Cholesterol based mesogenic Schiff’s base derivatives with carbonate linkage: Synthesis, characterisation and photoluminescence study

ABSTRACT The steroidal derivatives have been found to be extremely good mesogens since their origin. Due to their inherent chirality, they have the potential to induce a wide variety of liquid crystalline phases, including frustrated phases, depending on the structure of the steroidal skeleton and the substituents attached. In this report, thirteen new homologous Schiff’s base derivatives were synthesised by condensing 4-n-alkoxy aniline with 4-formyl phenyl cholesteryl carbonate. All the compounds were characterised using elemental analysis, FT-IR, 1H-NMR and 13C-NMR. In order to study the liquid crystalline behaviour of the synthesised compounds, optical texture studies were carried out using polarising optical microscope in heating and cooling cycles. The derivatives showed a variety of mesophases, including chiral nematic (N*), twist grain boundary-A (TGBA), smectic A (SmA) and chiral smectic C (SmC*) phases. The thermal behaviour was determined using a differential scanning calorimeter and thermogravimetric analysis. All the synthesised compounds are UV-active and show photoluminescence in the blue emission region with good quantum yield. GRAPHICAL ABSTRACT


Introduction
Mesogens made of cholesterol are the first liquid crystals that have ever been discovered.Since they were the first species discovered more than a century ago and because their derivatives are probably the most studied mesogens, they continue to hold promise for fresh research in a variety of fields from the development of electronic devices to optical technology [1,2].Cholesterol-based liquid crystals have gained considerable attention from researchers not only due to their natural and commercial availability but also because the helical supermolecular structure of cholesterol-based liquid crystals imparts some unique optical properties, such as selective reflection of circularly polarised light, high optical rotatory power, circular dichroism and electro-optic effect [3].Additionally, because these characteristics vary with variables like temperature, pressure and electric field, they might find use in optical storage, colour display technology, and full-colour rewritable recording systems [4][5][6].
The synthesis of different cholesteryl esters, ethers, carbonates, carbamates, etc. is made possible by the C3hydroxyl group of cholesterol.These substances are frequently used in pharmaceuticals, the chemical industry, and toiletries and cosmetics [7].Such molecules have a propensity to aggregate into sizeable threedimensional structures in which the location and orientation of the molecule are organised due to both the stiff CONTACT R. C. Tandel rctandel-appchem@msubaroda.ac.inSupplemental data for this article can be accessed online at https://doi.org/10.1080/02678292.2023.2260773.
Cholesterol has been widely used in chiral liquid crystalline materials due to its rigid structure with eight chiral centres and the simplicity with which the structure can be derivatised.They have the ability to generate a broad range of liquid crystalline phases, including frustrated phases, due to their intrinsic chirality, depending on the structure of the steroidal skeleton and the substituents attached [2,30].The frustration of the phases is generally observed only in chiral systems with high enantiomeric excess and strong molecular chirality (short pitch).Despite the fact that many conventional (over 3000) monomeric liquid crystals consisting of cholesteryl ester unit as a chiral part of the molecule have been reported, there are only a few cholesterol-based mesogens exhibiting the TGB A or related phases [31][32][33][34].Many researchers have created thousands of monomers, oligomers and polymers based on cholesterol due to the substance's capacity to induce a liquid crystalline characteristic in its many derivatives [35].
We hereby report the synthesis, characterisation, mesomorphic and thermal studies of newly synthesised cholesteryl carbonate-based liquid crystal with Schiff's base linkage and terminal alkoxy chains.

Materials
Cholesteryl chloroformate was purchased from Sigma-Aldrich Chemicals, USA.4-Hydroxy benzaldehyde, paracetamol, alkyl bromides were purchased from Loba Chemie Pvt. Ltd.Tetrahydrofuran (THF), acetone and ethanol underwent drying treatment with standard methods.All other solvents and reagents were AR grade and used without further purification.

