We observed phosphorescence from a ladder-type poly-(para-phenylene) and an analogous oligomer containing five phenylene rings. The spectra are similar to the intrinsic fluorescence spectra and bear out a singlet-triplet splitting of $5000\mathrm{cm}{}^{-1\mathrm{}}$ (polymer) and $6800\mathrm{cm}{}^{-1\mathrm{}}$ (oligomer). Phosphorescence decay of the polymer occurs on a $10–100-\mu \mathrm{s}$ scale obeying a power law and suggestive of nonradiative quenching, while that of the oligomer is asymptotically exponential with an intrinsic decay time of $\simeq 250\mathrm{ms}$. The polymer also exhibits delayed fluorescence. It originates from delayed recombination of geminate electron-hole pairs rather than from triplet-triplet annihilation.

• Received 28 June 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.1027