Two-Dimensional Confinement for Generating Thin Single Crystals for Time-Resolved Electron Diffraction and Spectroscopy: An Intramolecular Proton Transfer Study

Thin single organic crystals (≤1 μm) with large area (≥100 × 100 μm2) are desirable to explore their photoinduced processes using transmission-based ultrafast spectroscopy and electron-diffraction techniques. Here we present a method to grow thin large area single crystals of a prototypical proton transfer system, 1,5-dihydroxyanthraquinone. As a proof of concept, we perform optical measurements on as-grown samples and recorded the data in transmission mode

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

Ultrafast ring-opening and solvent-dependent product relaxation of photochromic spironaphthopyran

The ultrafast dynamics of unsubstituted spironaphthopyran (SNP) were investigated using femtosecond transient UV and visible absorption spectroscopy in three different solvents and by semi-classical nuclear dynamics simulations. The primary ring-opening of the pyran unit was found to occur in 300 fs yielding a non-planar intermediate in the first singlet excited state (S-1). Subsequent planarisation and relaxation to the product ground state proceed through barrier crossing on the S-1 potential energy surface (PES) and take place within 1.1 ps after excitation. Simulations show that more than 90% of the trajectories involving C-O bond elongation lead to the planar, open-ring product, while relaxation back to the S-0 of the closed-ring form is accompanied by C-N elongation. All ensuing spectral dynamics are ascribed to vibrational relaxation and thermalisation of the product with a time constant of 13 ps. The latter shows dependency on characteristics of the solvent with solvent relaxation kinetics playing a role.Funding Agencies|Max Planck Society</p