Formation of highly ordered self-assembled monolayers of alkynes on Au(111) substrate.

Carleen Morris Bowers,Agnieszka Noworolska,Piotr Cyganik,Benjamin Breiten,Tomasz Zaba,George M Whitesides

doi:10.1021/ja506647p

Abstract

Self-assembled monolayers (SAMs), prepared by reaction of terminal n-alkynes (HC≡C(CH2)nCH3, n = 5, 7, 9, and 11) with Au(111) at 60 °C were characterized using scanning tunneling microscopy (STM), infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), and contact angles of water. In contrast to previous spectroscopic studies of this type of SAMs, these combined microscopic and spectroscopic experiments confirm formation of highly ordered SAMs having packing densities and molecular chain orientations very similar to those of alkanethiolates on Au(111). Physical properties, hydrophobicity, high surface order, and packing density, also suggest that SAMs of alkynes are similar to SAMs of alkanethiols. The formation of high-quality SAMs from alkynes requires careful preparation and manipulation of reactants in an oxygen-free environment; trace quantities of O2 lead to oxidized contaminants and disordered surface films. The oxidation process occurs during formation of the SAM by oxidation of the -C≡C- group (most likely catalyzed by the gold substrate in the presence of O2).

Highlights

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Contact angle analyses of increasing lengths of alkynes (HCŁC(CH2)nCH3, n = 5, 7, 9 and 11) suggest[4] that the quality of these SAMs is lower than those based on n-alkanethiols
Our results show that the disorder and mixed organic functionality implied by previous work[4,5,6] are artefacts reflecting oxidation of the terminal acetylene by oxygen in solution during formation of the SAM,[15] and that using appropriate experimental conditions results in well-organized SAMs of alkynes that have qualities similar to that of alkanethiols on Au(111)

Summary

Introduction

The Harvard community has made this article openly available. Please share how this access benefits you. Au(111) indicate a “liquid-like” structure of the monolayer,[6] and XPS analyses of SAMs formed from alkynes[4,5] suggest that these

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Conclusion