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Article

Investigating the Electro-Optic Response of Steroid Doped Liquid Crystal Devices

by
Steven M. Wolf
1,2,†,
Zachary M. Marsh
1,2,†,
Steven M. Quarin
1,2,
Kyung Min Lee
1,2,
Sushma Karra
1,2,
Michael E. McConney
2,
Tod A. Grusenmeyer
2 and
Nicholas P. Godman
2,*
1
Azimuth Corporation, 2970 Presidential Drive, Fairborn, OH 45324, USA
2
Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, OH 45433, USA
*
Author to whom correspondence should be addressed.
†
These authors contributed equally.
Appl. Sci. 2023, 13(8), 5054; https://doi.org/10.3390/app13085054
Submission received: 3 February 2023 / Revised: 23 March 2023 / Accepted: 7 April 2023 / Published: 18 April 2023
(This article belongs to the Section Materials Science and Engineering)
Browse Figures
Versions Notes

Abstract

:
Nature is highly efficient at producing chiral compounds that are enantiomerically pure. The inherent chirality of naturally occurring biomolecules means that many have the potential to be used as chiral dopants for cholesteric liquid crystal (CLC) systems. Though many biomolecules have been identified as chiral dopants, many remain yet to be probed for their ability to function as chiral dopants. Here, 10 naturally occurring biomolecules comprised of steroids and bile acids were tested as chiral dopants for CLCs. Progesterone was identified as having high miscibility with nematic liquid crystals and was used in responsive liquid crystal devices. Progesterone-doped CLC devices were fabricated to exhibit either normal mode or reverse mode switchable behavior. Polymer stabilized CLCs (PSCLC) devices exhibiting dynamic electro-optic red- and blue-tuning behaviors were also fabricated. Furthermore, immiscible lithocholic acid was synthetically modified to afford two derivatives that were miscible at 10 wt. % in nematic liquid crystals. The two lithocholic acid derivatives were used as chiral dopants and incorporated into polymer stabilized CLCs which exhibited blue tuning behavior.

1. Introduction

The cholesteric liquid crystal (CLC) phase is a chiral, self-assembled helicoidal superstructure that exhibits a circularly polarized reflection of light on the nano- to micrometer length scale. The characteristic helical structure is formed through the intrinsic molecular chirality of the liquid crystals (LC), also called mesogens, or through the mixing of achiral mesogens with nonracemic chiral molecules [1,2]. Pioneering work on cholesterol benzoate led to the initial discovery of “flowing crystals” in 1888 by botanist Friedrich Reinitzer and physicist Otto Lehmann [3,4]. In this system, the CLC phase spontaneously formed through the molecular chirality of the mesogenic cholesterol benzoate itself. Later work on achiral nematic liquid crystals (NLCs) indicated that a transformation to the CLC phase occurred upon the addition of a non-racemic chiral molecule [5]. Here, the molecular interactions between the chiral molecule and the nematic mesogens induce a chirality transfer process to form a CLC phase that has the same handedness as the added chiral dopant [6].
The ability to form the CLC phase, sometimes referred to as the chiral nematic phase, by combining chiral molecules and NLCs has enabled a plethora of combinations to be explored with customizable properties. NLCs possess long-range orientational order and exhibit dielectric anisotropy. The anisotropy of the NLCs enables the modulation of their orientation via external stimuli such as temperature, electric fields, or mechanical pressure. The stimuli responsive property is also observed in CLC systems. These materials have been used industrially in LCDs [7] and reflective displays/optics [8,9], temperature responsive photonic devices [10], rewritable color recording [11], tunable band pass filters [12,13], and smart windows [14,15]. CLCs can also be polymer stabilized to improve the thermal stability and response time of electrically responsive devices (i.e., reflection tuning or switching), making them promising materials for displays [16].
The choice of chiral dopant is significant for determining the handedness of the helicoidal superstructure and reflection wavelength of a CLC. The selective reflection of a CLC system is determined by the ability of a chiral dopant to induce a twist in the nematic phase, referred to as the helical twisting power (HTP), according to the equation:
β = (pCr)−1
where β is the HTP of the chiral dopant, p is the pitch of the CLC, C is the dopant concentration, and r is the enantiomeric excess of the chiral dopant [17]. Increasing the chiral dopant concentration typically blue shifts the reflection peak as the CLC pitch length decreases. Subsequently, dopants with higher HTP values enable a selective reflection notch to be generated at lower concentrations. The HTP is dependent on the NLC matrix and will have slight variations across mesogenic systems. Other important properties are the chiral dopant’s phase stability and miscibility as phase separation can be a limiting issue. Therefore, the selection of a chiral dopant is crucial for the creation and utilization of CLC systems.
The stability and miscibility of the dopants are controllable through the structural modification of a chosen chiral molecule. The HTP of a chiral dopant can be increased by affixing aromatic moieties that are quasi-planar to increase interactions with the rigid nematic mesogens [16]. An increase in the aromaticity and rigidity of a chiral dopant often lead to a reduction in the miscibility of the dopant and must be balanced through the addition of alkyl chains. Many chiral dopants are fully synthetic, needing numerous synthetic modifications to meet the design rules [17]. Naturally occurring molecules have garnered attention as potential dopants due to their inherent chirality, high enantiomeric purity, and functional groups that enable chemical modification. The naturally occurring molecules that have been studied as potential chiral dopants include cholesterol and its derivatives, which are still widely used as chiral dopants in LC systems due to their HTP values, liquid crystalline properties, and ability to function as thermochromic liquid crystals [18,19,20]. Steroidal compounds exhibiting structural similarities to cholesterol have been probed as potential dopants. These steroids include estradiol [21,22], which was utilized to form thin CLC films that were sensitive to temperature changes making them a potential sensor materials. The steroid estrone [22,23] was used to synthesize high HTP chiral dopants that show minimal temperature dependence of the selective reflection making them potential dopants in bistable CLC displays. Other steroids studied include androstenalone [21] and pregnenolone [21]. Non-steroidal biomolecules used as chiral dopants include the monoterpene D-limonene, which slightly reduced the switching speed and voltage in twisted nematic systems [24,25]. Other biomolecules evaluated for their chiral properties include allobetulon [26] and botulin [27], which were synthetically modified to produce chiral dopants with high HTP values due to the easily modified functional groups present. Other chiral molecules studied for use as chiral dopants due to their inherent chirality include amino acids [28] and cholenic acid [29].
Using naturally occurring biomolecules as chiral dopants remains an under-investigated field of research. Although numerous biomolecules have been used as chiral dopants, a plethora have not been studied in LC systems. Furthermore, a large portion of naturally occurring materials can function as synthetic substrates due to the functional groups present. In this study, a series of steroids and bile acids were doped into nematic LCs to determine if the biomolecules can be utilized as chiral dopants. The steroids studied include androsterone, β-estradiol, cholesterol, cholic acid, epiandrosterone, hydrocortisone, prednisone, progesterone, and stigmasterol. The bile acid lithocholic acid was chosen as a potential chiral dopant due to its structural similarity to cholesterol and asymmetric functional groups that can be modified. The biomolecules were preliminarily screened for their solubility and miscibility in common nematic LCs. Lithocholic acid was used as a test case for synthetic modification to enhance the solubility and improve the HTP. After preliminary screening, three biomolecules were used as chiral dopants in responsive CLC devices. The three biomolecules were used in normal mode switching devices to test the responsiveness of the bulk CLC mixtures. Next, more complex polymer stabilized CLCs (PSCLCs) were formulated using the miscible biomolecules to create demo devices with dynamic electro-optical responses, including blue tuning, red tuning, and reverse mode switching of the selective reflection peak to highlight the potential applications.

