Naphthalenedicarboxylic acid esters.

0001The present invention relates to new chiral dopants for liquid crystals and their production, and to liquid-crystalline mixtures containing such dopants, and their use for optical and electro-optical purposes.

0002Liquid crystal materials for electro-optical display devices often contain one or more optically active additives for inducing a chiral structure. For example, for use in display devices with a twisted nematic structure, a nematic liquid crystal is preferably doped with an optically active additive, for example to avoid reversing the direction of twist (reverse twist) in TN cells (twisted nematic) or to ensure sufficient twisting in cells with a highly twisted nematic structure, such as STN cells (super twisted nematic), SBE cells (super birefringence effect) or OMI cells (optical mode interference). Furthermore, cholesteric liquid crystals for phase change cells can preferably consist of a nematic base material and one or more optically active dopants, and ferroelectric liquid crystals for display devices based on chirally tilted smectic phases can preferably consist of a material with tilted smectic phase and one or more optically active dopants exist.

0003The electro-optical characteristics of liquid crystal display devices are temperature-dependent, which is particularly troublesome in multiplex operation. However, it is known that this temperature dependency can be at least partially compensated for by adding chiral dopant, which induces a pitch that decreases with increasing temperature. Such an inverse temperature dependency was only found for a few compounds. However, it can also be achieved by using at least 2 chiral dopants which have a different relative temperature dependency and induce different twisting directions (DE-A-2827471). However, this usually requires a relatively high proportion of chiral dopants.

0004Cholesteric liquid crystals reflect light in a wavelength range for which the wavelength is approximately equal to the helical pitch. The spectral width of this reflection light can be varied by a suitable choice of the liquid crystal. The reflected light is completely circularly polarized. The direction of rotation of the reflected light depends on the direction of rotation of the cholesteric helix structure. The opposite circularly polarized light is transmitted undiminished. These properties can be used to manufacture optical filters, polarizers, analyzers etc. Furthermore, cholesteric liquid crystals have also been used variously for thermochromic applications and in cosmetic preparations.

0005Cholesteric liquid crystals for the above applications can preferably consist of a nematic or cholesteric base material and one or more chiral dopants, which allows easy adjustment of the desired helical pitch.

0006In order to achieve cholesteric mixtures with a pitch in the range of the wavelength of visible light, the chiral dopants should have the highest possible twisting power and be readily soluble in conventional liquid crystal materials. Furthermore, the chiral dopants should have sufficient stability, be well compatible with the mesophase type of the liquid crystal material and not restrict the mesophase range too much. Such properties would also be desirable for chiral dopants in order to achieve the twisted nematic structures mentioned at the outset, since their proportion could be kept so low that the properties of the liquid crystal material are only insignificantly influenced.

0007The invention relates to optically active naphthalene-2,6-dicarboxylic acid esters of the general formula<chemistry id="chem0001" num="0001"><img file="EP0449049A2_D0001.tif" /></chemistry>    wherein C * denotes a chiral carbon atom and R¹ is C₂-C₁₂-alkyl, C₂-C₁₂-alkenyl, C₂-C₁₂-alkoxycarbonyl or C₃-C₁₂-alkenyloxycarbonyl.

0008The compounds of the formula I are readily soluble in customary liquid crystal materials and enable the liquid crystal structure to be twisted to a high degree. The clearing points of liquid crystals are generally not or only slightly reduced when compounds of the formula I are added. The compounds of the formula I are easy to prepare, relatively low-viscosity and sufficiently stable against electrical and magnetic fields. They therefore optimally meet the above requirements.

0009It is understood that in order to achieve optical activity and a high twisting capacity, both chiral carbon atoms shown in formula I should have the R configuration or both the S configuration.

0010R1 can be branched-chain and optionally chiral. In general, however, straight-chain R 1 radicals are preferred.

0011The above term "C₂-C₁₂ alkyl" includes radicals such as ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl and the like.

0012The term "C₂-C₁₂ alkenyl" includes residues such as vinyl, allyl, propenyl, butenyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl, decenyl and the like.

