Unravelling Complex Photo-Induced Dynamics by Femtosecond Time-Resolved Vibrational Absorption Spectroscopy

The present Thesis presents a study on the femtosecond time-resolved vibrational absorption spectroscopy (TVAS) of electronically excited molecules. The experimental setup generated probe pulses in the mid-infrared spectral region which allowed for structure-sensitive detection. Four molecular systems were selected for detailed investigation, which are highly relevant, since key steps in their photo-induced deactivation and reaction dynamics are still poorly understood. The molecules 7- and 9- methylpurine (7-MePur, 9-MePur) as well as N6,N6 dimethyladenine (DMAde) have a purine scaffold as chromophore in common. However, completely different relaxation processes were detected by TVAS after photoexcitation. For 7- and 9-MePur, efficient population of long-lived excited states was observed. After photoexcitation, deactivation via the optically dark S1(nπ*) state occurred followed by intersystem crossing to the T1(ππ*) state. In contrast, the deactivation of electronically excited DMAde proceeded via a structural rearragement in the S1(ππ*) state leading to an intramolecular charge transfer state (ICT). These results showed that TVAS can provide detailed insights to better understand the relationship between chemical and electronic structure that regulate the accessibility and efficiency of deactivation pathways. Furthermore, TVAS measurements of acetylacetone (AA) and methyl salicylate (MS) in acetonitrile (ACN) and cyclohexane (CHX) solutions were performed. For AA, photoproduct formation took place via two distinct pathways.The photoproduct was identified as the CTC isomer which arised from cis-trans isomerisation around the C=C double bond. For MS, photoproduct formation was found in both solvents as a novel deactivation channel. The photoproduct was assigned to a rotamer in the S0 state and was formed by a twist of 180° of the ester moiety.


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