The lowest triplet states of anthraquinone and chloroanthraquinones: The 1-chloro, 2-chloro, 1,5-dichloro, and 1,8-dichloro compounds

From the measurements of phosphorescence spectra and triplet–triplet absorptions of the title compounds, it has been proposed that the lowest triplet states (T1) of the α-chloro compounds are of mixed nπ*–ππ* or ππ* character, while the nπ* triplet states are the lowest ones for anthraquinone and 2-chloroanthraquinone (the β-chloro compound). Compared with anthraquinone and the β-chloro compound, much shorter lifetimes of the T1 states and small phosphorescence quantum yields were obtained for the α-chloro compounds. These results have been interpreted in terms of the modification of the geometrical molecular structure by the interaction of the carbonyl group with chlorine atom(s) at the α position(s), causing the T1 states to be of ππ* character with short lifetimes.