Techniques and measurements
The synthetic route adopted for the series of mesogens is given in Scheme 1.The structures of the compounds were determined using standard spectroscopic methods.Thin-layer chromatography (TLC) was performed on silica gel plates (Merck).FT-IR spectra were recorded on a Bruker spectrometer as KBr Pellets. 1 H NMR and 13 C NMR spectral data were recorded on an Avance Bruker 400 spectrometer (400 MHz) with Deuterated Chloroform (CDCl 3 ) as solvent and TMS as Internal Standard.The polarised optical microscopy (POM) study was observed with a Nikon Eclipse Ci-Pol microscope equipped with a Linkam (Linkam, Surrey, England) heating stage.Phase transition temperatures and thermodynamic parameters were determined by using differential scanning calorimetry (DSC-822, Mettler Toledo, having Stare software).The heating and cooling rates were 10°C/min.The instrument was calibrated using indium as a standard.The thermal stability of the compounds was recorded using a Thermogravimetry Analyzer (TGA-50, Shimadzu, Japan) with 3-7 mg of the sample in platinum pan at a heating rate of 10°C/min.Thermo Finnigan's (Flash 1112 series EA) CHN analyser was used to carry out elemental analyses.Absorbance was recorded using a Shimadzu UV-1800 spectrophotometer (Japan).Photoluminescence and quantum yields (Φ PL ) were measured using a Shimadzu RF 6000 Spectro fluorophotometer (Japan).

Synthesis and characterisation
The synthetic routes of the mesogenic Schiff's base are shown in Scheme 1.

Synthesis of 4-formyl phenyl cholesteryl carbonate (4)
In a round-bottom flask, cholesteryl chloroformate (0.01 mol) (3) and dry THF (50 ml) were taken.To the reaction mixture, 4-hydroxy benzaldehyde (0.011 mol) (2), anhydrous pyridine (4.0 ml) and an additional dry THF (40 ml) were added.After stirring for 2-3 h at 38-40°C, the mixture was filtered to isolate pyridinium chloride and other solids.The filtrate was concentrated under vacuum to remove excess THF, and n-hexane was added.After filtration, the resulting precipitates were crystallised from an ethyl acetate-methanol mixture (70:30) until a consistent transition temperature was obtained.This compound (4) was found to be mesogenic in nature.

Synthesis of 4-n-alkoxy acetanilide (6 a-m)
In a 100 ml three necked round-bottom flask, paracetamol (0.1 mol) (5), anhydrous potassium carbonate (0.15 mol) and dry acetone (60 ml) were taken.The reaction mixture was heated to 70-80°C and stirred.To the warm solution, the appropriate alkyl bromide (0.15 mol) was added dropwise over 1 h.The mixture was then refluxed with continuous stirring at 70-80°C for 8 h.After cooling down to room temperature, the mixture was diluted with cold water.The solid mass obtained was filtered, washed with water and directly used for hydrolysis [46].Yield: 80-90%.

Synthesis of 4-n-alkoxy aniline (7 a-m)
A mixture of 4-n-alkoxy acetanilide (0.014 mol) (6 a-m), water (7 ml) and concentrated hydrochloric acid (4.5 ml) was stirred for 10 to 12 h at 90-95°C and then cooled to room temperature.The mixture was made alkaline with 50% sodium hydroxide solution at 20°C.The oily/waxy product was extracted with diethyl ether.After evaporation of solvent on a rotary evaporator, the corresponding products were obtained as pale-yellow oils and higher member were obtained as light brown solids.Boiling/melting point agrees with the reported value [46].Yield: 75-80%.

Result and discussion
The final Schiff's bases (1a-m) were prepared by condensation of 4-formyl phenyl cholesteryl carbonate (4) with the appropriate 4-n-alkoxy aniline (7a-m).The synthetic route adopted is given in Scheme 1.All the compounds in the series exhibit mesomorphic properties.The methyl to n-butyl derivatives of Series I show oily streaks texture of cholesteric (chiral nematic) phase.The n-pentyl and n-hexyl derivatives show oily streaks of the N* phase, while on cooling from isotropic melt, in addition to the cholesteric phase, they also show the SmA mesophase.The n-heptyl to n-decyl derivatives show enantiotropic SmA-N*-Iso transition.The SmA to N* transition is accompanied by an interceding TGB A phase.The n-dodecyl derivative shows only enantiotropic SmA phase in both heating and cooling cycles.The higher derivatives from n-tetradecyl to n-octadecyl show enantiotropic SmC*-SmA-Iso transition.The DSC thermograms are in comparable agreement with the polarising optical microscopy.Compounds were also thermally stable up to 330-350°C as analysed thermogravimetrically.
The FT-IR and NMR spectra, as well as the elemental analyses, are in complete agreement with the structure.
All other members of the series (1b-1 m) exhibit similar IR, NMR ( 1 H and 13 C) bands and are listed in the synthesis.

