Comparison of Exciton Annihilation Processes in Prototypical Charge-Transfer Molecules

In this study, we reveal the origin of exciton annihilation processes in condensed solid-state systems, focusing on a series of archetype charge transfer (CT) molecules, i.e., 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN), showing a thermally activated delayed fluorescence (TADF) and fac-tris[2-phenylpyridine]iridium(III) (fac-Ir(ppy)(3)) characterized by a phosphorescence luminescent property, termed metal-to-ligand CT behaviors. Using the classical rate equations derived from the respective luminescence behaviors in the condensed solid-states, we constructed accurate exciton quenching models for both TADF (System A) and phosphorescence dopants (System B). We found that the origin of optical roll-off behavior in the 4CzIPN-doped films is the singlet-triplet annihilation process, whereas in the case of fac-Ir(ppy)(3), it is attributed to triplet-triplet annihilation through biexcitonic collisions. This provides experimental and theoretical insights into the understanding of roll-off behaviors in a series of CT-type molecules, thus advancing toward the development of a new exciton quenching model for organic luminescent candidates.