One of the most intriguing possibilities offered by organothiol (OT)-based self-assembled monolayers (SAMs) adsorbed on solid substrates is to create organic surfaces, the properties of which can be tailored by choosing organothiols with appropriate groups at the w-position.[1] The large variety of suitable functional groups allows for control of the physico-chemical properties of the surface. For example, the wettability can be varied smoothly between hydrophobic and hydrophilic. Presently, the possibility of chemically binding other moieties to the organic surface exposed by the SAM is attracting an increasing amount of attention, for example, in connection with the coupling of biomolecules, [2] in model studies regarding biomineralization [3] and in anchoring zeolites [4] or metal-organic frameworks.[5] A special type of surface termination, ÀSH groups, has recently attracted considerable attention with regard to metallization of SAMs. [6] In many cases the desired organic surface can be obtained by using an appropriately w-functionalized organothiol, but there are often complications resulting from undesired interactions either between the corresponding functional groups (as in the case of COOH···HOOC hydrogen bonding [7,8] or the functional group and the Au substrate [9,10] ). Recently, we have demonstrated that a protecting-group strategy can be used to avoid such problems by first fabricating a SAM from protected organothiols.[10] A subsequent deprotection carried out by immersing the SAM in a corresponding solution then yields an organic surface with the desired functionality.Herein, we demonstrate that the chemical reactions at the organic surface of the SAM proceed quite differently to the corresponding reactions in solution. Although these types of surface reactions have been studied earlier by several groups (see the papers by Sullivan and Huck [11] and Love et al. [12] for recent reviews of this field), still a careful and systematic understanding is lacking. A detailed analysis of our findings reveals that there are general phenomena resulting from confining the occurrence of a particular chemical reaction to two dimensions, which can affect reactions on highly ordered organic surfaces. We demonstrate the importance of this reduction in dimensionality for the chemical reactivity by investigating the case of removing an acetate group by a basic agent, shown schematically in Figure 1. This reaction, which is standard in solution chemistry, is found to be significantly hindered when confined to a two-dimensional system. Whereas the reaction in solution proceeds in minutes, the corresponding reaction at the organic surfaces requires, depending on the conditions, up to 84 hours. This is a surprising observation considering that the agent used for removing the protecting COCH 3 -group is a very small molecule, a hydroxide ion (OH À ). The reasons for this unexpected behavior are unraveled using IR spectroscopy, near edge X-ray absorption fine structure spectroscopy (NEXAFS) and scanning electron microscop...

Lying low: Alkanethioacetates do not form the same dense thiolate monolayers on gold that are obtained from alkanethiols. A multitechnique study shows instead the formation of a kinetically stable monolayer with mostly flat‐lying thiolate molecules (see model; blue S). This extraordinary structure results from the diminished reactivity of the thioacetate group towards the gold surface.

We have prepared a range of azobenzene derivatives equipped with an asparagusic acid-based 1,2-dithiolane headgroup suitable for chemisorption on solid gold substrates. The formation of self-assembled monolayers (SAMs) of the amide cyclo-S2C3H5-4-C(O)NH-p-C6H4-N=N-Ph (1) and the ester cyclo-S2C3H5-4-C(O)O-p-C6H4-N=N-Ph (2) on gold was monitored in situ and in real time by optical second harmonic generation (SHG). The structure and composition of these SAMs was investigated by a range of ex situ methods, viz. ellipsometry, X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and Fourier transform infrared reflection absorption spectroscopy (FTIRRAS). Reversible, but moderate, photoswitchability was observed for these one-component SAMs by ellipsometry and dynamic contact angle measurements. Use of a second 1,2-dithiolane component for lateral dilution of the photoactive terminal groups resulted in a much more pronounced photoresponse.

Stabile Seitenlage: Alkylthioacetate bilden auf Goldsubstrat nicht dieselben dichten Thiolat‐Monoschichten, wie sie aus Thiolen erhalten werden. Mit mehreren Messmethoden wurde die Bildung einer kinetisch stabilen Monoschicht mit größtenteils flach liegenden Molekülen nachgewiesen (siehe Modell; blau S). Diese außergewöhnliche Struktur entsteht infolge der geringeren Reaktivität der Thioacetate gegenüber der Goldoberfläche.