Fluorescent J‑Aggregates in the NIR-II Window: Unlocking Frontiers in Biomedical Imaging

Fluorescent organic dyes have long served as versatile scaffolds for developing multifunctional imaging tools in biomedicine. The pursuit of fluorophores in the second near-infrared window (NIR-II) represents a major Frontier, aiming to deepen our view into living systems. Current paradigms, predominantly focused on extending π-conjugation, are frequently hampered by synthetic complexity and aggregation-caused quenching (ACQ), creating a barrier to clinical translation. In this perspective, we contend that J-aggregation offers a promising and alternative strategy: a more facile and spectrally tunable route to high-performance NIR-II emission. We analyze the design principles regulating J-aggregate formation across diverse molecular scaffolds, from cyanines and boron-dipyrromethene (BODIPY) derivatives to emerging fluorophore scaffolds, and synthesize their recent applications in bioimaging, biosensing, and theranostics. We also discuss the key challenges of current NIR-II emissive J-aggregates (e.g., low fluorescence quantum yield, poor in vivo stability) in biomedical applications, alongside prospects for advancing their development toward routine use in biomedicine.