Emission of substituted pyrylium and thiapyrylium salts: Phosphorescence and delayed fluorescence emission in polymeric matrices

Pyrylium and thiapyrylium salts show interesting emission properties when incorporated in a rigid PMMA matrix. In addition to a strong prompt fluorescence emission, the 18 compounds studied show a delayed fluorescence which cannot be observed in solution, even at 77 K. The delayed fluorescence/phosphorescence ratio increases with the temperature. The activation barrier as well as the intersystem crossing from the lowest excited triplet state to the lowest excited singlet state have been measured for compounds 4, 8, 11, and 17. An absence of crossing point between the potential energy surfaces of the two excited states is put forward to explain the low value (ca. 103 s−1) of the intersystem crossing rate constant.