Spontaneously homogeneous alignment of liquid crystals on self-assembly organic rubrene

Abstract

A novel spontaneously homogeneous liquid crystal (LC) alignment with self-assembled organic rubrene layer on the substrate surface has been reported. The spontaneous alignment technique without surface pre-treatment results in low cost, short processing time, and the room-temperature processing extends application to plastic device. The rubrene-nematic LC mixture is used to fabricate the LC cell. During the capillary filling, the LC molecules align in the flow direction. Meanwhile, the flow-induced LC alignment enforces the tetracene of the rubrene dopants along the flow direction because of the van der Waals and π–π interactions between the rubrene and LC molecules. Afterwards, the rubrene dopants spontaneously migrate onto substrate surface and self-assemble as the triclinic crystalline rubrene (TCR) with ribbon morphology. The LC alignment on TCR has a low pretilt angle, thereby providing a homogeneous LC alignment spontaneously. LC alignment on TCR shows a higher thermal stability compared to conventional polyimide due to the strong interactions of the rubrene and LC molecules. Moreover, the rubrene molecules remain in the bulk suppress the free-ions, decrease the rotational viscosity of LCs and hence shortening the response time of cell. The suppressed free-ions also decrease the screening effect on the substrate surfaces and hence the operation voltage of cell. The TCR structure on the substrate surface and the residual rubrene molecules dispersed in the bulk simultaneously contribute to the enhancement in electro-optical performance of the LC cell.