Influence of the hemisphere on Janus structures containing cyanobiphenyl units

ABSTRACT New Janus-type compounds were synthesised with two different hemispheres structures and properties. The structures contain cyanobiphenyl units linked with 3,4,5-tribenzyloxybenzene or 3,4,5-tridecyloxybenzene units. Also, the influence of the 1,3,4-thiadiazole heterocycle was studied. The mesomorphic properties were studied by differential scanning calorimetry, polarised light optical microscopy and thermal gravimetric analysis. The mesophases obtained depend on the structure of the hemispheres, even obtaining mesophases different from the initial ones. The new materials synthesised presented photoluminescent properties, which were studied in solution. GRAPHICAL ABSTRACT

Multifunctional chemical systems are designed to perform multiple tasks at the same time by combining different chemical moieties or components.These systems are designed to be versatile and adaptable, and can be used to create advanced materials for a wide range of applications, such as biology [1], pharmaceuticals [2], electronics materials [3][4][5].In the field of materials science, Janus materials have garnered significant interest in various scientific and engineering fields.Janus materials possess two distinct sides or regions with different properties or functionalities.The two sides of Janus materials can differ in terms of chemical composition, physical properties, surface functionalities, or even topography.This asymmetry allows Janus materials to perform different functions simultaneously or exhibit enhanced properties compared to traditional homogeneous materials.The distinct properties of each side can be utilised for specific applications or combined synergistically to achieve multifunctionality [6,7].
An advantage of Janus-type structures is that each hemisphere can be synthesised independently which offers high precision and control over the size, shape, and composition of each hemisphere.This design prevents macrosegregation caused by functional incompatibility, which is a common issue in physical mixtures [8][9][10].An important challenge is to obtain Janus-like structures with liquid crystal properties.Janus liquid crystals possess anisotropic behaviour, meaning they exhibit different properties in different directions.This anisotropy can arise from the asymmetry in molecular structure, surface interactions, or external stimuli.Therefore, comprehending the fundamental principles behind the formation, selfassembly, and phase behaviour of Janus liquid crystals is crucial.The kind of organisation will be determined by each hemisphere of the Janus.In this way, controlling the structure of the molecule various organisations such as nematic, lamellar, discotic and even cubic can be obtained [11][12][13][14].Columnar order is favoured by polycatenar, especially if benzene is trisubstituted at position 3,4,5 with alkoxylic chains [15][16][17][18][19][20].Moreover, the cyanobiphenyl group has been widely studied in the field of liquid crystals since gives rise to lamellar or nematic order [21][22][23][24].These units have been included in several studies due to their high polarity in the longitudinal axis of the molecule and photoluminescent properties [25][26][27].Also, this group has been utilised in a variety of structural systems such as calamitic [28], dimers [29,30], and dendrimers [31], as well as multifunctional systems including fullerene units [32][33][34][35][36].The diversity in structure has enabled the development of various systems that possess interesting properties suitable for potential technological applications [37,38].
On the other hand, heterocyclic systems are excellent building blocks for the development of molecular materials [39].The incorporation of heterocyclic rings in liquid crystal compounds can lead to modifications in the physical properties of these materials, such as thermal stability, electrical conductivity, and optical properties [17,31,[40][41][42][43][44][45].The 1,3,4-thiadiazole heterocycle has been widely studied in our group due to its high polarisability, electron-withdrawing nature, and luminescent properties, as well as the chemical and thermal stability [46][47][48].
