Bent-shaped liquid crystal dimers. Influence of the direction of the oxybiphenylenecarboxyl groups on their mesomorphic behavior

In order to study what role the direction of the whole mesogenic groups play on their mesomorphic behavior we synthesized a series of bent-shaped dimers in which the mesogens differ in their relative orientation along the long molecular axis of the dimers. A tilted anticlinic smectic mesophase is presented exclusively by the dimer where the mesogens are tethered in a head-to-head fashion. The head-to-tail and tail-to-tail organizations of the mesogens preclude any kind of mesophase formation. The mesomorphic behavior of dimers was characterized by a combination of polarizing optical microscopy, differential


Introduction
Two rigid mesogenic units connected by a conformationally flexible spacer constitutes a liquid crystal dimer (LCD) whose mesomorphic behavior manifests unique liquid crystal characteristics or resembles that observed in polymeric systems. 1,24][5] These methylene chains are considered mostly extended in an all-trans conformation within the mesophases. 4Thus, as shown in Figure 1a, dimers with even-membered spacers are attributed zigzag molecular shapes with the long molecular axis of the mesogens oriented parallel to the longitudinal molecular axis.In contrast, odd-membered dimers are considered to have bent molecular shapes with the long molecular axis of the mesogens inclined with respect to the longitudinal axis of the dimer (Figure 1b).7][8] Evidently, the mesogenic behavior depends much more on structural variations of dimers in comparison with monomeric calamitic compounds.Normally the mesophase behavior is influenced by the length and structure of the mesogenic units, the nature and length of the spacer and terminal chains, the existence of lateral groups and by the structure, position and direction of the linking groups.The last structural change has hardly been explored. 9,101) (a2) (b1) (b2) Our aim was to study what role the direction of the whole mesogenic groups play on the mesomorphic behavior of dimers with biphenylene rigid units.11 We synthesized a series of structurally related bent-shaped dimers in which both oxybiphenylenecarboxyl groups differ in their relative orientation along the long molecular axis of the dimer.The bent dimers have symmetric two-block molecular architectures with alternating non-polar aliphatic chains and rigid biphenylenes that are favorable for the generation of lamellar phases.They posses mesogen arrangements that may be regarded as built up from 4´-oxybiphenylene-4-carboxyl units linked either "head to head" as in 6D5-hh; that is, both carboxylic groups are linked by a fivemembered spacer and bear six-membered terminal chains (see Scheme 1), or "head to tail" in 6D6-ht, or "tail to tail" in 6D5-tt and 4D5-tt.A spacer with six methylene groups instead of five was used in 6D6-ht to compensate the lack of a carbonyl group in one of the linking groups and hence to retain the bent nature of the dimer structure.
The synthesis of the monomeric 6M5, bent dimers 6D5-hh, 6D6-ht, 6D5-tt, 4D5-tt and trimer 6T6-ht is outlined in Scheme 1.The synthetic routes involve standard alkylation and esterificacion reactions.It is noteworthy that a small amount of the -hydroxylated dimeric 2b, that is, the product of the self-transesterification of 2a, was unexpectedly formed during the synthesis of 2a, possibly as a result of excess reflux time.Thus, we were able to obtain the trimer 6T6-ht by a relatively short route.Overall, emphasis was placed on purity rather than yield since it is known that presence of minute quantities of residual solvent or impurities significantly affect the thermal behavior of mesogenic compounds. 12Thus, all final esters were recrystallized at least three times even though their 1 H NMR spectra corresponded to a pure substance and were thoroughly dried under vacuum.The phase transition behavior of compounds was investigated using differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) methods.Transition temperatures and associated enthalpies obtained from DSC thermograms of esters observed at cooling rates of 5 °C/min are shown in Table 1.  Figure 2 shows a set of DSC diagrams for the first cooling (1C) and subsequent second heating cycles (2H) for the 6D5-hh, 6D6-ht and 6D5-tt dimers as well as those corresponding to the monomeric 6M5 and the 6T6-ht trimer.In this set of DSC diagrams, a quite distinct thermal behavior for the three dimers is evident.However, most compounds share an enantiotropic behavior, i.e.: the exothermic phase transitions processes observed during cooling are mirrored in the subsequent heating cycle by endothermic processes that reversed those phase transitions that occurred during cooling; the exception being dimer 6D6-ht which shows an additional exothermic transition in the cooling cycle.
The DSC thermogram of 6D5-hh shows two first-order phase transitions in both heating and cooling cycles with rather large enthalpy changes indicating that appreciable molecular reorganization takes place at these two temperatures.The enthalpy change associated to the last transition at 136 °C is too large to be ascribed to a transition from a low-order nematic phase to the isotropic phase.It could instead correspond to an isotropization process from a more ordered mesophase.Indeed, POM observations of thin samples of 6D5-hh that were prepared by melting a minimum amount of compound between a clean glass slide and a cover slip, clearly showed that on cooling the isotropic liquid a mesophase was formed that exhibited a broken fan-shaped texture with a few small schlieren areas (Figure 3a).In fact, it is expected from its bent dimer structure that 6D5-hh would tend to form tilted SmC phases which appear as typical fan-shaped or as schlieren textures.Further cooling resulted in a transition to a phase which exhibited a paramorphotic focal-conic fan texture with concentric arcs (Figure 3b).The appearance of these permanent arcs is a signature of the untilted soft crystal E phase.Observations that confirmed these mesophase observations were done using DTS-treated glass substrates.These aliphatic surfaces promote the formation of planar smectic organizations where the smectic layers are parallel to the glass surface.In this case, they favor the formation of the planar schlieren textures over the focal conic fan textures.Thus, a schlieren texture which covers all the optical field was formed (Figure 3c).The texture exhibits at 132 °C not only singularities with four brushes (s= ±1) which are typical for a synclinic SmC texture, but in addition, many singularities with two brushes (s= ± 1/2) occur which have been observed in intercalated phases of smectogenic dimers. 13,14In anticlinic SmCA phases, the manifestation of such singularities is the result of an opposite tilt direction of the mesogenic groups between adjacent layers. 15,16On further cooling, the soft crystal E phase shows another of its characteristic textures, i.e., the schlieren texture of the SmCA phase changed to a 'scale'-like mosaic texture (Figure 3d).On the other hand, the monomeric 6M5 shows only an untilted SmA phase.Optical microscopy revealed that a typical smectic A phase, which exhibited typical focal-conic fan and homeotropic textures, was formed from the isotropic liquid on cooling.Further cooling produced a crystalline phase.
