Enhanced two-photon-excited fluorescence from electron donor-acceptor exciplex

Exciplex-based thermally activated delayed fluorescence (TADF) materials show long luminescence lifetimes and high quantum efficiency by converting the non-radiative triplets into the radiative singlets. Here, strong upconverted fluorescence from the amorphous thin films of TADF donor-acceptor (D-A) exciplexes is observed under near-infrared excitation. The spectroscopic studies demonstrate that the enhanced upconversion in exciplex emission combines a series of excited state processes including the two-photon absorption (TPA) of donors and the TADF of D-A pairs. The comparison between donor-only and D-A systems indicates that the upconversion is greatly facilitated by the energy harvesting mechanisms existing in the exciplex-based TADF materials. The results reveal the formation mechanism of exciplex excited-states following the two-photon excitation in D-A blends, and consequently the potentials of exciplex materials in nonlinear optics applications. Moreover, a maximum D-A distance of similar to 6.9 nm for two-photon excited exciplex formation is obtained.