Synthesis of tris­, tetrakis­, and pentakisazo dyes and their application to guest–host liquid crystal displays

Abstract

Tris-, tetrakis-, and pentakisazo dyes have been prepared by the oxidation of 4-(arylazo)arylamines. Iodobenzene diacetate was the preferred oxidant for preparation of 4,4'-bis(arylazo)azobenzenes and 4,4'bis(arylazo)-2,2',3,3',5,5',6,6'-octa¯uoroazobenzenes, whereas silver(II) oxide proved a satisfactory oxidant for synthesis of 4,4'-(arylazo)(methyl-substituted)azobenzenes. Unsymmetrical tris-and tetrakisazo dyes were also obtained by cross oxidation. The absorption bands of these azo dyes were observed in the range of 381±472 nm in liquid crystals. The solubility of the trisazo dyes in hexane was the highest, followed by the tetrakis-and pentakisazo dyes. The unsymmetrical derivative having methyl groups in the central p-phenylene moieties was most soluble among the trisazo dyes. The order parameters (S) of the tris-, tetrakis-, and pentakisazo derivatives were in the range 0.66±0.81, suf®ciently high for the practical application of these dyes in liquid crystal displays.

Orientation and Solvatochromism of Dyes in Liquid Crystals

“…The first relates to the length of the dye molecule. It has previously been observed that long azo dyes are more difficult to dissolve than the shorter ones [10]. In view of this one might expect MORPIP to have the highest solubility since it is the shortest of the four compounds studied…”

“…In general there are several factors, which govern the solubility of dichroic dyes in LCs. These include the shape and symmetry of the dye molecule, the presence and nature of substituents and side groups, and temperature [7,9,10]. The first relates to the length of the dye molecule.…”

“…Prior studies have shown that azo-compounds can align well with the director of the LC resulting in relatively high order parameters [8,10]. This is also the case for some anthraquinone compounds 3 although the order parameters in these cases are generally less than for the azo compounds because of the smaller l/b ratio of the anthraquinone systems.…”

“…Within this context anthraquinone compounds have been extensively investigated [6,7]. Another family of compounds, which have been found to have excellent solubility, are the azo chromophores [8][9][10]. However, this class of material absorbs mainly in the blue and UV part of the electromagnetic spectrum, which is a serious limitation for their usage in a potential full colour LCD.…”

“…Within this context a structural criteria has been defined as a guide, and this is that the length to breadth (l/b) ratio should be large. A class of compounds where this condition is met is the linear polyenes [7][8][9][10][11]. Those dyes are also termed pleochroic dyes.…”