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

Abstract

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.

Femtosecond Charge Transfer Dynamics in Monomolecular Films in the Context of Molecular Electronics

“…Whereas nitrile represents a most suitable resonantly addressable group for the RAES-CHC experiments, in view of its small size, strong absorption resonance (see Figure ), and the possibility of the MO-dependent ET, other groups can be used as well. A representative example is pyridine, exhibiting a strong [N 1s]­π 1 * feature at 398.5–398.9 eV . The respective excitation can be utilized within the applied RAES-CHC scheme to monitor the ET dynamics in molecular assemblies.…”

“…Similar to the nitrile case, these spectra could be decomposed into the ET and pure resonant decay contributions and τ ET could be calculated, viz. 12 ± 4 fs and 49 ± 8 fs, respectively . These values can be compared to τ ET for the analogous nitrile-substituted SAMs, viz.…”

“…A representative example is pyridine, exhibiting a strong [N 1s]π 1 * feature at 398.5−398.9 eV. 38 The respective excitation can be utilized within the applied RAES-CHC scheme to monitor the ET dynamics in molecular assemblies. As an example, the [N 1s]π 1 * decay spectra of two pyridinesubstituted SAMs, Py-PT and Py-P1T, are presented in Figure 11a,b, respectively.…”

“…12 ± 4 fs and 49 ± 8 fs, respectively. 38 These values can be compared to τ ET for the analogous nitrile-substituted SAMs, viz. NC-PT (see Figure 4) and NC-PT1 (NC−C 6 H 4 −CH 2 −S/Au).…”

“…Obviously, this unit decouples the electronic subsystems of metal and π-conjugated segment providing an effective barrier for ET. 38,39 The second example is the nitro group, with the [O 1s]π* decay spectra of several nitro-substituted SAMs presented in Figure 12a (an excitation energy of 530.95 eV). 40 Because of the strong electronegativity of the nitro group, ET from this moiety to the substrate is energetically forbidden, but the inverse ET (IET), from the molecular backbone to the nitro group is in principle possible.…”