Highly Effective Near‐Infrared Light‐Activatable Triplet Fusion Upconversion with BODIPY Annihilators: Triplet Modulation via Non‐Conjugated n–π Spatial Electron Coupling

Abstract Effective strategies to manage the triplet state of chromophores are desirable in terms of their crucial role in photonic and optoelectronic technologies. However, triplet excited‐state energy level regulation of organic molecules remains elusive. Here, non‐conjugated n–π spatial electron coupling is demonstrated within boron‐dipyrromethene (BODIPY) and can greatly reduce the low‐lying triplet state (T₁) to 1.34 eV without the conventional need for π‐conjugation extension. Moreover, the close alignment between T₁ states of this BODIPY derivative and the photosensitizer PtTNP contributes to the reversible triplet energy transfer, namely the intermolecular triplet excited‐state equilibrium. Furthermore, to the best of the knowledge, the first BODIPY‐based near‐infrared light‐activated triplet fusion upconversion (NIR TF‐UC) with an upconversion quantum efficiency of 13.3% is implemented. On this basis, the air‐stable solid‐state NIR TF‐UC material is successfully fabricated, whose upconverted emission can still be seen at an ultralow NIR power density of 6.4 mW cm⁻². This new strategy to implement triplet state modulation will accelerate the development of NIR TF‐UC and other important areas involving triplet state, such as photoredox catalysis, photodynamic therapy, and photovoltaic devices.