WO2024130989A1 - Liquid crystal composition and dimming device comprising same - Google Patents

Abstract

The present invention relates to a liquid crystal composition. The liquid crystal composition comprises one or more bimesogenic compounds of general formula (I), one or more chiral compounds and one or more nematic liquid crystal compounds of general formula (II). The present invention also relates to a dimming device having at least two stable states, wherein at least one state is a transmissive state in which incident light is substantially transmitted, and at least one state is a haze state in which the incident light is substantially scattered; the transmissive state and the haze state can be switched by means of an external electric field, and do not need to be continuously electrified to be maintained; the transmissive state is clear and highly transmissive, and the scattering state is privacy protective and obscure. According to the present invention, by optimizing the components of the liquid crystal composition, the dimming device has a higher haze in the obscure state, and thus can reduce the cell gap while achieving equivalent obscuring effects, thereby reducing driving voltage and becoming more energy-saving. In addition, the defective rate of device preparation is reduced, the amount of the liquid crystals is reduced, costs are reduced, and the value of the liquid crystal dimming device in practical applications is improved.

Description

Liquid crystal composition and dimming device thereof Technical Field

The present invention relates to the field of liquid crystals, and in particular to a liquid crystal dimer composition comprising a bi-mesogenic compound having an NCS group and a bistable liquid crystal composition derived therefrom, as well as applications of the liquid crystal composition in dimming devices such as light valves, switchable windows, switchable glasses, and switchable rearview mirrors.

Background technique

In recent years, with the improvement of people's demand for quality of life and energy saving and green environmental protection, dimming glass has attracted wide attention and been widely studied for its more convenient, comfortable and energy-saving characteristics. Among them, bistable intelligent dimming glass composed mainly of cholesteric liquid crystal composition materials is one of the important dimming technologies. The important feature of the bistable dimming glass technology of cholesteric liquid crystal composition is that it can maintain more than two stable optical states under zero electric field conditions, and a short pulse of seconds is used when switching the state, which greatly saves the electric energy consumed by driving. The shielding state (i.e., optical FC state) of the bistable dimming glass is the stable existence state of the bistable dimming under zero field conditions. It can not only minimize the propagation of parallel light to protect the privacy of users, but also reduce the transmittance of light and adjust the ambient light. The haze of the FC state is generally used to characterize its scattering degree of light and the efficiency of parallel light propagation. It is well known that the haze is related to the refractive index anisotropy (i.e., Δn, hereinafter referred to as Δn) of the liquid crystal composition, the pitch and the box thickness, especially the haze is proportional to the square of the Δn of the liquid crystal.

A bi-mesogen material refers to a liquid crystal compound containing two relatively independent liquid crystal mesogens in the molecule. Due to its special molecular structure, the elastic constant of the liquid crystal composition can be adjusted in the liquid crystal composition, thereby changing the uniformity of the plane arrangement of the liquid crystal composition molecules, reducing texture defects, thereby reducing the haze of the device containing the liquid crystal composition in the transparent state, and increasing its haze in the foggy state. For example, as reported in patents CN109825309A and CN110467926A, the Δn of the bi-mesogen materials and their compositions disclosed therein are relatively low, so it is necessary to increase the cell thickness to compensate for the optical scattering effect, which not only leads to an increase in the amount of material used, but also increases the energy consumption due to a significant increase in the driving voltage, and at the same time greatly increases the breakdown failure ratio of the dimming device.

Therefore, there is an urgent need to develop a liquid crystal composition and a dimming device thereof to solve the above limitations.

Summary of the invention

We found that NCS and its derivative dimers and compositions composed of them can greatly improve the Δn of the cholesteric phase bistable composition, thereby increasing the FC haze of the dimming glass, making the privacy protection effect more obvious, and at the same time reduce the spacing (box thickness) of the dimming device and reduce the driving voltage.

By optimizing the structure and content of the bimesogenic compound, a cholesteric phase liquid crystal composition containing at least one bimesogenic compound of a double-terminal isothiocyanate is obtained, and then the liquid crystal composition is combined with specific interface conditions to prepare a high-fog Bistable dimming device.