2. Materials and Methods

Lithocholic acid was purchased from Cayman Chemicals. Androsterone, β-estradiol, cholic acid, epiandrosterone, hydrocortisone, prednisone, progesterone, and stigmasterol were acquired from Milipore Sigma. Benzyl bromide, magnesium sulfate, methyl iodide, and 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) were also purchased from Millipore Sigma. Dichloromethane (DCM), ethyl acetate, hexanes, and hydrochloric acid were acquired from Fisher. The photoinitiator I-369, commercial chiral dopants R811 and R101, and MLC MLC-2079 were purchased from Merck. LC monomer RM82 was purchased from Synthon chemicals. LC E7 was purchased from BOC Chemicals. DCM was dried over 3M molecular sieves. All other chemicals were used as received. ITO-coated glass slides were purchased from Colorado Concepts. Optical adhesives 61 and 63 were purchased from Norland Optics.

2.1. Chiral Dopant Characterization

LC alignment cells were prepared using ITO-coated glass slides rinsed with acetone and methanol. The clean slides were flushed with nitrogen gas to remove dust or debris from the surface prior to placement in a plasma cleaner. The glass slides were exposed to nitrogen-rich plasma for 10 min then removed. The ITO faces of the glass slides were coated with a thin polyimide layer using a spin coater set to 4000 rpm for 20 s. The polyimide layer was rubbed with velvet to generate planar alignment. Cells were fabricated using clear optical adhesive 61 doped with 12 μm glass spacers. The optical adhesive was cured using 365 nm irradiation for 5 min.
Cano–Grandjean wedge cells were prepared using ITO-coated glass slides. The methodology for cleaning and alignment is described previously. The wedge cells were fabricated by affixing the glass slides using optical adhesive mixtures containing 50 and 10 µm glass spacers in optical adhesives 63 and 61, respectively. The optical adhesive mixtures were coated on opposite ends of the cell to generate a change in thickness across the length of the cell, enabling the formation of disinclinations. The optical adhesive was then cured using 365 nm UV light for 5 min. Samples were prepared by mixing the nematic LC E7 with the biomolecule chiral dopants at concentrations of 5 or 10 wt. %. All cells were filled utilizing capillary action and rubbed to remove defects prior to analysis. Analyses of the disinclination lines were carried out using a ZEISS Axio Imager 2 Polarized Light Microscope at 5× magnification. A Thorlabs variable line grating target was used for calibration.
Solubility/miscibility characterization was performed using a ZIESS Axio Imager 2 Polarized Light Microscope at 10× magnification. In a common procedure, the CLC solution was prepared by combining the LC E7 with the biomolecule chiral dopant at either 5 or 10 wt. %. The chiral dopant was solvated using a Vortex mixer while heating the solution above the nematic to isotropic transition temperature of LC E7 to form the CLC solution. The sample was then allowed to cool to room temperature prior to a second mixing cycle. This procedure was carried out a third time to ensure complete solvation of the chiral dopant. The solution was then applied to planar-aligned cells and left to fill via capillary action. Once filled, the cells were rubbed to remove defects.

2.2. Responsive PSCLCs

The responsive red tuning PSCLC solutions were mixed using a Vortex mixer while heating the solution above the nematic to isotropic transition temperature of MLC-2079. Red tuning formulations for sample RT1 are listed in Table S2. The heating of the samples was conducted in red light to avoid the auto-polymerization of I-369 prior to UV curing. Once the samples were mixed, ITO-coated planar-aligned cells were filled via capillary action. The cells were then exposed to 120 mW/cm2 365 nm UV light for 3 min to form the polymer stabilization network.
The responsive blue tuning PSCLC solutions were mixed using a Vortex mixer while heating the solution above the nematic to isotropic transition temperature of MLC-2079. Blue tuning formulations for samples BT1, BT2, and BT3 are listed in Table S2. The heating of the samples was conducted in red light to avoid the auto-polymerization of I-369 prior to UV curing. Once the samples were mixed, ITO-coated planar-aligned cells were filled via capillary action. The cells were then exposed to 250 mW/cm2 365 nm UV light for 30 min to form the polymer stabilization network. The cells were rotated and flipped every 5 min to avoid thermal heating of the sample.

