Unsymmetrical liquid crystalline dimers containing biphenyl moiety: synthesis, characterisation, mesomorphic study and DFT studies

ABSTRACT In the current study, two new unsymmetrical liquid crystalline homologous series of dimers with a rigid biphenyl core were synthesised using an ether-linking group. Several spectroscopic methods, including FT-IR, 1H-NMR, 13C-NMR, elemental analysis and mass spectrometry, were used to describe the synthesized dimers. Differential scanning calorimetry (DSC) and polarised optical microscopy (POM) were used to assess their mesomorphic properties. The results showed that the number of methylene spacers significantly affected the thermal properties of the synthesized dimers. To ascertain photophysical behaviour, UV–Visible spectroscopy was employed. Nematic-type mesophase patterns with good mesophase range and thermal stability were seen in both series. Dimers with flexible spacers (n = 2, 4, 6) had the best liquid crystalline nature out of all the dimers. The relationships between structure and its mesogenic property, the effects of various flexible spacers, the mesophase ranges, and the photo-isomerization of liquid crystalline dimers were all investigated. DFT Studies were carried out to gain a better understanding of structural features. Graphical abstract


Introduction
Liquid crystal (LC) oligomers have gained a lot of research attention among the wide range of low molar mass molecules that are known to promote liquid crystallinity and can exhibit a variety of mesophases, including nematic, smectic and cholesteric phases [1][2][3].Liquid crystal dimers are the most basic oligomers which is made up of two identical or different mesogenic units connected by a flexible spacer.The majority of LC dimers have typically unique LC characteristics that set them apart from the individual mesogenic components, making them a valuable source for the identification of novel LC phases [4].The most researched LC dimers are calamitic or rod-shaped LC dimers due to their ability to align with an electric field, these rod-like dimers are frequently used in the creation of liquid crystal displays (LCDs) [5].Calamitic dimers can be grouped into five categories based on the various potential relative positions of both rods.Linear calamitic [6][7][8][9][10] LC dimers consist of two rod-shaped mesogens joined together by a flexible spacer.H-shaped [11][12][13][14], T-shaped [15,16], U-shaped [17,18] and O-shaped [19] calamitic LC dimers are also well known.
Azobenzene is a well-known motif among the chromophores known to exhibit liquid crystalline behaviour [20].Due to their proper chemical balance and capacity for reversible photoisomerization, azobenzene compounds have recently attracted a lot of attention.The liquid crystalline dimers with biphenyl and azo benzene segments connected by flexible methylene spacer were reported by Subala et al. [21].Imrie et al. [22][23][24][25] reported many symmetrical and unsymmetrical dimers containing cyanobiphenyl moiety and azobenzenes containing various terminal groups and different flexible spacers.
The inclusion of a biphenyl group imparts structural rigidity to the molecule, a feature especially significant within liquid crystalline dimers.A diverse range of LC dimers, encompassing the biphenyl motif, have been documented in scientific literature.Mohammad et al. [26] reported unsymmetrical calamitic compounds containing heterocycles with biphenyl cores at one end and 6-amino-1,3-dimethyluracil units at the other end.Onepot synthesis of some new mesogenic biphenyl derivatives were reported by Gibb et al. [27].Cruickshank et al. [28] and Arakawa et al. [29] reported many series of sulphur-linked liquid crystalline dimers containing biphenyl core which includes methylene, ether, and thioether units in a spacer that has undergone systematic variation.Imrie et al. [30] have reported homologous series of LC dimers known as, α,ω-bis(4-cyanobiphenyl-4′-yl) alkanedioates which are composed of two cyanobiphenyl units connected by an alkyl spacer attached via ester linkages.Investigation of the phase behaviour and characteristics of 1,7-bis(4-cyanobiphenyl-4′-yl)heptane (CB7CB) was reported by Cestari et al. [31].Grunwald et al. [32] synthesised a series of cyanobiphenyl dimers connected to a central malonate unit via alkoxy spacers.Arakawa et al. [33] discovered a developing helical twist-bend nematic (N TB ) phase in fluorenone-incorporated LC trimers which have a core of 2,7 disubstituted fluorenone and bilateral cyanobiphenyl groups with ether and thioether linkages.Recently, Mohammad et al. [34] reported new liquid crystalline dimers containing coumarin and biphenyl moieties.Biphenyl derivatives with a central unit based on oligomeric 1,6-Hexamethylene diisocyanate have been synthesised by Gorbachev et al. [35].
The inclusion of Schiff base or imine linkages within a molecular structure offers dual benefits of enhanced stability and increased structural rigidity.This characteristic is particularly evident in numerous instances of liquid crystalline dimers that feature a biphenyl segment combined with azomethine linkages.These occurrences have been extensively documented in prior research [36][37][38][39][40].
Previously, we have reported the mesogenic biphenyl derivatives [41] with azo and ester linkages.Until now, numerous LC dimers have been reported featuring a 4-substituted biphenyl unit, encompassing cyano or alkoxy functional groups as a substituent.In contrast, our current series revolves around biphenyl core devoid of any substitutions, intriguingly exhibiting mesogenic properties.So the goal of the current work is to synthesize two homologous series of liquid crystalline dimers with a rigid biphenyl core and (E)-1-(4-butylphenyl)-2-phenyldiazene (series 1-BpnAz)/(E)-N-(4-butylbenzylidene)aniline (series 2-BpnSB) moiety with an ether linking group.The synthesized dimers were characterized using various spectroscopic techniques, such as FT-IR and NMR, and photophysical properties were studied by UV-Visible spectroscopy.The effect of the number of flexible methylene spacers (n = 2, 3, 4, 5, 6) on transition temperatures and their thermal stability were studied.For the series we have synthesised, with the aid of density functional theory (DFT) calculations, a variety of theoretical parameters were examined.

