from fluorescence intensities to quantum yields is not usually done.
Nevertheless,theuseofspecificfluorescencemarkershasbeenused
to correlate the relative amounts of^1 O 2 and O 2 generated by
photosensitizers under the same experimental conditions with
their phototoxicities ( 85 ). It was found that in a series of three bac-
teriochlorins with a geminal dimethyl group in each pyrroline ring,
the best performing bacteriochlorin in PDT of melanoma cell lines,
1,13-bis(3,5-dihydroxyphenyl)-8,8,18,18-tetramethylbacteriochlorin
(H 2 DOHPTMB), shown in Fig. 8, was also the one that had the
highest fluorescein emission( 85 ).
Finally, PAC was recently employed to obtain the quantum yields
of long-lived species, such as^1 O 2 and O 2 , formed after laser pulse
excitation of halogenated bacteriochlorins in aerated solutions. As
mentioned before, the values ofFDinitially reported with PAC
seemed to be systematically higher than the values obtained by sin-
glet oxygen phosphorescence ( 66 ). This discrepancy is removed
when it is considered that triplet bacteriochlorins generates both
(^1) O
2 and O 2
in aerated solutions. It should be remembered that
PAC measures the amount of heat released in the formation of a
transientandthatfractionsofheatreleasedinthesequentialforma-
tionoftransientswithdifferenttimescalescanbeseparated(86,87).
In the time window of a PAC experiment, the triplet state of the
bacteriochlorin is formed“instantaneously”and the first heat decay
corresponds to the difference between the energy of the light
absorbed and the triplet state energy, with appropriate consider-
ation of the triplet quantum yield and of the energy lost by fluores-
cence. The energy balance of the various fast processes gives
f 1 Ehn¼EhnFTETFFES (18)
The second heat decay corresponds to the reactions of the triplet.
When the triplet reacts by energy and electron transfer with
molecular oxygen, the energy balance is( 65 )
f 2 Ehn¼FDð ÞþETED FCTð ÞþETECT ðÞFTFDFCTET
(19)
The last term represents nonproductive channels such as the
fraction of triplet states that decays by process other than
quenching by molecular oxygen or the fraction of
bacteriochlorinþ/O 2 ion pairs that undergo charge recombina-
tion before they are fully solvated. Additionally, in some cases,
there is evidence for a long-lived CT complex with O 2 ( 81 ),
which may not decay in the time window of the PAC experiment.
The halogenated and sulfonated bacteriochlorins shown in Fig. 8
have long-lived triplet states and fast quenching rates with
DESIGN OF PORPHYRIN-BASED PHOTOSENSITIZERS 217