Orientational ordering of 4-pentyl-4′-cyanobiphenyl molecules evaporated on multi-layered polyimide film

Using optical second-harmonic (SH) generation and polarized absorption (PA) measurements, the orientational ordering process of 4-pentyl-4′-cyanobiphenyl (5CB) was studied during evaporation onto the alignment layer of polyimide (PI) Langmuir-Blodgett films. The intensity of SH signal increased in the beginning of the deposition, but it saturated after the first 5CB monolayer was formed. The PA measurement suggested that a layer-by-layer growth was induced after the formation of 5CB monolayer with accompanying the orientational ordering. The orientational order of the first 5CB monolayer was studied in terms of the orientational order parameters, S1 = 〈cosθ〉 and S2 = 〈(3cos2θ − 1)/2〉, using the SHG and PA measurements. It was found that S1 decreased as the thickness of PI alignment layer increased from 0.4 to 4.4 nm, while S2 was nearly constant. These results indicate that the Coulomb interaction between the permanent dipole of 5CB and its image dipole makes a significant contribution to the orientational ordering of deposited 5CBs.