On the contrary, the DSC thermogram of 6D5-tt shows only one transition in both heating and cooling cycles indicative of the absence of liquid crystallinity.Moreover, on cooling the isotropic liquid a highly birefringent spherulitic growth typical of crystal-to-isotropic transitions was observed by POM.Finally, no evidence of liquid crystalline behavior was gathered from the thermal and optical studies of the head-to-tail dimer 6D6-ht as well as from the trimer 6T6-ht.The DSC diagrams of 6D6-ht show two types of phase transitions in the cooling cycle.The first transition has a rather small enthalpy change but the one associated to the last broad transition in the heating cycle at 126 °C is large and showed an undercooling effect.POM observations indicated that the isotropization process occurs at 116 °C.Surprisingly, very little birefringence emerged from the black isotropic melt on cooling, though the sample turned out not to be fluid.The texture appeared to be composed of a few dots and thin bright rods on a black field representative of a homeotropic alignment of the mesogens.On further cooling, only a small increase in the birefringence was observed at 94 °C.Observations carried out using DTS-treated glass substrates gave similar results.In order to clarify the nature of the transition to the isotropic state, cooling and subsequent heating rates of 2.5, 5, 10 and 20 °C/min were used to record the DSC traces of dimers 6D5-hh, 6D6-ht and 6D5-tt.While the liquid crystalline 6D5-hh showed a small undercooling of ca.2-3 °C in the last transition as it is expected for a mesophase-toisotropic transition, an evident cooling-rate dependence of ca.7-11 °C, which is typical of crystal-to-isotropic transitions, was found for the crystalline 6D5-tt and as well as for 6D6-ht.Therefore, the optically uniaxial solid phase of 6D6-ht most probably has a soft crystal B organization.A thermal behavior similar to that observed for the dimer 6D6-ht and evidence of the presence of an optically axial phase was gathered for the trimer 6T6-ht.Thus, we found that the tail-to-tail organization of the mesogens in 6D5-tt precludes any kind of mesophase formation.Moreover, the shorter tail-to-tail dimer 4D5-tt was prepared to test whether the absence of mesomorphism in 6D5-tt could be due to a mismatch between the length of the pentylene spacer and hexyloxy terminal chains.Indeed, similar thermal and optical behaviors were observed for both tail-to-tail dimers, thus indicating that the length of terminal chains plays no role in their lack of liquid crystallinity.Similarly, the head-to-tail organization of the mesogens in 6D6-ht is not conducive to the formation of fluid mesophases.POM observations and DSC traces indicated that only optically uniaxial soft crystalline phases of the B type were present in dimer 6D6-ht, the related hydroxylated compound 2b, and trimer 6T6-ht.A previous study 17 reports that poly(hexyleneoxybiphenylenecarboxy) exhibited a fluid tilted mesophase SmCA, thus suggesting that stabilization of a smectic phase in the head-to-tail organization can only be achieved by the polymeric state.
Indeed, it is apparent that a fluid smectic mesophase is presented exclusively by the dimer where the oxybiphenylenecarboxyl mesogens are tethered in the head-to-head fashion.This mesophase is the anticlinic tilted SmCA although its monomeric precursor 6M5 forms an untilted SmA phase.The increase in TSI on passing from 6M5 to the dimer 6D5-hh evidences that the mesogenic units in the dimer are much more correlated than in the monomeric compound given the enhanced shape anisotropy of the dimer (see Figure 4); likewise the two-fold increase in SSI/R demonstrates a significant increment in the orientational order of the mesogenic groups (see Table 1 However, the inherent collective nature of the lamellar organization could be rather susceptible to small perturbations.Thus, the stability of the smectic mesophase was tested through the mesomorphic behavior of 6D5-hh/6M5 binary mixtures whose phase diagram is shown in Figure 5.The mixing of the bent dimer and the linear monomeric compound does not originate a less ordered nematic phase to the detriment of the smectic phases.Moreover, although this phase diagram does not show continuous miscibility across the full composition range, it does show an overall stabilization of the smectic phases and, in particular, a stabilization of the SmA at the cost of the SmC phase.Analogous results have been observed in binary mixtures of chemically similar compounds which carry a single smectogen but have widely different molecular lengths and had been attributed to the avoidance of free volume by out-oflayer fluctuations. 18,19Here, we first observed that the tilted SmCA phase is stable in mixtures with up to ~20% weight fraction of the linear 6M5.The tilted and untilted mesophases then coexist in the ~20% -~30% range and finally the SmA is the only low ordered smectic phase present on increasing concentration of the short smectogen.In addition, an injected SmB mesophase not present in either smectogen appeared below the entire SmA range.Possibly, the anticlinic interlock of the bent dimer is destroyed after the addition of ca.20% weight fraction of the linear compound by increased out-of-layer fluctuations that led to an untilted orientation of the mesogens in the SmA and SmB phases.In summary, the highest clearing temperature and mesomorphism is observed for the dimer having two inner polar carboxyl groups but increasingly lower isotropization temperatures and higher tendency to crystalline order occur when the carboxyl groups move out from these positions to the periphery of the mesogenic groups.

