RA-IR Studies on the Structure of Mercapto-Ended Azobenzene Derivatives in Self-Assembled Monolayers

Self-Assembled Monolayers (SAMs) were prepared from mercapto-ended azobenzene derivatives with the structure of n-C_nH_2n+1, AzoO(CH₂)_mSH (n=4,6,8,10,12; m=3,5). the structure of these SAMs was thoroughly studied with grazing-angle incident reflection absorption FT1R technique and wettability measurement. the results suggested that the plane of Azobenzene system of the assembling molecules in the SAMs lies on its back with an approximate angle of 22° included between the substrate surface normal and the Azobenzene plane. Tail alkyl groups (n-C_nH_2n+1) in these assembling molecules 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 tail alkyl groups, while n≤8, lies also on its back with an angle about 70° between its plane and the substrate surface normal. the conformation of these head groups (-O(CH₂)_mSH) in SAMs are disturbed 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 orientation of the Azobenzene plane. the packing density was investigated by measuring the contact angle of water on these SAMs. the results showed that the self-assembled monolayer films are perfectly packed and the coverage density might be improved with increasing the length of both tail and head alkyl chains.