Anti-Stokes Emission Utilizing Reverse Intersystem Crossing

Photon-upconversion (PUC) processes in organic molecular systems, such as triplet-triplet upconversion and hot-band absorption, are promising technologies for future energy harvesting. Although these processes can generate high-energy excitons compared to excitation energy, a PUC process with a high yield and no energy loss has not been established and, therefore, is highly desired. Here, we propose an alternative PUC mechanism that uses reverse intersystem crossing on thermally activated delayed fluorescence (TADF) molecules. This process combines a triplet sensitizer and a TADF molecule, generating a triplet in the former and transferring it to the latter. Specifically, the triplet energy transfer from Ir(ppy)₃ (sensitizer) to CzBSe (TADF) results in anti-Stokes emission with an anti-Stokes energy of 0.18 eV. We found that the triplet energy transfer rate strongly depends on the triplet radiative decay rate of TADF molecules and the difference in Gibbs energy between the energy acceptor and donor. Our findings will contribute to understanding triplet energy transfer dynamics in organic energy donor-acceptor systems and will lead to various applications, such as future optical cooling systems.