Measurements
Thin-layer chromatography (TLC) was used to separate organic compounds using Kiesel 60F254 silica gel precoated on aluminium sheets.A Bruker Alpha II spectrometer was used to gather the Fourier Transform Infrared Spectroscopy (FT-IR) spectra of compounds as pellets of potassium bromide (KBr).On an Advance Bruker 400 spectrometer, proton nuclear magnetic resonance ( 1 H-NMR) and carbon-13 nuclear magnetic resonance ( 13 C-NMR) spectrum data were obtained using deuterated chloroform (CDCl 3 ) solvent and internal standard tetramethyl silane (TMS).ESI-MS was used to determine mass spectra using a Shimadzu LCMS 2020 equipment.Thermosinnigan Flash 11-12 series EA was used to record elemental analyses.The optical textures of dimers were examined on the Leica DM 2500P POM.Differential Scanning Calorimetry (DSC) instrument from Mettler Toledo with Stare software was used to determine the transition temperatures and enthalpies (in kJ mol −1 ).All measurements were made in N 2 atmosphere with a heating and cooling rate of 10°C min −1 .We conducted TG-DTA analyses using the SII EXSTAR6000 TG-DTA instrument.The experiments were carried out in an N 2 atmosphere between 30°C and 550°C using an aluminium pan and a heating rate of 10°C min −1 .UV-visible spectra were recorded on PerkinElmer UV-Vis Spectrometer Lambda 35 with WinLab software.The molecular properties in the current work are computed using the DFT with method B3LYP, basis set 6-31 G (d, p), polarised, and diffuse functions with the aid of the Gaussian 09 computer programme.

Synthesis of unsymmetrical dimers (BpnAz/ BpnSB)
Take C n (10 mmol), A (4AzOH, for series 1) or B (4SBOH, for series 2) (10 mmol) were dissolved into dry acetone in a round bottom flask.Into this solution, 4.05 g anhydrous K 2 CO 3 (29 mmol) was added and the system was refluxed from 15 to 48 hr (monitored by TLC).After the reaction is done, the still hot solution is filtered and washed with hot acetone.The filtrate was collected and evaporated in a rotary evaporator, adding cold petroleum ether to the concentrated extracts.The resulting precipitate was filtered and washed three times with this same solvent.The crude dimers BpnSB were purified through recrystallization in ethanol until consistent transition temperatures were obtained.

