Self-Assembled Monolayers of Pseudo-C_2v-Symmetric, Low-Band-Gap Areneoxazolethiolates on Gold Surfaces

Abstract

A series of three homologous arene[2,3-d]-oxazole-2-thiols (benzoxazole-2-thiol (BOxSH), naphthaleneoxazole-2-thiol (NOxSH), and anthraceneoxazole-2-thiol (AOxSH)) were deposited onto Au(111) to obtain surfaces suitable as injection layers for organic electronics. The guiding idea was that the increasingly extended conjugated system would lower the band gap of the films while the introduction of the annulated heteroaromatic ring would provide the opportunity for pseudosymmetric attachment of the sulfur anchor, what should lower the conformational freedom of the system. In fact, the annulation of the oxazole ring lowers the optical band gaps of the parent compounds to 3.1-4.0 eV, depending on the number of benzene rings. To characterize the respective monolayers, a variety of spectroscopic techniques such as ellipsometry, infrared reflection-absorption spectroscopy, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure spectroscopy have been utilized. The monolayers of BOxS exhibit a lower film quality than those of NOxS and AOxS, with enhanced molecular density and more upright molecular orientation with increasing molecular length. Infrared spectroscopy suggests that the nitrogen atoms of the oxazole rings are located more closely to the Au(111) surface than the oxygen atoms, although no hints for an electronic interaction between the N atoms and the gold surface could be found. This preferred orientation could be tentatively traced to packing effects, solving a conundrum of the literature.

Reestablishing Odd–Even Effects in Anthracene-Derived Monolayers by Introduction of a Pseudo-C2v Symmetry

“…38,39 For comparison, we also measured the spectra of the AOx SAM, which differs from the AOxCn monolayers by the lack of the aliphatic linker. 34 The measurements were repeated several times to achieve consistent data over the entire series. 2.6.…”

“…Because of this limitation, we were looking for a way to “symmetrize” the π-system, while basically leaving the electronic properties unchanged. A useful approach in this context is the annellation of acene systems with an oxazole ring, as was demonstrated in a previous publication, where we reported the preparation of purely aromatic SAMs with an anchoring group on the long axis of the acene backbone …”

“…A useful approach in this context is the annellation of acene systems with an oxazole ring, as was demonstrated in a previous publication, where we reported the preparation of purely aromatic SAMs with an anchoring group on the long axis of the acene backbone. 34 On this basis, in the present study, we designed and synthesized a series of (pseudo)symmetrical anthracene [2,3d]oxazolyl-2-(CH 2 ) n -thioacetates (AOxCnSAc) with n = 2−6 (general structure shown in Figure 2) serving as precursors for the SAMs with the almost symmetric attachment of anthracene to the adjacent alkyl linker. The respective SAMs were prepared on Au(111) substrates and characterized in detail by ellipsometry, X-ray photoelectron spectroscopy (XPS), nearedge X-ray absorption fine structure (NEXAFS) spectroscopy, and IR spectroscopy to test the effect of the symmetric attachment of anthracene on the structural properties of the monolayers, making them promising candidates for application in organic semiconductor devices.…”

Two-Step Nanoscale Approach for Well-Defined Complex Alkanethiol Films on Au Surfaces

“…Here we report on the nanoscale formation of complex thiol films beyond a simple homogeneous monolayer which are formed on model surface substrates of ultraflat polycrystalline gold, UFG (Figure 1). The control of molecular order and disorder of selfassembled monolayers and interfaces is important in many fields of nanophysics, molecular electronics, 20,21 and surface science, yet rarely studied in detail. Beyond our own ultimate goal to understand and control the origin of localized breakdown of organic-molecule-inhibited (dealloying) corrosion, detailed insights into the molecular interactions and structure formation are therefore widely necessary.…”

Potential- and Anion-Controlled Organization of 2-Mercapto-5-Benzimidazolesulfonate on the Au(111) Electrode in Acidic Media

“…A conjugation in the molecular skeleton of a thoil molecule can be advantageous in this regard. Some progress in the study of thiols with aromatic functionality and heterocyclic structure adsorption on Au and Cu electrodes has been reported. , For example, a homologous series of thiol molecules with various extents of conjugation were synthesized and deposited on an Au electrode to study their effects on the rate of the electron transfer reaction. , A series of pyridine thiol molecules were examined to reveal their effects as the electron-transfer promoters for cytochrome c . − …”