Electronic properties of functional molecules

For improvement and optimization of the performance of organic molecule-based devices, such as photovoltaic cells and thin-film transistors comprehensive insight into the physical and chemical properties of the organic molecules at surfaces is necessary. N-heteropolycyclic aromatic compounds are promising candidates for n-channel semiconductors. The introduction of nitrogen atoms into the p-backbone of the polycyclic aromatic hydrocarbons stabilizes the frontier orbitals and increases the electron affinity, while the size of the HOMO−LUMO (optical) gap is nearly unaffected. By using two-photon photoemission spectroscopy (2PPE) and high resolution electron energy loss spectroscopy (HREELS) we determined quantitatively transport, singlet and triplet states as well as electronic spectra of serval N-heteropolycyclic molecules adsorbed on Au(111) [1-4]. Moreover their nonlinear optical properties such as the two-photon absorption (2PA) ability have been investigated [5, 6]. Using time-resolved 2PPE we gained insights into the exciton dynamics and resolved processes on the femtosecond to picosecond timescale [2]. In addition, transient absorption (TA) spectroscopy has been applied to illuminate the excited state dynamics of orthogonal photoswitches and pentacene embedded in a surface anchored metal organic framework (SURMOF) [7, 8].