Variables Affecting Phosphorescence and Fluorescence

The molecular structure and chemical environment determine the luminescence of a substance.

The number of luminescing molecules to the number of excited molecules represents quantum efficiency for phosphorescence or fluorescence.

Aromatic compounds with low-energy π-π* transitions exhibit intense fluorescence, whose quantum efficiency increases with the number of rings and degree of condensation.

Substitution on the benzene ring impacts the absorption maxima and fluorescence emission.

For instance, fluorescence decreases with increasing molar mass of the substituted halogen due to the heavy-atom effect.

Conversely, carboxylic acid or carbonyl groups often inhibit fluorescence.

Rigid molecules exhibit stronger fluorescence than flexible molecules due to a reduced rate of nonradiative relaxation, allowing sufficient time for fluorescence.

So, chelated metal complexes show enhanced fluorescence.

Increasing temperature and decreasing solvent viscosity increase collision frequency, enhancing the probability of deactivation through external conversion, which reduces fluorescence.

Fluorescence in aromatic compounds with acidic or basic ring substituents is pH-dependent, resulting in emission variations arising from differences in their resonance structures.