Influence of the length of end-group double bond substituents on the temperature change of phase transition in liquid crystals

ABSTRACT A series of nematic liquid crystal (LC) monomers, containing a reactive group in the lateral substituent, were designed and synthesised through an organic synthesis reaction. Considerable changes were made to the properties of the liquid monomers by varying the terminal alkyl chain lengths. The molecular structures of the intermediates and the LC monomers were characterised by flourier infrared absorption spectrometry and proton nuclear magnetic resonance (1H NMR) spectroscopy, leading to the identification of the target product. The phase transition temperatures of the LC monomers were observed and analysed using differential scanning calorimetry (DSC) and polarising optical microscopy (POM) with a hot stage. The two-way thermotropic liquid crystalline behaviour was observed during the heating process, providing insight into the effect of substituent changes on the phase transition temperature. Graphical abstract


Introduction
The application of liquid crystals (LC) in active-matrix display technologies and membrane applications has fostered their synthesis and integration as functional materials [1][2][3][4][5].Double-bonded liquid crystals are noteworthy for their exceptional low-temperature stability, melting points, and reduced viscosity.The reactivity of the double bond is also significant [6].These materials can be combined with polymethylhydrosiloxane (PMHS) to create side-chain liquid crystal elastomers through hydrosilylation.However, little is known about liquid crystal monomers that have a double bond on the side chain, as it is typically found in the terminal substituent [7].The location of the waist attachment allows liquid crystal elastomers to respond more quickly to external changes than traditional side-chain liquid crystal polymers [8][9][10].Concurrently, side alkyl chains in low molecular weight liquid crystals have garnered considerable interest [11][12][13].They have also re-emerged in the design of new ferroelectric dielectrics [14][15][16][17].
This report discusses the synthesis of novel doublebonded liquid crystal monomers, liquid crystal molecules with double-bonded substituents containing the number of methylene groups n = 1,2,4 and terminal alkyl chains containing the number of methylene groups m = 1,2,4.Some monomers exhibit isotropic nematic mesophases, while others display monotropic nematic mesophases during cooling.We investigated the influence of substituents on the phase transition temperature.The implications of this discovery could extend to potential designs of liquid crystal formations.

Experimental
Liquid crystals were synthesised through organic processes following the same steps detailed in our previous work [18].As shown in Figure 1(a), as an example, X 11 , protocatechuic acid as raw material was reacted with alcohol and concentrated sulphuric acid to obtain ethyl protocatechuic acid, ethyl protocatechuic acid was reacted with 1-bromooctane to obtain ethyl 2-hydroxy-4-octyloxybenzoate, and then reacted with allyl bromide to obtain ethyl 2-allyloxy-4-octyloxybenzoate, and 2-allyloxy-4-octyloxybenzoic acid was treated with NaOH to obtain 2-allyloxy Octyloxy-4-octyloxybenzoic acid, reacting 2-allyloxy-4-octyloxybenzoic acid and p-ethylcyclohexylphenol to obtain the target product X 11 , and X nm was prepared in a similar manner to X 11 .

Result and discussions
The NMR hydrogen spectral data and IR spectra suggest consistency between the chemical shifts and characteristic IR peaks of the product and the target product.The pertinent data can be found in the supplementary documentation.
The mesomorphic properties of our targets, called X nm , were investigated in detail using a polarisation microscope (POM) equipped with a heating and cooling stage and a differential scanning calorimeter (DSC).Figure 2(b) displays the DSC curves of the X nm compounds, captured during the initial heating and cooling cycle at a rate of 10°C/min.Additionally, Figure 2(a) presents optical polarising micrographs of X nm .For clarity, we refer to the heating process as 'h' and the cooling process as 'c'.
As illustrated in Figure 2(b), the compounds X 11 , X 12 , X 22 , X 24 , and X 44 displayed two endothermic peaks during the heating process.Observations made through a polarising microscope confirmed the occurrence of a phase transition.The optical textures indicative of the standard nematic phase for compounds X 11 , X 12 , X 22 , X 24 , and X 44 are depicted in Figure 2(a).The evidence suggests a transition of these compounds from a crystalline state, through a liquid crystal phase, and ultimately into an isotropic state.
Furthermore, the transition temperature of compound X 14 from the liquid crystal phase to the isotropic phase was observed using a polarising microscope, as depicted in Figure 2(a).The cooling process yielded two exothermic peaks for all compounds, except for X 11 and X 14 , as depicted in Figure 2(b).The phase transition was identified through observations made under a polarising microscope, with the optical textures corresponding to the characteristic nematic phase.
It is worth noting that the compounds exhibited similar phase-transition behaviours.Simultaneously, the transition temperatures of compounds X 11 and X 14 , from the isotropic phase to the liquid crystal phase, and from the liquid crystal phase to the crystalline state, were observed, as depicted in Figure 2.
The associated enthalpy values of compounds X 11 and X 14 were relatively small, hence undetectable by the instrument.This results in a reduction of order in the nematic phase, thereby diminishing the strength of the transition [19][20][21].T The target products X 11 , X 12 , X 14 , X 22 , X 24 , and X 44 displayed an enantiotropic nematic mesophase, undergoing a crystal-tomesophase and mesophase-to-isotropic transition during heating and cooling cycles.During the cooling process, compounds X 21 , X 41 , and X 42 exhibited a monotropic nematic phase.
When the double bond substituted group, n = 1, the transition temperature of the nematic mesophase to the isotropic liquid increases with the lengthening of the alkyl chain upon heating.This phenomenon might be ascribed to the addition of an alkyl carbon, which intensifies the intermolecular forces.When the alkyl chain, m = 4, the transition temperature of the nematic mesophase to the isotropic liquid first increases and then decreases during heating as the double bond substituted group increases.As the alkyl chain lengthens, intermolecular forces increase, resulting in a relative decrease in flexibility.Once the length of the alkyl carbon increases to an appropriate length, the intermolecular forces will decrease.
The phase-transition temperatures of the target products (X nm ), which are tabulated in Table 1, result from a thorough analysis of Figure 2. As the alkyl chain length increases, it is noteworthy that the melting point of the compounds (X nm ) initially increases and then decreases (T m (X 11 ) < T m (X 12 ) > T m (X 14 ); T m (X 21 ) < T m (X 22 ) > T m (X 24 ); T m (X 41 ) < T m (X 42 )> T m (X 44 ).This phenomenon is attributed to the interplay between molecular weight and flexibility.Specifically, the addition of one alkyl carbon can increase intermolecular forces, thus relatively reducing flexibility.Once the of the alkyl carbon was increased to a suitable length, the intermolecular forces were weakened and showed

Conclusions
This study successfully synthesised a series of ester liquid crystal monomers, characterised by double bonds in the side chain.The structures of these monomers were characterised using Fourier transform infrared, and nuclear magnetic resonance spectroscopy.Compounds X 14 and X 24 among these, displayed an enantiotropic nematic mesophase with a high aspect ratio.On the other hand, X 12 , X 22 , X 42 , and X 43 exhibited a monotropic nematic mesophase upon cooling.The study observed that a single methylene increases in the terminal substituent primarily affected the rise in the melting point due to changes in molecular weight.However, the addition of two or more methylene groups to the terminal substituent resulted in the molecule's flexibility becoming a key determinant of melting point variations.Essentially, enhanced flexibility was found to correspond to a lower melting point.It was found that for X nm , the melting point decreased as the length of the lateral substituent increased, provided that the molecule's long axis had sufficient flexibility.

Figure 2 .
Figure 2. (Colour online) (a) Polar optical microscope image of X nm and (b) DSC curve of X.