The Balance Effect of π–π Electronic Coupling on NIR‐II Emission and Photodynamic Properties of Highly Hydrophobic Conjugated Photosensitizers

Abstract Deep NIR organic phototheranostic molecules generally have large π‐conjugation structures and show highly hydrophobic properties, thus, forming strong π–π stacking in the aqueous medium, which will affect the phototheranostic performance. Herein, an end‐group strategy is developed to lift the performance of NIR‐II emitting photosensitizers. Extensive characterizations reveal that the hydrogen‐bonding interactions of the hydroxyl end group can induce a more intense π–π electronic coupling than the chlorination‐mediated intermolecular forces. The results disclose that π–π stacking will lower fluorescence quantum yield but significantly benefit the photodynamic therapy (PDT) efficiency. Accordingly, an asymmetrically substituted derivative (BTIC‐δOH‐2Cl) is developed, which shows balanced phototheranostic properties with excellent PDT efficiency (14.6 folds of ICG) and high NIR‐II fluorescence yield (2.27%). It proves the validity of the end‐group strategy on controlling the π–π interactions and rational tuning the performance of NIR‐II organic phototheranostic agents.