Phosphorescence of Large Polycyclic Aromatic Ketones

Phosphorescence measurements were performed for 6H-benzo[cd]-pyren-6-one (l), 2-nitro-benzo[hi]chrysen- 13-one (2), 2-bromo-di-benzo[a, de]anthracen- 13-one (3), and 7H-benzo[hi]chrysen-7-one (4) in an ethanol-ether-toluene mixed solution and the character of their lowest excited triplet (T₁) states were investigated. The phos horescence of the compounds appears at about 5,200 – 6,000 cm^?1 to the red of their fluorescence, indicating that the T, states have ππ^* character. On the other hand, the short phosphorescence lifetimes, 10 – 60 ms, suggest nπ^* transitions. Such a duality of the T₁ state seems to be characteristic of large polycyclic aromatic ketones. The polarized emission study revealed that the duality is ascribed to strong mixing of ππ^* and nπ^* natures.