IMPLEMENTACION

Anthraquinone dyes are another important class of commercial dyes, and they are based upon the fused ring chromophore shown in Figure 1.16. A range of substituents may be used around the structure to obtain a full range of colours, although typically the colours of anthraquinone dyes are weaker than those of azo dyes.⁴⁸

Figure 1.16 The structure of the anthraquinone chromophore.

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Anthraquinone itself does not exhibit a strong colour,⁵⁴ but the addition of electron-donating groups to the chromophore results in relatively strong visible absorption bands, attributable to π→π* intramolecular charge-transfer transitions between the substituent(s) and the carbonyl groups. This colour may be observed in the context of the absorption wavelengths of simple anthraquinone dyes such as those shown in Figure 1.17. The addition of more electron-donating substituents results in a red shift of the visible absorption band, as does the presence of hydrogen bonds with the carbonyl groups, which can be obtained by using appropriate donors in the 1-, 4-, 5-, and 8- positions, stabilising the charge-transfer state.⁵⁵

λmax = 410 nm λmax = 465 nm

λ_max = 550 nm λ_max = 620 nm Figure 1.17 Structures and λmax values of some anthraquinone dyes in DCM.⁵⁶

Red shifts are also observed on going to more polar solvents, again as a result of stabilisation of the charge transfer excited state, as shown in Table 1.3,⁵⁷ although deviations from this trend are observed in structures that can form intramolecular hydrogen bonds.⁵⁸

Table 1.3 Visible λmax values of 1-aminoanthraquinone in several solvent systems with their solvent polarity functions (Δf) listed.⁵⁷

Solvent Δf λmax / nm

Cyclohexane 0.000 452

Decalin 0.002 454

Cyclohexane: Ethyl Acetate (90:10) 0.046 463

Ethyl Acetate 0.201 467

In the context of guest-host systems, anthraquinone dyes are attractive due to their stability. In particular, their light-fastness properties are generally superior to those of

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azo dyes,⁴⁸ resulting in a significant amount of research into anthraquinone dyes in the context of liquid-crystal applications.

The molecular structure of the anthraquinone chromophore does not lend itself as readily to creating rod-like structures as the azo chromophore does. This possible drawback is evident from order parameters in the region of 0.6 in nematic hosts obtained for anthraquinone dyes with a range of colours, obtained from using various hydroxyl and amine substituents in the 1-,4-,5-,8- positions.²⁴

Much of the early work on anthraquinone-liquid crystal mixtures was carried out on compounds based on 1-phenylamino-4-hydroxy-anthraquinone, as shown in Table 1.4, after it was found that they exhibited both satisfactory solubility in liquid crystals and higher order parameters than their alkylamino counterparts.⁵⁹ Both elongation of the phenyl alkyl substituent and addition of a second phenylamino substituent in the 5- position were shown to result in an increased order parameter, as can be seen from the values in Table 1.4, and order parameters of up to 0.7 were achieved for compounds with a range of colours in nematic hosts.

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Table 1.4 Structures, λmax values and order parameters of selected amino substituted anthraquinones in the nematic host E7.⁵⁹

λmax / nm Sexp

570^a 0.43^a

596 0.62

596 0.65

544 0.58

556 0.66

It was subsequently found that similar structures with a sulfide linking group instead of an amine linking group yielded materials exhibiting order parameters higher than 0.8 in nematic hosts in some cases, as shown in Table 1.5.⁶⁰ The complex nature of alignment in anisotropic systems is illustrated by these structures, because the tetra-substituted compound exhibits the same experimental order parameter as the di-substituted compound, but is much less rod-like in shape.

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Table 1.5 Structures, λmax values and order parameters of selected sulfide substituted anthraquinones in the nematic host E43.⁶⁰

λmax / nm Sexp

465 0.80

520 0.79

550 0.80

Anthraquinone compounds that are more rod-like have been investigated by adding substituents in the 2-, 3-, 6- and 7- positions, providing relatively high order parameters in nematic hosts, as shown in Table 1.6. However, the high degree of alignment displayed by these compounds is not obtained as readily with as wide a range of colours as that obtained with azo dyes.^{60, 61}

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Table 1.6 Structures, λmax values and order parameters of selected 2,6- and 2,7- disubstituted anthraquinones in the nematic hosts E43^a and NP1132^b.^{60, 61}

