LUIS G. ARNAUT