Molecular orientation determined by second-harmonic generation: Self-assembled monolayers

Monolayers of 12-(4-nitroanilino)-dodecane thiol [O₂N--C₆H₄-NH(CH₂)₁₂SH](NAT) adsorbed on polycrystalline gold substrates were investigated in a gaseous environment and in ethanol. Contributions of comparable magnitude from the metal substrate, the adsorbate-substrate interaction, and the p-nitro aniline moiety (pNA) require phase-sensitive and polarization-dependent second-harmonic-generation (SHG) measurements. The susceptibility of the pNA end group is separated by comparative experiments with n-alkane thiols. Evaluation of the orientation of the pNA moiety requires phase information about the SHG signal. Intensity data alone produce ambiguous results. The tilt angle determined by SHG is shown to be strongly dependent on the model applied. For the NAT film in an inert gas atmosphere, the tilt angle {ϑ} of the pNA group calculated from the SHG experiments is compared with the value of 53^∘{} obtained by linear techniques. Agreement is only achieved if local-field factors are neglected and if a two-layer model is assumed to describe the linear optical properties of the system. Immersion of the NAT film in ethanol causes a change of the tilt angle of the pNA end group to a more upright position of about {ϑ}=36^∘{}. A detailed discussion reveals that the accuracy and reliability of SHG data to determine the orientation of molecules depend critically on the precision by which molecular properties are known and on the model describing the linear optical properties.