Chemistry
Synthesis of the unsymmetrical dimers were done by the simple Williamson etherification between the corresponding C n and (E)-4-((4-butylphenyl)diazenyl)phenol-series-I or (E)-4-((4-butylphenyl)imino) methyl)phenol-series-II. Unsymmetrical dimers (BpnAz/BpnSB) showed three peaks in their FT-IR spectra for the alkyl chain between 3095 and 2840 cm −1 , while the azo group's -N=N-stretching frequency and Imine group's -CH=N-stretching frequency was found between 1600 and 1610 cm −1 .The ether group experienced -C-O-C-stretching between 1242 and 1252 cm −1 , and the lack of a broad peak in the region of 3430-3450 cm −1 indicates that there is no free -OH group.Methylene protons (-CH 2 -CH 2 -) in the chain were seen as a multiplet at δ 1.0-2.5 in the 1H-NMR spectra of compounds and the triplet seen in the range of 0.95-0.98 for the terminal methyl (-CH 3 ) protons.When methylene protons are connected to oxygen, triplets are seen in around δ 4.0-4.5 ppm.The aromatic protons of the biphenyl ring and phenyl ring had a chemical shift range of δ 6.5-8.0 ppm.One proton in the imine linkage (-CH=N-) shows singlet around δ 8.2-8.5 ppm.Alkyl carbon peaks were found in the range of 10 to 35 ppm in the 13C-NMR spectra dimers, while peaks for methylene carbons which is directly connected to oxygen (-O-C-) were found at 68 ppm.Carbon of imine linkage (-CH=N) shows peak around δ 155-165 ppm All aromatic carbons were found around δ value of 110-165 ppm.

Mesomorphic properties
DSC thermograms and POM measurements allowed the identification of distinct transition temperatures and mesophase textures for dimers throughout the various heating/cooling cycles.

Differential scanning calorimetry study (DSC)
The thermograms were calculated using DSC (DSC-822, Mettler Toledo having Stare software).All phase transitions of the unsymmetrical dimers were tracked from ambient temperature to clearing temperature at a rate of 10°C min −1 during both heating and cooling cycles.The number of methylene units in BpnAz and BpnSB has an impact on the phase transitions that occur during heating and cooling cycles.Table 1 mentions the corresponding transition temperatures in ℃ and transition enthalpies in kJ mol −1 of the dimers.Dimers with n = 3 and 5 of both the series showed one endotherm on heating for crystalline to isotropic liquid (Cr-Iso) transition and one exotherm on cooling for isotropic liquid phase to crystalline (Iso-Cr) transition without any mesogenic property.DSC thermogram of Bp4Az and Bp4SB were depicted in Figure 1.
Figure 1(a) show DSC thermogram of Bp4Az.On heating, Bp4Az showing enantiotropic mesophase showed an exothermic peak at 116.5.0°C and 126.8°C with enthalpy changes of 33.3 kJ mol −1 and 1.29 kJ mol −1 , respectively, corresponding to a transition from Cr-N and N-Iso.In the cooling cycle, isotropic liquid slowly transformed into a nematic mesophase at 128.6°C, and it crystallised out on further cooling at 108.2°C.Corresponding enthalpy change was 1.04 kJ mol −1 and 32.7 kJ mol −1 for Iso-N and N-Cr transitions, respectively.Bp4SB shows enantiotropic mesophase.It shows two endotherms at 104.7°C and 129.2°C and two exotherms, which are the transitions from the isotropic liquid phase to the nematic phase (Iso-N) at 124.2°C and from the nematic to crystalline solid state (N-Cr) at 76.0°C, respectively.Enthalpy changes corresponding to these transitions are already shown in Table 1.

Polarising optical microscopy (POM)
The mesogenic properties of the dimers were observed using polarised light microscopy and a separate heating plate allowed for different temperatures to be achieved and monitored for each sample.Leica DM2500P with phase contrast was used to observe the mesogenic characteristics, and the Leica DFC295 camera was used to capture the details of each compound.All the dimers of both series were examined using a POM.During the heating and cooling cycle, the images of POM textures were captured (Figure 2).
On heating, from a crystalline solid in an ordinary slide, Bp4Az shows nematic phase and then turns into an isotropic liquid.On cooling from an isotropic liquid (Figure 2(a)), it shows nematic droplets around 128.5°C.As soon as the transition temperature was reached, the nematic droplets started to coalesce and merge, eventually giving rise to the Schlieren texture of the nematic phase at 119.2°C (Figure 2(b)).Eventually, as the compound cooled further, the Schlieren texture vanished and crystallisation began to take place.Similar enantiotropy is observed for Bp4SB (Figure 2(c,d)).

