Fluorophores with emission in the second near‐infrared (NIR‐II) window have displayed salient advantages for biomedical applications. However, exploration of new luminogens with high NIR‐II fluorescent brightness is still challenging. Herein, based on the “ring‐fusion” strategy, a series of heteroatom‐inserted rigid‐planar cores is proposed to achieve the bathochromic NIR‐II fluorophores with aggregation‐induced emission (AIE) performance. Interestingly, one of the representative fluorophores, 4,4′‐(5,5′‐(1,2,5thiadiazolo3,4‐idithieno2,3‐a:3′,2′‐cphenazine‐8,12‐diyl)bis(4‐octylthiophene‐5,2‐diyl))bis(N,N‐diphenylaniline) (TTQiT), enjoys a maximum emission beyond 1100 nm because of the efficiently narrowed energy bandgap by electron‐rich sulfur‐atom‐inserted core, which is verified by theoretical calculation. Taking advantage of the bright NIR‐II emission of TTQiT nanoparticles, the desirable in vivo NIR‐II imaging with high signal‐to‐background ratios is successfully performed and a long‐term stem cell tracking in the detection of acute lung injury is further realized. Therefore, it is anticipated that this work will provide a promising molecular engineering strategy to enrich the scope of NIR‐II fluorophores for catering to diverse demands in biomedical applications. A heteroatom‐inserted electron delocalization (HEED) strategy is facilitated to construct a series of second near‐infrared (NIR‐II) fluorophores. One representative AIEgen (TTQiT) displays a maximum emission wavelength beyond 1100 nm with desirable fluorescent brightness, which is successfully used as the NIR‐IIa cell trackers to dynamically detect the homing of stem cells in mice with acute lung injury.