Thermally Activated Dual‐Room‐Temperature Phosphorescence: Triplet Up‐Converted Anti‐Kasha in Dendrimers Enabled by Rigid Matrix Stabilization

ABSTRACT Room‐temperature phosphorescence (RTP) materials are a significant research field for anti‐counterfeiting, bioimaging, and optoelectronic devices, but anti‐Kasha RTP still lacks clear molecular design strategies, and conventional rigid polymer matrices fail to break Kasha's rule. To address these issues, this work proposes an acceptor dendronization strategy to synthesize the dendrimer emitter dTC‐BPSAF. Carbazole dendrons regulate triplet hybridization, enabling dTC‐BPSAF to form near‐degenerate lowest triplet ( T 1 ) and high‐lying triplet ( T 2 ) states with hybrid local‐charge transfer (HLCT) character and large spin‐orbit coupling. Integrating dTC‐BPSAF into rigid polymers further stabilizes T 2 by suppressing non‐radiative decay. Temperature‐dependent time‐resolved phosphorescence spectra and transient absorption spectra confirm that rigid matrix‐based films exhibit thermally activated endothermic T 1 → T 2 up‐conversion and dual‐band anti‐Kasha RTP. In contrast, a moderately rigid polymer matrix shows weak T 2 emission, while a soft polymer matrix only produces T 1 emission. This study establishes a dendrimer‐matrix synergy strategy combining molecular‐level triplet engineering, providing a generalizable approach for efficient anti‐Kasha RTP materials and new avenues for advanced photonics.