Delayed fluorescence and phosphorescence from polyphenylquinoxalines

We report on phosphorescence and delayed fluorescence from polyphenylquinoxalines in frozen solution and in the film at 77 K. Upon pulsed optical excitation, the following types of experiments have been carried out: (a) optical spectra detected by means of a gated optical multichannel analyzer, (b)-transient measurements employing a single photon counting setup, and (c) photoluminescence excitation spectra. We will argue that in a polymer in which the quinoxaline units are directly linked together via C-C-bonds, the delayed fluorescence is due to the recombination of Coulombically bound electron-hole pairs, which on their part are generated by direct optical excitation. This is deduced from the facts that (a) the intensities of delayed fluorescence, phosphorescence and prompt fluorescence all vary linearly with the pump intensity; that (b) both delayed fluorescence and phosphorescence decay according to a power law up to 1 s, i.e., at times exceeding the triplet lifetime; and that (c) the photoluminescence excitation spectrum of the delayed fluorescence shows an onset at 0.4 eV above the absorption edge. In an oxygen-bridged polyphenylquinoxaline, on the other hand, both geminate pair recombination and triplet-triplet annihilation play a role in the generation of delayed fluorescence.