2.3. Switchable CLCs

CLC solutions were mixed using a Vortex mixer while the solution was heated above the nematic to isotropic transition temperature of E7. Formulations for the samples NM1, NM2, and NM3 are listed in Table S2. Once the samples were mixed, unaligned cells were filled via capillary action. Unaligned cells coated with ITO were fabricated to achieve the focal conic texture at ambient conditions.
The reverse mode PSCLC solution was mixed using a Vortex mixer while heating the solution above the nematic to isotropic transition temperature of MLC-2079. The formulation for sample RM1 is listed in Table S2. The heating of the samples was conducted in red light to avoid the auto-polymerization of I-369 prior to UV curing. Once the samples were mixed, ITO-coated planar-aligned cells were filled via capillary action.

2.4. Synthetic Modification

Methyl lithocholate (LA-Me): 1.0 g of lithocholic acid (1.0 mmol) was combined with 50 mL of dry DCM and stirred at 450 rpm. Next, 0.4 mL of DBU (1.0 mmol) was added dropwise; upon addition of the base, the LA dissolves and the solution turns to a clear yellow color. Next, 1.50 mL of iodomethane (3.0 mmol) was added dropwise to facilitate esterification. After addition of the iodomethane, a white precipitate was formed. The reaction was stirred overnight at room temperature and then filtered to remove the precipitate. The filtrate was then washed with one 40 mL portion of 2M HCl then with two 50 mL portions of deionized water. The organic layers were combined, dried over magnesium sulfate, and filtered. The filtrate was concentrated through the removal of the organic solvent via rotary evaporation. The concentrated product was then run through a silica gel column on a Yamazen Smart Flash EPCLC W-Prep 2XY flash column using a solvent phase of 70:30 hexanes–ethyl acetate. The synthesis was adapted from previously reported procedures, and the 1H NMR spectrum (Figure S1) matches the literature [30].
Benzyl lithocholate (LA-Bn): 1.0 g of lithocholic acid (1.0 mmol) was combined with 50 mL of dry DCM and stirred at 450 rpm until all of the solid was dissolved forming a clear yellow solution. Next, 0.4 mL of DBU (1.0 mmol) was added dropwise forming a white salt that precipitated from the stirring solution. Next, 0.85 mL of benzyl bromide (3.0 mmol) was added dropwise to facilitate esterification. The reaction was stirred overnight at room temperature. The crude product was then washed with 40 mL of 2M HCl to remove excess base and two washing cycles consisting of 50 mL deionized water. The aqueous wash was then extracted with three 50 mL aliquots of DCM to collect crude product lost during the aqueous washing cycle. The aqueous layers were combined and dried over magnesium sulfate. The magnesium sulfate was removed via gravity filtration, and the product was concentrated through evaporation of the organic solvent via rotary evaporation. The concentrated product was then run through a silica gel column on a Yamazen Smart Flash EPCLC W-Prep 2XY flash column using a solvent phase of 70:30 hexanes–ethyl acetate. The synthesis was adapted from previously reported procedures, and the 1H NMR spectrum (Figure S2) matches the literature [31].

