Polycatenar liquid crystals based on bent-shaped chalcone and cyanopyridine molecules

ABSTRACT In this study, the synthesis, structural characterisation and mesomorphic and optical properties of seven new bent-shaped and polycatenar bent-shaped compounds derived from chalcone and cyanopyridine are reported. The mesomorphic behaviour was investigated by differential scanning calorimetry (DSC), polarised optical microscopy (POM) and X-ray diffraction (XRD) and correlated with the molecular structure. Two bent-core hexacatenars molecules (Ic and IIc) presented liquid crystalline properties, showing a hexagonal columnar (Colh) phases at room temperature, being each disc constituted by two mesogens. Optical studies were also performed for the final molecules, being conducted by ultraviolet-visible and fluorescence spectrometry. The cyanopyridine derivatives show moderate luminescence quantum yields, ranging between 18% and 27%, with emission maxima around 371 nm. It is also shown that while the chalcone central unit favours a calamitic liquid crystalline behaviour in molecules with lower number of aliphatic chains, a polycatenar structure with cyanopyridine as the central unit favours a Colh arrangement, also providing luminescence properties to the molecule. GRAPHICAL ABSTRACT


Introduction
Organic materials with low molecular mass form the basis of a class of compounds which are currently of great interest, gaining increasing importance. These materials, most of them synthetic, provide the advantages of lightness and flexibility along with unconventional optical, electro-optical, magnetic and/or electrical physical properties. These characteristics allow these compounds to be employed in electronic and optoelectronic devices [1][2][3][4][5].
Among these materials, liquid crystals (LCs) have a prominent position, being one of the first organic materials to be widely applied in electronic and optoelectronic devices [6,7], resulting in a multi billionaire industry. Besides the use in displays, the applicability of liquid crystalline materials as organic semiconductors for application in organic field effect transistors (OFETs) [8], organic light emitting diodes (OLEDs) [9] and organic photovoltaic cells (OPV) [10], has already been shown. Furthermore, the development of new, more robust and more efficient applications is strongly dependent on the synthesis of materials with molecular anisometry, which is a major factor in the design of new liquid crystalline materials. In this regard, it is essential to have a deep understanding of the relationship between the molecular structure, the structure of mesophases and other properties of the materials.
Polycatenar LCs represent an intermediate structure, between calamitic and discotic mesogens, with interesting behaviour, being therefore intensively investigated in the past two decades [11,12]. Calamitic polycatenar LCs have polyaromatic cores with up to three or four rings linked to each other through the para positions of the aromatic ring, and at the extremities aromatic terminal units are substituted by more than two aliphatic chains. Due to its structural hybrid form, polycatenar LCs can exhibit a rich polymesomorphism with lamellar and columnar phases in the same analogue series or molecule, as a result of the incompatibility of the volume of the different molecular sections and microsegregation phenomena [12].
The use of different functional groups or different heterocycles may result in additional interesting properties to the molecules, like luminescence, for example, expanding the applicability range of the material. Heterocycles such as tristriazolotriazine [13], 1,2,3-triazole [14], 1,3,4-oxadiazoles [15], 1,2,4-oxadiazoles [16,17], boron dipyrromethene [18] and 1,3,4-thiadiazole [19,20], have a great influence over the molecular shape, dipole moment and other molecular characteristics [21]. Cyanopyridines [22] are also an interesting alternative, as already demonstrated by their induction of luminescence [23,24], liquid crystalline behaviour [24][25][26] and as a bent-shaped design promoter [26][27][28]. Furthermore, the synthetic route to obtain a cyanopyridine core involves the preparation of chalcones which are widely known to have promising liquid crystalline activity when linked to 4-substituted phenyl esters with long aliphatic chains in symmetrical [29] or asymmetric [30][31][32] structures, usually featuring calamitic phases. Therefore, in this paper, we describe the synthesis, characterisation and study of the liquid crystalline and photophysical properties of two new series of bentshaped and polycatenar bent-shaped liquid crystalline molecules. These compounds were based on chalcone Ia-c and cyanopyridine IIa-d with a different number and position of long alkoxy chains ( Figure 1). The observed properties were correlated to the structure of the respective molecule and the advantages of the cyanopyridine over the chalcone core through the phase stability and luminescence of the final compounds was demonstrated.

