Quantitative Study of Luminescence in Aromatic and Heteroaromatic Molecules

Quantum yields of fluorescence and phosphorescence were determined for about 20 organic compounds (and their deuterated analogs) using fluorescence yield of 9, 10‐diphenylanthracene as a reference standard. The emission yields so obtained are combined with appropriate phosphorescence lifetime data to deduce conclusions related to radiative and radiationless transitions in these molecules. Evidence is presented which indicates that the dominant singlet→triplet intersystem crossing is $S_1{\mathrm{\to \; T}}_1$ in benzene but $S_1{\mathrm{\to \; T}}_j\text{(j>1)}$ in larger aromatic hydrocarbons. The results also indicate that deuteration of polycyclic hydrocarbons significantly increases the radiative lifetime of the lowest state, contrary to previously held views. Solvent and temperature dependence of fluorescence in nitrogen‐heterocyclic compounds and phosphorescence characteristics of aromatic carbonyl compounds are discussed in terms of vibronic interactions between nearby ππ^* and nπ^* states.