Amplification of Light‐Induced Helix Inversion of Intrinsically Chiral Diarylethene Molecular Switches

Photonics and tunable optics are rapidly developing fields that require materials with programmable properties and advanced functionalities. Cholesteric liquid crystals (CLCs) are unique materials that exhibit selective light reflection and can be tuned using stimuli‐responsive small organic molecules. The challenge lies in designing molecules that can convert external signals, such as light, into dynamic and invertible chiral states, which can be transduced to the CLC supramolecular structures inducing large differences in helicity and eventually to macroscopic properties. Here, novel intrinsically chiral phenanthrene‐based diarylethenes as light‐responsive chiral dopants for controlling the supramolecular helical architectures of CLCs are introduced. The substitution pattern and light‐invertible axial chirality of these diarylethenes make them highly compatible with liquid crystals and provide high twisting power. The light‐induced cyclization and molecular chirality transformation result in a wide tunability of the cholesteric helix pitch (reflection colors) and reversible inversion of helical handedness. These findings provide a powerful tool for controlling and manipulating the macroscopic properties of CLCs, opening new avenues for a range of applications, including diffractive optics and photonics, anticounterfeiting tags, and displays.


Introduction
Cholesteric liquid crystals (CLCs) with their unique supramolecular helical architectures and sophisticated optical properties are recognized as the most promising materials for applications in displays, [1][2][3][4] diffraction gratings [5][6][7] and smart windows, [8][9][10] among others.One of the ways to achieve CLCs is the use of chiral dopants where the chirality of a small molecule is transduced and amplified into liquid crystals.Overall the transformation of chirality from artificial molecules at the molecular length DOI: 10.1002/adfm.202312831[44][45] By introducing chiral side groups to achiral diarylethenes, for example, binaphthyl moieties, [37,38] robust and thermally stable chiral dopants can be achieved.However, the relatively small conformation change during the photocyclization reaction could hardly be transmitted to the pendant side groups in CLCs, resulting in a minor variation in helicity, and as a consequence achieving helical inversion in liquid crystals remains challenging.
Light-induced inversion of helicity in liquid crystal (LC) represents more efficient translation and amplification of tunable molecular conformations to dynamic macroscopic handedness, offering unique opportunities for advanced applications. [46]n a previous strategy, by grafting chiral binaphthyl moieties with a butylenedioxy bridge [47] or non-bridged chiral binaphthyl derivatives [48] in the diarylethene dopants, the CLCs exhibit reversible helical inversion and phase transition by the control of the dihedral angle between two naphthyl rings [49][50][51] through the conformational changes of the switches (Figure 1a), which have enabled the application of the resulting systems in controllable diffractive optics. [52,53]However, the multi-domains in LCs generated by the opposite helical conformation of the binaphthyl units will suppress the efficiency of chirality transfer from the pendent side group to the macroscopic handedness.In addition to developing functionalized diarylethenes with chiral pendant groups, a new strategy to achieve helix inversion in LCs is to transfer the dynamically intrinsic chirality of diarylethenes to LCs.So far, the limiting factor for the use of intrinsically chiral diarylethenes has been the low racemization barrier, which suppresses exploiting their chirality in liquid crystals.In 2007, our group developed the first series of intrinsically chiral diarylethenes containing phenanthrene as the core fragment to which the two reacting thienyl groups are attached and exhibit robust and reversible photoresponsive properties and photo-modulation of chirality. [54]ue to the bulk of the phenanthrene, the rotation around the thiophene-phenanthrene single bond is hindered, and the resulting enantiomers are stable enough to be isolated and studied at room temperature.In addition, the cyclization of such chiral diarylethenes results in dramatic conformational changes with the enantioselective transformation from axial chirality to the combination of central and helical chirality.
