Side‐Group‐Induced Polymorphism in Self‐Assembled Monolayers: 3,5‐Bis(trifluoromethyl)benzenethiolate Films on Au(111)

Abstract

The structure and molecular organization of self-assembled monolayers (SAMs) depend on a complex interplay of intermolecular and molecule-substrate interactions, so that even a small change in molecular composition can result in noticeable changes in the SAM structure. Herein we show that decoration of the most basic aromatic SAM constituent, benzenethiol, with two trifluoromethyl groups leads to distinct polymorphism in the respective SAMs, in which the appearance of a specific structural phase or a combination of several different phases is dependent on the parameters of the preparation procedure. High-quality films with a single crystallographic phase and significantly large domains could only be prepared after a short immersion time (5 min) and an additional re-immersion of the sample in pure ethanol at an elevated temperature. A standard 24 h immersion at room temperature led to poorly defined films with a large defect density and only a small portion of the surface covered by well-ordered molecular domains.

Charge Transport Properties of Single-Component and Binary Aromatic Self-Assembled Monolayers with Methyl and Trifluoromethyl Tail Groups

“…53 The analogous spectrum of the single-component CF 3 -BPT SAM is dominated by an intense peak at ∼284.3 eV, stemming from the biphenyl backbone (C−C), and a lowintensity peak at ∼292.0 eV, associated with the terminal trifluoromethyl group (−CF 3 ). 54 The spectra of the binary SAMs exhibit these two peaks as well, but the intensity of the −CF 3 feature decreases progressively with decreasing portions of CF 3 -BPT in the solutions from which the SAMs were formed.…”

“…The C 1s spectrum of the single-component CH 3 -BPT SAM in Figure a exhibits a single peak at ∼284.1 eV (and a small shoulder at ∼285 eV), representing a joint contribution of the biphenyl backbone and the terminal methyl group . The analogous spectrum of the single-component CF 3 -BPT SAM is dominated by an intense peak at ∼284.3 eV, stemming from the biphenyl backbone (C–C), and a low-intensity peak at ∼292.0 eV, associated with the terminal trifluoromethyl group (−CF 3 ) . The spectra of the binary SAMs exhibit these two peaks as well, but the intensity of the −CF 3 feature decreases progressively with decreasing portions of CF 3 -BPT in the solutions from which the SAMs were formed.…”

A dithiocarbamate anchoring group as a flexible platform for interface engineering

“…Generally, in all cases two molecules per unit cell arranged in a herringbone pattern are more stable than a cofacial arrangement, consistent with what is known for oligophenylenethiol SAMs. 87,88 Especially for SAMs containing aromatic entities this packing can be traced back to multipole interactions between the negative charge in the p-system and the partially positive hydrogen atoms, as explained in detail for biphenylthiolates in ref 89. The average angle of the p* orbitals of the terminal rings relative to the substrate surface (the values of a from Section 3.2) has been obtained via cos 2 (a) = 0.5Á(cos 2 (a 1 ) + cos 2 (a 2 )) from the angles of the individual molecules, a 1 and a 2 .…”

Importance of Long-Term Storage for Fluorine-Substituted Aromatic Self-Assembled Monolayers by the Example of 4-Fluorobenzene-1-Thiolate Films on Au(111)

“…23 Further interesting examples are SAMs generated from 3,5bis(trifluoromethyl)benzenethiolate (TFBT) molecules on Au(111), the structure and morphology of which were found to depend strongly on the IT, post-preparation storage time, and post-preparation treatment. 38 The standard preparation conditions (i.e., an IT of 24 h and RT) resulted in formation of poorly ordered films having a large defect density and only a small portion of the surface covered by ordered molecular domains. Well-ordered monolayers, with extraordinary length of the ordered domains, could only be prepared at a short IT (5 min) and an additional reimmersion of the sample into pure ethanol at high temperature.…”

“…Well-ordered monolayers, with extraordinary length of the ordered domains, could only be prepared at a short IT (5 min) and an additional reimmersion of the sample into pure ethanol at high temperature. 38 Interestingly, these monolayers exhibited the formation of etch-pits rather than gold islands, which are usually formed after the adsorption of purely aromatic thiols 3,5,23,40−42 and selenols 1,43 on Au(111).…”