Triplet-triplet annihilation after biexciton quenching of singlets

The rates and kinetics of delayed fluorescence resulting from triplet-triplet annihilation was studied within the original and universal integral encounter theory. The triplets appear due to inter-system crossing, simultaneous with fast singlet decay, accompanied by singlet-singlet and singlet-triplet quenching. The shortage of near-contact excitations resulting from the quasi-static singlet quenching shows up in the appearance of singlet and triplet phantoms. Though negative, their densities yield the same diffusional equations with recombination as real excitons. These densities may be built into the initial conditions for triplet annihilation if the latter is much slower than the preparation of the system. We justified the shape of the initial triplet distribution generated by instantaneous excitation, but corrected the distribution created by a longer pulse when annihilation during excitation should be accounted for. The near-contact gaps in initial distributions lead to the 'anti-Smoluchowski' acceleration of diffusional or hopping annihilation of mobile triplets and to the hindering of their static annihilation at earlier times. This enables a quantitative analysis of delayed fluorescence in the liquid and solid phase and a consistent determination of the annihilation rate parameters. (C) 1998 Elsevier Science B.V. All rights reserved.