Abstract
Aggregation-induced emission (AIE) is a cutting-edge fluorescence technology, giving highly-efficient solid-state photoluminescence. Particularly, AIE luminogens (AIEgens) with emission in the range of second near-infrared window (NIR-II, 1000–1700 nm) have displayed salient advantages for biomedical imaging and therapy. However, the molecular design strategy and underlying mechanism for regulating the balance between fluorescence (radiative pathway) and photothermal effect (non-radiative pathway) in these narrow bandgap materials remain obscure. In this review, we outline the latest achievements in the molecular guidelines and photophysical process control for developing highly efficient NIR-II emitters or photothermal agents with aggregation-induced emission (AIE) attributes. We provide insights to optimize fluorescence efficiency by regulating multi-hierarchical structures from single molecules (flexibilization) to molecular aggregates (rigidification). We also discuss the crucial role of intramolecular motions in molecular aggregates for balancing the functions of fluorescence imaging and photothermal therapy. The superiority of the NIR-II region is demonstrated by fluorescence/photoacoustic imaging of blood vessels and the brain as well as photothermal ablation of the tumor. Finally, a summary of the challenges and perspectives of NIR-II AIEgens for in vivo theranostics is given.
Highlights
Light is one of the most fundamental branches of science
We outline the latest achievements in the molecular guidelines and photophysical process control for developing highly efficient NIR-II emitters or photothermal agents with aggregation-induced emission (AIE) attributes
We provide insights to optimize fluorescence efficiency by regulating multi-hierarchical structures from single molecules to molecular aggregates
Summary
Light is one of the most fundamental branches of science. Recent years have witnessed the establishment of strong ties between light and life by being able to visualize and detect many previously unknown life processes. Understanding photophysical behaviors is crucial for the design and interpretation of light-based applications. According to the classical photophysical diagram (Fig. 1a), matter absorbs light energy into excited energy states before immediately falling back to lower energy states mainly through radiative (R) and non-radiative (NR) decays.[1] R is mostly in the form of uorescence, which can be utilized for bioimaging and biosensing. Heat generation is the main outcome of NR, which has numerous applications in photothermal therapy (PTT), photoacoustic (PA) imaging, laser resurfacing, desalination of seawater, etc.[2,3] these two critical processes compete against one another: the dominant R will suppress NR and vice versa. How can we regulate these two processes to attain desirable properties?
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