3. Results and Discussion

A large portion of biomolecule chiral dopants reported previously are steroids or have the same core structure common in steroids. Although many steroids have been studied in previous reports, numerous remain untested. In order to identify new steroidal chiral dopants, nine steroids were investigated for their ability to act as chiral dopants for LC systems. In addition to the nine steroids, the bile acid lithocholic acid was chosen due to its structural similarity to the other biomolecules tested. These molecules were chosen for their structural similarity to cholesterol benzoate, the original LC. For comparison, cholesterol and β-estradiol were included in the study as both have been used as chiral dopants in previous studies [18,19,20,21,22]. The structures of the steroids chosen for this study are shown in Figure 1.
To determine the miscibility of the steroids with common nematic LCs, mixtures containing 5 wt. % steroid in LC E7 were prepared, loaded into glass cells, and viewed with a polarized optical microscope (POM). The POM images shown in Figure 2 indicate that two-thirds of the steroids chosen were immiscible with nematic LC E7. Notably only progesterone, cholesterol, and β-estradiol were miscible. To further explore the miscibility of progesterone, cholesterol, and β-estradiol, the loading of these steroids in E7 was increased to 10 wt. %. POM images revealed that only progesterone remained miscible while cholesterol and β-estradiol began to crystallize out of the solution, shown in Figure S4. Lithocholic acid will be discussed below.
Once phase compatibility had been established and progesterone was identified as a suitable chiral dopant, the utility of biomolecules in responsive LC devices was investigated. Progesterone was chosen as the bio-chiral dopant due to its superior miscibility relative to the other steroids, allowing for a greater loading. The selective reflection of progesterone in E7 is shown in Figure S5a. Progesterone was incorporated into a switchable CLC devices that exhibited either “normal mode” or “reverse mode” behavior. In “normal mode” switchable CLCs, the sample is opaque due to the sample adopting the focal conic texture as unaligned cells are used, and, when a voltage is applied, the LCs reorient to form a transparent state as the molecules adopt a homeotropic texture [32,33,34]. “Reverse mode” CLCs are aligned in a planar texture in the absence of an electric field but are transparent as the selective reflection is placed in the infrared region. Upon the application of an electric voltage, the planar texture is lost and the sample adopts a focal conic texture leading to high scattering. The planar texture can be recovered when a sufficiently high electric field is applied [35]. The difference between the two modes is dictated by the sample preparation. It is important to note that ternary mixtures of the biodopants with small concentrations of commercially available dopants are used for all responsive CLC systems. The ternary mixtures reduce the risk of crystallization when using commercially available dopants while placing the selective reflection in the near-IR for greater electro-optic properties.
A “normal mode” switchable LC was fabricated using 10 wt. % progesterone; the full sample formulation for the NM1 sample is shown in Table S2. An LC with a positive dielectric anisotropy was chosen as the mesogens will reorient with the applied electric field, enabling the LCs to switch from the focal conic state to the homeotropic state. The “normal mode” LC device, shown in Figure 3a, exhibits low light transmittance (<20%) across the visible spectrum due to the focal conic orientation of the LCs scattering light. When a voltage of 95 V AC is applied, the LCs reorient with the electric field, adopting a homeotropic orientation that leads to an optically transparent clear state with ~60–80% transmittance across the visible spectrum. When the voltage is removed, the system returns to its initial scattering state as the LCs relax back to the focal conic state. The transparent state is only achieved in the presence of an electric field, and is not stable due to the relaxation of the nematic LC E7 when the stimuli is removed.
Next, an LC sample exhibiting “reverse mode” switching was fabricated using progesterone. To obtain a “reverse mode” device, the positive dielectric host E7 was replaced with MLC-2079 which has a negative dielectric anisotropy. LCs with a negative dielectric anisotropy do not reorient when an external stimuli is applied, unlike positive dielectric LCs. In “reverse mode” devices, the planar CLC texture is stabilized, and the selective reflection notch is present in the transmittance spectra; generally, it is placed in the infrared to obtain a sample that is transparent in the visible region. Upon the application of sufficiently high electric stimuli, the planar structure is lost, and the focal conic state is formed leading to significant scattering. The focal conic state is stable when the electric field is removed. The planar texture can be regenerated upon the application of sufficiently high electric fields [35]. This sample is considered bistable as the focal conic state and the planar state are stable when the electric field is removed [36]. A “reverse mode” CLC containing progesterone is highlighted in Figure 3b. Here, the initial sample is optically transparent with high transmittance from 400 to 800 nm, with the selective reflection placed in the near-infrared region at 950 nm. The application of DC voltage triggered the switching of the sample to the focal conic scattering state which was retained when the voltage was removed. The planar texture was recovered by applying 40 V of AC and was stable once the electric field was removed.
To further demonstrate the applicability of progesterone in LC devices, reactive monomers were added to the formulations to generate PSCLCs. The reactive monomers are mesogenic and adopt the orientation of the director; in CLCs, the monomers are distributed throughout the helical superstructure. As the system is exposed to UV light, the polymerization occurs and the resulting polymer network behaves as a template for the LC mesogens, acting as an interwoven anchoring layer [37,38]. The anchoring of the LCs to the polymer network enables unique responsive behavior that is not observed in most LC systems. The unique behavior is attributed to the deformation of the polymer network as ions trapped within the polymer are pulled towards the electrode when a voltage is applied. The ions in the solutions come from the added photoinitiator and ionic impurities in the nematic liquid crystals themselves. The deformation of the polymer network results in a change in the helical pitch and a shifting of the selective reflection. If the pitch distortion is uniform, broadening of the selective reflection can occur; if distortion of the pitch is non-uniform, tuning of the selective reflection is observed [39,40].
PSCLC formulations exhibiting both red and blue tuning behavior containing progesterone as the primary chiral dopant were fabricated. The formulations for both RT1 and BT1 are listed in Table S2. The structures of the chemicals used to fabricate the PSCLCs are shown in Figure S4. The LC monomer RM82 was chosen to form the polymer stabilizing network as the six carbon chain extender affords greater notch tuning ability [15]. MLC-2079 was chosen as the nematic LC host due to the negative dielectric anisotropy, which enables the distortion of the polymer network [39]. To ensure red tuning of the selective reflection, a low concentration of initiator was added to the formulation; additionally, the sample was cured for 3 min at a lower UV light intensity to reduce the ion concentration [41]. The red tuning PSCLC is shown in Figure 4a; the initial reflection peak is located in the near-infrared region at 1250 nm. Upon application of a DC voltage, the selective reflection shifts; at 80 V, the selective reflection is shifted to 2300 nm. This represents an increase in the selective reflection position of 13 nm/V. Red tuning is representative of an elongation of the helical pitch as the polymer network is deformed.
To generate a PSCLC with blue tuning behavior, a photoinitiator with a higher ion concentration and an increase in curing time was employed. The photoinitiator chosen, I-369, has a higher ion concentration when exposed to UV light than I-651, which was used for the red-tuning sample [41]. The sample was then cured at a higher UV light intensity over a time period of 30 min. The blue tuning of the PSCLCs selective reflection is shown in Figure 4b. The initial reflection is placed at 950 nm; as the voltage is applied to the sample, the selective reflection shifts to shorter wavelengths. At the highest applied voltage of 100 V DC, the selective reflection is centered near 450 nm, a shift of 5 nm/V.
In addition to the chirality present in steroids (and other biomolecules in general), many also possess functional groups that can be synthetically modified to change the properties of the compound. In this way, a host of new chiral dopants can be developed from widely available biomolecules. Lithocholic acid (LA), for instance, is sparingly soluble in many LCs due to the two polar functional groups, an alcohol and a carboxylic acid. However, these functional groups can be chemically modified to improve the miscibility of LA, potentially creating chiral dopants for CLCs. To test this hypothesis, two LA derivatives, LA-Bn and LA-Me, were synthesized via nucleophilic substitution of LA onto alkyl halides (benzyl bromide and iodomethane, respectively). These new derivatives serve as excellent model compounds due to their facile, one step synthesis. The synthetic scheme for the synthetic modification of LA is shown in Figure 5a. In each case, the phase compatibility of the derivatives is superior to that of the parent compound. LA is immiscible in E7 at 5 wt. %, and POM images show irregular CLC reflections and numerous oily streak defects originating in phase separated crystallites (Figure 5b). LA-Bn and LA-Me are both miscible at 5 wt. % with uniform reflection colors and few defects (Figure 5c,d). Similar to progesterone, both LA derivatives show miscibility with LC E7 at 10 wt. % (Figure S1). Helical twisting power values for the two derivatives are shown in Table S1.
For the modified lithocholic acid derivatives to be used as chiral dopants, their incorporation into responsive CLC devices was explored. The electro-optic responses of both bulk CLCs and PSCLCs were examined in mixtures formulated with LA-Bn and LA-Me, respectively. The selective reflection of LA-Bz and LA-Me in E7 are shown in Figure S5b and c, respectively. First, the electro-optic response of LA-Bn and LA-Me in normal mode switching CLCs was investigated as these samples require the least complex formulation. To achieve the focal conic texture, CLC mixtures were loaded into ITO-coated glass cells with no alignment layer. At 0 V, the CLC containing LA-Bn as the chiral dopant had transmittance values under 10%, as shown in Figure 6a. Once 95 V AC was applied, the mixture became homeotropic with the transmittance increasing to nearly 60% across the entire visible region. The normal mode switchable CLC containing LA-Me had superior properties with a larger change in transmittance at a similar applied voltage. At 0 V AC, the cell had transmittance values below 20%, and when 95 V AC was applied the transmittance rose to nearly 80% across the visible spectra. Compared to the normal mode switchable CLC containing progesterone, LA-Bn had diminished switching properties. However, the normal mode CLC containing LA-Me has similar properties to progesterone with higher transmittance across the visible spectrum.
Similar to progesterone which exhibited high miscibility in LCs, responsive PSCLCs were fabricated to examine the abilities of LA-Bn and LA-Me to function as chiral dopants. Sample formulations included 0.5 wt. % I-369, 3.1 wt. % R1011, 9.9 wt. % LA-Bn, 5.9 wt. % RM82, and 80.6 wt. % MLC-2079. For the LA-Me a formulation containing 1.0 wt. % I-369, 3.1 wt. % R1011, 10.2 wt. % LA-Bn, 6.1 wt. % RM82, and 79.6 wt. % MLC-2079 were used. The PSCLC containing LA-Bn had a selective reflection centered in the infrared at 1550 nm, shown in Figure 6a. At a maximum applied voltage of 85 V DC, the reflection notch was located at a wavelength of 850 nm. This corresponds to a blue shift of 700 nm, equivalent to 8 nm/V. The PSCLC containing LA-Me as the primary chiral dopant has a selective reflection centered at 1300 nm, shown in Figure 6b. At a maximum applied voltage of 50 V DC, the selective reflection was centered at 550 nm. The total blue shift of the selective reflection was 750 nm, corresponding to a 15 nm/V shift.

