Excimer‐Mediated Ultrafast Intermolecular Nonradiative Decay Enables Giant Photothermal Performance for Optimized Phototheranostic

Abstract Developing organic photothermal materials (OPMs) with high photothermal performance for phototheranostic mainly focus on the manipulation of intramolecular nonradiative ( intra NR) decay, which often requires quite complicated and time‐consuming molecular engineering. In addition to intra NR decay, intermolecular nonradiative ( inter NR) decay is equally important and more convenient in governing photothermal performance. However, controlling inter NR decay remains challenging due to the limited understanding of their origin and dynamics. Here, systemic investigation of intra NR and inter NR decay directs the first demonstration of simple manipulation of inter NR decay to produce a giant photothermal performance for optimized phototheranostic. Among three designed polymers with varying fluorine substitution, structure–performance studies reveal a dimer‐initiated inter NR decay to improve photothermal performance. Dimer is formed by intermolecular CF···H hydrogen bond. This finding inspires a simple aggregation control strategy to form excited dimer, namely, excimer. It initiates an ≈100‐fold enhancement in inter NR decay rate over conventional intra NR decay to produce ultrahigh photothermal conversion efficiency of 81% for efficient photoacoustic imaging‐guided photothermal therapy in vivo. This study provides insights into inter NR decay in achieving a giant photothermal effect and paves a convenient way to develop high‐performance OPMs.