Triplet-triplet annihilation photon up-conversion: Accessing triplet excited states with minimum energy loss

Triplet-triplet annihilation photon up-conversion (TTA-PUC) has gained immense attention among the scientific community in the last decade due to its application in the fields of energy, biology, and photocatalytic organic synthesis. One of the main aims to improve the efficiency of these low-to-high photon-energy conversion is to reduce energy losses during the intersystem crossing (ISC). Since 2015, many strategies have been reported to address this challenge and a significant update has been noticed in this field. This review is aimed to critically analyze these updates and provide an outlook for the future. A detailed mechanism of ISC in thermally activated delayed-fluorescence (TADF) molecules that possess a small singlet−triplet energy gap, is discussed with a focus on its deeper understanding and the impact of molecular design. In this context, a range of selected organic and inorganic TADF molecules are thoroughly evaluated. Osmium(II) complexes that exhibit a spin-forbidden metal-to-ligand charge-transfer (3MLCT) transition in their Vis-NIR-IR absorption spectra and can be excited directly into their triplet state, thereby bypassing the energy loss during ISC, are also debated in sufficient detail for their advantages as well as shortcomings in being used in TTA-PUC. This work aims at reviewing the latest progress in this field, understanding the fundamental ISC mechanism of these photosensitizers, and critically addressing the challenges that are faced in this field. This review is anticipated to serve as a helpful script for identifying future directions and designing molecular sensitizers for TTA-PUC, which can sensitize the triplet state with minimum energy loss during ISC and can be helpful for increasing the anti-Stokes shift in TTA-PUC.