To achieve the purpose of the present invention, the present invention provides a liquid crystal composition, which comprises:

One or more bimesogenic compounds of the general formula I:
NCS-MG1-X1-MG2-NCS I;

one or more chiral compounds; and

One or more nematic liquid crystal compounds of formula II:

in:

MG1 and MG2 each independently represent a mesogen, the mesogen being selected from the group consisting of mesogens of the general formula III:

Wherein, H1-H7 each independently represents a ring structure, and the ring structure is selected from

The group consisting of, wherein 1-4 H atoms in the ring structure can be substituted independently by halogen, CN, or alkyl radical having 1-7 carbon atoms, at least one -CH ₂ - in the alkyl radical can be substituted by -CHO-, -CO-, -COO- or -OCO-, and at least one H atom in the alkyl radical can be substituted by F or Cl;

A ₁ to A ₅ each independently represent -OCOO-, -OCH ₂ -, -CH ₂ O-, -CF ₂ O-, -(CH ₂ ) ₂ -, -(CH ₂ ) ₄ -, -C≡C-, -CH＝CH-, -CF ₂ CF ₂ -, -CF＝CF-, -CH＝CH-COO-, -OCO-CH＝CH- or a single bond; p, q and r are 0 or 1;

X1 is selected from the group consisting of groups of the general formula IV:

wherein Y1 and Y2 each independently represent -O-, -CH=CH-, -CF2O-, -OCF ₂ -, -CF ₂ CF ₂ - or a single bond;

_R1 and _R2 each independently represent -H, -F, -Cl, -NCS, _-OCF3 , _-CF3 , -CN or an alkyl group having 1 to 25 carbon atoms, wherein one or more H atoms in the alkyl group having 1 to 25 carbon atoms may each independently be substituted by halogen, and one or more non-adjacent _-CH2- in the alkyl group having 1 to 25 carbon atoms may each independently be substituted by -O-, -CH=CH-, -CH=CF- or -CF=CF-;

_R10 is a hydrogen atom, and _R11 is a hydrogen atom or an alkyl group having 1 to 5 carbon atoms;

m is 0, 1 or 2, n is 1, 2 or 3, o is 1 or 2, and m+n+o is not greater than 5, and s is an integer of 1-12.

In some embodiments of the present invention, the liquid crystal composition further comprises one or more compounds of Formula V:
R3-MG3-X2-MG4-R4 V;

wherein R3 and R4 each independently represent H, F, Cl, CN, NO ₂ or an alkyl radical having 1 to 25 carbon atoms, wherein one or more H atoms in the alkyl radical having 1 to 25 carbon atoms may each independently be substituted by halogen or CN, and one or more non-adjacent -CH ₂ - in the alkyl radical having 1 to 25 carbon atoms may each independently be substituted by -O-, -S-, -NH-, -N(CH ₃ )-, -CO-, -COO-, -OCO-, -OCOO-, -SCO-, -COS-, -CH＝CH-, -CH＝CF-, -CF＝CF- or -C≡C-, and the substitution does not include two -O- being adjacent to each other;

MG3 and MG4 each independently represent a mesogen selected from the group consisting of mesogens of Formula III; and

X2 is selected from the group consisting of groups of formula IV.

In some embodiments of the present invention, each of the mesogens MG1-MG4 independently comprises at least 2 six-membered rings.

In some embodiments of the present invention, each of the mesogens MG1-MG4 is independently selected from and a group consisting of mirror-image structures thereof, wherein 1-4 H atoms in the six-membered ring can be independently substituted by halogen, CN or an alkyl radical having 1-7 C atoms, at least one -CH ₂ - in the alkyl radical can be replaced by -CHO-, -CO-, -COO- or -OCO-, and at least one H atom in the alkyl radical can be substituted by F or Cl.