Texture analysis
To investigate the liquid crystalline behaviour of the synthesised compounds, the compounds were sandwiched between two untreated glass slides, and optical texture investigations were performed on heating and cooling cycles using an optical polarising microscope equipped with a heating stage.During the repeated heating and cooling processes, all the compounds remained stable.All the synthesised compounds are mesogenic in nature.In the series of compounds, the lower homologous from 1a-1d melted and exhibited a typical oily streaks texture of cholesteric [47] (chiral nematic mesophase) phase Figure 1(a) that maintained until the compounds became isotropic.As it cooled from isotropic melt, the droplet cholesteric texture exhibited and gradually coalesced into the cholesteric schlieren texture.The texture did not disappear until the sample crystallised.It belonged to thermotropic enantiotropic chiral nematic LCs.Compounds 1e and 1f on heating showed oily streaks of the cholesteric phase until isotropic, similar to 1a-1d while on cooling from isotropic melt, in addition to the cholesteric phase, a dark-field view of the SmA mesophase was also observed, which proceeds through a TGB A phase.The compounds 1g-1i displayed an enantiotropic N* and smectic phase seen as black area because here the optic axis is oriented along the direction of light propagation [48], with an interceding TGB A phase.The Cr-Sm transition is easily visible in the DSC thermogram, while the transition involving the Sm-TGB A mesophase Figure 1(b), which is associated with the minimal enthalpy change, is not witnessed in the DSC thermogram, but this transitory TGB A phase is evidenced optically by their typical filament texture as observed under the cross polarisers [49,50].The dodecyl derivative 1j exhibited only an enantiotropic SmA phase Figure 1(c).The higher derivatives of the series 1k-1m on the heating cycle first exhibited the SmC* mesophase, which on further heating changes from tilted helical structure to the untwisted parallel structure of the SmA phase observed as fan shaped texture and maintained until isotropic.On cooling from isotropic melt, the compounds exhibited SmA phase Figure 1(d), even though not observed from DSC; on further cooling from SmA phase under POM, these compounds exhibited broken fanshaped texture with equidistant line pattern due to the helical superstructure resembling SmC* mesophase Figure 1(e) throughout a somewhat large temperature range which resembles the textures observed in the literature [51][52][53].