4. Conclusions

A series of steroidal compounds were tested as chiral dopants for LC systems. One steroid that had yet to be reported as a chiral dopant, progesterone, was found to be miscible with common nematic LC E7 and MLC-2079 at concentrations of 10 wt. %. The use of progesterone as a co-dopant for commercially available R1011 in PSCLCs with red- and blue-tuning behavior was successfully fabricated. Progesterone also functioned as a chiral dopant for normal and reverse mode switchable CLCs and exhibited the desired responsive behavior. To further demonstrate the utilization of different classes of biomolecules, the bile acid lithocholic acid was probed as a potential chiral dopant. Although lithocholic acid is insoluble, simple synthetic modification was able to improve the miscibility in common LCs, as the two derivatives were stable at concentrations of 10 wt. %. Future studies will focus on synthetic modification using targeted side chains to improve the HTP of lithocholic acid derivatives to achieve values on par with high HTP commercially available dopants. Additionally, studies will be conducted to identify new classes of biomolecules that have potential usage as chiral dopants in CLCs and relevant CLC technologies.

Supplementary Materials

The supporting information can be downloaded at https://www.mdpi.com/article/10.3390/app13085054/s1. Figure S1: 1H NMR of LA-Me; Figure S2: 1H NMR of LA-Bn; Figure S3: POM images of (a) β-estradiol, (b) cholesterol, (c) progesterone, (d) LA-Bn, and (e) LA-Me at 10 wt. % in nematic LC E7; Figure S4: Chemical structures of the photoinitiator I-369, chiral dopant R1011, liquid crystal monomer RM82, and the eutectic liquid crystal mixture E7; Figure S5: Reflection notch data for (a) 9.1 wt. % progesterone, (b) 9.3 wt. % LA-Bn, and (c) 10.1 wt. % LA-Me in nematic LC E7; Table S1: Helical twisting power of the bio-chiral dopants used in the responsive CLC samples; Table S2: Formulations for the normal mode (NM), reverse mode (RM), blue tuning (BT), and red-tuning (RT) CLCs and PSCLCs containing progesterone, LA-Bn, and LA-Me.

Author Contributions

Conceptualization, N.P.G., S.M.Q., S.M.W. and Z.M.M.; validation, K.M.L., S.M.W. and Z.M.M.; formal analysis, S.M.W., Z.M.M., S.K., K.M.L. and N.P.G.; investigation, S.M.Q., S.K., K.M.L., S.M.W., Z.M.M. and N.P.G.; resources, N.P.G., T.A.G. and M.E.M.; data curation, Z.M.M., S.M.W. and K.M.L.; writing—original draft preparation, S.M.W. and Z.M.M.; writing—review and editing, N.P.G., S.M.W. and Z.M.M..; visualization, N.P.G., S.M.W. and Z.M.M.; supervision, N.P.G. and T.A.G.; project administration, N.P.G.; funding acquisition, N.P.G. and T.A.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the AFRL Materials and Manufacturing Directorate, Grant Number: FA8650-22-F-5406, and DARPA Living Foundries.