Structural dependence on mesomorphism
We investigated the effect of structural modifications on the mesomorphic properties by plotting the temperature at which phase transitions occurred against the number of carbons in the flexible spacer.The bar graph in Figure 3 of temperature versus the number of methylene flexible spacers in synthesised dimers demonstrates the thermal stability of mesophase during the cooling cycle.In Figure 3, the bar graph demonstrates that dimers of both the series have comparable thermal stability.The graph also shows that the odd-membered dimers in series 1 and 2 have lower Cr-Iso transition temperatures than the even-membered dimers.This suggests that the length of the carbon chain has an effect on the thermal stability and transition temperatures of the dimers.The dimers Bp4Az and Bp4SB are having even number of carbon atoms, whereas Bp3Az and Bp3SB contain an odd number of carbon atoms in the flexible spacers.Odd-membered dimers are bent, whereas the even-membered dimers are somewhat linear, which may also be responsible for the mesogenic properties of the dimers BpnAz and BpnSB (n = 2, 4, 6).Dimers in both the series show a clear odd-even effect.
From the comparison of T N-I of each series, it is found that the azo values appear to be similar to that of the imine values.This behaviour of the present series is in good agreement with the literature [4,22,45] and CB6O.m vs CB6OAzm [46].

Thermogravimetric analysis
The dimers Bp4Az and Bp4SB were subjected to TGA measurements in a nitrogen atmosphere at temperatures ranging from 50°C to 550°C.Figure 4 presents the TG curves for both the dimers.
Because there is no mass loss up to 350°C and a steady loss of mass up to 80-90% between 320°C and  450°C, which may be attributed to the compound's thermal decomposition, the TG curves demonstrate the thermal stability of the synthesised dimers.
Thermal decomposition manifested in the temperature span of 300-320°C, commencing at 304°C and 319°C for Bp4Az and Bp4SB, respectively.The temperatures of maximum degradation rate (Tmax) were noted as 366.8°C and 369.5°C for Bp4Az and Bp4SB, respectively.Furthermore, even at temperatures as high as 600°C, the degradation process remained incomplete, yielding residual quantities ranging from 19.9% to 6.45% for Bp4Az and Bp4SB, respectively.

Photochromic behaviour
Azobenzene is categorised as a dynamic substance because photoisomerization is typically triggered by the π-π* transition and leads to interconversion between cis and trans isomers.Since light irradiation can be used to regulate the inter-conversion between cis and trans isomers, it is a promising candidate for the development of light-responsive materials.A dual beam spectrophotometer equipped with a 1 cm quartz cuvette and a 365 nm filter was used to measure photoisomerization in Bp4Az.
The UV-Vis spectra for Bp4Az with various UV exposure times are shown in Figure 5(a).This suggests that the trans(E)-cis(Z) isomerization of Bp4Az occurs upon UV exposure, leading to a decrease in the π-π* transition at 355 nm and an increase in the n-π* transition at 440 nm.The steric hindrance caused by the cis configuration makes it more likely to revert back to its trans form.The compound was kept in the dark, devoid of any external energy, and its absorbance was then monitored until it had fully transformed into the stable trans form.This process is known as thermal back relaxation (Figure 5(b)), which is a spontaneous process that occurs due to the release of energy stored in the cis form.It shows an increase in the absorption maxima followed by a slight decrease in absorbance at the longer wavelength as indicated by the arrows in Figure 5(b).The presence of two isosbestic points in the reverse isomerisation process of Bp4Az confirms the absence of any side reaction during the conversion.The stability of the trans configuration was regained after 3.5 h, indicating a slow but complete recovery process.The kinetics of this process was studied by monitoring the changes in UV-vis spectra over time after turning off the UV light.Figure 5(c) shows a time-dependent plot of Bp4Az during UV illumination and thermal back relaxation from a photo-stationary state.
The photo conversion efficiency (CE) of the trans-cis photoisomerization process in Bp4Az was determined using equation [CE = (A t 0 À A t 1 Þ=A t 0 �, where A t 1 is absorbance after UV and absorbance before UV is A t 0 and was found to be 77.9% which is fairly high.It was found that ~3 min was needed to reach a photo stationary state which is due to the photo isomeric equilibrium of the trans-cis form of the Bp4Az [47,48].For Z-E and E-Z photoisomerization, the first-order plot should always be measured so it is shown in Figure 5(d) and was created by fitting the experimental data to equation À k cÀ t t = ln (A t 1 À A t /A t 1 À A t 0 ) [49].The reaction was a first-order reaction during a specified period of time.Later on, though, the response deviated from the initial order.The extended thermal back relaxation may have had an impact on the experimental temperature conditions because this experiment was carried out in a solution.