Thermal properties
The phase transition temperatures, enthalpy variations, and mesophase textures of the pure compounds 1a-m are summarised in Table 1.DSC curves (S43-S45) and POM observations provided clear transition temperatures and textures for all of the compounds, and they were in reasonable agreement with each other over the many heating/cooling cycles.Compounds 1a-d displayed two endotherms from crystalline solid to chiral nematic phase (Cr-N*) and chiral nematic phase to isotropic liquid phase (N*-Iso) on heating.Similarly, on cooling, two exotherms were observed, corresponding to isotropic liquid phase to chiral nematic phase (Iso-N*) and chiral nematic phase to crystalline solid state (N*-Cr).Compounds 1e and 1f showed two endotherms from the heating cycle and three exotherms on cooling cycle, while on heating, the first transition was observed from crystalline solid to chiral nematic mesophase (Cr-N*), and the second transition was observed for chiral nematic mesophase to isotropic liquid phase (N*-Iso).In the cooling cycle, three exotherms correspond to isotropic liquid phase to chiral nematic phase (Iso-N*), nematic to smectic A phase (N*-SmA) interceding with TGB A, and smectic A phase to crystalline solid (SmA-Cr).Compound 1g-i showed three exotherm and three endotherm peaks corresponding to crystal to twist grain boundaries A-smectic A (Cr-TGB A -SmA), twist grain boundaries A-smectic A-chiral nematic (TGB A -SmA-N*) and chiral nematic-isotropic (N*-Iso) in both heating and cooling cycles.Compound 1j exhibited enantiotropic mesophase with two endotherm and two exotherm peaks for the crystal-smectic A (Cr-SmA) and smectic A-isotropic (SmA-Iso) transitions.Compound 1k-m showed two endotherm and two exotherm peaks for crystal-smectic A (Cr-SmA) and smectic A-isotropic transition (SmA-Iso).An additional SmC* phase was observed for 1k-m using polarising optical microscopy but was not detected in DSC as it is associated with the minimal enthalpy change.
Enthalpy changes are fairly predicted during the initial transition of crystal-chiral nematic/smectic A for all of the compounds indicated in Table 1.The enthalpy changes during the chiral nematic-isotropic transition, on the other hand, are smaller than predicted.Again, this is to be expected with these sorts of mesogens [54].
The Smectic A (SmA) and TGB A (twist grain boundary A) phases typically do not show an enthalpy change because the transitions between these phases are due to competing intermolecular interactions and strong molecular chirality, as observed in the thermograms of compounds 1e-i.Likewise, for the transition from Smectic A (SmA) where the molecules are uniformly aligned to each other in the layers to Smectic C* (SmC*) where they exhibit a tilted orientation, the primary change occurs in Temperature in parenthesis ( ) indicates monotropic transition; Cr = crystal, SmA = smectic A phase, SmC* = chiral smectic C phase, N* = chiral nematic/ cholesteric phase, TGB A = twist grain boundary A phase, I = isotropic phase.a Phase transition temperatures were determined by both Polarising optical microscope (POM) and differential scanning calorimetry (DSC) studies: peak temperatures in the DSC thermograms obtained during the first heating and cooling cycles (scanning rate = 5°C min −1 ) coupled with POM measured temperatures are given; b Transition temperatures of some of the compounds were determined with the aid of a POM study as the expected well-resolved thermograms of both heating and cooling cycles could not be obtained; c Although TGB A -SmA/SmA-TGB A phase transitions were observed in POM, they were not resolved in DSC traces; hence the enthalpy value represents the combined enthalpy for TGB A -SmA/SmA-TGB A transitions; d Phase transition was observed under POM; enthalpy change too weak to be detected by DSC.ing optical microscope (POM) and differential scanning calorimetry (DSC) studies: peak temperatures in the DSC thermograms obtained during the first heating and cooling cycles (scanning rate = 5°C min−1) coupled with POM measured temperatures are given; bTransition temperatures of some of the compounds were determined with the aid of a POM study as the expected well-resolved thermograms of both heating and cooling cycles could not be obtained; cAlthough TGBA-SmA/SmA-TGBA phase transitions were observed in POM, they were not resolved in DSC traces; hence the enthalpy value represents the combined enthalpy for TGBA-SmA/SmA-TGBA transitions; dPhase transition was observed under POM; enthalpy change too weak to be detected by DSC.
the molecular arrangement and orientation within the layers rather than a disruption or forming of the interactions, which does not involve a substantial enthalpy change as observed in thermograms for compound 1k-m.Compounds 1a,1c,1d,1 g,1 h,1j and 1 l were subjected to thermogravimetric analysis (TGA) in order to comprehend the thermal stability.While examining the mesomorphic behaviour, it was discovered that all the cholesterol derivatives were stable up to 330-350°C, ruling out the possibility of thermal decomposition of these derivatives (Figure 2).However, due to extremely high isotropic temperatures, repeated heating and cooling of the cholesterol derivatives caused decomposition.