In this study, we have developed a novel series of phenanthrene-based diarylethenes as photoresponsive chiral dopants for modulating the supramolecular helical architectures of CLCs.The distinctive design feature of our diarylethenes is the fusion of the ethene bridge with a phenanthrene unit, which could transform the (M) helical chiral motif in the open form into the (P, P) helical chirality with the two (S, S) stereogenic centers in the closed form (Figure 1b).Additionally, we have introduced rigid aromatic moieties (S1-S3) as pendant side groups to the thiophenes, which has enabled high twisting power and miscibility with liquid crystals (Figure 1c).The synthesized diarylethenes S1-S3 undergo light-induced photocyclization accompanied by building up of new helical chiral molecular fragments, resulting in wide tunability of the helix pitch (P) of the cholesteric material and, more importantly, the inversion of the handedness of supramolecular cholesteric helices, as schematically shown in Figure 1b.The ability to induce a large change in helical twisting power (HTP) and a reversible inversion of helical handedness upon light irradiation provides a powerful tool for controlling and manipulating the macroscopic properties of the material.

Switching Process of S1-S3 in Solution
The phenanthrene-based diarylethenes were synthesized in three steps (Scheme S1, Supporting Information). [54]Toward the synthesis of diarylethenes S1-S3 (Figure 1c), we employed Friedel-Crafts acylation to obtain the diketone precursor 6.Subsequently, we utilized an intramolecular McMurry reaction to synthesize the phenanthrene-based photoresponsive core 7. Finally, we performed a twofold Suzuki coupling between the core 7 and the corresponding aryl-bishalide to obtain the diarylethenes S1-S3 as racemic and meso stereoisomers in yields of 22%-50%.We efficiently separated the racemic mixture of two photoactive enantiomers of diarylethenes and the photoinactive meso form by conventional column chromatography.The racemate and meso compounds were distinguished by 1 H-NMR based on the chemical shifts of the methyl proton H a located at the -position of the thiophenes.As the two methyl groups of the meso form are in closer proximity with each other, H a was significantly shifted upfield ( ≈2.2 ppm) compared to that of the racemate ( ≈2.1 ppm) (Figures S1-S2, Supporting Information).Since the meso forms of the switches are photochemically and optically inactive, we will only discuss the photoactive pairs of diarylethene enantiomers in our present study.
The reversible interconversion of the open (o) to closed (c) forms of the photoactive switches is achieved through sequential illumination with UV and visible light (Figure 2a).The isomerization process was investigated using 1  ) were determined to be 0.16 and 0.06, respectively (details are in Supporting Information, Figures S33-S36 and Table S2, Supporting Information).The photoconversion from S1-(o) to S1-(c) and S2-(o) to S2-(c) were determined to be 85% and 90%, respectively (Figures S3 and S4, Supporting Information).The photocyclization process of the switches in solution was monitored by UV-Vis spectroscopy and supported by density functional theory (DFT) calculations.Upon exposure of S3-(o) to UV light at 325 nm, the absorbance band gradually decreases, accompanied by two ascending absorption maxima centered at 404 and 600 nm assigned to S3-(c).The absorption at 404 nm is attributed to HOMO (highest occupied molecular orbital) →LUMO+1 (lowest unoccupied molecular orbital) and HOMO−1→LUMO excitations and the absorption ≈600 nm corresponds to HOMO→LUMO excitation, where MO 184, 185 and MO 186, 187 are the  and * orbitals, respectively (Table S7, Figure S42, Supporting Information).The visible light exposure at 595 nm results in the cycloreversion reaction recovering the initial spectrum of S3-(o) (Figure 2c).