In some embodiments of the present invention, the nematic liquid crystal compound of Formula II is selected from one or more compounds of Formula VI and/or Formula VII:

wherein R ₅ , R ₆ and R ₇ each independently represent -H, -F, -Cl, -NCS, -OCF ₃ , -CF ₃ or an alkyl group having 1 to 25 carbon atoms, and one or more non-adjacent -CH ₂ - in the alkyl group having 1 to 25 carbon atoms may each independently be replaced by -O--CH=CH-, -CH=CF- or -CF=CF-;

_A6 and _A8 each independently represent -OCOO-, _-OCH2- , _-CH2O- , _{-CF2O-, -(CH2)2-, -(CH2)4- , -C≡C- , -CH = CH-} , -CF2CF2-, -CF=CF-, -CH=CH-COO-, -OCO-CH=CH- or a single bond;

H ₈ , H ₉ , H ₁₀ , H ₁₁ and H ₁₂ each independently represent a cyclic structure, and the cyclic structure is selected from the group consisting of

The group composed of;

a is 1, 2, 3 or 4, b is 0, 1 or 2, c is 1 or 2, and a+b+c is not greater than 5;

d is 0, 1, 2, 3, or 4, and

e is an integer not less than 2.

In some embodiments of the present invention, the compound of formula I is selected from the group consisting of:

In some embodiments of the present invention, the chiral compound is selected from one or more of the following compounds:

In some embodiments of the present invention, the nematic liquid crystal compound is selected from the group consisting of the following compounds:

In some embodiments of the present invention, the compound of general formula I accounts for 10% to 40% of the total mass of the liquid crystal composition. Further preferably, the compound of general formula I accounts for 15% to 35% of the total mass of the liquid crystal composition. Even more preferably, the compound of general formula I accounts for 20% to 30% of the total mass of the liquid crystal composition.

In some embodiments of the present invention, the helical pitch P of the liquid crystal composition is 0.2 μm-4 μm; more preferably, the helical pitch P of the liquid crystal composition is 0.8 μm-2 μm.

In some embodiments of the present invention, the compound of formula II accounts for 1% to 20% of the total mass of the liquid crystal composition.

In some embodiments of the present invention, the chiral compound accounts for 40%-85% of the total mass of the liquid crystal composition; more preferably, the chiral compound accounts for 50%-70% of the total mass of the liquid crystal composition.

In some embodiments of the present invention, the liquid crystal composition further comprises one or more dye compounds.

The present invention also provides a dimming device including the liquid crystal composition of the present invention, the dimming device has at least two stable states, including at least one transparent state that allows incident light to be substantially transmitted and at least one foggy state that allows incident light to be substantially scattered, the transparent state and the foggy state can be switched by an external electric field without continuous power-on maintenance, the transparent state is clear and highly transparent, and the scattering state is privacy shielding. The transparent state needs to be switched by a first voltage, and the foggy state needs to be switched by a second voltage.

In some embodiments of the present invention, the haze of the dimming device in a transmitted state is no greater than 10%.

In some embodiments of the present invention, the haze of the dimming device is not less than 80%.

On the other hand, to achieve the above-mentioned purpose, the present invention also provides a novel liquid crystal dimming device, which meets the above-mentioned needs through a combination of liquid crystal composition, device structure, and driving method.

In some embodiments of the present invention, the liquid crystal dimming device includes a first transparent conductive substrate, a second transparent conductive substrate, and a liquid crystal layer disposed between the first transparent conductive substrate and the second transparent conductive substrate, wherein the first transparent conductive substrate includes a first transparent substrate and a first transparent conductive layer disposed adjacent to the liquid crystal layer, the second transparent conductive substrate includes a second transparent substrate and a second transparent conductive layer disposed adjacent to the liquid crystal layer, and the liquid crystal layer includes a liquid crystal composition, wherein the liquid crystal composition is a bile The cholesteric liquid crystal composition has a helical pitch P < 4 μm, and the cholesteric liquid crystal composition includes an elastic constant regulator; the liquid crystal layer is subjected to a voltage applied between the first transparent conductive substrate and the second transparent conductive substrate to change the arrangement state of the liquid crystal molecules in the liquid crystal layer, and the change in the arrangement of the liquid crystal molecules in the liquid crystal layer causes the change in the propagation characteristics of the light incident on the liquid crystal layer, including transmission, scattering, reflection, etc., and the arrangement state of the liquid crystal molecules has at least two stable states that remain basically stable after the voltage is removed, at least one of the stable states is a transmission state, which transmits the light incident on the liquid crystal layer, and at least one of the stable states is a shielding state, which can scatter the light incident on the liquid crystal layer. The haze of the transmission state is not more than 10%, the haze of the fog state is not less than 80%, and the absolute change of the haze of the fog state within 5 minutes does not exceed 10%.

