Five di- and tri-substituted natural anthraquinones, i.e. nordamnacanthal (1), damnacanthal (2), rubiadin (3), 1-methoxy2-methyl-3-hydroxyanthraquinone
(4) and 1-hydroxy-3-methoxyanthraquinone (5) were subjected to photophysical
studies. The results indicated that steric hindrance and intramolecular hydrogen bonding are important factors that
affect absorption and emission spectral of these natural anthraquinones. Besides that, emission properties were
significantly enhanced with formation of intramolecular hydrogen bonding in 1,3-dihydroxy-2-aldehyde tri-substituted
anthraquinone 1. This gave rise to formation of two additional quasi aromatic rings extending the π-conjugation system
in the anthraquinone structure.