For exploring new synthetic strategies to expand the variety of Janus liquid crystal materials and their properties the goal of this work is the synthesis, characterisation and study of the mesogenic and luminescent properties of three new molecules of Janus-like structure.A common hemisphere for the three Janus molecules has been used in order to compare the influence of different functional groups on the properties of the system: a benzene group with three octyloxycyanobiphenyl units at the positions 3,4 and 5.This unit favours the appearance of lamellar arrangements in the mesophase and gives fluorescent properties.In the first Janus system a benzene molecule substituted at positions 3,4 and 5 with deciloxy chains was used as the second hemisphere in order to obtain a columnar order.The second Janus system has a benzene molecule substituted at positions 3,4 and 5 with benzyl units as the second hemisphere.1,3,4-thiadiazole heterocycle can increase the stability of Janus liquid crystals by improving the ππ stacking interactions between molecules.This increased stability can prevent phase separation or degradation, leading to more robust and long-lasting liquid crystal properties [49,50].To study the effect of the incorporation of the 1,3,4-thiadiazole heterocycle on the molecule, the third system was synthesised.

Synthesis and characterization
The synthesis of Janus compounds 7-9 is outlined in Scheme 1.The first path of the synthesis is a Mitsunobu reaction between 4′-hydroxybiphenyl -4-carbonitrile and 8-bromineoctanol [51], using CH 2 Cl 2 as solvent in order to obtain compound 1.The FT-IR shows bands at 2932 and 2216 cm −1 typical of C sp3 − H and the nitrile group, respectively, in addition to the band corresponding to the C-O bond at 1255 cm −1 , confirming the ether group.Compound 2 is obtained by Williamson's alkylation of compound 1 with methyl gallate in 2-butanone, using K 2 CO 3 and 18-crown-6.The 1 H NMR spectrum show a signal at δ = 4.01ppm corresponding to the -OCH 2 integrating by 12 H and a singlet with a displacement of δ = 3.89ppm integrating by 3 H corresponding to the methyl group (-OCH 3 ).Acid 3 is obtained by basic hydrolysis with KOH and a THF/ethanol mixture as solvent and subsequent acidification with HCl.FT-IR shows a shift of the band from 1705 to 1608 cm −1 corresponding to the carbonyl group of the carboxylic acid, also in 1 H-NMR the disappearance of the singlet corresponding to the methyl group of compound 2 is observed.
Compounds 4 and 6 have been previously synthesised while compound 5 was synthesised as indicated in Scheme 2 [46,52].Esterification of 3,4,5-trisbenzyloxybenzoic acid with 4-hydroxyphenoxy-tertbutyldimethylsilane and DCC/DMAP [53,54].In the 1 H NMR spectrum, the shift in δ = 0.22 ppm per 9 H and δ = 1.01 ppm per 6 H corresponding to the TBDMS protective group are observed.In addition, a signal in δ = 5.17 ppm per 6 H corresponding to the O-CH 2 -Ar hydrogens from the benzyl group is founded.The deprotection of the TBDMS group of compound 5a is carried out with HF and DMF as solvent obtaining compound 5.The 1 H-NMR spectroscopic data show the disappearance of all the signals of the TBDMS group and a band in 3326 cm −1 is observed by FTIR corresponding to the -OH vibrations of the phenol.
The final Janus compounds have been obtained by esterification via DCC/DMAP with CH 2 Cl 2 as solvent between acid 3 and the previously synthesised phenols 4 [47], 5 and 6 [52].All synthesised compounds were characterised by FT-IR and 1 H and 13 C NMR spectroscopy and high resolution mass spectrometry (HR-MS).
In the 1 H NMR spectrum of compound 7 a chemical shift at δ = 0.88ppm corresponding to the methyl groups and δ = ~4.00ppm signals corresponding to -OCH 2 hydrogens are observed.
Additionally, the signals of aromatic hydrogens that match the multiplicity and quantity of hydrogens in the structure can be observed (Figure S1(a)).For compound 8 at δ = 5.18ppm the characteristic signal of the Ar-CH 2 -O hydrogens of the benzyl group (6 H) are observed, likewise the hydrogens of the OCH 2 groups of the alkoxylic chains are observed at δ = 4.00 ppm (Figure S1(b)).Finally, for compound 9, the spectrum is very similar to compound 7 showing a shift in δ = 0.89ppm.This signal corresponds to the terminal 9 H of the methyl groups and another in δ = ~ 4.00 ppm is associated with the 16 H of the OCH 2 groups (Figure S1(c)).In all the spectra, there is a concordance of two doublets at δ = 7.60 ppm corresponding to the biphenyl system.

