A cinnamate liquid crystal for rapid optical recording

ABSTRACT Photorecording materials have been applied for information recording. Herein, a cinnamate liquid crystal with an enantiotropic nematic phase was synthesised, which can be isomerised and polymerised under the irradiation of the 365-nm UV light. Cholesteric liquid crystal polymer network (CLCN) films were prepared using the mixtures of it, LC242, a chiral dopant and a photoinitiator. The CLCN films possess a slight gradient helical pitch which increases from the bottom to the top of the films. The formation of this structure should be driven by the photoisomerisation of the cinnamate. Under the irradiation of 365-nm UV light with a low intensity, the CLC mixtures show a photochromic behaviour which is proposed to be driven by the formation of oligomers. Based on this, the CLC mixtures can be applied for optical recording. GRAPHICAL ABSTRACT


Introduction
Due to the requirements of the information recording and storage, the study of photorecording materials has become a research hotspot during the last decades [1,2].Organic photochromic materials have been widely used in many fields such as data storage, bioimaging, and holographic recording, due to their large molecular diversity, high sensitivity, and diverse photochromic behaviours [3][4][5][6].The photochromic materials can perform photoinduced switching between two isomers [7][8][9].During this process, physical or chemical properties, including absorption spectra, electronic conductivity, dipole moments, dielectric constants, geometries, and other properties may be altered [10].This has inspired the application of such compounds as photochemical molecular switches [11,12].Cholesteric liquid crystals (CLCs) are one-dimensional photonic crystals generally formed by the incorporation of chiral molecules with helical twisting forces into nematic liquid crystals [13,14].The unique supramolecular helical structure allows the selective Bragg reflection of incident light, resulting in visually structural colours [15][16][17][18].The tunable reflected colours and wide available wavelength window of CLCs make them advanced materials for colour display and optical recording.When there is an inhomogeneous helical spacing in CLC polymer films, a wide reflective spectral band is created [19,20].The broadband reflective films may be attractive for reflective LC displays [21,22], brightness enhancement films [23] and Infrared shielding films [24][25][26].Liquid crystal displays with reflective bands in the infrared and far infrared regions have potential applications in military shielding and stealth [27].
There are various ways to induce an inhomogeneous helical gradient in CLC polymer films, such as directly stacking multiple layers of CLC polymer films with different pitches [28,29], or realising the inhomogeneity of the internal pitch through the addition of responsive chiral dopants which can be externally stimulated to change the helical twisting power (HTP) [30][31][32][33][34].In addition to these, the helical pitch can be adjusted by controlling the UV light intensity in the vertical direction of the film of the CLC mixture to regulate the polymerisation rate of the monomer [35,36].Uneven helical pitch can also be produced in thermally induced molecular diffusion in two-phase coexisting material systems [37].
Photochromic CLC mixtures have been prepared using the cinnamate chiral dopants [38].Due to the photoisomerisation of the cinnamates, the CLC polymer network (CLCN) films with a broad reflection band were prepared [39].Herein, a reactive and photoisomerizable cinnamate liquid crystal was synthesised.The CLCN films with a structural colour were prepared using the CLC mixtures of it through photopolymerisation.The formation of oligomers during the polymerisation process was proposed to drive the photochromic behaviour.Based on this photochromic behaviour, the CLC mixtures are suitable for photorecording and decoration.

