A Planarity‐Hindrance Co‐Balance Strategy to Develop Antiparallel H‐Aggregates With Minimal Absorbance Blueshift for Type I Photodynamic Therapy

H-aggregates offer intrinsic features for type I photodynamic therapy (PDT) by concurrently promoting triplet state formation and strengthening charge transfer ability. However, their exploitation remains limited by the inherently large absorption blueshift (usually >100 nm) arising from strong H-type excitonic coupling in conventional parallel-packed H-aggregates, forcing short-wavelength laser excitation with poor tissue penetration. Herein, this study reports a planarity-hindrance co-balance strategy to develop donor-π-acceptor-based antiparallel-packed H-aggregates with minimal absorption blueshift for type I PDT. The results demonstrate that π-bridge planarization drives H-packing, while donor-site steric tuning dictates the blueshift by modulating slipping angles and π-π overlapping degree, and a steric threshold (Me/OMe) is identified beyond which blueshift becomes invariant. The optimized MTBSIC molecules form H-aggregates with an exceptionally small blueshift of 15 nm over its monomers. MTBSIC H-aggregates further display markedly enhanced type I ROS generation and improved photothermal conversion ability over their amorphous counterparts possessing similar monomeric photophysical properties. Mechanistic analyses reveal that H-packing promotes both intersystem crossing and intermolecular charge transfer/separation, synergistically boosting type I ROS production. MTBSIC H-aggregates further achieve potent tumor inhibition with high biocompatibility both in vitro and in vivo. This work establishes a generalizable molecular design paradigm for near-monomer-like H-aggregates for high-performance phototheranostics.