Pyridine as a Resonantly Addressable Group to Study Electron-Transfer Dynamics in Self-Assembled Monolayers

Resonant Auger electron spectroscopy (RAES) in combination with the core-hole clock (CHC) approach represents a unique tool to study femtosecond electron transfer (ET) dynamics in molecular systems and self- assembled monolayers (SAMs) in particular. A most promising experimental strategy to apply this approach to SAMs involves the decoration of the target films with a group, which can be resonantly excited by narrow-band X-rays. Using a series of well-defined SAMs with a terminal pyridine moiety, we demonstrate here that pyridine can be used as such a group, as an alternative to the nitrile group, which has been utilized for this purpose in the past. For these SAMs, we evaluate the characteristic time for ET from the terminal nitrogen atom of the pyridine group through the molecular framework to the substrate and discuss the results in terms of conjugation and coupling of the relevant electronic states in the molecules. The obtained ET times correlate well with the literature values obtained for the analogous nitrile-terminated monolayers, showing, however, some differences associated with the character of the involved molecular orbitals and the larger size of the pyridine moiety as compared to nitrile. The results extend the possibilities of the RAES CHC approach in its specific application to SAMs providing a larger flexibility for the choice of suitable molecules and pointing out other potential candidates as resonantly addressable groups.