0013The term "C₂-C₁₂ alkoxycarbonyl" includes radicals such as methoxycarbonyl, ethoxycarbonyl, propyloxycarbonyl, butyloxycarbonyl, pentyloxycarbonyl, hexyloxycarbonyl, heptyloxycarbonyl, octyloxycarbonyl and the like.

0014The term "C₃-C₁₂ alkenyloxycarbonyl" includes radicals such as vinyloxycarbonyl, allyloxycarbonyl, propenyloxycarbonyl, butenyloxycarbonyl, pentenyloxycarbonyl, hexenyloxycarbonyl, heptenyloxycarbonyl, octenyloxycarbonyl, 2,6-dimethyl-5-hepten-1-yloxycarbonyl and the like. In general, 2E-alkenyloxycarbonyl groups and alkenyloxycarbonyl groups with a terminal double bond are preferred.

0015In general, particular preference is given to those compounds of the formula I in which R 1 has at most 7 carbon atoms.

0016According to the invention, the compounds of the formula I can be prepared by naphthalene-2,6-dicarboxylic acid or a suitable derivative of naphthalene-2,6-dicarboxylic acid with an optically active compound of the general formula<chemistry id="chem0002" num="0002"><img file="EP0449049A2_D0002.tif" /></chemistry>    in which C * and R 1 have the meanings given above, esterified.

0017The esterification can be carried out in a manner known per se. Suitable derivatives of naphthalene-2,6-dicarboxylic acid are, for example, the acid halides, for example naphthalene-2,6-dicarboxylic acid chloride. A preferred method is the reaction of a naphthalene-2,6-dicarboxylic acid halide in an inert organic solvent (e.g. an aromatic or saturated, optionally chlorinated hydrocarbon, such as benzene, toluene, hexane or carbon tetrachloride) in the presence of an acid-binding agent (for example pyridine).

0018The starting materials are known or analogs of known compounds.

0019The invention also relates to a liquid-crystalline mixture comprising a liquid-crystalline carrier material and one or more optically active compounds of the formula I. In principle, all liquid-crystal materials which have a twistable liquid-crystal phase with an adequate mesophase range are suitable as carrier material. The compounds of the formula I are particularly suitable as chiral dopants for nematic or cholesteric carrier materials. The liquid-crystalline carrier material can be a single compound or a mixture and preferably has a clearing point of at least about 60 ° C.

0020The proportion of chiral dopant of the formula I is essentially determined by its twisting power and the desired pitch. The proportion of chiral dopant can therefore vary within a wide range depending on the application and can be, for example, approximately 0.1-30% by weight. For display devices based on liquid crystals with a twisted nematic structure, depending on the cell type and thickness, a pitch of about 3-40 µm is usually required and therefore a correspondingly small proportion is sufficient (usually about 0.1-1% by weight). In contrast, pitches of less than 2 μm, for example about 0.4-0.6 μm, are required for applications which are based on the reflection of visible light by cholesteric layers, which requires a correspondingly higher proportion of chiral dopant (usually about 2- 20% by weight). For cells with a highly twisted nematic structure, the chiral dopant content is usually about 0.3-3% by weight.

0021A large number of suitable liquid-crystalline support materials are known and are commercially available. As a rule, liquid-crystalline mixtures containing 2 or more components are preferred as carrier materials. In principle, however, a liquid-crystalline compound can also be used as the carrier material if it has a sufficiently wide mesophase.

0022Compounds of the following general formulas are particularly suitable as components for liquid-crystalline support materials<chemistry id="chem0003" num="0003"><img file="EP0449049A2_D0003.tif" /></chemistry><chemistry id="chem0004" num="0004"><img file="EP0449049A2_D0004.tif" /></chemistry>    wherein R² and R⁶ independently denote alkyl or alkenyl; R³ is cyano, alkyl, alkoxy, alkenyl, alkenyloxy, alkoxymethyl or alkenyloxymethyl; Z¹ represents a simple covalent bond or -CH₂CH₂-; n represents the number 0 or 1; R⁴ denotes alkyl, alkoxy, alkenyl or alkenyloxy; R⁵ is cyano, alkyl, alkoxy, alkenyl or alkenyloxy; Ring A represents 1,4-phenylene or trans-1,4-cyclohexylene; and Z² denotes a simple covalent bond, -COO- or -CH₂CH₂-.

