Anchoring Effects of Self-Assembled Monolayers for Polymer-Dispersed Liquid Crystal Films

Abstract

Polymer-dispersed liquid crystal (PDLC) films of 4-cyano-4‘-pentylbiphenyl (5CB) were fabricated between two quartz substrates, the surfaces of which had been modified with the self-assembled monolayers (SAMs) of CH3−(CH2)17−Si(OMe)3 (1), HS−(CH2)10−Si(OEt)3 (2), and NC−(CH2)11−Si(OEt)3 (3). The SAM-modification effects on the molecular aggregation of 5CB were investigated by steady-state and time-resolved fluorescence analysis for the PDLC films. Remarkably, it was found that selective excitation of the interface layer with the substrate surface gave both the monomer and excimer emissions of 5CB in relative intensities, depending on the chemical nature of the SAM surfaces. While the monomer and excimer emissions appeared in comparative intensities in the case of the unmodified quartz surface, the surface modification with the SAM of 1 resulted in a dominant contribution of the excimer emission. By contrast, the monomer emission was much stronger than the excimer emission in the case of the surface modified by the SAM of 2. The surface modification with the SAM of 3 gave a fluorescence spectrum very similar to that in the case of the unmodified surface. Fluorescence decay analysis for the PDLC films revealed that the excimer emission consists of two components with shorter (1.3−1.6 ns) and longer (10−12 ns) lifetimes, whose relative contributions depend on the SAM modifications. The molecular pictures of 5CB depicted from the decay dynamics are in good agreement with those derived from the steady-state fluorescence behavior of the PDLC films. Electrooptic devices based on the PDLC films were constructed by using indium−tin oxide transparent electrodes modified with the SAMs, and it was confirmed that the electrooptic responses again significantly depend on the modifications of the substrate surface. The dependency of the fluorescence and electrooptic behavior on the surface modifications for the PDLC films has been discussed in terms of anchoring effects of the substrate surfaces, which effectively work even in heterogeneous materials such as PDLCs.