Designing small organic molecular NIR-II fluorophores by ring strain modulation

Developing small molecular organic fluorophores in the second near-infrared window (NIR-II, 1000-1700 nm) with excellent photophysical properties is an ongoing pursuit for in vivo bioimaging and biosensing. Herein, we report a strategy to modulate the optical properties of xanthene-based fluorophores by manipulating their ring strain through fine-tuning the ring size, which strongly influences the rigidity and planarity of the conjugated structure, thereby impacting their optical characteristics. Additionally, the ring strain imparts varying responsiveness to the fluorophores by affecting the pK_cycl values through spirocyclization.