DFT studies
The characteristics of molecule and potential interactions between them can be predicted by computational studies, which can also shed light on structure-activity relationships.The chemical characteristics of a molecule, such as its electron densities and potential energies, can be better understood by using Frontier Molecular Orbital (FMO) contours, which are derived from quantum mechanical descriptors and Electrostatic Potential (ESP) contours.

Optimization of the molecular structure
For the dimers under consideration, the chemical quantum parameters were calculated by the DFT method.The estimated geometrical structures were computationally calculated at the B3LYP level in the gas phase (using the 6-31 G basis set).The present series showed LC traits, which suggested that they ought to be in a planar conformation.Each component of the system's optimised geometry proved to be stable because the imaginary frequency was absent (Figure 6).To represent molecular structures, carbon, nitrogen, oxygen, and hydrogen atoms are coloured grey, blue, red, and white, respectively.Sigma and pi bonds are then used to join these atoms together, constructing the molecule's three-dimensional structure.Figure 6 displays every optimised molecular structure for dimers of both series.
The electronic dipole moment (µ), as well as other thermodynamic properties (enthalpy of formation and entropy), were calculated using the relative energies of the series from the electronic structure theory computations.Table 2 shows the optimized energies of the dimers.With increasing molecule length, it was predicted that the zero-point energy and other calculated quantum thermal parameters would rise.
ZPVE stands for sum of electronic and zero-point energies and E tot stands for the sum of electronic and thermal energies, whereas H is the sum of electronic and thermal enthalpies and G is the total of the free electronic and thermal energies.Figure 7 depicts the relationship between the no. of carbon in a flexible spacer in dimers versus the energy minima in Hartree.The plot shows that as the number of carbons in the flexible spacer increases, the energy minima decrease, indicating a more stable dimer structure.This trend can be attributed to the increased flexibility and conformational freedom of longer spacers.

Infrared spectral analysis
The theoretical FT-IR technique appears to be a promising tool for identifying the various types of stretching vibrational assignments in LCs.Analyses of computed infrared spectra illustrated in Figure 8 attribute responsible changes in electronic, vibrational and rotational mechanisms during the formation of compounds.A comparison between the experimental and calculated (B3LYP/6-31 G) FT-IR spectrum of dimers Bp4Az and Bp4SB are shown in Figure 8.
According to a wave number analysis, there are no negative wave numbers responsible for intermediate transitions, and wave numbers in the assigned region are consistent with the literature in the functional and finger print regions.Computed wave numbers are in agreement with experimental spectra for all the dimers in the proposed work [50].Figure 8 presents a comparative analysis of the experimental FT-IR spectrum of Bp4Az and Bp4SB, alongside their respective predicted gas-phase spectra using the B3LYP/6-31 G method.In these spectra, we observe distinct stretching modes: the C-H stretching modes linked to the aromatic ring (sp 2 ) at 3095 cm-1 and those tied to the aliphatic chain (sp 3 ) at 2840 cm −1 .Notably, the spectral range between 1600 cm −1 and 1610 cm −1 shows characteristic stretching frequencies attributed to the -N=N-and -CH=N-functional groups.These features suggest the presence of azo and imine connections in the dimer structures.Of significance is the spectral range from 1240 cm −1 to 1260 cm −1 , indicating the occurrence of -C-O-Cstretching modes, which serve as distinctive indicators of the ether group across all dimer configurations.As we move to the region from 1600 cm −1 to 1000 cm −1 , we encounter notable traits including C-C and C-N stretching modes, as well as distinctive deformations intrinsic to the biphenyl and phenyl rings of Bp4Az.Expanding further into the spectral range spanning 1000 cm −1 to 400 cm −1 , we gain valuable insights into stretching and rocking modes of CH and CH 2 groups, along with a variety of deformations and torsion modes.Tables 3 and 4 depict the observed and calculated wave numbers in cm −1 and assignments of bands for Bp4Az and Bp4SB, respectively, in gas phase by using the B3LYP/6-31 G method.
It is important to consider that the experimental spectrum was captured in the solid state, characterized by complex interactions between molecules and crystal packing effects.These factors introduce complexities such as molecular symmetry, lattice vibrations, and hydrogen bonding interactions, all of which significantly impact the observed spectrum.In contrast, these intricacies are not accounted for in gas-phase calculations, leading to an anticipated difference between the experimental and theoretical spectra.Notably, this divergence is most pronounced in the lower frequency range, where lattice vibrations play a more prominent role.Table 3. Observed and calculated wave numbers (cm −1 ) and assignments for Bp4Az in gas phase by using the B3LYP/6-31 G method.

