Triplet–Triplet Annihilation Upconversion: From Molecules to Materials

Conspectus Photon upconversion, particularly triplet–triplet annihilation upconversion (TTA-UC), converts low-energy photons into higher-energy ones through multiphoton fusion, offering unique photophysical properties with broad applications. As a new generation of organic upconversion materials, TTA-UC shows great promise in biosensing, biomedicine, photoredox catalysis, and solar energy harvesting. For example, TTA-UC-based nanoprobes enable highly sensitive, background-free biomarker detection; TTA-UC nanoparticles act as phototransducers that extend optogenetic responses into the near-infrared region; their nonlinear luminescence supports high-resolution 3D printing; and they enhance photon utilization in photovoltaics. Although significant progress has been made in the past decade, challenges remain, including developing high-performance near-infrared TTA-UC pairs with ultralarge anti-Stokes shifts, creating general strategies for synthesizing water-dispersible and oxygen-resistant nanoparticles, and expanding biomedical applications to promote clinical translation. This Account systematically introduces the breakthroughs achieved by my laboratory in TTA-UC. We successfully synthesized a B,N-doped near-infrared-absorbing photosensitizer with thermally activated delayed fluorescence properties, which prevent energy loss during the intersystem crossing process. Subsequently, we constructed TTA-UC with an anti-Stokes shift of up to 1.03 eV (near-infrared to blue light). Additionally, we discovered a novel ligand, thiophene-substituted diketopyrrolopyrrole (Th-DPP), and precisely regulated its conformation on the surface of lead sulfide quantum dots (PbS QDs). A triplet exciton transfer efficiency of up to 90% was still observed, regardless of the fact that the T1 energy level of the Th-DPP is only 0.05 eV lower than the bandgap of PbS QDs. These findings enabled TTA-UC using 1064 nm excitation and an anti-Stokes shift that approached the theoretical limit. A soft core–shell nanostructure has been offered as a versatile strategy for the preparation of small, uniform, and efficient water-dispersible TTA-UC NPs. A multistage assembly process that involved TTA-UC and proteins was employed to fabricate biocompatible photonic upconversion supramolecular assemblies. Porous aromatic framework (PAF)-based TTA-UC materials were synthesized by employing annihilators as linkers. In addition to suppressing the skeleton-vibration-induced loss of triplet exciton energy, the annihilator unit’s presence of methyl steric hindrance enables the annihilators to maintain a consistent T 1 energy level, thereby significantly enhancing the performance of PAF-based TTA-UC materials. Regarding applications, we employed photon upconversion supramolecular assemblies as nanoprobes to achieve the rapid and highly sensitive analysis of prostate tumor markers (sarcosine) in a background manner. We showed wide-range oxygen sensing by employing PAF-based TTA-UC materials as oxygen-sensitive units. In addition, we demonstrated the efficacy of porous TTA-UC materials as heterogeneous photocatalysts, resulting in the efficient transformation of a variety of photoredox reactions in a catalyst recovery manner. Additionally, we have analyzed the current challenges based on our experience and proposed potential solutions. At the same time, we have also proposed possible development trends of TTA-UC and its potential application scenarios in the near future. We are confident that the rational design and ongoing exploration of TTA-UC, from the molecular to the material scale, will result in substantial advancements for organic photonic upconversion materials. These advancements will facilitate their practical implementation in biomedicine, chemistry, materials science, and photonics.