0023Liquid-crystalline mixtures which contain one or more optically active compounds of the formula I and one or more compounds from the group of the compounds of the formulas III-XI are therefore particularly preferred.

0024R², R³, R⁴, R⁵ and R⁶ preferably have at most 12 carbon atoms, particularly preferably at most 7 carbon atoms. Preferred alkenyl groups are 1E-alkenyl, 3E-alkenyl and 4Z-alkenyl. Preferred alkenyloxy groups are 2E-alkenyloxy and 3Z-alkenyloxy.

0025The invention is further illustrated by the following examples. In connection with liquid crystal phases and phase transitions, C means a crystalline phase, N a nematic phase, N * a cholesteric (chiral nematic) phase and I the isotropic phase. The helical pitch is denoted by p and the wavelength of the selectively reflected, circularly polarized light by λ_Max. Optical antipodes each have "mirror-image properties", ie the same melting points etc., but lead to the opposite direction of helix rotation and the opposite circular polarization of the reflected light.

example 1

0026A mixture of 0.4 g of naphthalene-2,6-dicarboxylic acid chloride, 0.5 g of S (+) - lactic acid butyl ester, 1 ml of absolute pyridine and 50 ml of absolute toluene was heated overnight under gentle reflux and under a nitrogen atmosphere. The reaction mixture was then washed in succession with 500 ml of water, 100 ml of dilute hydrochloric acid and again with 500 ml of water, dried over magnesium sulfate, filtered and then concentrated. The residue was washed on silica gel with toluene / ethyl acetate (vol. 4: 1) chromatographically cleaned. Recrystallization from ethanol gave 0.4 g of bis - [(S) -1- (butyloxycarbonyl) ethyl] -2,6-naphthalenedicarboxylate with mp (CI) 81 ° C.

0027The naphthalene-2,6-dicarboxylic acid chloride used as the starting material was prepared as follows:    A mixture of 1 g of naphthalene-2,6-dicarboxylic acid and 100 ml of toluene was mixed with 2 ml of thionyl chloride under a nitrogen atmosphere and then heated under gentle reflux overnight. The reaction mixture was concentrated, then 50 ml of absolute toluene were added and the mixture was concentrated again. This procedure was repeated two more times. The solid residue was then sublimed in a Kugelrohr. This gave 0.8 g of naphthalene-2,6-dicarboxylic acid chloride.

0028The following connections can be made in an analogous manner:    Bis - [(S) -1- (methoxycarbonyl) ethyl] -2,6-naphthalenedicarboxylate, mp (CI) 120 ° C;    Bis - [(S) -1- (ethoxycarbonyl) ethyl] -2,6-naphthalenedicarboxylate, mp (CI) 74 ° C;    Bis - [(S) -1- (propyloxycarbonyl) ethyl] -2,6-naphthalenedicarboxylate, m.p. (CI) 121 ° C;    Bis - [(S) -1- (pentyloxycarbonyl) ethyl] -2,6-naphthalenedicarboxylate;    Bis - [(S) -1- (hexyloxycarbonyl) ethyl] -2,6-naphthalenedicarboxylate;    Bis - [(S) -1- (2E-hexenyloxycarbonyl) ethyl] -2,6-naphthalenedicarboxylate, m.p. (CI) 70 ° C;    Bis - [(R) -2-pentyl] -2,6-naphthalenedicarboxylate, m.p. (CI) 76 ° C;    Bis - [(R) -2-hexyl] -2,6-naphthalenedicarboxylate, m.p. (CI) 53 ° C;    Bis - [(R) -2-heptyl] -2,6-naphthalenedicarboxylate;    Bis - [(R) -2-octyl] -2,6-naphthalenedicarboxylate, m.p. (CI) 4 ° C;    Bis - [(R) -2-nonyl] -2,6-naphthalenedicarboxylate.

