Four-Photon Molecular Upconversion

Triplet-triplet annihilation (TTA) upconversion converts low-energy photons into higher-energy emission, but conventional TTA is limited by its two-photon nature, as the emitted photon energy cannot exceed twice the excitation photon energy. Here, a fully organic solid-state thin-film architecture cascades two TTA upconversion stages through an intermediate Förster resonance energy transfer (FRET) bridge to achieve molecular four-photon upconversion. The first sensitizer/annihilator bilayer upconverts near-infrared (NIR) excitation into intermediate singlets, which are transferred through the FRET bridge to the sensitizer of the second TTA stage. The second sensitizer/annihilator bilayer performs a second TTA step to generate blue emission. The device converts 904 nm excitation into 445 nm emission, corresponding to a 1.41 eV anti-Stokes shift and an emitted photon energy 2.03 times the excitation photon energy, exceeding the maximum possible energy gain with conventional two-photon TTA. Because two quadratic TTA stages are coupled in series, the ideal low-intensity response of the four-photon device is quartic when both stages operate below threshold, providing a steeper nonlinearity desirable for nonlinear sensing. Experimentally, power-dependent measurements reveal a superquadratic slope of 3.03, followed by quadratic and linear regimes as the constituent TTA stages cross their thresholds. Independent stage measurements and time-resolved photoluminescence dynamics support the four-photon mechanism.