The origin of the delayed emission in films of a ladder-type poly(para-phenylene)

We report on the observation of delayed fluorescence (DF) and phosphorescence (P) from films of the ladder-type poly(para-phenylene) (MeLPPP). Upon pulsed optical excitation into the S1←S0 transition of the MeLPPP, delayed emission occurs on a time scale of μs to ms. The DF varies linearly with pump light intensity, while the phosphorescence shows a sublinear intensity dependence. We will argue that singlet–singlet (SS) annihilation is responsible for the sublinear phosphorescence intensity (Pint) dependence, since the intensity dependence of the prompt fluorescence shows the same behaviour above excitation intensities of 1 μJ/pulse. Both, DF and P decay according to a power law. The data analysis reveals that the DF is caused by recombination of geminate electron hole pairs rather than triplet–triplet (TT) annihilation. This conclusion is supported by investigations of the response of the DF to an applied electric field.