Thermal properties
The thermal behaviour of the compounds was investigated by TGA and DSC.All compounds are stable above 350°C and exhibit similar decomposition pathways (Table 1).The temperatures aforementioned are significantly higher than the isotropic transition, which are below 160°C.(Figure S2 in SI).
Liquid crystal properties were studied by polarised light microscopy (POM) and differential scanning calorimetry (DSC).Transition temperatures can be founded in Table 1. Figure 1 shows a graph depicting the transitions for all liquid crystal compounds.
The thermal properties of compound 6 have been studied previously by Parra et al. [47].Compound 2 has enantiotropic liquid crystal properties with a nematic type mesophase, with a schlieren texture observed by POM, Figure 2(a).For this compound, a nematic-isotropic transition enthalpy of 4.1 kJ mol −1 was observed by DSC.By cooling, a transition to the solid state is not appreciated by DSC, while by POM a solidification of the mesophase is observed, which can be attributed to a second-order transition (glass transition).The N-I transition enthalpies exhibit higher values compared to what would typically be anticipated for conventional compounds with low molecular mass.This is attributed to the presence of three interacting promesogens within each molecule [55].Compound 3 has a monotropic liquid crystal properties.On heating, the Cr-I transition is observed at 159°C, while on cooling two transition temperatures are observed: 158°C corresponding to an isotropic-nematic transition, and 120°C corresponding to crystallisation from the mesophase.By POM a marble-like texture is observed in the liquid crystal state (Figure 2(b)), as a compound 2. The nematic-crystal transition is only observed by POM due to the low enthalpy value.For the final compounds, just 7 and 9 showed liquid crystal properties.Compound 7 exhibited enantiotropic liquid crystal characteristics, displaying a SmA mesophase.When the isotropic melt was cooled, the formation of bâtonnets was observed using POM (Figure 2(c)).On heating, a short mesomorphic range of 4°C is observed, while on cooling, the mesomorphic range is 19°C.The transition enthalpy determinated in the Cr-SmA transition is 89.1 kJ mol −1 while the transition enthalpy SmA-I and then I-SmA are similar (13.6 kJ mol −1 ).Upon cooling, a slow crystallisation occurs that is only observed by POM.Compound 9 shows enantiotropic liquid crystal properties, the transition occurs upon heating, with a melting temperature of 116°C and a subsequent clearing temperature of 125°C.A homeotropic SmA mesophase with a mesomorphic range of 9°C was observed using POM Table 1.Phase transition temperatures (°C) and enthalpy (kJ mol −1 in parentheses) for compounds determined by DSC for the second heating and cooling cycles, scanning rate 10°C min −1 .ΔT is the mesomorphic range (°C), and decomposition temperatures determined by TGA.  as is shown in Figure 2(d).The DSC curve exhibits a single transition peak when heated because the compound has a short mesomorphic range.This peak represents both the melting and clearing processes and is characterised by a high energy value of 103 kJ mol -1 .Additionally, during the second heating cycle, a cold transition occurs at 54°C due the compound transitions from a glassy to a crystalline state.On cooling, the mesomorphic range is broader, appearing from the isotropic state at 117°C until crystallisation at approximately 60°C.The transition is not observed in DSC and can only be observed by POM.
Although the hemisphere with cyanobiphenyl units is the same for the three final compounds and has a nematic type arrangement, by incorporating a different hemisphere with another type of arrangement, the resulting organisation is totally different.The final compound 7 with a SmA mesophase is thus obtained which included a conical alkoxylic end (4) that lacks liquid crystal properties.Furthermore, a more ordered type of organisation, such as the smectic A, replaces the initial compound's nematic mesophase in this case.However, compound 8 -which contains a hemisphere with benzyl group units -does not exhibit liquid crystal properties; rather, its mesomorphic characteristics are entirely lost, resulting in a Cr-I transition.In this case, the unrestricted movement of the benzyl groups hinders the formation of a suitable configuration for the formation of a mesophase.
