Fluorene Thiophene α-Cyanostilbene Hexacatenar-Generating LCs with Hexagonal Columnar Phases and Gels with Helical Morphologies as Well as a Light-Emitting LC Display

Two series of novel synthesized hexacatenars, O/n and M/n, containing two thiophene-cyanostilbene units interconnected by central fluorene units (fluorenone or dicyanovinyl fluorene) using a donor–acceptor–acceptor–donor (A–D–A–D–A) rigid core, with three alkoxy chains at each end, can self-assemble into hexagonal columnar mesophases with wide liquid crystal (LC) ranges and aggregate into organogels with flowerlike and helical cylinder morphologies, as revealed via POM, DSC, XRD and SEM investigation. Furthermore, these compounds were observed to emit yellow luminescence in both solution and solid states which can be adopted to manufacture a light-emitting liquid crystal display (LE-LCD) by doping with commercially available nematic LC.

Conjugated donor-acceptor (D-A) molecules could give rise to efficient fluorescence and charge transport and provide low band gap semiconductors due to intramolecular D-A interactions [16][17][18][19][20][21][22]. As one of the most extensively studied acceptor moieties, fluorene moieties are famous due to their high blue-emitting efficiency and good thermostability [23,24]. Among many donor moieties, thiophene is often used because of its unique optical property and electron transport capability [25,26].

Synthesis
Final products O/n and M/n were prepared according to the method of synthesis shown in Scheme 2. Firstly, fluorene was bromided with bromine in chloroform yielding 2,7-dibromofluorene, followed by oxidation with chromium trioxide, producing 2,7dibromofluorenone 3 [55,56]. Then, compound 3 was further reacted with bis(pinacolato) diboron, leading to 2,7-fluorenone borate 4. 4-hydroxyphenylacetonitrile was etherified with benzyl chloride 8/n [57] to afford intermediate 9/n. A Knoevenagel reaction between intermediate 9/n and 5-bromo-2-thiophene carbaldehyde produced intermediate 10/n. A Suzuki coupling reaction between 10/n and 4 generated the target compounds O/n. O/n were converted to M/n via a condensation reaction with malononitrile. The synthesis methods and structural identification data are displayed in Supporting Information.

Mesomorphic Properties
Polarized optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) were used to investigate the LC properties of the synthesized compounds O/n and M/n (Table 1). Under POM, compounds O/n and M/n exhibited mosaic and spherulitic textures as specialized for columnar phases in their LC ranges (Figures 1a, 2a and S1). The small angle X-ray diffraction (SAXS) pattern of M/14 at 180 • C ( Figure 1b) showed d −2 values of the three peaks in a ratio of 1:3:4, with the respective Miller indices, (10), (11) and (20), indicating a two-dimensional hexagonal lattice. The lattice constant of M/14 calculated using the XRD results was 5.65 nm (Tables 1 and S1). The number of molecules in the cross section of column was calculated to be about three (Table 1). To satisfy the maximum amount of space filling, three molecules should be parallelly aligned in a disc. Afterwards, these discs were stacked into a cylinder, and the formed cylinders further arranged into a columnar liquid crystal phase with a p6mm lattice (Figure 1b, insert map). Micro-segregation of the central rigid aromatic core from the peripheral flexible alkyl chains and π-π stacking are the driving forces for such colum-3 of 12 nar packing. A snapshot of the molecular dynamic simulation for the Col hex /p6mm phase ( Figure 1d) as well as the reconstructed ED map (Figure 1c) also demonstrated such packing.

