Photochromism of pyridine-substituted merocyanine through reversible C-N bond formation

The reversible interconversion between spiropyran and merocyanine constitutes one of the most intensively investigated photochromic systems, enabling diverse applications in stimuli-responsive materials and optical imaging. Here, we report pyridine-substituted merocyanines (PMCs) as a class of photochromic molecules. Upon visible light irradiation, these compounds undergo an uncommon photoinduced C-N bond formation to yield spiroindolizine (SIZ) structures, accompanied by the color fading. The SIZs can thermally revert to the colored PMC form. This photochromism is marked by rapid response, high photo-switching ratio, robust water compatibility, and fatigue resistance. Furthermore, we have synthesized quinoline-substituted merocyanines and found that they can be bidirectionally switched by 660 nm and 500 nm light to form SIZ and PMC structures. Finally, we showed the photochromism of these molecules in solid matrices and demonstrated a light-controlled sequential switching function, highlighting their potential for advanced photonic applications. Spiropyran and merocyanine systems are useful due to the photoswitching properties, but do have some limitations. Here, the authors report pyridine-substituted merocyanines as photochromic compounds, with a photoinitiated C-N bond formation.