The incorporation of hemispheres with different mesomorphic organisations in compound 9 leads to a noticeable change in its molecular organisation.In this compound the mesomorphic organisation of the cyanobiphenyl hemisphere is nematic (4), and the hemisphere with the unit of 1,3,4-tiadiazol presents a columnar organisation (6) [46], which is preferred due to the π-π interactions between the aromatic and the thiadiazole rings.The resulting organisation of the final compound is a SmA mesophase, and by cooling it presents a wide mesomorphic range of 57°C.This may be attributed to the fact the position of the 2,5-diphenylthiadiazole system in the new molecule is not sufficient to allow pi-pi stacking.However, the high dipole moment associated with the heterocycle favours lateral interactions that stabilise the layer ordering (SmA).The behaviour of compound 9 indicates that the calamitic order prevails when both hemispheres are joined.However, if we carefully examine these results, the transition temperatures and mesomorphic properties of compounds 7 and 9 are very similar, both exhibiting an SmA phase.The transition enthalpy for 7 is 89.1 kJ mol −1 for Cr-SmA and 13.6 kJ mol −1 in the SmA-I.The total enthalpy changes for compound 7 is 102.7 kJ mol −1 .On the other hand, compound 9 has a transition enthalpy of 103 kJ mol -1 , encompassing both the SmA-I and Cr-SmA transitions.The above suggests that the mesomorphic properties are governed by the same interactions rather linked to the segregation between the two types of mesogens (hemispheres).Previous studies have described the various arrangements of smectic layers in heterolytic systems using traditional concepts, such as partial molecular volumes and transverse crosssections of molecular segments [56,57].In the specific cases of systems containing OCB, the suppression of the terminal chains allows the possibility of a monolayer packing or a bi-layer packing through cyano pairing, or a combination of both [58].The inclusion of thiadiazole instead of the ester group does not result in any notable in the mesomorphic properties.

UV-Vis spectroscopy and fluorescence
In addition to the liquid crystal properties [53], cyanobiphenyl is also widely studied for the fluorescent properties [23].Fluorescence studies were performed for the precursors with cyanobiphenyl units and the final compounds (Figure 3).
The UV-Vis absorption and emission measurements were performed using dichloromethane as solvent to concentrations of 2.0 × 10 −4 mol L −1 for the absorption study and 2.0 × 10 −7 mol L −1 for the emission study.Absorption spectrums are shown in Figure 3(a) while the normalised absorption and emission spectrum are showed in Figure 3(b).Data from the photophysical studies are   summarised in Table 2.The absorption behaviour is similar in all the compounds studied, with a maximum absorption at wavelengths above 290 nm attributed to the π-π* transitions due to their large molar absorption coefficient (Table 2).The combination of electron withdrawing groups (cyanobiphenyls and thiadiazole) and electron donor groups (alkyloxy chains) induces a charge transfer corresponding to the energy of transition π-π* [46].The precursors and final compounds containing cyanobiphenyl units present interesting luminescent properties in dichloromethane.Thus, compounds that only contain cyanobiphenyl units (2 and 3) displayed a maximum absorption wavelength of 296 nm.The maximum absorption wavelength is shifted to 291 nm for the final Janus when the hemisphere with the alkyloxy chains (7) and the benzyl groups (8) are included.However, adding a hemisphere with alkoxy chains and the 1,3,4-thiadiazole heterocycle (9) causes a bathochromic effect at 297 nm.This is because the heterocycle increases the conjugated pi system lengthens which in turn reduces the HOMO-LUMO gap (Figure 3).