Mesomorphic Properties
Polarized optical microscopy (POM), differential scanning calorimetry (DSC) and Xray diffraction (XRD) were used to investigate the LC properties of the synthesized compounds O/n and M/n ( Table 1) (Tables 1 and S1). The number of molecules in the cross section of column was calculated to be about three (Table 1). To satisfy the maximum amount of space filling, three molecules should be parallelly aligned in a   Figure S2). Cr = crystalline solid; Iso = isotropic liquid; Col hex /p6mm = hexagonal columnar phase; a = lattice parameter determined via SAXS; µ = number of molecules in the cross section of the column (µ = (a 2 /2) √ 3h(N A /M)ρ, presuming density of ρ = 1 g/cm 3 and a columnar disk height of h = 0.43-0.44 nm equal to the measured maxima of the diffuse wide-angle scatterings, as displayed in Figures S3, S4, S5b and S6b). b was identified via contact experiment, which showed that all compounds are of a hexagonal columnar phase. c Transition temperatures were identified via POM. disc. Afterwards, these discs were stacked into a cylinder, and the formed cylinders further arranged into a columnar liquid crystal phase with a p6mm lattice (Figure 1b, insert map). Micro-segregation of the central rigid aromatic core from the peripheral flexible alkyl chains and π-π stacking are the driving forces for such columnar packing. A snapshot of the molecular dynamic simulation for the Colhex/p6mm phase ( Figure 1d) as well as the reconstructed ED map (Figure 1c) also demonstrated such packing.  Figure S2). Cr = crystalline solid; Iso = isotropic liquid; Colhex/p6mm = hexagonal columnar phase; a = lattice parameter determined via SAXS; μ = number of molecules in the cross section of the column (μ = (a 2 /2)√3h(NA/M)ρ, presuming density of ρ = 1 g/cm 3 and a columnar disk height of h = 0.43-0.44 nm equal to the measured maxima of the diffuse wide-angle scatterings, as displayed in Figures S3, S4, S5b and S6b). b was identified via contact experiment, which showed that all compounds are of a hexagonal columnar phase. c Transition temperatures were identified via POM.  The SAXS pattern for O/12 at 200 °C is shown in Figure 2b, the d −2 values of the three peaks in the ratio of 1:3:4, which can also be indexed to the 10, 11 and 20 reflections of hexagonal lattice with p6mm symmetry. The lattice constant of O/12 calculated using the XRD results is 5.37 nm (Tables 1 and S2). The number of molecules (μ) in the cross section of the columns was approximated as three (Table 1, Table S3 and S4). The suggested packing model for such a hexagonal columnar phase is shown in the insert of Figure 2b, which is in line with the molecular dynamics (MD) annealed model (Figure 2d) as well as the reconstructed electron density map (Figure 2c). The optimized molecular structures of O/n and M/n, calculated using density functional theory (DFT) (Gaussian 09W B3LYP/(6-31G, d)), are shown in Figure S11. It can be seen that the two cyano groups from CS, the carbonyl group of fluorenone and the cyano groups of fluorenone malononitrile are located on the same side of the π-conjugated rigid core.
By comparison with the former reported fluorene polycatenars FC/n [46], FO16 and FCN16 [47] reported here have compounds with broader LC ranges ( Figure 3); the introduction of thiophene α-increases greatly increases greatly the clear points, and the mesophase ranges are also widened greatly. Clearly, this should be due to the extended conjugated units reinforcing the π-π stacking, with the introduction of α-cyanostilbene group producing a strong dipole-dipole interaction. The optimized molecular structures of O/n and M/n, calculated using density functional theory (DFT) (Gaussian 09W B3LYP/(6-31G, d)), are shown in Figure S11. It can be seen that the two cyano groups from CS, the carbonyl group of fluorenone and the cyano groups of fluorenone malononitrile are located on the same side of the π-conjugated rigid core.
By comparison with the former reported fluorene polycatenars FC/n [47], FO16 and FCN16 [48] reported here have compounds with broader LC ranges ( Figure 3); the introduction of thiophene αincreases greatly increases greatly the clear points, and the mesophase ranges are also widened greatly. Clearly, this should be due to the extended conjugated units reinforcing the π-π stacking, with the introduction of α-cyanostilbene group producing a strong dipole-dipole interaction.

Gelation Behavior
Compounds O/16 and M/14 were chosen as representatives to study the gel properties of these compounds. Investigation with "stable to inversion of the container" methods [57] indicated that both O/16 and M/14 could generate organogels in 1,4-dioxane (10

