Assembly of photopolymerizable discotic molecules on an aligned polyimide layer surface to form a negative retardation film with an oblique optical axis

Ultraviolet (UV) polymerizable discotic liquid-crystalline (DLC) molecules (2,3,6,7,10,ll-hexakis(4′-acryloy-m-alkyloxyben- zoyoxy)triphenylene [HAHBT-m, where m was the number of methylene units, and here m = 6 (HAHBT-6)]) were assembled to form a negative retardation film with an oblique optical axis on a specifically designed rubbing-aligned polyimide layer surface [6FDA-11CBBP (where 11 is the number of methylene units in the side chains)]. The side chains of this polyimide were terminated by cyanobiphenyl groups. Surface-enhanced Raman scattering (SERS) and optical second harmonic generation results showed that rubbing caused a surface structural re-arrangement in the alignment layer resulting in a negative pre-tilt angle (θ_s) of -8.5° (which was in the direction opposite to the rubbing direction). The molecular topology at the rubbed surface was governed by a stable fold-like bent structure of the cyanobiphenyl side chains, in which the CN groups preferentially pointed down towards the surface. When the DLC molecules were deposited onto the alignment surface and polymerized via UV irradiation to generate a new optical film, an oblique optical axis with an average tilt angle of -18.6° with respect to the film normal was detected using ellipsometric measurements. This tilted optical axis was developed by the DLC molecules being wedged on top of the cyanobiphenyl groups when in the bent conformation. Furthermore, the tilt angle difference between the θ_sat the alignment surface and at the air interface of the DLC molecules was attributed to a splay deformation of the DLC molecules along the film surface normal. Optical modeling has also confirmed our experimental observations.