Novel liquid crystalline fluorene and fluorenone MIDA boronates: influence of alkyl vs alkoxy side chains on the mesomorphic properties

ABSTRACT To obtain novel boron liquid crystals, we successfully combined MIDA boronates with chromophoric fluorene and fluorenone mesogenic units and studied the influence of alkoxy versus alkyl side chains by differential scanning calorimetry (DSC), polarising optical microscopy (POM) and X-ray diffraction (XRD). The fluorenes showed broader temperature ranges of the SmA phase compared to their fluorenone counterparts. By replacing the alkoxy side chains with alkyl side chains, the mesophase ranges could be significantly enlarged and the thermal stability increased. Graphical abstract

We have recently developed thermotropic liquid crystals 4, carrying the bulky and highly polar N-methyl-iminodiacetic acid (MIDA) boronate group [57][58][59].In contrast to non-mesomorphic precursors such as pinacol borolanes and boronic acids, it was found that the pronounced polarity of the MIDA group, indicated by the high dipole moment of 7.6 D as compared to 2.8 D for the corresponding boronic acid was essential for mesophase formation and stabilisation [57].While the previous studies mostly focused on phenyl, biphenyl and heteroaryl mesogenic units [57][58][59], we were curious how the combination of fluorene or fluorenone moieties with the MIDA unit would affect the liquid crystalline self-assembly of the resulting fluorene or fluorenone MIDA boronates (Scheme 1).Our study revealed that the merging of the two molecular entities is indeed beneficial for CONTACT Sabine Laschat sabine.laschat@oc.uni-stuttgart.deSupplemental data for this article can be accessed online at https://doi.org/10.1080/02678292.2024.2319626.