Experimental Section
General.4'-hydroxybiphenyl-4-carboxylic acid was a gift from Nippon Steel Chemical Ltd..All other reagents and solvents were purchased from Aldrich and used without further purification unless otherwise specified.Melting points reported are not corrected. 1H NMR and 13 C NMR spectra were recorded on a Bruker ARX300 spectrometer at 25 °C.The initial phase assignments and corresponding transition temperatures for the final products were determined and imaged by thermal optical microscopy using a Leitz (Model Ortholux) polarizing microscope equipped with a pair of crossed polarizers and a hot stage (Mettler).Thin samples were prepared by melting a minimum amount of compound between a clean glass slide and a cover slip, which could be virgin or dodecyltrichlorosilane (DTS)-coated.Heating and cooling rates varied from 0.1 to 5 ºC min -1 .Temperatures and enthalpies of transitions were investigated by differential scanning calorimetry (DSC) using a TAQ20 instrument.Samples of about 5 mg were studied under a nitrogen flow at scanning rate of 5 ºC min -1 , for both heating and cooling cycles, after being encapsulated in aluminum pans.The entropy changes at the phase transition temperatures are expressed as S/R, in which S is calculated from S = H/T.H is calculated in kJ mol -1 and T is the corresponding phase transition temperature in Kelvin.

Preparation of dodecyltrichlorosilane (DTS)-coated glass slides for POM observations.
The microscope slides were cleaned in freshly prepared piranha solution (70% H2SO4, 30% H2O2) for 30 min at 80 °C using nitrogen to provide agitation, and then exhaustively rinsed with deionized water.Warning: Piranha solution should be handled with caution.It has detonated unexpectedly; likely, it has been inadvertedly mixed with significant amounts of an oxidizable organic material.Next, the slides were cleaned in base solutions (70% KOH, 30% H2O2) for 30 min at 80 °C, and then exhaustively rinsed with deionized water, ethanol and finally methanol.The clean slides were dried under a stream of nitrogen and stored overnight at 110 °C.The slides were immersed in 10 mM solution of dodecyltrichlorosilane (DTS) in CH2Cl2 for 30 min at room temperature, rinsed with CH2Cl2 and dried under nitrogen.

Figure 2 .
Figure 2. DSC termograms of first cooling and second heating cycles.

6 Figure 4 .
Figure 4. RM1 gas-phase structures of a) 6M5 and b) 6D5-hh.The aspect ratio R = L/D was calculated considering the molecular length, L, and the largest diameter, D, found along the molecular inertial axes.

Figure 5 .
Figure 5.The composition-temperature phase diagram for the binary system with compounds 6D5-hh and 6M5.The dotted lines were added as a visual aid to circumscribe the region of nonmiscibility.