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Photoacoustic calorimetry was used to measure the quantum yields of singlet molecular oxygen production by the triplet states of tetraphenylporphyrin (TPP), ZnÀTPP and CuÀTPP in toluene, yielding values of 0.67 AE 0.14, 0.68 AE 0.19 and 0.03 AE 0.01, respectively. We show that a novel dichlorophenyl derivative of Zn À TPP is capable of singlet-oxygen production with a 0.90 AE 0.07 quantum yield. The synthesis and characterisation of a new photostable chlorin with high absorptivity in the red that is capable of singlet-oxygen production with 0.54 AE 0.06 quantum yield is described. Our results suggest that chlorinated chlorins may be interesting new sensitisers for photodynamic therapy.

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New halogenated and sulfonated bacteriochlorins and their analogous porphyrins are employed as photosensitizers of singlet oxygen and the superoxide ion. The mechanisms of energy and electron transfer are clarified and the rates are measured. The intermediacy of a charge-transfer (CT) complex is proved for bacteriochlorins, but excluded for porphyrins. The energies of the intermediates and the rates of their interconversions are measured, and are used to obtain the efficiencies of all the processes. The mechanism of formation of the hydroxyl radical in the presence of bacteriochlorins is proposed to involve a photocatalytic step. The usefulness of these photosensitizers in the photodynamic therapy (PDT) of cancer is assessed, and the following recommendations are given for the design of more effective PDT protocols employing such photosensitizers: 1) light doses should be given over a more extended period of time when the photosensitizers form CT complexes with molecular oxygen, and 2) Fe(2+) may improve the efficiency of such photosensitizers if co-located in the same cell organelle assisting with an in vivo Fenton reaction.

The photophysics of halogenated and metallated tetrakisphenylporphyrins is investigated using single-photon counting, photoacoustic calorimetry and luminescence techniques. The radiationless transition rates in these and related molecules are interpreted with a quantum-mechanical tunnelling model modified to include the effect of spin-orbit coupling in the intersystem crossing rates. It is shown that tetrakisphenylporphyrins with halogens in the ortho positions have long-lived triplet states that are formed in high yields. Ó