Mesomorphic properties of alkoxybromofluorenes, alkyl-and alkoxy-substituted fluorene and fluorenone MIDA boronates
Preliminary investigations of all alkyl-and alkoxy fluorene and fluorenone derivatives by polarising optical microscopy (POM) revealed that most precursors and fluorene or fluorenone intermediates 10(n), 12(n), 18 (O-n) were non-mesomorphic and showed only isotropic melting.The only exception were 2-alkoxy-7-bromofluorenes 17(O-n), which showed mosaic-like textures upon cooling (Figure 1).In addition, for derivatives with side chains C 12 or C 14 17(O-12) and 17(O-14) respectively, at higher temperatures Maltese crosses with homeotropic alignment were visible (Figure 1).Differential scanning calorimetry (DSC) of 17(O-10) revealed upon heating an endothermal crystal to crystal transition at 67°C, followed by a hardly visible endothermal melting transition at 71°C and endothermal clearing transition at 96°C (Figure 2).In the subsequent cooling cycle, the isotropic to mesophase transition appeared at 94°C and crystallisation at 27°C with a strong hysteresis due to supercooling.In case of 17(O-12) an endothermal melting transition at 74°C, followed by an endothermal mesophase to mesophase transition at 81°C and clearing transition at 90°C were visible upon heating, which appeared in the cooling cycle at 89°C, 78°C and 46°C, respectively.Similar behaviour was found for 17(O-14), while the highest homologue 17(O-16) displayed only a monotropic mesophase.The DSC results are summarised in Table 1.
In order to assign mesophase geometries compound 17(O-12) was examined by X-ray diffraction (XRD) experiments (Figure 3, Table 2).At 85°C, a sharp reflection at 27.24 Å was detected in the small-angle section, which was assigned as the (001) layer reflection of a SmA phase (d = 28.34Å).In the wide-angle section, a diffuse halo around 4.6 Å was observed, which was caused by the fluid alkoxy side chains (Figure 3).The SAXS diffractogram at 57°C also showed the distinct (001) layer reflection at 27.24 Å and in the WAXS the halo was visible.However, the halo was superimposed by three sharp wide-angle reflections at 4.81 Å, 4.26 Å and 3.50 Å, which were indexed as (110), ( 200) and (210) reflection of a smectic E phase.Thus, the low-temperature phase was assigned as SmE phase with lattice parameters a = 8.52 Å, b = 5.83 Å, c = 27.24Å in agreement with the known 4-pentyloxy-4'-bromo-benzylidene aniline [68] and 2-bromo-6-dodecyloxyazulene [69] displaying SmE phases.By using the formula [70] the number Z of molecules per unit cell was calculated to Z = 2.The observed layer distance of 27.24 Å is somewhat larger than the calculated molecular lengths of 17(O-12) in the all-trans conformation (L calc = 25.74Å).In case of a smectic monolayer a smaller d-value as compared to L calc would be expected.In order to rationalise the experimentally observed results, a smectic bilayer with partial interdigitation of the side chains might be assumed.According to the literature in soft crystalline SmE phases, the aromatic mesogens are oriented in an    antiparallel herringbone pattern [71,72].Our proposed packing model with antiparallel fluorene units is shown in Figure 4.  3) and POM (Figure 8).A typical DSC curve is shown for alkyl fluorene MIDA boronate Flu(12)MIDA in Figure 5. Upon 1 st heating an endothermal melting transition at 186°C and a clearing transition at 257°C were observed.Upon 1 st cooling, the mesophase reappeared at 250°C followed by a glass transition at 128°C, which was hardly visible in the DSC.Upon subsequent heating/cooling cycles, decomposition was observed.Similar results were obtained for the higher homologues Flu(n)MIDA (Figure S1-S4, ESI).In case of the alkoxy fluorene MIDA boronates, Flu (O-n)MIDA peaks were sometimes visible in the cooling cycle (Figure 5b) and sometimes not due to thermal decomposition (Figure S1-S4, ESI).The DSC results are summarised in Table 3.For some samples, glass transitions could not be observed during the DSC measurements (Figure 5c).These transitions were determined via POM.This phenomenon was also observed during our previous studies on MIDA   boronates [57][58][59].For this purpose, the corresponding sample was heated and cooled with the same rates as in the DSC.If a phase transition occurred, the heating and cooling was reiterated with a rate of In the alkoxy-fluorenone MIDA boronate series FluO(O-n)MIDA melting transitions were significantly higher as compared to the alkyl series FluO(n)MIDA, but remained independent of the chain lengths.On the other hand, clearing transitions increased significantly with increasing chain lengths.Thus, FluO(O-16)MIDA showed the broadest and most stable mesophase (58 K) within the series.
When comparing MIDA boronates with alkyl vs alkoxy side chains (Figure 6), the most obvious influence of the alkyl side chains is the significant decrease of the melting points.A similar observation was reported previously for the melting into the nematic phase of calamitic alkyl-cyanobiphenyls (vs alkoxycyanobiphenyls) [73][74][75][76][77] and cyanophenyl-4-alkylbenzoates (vs cyanophenyl-4-alkoxy-benzoates) [8,[78][79][80][81][82][83][84].These experimental results might be rationalised by the lower polarisability of the alkyl chain as compared to the alkoxy chain, resulting in lower anisotropy of the connection between aryl moiety and alkyl side as compared to aryl -alkyloxy connections [85].Upon transition from the crystalline to the mesophase these dipolar interactions must be overcome in order to self-assemble into the liquid crystalline phase.The lower anisotropy of derivatives with alkyl chains thus results in reduced transition temperatures.Furthermore, according to Arakawa, the different bond geometries of aryl unit plus side chains have to be considered (Figure 7) [85].In case of alkyl-substituted aryl moieties an out-of-plane conformation is preferred in order to avoid unfavourable steric interactions between the CH 2 unit and the ortho-H atoms.In contrast, for alkoxy-substituted aryl moieties an in-plane conformation is preferred because both lone pairs at the O atom and the C-H bonds of the neighbouring CH 2 unit can adopt a gauche  The following phases were observed: Crystalline (Cr); glass-like (G); smectic A (SmA); isotropic liquid(I).b Values from DSC 1 st or 2 nd cooling cycle with cooling/heating rates of 10 K • min −1 ; c Phase transitions determined via POM due to the absence in DSC traces (cooling/heating rates of 2 K • min −1 ).d Decomposition during first heating cycle.
conformation with respect to the ortho-H atoms, thus minimising any steric interactions [85].Thus, the out-of -plane conformation (i.e. the orthogonal arrangement of the alkyl chain with respect to the plane of the aryl ring) results in a decreased tendency for crystallisation, lower crystallisation temperatures or even glass transitions and vitrification.It should be noted that Imrie recently discussed similar arguments to rationalise the influence of alkyl vs alkoxy vs thioether side chains in calamitic dimers with cyanobiphenyl [86,87] and/or imine unit [88] showing twist-bend nematic phases.Significantly higher values of T NI were found for dimers with alkoxy or thioether chains, which was explained by the in plane conformation of the alkoxy chain and the higher shape anisotropy of these dimers [86,87].DFT calculation revealed bond angles of C-O-C (119°) > C-C-C (113.5°)> C-S-C (100.5°) in agreement with the observed trend of T NI values [88].
Under the POM fan-shaped textures, Maltese cross defects as well as areas with homeotropic alignment were observed for all four MIDA boronate series (Figure 8).
In Flu(O-12)MIDA showed a sharp reflection in the small-angle region at 2.03, which was assigned as (001) reflection of a SmA phase with a layer distance d = 43.4Å (Figure 8, Table 4).In the wide-angle region, the broad halo around 5.