Data Availability Statement

Data is contained within the article or Supplementary Materials.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Mitov, M. Cholesteric Liquid Crystals with a Broad Light Reflection Band. Adv. Mater. 2012, 24, 6260–6276. [Google Scholar] [CrossRef] [PubMed]
  2. McConney, M.E.; Rumi, M.; Godman, N.P.; Tohgha, U.N.; Bunning, T.J. Photoresponsive Structural Color in Liquid Crystalline Materials. Adv. Opt. Mater. 2019, 7, 1900429. [Google Scholar] [CrossRef]
  3. Reinitzer, F. Contributions to the Knowledge of Cholesterol. Liq. Cryst. 1989, 5, 7–18. [Google Scholar] [CrossRef]
  4. Lehmann, O. Über fliessende Krystalle. Z. Phys. Chem. 1889, 4, 462–472. [Google Scholar] [CrossRef]
  5. Friedel, G. Les États Mésomorphes de La Matière. Ann. Phys. 1922, 9, 273–474. [Google Scholar] [CrossRef]
  6. Pieraccini, S.; Masiero, S.; Ferrarini, A.; Piero Spada, G. Chirality Transfer across Length-Scales in Nematic Liquid Crystals: Fundamentals and Applications. Chem. Soc. Rev. 2011, 40, 258–271. [Google Scholar] [CrossRef] [PubMed]
  7. Chen, R.H. Liquid Crystal Displays: Fundamental Physics and Technology; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2011. [Google Scholar] [CrossRef]
  8. Yang, D.-K. Review of Operating Principle and Performance of Polarizer-Free Reflective Liquid-Crystal Displays. J. Soc. Inf. Disp. 2008, 16, 117–124. [Google Scholar] [CrossRef]
  9. Zhu, X.; Ge, Z.; Wu, T.X.; Wu, S.-T. Transflective Liquid Crystal Displays. J. Disp. Technol. 2005, 1, 15–29. [Google Scholar] [CrossRef]
  10. Zhang, W.; Froyen, A.A.F.; Schenning, A.P.H.J.; Zhou, G.; Debije, M.G.; de Haan, L.T. Temperature-Responsive Photonic Devices Based on Cholesteric Liquid Crystals. Adv. Photo Res. 2021, 2, 2100016. [Google Scholar] [CrossRef]
  11. Tamaoki, N. Cholesteric Liquid Crystals for Color Information Technology. Adv. Mater. 2001, 13, 1135–1147. [Google Scholar] [CrossRef]
  12. Fuh, A.Y.-G.; Ho, S.-J.; Wu, S.-T.; Li, M.-S. Optical Filter with Tunable Wavelength and Bandwidth Based on Phototunable Cholesteric Liquid Crystals. Appl. Opt. 2014, 53, 1658. [Google Scholar] [CrossRef] [PubMed]
  13. Worth, B.; Lee, K.M.; Tondiglia, V.P.; Myers, J.; Mou, S.; White, T.J. Dynamic, Infrared Bandpass Filters Prepared from Polymer-Stabilized Cholesteric Liquid Crystals. Appl. Opt. 2016, 55, 7134. [Google Scholar] [CrossRef] [PubMed]
  14. Li, C.-C.; Tseng, H.-Y.; Chen, C.-W.; Wang, C.-T.; Jau, H.-C.; Wu, Y.-C.; Hsu, W.-H.; Lin, T.-H. Versatile Energy-Saving Smart Glass Based on Tristable Cholesteric Liquid Crystals. ACS Appl. Energy Mater. 2020, 3, 7601–7609. [Google Scholar] [CrossRef]
  15. Lee, K.M.; Tondiglia, V.P.; McConney, M.E.; Natarajan, L.V.; Bunning, T.J.; White, T.J. Color-Tunable Mirrors Based on Electrically Regulated Bandwidth Broadening in Polymer-Stabilized Cholesteric Liquid Crystals. ACS Photon. 2014, 1, 1033–1041. [Google Scholar] [CrossRef]
  16. Pieraccini, S.; Ferrarini, A.; Spada, G.P. Chiral Doping of Nematic Phases and Its Application to the Determination of Absolute Configuration. Chirality 2008, 20, 749–759. [Google Scholar] [CrossRef] [PubMed]
  17. Eelkema, R.; Feringa, B.L. Amplification of Chirality in Liquid Crystals. Org. Biomol. Chem. 2006, 4, 3729. [Google Scholar] [CrossRef]
  18. Gvozdovskyy, I.; Lisetski, L. Rotation of Single Crystals of Chiral Dopants at The Top of a Nematic Droplet: Factors Affecting the Angular Velocity. Mol. Cryst. Liq. Cryst. 2007, 475, 113–122. [Google Scholar] [CrossRef]
  19. Luo, Z.-W.; Tao, L.; Zhong, C.-L.; Li, Z.-X.; Lan, K.; Feng, Y.; Wang, P.; Xie, H.-L. High-Efficiency Circularly Polarized Luminescence from Chiral Luminescent Liquid Crystalline Polymers with Aggregation-Induced Emission Properties. Macromolecules 2020, 53, 9758–9768. [Google Scholar] [CrossRef]
  20. Xia, Q.; Meng, L.; He, T.; Huang, G.; Li, B.S.; Tang, B.Z. Direct Visualization of Chiral Amplification of Chiral Aggregation Induced Emission Molecules in Nematic Liquid Crystals. ACS Nano 2021, 15, 4956–4966. [Google Scholar] [CrossRef]
  21. Obadović, D.Ž.; Stojanović, M.; Jovanović–Šanta, S.; Cvetinov, M.; Lazar, D.; Vajda, A.; Éber, N.; Ristić, I. Study of Binary Cholesteric Liquid Crystalline Mixtures Doped with Some Chiral Nonmesogenic Estradiol Derivatives. Mol. Cryst. Liq. Cryst. 2011, 547, 46/[1736]–53/[1743]. [Google Scholar] [CrossRef]
