Integration of Motion and Stillness: A Paradigm Shift in Constructing Nearly Planar NIR-II AIEgen with Ultrahigh Molar Absorptivity and Photothermal Effect for Multimodal Phototheranostics

The two contradictory entities in nature often follow the principle of unity of opposites, leading to optimal overall performance. Particularly, aggregation-induced emission luminogens (AIEgens) with donor-acceptor (D-A) structures exhibit tunable optical properties and versatile functionalities, offering significant potential to revolutionize cancer treatment. However, trapped by low molar absorptivity (ε) owing to the distorted configurations, the ceilings of their photon-harvesting capability and the corresponding phototheranostic performance still fall short. Therefore, a research paradigm from twisted configuration to near-planar structure featuring a high ε is urgently needed for AIEgens development. Herein, by introducing the strategy of "motion and stillness" into a highly planar A-D-A skeleton, we successfully developed a near-infrared (NIR)-II AIEgen of Y5-2BO-2BTF, which boasts an impressive ε of 1.06 × 10⁵ M^-1 cm^-1 and a photothermal conversion efficiency (PCE) of 77.8%. The modification of steric hindrance on the benzene ring in the acceptor unit of the aggregation-caused quenching counterpart Y5-2BO, to a meta-CF₃-substituted naphthyl, leads to reversely staggered packing and various intermolecular noncovalent conformational locks in Y5-2BO-2BTF ("stillness"). Furthermore, the -CF₃ moiety acted as a flexible motion unit with an ultralow energy barrier, significantly facilitating the photothermal process in loose Y5-2BO-2BTF aggregates ("motion"). Accordingly, Y5-2BO-2BTF nanoparticles enabled tumor eradication and pulmonary metastasis inhibition through NIR-II fluorescence-photoacoustic-photothermal imaging-navigated type I photodynamic-photothermal therapy. This work provides the first evidence that the highly planar conformation with a reversely staggered stacking arrangement could serve as a novel molecular design direction for AIEgens, shedding new light on constructing superior phototheranostic agents for bioimaging and cancer therapy.