Delayed Fluorescence and Triplet-Triplet Annihilation inπ-Conjugated Polymers

The delayed fluorescence of poly( $p$-phenylene vinylene) (PPV) and poly( $p$-phenylene ethynylene) (PPE) derivative solids and frozen solutions at 20 K is described. It provides strong evidence for triplet-triplet annihilation to singlets excitons, accounting for up to $\sim 3\%$ of the total emission in PPV films and $\sim 1.5\%$ in PPE powder. It also yields triplet lifetimes of 70 and $110\mu \mathrm{s}$ in PPV films and frozen solutions, and $\sim 200$ and $\sim 500\mu \mathrm{s}$ in PPE powder and frozen solutions, respectively.