Structural Investigation of a Self-Assembled Monolayer of a p-Nitroanilino-Terminated Thiol A. Wesch, †,‡ O. Dannenberger, † Ch. Wo ¨ll,* ,† J. J. Wolff,* ,§ and M. Buck* ,† Institut fu ¨ r Angewandte Physikalische Chemie, Universita ¨ t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany, and Organisch-Chemisches Institut, Universita ¨ t Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany Received January 17, 1996. In Final Form: July 15, 1996 X Monolayers of an end-group-modified thiol, 4-(12-mercaptododecyl)aminonitrobenzene (p-NO2-C6H4- NH(CH2)12SH) 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° with respect to the surface normal, and the plane of the benzene ring is rotated by 58° 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. 1. Introduction The technological interest in tailoring surface proper- ties, e.g., adhesion, wetting behavior, or chemical reactiv- ity, makes ultrathin organic films of self-organizing organosulfur compounds on metals particularly attractive systems. The systems which have been studied most extensively so far are monolayers of n-alkanethiols, H 3 C- (CH 2 ) m SH. Almost all techniques available have been applied to unravel their structure. 1-9 Above a minimum chain length (m > 14) the molecules order and form a dense monolayer with the main part of the alkane chain in an all-trans conformation. Their angle of inclination depends on the substrate and varies between 0° and 35°. However, the unreactive methyl-terminated organic sur- face of the alkanethiols is of limited interest with regard to applications, and therefore, thiols with modified end groups have been investigated. They have been used to modify electrodes in electrochemistry, 10,11 to control wet- ting, 12 for biomimetics, 13 for protein adsorption, 14,15 and for patterning of surfaces. 16 In contrast to n-alkanethiols much less is known about the structure of films consisting of end-group-modified thiols. Relatively few of them have been structurally characterized in detail. Whether they self-organize like n-alkanethiols to form well-ordered systems or whether they exhibit a rather random orien- tation depends on the particular system, and at present, the degree of self-organizationsin analogy to the three- dimensional case of crystallizationscannot be predicted generally from the molecular structure. A possible route to achieve ordered structures irrespective of the end group might be to modify the hydrocarbon backbone by intro- ducing appropriate functional groups. So far amide groups, 17 aromatic groups such as bi/terphenyl or naph- thalene, 18,19 or staffandithiols 20 have been investigated. Another possibility is the stabilization by coadsorption of self-assembling molecules with different end groups. 21 In the studies presented here we have used 4-(12- mercaptododecyl)aminonitrobenzene (p-NO 2 -C 6 H 4 - NH(CH 2 ) 12 SH, NAT) as a model system of an end-group- modified thiol (see Figure 1). The molecule combines various properties which allow investigation of different aspects of self-assembled monolayers (SAM). Since the end group is significantly larger than a methyl group, a deviation from the dense packing of the hydrocarbon chains in alkanethiols is expected. The large dipole moment of 6.3 D and the possibility of hydrogen bond formation of the p-nitroanilino (pNA) group could also affect the structure of the film. Since pNA is a well- † Institut fu ¨ r Angewandte Physikalische Chemie. ‡ Present address: Institut fu ¨ r Technische Chemie, Forschungs- zentrum Karlsruhe, Postfach 3640, 76021 Karlsruhe, Germany. § Organisch-Chemisches Institut. X Abstract published in Advance ACS Abstracts, October 1, 1996. (1) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (2) Yeganeh, M. S.; Dougal, S. M.; Polizzotti, R. S.; Rabinowitz, P. Phys. Rev. Lett. 1995, 74, 1811. (3) Camillone, N., III; Chidsey, C. E. D.; Eisenberger, P.; Fenter, P.; Li, J.; Liang, K. S.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 99, 744. (4) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (5) Ha ¨ hner, G.; Wo ¨ll, Ch.; Buck, M.; Grunze, M. Langmuir 1993, 9, 1955. (6) Edinger, K.; Go ¨lzha ¨ user, A.; Demota, K.; Wo ¨ll, Ch.; Grunze, M. Langmuir 1993, 9, 4. (7) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (8) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383. 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