Self-Assembled Monolayers Generated from Unsymmetrical Partially Fluorinated Spiroalkanedithiols

Abstract

Self-assembled monolayers (SAMs) were prepared on gold substrates from an unsymmetrical partially fluorinated spiroalkanedithiol adsorbate with the specific structure of [CH3(CH2)7][CF3(CF2)7(CH2)8]C[CH2SH]2 (SADT) and compared to SAMs formed from the semifluorinated monothiol F8H10SH [CF3(CF2)7(CH2)10SH] of analogous chain length and n-octadecanethiol. The adsorbate with two alkyl chains, one terminally fluorinated and the other nonfluorinated, was designed to form monolayers in which the bulky helical fluorocarbon segments assemble on top of an underlying layer of well-packed trans-extended alkyl chains. Different combinations of deposition solvents and temperatures were used to produce the bidentate SAMs. Characterization of the resulting monolayers revealed that SAMs formed in DMF at room temperature allow complete binding of the sulfur headgroups to the surface and exhibit higher conformational order than those produced using alternative solvent/temperature combinations. The reduced film thicknesses and enhanced wettability of the SADT SAMs, as compared to the SAMs generated from F8H10SH, suggest loose packing and an increase in the tilt of the terminal fluorocarbon chain segments. Nevertheless, the density of the underlying hydrocarbon chains of the SADT SAMs was higher than that of the F8H10SH SAMs, owing to the double-chained structure of the new adsorbate. The conformational orders of the SAM systems were observed to decrease as follows: C18SH > F8H10SH > SADT. However, the SAMs formed from this new double-chained bidentate adsorbate in DMF expose a fluorinated interface with a relatively low surface roughness, as determined by contact-angle hysteresis.

Unsymmetrical Spiroalkanedithiols Having Mixed Fluorinated and Alkyl Tailgroups of Varying Length: Film Structure and Interfacial Properties

“…Previous work on related SADT SAMs developed in DMF yielded films with optimal qualities (i.e., relatively ordered, less wettable with a well-packed monolayer film) [50].…”

“…Preliminary confirmation of SAM formation on metal substrates can be determined by measuring the thickness of the film using ellipsometry [50]. Figure 3 provides the XPS spectra of the S 2p, C 1s, and F 1s regions for the SAMs derived from F8H10SH and the F8H10-Cm adsorbates.…”

“…Several examples of designs of such bidentate adsorbates, which have been used to enhance film stability on metal substrates, include aromatic-based bidentate adsorbates [44][45][46][47], spiroalkanedithiolate-based adsorbates [48][49][50][51], and trithiolate-based adsorbates [52][53][54][55][56][57]. Using the spiroalkanedithiol (SADT) structure, well-packed and highly oriented SAMs can be generated by utilizing an adsorbate structure having a pair of relatively long alkyl chains [48].…”

“…A major attractive feature of this design is the relatively simple synthetic approach, which can open new avenues for the synthesis of a large variety of adsorbates. A recent example includes the development of film compositions where two phase-incompatible groups were incorporated into the same molecule, yielding homogeneously mixed SAMs [50,51].…”

“…In a previous report, we studied the synthesis and characterization of SAMs generated from a prototype unsymmetrical partially fluorinated SADT [50]. The structure consisted of the spirodithiol headgroup connected with two different tailgroups (i.e., one hydrocarbon and one partially fluorinated), as shown in Figure 1.…”