Appropriate introduction of nitrile groups to balance NIR-II fluorescence imaging with photothermal therapy/photoacoustic imaging

Phototheranostics, especially the combination of second near-infrared window (NIR-II) fluorescence imaging and photothermal therapy (PTT), is a novel biotechnological approach that integrates diagnosis and therapy to enable accurate and effective tumor killing. However, it is difficult to balance the competition between generating fluorescence by radiation decay and generating heat by non-radiation decay. In this paper, we propose a nitrilation strategy to design A-D-A'-D-A photothermal agents (PTAs) to balance this inherent conflict and achieve both high fluorescence quantum yield (FLQY) and good photothermal conversion ability. Our experimental results show that the photothermal conversion efficiency (PCE) and FLQY of nanoparticles (NPs) were significantly enhanced after the induction of nitrile groups and appropriate nitrilation. Theoretical calculations demonstrate that moderate induction of nitrile groups can maximize electrostatic potential difference (?ESP) and increase intramolecular/intermolecular D-A interactions, resulting in tighter molecular packing and further promoting an increased molar extinction coefficient. Among the three types of nanoparticles we investigated, the NIR-II emission of BTP-TCID-2CN NPs with two nitrile groups extended to 1200 nm, exhibiting the highest FLQY (3.7%), good PCE (33.5%), and excellent photoacoustic imaging capacity. In addition, both biological safety and cellular internalization ability were significantly improved after appropriate nitrilation. These characteristics enable BTP-TCID-2CN NPs to support high-resolution NIR-II FLI/PAI-guided PTT in vivo. This study provides a feasible basis for the future research and development of NIR-II organic PTAs with excellent overall performance.