unchanged. The intensity of the 'Ag •*-'Bu* emission band also increases relative to those from 'Ag •*-'Ag* with increasing solvent polarizability and with increasing temperature.We have observed similar behavior upon changing the solvent from hexane (as = 0.31) to hexadecane (as = 0.35). Table II lists the shifts in the absorption and fluorescence maxima as well as those of the 'Bu* emission band for the arene probes. As expected for excitation to a 'Bu* state, the absorption maximum of each probe is most sensitive to the solvent change. The emission maxima of the DPH probes do not shift upon changing the solvent. This can be attributed to the fact that this emission is predominantly from an 'Ag* state. The emission band on the blue edge of the fluorescence envelope, however, does show sensitivity to the solvent. Not only are substantial shifts of the peak at -385 nm observed (210 and 270 ± 30 cm"' for DPH and 4H4A, respectively) but the intensity of this peak relative to that of the maximum also increases with increasing as. There is no corresponding emission band observed in the spectra of the DPB compounds.

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Abstract— The o‐, m‐, and p‐isomers of 5, 10, 15, 20‐ tetra(hydroxyphenyl)‐porphyrin have been of recent interest as potential second‐generation sensitisers in tumour phototherapy. Fluorescence spectroscopy, nanosecond laser flash photolysis and pulse radiolysis have been used to characterise the singlet and triplet excited states of tetraphenylporphyrin and the o, m‐, and p‐isomers of tetra(hydroxyphenyl)porphyrin. This has included evaluation of fluorescence yields and lifetimes, triplet spectra, lifetimes, oxygen quenching rate constants, extinction coefficients, and yields and singlet oxygen yields. Whilst the fluorescence quantum yields were low, the triplet yields were all 0.7 ± 10% and the singlet oxygen yields 0.6 ± 10%: all these parameters are in the ranges shown by other efficient porphyrin photosensitisers. The similar photophysical properties found for these compounds suggest that their differing tumour sensitising efficiencies are likely to be due to other factors.

Gadolinium(III) texaphyrin (Gd-tex2+) is representative of a new class of radiation sensitizers detectable by magnetic resonance imaging (MRI). This porphyrin-like complex has a high electron affinity [E1/2 (red.) approximately = -0.08 V versus normal hydrogen electrode] and forms a long-lived pi-radical cation upon exposure to hydrated electrons, reducing ketyl radicals, or superoxide ions. Consistent with these chemical findings, Gd-tex2+ was found to be an efficient radiation sensitizer in studies carried out with HT29 cells in in vitro as well as in in vivo single and multifraction irradiation studies with a murine mammary carcinoma model. Selective localization of Gd-tex2+ in tumors was confirmed by MRI scanning.

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