As we previously reported, the open form of phenanthrenebased diarylethenes has a high racemization barrier in solution (≈110 kJ mol −1 ), which means that the interconversion between racemate and meso form has a lifetime exceeding 1000 h at 293K. [54]This high racemization barrier facilitates the separation of the enantiopure switches with chiral high-performance liquid chromatography (Figure S7, Supporting Information).The enantiomers of S1-3-(o) exhibit mirror-imaged electronic circular dichroism (CD) at 310-330 nm (Figure S8, Supporting Information), and their absolute configuration has been confirmed with DFT calculations (Figures S43-S44, S47-S48, Supporting Information).Upon UV light irradiation of enantiopure switch (M)-S3-(o), a negative Cotton effect was observed at 600 nm in accordance with the calculated results (Figure 2d S3, Figure S38, Supporting Information).Furthermore, S3 exhibits excellent fatigue resistance over several cycles of photoswitching between open and closed forms, as evidenced by UV-Vis and CD spectral data (Figure 2e, the switching behavior of (M)-S1 and (M)-S2 compounds is shown in Figure S6, Supporting Information).Therefore, the effective and robust operation of the chiral optical switches S1-S3 and the inversion of helical chirality in solution suggest that this phenomenon can be transduced and amplified across length scales toward dy-namic supramolecular helices in cholesteric liquid crystalline materials.

Chirality Transfer from Photoswitches to Liquid Crystals
HTP is a phenomenological parameter that reflects the ability of chiral molecules/dopants to twist the nematic liquid crystalline phase into the cholesteric phase (also known as the chiral nematic phase).HTP can be calculated using the following equation: HTP = 1/(P×c×ee), where P is the helical pitch, c is the concentration of the chiral dopant in a nematic LC host and ee is the enantiomeric excess of the chiral dopant.By convention, the HTP is positive for right-handed helices and negative for left-handed helices.HTP depends only on the intrinsic chirality of the dopant and its interactions with the host's liquid crystal molecules. [25,55,56]he HTP values of S1-S3 and sense of helicity of the induced cholesteric mesophase were determined using the Grandjean-Cano wedge cell method. [57]A small amount of the enantiopure switches (M)-S1-3-(o), ranging from 0.4 mol % to 0.9 mol % were doped in ZLI-1083, a nematic liquid crystal mixture (Figure 1c) composed of molecules that do not absorb light at  > 300 nm.All three switches in the initial open form induced left-handed helical structures in liquid crystal (Figure 3a, Figure S9, Supporting Information).We found that increasing the length of rigid substituents at the thiophene rings (S1→S3) gradually increased the HTP values, from −38.7 μm −1 for (M)-S1-(o) till -242.8 μm −1 for (M)-S3-(o).Interestingly, UV light-induced cyclization of diarylethene switches resulted in the inversion of the chiral sense of the supramolecular helices from left-to right-handed ones.The experimentally achieved HTP values increased in a series of (P,P)-S1-3-(c) from +43 to +203.9 μm −1 , similar to the trend for (M)-S1-3-(o).We additionally estimated the HTP values of the "pure" closed forms of the switches based on ratios in UV photostationary states extracted from 1 H-NMR data.From the bar chart in Figure 3a, it can be seen that the closed forms of the switches have a slightly higher HTP value compared to their open forms.A considerable difference of HTPs, ΔHTP = 636.6 μm −1 (or ≈263%), for switch S3 can be rationalized in terms of the increased rigidity of the intrinsically chiral switch upon photocyclization which increases the interactions with liquid crystal molecules, as evidenced by high values of induced linear dichroism (see details in Figure S10, Supporting Information).][41][47][48] Due to the excellent compatibility of the designed chiral switches with the liquid crystal host, a mixture containing 1.5 mol% of (M)-S3-(o) was prepared to demonstrate the lightdriven operation of the microscopic chiral system.Figure 3b shows a polarized optical image of a close-up view of the boundary where UV-exposed (right-handed helical structure) and nonexposed (left-handed helical structure) areas of the cholesteric layer meet.Moving from the left side of the image to the right, a sequence of parallel vertical defect lines can be observed, corresponding to the stepwise unwinding of the left-handed helices until a fully unwound state, which is represented by the black-colored area (Figure S11, Supporting Information).This boundary distinguishes the areas with left-and right-handed helical structures.Moving further to the right side shows a sequence of parallel defect lines corresponding to the stepwise winding of the right-handed helices.Thus, it clearly demonstrates that photocyclization of the switch results in the inversion of supramolecular chirality of the liquid crystal material, which has been further confirmed by the CD study (Figure S12, Supporting Information).

