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 (intraNR) decay, which often requires quite complicated and time‐consuming molecular engineering. In addition to intraNR decay, intermolecular nonradiative (interNR) decay is equally important and more convenient in governing photothermal performance. However, controlling interNR decay remains challenging due to the limited understanding of their origin and dynamics. Here, systemic investigation of intraNR and interNR decay directs the first demonstration of simple manipulation of interNR 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 interNR 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 interNR decay rate over conventional intraNR decay to produce ultrahigh photothermal conversion efficiency of 81% for efficient photoacoustic imaging‐guided photothermal therapy in vivo. This study provides insights into interNR decay in achieving a giant photothermal effect and paves a convenient way to develop high‐performance OPMs.