The Two n,π* Transitions of Monohalogenoanthraquinones

The n,π* absorption and phosphorescence spectra of 1-fluoro-, 2-fluoro, 1-chloro-, 2-chloro-, 1-bromo-, and 2-bromoanthraquinones were studied in solutions, mainly at 77 K. These phosphorescence spectra and their lifetimes resemble those of anthraquinone (AQ), except for those of 1-F-AQ. The phosphorescence spectrum of 1-F-AQ is at longer wavelengths than that of AQ, and its lifetime is 0.8 ms in the methylcyclohexane solution, much shorter than that of AQ. All of the 1-halogenoanthraquinones showed two distinct n,π* singlet-singlet absorption bands at 77 K, and 1-Br-AQ showed an additional band of the n,π* singlet-triplet transition at 492 nm. The low-energy n,π* singlet and triplet states of these compounds are discussed on the basis of these experimental results.