Triplet-triplet absorption studies on related heterocyclic molecules

A study of the triplet-triplet (T-T) absorption spectra of a series of aromatic heterocyclic molecules in EPA glass at 77°K has been carried out in the region 11 000–27 000 cm−1 using a steady cross illumination technique. Two transitions have been observed in each of dibenzofuran and dibenzothiophene and assigned as ππ ∗ ← ππ ∗ in nature. A similar assignment has been made for the two transitions observed in 5,6-benzoquinoline and the one transition observed in 7,8-benzoquinoline. A single transition has been observed in phthalazine. Measurements of the decay of T-T absorption have been carried out and, in all cases, the correspondence with the phosphorescence lifetimes verified the triplet nature of the observed absorptions. A comparison of the information obtained from T-T spectra with phosphorescence and/or ESR data is made in order to demonstrate how T-T spectra can be used to aid in the understanding of the properties of the lowest triplet state. Thus for dibenzofuran and dibenzothiophene, a comparison of upper triplet state energies suggests that the heteroatoms do not provide effective conjugation with the hydrocarbon rings in the lowest triplet state. For the benzoquinolines, the T-T spectra suggest a close analogy with the triplet state properties of phenanthrene although 4-aza substitution (7,8-benzoquinoline) seems to produce an increased perturbation of the ππ ∗ states by nπ ∗ states as compared with 1-aza substitution (5,6-benzoquinoline).