Charge Transfer in Monomolecular Films and Metal-Organic Frameworks

Abstract

Characterization and understanding of electronic properties of nanoscale systems is an important issue in modern nanotechnology including molecular and organic electronics. To advance in this topic, charge transfer (CT) properties of two specific nanoscale systems were analyzed in detail in this work. First, electron transfer (ET) dynamics in supported 2D assembles of molecular wires, self-assembled monolayers (SAMs), were studied by resonant Auger electron spectroscopy (RAES) in a combination with a so-called core hole clock (CHC) approach. A variety of suitable SAMs were custom-designed to address specific questions within the general framework of ET dynamics; most of these SAMs were equipped with nitrile tail groups, serving as a predefined site for the resonant excitation of an electron making the ET. The experiments showed a similar electronic coupling efficiency to coinage metal surfaces for the most frequently used S and Se anchors, solving a long-term controversy. Further, an efficient ET was found in acene-based SAM constituents, manifested by a quite low tunneling decay constant (beta) of 0.25 1/A, similar to that of oligophenyls. In subsequent experiments on an analogous non-benzenoid system, the same ET properties as for its benzenoid isomer were found. As an ultimate proof of the approach, the nitrile groups were attached directly to the substrate, showing an ET time in the sub-fs region, as has been expected. A well-perceptible contribution of the ET process in the RAES [N1s]pi* spectra of pyridyl-substituted molecules revealed that pyridyl is a suitable resonant group for CHC and can be efficiently used as an alternative to nitrile, while NO2-functionalized SAM constitutents exhibited an inverse ET process. Second, static CT properties of surface-anchored metal-organic frameworks (SURMOFs) were studied, taking the basic and well-known HKUST-1 framework as a most suitable reference system. The measurements were performed with the custom-designed two-terminal junction setup and both pristine and guest-molecule loaded SURMOFs were investigated. The pristine SURMOFs showed CT properties similar to hybrid metal-organic molecular wires, as manifested by avery low beta value of 0.0006 1/A. The CT experiments performed after the incorporation of the guest molecules, viz. ferrocen, TCNQ and its fluorinated analog F4-TCNQ, into the pores of the framework showed a significant increase in the current density. This increase was especially dramatic in the case of TCNQ, achieving up to 6 orders of magnitude. This finding verified a previously reported and highly announced result for this particular guest molecule, obtaining it, however, for the samples of well-controlled thickness, quality and orientation. At the same time, in contrast to the previous report, loading with F4-TCNQ resulted in a similar increase in the current density as for TCNQ, questioning the proposed CT model. These observations were made for several orientations of the SURMOF and different solvents used for the loading. Based on the experimental data, a novel superexchange mechanism for CT in the redox.-molecule-loades SURMOFs was proposed.