Investigation of charge localization and charge delocalization in model molecules by multiphoton ionization photoelectron spectroscopy and DFT calculations

In this work we focus on the question to which degree a surplus charge is l ocalized or delocalized in extended molecular systems. Molecules consisting of a flexible tail and the benzene chromophore, such as n-propylbenzene, 2 -phenylethyl alcohol and 2-phenylethylamine, are used as model molecules. T heir S-0-S-1 resonance enhanced multiphoton ionization (MPI) spectra contai ning origin transitions of different conformers appear at similar wavelengt hs. This shows, that in the neutral the electronic excitation is localized at the benzene chromophore. Geometry differences between the neutral and th e cation can be qualitatively derived from intensities of vibrational trans itions or the onset behavior in MPI high-resolution photoelectron (MPI-PE) spectra. We identify two possible reasons for structural changes: Charge-di pole interaction and charge delocalization. Whereas both effects can be act ive for the folded gauche conformers, the charge-dipole interaction is expe cted to be small for the extended anti conformers and geometry changes are attributed to charge delocalization. Density functional calculations of str uctures and energies qualitatively confirm the experimental results for all molecules and their conformers. They predict charge delocalization into th e end group of below 20% for n-propylbenzene and 2-phenylethyl alcohol. In the case of 2-phenylethylamine the charge is equally shared by the near-iso energetic charge sites of the benzene chromophore and the amine group.