Decomposition of a phthalocyanine dye in various conditions under UV or visible light irradiation

The photostability of a copper complex of a phthalocyanine dye at 313 nm and under visible light was investigated in various solvents both in the presence or absence of O2. The dye was found relatively stable in water, while substantial decomposition, in different times, occurs in methanol, ethanol, acetone and dimethylformamide. A study of the products formed in some of these solvents was performed, allowing to elucidate the mechanism of the colour fading. An interesting and unexpected effect was found in the case of mixed solvents. The presence of acetone in the hydroxylated solvents, even at very low concentration, increases dramatically the rate of decomposition of the dye. This effect is strongly enhanced with increasing of the aliphatic chain size of the ketone. A hypothesis on this effect is put forward.