Electron Transfer Dynamics and Structural Effects in Benzonitrile Monolayers with Tuned Dipole Moments by Differently Positioned Fluorine Atoms

To understand the influence of the molecular dipole moment on the electron transfer (ET) dynamics across the molecular framework, two series of differently fluorinated, benzonitrile-based self-assembled monolayers (SAMs) bound to Au(111) by either thiolate or selenolate anchoring groups were investigated. Within each series, the molecular structures were the same with the exception of the positions of two fluorine atoms affecting the dipole moment of the SAM-forming molecules. The SAMs exhibited a homogeneous anchoring to the substrate, nearly upright molecular orientations, and the outer interface comprised of the terminal nitrile groups. The ET dynamics was studied by resonant Auger electron spectroscopy in the framework of the core-hole clock method. Resonance excitation of the nitrile group unequivocally ensured an ET pathway from the tail group to the substrate. As only one of the π* orbitals of this group is hybridized with the π* system of the adjacent phenyl ring, two different ET times could be determined depending on the primary excited orbital being either localized at the nitrile group or delocalized over the entire benzonitrile moiety. The latter pathway turned out to be much more efficient, with the characteristic ET times being a factor 2.5-3 shorter than those for the localized orbital. The dynamic ET properties of the analogous thiolate- and selenolate-based adsorbates were found to be nearly identical. Finally and most importantly, these properties were found to be unaffected by the different patterns of the fluorine substitution used in the present study, thus showing no influence of the molecular dipole moment.

Ultrafast Electron Transfer at Organic Semiconductor Interfaces: Importance of Molecular Orientation

Much is known about the rate of photoexcited charge generation in at organic donor/acceptor (D/A) heterojunctions overaged over all relative arrangements. However, there has been very little experimental work investigating how the photoexcited electron transfer (ET) rate depends on the precise relative molecular orientation between D and A in thin solid films. This is the question that we address in this work. We find that the ET rate depends strongly on the relative molecular arrangement: The interface where the model donor compound copper phthalocyanine is oriented face-on with respect to the fullerene C60 acceptor yields a rate that is approximately 4 times faster than that of the edge-on oriented interface. Our results suggest that the D/A electronic coupling is significantly enhanced in the face-on case, which agrees well with theoretical predictions, underscoring the importance of controlling the relative interfacial molecular orientation.