Three potential photosensitizers for photodynamic therapy: tetra-t-butyl phthalocyaninato Zn(II), tetrakis-(1,ldimethyl-2-phthalimido)ethylphthalocyaninato Zn(II), and tetrakis-(l,l-dimethyl-2-amino)ethylphthalocyaninato Zn(II), have been studied in homogeneous organic media. Dimerization constants and monomer and dimer spectra have been determined. Fluorescence, triplet and singlet oxygen quantum yields were measured and the photophysical characterization of the triplet state was performed for the three dyes. These parameters were evaluated in connection with aggregation results.

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This work describes a systematic comparison of oxygen and sulfur as covalent linkers on octasubstituted zinc(II) phthalocyaninates. Most photophysical parameters that make phthalocyanines technologically relevant, e.g. molar absorption coefficients, fluorescence, triplet and singlet oxygen quantum yields, are essentially unaffected by the substitution. The energy content of the first triplet state was observed to be close to the first singlet state of molecular oxygen for both spacers, as follows from photoacoustic determinations. Nonetheless, a bathochromic shift of 30 nm in the absorption and emission maxima, and of 60 nm in the triplet-triplet absorption spectra were observed when alkyloxyl and alkylsulfanyl moieties were alternatively present. Fluorescence quantum yields proved to be much more sensitive towards aggregation than the absorption spectra. Therefore, a novel fluorescence data analysis provided aggregation parameters and photophysical properties of the monomeric species. It was observed that the tendency towards dimerization is slightly higher with sulfur linkers. These results set a foundation for the rational design of conveniently substituted phthalocyaninates with different connectors between the macrocycle and the side chains.

Factors such as charge, aggregation and lipophilicity influence photosensitiser localisation. The lipophilic octasubstituted sensitiser 2,3,9,10,16,17,23,24-octakis(decyloxy)phthalocyaninato zinc(II) was incorporated into liposomes of dimyristoyl-L-alpha-phosphatidylcholine (DMPC), non-ionic micelles of Tween 80 and the hydrosoluble polymer Solutol HS 15 in order to investigate how these different environments affect the photophysical properties and phototoxicity of the photosensitiser. Fluorescence quantum yields and singlet molecular oxygen generation are enhanced in the presence of Solutol HS 15. Phototoxicities were calculated by employing a concentration of 10(-7) M of the dye against the Hep-2 cell line, which showed a viability of 53 and 30% in DMPC and Solutol HS 15, respectively. After 24 h of photodynamic therapy with 15 min irradiation, apoptotic and necrotic cells were observed.

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