Effects of halogen substitution on the photostability and thermal degradation of boron(III), zinc(II) and cadmium(II) dipyrrinato complexes

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The results of investigation of the photostability and thermal degradation of B(III), Zn(II), Cd(II) complexes with monoiodo- and dibromosubstituted dipyrrins of the composition [BF2L] and [ML2] were presented. The processes of photo- and thermal degradation of complexes includes the initial stages of dye molecules dehalogenation. In general, the replacement of 4-iododipyrrin ligand by 5,5′-dibromo- and 4,4′-dibromosubstituted analogs in the composition of complexes [BF2L] and [ML2] promotes an increase in the photo- and thermal stability of the dyes. The replacement of the saturate hydrocarbon (cyclohexane) with the aromatic analogue (benzene) leads to a significant decrease in the photostability of [BF2L] and [ML2] because of polarization of the chromophore systems of dye molecules due to π-π stacking with aromatic solvent molecules. The thermal destruction start temperatures [ML2] and [BF2L] in the argon atmosphere range from 179 to 275 °C and strongly depend on the nature and efficiency of the participation of complexing and halogen atoms in intramolecular redox reactions. Complexes [BF2L], which showed the greatest photostability in solutions, differ the least thermal stability in the solid phase. The observed differences are due to the specific structural effects and different mechanisms of the photo- and thermal degradation processes under various environmental conditions.