Molecular orientation of self-assembled monolayers of azobenzene derivatives

Self - assembled monolayers (SAMs) of a series of azobenzene derivatives on gold were characterized with reflectance absorption infrared spectroscopy (RA - IR). The effect of molecular structure of azobenzene derivatives on molecular orientation of their SAMs was investigated by comparison of the RA - IR spectra with the corresponding transmission spectra of the isotropic samples. Mathematical expressions were explored for orientation evaluation of the alkly chain and the azobenzene group, respectively. Our investigation reveals that both the alkyl chain and the azobenzene chromophore tilt away dramatically from the surface normal direction with lengthening of the alkyl chain. The molecular orientation changes in the series of SAMs were attributed to the changes of van der Waals interaction among alkyl chains. The influences of alkyl chain lengthing on alkyl chain and azobenzene group are not the same. While the number of the methylene moiety in molecule increases, the tilting angle of alkyl chain increases rapid but the changes of tilting angle of azobenzene moiety is relative insignificant. Such difference between alkyl chain and azobenzene group is attributed to their difference in rigidity.