Triplet-harvesting materials are a class of functional materials designed to utilize triplet excited states (T₁) for efficient light emission or energy conversion1,2,3. These materials play a crucial role in advanced optoelectronic applications by overcoming the spin-forbidden nature of triplet transitions through strategic molecular design. Generally, the luminescent mechanisms of triplet-harvesting materials can be divided into three types: phosphorescence4,5,6, thermally activated delayed fluorescence (TADF)7,8 and triplet-triplet annihilation (TTA)9,10 (Fig. 1).

Fig. 1: Luminescence mechanisms of triplet-harvesting materials.
figure 1

a Phosphorescence; b thermally activated delayed fluorescence; c triplet-triplet annihilation.

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Phosphorescence is a radiative transition from the triplet state (T1) to the ground state (S0) (Fig. 1a). Unlike fluorescence, with nanosecond-scale emission lifetimes, phosphorescence can last from microseconds to several seconds due to the involvement of triplet (or triplet excited) states11. This longer-lived emission arises because the transition between T1 and S0 is spin-forbidden, making it a slower process. Due to the weak spin-orbit coupling (SOC) and the fast non-radiative decay of the triplet excited state, phosphorescence could historically only be observed and detected at low temperature (77 K)12. In recent years, researchers have developed room temperature phosphorescence (RTP) through the introduction of heavy atoms (such as Br, Cl, and Se), heteroatoms (such as N, O, and S), and aromatic carbonyl groups into organic compounds in order to promote SOC and thus enhance the intersystem crossing (ISC) rate. Another effective strategy to achieve RTP is to build a relatively rigid molecular environment, through crystal engineering, host–guest doping, supramolecular self-assembly, and so on, in order to hinder the non-radiative transition of the triplet state. Based on these principles, RTP materials with high quantum yields, long emission lifetimes, and multiple emission colors have been obtained, showing promising applications in organic light-emitting diodes (OLEDs), bioimaging, photodynamic therapy (PDT), sensors, and so on. Phosphorescent materials have revolutionized OLED technology by significantly improving device efficiency (internal quantum efficiency can reach 100% in theory) and extending operational lifetimes. They are extensively used in displays and lighting. Phosphorescent dyes with longer emission lifetimes (from microseconds to seconds) offer advantages over fluorescent counterparts, providing a better contrast and reduced background signal interference, making them suitable for cellular and tissue imaging with high sensitivity. Due to their intrinsic triplet-state emission and sensitivity to the environment, phosphorescent materials can be used as photosensitizers in PDT, as well as in oxygen or temperature sensing.

Thermally activated delayed fluorescence is a type of delayed fluorescence from a singlet excited state to the ground state through an effective reverse intersystem crossing (RISC) from the triplet excited state (T1) to the singlet excited state (S1), as shown in Fig. 1b. It is a phenomenon that allows organic materials to achieve nearly 100% internal quantum efficiency in light-emitting devices without the need for heavy metal elements. This mechanism leverages the unique properties of certain molecular structures to enable efficient up-conversion of triplet excitons to singlet excitons, thereby facilitating delayed fluorescence emission13,14. Its lifetime is normally on the microsecond scale. For TADF materials, this energy gap (ΔEST) between S1 and T1 is minimized to facilitate RISC, where triplet excitons are converted back to singlet excitons. Efficient RISC requires small ΔEST values, typically less than 0.3 eV, allowing thermal energy to promote the transition. TADF materials are often designed with a donor-acceptor (D-A) architecture, where one part of the molecule acts as an electron donor and the other as an electron acceptor. This arrangement helps the systems localize the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), effectively reducing ΔEST. Thus, TADF materials show great promise in OLEDs due to their high efficiency, broad color tunability, and cost-effectiveness.

The third mechanism for triplet-harvesting materials is triplet-triplet annihilation. It is a significant process in the fields of photophysics and photochemistry, where two triplet excited states interact to produce a higher-energy singlet excited state (see Fig. 1c)15. This phenomenon is particularly useful in applications like up-conversion luminescence and enhancing the efficiency of energy harvesting systems such as solar cells.

This Collection illustrates the breadth and creativity of the triplet-harvesting approaches in the field. For example, the work on carbonyl-nitrogen multi-resonance emitters demonstrates how precise electronic structure design enables high color purity and efficient triplet management in hyperfluorescent OLEDs (https://doi.org/10.1038/s42004-025-01435-z)16. Meanwhile, newly developed host materials with high triplet energy, those containing 1,3,5-oxadiazine cores, could effectively confine triplet excitons within phosphorescent and TADF-type emitters, suppressing exciton quenching processes (https://doi.org/10.1038/s42004-024-01377-y)17. The studies presented in this Collection collectively highlight the transformative role of triplet-harvesting strategies in advancing luminescent materials. From fundamental design principles to device-level implementations, they underscore how control over triplet excitons is shaping the future of exciton-management technologies. As the field moves forward, continued interdisciplinary efforts will be essential to translate photophysical insights into durable, high-performance materials capable of meeting both technological demands and emerging applications.

Looking forward, it is anticipated that the continued integration of synthetic ingenuity, photophysical understanding, and device engineering will yield a new generation of triplet-harvesting materials. These will not only bridge the gap between the efficiency and stability in OLEDs but also unlock novel functionalities in areas such as PDT, sensing, and bioimaging. The strategic control of triplet states through orbital engineering, supramolecular interactions, or external stimuli stands as a compelling frontier in the evolution of photoluminescent and electroluminescent materials.