Photoreversible Ultrafast Dynamics of Ring Opening and Increased Conjugation under Spatial Confinement

Isomerization through stereochemical changes and modulation in bond order conjugation are processes that occur ubiquitously in diverse chemical systems and for pho- tochromic spirocompounds, it imparts them their functionality as phototransformable molecules. However, these transformations have been notoriously challenging to observe in crystals due to steric hindrance but are necessary ingredients for the development of reversible spiro-based crystalline devices. Here we report the detection of spectroscopic signatures of merocyanine due to photoisomerization within thin films of crystalline spiropyran following 266 nm excitation. Our femtosecond spectroscopy experiments reveal bond breaking, isomerization, and increase in bond order conjugation to form merocyanine on a time scale of < 2 ps. They further unveil a lifetime of several picoseconds of this photoproduct, implying that the system is highly reversible in the solid state. Preliminary femtosecond electron diffraction studies suggest that lattice strain favors the return of photoproduct back to the closed spiroform. Our work thus paves the way for spiropyran-derived compounds for ultrafast studies and applications.