Knowledge Bank

The low field Zeeman effect on the phosphorescence intensity emitted from the lowest triplet state of a molecule cooled to liquid helium temperatures can give information concerning the radiationless pathway of intersystem crossing from the singlet manifold to the triplet manifold. In certain cases, it is possible to distinguish whether the intersystem crossing occurs directly to the lowest triplet $(S1⟶T1)$ or indirectly through an intermediate triplet, $Tx$, located in energy between $S1$ and $T1$, followed by internal conversion to $T1(S1⟶Tx⟶T1)$. Once the pathway of intersystem crossing is found and if the symmetries of the states involved are known, it is possible to draw conclusions concerning the symmetry of the most effective promoting vibrational mode(s) for the inter-system crossing process. Recent results for aromatic and substituted aromatic molecules will be discussed.