Gelation Behavior
Compounds O/16 and M/14 were chosen as representatives to study the gel properties of these compounds. Investigation with "stable to inversion of the container" methods [58] indicated that both O/16 and M/14 could generate organogels in 1,4-dioxane (10 mg/mL) ( Table 2). The morphologies of the xerogels were studied via SEM investigation. Xerogel formed by O/16 in 1,4-dioxane displayed a microsphere with flower-like morphologies. The sizes of the spheres were non-uniform, with the diameter ranging from 1 µm to 2.72 µm (Figure 4a,b). The enlarged image of an individual sphere indicated that the spheres were constituted by a nanosheet with a thickness from 30 nm to 100 nm and an average diameter of 0.4-0.6 µm, as shown in Figure 4b. Interestingly, the gel morphology of M/14 in 1,4-dioxane showed the typical spiral cylindrical morphologies, with diameters of about 3.75-12.5 µm (Figure 4c,d). The dipolar-dipolar interaction, π-π interaction, Var der Waals force, etc., could all promote the formation of gels. The possible gel formation process is demonstrated in SI ( Figure S7).

Photophysical Properties
The UV-vis and fluorescence spectra of the representative compounds O/12 and M/12 were investigated (Table 3, Figures 5 and S8). Compared with the UV-vis spectra in the solution, the UV-vis spectra of O/12 and M/14 in the thin film displayed red-shifts from 425 to 437 nm and 429 to 473 nm, respectively (Table 3, Figure 5), and the fluorescence

Photophysical Properties
The UV-vis and fluorescence spectra of the representative compounds O/12 and M/12 were investigated (Table 3, Figures 5 and S8). Compared with the UV-vis spectra in the solution, the UV-vis spectra of O/12 and M/14 in the thin film displayed red-shifts from 425 to 437 nm and 429 to 473 nm, respectively (Table 3, Figure 5), and the fluorescence spectra of the films were also red-shifted (602 to 666 nm for O/12 and 493 to 532 nm for M/12). Thus, a J-type parallel π-π aggregating style in the solid was suggested [59]. The emission spectra of O/12 in the dilute solution and solid state displayed emissions at 602 nm and 666 nm, respectively. This means a large Stokes shift (177 nm in the THF solution, and 229 nm in solid state). The energy gap calculated using the UV spectra E opt g (eV), DFT calculations (B3LYP/6-31G) E cal g (eV) and cyclic voltammetry E CV g (eV) (Figures S9 and S10) are almost identical, with the value about 2.60 eV for O/12 and 2.20 eV for M/12. The energy gap of M/n is smaller than that of O/n; this may be due to the stronger electron drawing of malononitrile in M/n. This investigation indicated that these compounds had potential as semiconductors. a Excited with absorption maxima band; b Stokes shifts: λ em -λ Abs ; c measured in the THF solution; d measured in solid state; relative to quinine sulfate in 0.1 M H 2 SO 4 (Φ FL = 0.54) as the standard; E OPT g energy band gap in THF solution calculated using the UV-vis spectrum; according to hν − (Ahν) 2 , the tangent of the curve edge is made, and the intersection with the X axis is the spectral energy gap of the compound; E CV g genergy band gap in film calculated using CV on glassy carbon electrode in 0.1 mol/L of Bu 4 NBF 4 in CH 3 CN with a scan rate of 100 mVs −1 , E g 2 = E ox − E red ; E g cal energy band calculated using DFT/B3LYP, 6-31G (d), E g cal = E LUMO − E HOMO . The solvent effects on the photophysical properties of representative compound O/12 were studied. The absorption spectra changed little by increase of the solvent polarity, meaning that the solvent polarity had little influence on the structure and electronic properties of the ground state ( Figure S14a), while the fluorescence spectra changed largely with increasing solvent polarity ( Figure S14b). Fluorescence quantum yields (ΦFL) measured in different solutions ranged from 0.061 to 0.579 (Table 4) [59]. The ΦFL of O/12 was 0.251 in solid state, indicating that O/12 could be suitable for LE-LCD and bioimage application (Table 4) [60][61][62][63][64], while the fluorescence of M/14 was very weak in both the solution and solid state, which is unsuitable for LE-LCD studies ( Figure S8).  The solvent effects on the photophysical properties of representative compound O/12 were studied. The absorption spectra changed little by increase of the solvent polarity, meaning that the solvent polarity had little influence on the structure and electronic properties of the ground state ( Figure S14a), while the fluorescence spectra changed largely with increasing solvent polarity ( Figure S14b). Fluorescence quantum yields (Φ FL ) measured in different solutions ranged from 0.061 to 0.579 (Table 4) [60]. The Φ FL of O/12 was 0.251 in solid state, indicating that O/12 could be suitable for LE-LCD and bioimage application (Table 4) [61][62][63][64][65], while the fluorescence of M/14 was very weak in both the solution and solid state, which is unsuitable for LE-LCD studies ( Figure S8). Surprisingly, the AIEE phenomena usually observed in CS compounds are only observed for intermediate 10/12, but not for the final product O/12; the corresponding experimental data and explanation are shown in SI (Figures S12 and S13).

