Asymmetric organic NIR chromophores for bioimaging and phototherapy

The main aim of this review is to describe asymmetric near-infrared (NIR) I/II small-molecule fluorescent probes that have shown promising applications in bioimaging and phototherapy. NIR-I and NIR-II probes offer superior performance in terms of deeper tissue penetration, higher signal-to-background ratio (SBR), improved spatial resolution, reduced autofluorescence and temporal resolution, reduced phototoxicity/photobleaching, and support for image-guided surgery. Many organic probes have been reported for bioimaging and phototherapy. Among them, asymmetric probes, in particular, are emerging to meet current needs, mainly due to strong intramolecular charge transfer (ICT) that leads to large Stokes shifts, suppresses aggregation-induced quenching, and yields a high extinction coefficient, together with a high photoluminescence quantum yield. As a result, it is beneficial to summarise the current development of asymmetric probes. This review focuses on asymmetric molecular design, NIR-I/II fluorescence emission, quantum yield, and emerging applications in bioimaging and phototherapy. Based on the discussions extracted from the collected literature, such chromophores efficiently generate reactive oxygen species (ROS) or induce photothermal effects under NIR irradiation, thereby enabling effective phototherapy. Compared with other chromophore classes, these chromophores offer biocompatibility, molecular tunability, and reduced long-term toxicity. Collectively, asymmetric organic NIR chromophores provide a versatile platform for imaging-guided therapy, facilitating the development of next-generation theranostic drugs in precision medicine. Furthermore, the review proposes the future directions of the asymmetric design strategy.