Example 2

0029The following basic BM-1 liquid crystal mixture was used to measure the induced pitch and its temperature dependence in liquid crystal materials:<tables id="tabl0001" num="0001"><img file="EP0449049A2_D0005.tif" /></tables> Mp &lt;-30 ° C, clp. (NI) 90 ° C; Δε = 8.5, Δn = 0.139 and η = 22 mPas measured at 22 ° C.

0030The twisting power of the optically active dopant and its temperature dependence is characterized by the parameters A, B and C according to the series development.<maths id="math0001" num=""><img file="EP0449049A2_D0006.tif" /></maths>    wherein and p, c and T₁ have the following meanings:<dl id="dl0001"><dt>T₁</dt><dd>= T-22 ° C</dd><dt>T</dt><dd>= Temperature in ° C</dd><dt>p</dt><dd>= Pitch in µm (a positive value means clockwise helix structure and a negative value means counterclockwise helix structure)</dd><dt>c</dt><dd>= Concentration of the optically active dopant in% by weight.</dd></dl>

Mixture M-1

003199 Wt% BM-1, 1% by weight bis - [(S) -1- (butyloxycarbonyl) ethyl] -2,6-naphthalenedicarboxylate; Klp. (N * -I) 86.7 ° C; p (22 ° C) = -5.4 µm; A = -0.1863 · µm⁻¹ · wt% ⁻¹, B = 3.605 · 10⁻⁴ · µm⁻¹ · wt .-% ⁻¹ · ° C⁻¹, C = 2.838 · 10⁻⁶ · µm⁻¹ · wt% ⁻¹ · ° C⁻².

Mixture M-2

003299 Wt% BM-1, 1% by weight bis - [(S) -1- (ethoxycarbonyl) ethyl] -2,6-naphthalenedicarboxylate; Klp. (N * -I) 90 ° C; p (22 ° C) = -4.7 µm; A = -0.2133 · µm⁻¹ · wt% ⁻¹, B = 4.503 · 10⁻⁴ · µm⁻¹ · wt .-% ⁻¹ · ° C⁻¹, C = 2.416 · 10⁻⁶ · µm⁻¹ · wt% ⁻¹ · ° C⁻².

Mixture M-3

003399 Wt% BM-1, 1 wt% bis - [(R) -2-pentyl] -2,6-naphthalenedicarboxylate; Klp. (N * -I) 89.7 ° C; p (22 ° C) = +24.8 µm; A = 0.04039 · µm⁻¹ · wt% ⁻¹, B = -0.341 · 10⁻⁴ · µm⁻¹ · wt .-% ⁻¹ · ° C⁻¹, C = -1.156 · 10⁻⁶ · µm⁻¹ · wt% ⁻¹ · ° C⁻².

Mixture M-4

003499 Wt% BM-1, 1 wt% bis - [(R) -2-hexyl] -2,6-naphthalenedicarboxylate; Klp. (N * -I) 89.8 ° C; p (22 ° C) = +11.7 µm; A = 0.0853 · µm⁻¹ · wt% ⁻¹, B = -2.711 · 10⁻⁴ · µm⁻¹ · wt .-% ⁻¹ · ° C⁻¹, C = -1.710 · 10⁻⁶ · µm⁻¹ · wt% ⁻¹ · ° C⁻².

Mixture M-5

003599 Wt% BM-1, 1 wt% bis - [(R) -2-octyl] -2,6-naphthalenedicarboxylate; Klp. (N * -I) 88.6 ° C; p (22 ° C) = +9.4 µm; A = 0.1059 · µm⁻¹ · wt% ⁻¹, B = -4.245 · 10⁻⁴ · µm⁻¹ · wt .-% ⁻¹ · ° C⁻¹, C = -3.163 · 10⁻⁶ · µm⁻¹ · wt% ⁻¹ · ° C⁻².