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 int egral encounter theory. The triplets appear due to inter-system crossi ng, simultaneous with fast singlet decay, accompanied by singlet-singl et and singlet-triplet quenching. The shortage of near-contact excitat ions resulting from the quasi-static singlet quenching shows up in the appearance of singlet and triplet phantoms. Though negative, their de nsities yield the same diffusional equations with recombination as rea l excitons. These densities may be built into the initial conditions f or triplet annihilation if the latter is much slower than the preparat ion of the system. We justified the shape of the initial triplet distr ibution generated by instantaneous excitation, but corrected the distr ibution created by a longer pulse when annihilation during excitation should be accounted for. The near-contact gaps in initial distribution s lead to the 'anti-Smoluchowski' acceleration of diffusional or hoppi ng annihilation of mobile triplets and to the hindering of their stati c 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.