 J-aggregation is an effective strategy to extend the wavelength of organic NII florescence dyes, but weak intermolecular interactions often lead to decomposition of the aggregates in biological environment. Here, the authors demonstrate a series of activatable quinazoline derivatives, which can self-assemble in vivo into highly stable NII J-aggregates. This article was authored by Toli, Ping Jiao Liang, Li Xu, and others.