NIR-II Organic Phototheranostics: Matching Rotatable Donors and Shielding Acceptors for Enhanced Imaging and Phototherapy by Stabilized Triplet Excitons and Optimized Radiative Decay.

Second near-infrared (NIR-II) organic phototheranostic agents (OPTAs) have received significant interests in cancer treatment by enabling precise positioning of deep-seated lesions and highly efficient ablation of tumors. However, competition between radiative and nonradiative pathways of the excitons sets great obstacles for obtaining robust OPTAs. Herein, we propose to fabricate high-performance NIR-II OPTAs through molecular engineering of acceptor shielding units as well as manipulating molecular conformations by donor rotation. Accordingly, two series of OPTAs (BPN series: 4,4'-(ethene-1,1-diyl)bis(N,N-dimethylaniline) as rotatable donor and butadiene as π-bridge, BPTQ series: julolidine as donor and 2-vinylthiophene as π-bridge) with different acceptor shielding units were designed. Photophysical data and theoretical calculations verified the introduction of rotatable donors can help to realize highly twisted molecular conformations, resulting in enhanced NIR-II emission and photothermal conversion efficiency (PCE). Moreover, the adoption of acceptor shielding units is advantageous for stabilizing triplet excitons for radical reactive oxygen species (ROS) generation. As a result, a balance was achieved for BPN-P nanoparticles (NPs) in the BPN series, which exhibited high NIR-II emission, good radical ROS generation and highest PCE of 63%. BPN-P NPs achieved both high-resolution NIR-II angiography and imaging-guided synergistic photodynamic/photothermal cancer treatment in live mice model, showcasing the effectiveness of the strategy.