Self-assembled oligo(phenylene-ethynylene) molecular electronic switch monolayers on gold: Structures and chemical stability

Self-assembled monolayers (SAMs) of the nitro-substituted oligo(phenylene-ethynylene) (OPE) 4,4′-(diethynylphenyl)-2′-nitro-1-benzenethiolate on Au{111} were prepared, and the structures were characterized by multiple techniques, including infrared spectroscopy, ellipsometry, and X-ray photoelectron spectroscopy. Assembly of the nitro-OPE SAM, either via acidic hydrolysis of the thioacetate derivative or from the thiol in pure solvent, produces a well-ordered SAM with a (√3 × √3) superlattice structure and an average molecular tilt of 32-39° from the surface normal. In comparison, SAMs prepared from the unsubstituted OPE show the same lattice structure and a similar tilt of ∼33°. In contrast, when the nitro-OPE SAM is assembled by hydrolysis of the thioacetate derivative under basic conditions, extensive redox reactions arise in which oxidation of the S atoms occurs with accompaying reduction of -NO₂ to -NH₂, apparently via intermediates including -NH(OH), to form mixed composition SAMs typically containing ∼30% of the amino-substituted molecule. Further, the nitro-OPE SAM, regardless of the preparation method, shows significant chemical instability under storage in air and/or light exposure. Since the nitro-OPE molecule and molecules with related structures are of considerable interest for molecular electronics applications, these results indicate that extreme diligence must be used in designing conditions for the fabrication of devices utilizing these SAMs.