Frontier molecular orbital contours
The geometry of each molecule was optimised at the B3LYP/6-31 G (d,p) level.The optimized geometry was used to calculate the electronic properties of the molecule, including its HOMO-LUMO gap and dipole moment [51].The HOMO-LUMO gaps were calculated for the dimers of series 1 and 2 at the B3LYP/ 6-31 G (d,p) level, and the results are summarised in Table 5. Figure 9 shows the analyses of the distributions of the highest-occupied and lowest-unoccupied frontier molecular orbitals for the dimers of series 1 and 2. The HOMO-LUMO gap is an important parameter for predicting the electronic properties of molecules, and its value can be used to estimate the reactivity and stability of the compounds.The results showed that the HOMO-LUMO gap for the dimers of series 1 was smaller than that of series 2 dimers, indicating that series 1 dimers may have higher reactivity and lower stability.
Table 5 indicates that the energy of the HOMO and LUMO for dimers of individual series is comparable, when the number of carbons in the flexible spacer were increased from n = 2 to n = 5.According to the observation, the electronic properties of the carbon chains in the flexible spacer had no effect on the FMOs of HOMO and LUMO.This can be explained by the fact that HOMO and LUMO orbitals are not used by the flexible spacer.Additionally, all of the designed homologues displayed comparable electron cloud distributions over the azo and azomethine linkage's π-system, and phenyl core system.Therefore, the presence of different flexible spacers does not significantly affect the electronic properties of the molecule.Quantum mechanical descriptors are the ionization potential (I), electron affinity (A) and electrophilicity index (ω) with changes in electronic densities during the formation of compounds.Ionisation potential corresponds to the highest occupied molecular orbital (HOMO) as electron donating sites and the lowest unoccupied molecular orbital (LUMO) as electron receiving sites, with energies in electron volts (eV) and energy difference (ΔE).Additionally, since the energy gap and softness are inversely related (softness = 1/η), they share the same softness.
Table 5 also shows the optical properties of the dimers.Changes in electron density contribute to changes in optical properties as a result of intermolecular interactions with reduced molecule opposition, resulting in altered charge transfer and polarity of the molecule.The distribution of the electrons at the atomic sites of LC materials affects a number of parameters, including the electronic configuration, molecular dipole moment and polarizability.The no. of carbon atoms present in the flexible spacer has a notable influence on the calculated dipole moments, as indicated in Table 5. Particularly, an elevated dipole moment is observed in dimers with an even number of carbons in spacer, which corresponds to their display of mesogenic properties.Conversely, dimers with lower dipole moments exhibit no mesogenic properties.However, both the mesophase range and thermal stability were significantly impacted by the flexible spacer's chain length.This observed correlation between the dipole moment and mesogenic behaviour implies that the dipole moment associated with the flexible spacer in these dimers impacts the arrangement of the liquid crystal phases.Notably, a substantial dipole moment appears to facilitate the development of structured phases, whereas dimers with diminished dipole moments do not manifest such mesogenic traits.

Relationship between theoretical and experimental variables
The experimental values for the mesophase stability and mesophase ranges were correlated with the theoretical dimensional parameters of the dimers.Table 6 shows the dimensional parameters of the dimers.Figure 10 presents a line graph that illustrates the relationship between the aspect ratio and the number of carbons in the flexible spacer for dimers in both series.This graph provides valuable insights into the structural characteristics of these molecules.Upon examining the graph, all the odd-numbered dimers in both series exhibit lower aspect ratios.This lower aspect ratio is primarily attributed to the increase in the width of the molecule, which occurs due to the presence of an odd number of carbons in the flexible spacer.Conversely, all the even-numbered dimers consistently display higher aspect ratios.This trend is a consequence of the molecular structure, where an even number of carbons in the flexible spacer results in a narrower width.A clear correlation is observed between the aspect ratios of the dimers in both series 1 and 2. Specifically, dimers with higher aspect ratios such as BpnAz and BpnSB (n = 2, 4), tend to exhibit mesogenic behaviour, indicating their ability to form liquid crystal phases.On the other hand, all the odd-numbered dimers with lower aspect ratios are non-mesogenic.