  22. Brandenburger, F.; Matthes, B.; Seifert, K.; Strohriegl, P. Cholesteric Polymeric Networks with Steroids as Chiral Components. Liq. Cryst. 2001, 28, 1035–1039. [Google Scholar] [CrossRef]
  23. Shkolnikova, N.; Yaremenko, F.; Sheshenko, Z.; Vakula, V.; Kutulya, L.; Pivnenko, N.; Roshal, A. Influence of the Rigidity of the Steroid Core in the Structure of Chiral Dopants on the Temperature Dependence of Cholesteric Short Pitch. Displays 2015, 36, 34–40. [Google Scholar] [CrossRef]
  24. Zola, R.S.; Yang, Y.-C.; Yang, D.-K. Limonene as a Chiral Dopant for Liquid Crystals: Characterization and Potential Applications. J. Soc. Inf. Disp. 2011, 19, 410. [Google Scholar] [CrossRef]
  25. Zola, R.S.; Hurley, S.; Yang, D.-K. D-Limonene as a Chiral Dopant for Thermotropic Liquid Crystalline Systems. Appl. Phys. Express 2011, 4, 61701. [Google Scholar] [CrossRef]
  26. Semenenko, A.N.; Babak, N.L.; Shishkina, S.V.; Musatov, V.I.; Mazepa, A.V.; Lipson, V.V. Synthesis, Molecular and Crystal Structure of Spirocyclopropyl Derivatives of Lupane Series and Their Ability to Induce Cholesteric Mesophase in Nematic Solvents. J. Mol. Struct. 2018, 1171, 605–613. [Google Scholar] [CrossRef]
  27. Guimarães, V.; Svanidze, A.; Guo, T.; Nepal, P.; Twieg, R.; Palffy-Muhoray, P.; Yokoyama, H. Synthesis and Characterization of Novel Bio-Chiral Dopants Obtained from Bio-Betulin Produced by a Fermentation Process. Crystals 2021, 11, 785. [Google Scholar] [CrossRef]
  28. Komaba, K.; Jo, T.; Goto, H. Preparation of Helical Liquid Crystal Electrolyte with L-Isoleucine Derivatives and Molecular Asymmetric Imprinting Polymerization in Liquid Crystal to Produce Electro-Optically Coloration Active Polymers Having Chiral Charge Carrier Chiralions. Mol. Cryst. Liq. Cryst. 2022, 1–12. [Google Scholar] [CrossRef]
  29. Shen, Y.; Chen, X.; Wen, J. Liquid-Crystalline Derivatives of 3 β -Hydroxy-5-Cholenic Acid. Mol. Cryst. Liq. Cryst. 2011, 537, 76–84. [Google Scholar] [CrossRef]
  30. Mizushina, Y.; Kasai, N.; Miura, K.; Hanashima, S.; Takemura, M.; Yoshida, H.; Sugawara, F.; Sakaguchi, K. Structural Relationship of Lithocholic Acid Derivatives Binding to the N-Terminal 8-KDa Domain of DNA Polymerase β. Biochemistry 2004, 43, 10669–10677. [Google Scholar] [CrossRef]
  31. Valkonen, A.; Sievänen, E.; Ikonen, S.; Lukashev, N.V.; Donez, P.A.; Averin, A.D.; Lahtinen, M.; Kolehmainen, E. Novel Lithocholaphanes: Syntheses, NMR, MS, and Molecular Modeling Studies. J. Mol. Struct. 2007, 846, 65–73. [Google Scholar] [CrossRef]
  32. Yang, D.-K.; Chien, L.-C.; Doane, J.W. Cholesteric Liquid Crystal/Polymer Dispersion for Haze-free Light Shutters. Appl. Phys. Lett. 1992, 60, 3102–3104. [Google Scholar] [CrossRef]
  33. Zhang, F.; Yang, D.-K. P-69: Polymer Stabilized Cholesteric Dichroic Dye Displays. SID Symp. Dig. Tech. Pap. 2002, 33, 469. [Google Scholar] [CrossRef]
  34. Lin, Y.-H.; Ren, H.; Fan, Y.-H.; Wu, Y.-H.; Wu, S.-T. Polarization-Independent and Fast-Response Phase Modulation Using a Normal-Mode Polymer-Stabilized Cholesteric Texture. J. Appl. Phys. 2005, 98, 43112. [Google Scholar] [CrossRef]
  35. Yin, Y.; Li, W.; Cao, H.; Guo, J.; Li, B.; He, S.; Ouyang, C.; Cao, M.; Huang, H.; Yang, H. Effects of Monomer Structure on the Morphology of Polymer Network and the Electro-Optical Property of Reverse-Mode Polymer-Stabilized Cholesteric Texture. J. Appl. Polym. Sci. 2009, 111, 1353–1357. [Google Scholar] [CrossRef]
  36. Lee, K.M.; Tondiglia, V.P.; White, T.J. Bistable Switching of Polymer Stabilized Cholesteric Liquid Crystals between Transparent and Scattering Modes. MRS Commun. 2015, 5, 223–227. [Google Scholar] [CrossRef]
  37. Fung, Y.K.; Yang, D.-K.; Ying, S.; Chien, L.-C.; Zumer, S.; Doane, J.W. Polymer Networks Formed in Liquid Crystals. Liq. Cryst. 1995, 19, 797–801. [Google Scholar] [CrossRef]
  38. Dierking, I. Polymer-Modified Liquid Crystals; Royal Society of Chemistry: London, UK, 2019. [Google Scholar]
  39. Lee, K.M.; Tondiglia, V.P.; Lee, T.; Smalyukh, I.I.; White, T.J. Large Range Electrically-Induced Reflection Notch Tuning in Polymer Stabilized Cholesteric Liquid Crystals. J. Mater. Chem. C 2015, 3, 8788–8793. [Google Scholar] [CrossRef]
  40. Nemati, H.; Liu, S.; Zola, R.S.; Tondiglia, V.P.; Lee, K.M.; White, T.; Bunning, T.; Yang, D.-K. Mechanism of Electrically Induced Photonic Band Gap Broadening in Polymer Stabilized Cholesteric Liquid Crystals with Negative Dielectric Anisotropies. Soft Matter 2015, 11, 1208–1213. [Google Scholar] [CrossRef]
