Structural investigation of self-assembled monolayer of a p-nitroanilino-terminated thiol

Monolayers of an end-group-modified thiol, 4-(12-mercaptododecyl)aminonitrobenzene (p-NO2-C6H4-NH(CH2)(12)SH) formed by self-assembly on Au substrates, were investigated by grazing-incidence reflection absorption infrared spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and contact angle measurements. The monolayer exhibits a packing of the alkyl chains which is 10-20% less dense compared to a monolayer of docosanethiol. The orientation of the aromatic end group was determined completely by a combination of IRRAS and NEXAFS. The 1,4-axis of the benzene ring is inclined to 52 degrees with respect to the surface normal, and the plane of the benzene ring is rotated by 58 degrees away from the plane defined by the 1,4-axis and the surface normal. No hydrogen bonding between the p-nitroaniline moieties is detected. The hydrocarbon chains do not show a preferred orientation. The film is described by a layered structure consisting of a layer of alkane chains with a high degree of gauche conformation beneath an oriented layer of the nitroaniline. Shifts of the vibrations of the alkane chain and the nitroaniline group in opposite directions are observed upon change of the temperature. This can be explained by a balance of contributions from the layer of the nitroaniline groups and the layer of the alkane chains to the total free enthalpy of the monolayer.