Electrostatic potential contours
The MEP surface analysis provides valuable insights into the electrostatic potential distribution of the molecules, which is crucial for understanding their reactivity and binding properties.This method can be used to identify potential binding sites or to predict the behaviour of molecules in various chemical reactions.The MEP surface analysis of the dimers were determined using DFT calculations on an optimised structure with a B3LYP/6-31 G (d,p) basis set.For the representative dimers under study, the electrostatic potential surface  charges were mapped in Figure 11.The electrostatic potential observed in the dimers has a maximum and minimum limit that ranges from −2.662e-2 to +2.662e-2, and the MEP was measured at 0.0004 as its isovalue.
The red regions showed areas of high electron density and blue regions areas of low electron density.The least negatively charged atomic sites were found in the terminal butyl chain and the middle flexible spacer of all the dimers.The mesogenic cores, which are visible at the oxygen atoms of the ether linkage and nitrogen atoms of azo/azomethine linkages, are expected to be localized in the red regions, which indicate high electron density.

Conclusion
Two new series of unsymmetric liquid crystal dimers of general formula BpnAz (series 1) and BpnSB (series 2) (n = 2, 3, 4, 5, 6) possessing azobenzene and azomethine linkages and a biphenyl moiety have been synthesised and characterised.When exposed to 365 nm UV-Visible light, the azo chromophore undergoes significant photoisomerization.The study suggests that the length and structure of mesogenic units and the nature of the spacer play a crucial role in determining the phase behaviour of dimers.Additionally, the presence of flexible spacers (−methylene units) in even numbers appears to be a necessary condition for nematogenic behaviour.Due to the bent shape of the molecular structure, all dimers with an odd number of carbon atoms in the flexible spacer are nonmeogenic.In comparison to azomethine dimers (series 2), the unsymmetrical azo dimers (series 1) exhibit higher thermal stability.Both the series show almost comparable mesophase ranges.All the dimers in the suggested work have computed wave numbers that agree with experimental spectra.The fact that there is no mass loss in TGA up to 350°C proves that the synthesised dimers have higher thermal stability.The high thermal stability of the synthesised dimers makes them promising candidates for applications in high-temperature environments.
M. S. University of Baroda for providing laboratory facilities, DST-FIST for providing NMR, POM, DSC and ESI-MS analyses facilities.
Scheme 1. (Colour online) Synthesis scheme for the unsymmetrical dimers containing biphenyl moiety.

Figure 2 .
Figure 2. (Colour online) Optical photomicrographs of dimers (a) nematic droplets at 128.5°C on cooling for Bp4Az (b) Schlieren texture upon cooling to 119.2°C for Bp4Az (c) nematic droplets at 130.8°C for Bp4SB (d) Schlieren texture at 128.3°C on cooling for Bp4SB.

Figure 3 .
Figure 3. (Colour online) Bar graph of temperature against no. of carbon of synthesized dimers showing thermal stability of the mesophase in the cooling cycle.

Figure 5 .
Figure 5. (Colour online) Absorption spectra for Bp4Az (a) with various UV exposure times.The UV illumination of 0 sec ~ no UV (b) Thermal back relaxation process (c) plot of Bp4Az as a function of time during UV illumination (E→ Z) and during thermal back relaxation in dark (Z→ E) (d) first-order plots for E-Z and Z -E thermal isomerization for Bp4Az.

Figure 7 .
Figure 7. (Colour online) Flexible spacer dependence on energy minima for series 1 and 2.

Figure 10 .
Figure 10.(Colour online) Relationship between the aspect ratios of dimers of series 1 and 2.

Table 4 .
Observed and calculated wave numbers (cm −1 ) and assignments for Bp4SB in gas phase by using the B3LYP/6-31 G method.

Table 5 .
Quantum mechanical descriptors of dimers.

Table 6 .
Dimensional parameters of the dimers.