Observation of up-conversion fluorescence from exciplex of m-MTDATA:TPBi blend

Abstract Exciplex-type thermally activated delayed fluorescence (TADF) materials exhibit charming application potential in organic light-emitting diodes. However, investigations on their non-linear optical properties are rather rare. Here, we report up-conversion fluorescence from an electron donor-acceptor exciplex blend composed of the donor 4,4′,4″-Tris(N-3-methylphenyl-N-phenyl-amino)-triphenylamine (m-MTDATA) and the acceptor 1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi). Strong up-conversion fluorescence has been observed and a two-photon absorption (TPA) cross-section σ of ∼387 GM has been measured. The optical spectra analysis indicates that the TPA is dominated by the exciton absorption of m-MTDATA molecules. The observed up-conversion fluorescence is explained by a joint mechanism including two-photon absorption, exciplex formation, reverse intersystem crossing, and prompt or delayed fluorescence. The TADF and up-conversion fluorescence properties of this exciplex blend have great application potentials in linear and non-linear optics.