Optical Properties of Light-Responsive Liquid Crystal Material
The supramolecular helical structure of the cholesteric liquid crystals has a unique optical property, namely, it acts as Bragg diffraction grating that selectively reflects light of specific color and polarization, similar to structural colors of biological objects. [58]The center of the reflection band ( max ) of cholesteric material is proportional to the helix pitch (P) as follow:  max = n ×P, where n is the average reflective index of liquid crystal.We prepared planar-aligned layers of the material containing 1.5 mol% of (M)-S3-(o) which selectively reflects leftcircularly polarized blue light in the initial state.We then monitored the optical signature of the materials during exposure to UV light ( = 325 nm). Figure 4a shows transmission spectra of the layer upon UV light irradiation.One can observe the appearance of new bands ≈410 and 600 nm, while the other band at 450 nm corresponding to the light reflection gradually shifts to the red spectral region until it goes out of the measurable range (left panel in Figure 4a, sinusoidal shape of the transition curves originates from light interference).After longer exposure, the reflection band appears again in the near-infrared spectral range and continues moving toward shorter wavelengths until reaches PSS UV at ≈800 nm (right panel in Figure 4a).This process is caused by the winding of the newly formed right-handed helical superstructure.When the illumination wavelength is changed to  = 595 nm, the back photochemical process (cycloreversion) is initiated, resulting in the following sequence of processes: unwinding of right-handed helices, handedness inversion, and winding of left-handed helices, as shown in Figure 4b.In PSS Vis state, the material almost reaches its original color ( max ≈510 nm).The fatigue resistance study revealed a slight linear shift of selective reflection bands both in PSS UV and PSS Vis (Figure S14, Supporting Information), likely due to the gradual formation of photochemically inactive meso form of the switch (Figure S37, Supporting Information).Thus, the use of a single chiral dopant allows for reversible modulation of the structural colors of the material in the entire visible spectral range with the possibility to invert the chiral sense (Figure 4b).We can also conclude that the color change is directly coupled with the photochemical processes of ring closure -ring opening of the chiral switch, which is clearly indicated by the match of the evolutions of the helix pitch and absorbance maximum corresponding to the accumulation/consumption of the closed form (Figure 4c).
Additionally, we investigated the thermal stability of the closed form of the switch when embedded in the liquid crystal environment.The half-life time of (P,P)-S3-(c) at room temperature was determined to be 36 min (Figure S15, Supporting Information), which is comparable to previously reported phenanthrenebased diarylethenes in solution (23-33 min at 293 K. [54] ) However, the thermal stability of the closed form is lower than that of the intrinsically chiral diarylethenes reported recently. [59]This is likely due to the disruption of the highly aromatic phenanthrenebased diarylethenes, which increases the ground-state energy of the closed form, leading to a lower energy barrier for the cycloreversion reaction, as supported by DFT calculations (Figure S38, Supporting Information). [60]Surprisingly, by investigating the racemization barrier of (M)-S3-(o) in a liquid crystal medium we revealed that it is ≈101 kJ mol −1 , which is 10 kJ mol −1 lower than the barrier measured in methylcyclohexane solution (Figures S16 and S17, Supporting Information).In other words, the half-life time of racemization drops from 1000 to 35 h at 293K when the switch is embedded in liquid crystals.[63] In our case, these effects, together with the large interactions between the switch and liquid crystal molecules revealed by the linear dichroism (Figure S10, Supporting Information), likely reduce the bar-rier of racemization, although the exact mechanism of this process remains elusive.
