Two-dimensional confinement for generating thin single crystals for applications in 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 photoinduced processes using ultrafast spectroscopy and electron-diffraction. Here, we present a general method based on spatial confinement to grow such crystals using the prototypical proton transfer system, 1,5-dihydroxyanthraquinone, as an example, and provide the protocol for optically characterizing structural dynamics to enable proper assignments using diffraction methods.

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 (S1). Subsequent planarisation and relaxation to the product ground state proceed through barrier crossing on the S1 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 S0 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.