Synthesis
The target compounds were obtained through a convergent synthetic route (Scheme 1), starting by the synthesis of the central bent core and the carboxylic acids, followed by their connection by an esterification reaction.
First, the benzoic acids 3a-d, substituted with long alkoxy chains in different number and positions were prepared according to the methodologies described in the literature [33]. The synthetic route is based on an initial protection of the carboxylic acid functional group by a Fischer esterification, followed by alkylation of the hydroxyl via Williamson etherification using 1-bromododecane, K 2 CO 3 and butanone. In the case of the reagents containing more than one hydroxyl group, TBAB (tetrabutylammonium bromide) was employed to ensure complete alkylation. At the end, deprotection of the carboxylic acid by alkaline hydrolysis followed by acidification resulted in the substituted carboxylic acids 3a-d.
As described in Scheme 1, the final step consisted in the conversion of the carboxylic acids 3a-d into their respective benzoyl chlorides with SOCl 2 followed by an esterification reaction with the dihydroxyl bent cores 1 or 2. After the necessary purifications, two new series of compounds derived from 4,4ʹdi(benzoyloxy) chalcone Ia-c and 3-cyano-4,6-bis(4-benzoyloxyphenyl)-2-methoxypyridine IIa-d ( Figure 1) were obtained with yields of between 27% and 95%.
The structure and purity of the synthesised compounds were fully characterised by melting point (m. p.), Fourier transform infrared spectroscopy (FT-IR), 1 H and 13 C nuclear magnetic resonance (NMR) spectra and high resolution mass spectra (HRMS). The experimental procedures and characterisation data are described in the experimental section.