Structure-mesomorphic property relationship
A graphical representation of the transition temperature as a function of the number (n) of carbon atoms in the alkoxy chain is given in Figure 3. Based on Figure 3 and N*/SmA/SmC*-Iso were obtained.The N*/SmA/ SmC* to isotropic transition temperatures exhibits a sharp falling tendency and shows distinct odd-even effect only for the first five members of the series, while the SmA to N* transition shows an ascending trend with increasing terminal chain length.The SmC* to SmA transition shows a rising tendency with increasing terminal chain length.The curve of the Cr to first mesophase rises to maximum till the dodecyl, derivative and then a falling tendency is observed till the octadecyl derivative except for the heptyl derivative, which may be due to the commencement of the SmA phase.The nematicisotropic transition temperature tends to decrease as the terminal chain length increases in Series I compounds, and the onset of the smectic phase from middle member homologues is also a common trend.In a system like this, this specific pattern is expected [55,56].Lower homologues have the least separation of aromatic nuclei and the highest terminal cohesions, resulting in entirely nematogens.As we progress through the series, the lateral cohesive forces grow and the molecules align themselves in the layered structure before entering the nematic phase.As terminal intermolecular interactions are relatively weak to preserve the parallel molecular orientation, smectogenic properties should predominate at the expense of nematic phase stability as terminal chain length increases.Organic compounds have a close relationship between their molecular structure and their liquid crystalline characteristics, which in turn influences their thermal stability.One can establish a correlation between these compounds' liquid crystalline characteristics and a measurement of thermal stability by looking at their molecular constitution.The average thermal stabilities of different mesogenic series are compared in Table 2.
Comparison of molecular structure of present Series I with reported series: (1) 4,4´-alkoxy benzylidene amino phenyl cholesteryl carbonate; Series I (2) Cholesteryl 4-n-alkoxybenzylidene-4´aminobenzoates; Series A [57] (3) Cholesteryl 4-(4´-n-alkoxy benzoyl) amido benzoates; Series B [58] The geometry of these series is given in Figure 4.The molecules of Series I have the formyl cholesteryl carbonate moiety linked to n-alkoxy aniline, forming a Schiff 's base linkage, while in Series A the p-aminobenzoate of cholesterol is linked to n-alkoxy benzaldehydes to form the Schiff 's base linkage.The higher N*-Iso thermal stability of Series A can be attributed to the central ester linkage compared to carbonate in Series I. Compared to esters, carbonates often exhibit lower mesophase temperatures and a wider range of mesophases due to the carbonate group's relatively flexible character compared to the esters more rigid structure.While the higher thermal stability of the SmA/SmC*-N* of Series I may be due to the interceding TGB A mesophase.
The compound in Series I differs from Series B not only in the carbonate and ester linkage but also the reversible central azomethine linkage is replaced by an amide linkage.In comparison to carbonates, the molecule is more rigid due to the presence of the amide group.Due to resonance, the amide bond exhibits partial double bond characteristics, which restrict free rotation around the double bond.The mesomorphic characteristics of amides are impacted by this enhanced rigidity.A majority of the time, amides exhibit higher mesophase temperatures than carbonate, which is the case.As the length and symmetry of the substituents increase, the likelihood of the development of ordered mesophases increases, which is also evident as the SmA/ SmC*-N* thermal stability of Series B is very close to that of Series I.
As evident, the N*-Iso average thermal stability of Series B is the highest among all three series, further confirming that amide linkage is more conducive to mesomorphism as compared to azomethine linkage, while the thermal stability of Series I is the lowest, which may be due to the more flexible carbonate linkage.The forgoing discussion indicated that a slight modification in the linking groups in the molecular structure can adversely affect the mesomorphic properties of the system.

