Metal-free click and bioorthogonal reactions of aggregation-induced emission probes for lighting up living systems

In 2002, two transformative research paradigms emerged: ‘click chemistry’ and ‘aggregation-induced emission (AIE),’ both leaving significant impacts on early 21st-century academia. Click chemistry, which describes the straightforward and reliable reactions for linking two building blocks, has simplified complex molecular syntheses and functionalization, propelling advancements in polymer, material, and life science. In particular, nontoxic, metal-free click reactions involving abiotic functional groups have matured into bioorthogonal reactions. These are organic ligations capable of selective and efficient operations even in congested living systems, therefore enabling in vitro to in vivo biomolecular labelling. Concurrently, AIE, a fluorogenic phenomenon of twisted π-conjugated compounds upon aggregation, has offered profound insight into solid-state photophysics and promoted the creation of aggregate materials. The inherent fluorogenicity and aggregate-emission properties of AIE luminogens have found extensive application in biological imaging, characterized by their high-contrast and photostable fluorescent signals. As such, the convergence of these two domains to yield efficient labelling with excellent fluorescence images is an anticipated progression in recent life science research. In this review, we intend to showcase the synergetic applications of AIE probes and metal-free click or bioorthogonal reactions, highlighting both the achievements and the unexplored avenues in this promising field.