To demonstrate the remarkable tunability of the optical properties of the materials based on intrinsically chiral switches, we recorded a few colored images by irradiating a layer of the described above mixture (ZLI-1083+1.5 mol % of (M)-S3-(o)) with UV light through a designed mask.Initially, the layer reflected blue light.Then, the layer was exposed to UV light through a mask for 1, 2, and 3 s, revealing three images of the heart pattern colored green, yellow, and red, respectively (Figure 4d).The entire multicolored image selectively reflects left circularly polarized light, as evidenced by observing the layer using polarized filters.Subsequently, the layer was homogeneously exposed to UV light until the photostationary state was reached.No reflection colors could be observed by the naked eye, as the reflection band is centered at 800 nm.However, the layer turned intense blue due to the formation of a closed form of the switch, which strongly absorbs light at ≈600 nm.A hieroglyph was then written on top of the cell using a black marker, followed by visible light exposure for 6 min and wiping out the ink.The recorded optical information can be visualized in two ways.First, due to the different absorbance of visible light by open and closed forms (Figure 2c), the white hieroglyph appeared on a blue background when observed in transmission mode, as shown in Figure 4e.Second, in reflection mode, no colors were observed when using a right circularly polarized filter, while the hieroglyph appeared orange-green when using a left circularly polarized filter.This indicates that the area of the hieroglyph has a left-handed helical structure, reflecting visible light, while the area around it has a right-handed structure, reflecting near-infrared light.

Conclusion
In the present study, the chiral amplification and reversible helix inversion in liquid crystals through the use of intrinsically chiral phenanthrene-based diarylethene photoswitches were demonstrated.A series of novel diarylethenes were synthesized and their isomerization behaviors were fully characterized via spectroscopic analysis and DFT calculations.The chiral switches exhibited good solubility and compatibility with liquid crystals and had large initial HTP values -242.8 μm −1 with large variations up to +203.9 μm −1 that enable the transduction and amplification of their intrinsic chirality from the molecular level to the supramolecular helical architecture of the cholesteric phase.By modulating the geometry and helicity of the diarylethenes under light stimuli, reversible and controllable helix inversions in liquid crystals were achieved, resulting in materials with lightcontrollable optical properties.These highly efficient helicityinverted liquid crystal materials based on intrinsically chiral photoswitches hold great promise for various potential applications, among others materials with tunable circular polarized luminescence, materials for lasing and laser beam steering, and anticounterfeit materials.

Figure 1 .
Figure 1.Helical inversion in liquid crystals induced by photoresponsive diarylethene (DAE) based chiral dopants.a) Diarylethene-based chiral dopants with binaphthyl pendant side group can cause the inversion of supramolecular helicity of liquid crystals through conformational states of binaphthyl units.LH (RH) refers to the left (right)-handed helix in the liquid crystals.b) Schematic representation of chiral information transfer from molecular to supramolecular levels.Light-driven and reversible inversion of molecular axial chirality of the intrinsically chiral diarylethenes embedded in cholesteric liquid crystals causes inversion of handedness of supramolecular cholesteric helices.Here (M) helically chiral conformer of the open form of the diarylethene is composed of two (R a ) axially chiral moieties.The closed form combines (P,P) helical chirality and two (S) stereogenic centers.c) Chemical structures of intrinsically chiral diarylethenes S1-S3 and liquid crystal mixture ZLI-1083 (n = 3 (30 wt%), n = 5 (30 wt%), n = 7 (40 wt%)) used in the present study.

Figure 2 .
Figure 2. Switching behavior of S3 in solution.a) Reversible light-induced ring-closure reaction of diarylethene photoswitches.b) Partial 1 H-NMR spectra of S3-(o) (2 mm in toluene-d 8 ) before irradiation, after UV (PSS 325nm ) and after visible light irradiation (PSS 595nm ).The solution was in situ irradiated at 0 °C.c) Absorbance spectra of S3-(o) (10 mM, THF) before (orange solid) and after UV (blue solid) and visible light exposure (red dashed).Bars correspond to the calculated excitation energies and oscillator strengths.Spectra were recorded at 0 °C.d) Circular dichroism spectra of (M)-S3-(o) (1 mM, THF) before (orange solid) and after UV (blue solid) and visible light exposure (red dashed).Bars correspond to the calculated excitation energies and rotary strengths.Spectra were recorded at 0 °C.e) Fatigue resistance study.Absorbance and CD signals after sequential UV and visible light illumination of the (M)-S3-(o) solution in THF (at each step PSSs were reached).
