Intramolecular Steric Congestion Tailored Excited‐State Dynamics of C═C Photorotation for Effective Photothermal Tuning

ABSTRACT The design of high‐performance organic photothermal agents (PTAs) is often impeded by a limited understanding of their photothermal conversion mechanisms. Herein, through combined theoretical calculations with experimental investigations, we demonstrate a precise strategy for tuning photothermal conversion efficiency (PTCE) in donor–acceptor (D–A) molecules (namely, AD‐IO, AD‐NIO, and AD‐BIO) bridged by a photo‐driven rotatable C═C bond. By introducing tailored intramolecular interactions, molecular steric congestion is systematically increased, leading to enhanced structural nonplanarity, which in turn governs the excited‐state rotational dynamics around the pivotal C═C bond. Such controlled C═C photorotation significantly accelerates nonradiative excited‐state decay through conical intersections (CIs) to the ground (S 0 ) state, resulting in remarkably high PTCEs up to 69.9% under near‐infrared (NIR) irradiation. Notably, nanoparticles loaded with the photothermal agent AD‐BIO show superior NIR photothermal performance and excellent stability, enabling effective antibacterial applications. This work elevates intramolecular interaction manipulation into a general design paradigm for engineering excited‐state energy dissipation, enabling high‐performance PTAs with precisely tunable photothermal conversion for broad applications.