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

Abstract

Conspectus A key issue of molecular electronics (ME) is the correlation between the molecular structure and the charge transport properties of the molecular framework. Accordingly, a variety of model and potentially useful molecular systems are designed, to prove a particular function or correlation or to build a prototype device. These studies usually involve the measurements of the static electric conductance properties of individual molecules and their assembles on solid supports. At the same time, information about the dynamics of the charge transport (CT) and transfer in such systems, complementary in the context of ME and of a scientific value on its own, is quite scarce. Among other means, this drawback can be resolved by resonant Auger electron spectroscopy (RAES) in combination with core hole clock (CHC) approach, as described in this Account. The RAES-CHC scheme was applied to a variety of aliphatic and aromatic self-assembled monolayers (SAMs), adsorbed on Au(111) over the thiolate and selenolate docking groups. Electron transfer (ET) from a suitable terminal tail group to the substrate, across the molecular framework, was monitored, triggered by resonant excitation of this group (nitrile in most cases) by narrow-band X-ray radiation. This resulted in the quantitative data for the characteristic ET time, τET, in the femtosecond domain, with the time window ranging from ∼1 fs to ∼120 fs. The derived τET exhibit an exponential dependence on the molecular length, mimicking the behavior of the static conductance and suggesting a common physical basis behind the static CT and ET dynamics. The dynamic decay factors, β ET, for the alkyl, oligophenyl, and acene molecular “wires” correlate well with the analogous parameters for the static CT. Both τET and β ET values exhibit a distinct dependence on the character of the involved molecular orbital (MO), demonstrating that the efficiency and rate of the CT in molecular assemblies can be controlled by resonant injection of the charge carriers into specific MOs. This dependence as well as a lack of correlation between the molecular tilt and τET represent strong arguments in favor of the generally accepted model of CT across the molecular framework (“through-bond”) in contrast to “through-space” tunneling. Comparison of the SAMs with thiolate and selenolate docking groups suggests that the use of selenolate instead of thiolate does not give any gain in terms of ET dynamics or molecular conductance. Whereas a certain difference in the efficiency of the electronic coupling of thiolate and selenolate to the substrate cannot be completely excluded, this difference is certainly too small to affect the performance of the entire molecule to a noticeable extent. The efficient electronic coupling of the thiolate docking group to the substrate was verified and the decoupling of the electronic subsystems of the substrate and π-conjugated segment by introduction of methylene group into the backbone was demonstrated. No correlation between the molecular dipole or fluorine substi...

Cyano-Substituted Triptycene-Based Monolayers on Au(111): Tripodal Adsorption, Dipole Engineering, and Charge Transfer

“…This similarity allows us to assume that the π* systems of the terminal cyano group are coupled to (and aligned with) the π* systems of the respective blades of the adsorbed tripods in the same fashion as for molecular benzonitrile 70,71 and monodentate, benzonitrileterminated SAMs. 38,39 It is also of importance that the energies of the π 1 * and π 3 * resonances of the cyano groups at the N K-edge are higher than the binding energy of the cyano-group-related N 1s peak in the XP spectrum (∼398.75 eV; see section 3.1). Such a relation is an energetic prerequisite for the transfer of an electron excited into these orbitals at the SAM−ambient interface to the substrate, through the molecular framework and the anchoring group.…”

“…The π 1 * and π 3 * resonances correspond to the molecular orbitals, which are either delocalized over the entire benzonitrile moiety (π 1 *) or exclusively localized at the cyano group (π 3 *). , Consequently, the π 1 * orbital is aligned with the π* system of the adjacent phenyl ring, being perpendicular to the ring plane, whereas the π 3 * orbital is located in this plane. , The strong conjugation of the π 1 * orbital with the π* system of the ring causes a redistribution of the electron density, so the intensity of the respective resonance decreases, resulting in the characteristic shape of the joint π 1 */π 3 * feature, typical of benzonitrile. This similarity allows us to assume that the π* systems of the terminal cyano group are coupled to (and aligned with) the π* systems of the respective blades of the adsorbed tripods in the same fashion as for molecular benzonitrile , and monodentate, benzonitrile-terminated SAMs. , …”

“…The respective properties were studied by resonance Auger electron spectroscopy (RAES) following the established core–hole clock (CHC) approach, − specifically adapted to SAMs. − RAES generally involves a resonant excitation of a core level electron into a specific unoccupied electronic state (a molecular orbital in our case) and monitoring the Auger electron decay of the respective excited state. The lifetime of the latter state serves as internal clock as far as dynamic phenomena are studied.…”

“…Second, this group as a polar one , provides a means of dipole engineering of surfaces and interfaces, which is a frequently used approach for monodentate SAMs ,− but also a potential field of applications for tripodal monolayers. Finally, the decoration of NC-Trip-SH with cyano groups allows monitoring electron transfer dynamics in these systems, following the established approach. − It can then, in particular, be clarified whether such a transfer, triggered by the creation of electron at one of the cyano groups, occurs across the individual blades and the respective anchoring groups or is affected by the aliphatic bridge connecting these blades and providing an alternative pathway for the transfer.…”

Elastic Properties of Poly(ethylene glycol) Nanomembranes and Respective Implications

“…A possible explanation can be the difference in the electronic coupling to the substrate for these two kinds of systems. The SAMs couple strongly to the substrate [ 42 ], relying on the chemical bond between the anchoring group of the SAM-forming molecules and the substrate [ 43 ]. Consequently, the tunneling of “hot” electrons between the respective, strongly coupled electronic systems is likely possible, as long as empty states at the molecular side of the interface are available.…”