Femtosecond Time-of-Flight Mass Spectrometry and Photoelectron Imaging of Azobenzene Derivatives
To observe the pure ultrafast dynamics of photochromic molecular switches following photoexcitation in the absence of any overshadowing solvent effects, systematic investigations need to be conducted in the gas phase. In this Thesis, the archetype photoswitch azobenzene (AB) was chosen as benchmark molecule for the used femtosecond time-resolved time-of-flight mass spectrometry and photoelectron imaging (PEI) setup in combination with quantum chemical calculations. The influence of the bridging length of bridged AB derivatives was investigated by means of the -CH2-bridged 11H-dibenzo[c,f ][1,2]diazepine (Dzp) and -C2H4-bridged 5,6-dihydrodibenzo[c,g][1,2]diazocine (Dz). Upon S1 (nπ∗) photoexcitation, the boat-like Z-Dzp molecules isomerize via a prolonged consecutive isomerization pathway, while an ultrafast concerted one was found for Z-Dz. Owing to the ultrafast dynamics in this state, wavelength-dependent ionization mechanism were found for both molecules. An ultrafast reorganization within the ionization time window was proposed, enabling adiabatic transitions in addition to vertical transitions. Employing photoionization-photofragmentation spectroscopy, this reorganization motion was further found to be conserved upon ionization manifesting as a coherent motion of the wavepacket assigned to the butterfly vibration in the ionic ground state D0. Three heteroaryl azo dyes 2-, 3- and 4-phenylazopyridine were subject to a systematic experimental study. Upon Sn (ππ∗) photoexcitation, the molecules deactivate alike ultrafast through a conical intersection to the S1 (nπ∗) state. The S1 state lifetime, however, was found to be prolonged by one magnitude depending on the chosen isomer (2-PAPy > 3-PAPy > 4-PAPy) attributed to a height dependency of a potential energy barrier in this state.
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