Energy-Level Alignment and Orbital-Selective Femtosecond Charge Transfer Dynamics of Redox-Active Molecules on Au, Ag, and Pt Metal Surfaces

Abstract

Charge transfer (CT) dynamics across metal–molecule interfaces has important implications for performance and function of molecular electronic devices. CT times, on the order of femtoseconds, can be precisely measured using synchrotron-based core-hole clock (CHC) spectroscopy, but little is known about the impact on CT times of the metal work function and the bond dipole created by metals and the anchoring group. To address this, here we measure CT dynamics across self-assembled monolayers bound by thiolate anchoring groups to Ag, Au, and Pt. The molecules have a terminal ferrocene (Fc) group connected by varying numbers of methylene units to a diphenylacetylene (DPA) wire. CT times measured using CHC with resonant photoemission spectroscopy (RPES) show that conjugated DPA wires conduct electricity faster than aliphatic carbon wires of a similar length. Shorter methylene connectors exhibit increased conjugation between Fc and DPA, facilitating CT by providing greater orbital mixing. We find nearly 10-fold increase in the CT time on Pt compared to Ag due to a larger bond dipole generated by partial electron transfer from the metal–sulfur bond to the carbon–sulfur bond, which creates an electrostatic field that impedes CT from the molecules. By fitting the RPES signal, we distinguish electrons coming from the Fe center and from cyclopentadienyl (Cp) rings. The latter shows faster CT rates because of the delocalized Cp orbitals. Our study demonstrates the fine tuning of CT rates across junctions by careful engineering of several parts of the molecule and the molecule–metal interface.

Probing Matrix Effects in the Course of Electron Transfer across a Self-Assembled Monolayer

“…This SAM is characterized by an oblique structure with a herringbone molecular arrangement, a noticeable molecular inclination, and a molecular footprint of ∼0.24 nm 2 corresponding to a dense molecular packing . As the experimental approach, we used resonant Auger electron spectroscopy (RAES), relying on the established core-hole-clock (CHC) method, − specifically adapted to SAMs (see refs − for alternative approaches). − In contrast to the static conductance, studied in most CT experiments on SAMs, this method addresses the dynamics of electron transfer (ET) across the molecular framework, providing information on the characteristic time (τ ET ) required for ET between the exactly defined starting point within individual, SAM-forming molecules and the substrate. To localize the starting ET point, mimicking the top electrode in the standard, two-terminal CT approach, the molecular backbone is typically decorated with a suitable tail group, which can be resonantly excited by narrow-band X-rays, and the τ ET value is derived from the analysis of the resulting decay spectra (RAES).…”

Resolving charge transfer mechanisms in molecular tunnel junctions using dynamic charge transfer and static current–voltage measurements

“…S6 (ESI†) shows also some contribution to the LUMO from the Fe, as opposed to none in the previously studied S-CH 2 -DPA-C n -Fc series (further details of PDOS are in ESI† Section S3). 6,27 Peak II (LUMO+1) represents a mix of transitions to the Fe 3d xz /3d yz orbitals and the π* of the phenyl rings. In contrast to our earlier studies on S-CH 2 -DPA-C n -Fc, 6 the Cp rings and the OPE backbone in the S-CH 2 -OPE n Fc series are conjugated, and the LUMO+1 orbitals have more contributions from OPE units.…”

“…6,27 Peak II (LUMO+1) represents a mix of transitions to the Fe 3d xz /3d yz orbitals and the π* of the phenyl rings. In contrast to our earlier studies on S-CH 2 -DPA-C n -Fc, 6 the Cp rings and the OPE backbone in the S-CH 2 -OPE n Fc series are conjugated, and the LUMO+1 orbitals have more contributions from OPE units. Peak III corresponds to the transition to the delocalized LUMO+2 orbitals extending along the OPE wires and Cp rings, with minor contributions from the Fe 3d xy orbitals.…”

“…In CHC studies, CT time τ CT also follows an exponential decay with tunneling distance 6 τ CT = τ 0 e βd where τ 0 is the pre-exponential factor for CT time. CHC studies on oligophenyl, acene and alkyl backbones give β values of 0.55 Å −1 , 0.25 Å −1 and 0.72 Å −1 , 2,7,23 respectively, close to the β of 0.44 Å −1 , 0.20 Å −1 and 0.76 Å −1 observed in J ( V ) measurements for their corresponding molecular junctions.…”

“…4,5 The unique capability of CHC to quantify the femtosecond CT time scale from different orbitals within a single molecule makes it particularly valuable to study systems with multiple orbitals close to the Fermi level (E F ) of the metal substrate. [6][7][8] However, it is important to clarify that the orbitals involved with CT during CHC spectroscopy are not necessarily the same as those orbitals involved with charge transport in molecular junctions under static direct-current conditions, as observed in J(V) measurements of current density (J) vs. voltage (V). For instance, CHC exclusively explores dynamic CT from unoccupied molecular orbitals to the continuum states of the metal substrates, while occupied molecular orbitals are accessible in the J(V) measurements depending on the polarity of the applied bias voltage.…”