Results and discussion
The molecular structures of the compounds in the CLC mixtures are shown in Figure 1.LC242 is a nematic liquid crystal [40].CA-iso is a chiral dopant for inducing the formation of the cholesteric structure [41].Irgacure 369 acts as the photoinitiator [42].The synthetic procedure for the reactive cinnamate, C6C1, was shown in Scheme S1, Supporting Information.The differential scanning calorimetry (DSC) and polarised optical microscopy (POM) characterisations indicated that C6C1 showed an enantiotropic nematic phase and a monotropic SmX phase (Cr 84.9°C N 89.6°CI 87.7°-C N 44.7°CSmX 43.1°C Recr, Figure S1 and S2(a), Supporting Information).Due to the structure of the SmX phase is ultra-unstable, it was failed to characterise it using POM.For the LC242/C6C1/CA-iso/Irgacure 369 mixture prepared at the weight ratio of 81.5/10/ 4.0/4.5, an oily steak texture was identified at 85.0°C in the POM image, indicating a cholesteric phase (Figure S2(b), Supporting Information).Therefore, it is possible to prepare a CLCN film using this mixture.
The photoisomerisation of C6C1 was studied by taking the UV-vis spectra of C6C1 in CHCl 3 at a concentration of 7.9 × 10 −5 M under the irradiation of 365-nm UV light (400 mW/cm 2 ) (Figure 2(a)).C6C1 showed a maximum absorption band at 317 nm which was attributed to the π−π* transition of the E-form cinnamoyl group [43].With extending the UV light irradiation time, the intensity of the absorption band at 317 nm decreased.A photostable state reached at about 240 s.It was reported that the chiral cinnamates could control the helical pitch of CLC mixtures through photoisomerisation [44,45].To understand the effect of the isomerisation of C6C1, a CLC mixture was prepared at the LC242/C6C1/CA-iso weight ratio of 86.4/10/3.6.The CLC mixture showed a Bragg reflection band at 521 nm (Figure 2(b)).After the CLC mixture was irradiated under the 365-nm LED lamp (80 mW/cm 2 ) for 30 s or the high-pressure Hg lamp (1.0 kW) for 10 s at 80°C, the wavelength of the Bragg reflection band slightly shifted to short wavelength.Based on the equation of the selective Bragg diffraction, λ max = nP, (λ max , the reflection wavelength at the maximum; n, the average refractive index; P, the helical pitch), the isomerisation of C6C1 did not affect much on the helical pitch of the CLC mixture.It was reported that bent molecules tend to exhibit chiral conformations and stack into helical structures [46,47].In a CLC mixture, the helical pitch decreases with increasing the concentration of a compound with a bent configuration [48].Therefore, the blueshift of the Bragg reflection band was proposed to be driven by the formation of Z-isomer (Figure 2(b)).
The photopolymerisation of C6C1 was studied by taking the FT-IR spectra of C6C1 before and after photopolymerisation without the addition of any photoinitiators (Figure S3, Supporting Information) [49].This polymerisation should be driven by the high absorption efficiency of C6C1 at a short wavelength [50].C6C1 was heated to a hot temperature and then cooled down to 80°C between two quartz plates.Then, the photopolymerisation was carried out and under the irradiation of 365-nm UV light (400 mW/cm 2 ) for 30 min (Figure S4, Supporting Information).For C6C1, an absorption band was identified at 1414 cm −1 , which originated from the acrylate group.After polymerisation, a weak absorption band was identified at 1410 cm −1 , indicating that most of the monomers were polymerised.Therefore, photoisomerisation and photopolymerisation can carry out simultaneously under a suitable UV light irradiation condition.
For the preparation of the CLCN films, several mixtures of LC242/C6C1/CA-iso/Irgacure 369 were prepared by changing the CA-iso concentration and keeping the Irgacure 369 concentration at 4.5 wt% (Table S1).These mixtures were coated on the surface of a rubbing-oriented polyethylene terephthalate (PET) film.With increasing the CA-iso concentration from 2.5 to 4.3 wt%, the colour of the CLC mixture changed from pale red to purple (Figure 3(a)), and the selective reflection band shifted from 733 to 433 nm (Figure 3(c)).After photopolymerisation under the irradiation of the high-pressure Hg lamp, the CLCN films with a structural colour were obtained.With increasing the CA-iso concentration, the colour of the CLCN film also changed from pale red to purple (Figure 3(b)), and the reflection band underwent blue shift from 733 to 418 nm (Figure 3(d)).Comparison of the UV-vis spectra of the samples before and after polymerisation, the reflection bands slightly shifted to short wavelength.This blueshift was proposed to be driven by the isomerisation of C6C1 (Figure 2).It was also found that the Bragg reflection band was broadened after polymerisation.These CLCN films were proposed to possess a gradient helical pitch.
The baselines of the transmission bands were about 88%, which was mainly caused by the specular reflection of the CLCN films.The transmittances of the CLCN films were close to 45%, with that of the purple film approaching 40%.Since single-handed CLCN film can only reflect 50% of light, the reflectance of these CLCN films almost reached the maximum.To understand the optical activity of the CLCN films shown in Figure 3, the diffuse reflectance circular dichroism (DRCD) spectra of the CLCN films prepared using 3.2 and 3.6 wt% of CA-iso were taken (Figure S5, Supporting Information).The cross-sectional field-emission scanning electron microscopy (FE-SEM) images of the CLCN films prepared using 3.2 and 3.6 wt% of CA-iso are shown in Figure 4.The film thicknesses of them are about 3.8 μm.The helical pitches of the CLCN films prepared using 3.2 wt% of CA-iso are about in the range from 367 and 398 nm.And those of the CLCN films prepared using 3.6 wt% of CA-iso are about in the range from 305 and 376 nm.The helical pitch increased gradually from the bottom to the top of the films.The formation of this gradient helical pitch was proposed to be driven by the oxygen inhibition, photoisomerisation of C6C1 and polymerisation of the monomers.The CLCN films with different thicknesses were also prepared using a CLC mixture with 3.6 wt% of CA-iso.With increasing the film thickness from 0.84 to 4.3 µm, the reflectance increased from 24.7% to 43.7% (Figure S6, Supporting Information), and that reached a maximum value at about 2.4 µm (Figure S7, Supporting Information).However, the wavelength of the Bragg reflection band almost did not shift.Therefore, the oxygen inhibition almost did not affect the helical pitch under the irradiation of the high-pressure Hg lamp.