The arrangement of liquid crystal molecules in the transparent state is basically orderly, forming a basically uniform planar molecular arrangement with a spiral structure, which has low scattering of light incident on it, making the liquid crystal dimming device present a high transmittance state with light transmission as the main feature. The foggy liquid crystal molecules exist in the form of domains. Due to the relatively strong torque and elasticity of the liquid crystal molecules, the sudden application of an external electric field can cause the liquid crystal to form multiple molecular domains with disordered orientations. In each molecular domain, the liquid crystal molecules still have a basically orderly spiral arrangement, while the orientations between the molecular domains are basically disordered. When the liquid crystal layer is removed from the external electric field, this multi-domain disordered molecular arrangement can be maintained for a long time, forming a stable focal conic liquid crystal molecular arrangement. The multi-domain structure of the liquid crystal molecules can produce strong scattering of the light incident on it.

As mentioned above, in the foggy state, the liquid crystal molecules exist in the form of multiple domains. The liquid crystal molecules in the domains are basically ordered, and there is disorder between the domains. For the scattering of light incident on them, under the action of appropriate interface conditions, the scattering state can exist stably. Interface conditions, driving conditions, and the characteristics of the material itself are the key to the foggy haze and its stability. From the perspective of material properties, the existence state of the domains and the birefringence of the liquid crystal composition are the key factors affecting the foggy haze and stability. Among them, the existence state of the domains is closely related to the chiral compounds, and the existence of chiral compounds is the basis for the formation of the helical structure. Studies have found that when the helical pitch is ≥2μm, as the helical pitch increases, the initial foggy haze will gradually decrease, and the foggy stability will gradually weaken, resulting in a significant decrease in the foggy haze after a certain period of time. At the same time, it is found that when the helical pitch is too large, it will be difficult to form a domain structure, so a stable scattering state cannot be formed, and the foggy state is eventually lost. In order to achieve the purpose of the present invention, the helical pitch P of the liquid crystal composition must be less than 4μm. The size of the helical pitch P can be adjusted by the content of the chiral compound, following the formula P = 1/(HTP*c), where the HTP value is the helical twisting ability of the chiral compound, and c is the mass concentration of the chiral compound in the liquid crystal composition. Under suitable interfaces, driving conditions, and helical pitches, the focal conic texture of the FC state can exist stably. At the moment of switching to the FC state under the action of the electric field, the liquid crystal molecules quickly form a metastable multi-domain state, and then under the joint action of the interface conditions, this metastable state will gradually stabilize, and the haze of the fog state will also be maintained at a certain level, and can exist stably for a long time. Studies have found that under suitable driving conditions and interface conditions, the FC state haze generally tends to stabilize after 5 minutes, and this stable state can be maintained continuously. For the convenience of evaluation, we define the haze stability as the absolute change in haze between the haze of not less than 5 minutes after switching to the FC state and the initial haze at the moment of switching.

As is well known to professionals in the industry, the photoelectric properties of the composition, such as Δn, haze, and driving voltage, all have physical additive characteristics, that is, the properties of the composition are a physical addition of the properties of its components and their contents, such as (where i represents the i-th component, and xi represents the content of the i-th component). Replacing the components of the composition with materials with higher Δn can directly increase the Δn of the composition. At the same time, there is the following relationship between the physical properties: Hf = k*Δn ² *d (Hf is the FC state haze, k is the coefficient, and d is the cell thickness of the liquid crystal), that is, the increase in the Δn of the composition can increase the FC state haze of the liquid crystal cell, and the cell thickness can be reduced without reducing the haze. The direct impact of reducing the cell thickness is to reduce the amount of liquid crystal injected into the unit area of the liquid crystal cell. The usage amount reduces the cost of using LCD. According to the calculation formula of driving voltage: (Fundamentals of Liquid Crystal Devices, P323, by Yang Dengke and Wu Shicong), the reduction in cell thickness will also reduce the driving voltage and energy consumption.

