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Free keywords:
Transient absorption, Femtosecond electron diffraction, photochemical reaction, photochromic
Abstract:
Molecular crystals are at the forefront of research to find new technologies and to probe emerging phenomena such as superconductivity. Their studies are, therefore, important but also challenging, especially in the field of atomically-resolved dynamics due to low damage thresholds and related factors. Two such crystals, showing different phenomena, were the focus of this work. The dynamics of photochemical reaction in ultrathin crystals of spirooxazines were studied using a home-built transient absorption setup and 266 nm light for excitation. They were found not to be completely photochromic in crystalline state, in contrast to their behaviour in solution and amorphous films. A transient emerges after excitation, which evolves to form planar trans-merocynanine product within 80 ps in solution, but due to steric restrictions in the crystal, only the ring-opened form of spirooxazine was created without undergoing isomerisation. For these experiments, a methodology was developed to tackle the issue relating to the permanent formation of the photoproducts in crystals, which severely hampers reversibility. It made use of an additional ultrashort beam, making it a three-beam approach, with the third beam acting to arrest the accumulation of the products by removing them from the probe volume via the back photoreaction. Using this approach, it was possible to achieve over 10,000 pump-probe cycles, enabling time-resolved spectroscopy of this system and opens up prospect for femtosecond diffraction experiments. Femtosecond electron diffraction study was carried out on EDO-MeEDO, a doped crystal of EDO-TTF. The response of the system was found to be different to what is known for the pure EDO-TTF crystal; our findings could not confirm the occurrence of a cell doubling phase transition. At acoustic time scales, it is hypothesised that cooperative effects play a role, which lead to pronounced phonon signals. Further studies are needed to understand the dynamics of this system.
