Synthesis and Thermal Stability of Solvent Soluble Dyes Based on Dimerized Diketo-pyrrolo-pyrrole Pigment

Red, green and blue LCD filters are typically fabricated using pigments such as C.I. Pigment Red 254, C.I. Pigment Green 36 and C.I. Pigment Blue 15:6, respectively, by means of a photolithographic process. However the use of pigments can cause significant reflection and scattering of light, leading to reduced light transmission and giving rise to lower contrast ratios. These considerations are important because industry is targeting both improved contrast ratio (over 18,000) and higher transmittance for the next to 8generation of LCD filters. C.I. Pigment Red 254, which is based on DPP (Diketopyrrolo-pyrrole) and was first synthesized by Ciba-Geigy, has mainly found use in paints and inks as well as LCD color filters owing to its excellent thermal stability and bright red shade. DPP derivatives are known for their poor solubility in most organic solvent due to their tendency to form strong - interactions and intermolecular hydrogen bonds. Much effort has focused on functionalizing them to improve their solubility. In general, organic dyes exhibit lower light scattering and higher contrast ratios in color filters compared to organic pigments. However, dyes have inferior thermal and solvent stability, which has so far inhibited their commercial adoption in color filter fabrication. In our previous studies, N-alkylated DPP colorants were found to have enhanced solubility in several organic solvents, such as THF and NMP (1-methyl-2-pyrrolidone). Although our results revealed improvements in solubility and weight reduction at 250 C, it was recognized that further studies would be needed to tackle the problem of sublimation during color filter fabrication. In this study, therefore, three soluble dyes derived from a DPP-based pigment were prepared by N-alkylation and dimerization, as shown in Figure 1. The color properties and thermal stability of the synthesized dyes were investigated by spectroscopy and TGA (thermogravimetric analysis) respectively. The chromatic characteristics of films fabricated from the red dyes were examined. Colorants for LCD color filters should exhibit high solubility in organic solvents, such as PGMEA or NMP, which are commonly used commercially. Regardless of the compatibility with binders used in the color photo-resists, it must be possible to dissolve the dye molecules at concentrations greater than 5 wt % in these solvents in order to avoid aggregation and achieve the desired optical properties. The solubilities of the three synthesized dyes in PGMEA were compared with that of C.I. Pigment Red 254, as shown in Table 1. As anticipated, C.I. Pigment Red 254 and dyes 1 & 2 were only sparingly soluble: less than 1 wt % in PGMEA. However dye 3 which contains a longer alkyl group (octyl) between two DPP skeletons showed dramatically enhanced solubility (14.4 wt %) indicating that extended alkyl linking groups can efficiently prevent intermolecular - stacking of DPP aromatic moieties. Therefore, the resulting solubility of dye 3 is suited to the preparation of soluble dye based photo-