Luminescence of nitroaromatic molecules

Mono-nitroaromatic hydrocarbons exhibit phosphorescence but do not usually fluoresce in standard glassy media at 77°K. The inverse situation (i.e., fluorescence but no phosphorescence) dominates luminescent behavior in highly acidic glassy media at 77°K. Mono-nitroaromatic hydrocarbons containing a large aromatic system exhibit both fluorescence and phosphorescence in both neutral and acid media. These peculiarities are interpreted on the basis of the induction of intramolecular charge-transfer character into the lower-energy excited states of the molecule by either protonation of the nitro group or by increasing the size of the aromatic moiety.