The beneficial effect of the present application is that the present application provides a liquid crystal dimming device, which has a higher haze in the shielding state through a liquid crystal composition with optimized components, and can reduce the thickness of the liquid crystal box when achieving the same shielding effect, thereby reducing the driving voltage and being more energy-efficient. At the same time, the defective rate of device preparation is reduced, the amount of liquid crystal used is reduced, the cost is reduced, and the value of the liquid crystal dimming device in practical applications is improved.

Detailed ways

In the following description, a large number of specific details are set forth for the purpose of explanation so as to provide a comprehensive understanding of the present invention. However, it is apparent to those skilled in the art that the present invention can be implemented without these specific details. In other examples, well-known structures and devices are shown in block diagrams. In this regard, the illustrative example embodiments cited are only for illustration and are not intended to limit the present invention. Therefore, the scope of protection of the present invention is not limited to the above-mentioned specific embodiments, but is subject to the scope of the appended claims.

Each component used in the following examples can be synthesized by a known method or obtained through commercial channels. These synthesis techniques are conventional, and each liquid crystal compound obtained meets the standards of electronic compounds after testing.

In the present application and especially in the following examples, the group structures in the liquid crystal composition are coded; Table 1 shows the group structures and codes used for the liquid crystal composition and the bimesogenic compound.

Table 1 Liquid crystal composition group structure code

Among them, "5PPN", according to the naming principle of Table 1, its corresponding structure is:

"n=3", according to the naming principle of Table 1, its corresponding structure is: -C ₃ H ₇ .

(1) Preparation of Liquid Crystal Composition

According to the ratio of each liquid crystal composition specified in the following examples, a liquid crystal composition is prepared. The preparation of the liquid crystal composition is carried out according to the conventional method in the art. First, weigh each component compound according to the corresponding mass percentage, place the liquid crystal composition in a glass sample bottle, add a magnetic stirrer, place it on a heated magnetic stirrer, and heat and stir until the liquid crystal composition in the sample bottle is completely melted to form an isotropic transparent solution. The temperature at this time has reached the clearing point of the liquid crystal composition. Maintain this temperature and continue heating and stirring for 60 minutes to ensure that the sample is evenly mixed. Stop heating and continue stirring for 2 hours.

(2) Dimming device structure

The liquid crystal dimming device includes a first transparent conductive substrate and a second transparent conductive substrate, which are parallel to each other and arranged opposite to each other to form a liquid crystal box, the thickness of the liquid crystal box is 15μm and 20μm, and the supporting structure is a spherical polystyrene spacer, which accounts for 0.2wt.% of the liquid crystal composition. The transparent conductive substrate is ITO transparent conductive glass, the alignment layer is VA type, and the orientation is performed by a friction orientation method. The liquid crystal layer in the liquid crystal box filled between the first transparent conductive substrate and the second transparent conductive substrate includes the above-mentioned liquid crystal composition, and the liquid crystal composition may contain a dichroic dye, and the dichroic content is selected according to the transmittance requirement.

(3) Haze measurement method

The liquid crystal composition was vacuum-infused into 15μm and 20μm VA liquid crystal cells, respectively. After the sealing was completed, the liquid crystal dimming device was placed at a constant temperature of 25℃ for more than 30 minutes to prepare for the test. First, the WGT-S haze meter was turned on and the test started after the light source was stable for 30 minutes. A pulse voltage was used to drive it to the planar state and focal conic state respectively, and then the WGT-S haze meter was used to measure the transmittance and haze, and the phenomenon was visually observed at the same time.

