Delayed Fluorescence and Triplet-Triplet Annihilation in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi mathvariant="italic">π</mml:mi></mml:math>-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 $\ensuremath{\sim}3%$ of the total emission in PPV films and $\ensuremath{\sim}1.5%$ in PPE powder. It also yields triplet lifetimes of 70 and $110\ensuremath{\mu}\mathrm{s}$ in PPV films and frozen solutions, and $\ensuremath{\sim}200$ and $\ensuremath{\sim}500\ensuremath{\mu}\mathrm{s}$ in PPE powder and frozen solutions, respectively.