  41. Lee, K.M.; Tondiglia, V.P.; Godman, N.P.; Middleton, C.M.; White, T.J. Blue-Shifting Tuning of the Selective Reflection of Polymer Stabilized Cholesteric Liquid Crystals. Soft Matter 2017, 13, 5842–5848. [Google Scholar] [CrossRef]
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Figure 1. Chemical structures of the steroids used as chiral dopants.
Figure 1. Chemical structures of the steroids used as chiral dopants.
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Figure 2. POM images of (a) androsterone, (b) cholic acid, (c) epiandrosterone, (d) hydrocortisone, (e) prednisone, (f) stigmasterol, (g) β-estradiol, (h) cholesterol, and (i) progesterone at 5 wt. % in E7.
Figure 2. POM images of (a) androsterone, (b) cholic acid, (c) epiandrosterone, (d) hydrocortisone, (e) prednisone, (f) stigmasterol, (g) β-estradiol, (h) cholesterol, and (i) progesterone at 5 wt. % in E7.
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Figure 3. Switchable LC devices in (a) normal mode which is focal conic at 0 V and switches to a transparent homeotropic state when 95 V AC is applied. (b) Reverse mode switchable CLC that is (i) in the transparent planar state at 0 V, (ii) switches to the scattering focal conic state with 35 V DC, (iii) is stable in the focal conic state when the DC voltage is removed, (iv) switches back to the transparent planar state upon application of 40 V AC, and (v) remains stable in the planar state when the voltage is stable.
Figure 3. Switchable LC devices in (a) normal mode which is focal conic at 0 V and switches to a transparent homeotropic state when 95 V AC is applied. (b) Reverse mode switchable CLC that is (i) in the transparent planar state at 0 V, (ii) switches to the scattering focal conic state with 35 V DC, (iii) is stable in the focal conic state when the DC voltage is removed, (iv) switches back to the transparent planar state upon application of 40 V AC, and (v) remains stable in the planar state when the voltage is stable.
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Figure 4. (a) Red and (b) blue-tuning PSCLCs containing 10 wt. % progesterone.
Figure 4. (a) Red and (b) blue-tuning PSCLCs containing 10 wt. % progesterone.
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Figure 5. (a) Synthetic modification of LA using alkyl halides, (b) POM image of 5 wt. % LA in E7, (c) 5 wt. % LA-Bn in E7, and (d) 5 wt. % LA-Me in E7.
Figure 5. (a) Synthetic modification of LA using alkyl halides, (b) POM image of 5 wt. % LA in E7, (c) 5 wt. % LA-Bn in E7, and (d) 5 wt. % LA-Me in E7.
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Figure 6. Responsive CLC and PSCLC samples of (a) normal mode switchable CLC containing 9.5 wt. % LA-Bn in the focal conic state at 0 V AC and the homeotropic state at 95 V AC. (b) Normal mode switchable CLC containing 10 wt. % LA-Me in the focal conic state at 0 V AC and the homeotropic state at 95 V AC. (c) Blue tuning PSCLC containing 9.9 wt. % LA-Bn. (d) Blue tuning PSCLC containing 10.2 wt. % LA-Me.
Figure 6. Responsive CLC and PSCLC samples of (a) normal mode switchable CLC containing 9.5 wt. % LA-Bn in the focal conic state at 0 V AC and the homeotropic state at 95 V AC. (b) Normal mode switchable CLC containing 10 wt. % LA-Me in the focal conic state at 0 V AC and the homeotropic state at 95 V AC. (c) Blue tuning PSCLC containing 9.9 wt. % LA-Bn. (d) Blue tuning PSCLC containing 10.2 wt. % LA-Me.
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Wolf, S.M.; Marsh, Z.M.; Quarin, S.M.; Lee, K.M.; Karra, S.; McConney, M.E.; Grusenmeyer, T.A.; Godman, N.P. Investigating the Electro-Optic Response of Steroid Doped Liquid Crystal Devices. Appl. Sci. 2023, 13, 5054. https://doi.org/10.3390/app13085054

AMA Style

Wolf SM, Marsh ZM, Quarin SM, Lee KM, Karra S, McConney ME, Grusenmeyer TA, Godman NP. Investigating the Electro-Optic Response of Steroid Doped Liquid Crystal Devices. Applied Sciences. 2023; 13(8):5054. https://doi.org/10.3390/app13085054

Chicago/Turabian Style

Wolf, Steven M., Zachary M. Marsh, Steven M. Quarin, Kyung Min Lee, Sushma Karra, Michael E. McConney, Tod A. Grusenmeyer, and Nicholas P. Godman. 2023. "Investigating the Electro-Optic Response of Steroid Doped Liquid Crystal Devices" Applied Sciences 13, no. 8: 5054. https://doi.org/10.3390/app13085054

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