Structure λmax / nm Sexp

0.72^a

0.73^a

612^b 0.73^b

536^b 0.74^b

Another property that has proven challenging to optimise for anthraquinone-based dyes is their solubility in liquid crystal hosts. This property is particularly important because anthraquinone compounds commonly have significantly lower absorption coefficients than azo dyes,⁴⁸ consequently requiring higher concentrations than azo dyes to achieve the same colour intensity in a device. However, in general, anthraquinone dyes typically exhibit lower solubility than azo dyes,³⁷ especially in the case of sulfide substituted anthraquinones.⁶²

Due to the strong dependence of both order parameter and solubility on molecular structure, an alteration to optimise one parameter will generally have an effect on the other; for example, the addition of biphenyl substituents to anthraquinones has been shown to increase order parameters whilst decreasing the solubility.⁶³ However, in the case of some naphthylsulfide-substituted anthraquinones, both the order parameters and the solubilities were found to be higher than for the phenyl sulfide equivalents.⁶⁴

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Typically, the trends described above have not been rationalised quantitatively, although some properties have been linked to specific structural units. For example, it has been suggested that the typically lower order parameters of amino-anthraquinones than sulfide-anthraquinones arise from the sulfide linkage being more flexible than the amine linkage.^{65, 66} Greater flexibility has also been suggested to enhance solubility in para-substituted phenyl sulfide compounds over their ortho-para-substituted analogues.⁶⁶ A general effect that has been observed is the significant increase of solubility for asymmetric dyes compared with symmetrically substituted dyes, but at the cost of ease of synthesis;^{60, 65-67} examples of this effect are shown in Table 1.7. The variation in solubility of a number of anthraquinone compounds has been correlated with the enthalpies of fusion and melting points of the dyes.⁶⁸

Table 1.7 Structures and room temperature solubilities of selected anthraquinone dyes in the nematic host LIXON 5052XX.^{65, 67}

Structure Substituents Solubility / wt % R₁ = t-Bu (para) exhibit a variety of mesophases themselves, without the presence of any solvent.⁶⁹

69 1.4.3 Other dyes

In addition to research on azo and anthraquinone dyes, work has also been carried out on the behaviour of other chromophores in liquid crystal hosts, albeit to a lesser extent.

The scope of this reported work is very wide, but example structures of some different classes of dyes and their properties are given in Table 1.8, and each class is discussed briefly below.

Tetrazine dyes have typically been found to exhibit low absorption coefficients, but their high solubilities in nematic hosts of ca 20 wt % means that intense colours can still be obtained. Two-ring species tend to have low order parameters of ca 0.5, whereas three-ring species have been shown to have higher order parameters of up to 0.8. These compounds also offer better stability than azo dyes, although they do not tend to be as stable as anthraquinones.⁷⁰

Some naphthalimide dyes exhibit order parameters of between 0.4 and 0.6 in nematic hosts, with solubilities of up to ca 5 wt%, and have good stability. They also exhibit strong fluorescence in the visible region, which can enable more visually appealing displays.^71-74

Research into perylene dyes as guests have shown that order parameters of up to 0.7 may be achieved by elongation of the structures with substituents containing rigid aromatic groups. These dyes also exhibit fluorescence and have solubilities in nematic hosts of up to ca 5 wt %.⁴²

Another class of dye displaying strong emission characteristics that have been analysed in nematic mixtures are acenequinones, which are similar in structure to anthraquinone dyes. Yellow dyes have been produced with order parameters above 0.7 with tuning of the emission properties possible by functionalisation of the core.⁷⁵

Substituted benzothiadiazole yellow dyes studied in this context exhibit absorption and emission, with order parameters of over 0.7 and high quantum yields of emission.⁷⁶

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Table 1.8 Structures and properties of examples of different dye classes analysed in nematic liquid crystal mixtures.

Dye Colour S_exp Solubility

/ wt %

Tetrazine

red-violet 0.73 ≈ 20^a

Naphthalimide

yellow 0.53 ≈ 5^b

Perylene

yellow 0.69 ≈ 5^c

Acenequinone

yellow 0.75

Benzothiadiazole

yellow 0.73

a “Mi 24” host; ^b “ZLI 1695” host; ^c 8OCB host

71 1.5 Techniques