To understand the effect of oxygen inhibition, the CLCN films shown in Figure 5 were prepared using a LC242/C6C1/CA-iso/Irgacure 369 (w/w/w/w, 81.9/ 10/3.6/4.5)mixture through a two-step preparation approach.At the first step, the CLC mixture was coated on the surface of a PET film, a Bragg reflection band was identified at 500 nm.After the CLC mixture was irradiated under the irradiation of a 365-nm LED lamp (80 mW/cm 2 ) at 80°C for 3.0 s, a photochromic phenomenon was identified.A Bragg reflection band with a long tail was identified at 503 nm in the UV-vis spectrum (Figure 5(b)).Due to oxygen inhibition, the film surface was still viscous.Oligomers were proposed to be formed  at the top section of the film.However, the monomers near the PET film surface should have been solidified.To confirm the polymerisation, the film was washed with acetone.A CLCN film with a structural colour was obtained.A Bragg reflection band without tail was identified at 480 nm.Therefore, the top section of the film prepared under the irradiation of the LED lamp should possess the longer helical pitches.
Since the isomerisation of C6C1 tended to slightly shorten the helical pitch, the increase of the helical pitch should be not driven by the isomerisation.Moreover, it also should not be driven by the polymerisation-induced chiral dopant diffusion [51,52].Due to the polymerisation of the monomers near the PET film surface, the diffusion of the chiral dopant from the bottom to the top of the film will decrease the helical pitch of the film near the air.To understand the increase of the helical pitch of the film near the air, the film was washed with acetone, after the irradiation of the LED lamp.The compounds dissolved in acetone were characterised using gel permeation chromatography (GPC).The relative weight-average molecular weight and the molecular weight distribution were 1800 and 1.15, respectively.Therefore, the increase of the helical pitch was proposed to be driven by the formation of oligomers under the irradiation of the LED lamp with a weak light intensity.For a better understanding the non-uniform helical pitch, a CLCN film was prepared without the addition of C6C1.A Bragg reflection band with a shorter tail was identified in the UV-vis spectrum (Figure S8(a), Supporting Information).For the CLCN film prepared using a CLC mixture with 20 wt% of C6C1, a Bragg reflection band with a longer tail was identified (Figure S8(b), Supporting Information).However, due to the strong UV light absorption of C6C1, the CLC mixture cannot be solidified at the 30 wt% of C6C1 concentration.Therefore, both the oxygen inhibition and the UV light absorption of C6C1 play important roles in the formation of the non-uniform helical pitch.
At the second step, the CLCN film was obtained under the irradiation of the high-pressure Hg lamp (1.0 kW) for 10 s.The surface of the film was solidified.A Bragg reflection band with a long tail was identified at 503 nm (Figure 5(b)).Non-uniform helical pitch was identified at the top section of the CLCN film (Figure S9, Supporting Information).The formation of the CLCN film was studied by taking the FT-IR spectra at different steps.For the CLC mixture, two absorption bands were identified at 1406 and 810 cm −1 which originated from the acrylate group (Figure S10, Supporting Information) [52].After the CLC mixture was irradiated under the LED lamp, these two FT-IR absorption bands disappeared, indicating the polymerisation of the acrylate groups.Both cross-linked polyacrylate and polyacrylate oligomer were formed.After the film was irradiated under the high-pressure Hg lamp, the FT-IR spectrum of the film was similar to that of the film prepared under the irradiation of the LED lamp.The formation of the structure of a CLCN was briefly illustrated in Scheme 1.
Based on the photochromic property of the CLC mixture, a CLCN grating and a CLCN pattern were prepared at the LC242/C6C1/CA-iso/Irgacure 369 weight ratio of 81.9/10/3.6/4.5 and using photomasks through the two-step approach (Figure 6(a,b)).The CLC mixture was irradiated under the LED lamp and the high-pressure Hg one, subsequently.The POM image of the one-dimensional (1D) grating showed a high resolution (Figure 6(a)).Due to the irradiation of the LED lamp, the structural colour of the butterfly pattern turned from cyan to yellow (Figure 6(b)).Based on the high photochromic rate, the CLC mixtures can be used for rapid photorecording.Herein, a Wood's lamp (input power 5.0 W) was used as the pen.After the CLC mixture was coated on the surface of a PET film, a colour flower was drawn.During the drawing process, the structural colour of the irradiated area changed from cyan to orange.After photopolymerisation under the irradiation of the high-pressure Hg lamp, a colour CLCN pattern was obtained (Figure 6(c)).Since the CLC mixture shows rapid photorecording property, it is suitably applied for information storage.
A colourful pattern was prepared using CLC inks through inject printing.The cyan, green and red inks were prepared at the LC242/C6C1/CA-iso/Irgacure 369 weight ratio of 81.9/10/3.6/4.5, 82.3/10/3.2/4.5 and 82.7/ 10/2.8/4.5, respectively.They were printed on the surface of a black PET film and polymerised under the irradiation of the high-pressure Hg lamp (Figure 6(d)).Since the CLC mixtures are inject printable, varieties of colourful patterns can be prepared, which are potentially applied for anti-counterfeiting and decoration.