H-NMR spectroscopy, as shown in Figures S3-S5 in the supporting information.For instance, the characteristic shift of S3 at 2.03 ppm (Figure 2b, bottom orange curve) corresponds to the methyl proton H a of S3-(o).Upon irradiation with UV light at 325 nm, the peak gradually decreases with the appearance of a new peak at 2.41 ppm, assigned to the proton H b of S3-(c) (Figure S5, Supporting Information), allowing us to determine the ratio of S3-(o)/S3-(c) at the photostationary state (PSS) as 30%/70% (PSS UV , Figure 2b).Subsequent irradiation with visible light at 595 nm regenerates the initial state of the switch (PSS Vis , Figure 2b, top red curve), indicating complete back photoisomerization from the closed to open form.The quantum yields of cyclization and cycloreversion reactions (Φ 325 nm o→c , Φ 595 nm c→o , see also Figures S6, S45-S46, Supporting Information), revealing the exclusive transformation into (P,P)-S3-(c) with (S,S) configuration at the stereogenic centers and (P,P) helical chirality (Figures S49-S50, Supporting Information).Notably, along with the formation of two stereogenic centers, the photocyclization reaction of S3 results in the inversion of helical chirality involving the interconversion processes of (M)-S3-(o)→TS-(M)-S3-(o)→(M)-S3-(c)→TS-S3-(c)→(P,P)-S3-(c), as supported by DFT calculations (Table

Figure 3 .
Figure 3. Helical twisting powers (HTPs) of switches S1-S3.a) The HTP values of different states of (M)-S1-3 in ZLI-1083 liquid crystal.* HTP values of pure (P,P)-S1-3-(c) were calculated using the PSS ratios measured by 1 H-NMR spectroscopy.The positive values correspond to the right-handed cholesteric helix, while the negative values correspond to the left-handed helix.The values were calculated based on mol% concentration, the data for wt% concentration can be found in Table S1 (Supporting Information).b) Polarized optical image of the area where non-irradiated and UV light irradiated areas meet each other forming an achiral area of a "compensated" state.The gradual change of helix pitch is due to the diffusion of chiral species between two areas.Sample: glass cell promoting planar molecular alignment filled with a mixture of 1.5 mol% of (M)-S3-(o) in ZLI-1083.Cell gap: 8 μm.

Figure 4 .
Figure 4. Light-induced helix inversion in liquid crystals.a) Transmittance spectra of the sample upon light irradiation with UV light ( = 325 nm).The left panel shows the unwinding initial left-handed helical structure and the right panel shows the winding of the right-handed helical structure.b) Evolution of selective light reflection maximum ( max ) during exposure to UV light and visible light ( = 595 nm).c) Overlap of the kinetic profiles of helix pitch evolution and conversion of the closed form of the switch followed by the absorbance at 412 nm upon exposure to UV and visible light.d) Real color image of the sample with photorecorded patterns observed in right circularly polarized (RCP) light and in left circularly polarized (LCP) light.Patterns were recorded by UV light irradiation for 1s, 2s, and 3s.Corresponding polarized optical images taken in reflection mode are shown.e) Real color image of the sample with photorecorded pattern observed in transmission mode in natural light and in reflection mode using RCP and LCP light filters.The pattern was recorded by visible light exposure of the sample pre-irradiated with UV light through a mask for 6 min.In a-e, 8 μm thick cells filled with a mixture of ZLI-1083 and 1.5 mol% (M)-S3-(o) were used.Size of the cell: 2×2 cm.The intensity of the 325 nm UV light and 595 nm visible light were 3 and 15.5 mW cm −2, respectively.