Thiolate versus Selenolate: Structure, Stability, and Charge Transfer Properties

Abstract

Selenolate is considered as an alternative to thiolate to serve as a headgroup mediating the formation of self-assembled monolayers (SAMs) on coinage metal substrates. There are, however, ongoing vivid discussions regarding the advantages and disadvantages of these anchor groups, regarding, in particular, the energetics of the headgroup-substrate interface and their efficiency in terms of charge transport/transfer. Here we introduce a well-defined model system of 6-cyanonaphthalene-2-thiolate and -selenolate SAMs on Au(111) to resolve these controversies. The exact structural arrangements in both types of SAMs are somewhat different, suggesting a better SAM-building ability in the case of selenolates. At the same time, both types of SAMs have similar packing densities and molecular orientations. This permitted reliable competitive exchange and ion-beam-induced desorption experiments which provided unequivocal evidence for a stronger bonding of selenolates to the substrate as compared to the thiolates. Regardless of this difference, the dynamic charge transfer properties of the thiolate- and selenolate-based adsorbates were found to be nearly identical, as determined by the core-hole-clock approach, which is explained by a redistribution of electron density along the molecular framework, compensating the difference in the substrate-headgroup bond strength.

Triptycene-terminated thiolate and selenolate monolayers on Au(111)

“…Regarding the use of an acetyl-protected SAM precursor, i.e., Trp1SeAc for SAM formation, it can be noted that the acetyl group has been cleaved from Trp1SeAc, also leading to the formation of a selenolate SAM. This finding has been reported in earlier publications on preparation of selenium-anchored SAMs from selenoacetates [5,8]. The measured thicknesses of the triptycene-based films clearly indicate that they are monolayers.…”

“…Since the diselenide (Trp1Se) 2 is rather insoluble, we decided to turn it into the selenoacetate Trp1SeAc (compare Scheme S1 in the Supporting Information File 1). Selenoacetate compounds can be used to form selenolate SAMs on gold surfaces [8]. Earlier work on homologous sulfur compounds revealed that free thiol groups are incompatible with benzyne [28].…”

“…This third parameter, γ, is an obstacle to the determination of molecular tilt since in general it demands additional information (either geometric assumptions or input from complementary experimental methods or theoretical calculations). Only in rare cases, β can unambiguously be determined from NEXAFS data, e.g., by evaluation of two transitions with perpendicular TDMs [8,40]. Sometimes, the combination of IRRAS, NEXAFS experiments and theoretical calculations of NEXAFS spectra allows for reliable conclusions on the molecular tilt in SAMs [41].…”

“…The by far most intensely studied SAM systems are those of organothiolates on gold surfaces [1,4]. More recently, selenolate SAMs on gold have experienced considerable attention because of their superior degree of order in comparison to their thiolate analogues [5–8]. The structure and degree of order in SAMs strongly depend on the nature of the backbones of the SAM-building molecules [1,4].…”

Probing charge transfer dynamics in self-assembled monolayers by core hole clock approach

“…The future directions of research in the framework of the CHC approach in its specific application to SAM-like molecular assembles include comparison of different anchor groups [93], alternative resonantly excitable tail groups, ultimate cases of ET, and switchable systems. An interesting extension, which can also be applied to a variety of different systems, e.g.…”