Thermal and mesomorphic studies
The phase transition temperatures, liquid crystalline behaviour and mesophase textures of the two series of final compounds were initially investigated by polarised optical microscopy (POM) equipped with a hot stage. By differential scanning calorimetry (DSC) measurements, under nitrogen atmosphere and with heating/cooling cycles employing rate of 10°C min −1 , the transitions temperatures were ratified and the respective involved energies determined. Thermal stability was evaluated by thermogravimetric analysis (TGA) under nitrogen atmosphere, being considered the temperature in which 1% of mass was lost by the material. The data obtained in these measurements are summarised in Table 1.
As described in Table 1, compounds synthesised in this work with two (IId) and four (Ia,b and IIa,b) alkoxy chains do not show any liquid crystalline behaviour. On the other hand, when a total of six long alkoxy chains are present and a polycatenar bentshaped structure is formed, a liquid crystalline behaviour is observed, as noticed for Ic of the chalcones series and IIc of the cyanopyridines series.
When analysing by POM, a monotropic liquid crystalline behaviour was observed for compound Ic, melting directly to the isotropic liquid at around 73°C, on heating. On cooling, fan-shaped focal conic texture were observed around 37°C (Figure 2(a)). Interestingly, no indication of crystallisation at room temperature was noticed, even after 1 week. The texture allied to the high viscosity of the material are indicative of a hexagonal columnar (Col h ) organisation [34,35].
The thermal behaviour of compound Ic observed by POM was confirmed by DSC measurements, as shown in Figure 3. During the first heating/cooling cycle ( Figure 3, 1 st Run) only one endothermic signal at 72.9°C (111.7 kJ mol −1 ) was observed, which relates to the Cr-Iso transition. On cooling from the isotropic liquid, an exothermic signal corresponding to the Iso-Col h transition at 35.7°C (4.2 kJ mol −1 ) was observed. This large thermal hysteresis is commonly observed in monotropic (metastable) phases and may vary depending on the cooling rate employed and the conditions of the measurements (DSC, POM and XRD). Continuing with the cooling, a second exothermic transition (Col h -M x ) is observed at 2.4°C (21.4 kJ mol −1 ). Due the impossibility of POM and XRD measurements at the M x temperature range, no reliable phase identification can be made, being therefore described as an unknown M x phase. No other exothermic transition was observed when the sample was further cooled until −40°C. Interestingly, the energies involved in the exothermic peaks (4.2 and 21.4 kJ mol −1 ) are significantly lower than the energy involved in melting transition (111.7 kJ mol −1 ), suggesting that the material is not in its thermodynamically more stable form. On the heating process from the M x phase ( Figure 3, 2 nd Run), a complex thermal behaviour was noticed. First, an endothermic transition was observed at 14.4°C, which is followed by two exothermic signals, a crystallisation around 25°C and another transition around 41°C. At 73°C, the melting of the material is again observed, with an energy similar to the one measured in the first heating scan. Further heating/cooling   In contrast to compound Ic, compound IIc showed an enantiotropic behaviour. Due to its waxy appearance and through observation by POM, it was verified that the material was in the liquid crystalline state at room temperature, with a transition to an isotropic liquid at around 40°C. On cooling, there was slow growth of fanshaped focal conic texture (Figure 2(b)), again indicating the formation of a Col h organisation.
The DSC analysis of compound IIc (Figure 4) confirmed the room temperature liquid crystalline behaviour, with the material melting (Cr-Col h transition) at around −13°C (14.8 kJ mol −1 ) and remaining in this phase until the Col h -Iso transition at 41.6°C (2.7 kJ mol −1 ). On cooling, the Iso-Col h transition was observed at 35.5°C (2.5 kJ mol −1 ) and the Col h -Cr transition around −18°C (11.9 kJ mol −1 ).
For both series of molecules prepared in this work, the greater the number of alkoxy chains the lower the temperature at which the transition to the isotropic liquid occurs (melting points for compounds Ia,b and IIa,b,d and clearing point for compounds Ic and IIc). The additional chains hinder the molecular packing/ proximity necessary for the crystal or mesophase maintenance, resulting in lower thermal energy necessary to the isotropisation of the material. An exception to this behaviour was observed when the dodecyloxy chains are at positions 3 and 5 (compounds Ib and IIb). In these cases, the lowest temperatures for the transition to the liquid state were observed, with the materials being in the liquid form already at room temperature. It seems that the empty space between the alkoxy chains in the positions 3 and 5 (from the aromatic ring) increases the free volume, hindering the packing necessary for both mesophase formation and crystallisation, resulting in the liquid state at room temperature. In addition, it is interesting to note that the compounds derived from the cyanopyridine core IIa-d have lower transition temperatures when compared with derivatives from the chalcone core Ia-d, although the compounds of series II have an extra aromatic ring in their structure when compared with the compounds of series I. This may be the result of the greater rigid core curvature promoted by the pyridine ring allied to the reduction of co-planarity between the pyridine heterocycle and the benzene aromatic rings [24] due to steric effects. These factors hinder effective packing, lowering the temperature necessary to bring the material to the isotropic state. The more linear structure of the chalcone central group, allied to the absence of lateral substituents, may also explain the reason why a liquid crystalline behaviour is observed for compound Id but not for IId.
The products of both series showed good thermal stability, as determined by the TGA considering the temperature at which 1% mass loss of the material occurred. It was also noted that for the products derived from the cyanopyridine core, the decomposition temperatures were slightly lower than the values for the corresponding chalcone derivatives. Within each series of compounds, those containing alkoxy chains at positions 3 and 5 (Ib and IIb) showed slightly lower decomposition temperatures when compared with the other compounds in the same series.

