Photochemistry of thioxanthones VI: A polymerization, spectroscopic and flash photolysis study on novel water-soluble substituted 3-(9-oxo-9 H-thioxanthene-2,3-γ-4-yloxy)- N,N,N-trimethyl-1-propanaminium salts

Abstract

The photopolymerization activity and photochemistry of nine novel water-soluble 3-(9-oxo-9 H-thioxanthene-2,3-γ-4-yloxy)- N,N,N-trimethyl-1-propanaminium salts is examined in water and 2-propanol using absorption, luminescence and conventional microsecond flash photolysis techniques. For both the chloro and the methylsulphonate salts, photopolymerization activity in water follows the order of substitution 4- > 2- > 3-. Methyl substitution in the 1-position deactivates the molecule and this is reflected by a reduction in both the photopolymerization activity and luminescence (fluorescence and phosphorescence) quantum yields and the triplet lifetime. Intramolecular hydrogen-atom abstraction is the main competitive process suppressing photochemical activity. The longest wavelength absorption maxima and extinction coefficients and luminescence quantum yields are similar to those of water-soluble 2-substituted derivatives studied previously, indicating a lowest excited singlet state with strong n̄n̄ * character. On changing the solvent from 2-propanol to water all the fluorescence quantum yields are enhanced and this is consistent with a strong degree of charge transfer character in the lowest excited singlet state. From conventional microsecond flash photolysis results, transient absorptions below 400 nm are associated with the intermediate ketyl radical formed by the lowest excited singlet/triplet states of the thioxanthene molecule abstracting a hydrogen atom from the amine synergist (used in photopolymerization) while absorptions above 400 nm are associated with the radical anion intermediate formed by a concurrent electron abstraction process from the amine. The latter is confirmed through a detailed study on the effect of pH and amine concentration on transient production, as well as the effect of various amines of increasing ionization potential. Transient formation is only partly reduced in aerobic conditions and is consistent with our earlier findings and conclusions that photopolymerization activity is associated, in the main, with electron abstraction by the lowest excited singlet state of the thioxanthone molecule from the tertiary amine synergist. This is confirmed by a correlation between the photopolymerization activity and the ionization potential of different amines.