STRUCTURAL INVESTIGATION OF SELF-ASSEMBLED MONOLAYER OF A P-NITROANILINO-TERMINATED THIOL

Monolayers of an end-group-modified thiol, 4-(12-mercaptododecyl)amino nitrobenzene (p-NO2-C6H4-NH(CH2)(12)SH) formed by self-assembly on Au substrates, were investigated by grazing-incidence reflection absorpti on infrared spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XP S), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), a nd contact angle measurements. The monolayer exhibits a packing of the alkyl chains which is 10-20% less dense compared to a monolayer of do cosanethiol. The orientation of the aromatic end group was determined completely by a combination of IRRAS and NEXAFS. The 1,4-axis of the b enzene ring is inclined to 52 degrees with respect to the surface norm al, and the plane of the benzene ring is rotated by 58 degrees away fr om the plane defined by the 1,4-axis and the surface normal. No hydrog en bonding between the p-nitroaniline moieties is detected. The hydroc arbon chains do not show a preferred orientation. The film is describe d by a layered structure consisting of a layer of alkane chains with a high degree of gauche conformation beneath an oriented layer of the n itroaniline. Shifts of the vibrations of the alkane chain and the nitr oaniline 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 chain s to the total free enthalpy of the monolayer.