Nonlinear molecular spectroscopy on aromatic chromophores with homo- and heterodyne detection

In this thesis, three nonlinear optical techniques are utilized to spectroscopically explore the electronic and vibrational states of aromatic molecules, specifically arylazopyrazole, anthracene, and MOM-BINOL.
In the first section femtosecond transient absorption spectroscopy was employed to investigate the photoisomerization dynamics of planar and non-planar arylazopyrazole (AAP) molecular photoswitches. This revealed that the twisted configuration accelerates photoisomerization due to the pre-existing out-of-plane rotation of the phenyl and pyrazole rings, causing their π-systems to be out of conjugation with the p-orbitals of the azo groups. Moreover, the study underscores the significance of ground state ring vibrations in this flexible system, which influences the range of Franck-Condon states available in an ensemble and consequently affects the observed relaxation process.
Subsequently the electronic-vibrational interactions in the rigid molecule anthracene is studied using resonance Raman spectroscopy over its first excited state 1 L a using the Kerr-gate fluorescence suppression technique. Although most peaks in the Raman excitation profile align with the absorption maximum of the 1 L a ( 0 - 0 ) resonance at 376 nm, a notable exception is observed for the Raman peak detected at 785 c m - 1 . This Raman peak exhibits a Raman excitation profile that diverges significantly from the anthracene absorption spectrum, peaking instead at 370 nm, which corresponds to a minor shoulder in the absorption spectrum. This discrepancy suggests a nearly exclusive Raman enhancement by the dark state 1 L b for the corresponding vibrational mode.
Lastly, a novel nonlinear chirally sensitive method known as CARS-ROA is reproduced and the challenges associated with this highly sensitive experiment are detailed in the final section. Following this, a dissolved chiral substance (MOM-BINOL) is examined.


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