Exciton Management via Energy Gap Engineering and High‐Lying Triplet States for Efficient Phototheranostics

ABSTRACT

Conventional organic photosensitizers suffer from inefficient exciton utilization, where fluorescence imaging and reactive oxygen species (ROS) generation compete for the same excited‐state population. Charge‐transfer (CT) strategies can enhance intersystem crossing (ISC) but often induce severe non‐radiative decay due to the energy gap law. Here, an energy gap–engineered benzotriazole system with high‐lying triplet mediation is proposed. The electron‐withdrawing derivative pCHO maintains a large optical gap, suppressing internal conversion (IC) and achieving high photoluminescence quantum yield (PLQY) (64.83%), compared to ‐pNH ₂ (9.43%). Theoretical analysis reveals that the LE‐dominated S ₁ state and CT‐character T ₂ state create pronounced orbital heterogeneity, resulting in enhanced spin–orbit coupling (SOC = 0.9810 cm ⁻¹ ). This enables a dual‐channel exciton management mechanism: triplet excitons are efficiently populated via high‐lying states and stabilized in T ₁ to promote ROS generation. Meanwhile, a T ₂ ‐mediated pathway partially repopulates S ₁ , sustaining fluorescence for high‐contrast imaging. Formulated into PEGylated nanoparticles, ‐pCHO@PEG retains these photophysical properties in biological environments and exhibits effective photodynamic cytotoxicity. This work provides a general molecular strategy to decouple imaging brightness from ROS generation in phototheranostic systems.