The embodiments of the present invention and its purpose are described and illustrated by examples below. These examples are exemplary and illustrative only, and not restrictive. In the present application and especially in the following examples, the beneficial effect of the present application is that the present application provides a liquid crystal dimming device, which has a higher haze in the shielding state through a liquid crystal composition with optimized components, and reduces the thickness of the liquid crystal box while achieving the same shielding effect, thereby reducing the driving voltage and being more energy-efficient. At the same time, the defective rate of device preparation is reduced, the amount of liquid crystal used is reduced, the cost is reduced, and the value of liquid crystal dimming devices in practical applications is improved.

Comparative Example 1

Example 1

Example 2

Example 3

Example 4

Example 5

Example 6

Example 7

Example 8

Example 9

Example 10

Embodiment 11

Comparative Example 2

Example 12

Example 13

In Examples 1-4, by adding isothiocyanate-terminated bimesogenic compounds, the haze of the dimming device made from the liquid crystal composition of the present invention in the shielded state is significantly increased compared with Comparative Example 1; in Examples 5-11, by adding isothiocyanate-terminated bimesogenic compounds and a variety of nematic liquid crystal compounds and chiral compounds, good coordinated use effects can be achieved; in Examples 12-13, compared with Comparative Example 2, the haze of the 15μm dimming device made from the liquid crystal composition of the present invention in the shielded state is significantly increased, and compared with Comparative Example 1, the 15μm liquid crystal box and the 20μm liquid crystal box have lower driving voltage and are more energy-efficient while achieving the same shielding effect, and since the box thickness is reduced, the amount of liquid crystal used is reduced, thereby reducing costs and improving the value of liquid crystal dimming devices in practical applications.

Although several exemplary embodiments have been described above in detail, the disclosed embodiments are exemplary and non-restrictive only, and those skilled in the art will readily appreciate that many other modifications, alterations and/or substitutions are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of the present disclosure. Therefore, all such modifications, alterations and/or substitutions are intended to be included within the scope of the present disclosure as defined by the appended claims.

Industrial Applicability

The liquid crystal composition and the dimming device thereof involved in the present invention can be applied in the liquid crystal field.

Claims (12)

1. A liquid crystal composition, comprising:

   One or more bimesogenic compounds of the general formula I:
   NCS-MG1-X1-MG2-NCS I;

   one or more chiral compounds; and

   One or more nematic liquid crystal compounds of formula II:
   

   in:

   MG1 and MG2 each independently represent a mesogen selected from the group consisting of mesogens of formula III:
   

   Wherein, H ₁ -H ₇ each independently represent a cyclic structure, and the cyclic structure is selected from The group consisting of, wherein 1-4 H atoms in the cyclic structure can be substituted independently by halogen, CN, or an alkyl radical having 1-7 carbon atoms, at least one -CH ₂ - in the alkyl radical can be substituted by -CHO-, -CO-, -COO- or -OCO-, and at least one H atom in the alkyl radical can be substituted by F or Cl;

   A ₁ to A ₅ each independently represent -OCOO-, -OCH ₂ -, -CH ₂ O-, -CF ₂ O-, -(CH ₂ ) ₂ -, -(CH ₂ ) ₄ -, -C≡C-, -CH＝CH-, -CF ₂ CF ₂ -, -CF＝CF-, -CH＝CH-COO-, -OCO-CH＝CH- or a single bond; p, q and r are 0 or 1;

   X1 is selected from the group consisting of groups of the general formula IV:
   

   wherein Y1 and Y2 each independently represent -O-, -CH=CH-, -CF ₂ O-, -OCF ₂ -, -CF ₂ CF ₂ - or a single key;

   R ₁ and R ₂ each independently represent -H, -F, -Cl, -NCS, -OCF ₃ , -CF ₃ , -CN or an alkyl group having 1 to 25 carbon atoms, wherein one or more H atoms in the alkyl group having 1 to 25 carbon atoms may each independently be substituted by halogen, and one or more non-adjacent -CH ₂ - in the alkyl group having 1 to 25 carbon atoms may each independently be substituted by -O-, -CH=CH-, -CH=CF- or -CF=CF-;