Conclusions
A photopolymerizable and photoisomerizable cinnamate liquid crystal was synthesised.CLC mixtures were prepared using it, and the structural colour of them was able to be fixed after photopolymerisation.It was found that the distribution of the helical pitch of a CLCN film was broadened under the irradiation of a 365-nm light with a weak intensity.The formation of oligomers is proposed to drive this phenomenon, which has been studied using GPC.Based on this high photochromic rate, colourful patterns can be drawn using a Wood's lamp.Therefore, the CLC mixtures shown here are suitably applied for information storage and the obtained patterns are suitably applied for anti-counterfeiting and decoration.

Characterizations
The FT-IR spectra were performed on a Nicolet 6700 spectrometer at 2.0 cm −1 resolution by averaging over 16 scans.The 1 H NMR spectrum was taken on a Varian NMR (300 MHz) spectrometer in CDCl 3 using tetramethylsilane (TMS) as an internal standard at room temperature.The elemental analysis was measured on an EA-1106 instrument.The DSC measurement was conducted on a TA-Q200 under nitrogen at 10°C min −1 .The POM images were taken using a CPV-900C polarisation microscope fitted with a Linkam LTS420 hot stage.The transition temperatures reported in this paper were the peak values of the transition on DSC traces.The mass spectrum (MS) was measured with Ultraflextreme MALDI TOF/TOF spectroscope.The FE-SEM images were obtained using a Hitachi S-4800 operating (Ibaraki prefecture, Japan) at 5.0 kV.UV-vis spectra were obtained with a UV-vis spectrophotometer (UV-1900i, Japan).The DRCD spectra were measured by using a JASCO 815 spectrometer (JASCO, Japan), and anhydrous barium sulphate was used as the blank.The UV LED series equipment (UVSF81T) produced by FUTANSI Electronic Technology Co., Ltd (Shanghai, China).The 365-nm LED parallel light source is equipped with double aspherical quartz lenses to produce parallel light with a parallel half angle of less than 2°.The CLCN films were prepared under the irradiation of a high-pressure Hg lamp (MINHIO 4012-20, 1.0 kW) produced by MINHIO Intelligent Equipment Co., Ltd (Shenzhen, China).The UVA, UVB, UVC and UVV light intensities are 96.0,87.2, 0 and 75.4 mW/cm 2 , respectively.The Printer (A4UV) is produced by Sonpoo Electronic Co., Ltd (Guangdong, China).The hand-drawn pattern was obtained with a Wood's lamp (5.0 W).The relative weight-average and molecular weight distribution of the oligomer were estimated using a Waters 1515 GPC system.Monodisperse polystyrene acts as the calibration standard and THF acts as the eluent at a flow rate of 1.00 mL min −1 at 35°C.

