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 Si <-- S-0 transition of the MeLPPP, delayed emission occurs on a rime scale of <mu>s 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 (P-int) dependence, since the intensity dependence of the prompt fluorescence shows the same behaviour above excitation intensities of 1 muJ/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. (C) 2001 Elsevier Science B.V. All rights reserved.