Ultrafast signatures of merocyanine overcoming steric impedance in crystalline spiropyran.

Isomerisation through stereochemical changes and modulation in bond order conjugation are processes that occur ubiquitously in diverse chemical systems and for photochromic 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 photoisomerisation within crystalline spiropyran following 266 nm excitation. Our femtosecond spectroscopy experiments reveal bond breaking, isomerisation and increase in bond order conjugation towards the formation of merocynine on a sub-2 ps time scale. They further unveil a lifetime of several picoseconds for the initial open ring intermediate with subsequent relaxation to mercocyanine, with established back connversion pathways, which make the system highly reversible in the solid state. Supporting femtosecond electron diffraction studies suggest that lattice strain favours the return of photoproduct to the closed spiroform. Our work thus paves the way for novel ultrafast applications from spiropyran-derived compounds.