Harnessing the energy gap law for high NIR-II quantum yield at the molecular and aggregate levels

Fluorescence imaging in the second near-infrared window (NIR-II, 1000-1700 nm) has emerged as a powerful modality in biomedical research over the past decade, attracting broad scientific interest. Among various NIR-II emissive materials, small organic molecules have garnered particular attention due to their well-defined chemical structures, tunable photophysical properties and good biocompatibility. Despite notable progress in this field, most reported NIR-II fluorophores still suffer from low quantum yields (QYs), a fundamental limitation governed by the energy gap law, which significantly restricts their performance in advanced bioimaging applications. Overcoming the constraints imposed by this law on NIR-II QYs thus remains a critical and unresolved challenge. In this Focus Article, we first elucidate the photophysical origin of low QYs observed in NIR-II small molecules. We then systematically outline recent strategies aimed at enhancing their emission efficiency, both at the molecular and aggregate levels. Finally, we provide a forward-looking perspective on promising research directions, with the aim of stimulating further innovation and effort in this rapidly evolving frontier.