Molecular π‐Conjugation Engineering Enables Flexible and Water‐Resistant Organic RTP Materials for Wearable Applications

Organic room‐temperature phosphorescence (RTP) materials for flexible and aquatic applications are limited by triplet exciton instability under deformation and environmental perturbation. Here, molecular π‐conjugation engineering is integrated with polymer‐matrix confinement to enable RTP in elastomeric systems. Benzoindole derivatives with tunable π‐conjugation are synthesized and embedded into rigid PVB and flexible SEBS matrices, revealing a distinct host‐dependent mechanism. While hydrogen‐bonding‐dominated PVB exhibits a slight decrease in phosphorescence lifetime upon π‐extension, elastomeric SEBS shows a pronounced lifetime enhancement from 0.52 to 1.10 s. Theoretical calculations demonstrate that π‐extension intrinsically enhances excited‐state delocalization and improves singlet‐triplet energetic matching, whereas MD simulations reveal that physical confinement within SEBS more effectively restricts the extended chromophore and suppresses nonradiative decay. The resulting RTP elastomers sustain stable emission under 400% strain over ten cycles, support loads up to 1.5 kg, and retain phosphorescence after 2 months of seawater exposure. These features enable wearable and underwater operation, establishing a molecular‐level for mechanically adaptive and environment‐tolerant RTP materials.