Quantum yields of the studied compounds were determined by comparison with fluorescein (2.0 × 10 −7 mol L −1 , Φ = 0.95 in water + 0.1 mol L −1 NaOH) as a reference compound [46].Photoluminescence quantum yields (ΦPL) for the of these compounds range from 0.19 to 0.47.The quantum yields of the hemisphere with cyanobiphenyl units decrease when the other adduct is incorporated.Compound 8, which has benzyl groups, exhibits the highest quantum yield of the final compounds (Φ = 0.34), while compounds 7 and 9 exhibit comparable quantum yield (Φ = 0.20 nm).Compound 9 has a Stokes shift of 143 nm, which is greater than the shifts of 70 nm for the other compounds.This indicates that the addition of the heterocycle causes a loss of vibrational non-radiative energy in the system.Additionally, compound 9 has two emission maxima, the band of cyanobiphenyls to 360 nm and an intense band associated with the emission of the heterocyclic system to 440 nm (Figure 3(b)).The appearance of two absorption maxima is attributed to the fact that this compound contains two chromophore groups.The findings suggest that the inclusion of the heterocycle has a significant influence on the photophysical characteristics, resulting in the generation of a blue emission band (Figure 3(c)).

General
All the procedures can be found in SI.All reagents were obtained from commercial sources (Merck or Aldrich) and used without further purification.The organic solvents were analytical grade quality.The purity of the compounds was checked by thin-layer chromatography (Merck Kieselgel 60F254).1H NMR and 13C NMR spectra were recorded using a Bruker Ascend 400 MHz spectrometer, with CDCl 3 and DMSO-d 6 as solvents and tetramethylsilane (TMS) as an internal standard.FT-IR spectra were recorded with a Nicolet Magna 550 spectrometer.MALDI-TOF mass spectrometry was performed on an Autoflex Bruker mass spectrometer, employing ditrhranol as matrix.
Mesophases analysis was performed using an Olympus B × 41 optical microscope equipped with an Olympus U-TV0.5XC-3polariser and a Linkam THMS600 heat stage and a RTV QIMAGING digital camera.Transition temperatures and enthalpies were investigated by DSC using a NETZSCH DSC 204 F1 Phoenix calorimeter.Samples were encapsulated in aluminium pans and observed at scanning rate of 10°C min −1 on heating and cooling.The instrument was calibrated using an indium standard (156.6°C,28.44 J g −1 ) under nitrogen atmosphere.
Ultraviolet absorbance measurements were made with a Shimadzu UV-1800 spectrophotometer.Emission spectra in solution and in film were acquired in a Fluorometer (Photon Technology International Inc. QuantaMaster) using a 1 cm path cuvette and solution at concentrations of 2.0 × 10 −4 mol L −1 in dichloromethane for UV-Vis study and 2.0 × 10 −7 mol L −1 in dichloromethane for fluorescence study.

Conclusion
The most important aspect in the study of Janus liquid crystals is gaining a comprehensive understanding of their structure, self-assembly behaviour, and phase behaviour.This knowledge is crucial for harnessing the unique properties of Janus liquid crystals and developing innovative applications in various fields in this context by convergent synthesis is possible to obtain new Janus-type materials with two distinct structural systems, incompatible by physical bonding, but which can be chemically bound.To the nematic starting material trioctyloxycyanobiphenyl benzoate structure was incorporated three different hemispheres.In these new materials, a stabilisation of their mesomorphic properties can occur as in the case of compound 7 which has a SmA unlike the starting material 4 that does not have liquid crystalline properties.Also, new mesophases can be generated as the lamellar SmA mesophase showed for the compound 9 whose starting material presents a columnar phase.Besides, the cyanobiphenyl group gives photoluminescent properties with quantum yields between Φ = 0.20 and 0.34.In addition, the material with the 1,3,4-thiadiazole heterocycle has a Stokes displacement of 143 nm.From these results, future research on Janus liquid crystals will be focus on advancing the understanding of their properties and optimising the design and functionality of these materials.

Disclosure statement
No potential conflict of interest was reported by the author(s).
phase; SmA, smectic A phase; I, isotropic phase; N, nematic phase; g, glass transition.a Optical data, b Combined enthalpies Cr-SmA and SmA-I.
a Determined in dichloromethane solution