Construction and Applications of Efficient, Oxygen-Tolerant Triplet-Triplet Annihilation Upconversion Materials

Abstract

Triplet-triplet annihilation upconversion (TTA-UC) is an emerging class of photonic upconversion materials notable for low excitation power thresholds, high upconversion quantum yields, and tunable absorption and emission profiles. These unique features give TTA-UC materials significant potential across diverse fields such as chemistry, biology, and materials science. A typical TTA-UC system consists of sensitizers and annihilators, functioning through a sequence where the sensitizer absorbs photons and transfers triplet energy to the annihilator via triplet-triplet energy transfer, followed by triplet-triplet annihilation (TTA) that emits higher-energy photons. Because TTA-UC materials can be excited by long-wavelength light, they overcome the limitations in penetration depth of conventional fluorescence technologies, showing great promise for applications such as deeptissue imaging, targeted photodynamic therapy, and precise optogenetic modulation. However, molecular oxygen causes nonradiative decay pathways that severely quench upconversion efficiency, posing a major challenge for practical use. Over the past decade, researchers have developed various innovative strategies to counteract oxygen-induced quenching. This review systematically summarizes key scientific approaches to creating high-performance, oxygen-tolerant TTA-UC materials, with a focus on their underlying mechanisms. First, we discuss molecular engineering strategies involving electron-deficient groups and conformational control to improve the photostability of TTA-UC chromophores. Second, we describe the fabrication of oxygenresistant TTA-UC nanoparticles using reductive oil droplets as soft templates. Finally, we discuss nanostructure-mediated optimization of intermolecular triplet energy transfer dynamics to enhance oxygen resilience. A critical evaluation of the advantages and limitations of each approach is provided. Additionally, we highlight key challenges, including improving the upconversion efficiency of near-infrared-responsive TTA-UC, developing novel nanoparticle fabrication methods, and refining surface bioconjugation chemistry. We conclude by exploring prospects for integrating TTA-UC with synthetic biology techniques to design biosynthetic upconversion proteins, potentially establishing upconversion luminescence as a vital tool in fundamental life science research and accelerating its application in diverse biomedical fields.

Keywords

Triplet-triplet annihilation upconversion / Oxygen-tolerant / Nanoparticles / Biological applications

Author Contributions

Conceptualization, Z.-W.L., H.-J.F. and L.H.; Writing—Original Draft Preparation, Z.-W.L. and F.Q.; Writing—Review & Editing, J.-Y.L., M.-Y.Z., Y.P., C.-X.P., W.-Y.L. and H.-J.F.; Supervision, H.-J.F. and L.H.; Funding Acquisition, H.-J.F. and L.H.

Ethics Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data sharing is not applicable.

Funding

This research was funded by the National Natural Science Foundation of China (NSFC) [22377063, 224B2405], Haihe Laboratory of Sustainable Chemical Transformations for financial support [24HHWCSS00020], and the Research Start-up Fund of Nankai University.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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