Unraveling the Complex Vibronic Coupling in Highly Fluorinated Aromatic Systems
Highly fluorinated aromatic molecules exhibit a rather unique behavior: the
fluorination stabilizes πσ* states. This perfluoro effect was identified as the driving force in modulating the dynamics of the S1 (ππ*) state of pentafluorobenzene in the past. Vibronic coupling between the excited ππ* and higher-lying πσ* states, mediated by out-ofplane vibrational modes, yields oscillations of the time-resolved response signals. In this Thesis, the influence of the effects of number, position, and nature of substituent as well as the influence of a heteroatom in the aromatic system on the dynamics of the S1 state were systematically investigated by means of femtosecond time-resolved time-of-flight mass spectrometry and corresponding photoelectron imaging spectroscopy, supported by ab initio calculations. The set of molecules included 1,2,3-TriFB, all three TFB isomers, PFT, PFPh as well as ClPFB and BrPFB, and PFPy. Nearly all time-resolved parent ion yields showed intense oscillations. The photoelectron spectroscopy confirmed vibronic coupling to be the origin of the oscillations. The oscillations became more pronounced the more fluorine atoms were attached to the aromatic rings and the closer they were to each other. In contrast, different substituents damped the oscillations. A complex multi-mode oscillation was observed for 1,2,3,5-TFB, exhibiting two oscillational frequencies. Thus, the dynamics of 1,2,4,5-TFB (no oscillation), 1,2,3,5-TFB (multi-frequency oscillation), and 1,2,3,4-TFB (single-frequency oscillation) are strikingly different, despite nearly identical electronic structures. PFPy showed an even more intricate signal pattern. At least three oscillational frequencies could be attributed to genuine vibrational modes. Hence, the dynamics of PFPy demonstrates that one can map the motion of multiple vibrational modes of the coupled wavepacket at once in real time.
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