Conclusion
In order to understand the liquid crystalline self-assembly of mesogens carrying the highly polar MIDA group containing alkyl-or alkoxy-substituted fluorene and fluorenone core units four series of novel calamitic liquid crystals were synthesised starting from fluorenone.Merging flat fluorene or fluorenone cores with bulky MIDA head group leads to stable SmA phases.When comparing the influence of the additional lateral dipole moment in fluorenones as compared to fluorenes on the mesophase stability and temperature range with the influence of the side chain (alkyl vs. alkoxy), the side chain has a much more pronounced impact on the mesomorphic properties.In other words, MIDA boronates with alkyl side chains displayed broader and more stable SmA phases as compared to the MIDA boronates with alkoxy side chains.On the other hand, when side chains were kept constant, the replacement of the fluorenes by fluorenones had a much smaller influence.
In general, fluorenes displayed broader temperature ranges of the SmA phase in comparison with the fluorenones.More importantly, alkyl side chains resulted in significantly decreased melting transitions as compared to derivatives with alkoxy side chains, which was rationalised by the reduced anisotropy of the connecting unit between aryl moiety and side chain and the conformational preference of an out-ofplane conformation for alkyl-substituted derivatives vs. an in-plane conformation for the alkoxysubstituted derivatives.
Our results revealed that the polar MIDA unit can be successfully combined with chromophoric mesogenic units to obtain novel liquid crystals.Moreover, the replacement of the archaetypal alkoxy side chains by alkyl side chains provides a tool to broaden the mesophase range and to overcome thermal stability problems, which might be useful for other calamitic liquid crystal classes as well.
For the higher temperature SmA phase of 17(O-12) also a partially interdigitated bilayer SmA was proposed in order to explain the layer distance of 28.34 Å.After having solved the mesophase assignment of 2-alkoxy-7-bromofluorenes 17(O-n), we investigated the fluorene and fluorenone MIDA boronates Flu(n) MIDA, Flu(O-n)MIDA, FluO(n)MIDA, FluO(O-n) MIDA by DSC (see Figure 5, 6 and Table

Figure 4 .
Figure 4. (Colour Online) Proposed packing model of the SmE phase of 17(O-12) in (a) top view, (b) side view and the proposed packing model of the SmA bilayer (c).
2 K/min to determine the transition temperature as accurate as possible.Also the series of alkoxy fluorene MIDA boronates Flu(O-n)MIDA suffered from thermal decomposition upon prolonged heating/cooling cycles.With some exceptions, the alkyl-and alkoxy-substituted fluorenone MIDA boronates displayed more reproducible DSC curves.Typical examples are shown for FluO (12)MIDA and FluO(O-12)MIDA respectively (Figure 5).The series of alkyl-fluorene MIDA boronates Flu(n) MIDA showed melting points in range of 166-174°C and clearing temperatures close to 250°C resulting in broad mesophase widths of 71-89 K irrespective of the chain lengths (Figure6).In contrast, the corresponding alkoxy-fluorene MIDA boronates Flu(O-n)MIDA displayed higher melting and lower clearing transitions with much smaller mesophase widths of only 22-43 K as compared to the alkyl series.The maximum mesophase stability and temperature range was found for the C 12 and C 14 derivatives.For the alkyl-fluorenone MIDA boronates FluO(n) MIDA melting and clearing temperatures increased with increasing chain lengths up to C 14 and then decreased, resulting in phase widths of 57-77 K (Figure6).In general, alkyl-fluorenone MIDA boronates FluO(n)MIDA and their fluorene counterparts Flu(n)MIDA showed better thermal stability and reproducibility as compared to alkoxy-fluorenone MIDA boronates FluO(O-n)MIDA and alkoxy-fluorene MIDA boronates Flu(O-n)MIDA respectively.
order to assign the phase geometries of the fluorene and fluorenone MIDA boronates exemplarily SAXS and WAXS experiments were carried out for Flu(O-12)MIDA, FluO(O-12)MIDA, Flu(16)MIDA and FluO(14)MIDA.Unfortunately, decomposition of the samples Flu(16) MIDA and FluO(O-12)MIDA was observed during irradiation time and therefore XRD measurements were abandoned for these samples.

Figure 7 .
Figure 7. Out-of plane and in-plane conformations of alkyl and alkoxy chains on six-membered aromatic rings.The figure was adapted from ref [85].

Table 2 .
X-ray diffraction data of bromoalkoxyfluorene 17(O-12).The measurements were performed during cooling from the isotropic liquid phase.The halo was determined from WAXS.

Table 4 .
X-ray diffraction data for fluorene Flu(12)MIDA and fluorenone FluO(14)MIDA.The measurements were performed during cooling from the isotropic liquid phase.The halo was determined from WAXS.