A study on the fluorescence property and the solubility of the perylene derivatives and their application on the LCD color filter

Abstract

Liquid crystal displays (LCDs) have captured a larger portion of the display market than other types of displays. The LCD module consists of a backlight unit that radiates white light and a liquid crystal panel for producing imagery. One of the most essential component of LCD modules is the color filter, which converts the white backlight into red (R), green (G), and blue (B) colored lights. The LCDs operate by using liquid crystals and polarizers to control and assemble these colored lights into the correct patterns to produce images. Numerous studies have reported improvements to the performance of LCDs by enhancing the optical properties of the color filters. Traditional color filters use pigments which lead to light scatting and cause a small amount of light to leak into the full-black state due to their large particle size. This problem can be resolved by using dyes of smaller particle size. In general, dyes with smaller particle sizes have superior color purity and transmittance. Dyes to be used in LCD color filters should have sufficient thermal stability to withstand the color filter manufacturing process, be soluble in the relevant organic industrial solvents, and possess superior spectral properties. In this study, several substituents were introduced at terminal- and bay-positions of the perylene molecule to develop red dyes that can be used as colorants in LCD color filters. The feasibility of the newly synthesized dyes for use in LCD color filters was evaluated based on the spectral properties, thermal stability, and solubility of dyes. In addition, the relationship between the chemical structure of the dyes and their properties was analyzed using density functional theory (DFT) calculations. Perylene-based dyes generally exhibit strong fluorescence. Fluorescence of the dyes increases both the maximum and the minimum brightness of color filters, although the latter is more affected. Therefore, color filters containing highly fluorescent dyes exhibit a lower contrast ratio than those with dyes that are less fluorescent. The fluorescence properties of the color filters fabricated with the synthesized dyes showed same results. The fluorescence increased the minimum brightness of the color filters, and as a result, their contrast ratio decreased. To resolve this low-contrast ratio problem, new perylene-based dyes with high solubility and low fluorescence were developed. Accordingly, the color filters fabricated with these low-fluorescence dyes exhibited lower minimum brightness levels and improved contrast ratios compared to the color filters made with high-fluorescence dyes. Perylene-based dyes with methoxy groups at various positions and orientations were synthesized to understand the effect of methoxy groups on dye fluorescence. It was found that methoxy groups of terminal-substituents had a lesser effect on fluorescence quenching than methoxy groups of bay-substituents. Moreover, only the methoxy groups at the para-position of the bay-substituents showed fluorescence quenching ability. These results are interpreted to show that fluorescence quenching is greatly influenced when the methoxy groups are involved in the main conjugation systems, which are majorly attributed by the main body of the perylene molecule. The relationship between the positions and orientations of the methoxy groups and the molecular conjugation systems were further analyzed by molecular orbital modeling obtained by time-dependent density functional theory (TD-DFT) simulations, and the substituents containing methoxy groups are presumed to inhibit the fluorescence of dyes due to their the electron-donating effect.