Preparation of the CLCN films
A typical preparation procedure was shown as follows.A mixture of LC242/C6C1/CA-iso/Irgacure 369 was prepared at the weight ratio of 81.2/10/4.3/4.5, which was dissolved in a mixture of cyclohexanone/ethyl acetate (w/w, 4/1) to form a solution with 20 wt% of solid content.The solution was coated on the surface of a rubbing-oriented PET film using a 40-µm Mayer bar.After the solvents were removed at 120°C, polymerisation was carried out under the irradiation of the highpressure Hg lamp (1.0 kW) at room temperature for 10 s.Then, a purple CLCN film was obtained.The other CLCN films were prepared by changing the concentration of CA-iso.

Preparation of the CLCN grating and the colorful butterfly pattern prepared using photomasks
A typical preparation procedure was shown as follows.After the LC242/C6C1/CA-iso/Irgacure 369 (w/w/w/w, 81.9/10/3.6/4.5) was coated on the surface of a rubbingoriented PET film using a 20-µm Mayer bar as described above, a 1D grating mask or a butterfly one was covered on the film surface.Then, the photopolymerisation was carried out under the irradiation of a 365-nm LED lamp (80 mW/cm 2 ) at 80°C for 3.0 s.After removing the photomask, the film was solidified under the irradiation of the high-pressure Hg lamp for 10 s.

Preparation of the pattern drawn by hand
After the CLC mixture of above was coated on the PET film surface, the flower pattern was drawn using a Wood's lamp at 80°C.The picture with cyan background was obtained under the irradiation of the highpressure Hg lamp for 10 s.

Preparation of the colorful CLCN pattern by inject printing
Three inks were prepared at the LC242/C6C1/CA-iso/ Irgacure 369 weight ratio of 81.9/10/3.6/4.5, 82.3/10/3.2/4.5 and 82.7/10/2.8/4.5, which were dissolved in a mixture of DMAc/cyclopentanone (v/v, 4/6) to form solutions with a solid content of 30 wt%.After the pattern of a peacock was printed, the solvents were removed at 80°C for 8.0 min.Finally, the CLCN pattern was obtained under the irradiation of the high-pressure Hg lamp for 10 s.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Figure 1 .
Figure 1.Molecular structures of the compounds in the CLC mixtures.
The positive DRCD signals indicated a right-handed helical supramolecular structure.Namely, these CLCN films can selectively reflect right-handed circularly polarised lights.

Figure 3 .
Figure 3. (Colour online) Photographs of the CLC mixtures (a) before and (b) after polymerisation.UV-vis spectra of the CLC mixtures (c) before and (d) after polymerisation.

Figure 5 .
Figure 5. (Colour online) (a) Schematic diagram of the two-step preparation process of a CLCN film.(b) UV-vis spectra of the CLCN film prepared through the two-step approach.

Scheme 1 .
Scheme 1. (Colour online) Schematic representation of the structural transition of the two-step approach.

Figure 6 .
Figure 6.(Colour online) (a) POM image of the 1D grating taken in reflection mode, and pictures of (b) a butterfly pattern prepared using a photomask, (c) a flower drawn by hand and (d) a peacock pattern prepared by inject printing.