Real‐Time Monitoring of Excited‐State Ring‐Puckering Process in BODIPY Molecule Using Ultrafast Spectroscopy

Pyrromethene 597 (PM597), a widely utilized commercial laser dye, is known to exhibit a comparatively lower fluorescence quantum yield than its structural analogs such as PM546 and PM560. To elucidate the underlying mechanism for this lower emission efficiency in PM597, we conducted a comprehensive investigation of its excited‐state dynamics using femtosecond fluorescence upconversion and transient absorption spectroscopy. Our studies definitively reveal ultrafast decay pathways for the excited state of PM597 immediately following light absorption. Crucially, we were able to monitor, in real‐time, the formation of a new excited‐state species arising from an excited‐state ring‐puckering process. Our findings demonstrate that this ring‐puckering process is highly sensitive to the viscosity of the medium, occurring with a time constant of ≈1.3 ps in low‐viscosity solvents. These experimental results are in excellent agreement with previously proposed theoretical calculations. Furthermore, we have also elucidated the significant role played by substituents on the BODIPY ring in modulating this excited‐state ring‐puckering process.