RA-IR studies on the structure of mercapto-ended azobenzene derivatives inself-assembled monolayers

Self-Assembled Monolayers (SAMs) were prepared from mercapto-ended azobenze ne derivatives with the structure of n-CnH2n+1 AzoO(CH2)(m)SH (n = 4, 6, 8, 10, 12; m = 3, 5). The structure of these SAMs was thoroughly studied with a grazing-angle incident reflection absorption FTIR technique and wettabil ity measurement. The results suggested that the plane of the Azobenzene sys tem of the assembling molecules in the SAMs lies on its back with an approx imate angle of 22 degrees included between the substrate surface normal and the Azobenzene plane. Tail alkyl groups (n-CnH2n+1)in these assembling mol ecules were considered to be in an all-trans conformation, as if they were in a crystalline-like environment. And the C-C-C plane of these all-trans r ail alkyl groups, while n greater than or equal to 8, lies also on its back with an angle about 70 degrees between its plane and the substrate surface normal. The conformation of these head groups (-O(CH2)(m)SH) in SAMs are d isturbed by many structural factors. While m = 3 or 5, the head group chain was proposed to remain in a gauche conformation to fit the upright orienta tion of the Azobenzene plane. The packing density was investigated by measu ring the contact angle of water on these SAMs. The results showed that the self-assembled monolayer films are perfectly packed and the coverage densit y might be improved with increasing the length of both tail and head alkyl chains.