X-ray diffraction
In order to obtain more information on the molecular organisation in the liquid crystalline state and to unequivocally confirm the observations made by POM, powder XRD measurements were taken at various temperatures for both liquid crystalline compounds synthesised in this work (Ic and IIc).
The diffractograms presented in Figure 5(a,b) and summarised in Table 2 show typical patterns characteristic of hexagonal packing, ratifying the Col h organisation for both compounds (Ic and IIc). On cooling from the isotropic liquid, in the liquid crystalline state, the diffractograms of Ic and IIc show a strong diffraction peak at small angles region, which is assigned as the d 10 reflection. Still in the small angle region, a series of lower intensity peaks with a d-spacing ratio of √3 (d 11 ) and √4 (d 20 ) could also be observed, as typical for Col h phases. The broad reflection centred at 4.4 Å was assigned to the mean distance between the disordered aliphatic chains and the aromatic cores [33,36,37]. The reflection around 3.5 Å (d 01 ) normally present in organised columnar phases was not observed, indicating the absence of periodicity between the discs of a column, which is the result of disordered columnar packing [35,38]. For compound Ic, the presence of several diffraction peaks, including in the wide angles region, indicate the crystallisation of the material at room temperature, as opposed to what was observed by DSC and POM. The difference between the temperatures observed for the Col h -Cr transition in compound Ic is due to the different conditions under which the POM, DSC and XRD analyses were performed, this being common for monotropic phases due their instability.
Through the data obtained from the diffractograms for compounds Ic and IIc it was possible to calculate the lattice parameter values (a) for the Col h phases and, thereafter, the calculated values for the reflections (see Table 2). A good correlation between the experimental (d obs ) and calculated (d calc ) data can be observed. Besides, by using equation (1) [39][40][41], the number of mesogens present on each disc slice (Z) could also be estimated, where: Mm = molar mass; N = Avogadro's number; a = lattice parameter; h = distance between mesogens; ρ = density of the materials, assumed to be 1 g cm −3 . Due to the absence of the diffraction peak related to the periodicity in the discs packing (d 01 ), a value of 4.4 was adopted as the height of each disc slide [42], which is referent to the lateral mean distance of the aliphatic chains and mesogens, as discussed above.
The results indicate that the discs in the Col h phase for both compounds (Ic and IIc) are constituted by two mesogens. It is interesting that despite the change in the curvature from chalcone to cyanopyridine, there was no significant change in the cell parameter or the organisation of the mesogens. Since each disc slice is constituted by two mesogens, every molecule must adopt a half-disc shape, as schematically illustrated in Figure 6. It is important to note that liquid crystals are a dynamic system, and although the molecules were represented in such conformation, different conformations are likely to be present without changing significantly the disc size and shape. The molecular length (L) of the mesogens in their halfdisc shape in the most stretched conformation was estimated by minimising the energy using the MM2 method employing the ChemBio3D Ultra software (version 13.0), being the values described in Table 2.  As can be observed from the values reported in Table 2, there is a significant difference between the values for the cell parameter a (37-38 Å) and the size of the molecules in their most extended conformation (L ≈ 54.7 Å). Supposing that the disc diameter is not significantly greater than the molecular length, then the ratio of the cell parameter and the disc diameter (a/L) is approximately 0.7. This value, that is, the significant difference among the cell parameter (disc size) and the molecular length, suggests a strong interdigitation of aliphatic chains between the discs of adjacent columns, or even that the aliphatic chains are not found in their most extended conformation, filling in the empty spaces within and between the discs [41]. We can also cite dipole-dipole interactions [43] and microsegregation [44] for chalcone Ic, and also CN-CN interactions for cyanopyridine IIc, as being responsible for bringing the two molecular units together, resulting in the formation of a disc [45].

Optical properties
The photophysical properties of the compounds synthesised in this work were investigated in chloroform solution and the data are shown in Table 3. Compounds Ia-c showed absorptions maxima at 273 and 314 nm (Figure 7(a)) with molar absorptivity (ε) in the range of 2.80 × 10 4 -3.95 × 10 4 L mol −1 cm −1 , attributed to the π-π* transitions. No significant luminescence could be detected for the compounds Ia-c.
In contrast to the chalcone derivatives synthesised in this work, compounds IIa-d exhibited luminescence in the blue region of the visible spectrum, with emission maxima at around 371 nm. This resulted in Stokes shifts of 42-44 nm, which were calculated considering the difference between the wavelength of maximum absorption of the lower energy band and the maximum of the emission band of the respective molecule [46]. In addition, the quantum yields of fluorescence for IIa-d compounds had moderate values between 18% and 27%.
The study on the photophysical properties in solution revealed that the number of alkoxy chains, as well as their position, has little influence on the luminescence of compounds IIa-d. It was observed, however, that the change in the bent core from chalcone to cyanopyridine is determinant for the appearance of fluorescence in compounds IIa-d. Therefore, the cyanopyridine group is promising as luminescence inducer when included between π-conjugated units [22][23][24].

Conclusion
Two new series of bent-shaped compounds derived from chalcones and cyanopyridine were planned and prepared. From the synthesised compounds, just Ic and IIc present liquid crystalline behaviour, showing an hexagonal columnar arrangement at room temperature. However, Ic shows unstable monotropic liquid crystalline behaviour. The results of this study suggest that the number and arrangement of the alkoxy chains in the bent structures is decisive for the appearance of Col h phase, being favoured in a polycatenar (hexacatenar) structure. Results also suggest that, while the more linear structure derived from the chalcone unit is better for molecules containing a low number of aliphatic chains, the cyanipyridine bent core stabilises the liquid crystalline behaviour in hexacatenar systems.
The optical properties show that the inclusion of the cyanopyridine core between π-conjugated structures induces the fluorescence, as observed for the IIa-d compounds. Furthermore, although the position and amount of alkyl chains in the side groups may be determinant for liquid crystalline behaviour, they have little influence on the luminescence of these compounds.
Finally, the good thermal stability associated with the fluorescence induced by cyanopyridine core, in a curved design, has been shown to be effective both in relation to lamellar and columnar liquid crystalline behaviour. The polycatenar structure of compounds Ic and IIc are promising in the search of new and more efficient luminescent liquid crystalline structures.