Optical properties
The ultraviolet-visible absorption spectra of the compounds 1a-m in CHCl 3 solution are shown in Figure 5, and their corresponding photophysical data are mentioned in Table 3. Due to their structural similarity, the absorption spectra of compounds 1a-m were all quite similar in shape.In absorbance spectra, the λ max of all the compounds occurred near 339 nm.The highest absorption peaks of all the compounds were found to be near 269 nm.The band at higher energy (269 nm) is due to π-π* transition of benzene ring [59], while the band at lower energy (339 nm) is due to n-π* transition of the azomethine linkage (CH=N), with considerable charge transfer characteristic [60,61].With the increase in the alkoxy chain length, a slight red shift is observed in most of the compounds.The absorbance is maximum in the case of compound 1e and for compound 1k the absorbance is a minimum.
Photoluminescence (PL) is an important property of liquid crystalline materials used in displays [62].The photoluminescence spectra of synthesised compounds were done in CHCl 3 (1×10 −5 M) at wavelengths 350-750 nm with an excitation wavelength of 340 nm to investigate the link between structure and spectral characteristics.In all analyses, the emission and excitation bandwidths were set to 5 nm.All the compounds 1a-m are blue emitters in solution and exhibit essentially the same luminescence profile with three emission peaks as shown in Figure 6, and their corresponding photophysical data are mentioned in Table 3.They emit strong fluorescence at wavelength of 400-470 nm.The presence of 4-n-alkoxy group at the terminal end, along with a carbonate linkage, induces an intramolecular charge transfer (ICT) phenomenon in the same direction.The addition of the carbonate group and Schiff's base within the cholesterol moiety impacts this ICT effect, leading to the generation of a fluorescence emission spectrum [63].With the gradual increase in chain length of the alkoxy group, fluorescence intensity increases continuously.The fluorescence quantum yields in CHCl 3 (1 × 10 −6 M) were also investigated, showing a broad variance between 0.32 and 0.51 compared to the standard (quinine sulphate dissolved in 1 N sulphuric acid; Φ PL = 0.546) suggesting their good fluorescence properties, which not only reveal its unique molecular dynamics but also point towards its significant potential in applications related to optoelectronics.The higher homologues in the series lead to a stronger emission and a higher fluorescence quantum yield.The displayed stokes shifts are nearly the same for all the compounds, ranging from 67 to 73 nm (Table 3).
In conclusion, all of the synthesised compounds have PL spectra that show emission in the blue region = 400-470 nm, indicating that all of the materials have blue light emission properties that can be used in potential applications such as OLED materials, biotags for biological sensing applications and fluorescent probes in biological applications [64][65][66].

Conclusion
In conclusion, we have synthesised thirteen new homologous Schiff's base derivatives by condensing 4-n-alkoxy aniline with 4-formyl phenyl cholesteryl carbonate.All of the final derivatives 1a-m show mesomorphic properties.The methyl to n-hexyl derivatives show an enantiotropic chiral nematic mesophase except for the n-pentyl and n-hexyl derivatives, which also exhibit an additional TGB A -SmA monotropic mesophase on cooling.The n-heptyl to n-decyl derivatives show enantiotropic TGB A -SmA and N* phase transitions.The n-dodecyl derivative shows only the enantiotropic SmA phase, while the n-tetradecyl to n-octadecyl show the enantiotropic SmA and SmC* mesophases.In the plot of transition temperature versus number of carbon atoms in the alkoxy chain, the smectic A to chiral nematic curve rises to maximum, while the Smectic C* to Smectic A shows a falling tendency.The chiral nematic/SmA/SmC*isotropic curve shows the usual falling tendency.All the compounds are UV-active and show PL in the blue  emission region.With increasing terminal chain length, stronger emission and the higher fluorescence quantum yield was observed.The investigation of their application aspects would be quite fascinating.

Figure 1 .
Figure 1.(Colour online) Microphotographs of the textures observed under POM for the different LC phases of compounds placed between two untreated glass substrates (a) oily streaks texture of cholesteric or chiral nematic (N*) phase of 1b at 179°C, (b) SmA-TGB A -N* transition of 1 h at 256°C, (c) focal conic texture of SmA mesophase of 1j at 250°C, (d) fan shaped texture texture of SmA mesophase of 1 l at 249°C, (e) broken fan shaped texture with helix lines of SmC* phase of 1 l at 113°C, (f) filament texture of SmA-TGB A -N* transition of 1i at 260°C.

Figure 2 .Figure 3 .
Figure 2. (Colour online) TGA profile for some of the cholesterol derivatives.

Figure 4 .
Figure 4. Comparative geometry of series I, A and B.

Table 2 .
Average thermal stabilities (°C) of series I, a and B compounds.

Table 3 .
UV and fluorescence peaks for compounds 1a-m.Determined using quinine sulphate as standard (Φ PL = 0.546 in 1N H 2 SO 4 ). a