   _R10 is a hydrogen atom, and _R11 is a hydrogen atom or an alkyl group having 1 to 5 carbon atoms;

   m is 0, 1 or 2, n is 1, 2 or 3, o is 1 or 2, and m+n+o is not greater than 5;

   s is an integer from 1 to 12.
2. The liquid crystal composition according to claim 1, wherein the liquid crystal composition further comprises one or more compounds of formula V:
   R3-MG3-X2-MG4-R4 V;

   wherein R3 and R4 each independently represent H, F, Cl, CN, NO ₂ or an alkyl radical having 1 to 25 carbon atoms, wherein one or more H atoms in the alkyl radical having 1 to 25 carbon atoms may each independently be substituted by halogen or CN, and one or more non-adjacent -CH ₂ - in the alkyl radical having 1 to 25 carbon atoms may each independently be substituted by -O-, -S-, -NH-, -N(CH ₃ )-, -CO-, -COO-, -OCO-, -OCOO-, -SCO-, -COS-, -CH＝CH-, -CH＝CF-, -CF＝CF- or -C≡C-, and the substitution does not include two -O- being adjacent to each other;

   MG3 and MG4 each independently represent a mesogen selected from the group consisting of mesogens of formula III; and

   X2 is selected from the group consisting of groups of formula IV.
3. The liquid crystal composition according to claim 1 or 2, wherein the mesogens MG1-MG4 each independently contain at least two six-membered rings.
4. The liquid crystal composition according to claim 3, wherein the mesogens MG1-MG4 are each independently selected from and a group consisting of mirror-image structures thereof, wherein 1-4 H atoms in the six-membered ring can be independently substituted by halogen, CN or an alkyl radical having 1-7 C atoms, at least one -CH ₂ - in the alkyl radical can be substituted by -CHO-, -CO-, -COO- or -OCO-, and at least one H atom in the alkyl radical can be substituted by F or Cl.
5. The liquid crystal composition according to claim 1, characterized in that the one or more nematic liquid crystal compounds of formula II are further selected from one or more compounds of formula VI and/or formula VII:
   
   

   wherein R ₅ , R ₆ and R ₇ each independently represent H, -F, -Cl, -NCS, -OCF ₃ , -CF ₃ or an alkyl group having 1 to 25 carbon atoms, and one or more non-adjacent -CH ₂ - in the alkyl group having 1 to 25 carbon atoms may each independently be replaced by -O-, -CH=CH-, -CH=CF- or -CF=CF-;

   _A6 and _A8 each independently represent -OCOO-, _-OCH2- , _-CH2O- , _{-CF2O-, -(CH2)2-, -(CH2)4- , -C≡C- , -CH = CH-} , -CF2CF2-, -CF=CF-, -CH=CH-COO-, -OCO-CH=CH- or a single bond;

   H ₈ -H ₁₂ each independently represent a cyclic structure, wherein the cyclic structure is selected from The group composed of;

   a is 0, 1, 2, 3 or 4, b is 1 or 2, c is 1 or 2, and a+b+c is not greater than 5;

   d is 0, 1, 2, 3, or 4, and

   e is an integer not less than 1.
6. The liquid crystal composition according to claim 1, wherein the compound of formula I is selected from the group consisting of the following compounds:
   
   
   
7. The liquid crystal composition according to claim 1, characterized in that the compound of general formula I accounts for 10% to 40% of the total mass of the liquid crystal composition.
8. The liquid crystal composition according to claim 1, characterized in that the helical pitch P of the liquid crystal composition is 0.2-4 μm.
9. The liquid crystal composition according to claim 1, characterized in that the compound of general formula II accounts for 1% to 20% of the total mass of the liquid crystal composition.
10. The liquid crystal composition according to claim 1, characterized in that the chiral compound accounts for 40% to 85% of the total mass of the liquid crystal composition.
11. The liquid crystal composition according to claim 1, further comprising one or more dye compounds.
12. A dimming device, characterized in that the dimming device includes a first transparent conductive base layer, a second transparent conductive base layer and a liquid crystal layer arranged between the first transparent conductive base layer and the second transparent conductive base layer, and the liquid crystal layer includes the liquid crystal composition described in any one of claims 1-11.