Instrumentation
FT-IR spectra were recorded on a Bruker spectrometer (model ALPHA) in KBr discs. 1 H and 13 C NMR spectra were obtained with a Varian Mercury Plus spectrometer operating at 400 MHz ( 1 H) and 100.6 MHz ( 13 C) or, when indicated, with a Bruker AC-200F spectrometer operating at 200 MHz ( 1 H) and 50.4 MHz ( 13 C). The following abbreviations were used to designate multiplicities: s = singlet, d = doublet, t = triplet, m = multiplet, br = broad. Chemical shifts were expressed in ppm and through the coupling constant (J) in Hz. High resolution mass spectra were recorded on a MicrOTOF QII Bruker, using an atmospheric pressure photoionisation (APPI) source, and the samples being injected using a Hamilton 500 μL syringe (model 1750 RN SYR). Melting points and mesomorphic textures were determined using an Olympus BX50 microscope equipped with a Mettler Toledo FP-82 hot stage and an Olympus DP73 digital camera. Thermal transitions were determined by DSC measurements carried out on a Q2000 module (TA Instruments) using a heating/cooling rate of 10°C min −1 and a nitrogen flow of 50 mL min −1 . Thermal stability was investigated by TGA using a Shimadzu TGA-50 module, at a heating rate of 10°C min −1 and with a nitrogen flow of 20 mL min −1 . The measurements were performed in Pt crucibles and the temperature was varied from 30°C to 900°C. XRD experiments were performed with an X'Pert PRO (PANalytical) diffractometer using CuKα beam (λ = 1.5405 Å) and using the X'Celerator detector to collect the diffracted radiation. Films were prepared by depositing an amount of powder on a glass plate, where the temperature was controlled with a TCU2000 -Temperature Control Unit (Anton Paar), which allows the temperature to be controlled during the measurements. The films were heated until the isotropic phase was reached and the diffraction patterns were collected during cooling to room temperature. UV-visible and fluorescence spectra were obtained on Spectro Vision DB and Hitachi F7000 spectrophotometers, respectively, from chloroform solutions placed in quartz cuvettes.

Synthesis
The benzoic acids intermediates 3a-d, required for the synthesis of the final esters of series I and II were prepared according to literature procedures [33]. Since the synthesis of these benzoic acids is widely known, it will not be described in this paper. The synthesis of chalcone 1 is based in a literature procedure [27], with some minor modifications. The methodology as well as its characterisation data can be found in the supporting information.

3-Cyano-4,6-bis(4-hydroxyphenyl)-2methoxypyridine 2
Under constant stirring, solid sodium methoxide (248 mmol) was slowly added to a suspension constituted by chalcone 1 (12.4 mmol) and malononitrile (12.4 mmol) in methanol (40 mL). After complete addition, the mixture is stirred at room temperature for further 12 h. After this period, the formed precipitate was collected by filtration, washed with methanol and purified by column chromatography in CH 2  (E)-(1,3-diphenylprop-2-en-1-one)bis(4,1-phenylene)bis(3,4-bis(dodecyloxy)benzoate) Ia. In this method 1 eq. (0.6 mmol) of 1 (to chalcones I) or 2 (to cyanopyridines II), 2.5 eq. (1.6 mmol) of freshly prepared acid chlorides derived from acids 3a-d and 10 mL of dry CH 2 Cl 2 for the solubilisation of the reagents were placed in a bottle flask. Triethylamine was then added drop-to-drop and the solution was stirred under reflux for 24 h. At room temperature, the organic phase was then washed with water, 5% HCl and again with water. The organic phase was then dried, the solvent removed and the crude product purified by column chromatography with CH 2 Cl 2 / EtOAc 95:5 (v/v). Yield: 95%. IR (KBr) ν max cm