Polarized Emission Spectra, the Dichroic ratio and LE-LCD Device
The solid fluorescence quantum yield of compound O/12 was measured to be 25.1%. Thus, the potential application of O/n as an LE-LCD device was studied. The dichroic ratio of the polarized fluorescence spectra for the LC mixture of 0.5 wt% O/12 with nematic LC SLC9023 was measured to be 10.96 when the electric field was off (Figure 6a), and about 2.17 when the electric field was on (Figure 6b). Such values mean that the LC mixture of O/12 and SLC9023 were suitable for fabrication of the LE-LCD device. Thus, the LE-LCD device was homemade. Firstly, patterned ITO glass substrates were used to prepare the LC cell. Then, LC mixtures (SLC9023 + 0.5 wt% O/12) were poured into the LC cell. The unidirectional array of the LC mixtures was obtained by rubbing the polyimide (PI)-aligned layer (the device fabrication details are presented in SI). The LC mixtures were irradiated using a UV lamp. The variation in PL efficiency was detected using a polarizer, the transmission direction of which was transmitted parallel to the LC-aligned direction. When the electric field was turned off, the LC cell under UV illumination was bright. Under an electric field of 1 KHz and 5 v, only the central region without ITO was bright. Thus, switching from brightness to darkness was realized ( Figure  7). Thus, the LE-LCD device was homemade. Firstly, patterned ITO glass substrates were used to prepare the LC cell. Then, LC mixtures (SLC9023 + 0.5 wt% O/12) were poured into the LC cell. The unidirectional array of the LC mixtures was obtained by rubbing the polyimide (PI)-aligned layer (the device fabrication details are presented in SI). The LC mixtures were irradiated using a UV lamp. The variation in PL efficiency was detected using a polarizer, the transmission direction of which was transmitted parallel to the LCaligned direction. When the electric field was turned off, the LC cell under UV illumination was bright. Under an electric field of 1 KHz and 5 v, only the central region without ITO was bright. Thus, switching from brightness to darkness was realized (Figure 7). bing the polyimide (PI)-aligned layer (the device fabrication details are presented in SI). The LC mixtures were irradiated using a UV lamp. The variation in PL efficiency was detected using a polarizer, the transmission direction of which was transmitted parallel to the LC-aligned direction. When the electric field was turned off, the LC cell under UV illumination was bright. Under an electric field of 1 KHz and 5 v, only the central region without ITO was bright. Thus, switching from brightness to darkness was realized ( Figure  7).

Conclusions
Two novel series of A-D-A-D-A π-conjugated hexacatenars, consisting of a fluorenone or dicyanoethenyl fluorene core and with thiophene-cyanostilbene on both sides, can self-organize into columnar phases with a p6mm lattice and form organogels with nanoflower and helical cylinder morphologies. Finally, the potential of O/12 as a yellowemitting LE-LCD device has been realized. Thus, the design principle which combines

Conclusions
Two novel series of A-D-A-D-A π-conjugated hexacatenars, consisting of a fluorenone or dicyanoethenyl fluorene core and with thiophene-cyanostilbene on both sides, can selforganize into columnar phases with a p6mm lattice and form organogels with nanoflower and helical cylinder morphologies. Finally, the potential of O/12 as a yellow-emitting LE-LCD device has been realized. Thus, the design principle which combines central fluornene with thiophene-cyanostilbene on both sides of the rigid core is a successful strategy to obtain stable columnar mesophases with wide LC ranges, orgnogels with interesting chiral morphologies and semiconductor materials with interesting properties.
Author Contributions: Conceptualization, H.Z. and X.C.; methodology, H.Z. and X.C.; software, H.Z.; validation, H.Z. and X.C.; investigation, H.Z. and X.C.; writing-original draft preparation, H.Z. and X.C.; writing-review and editing, H.Z. and X.C. All authors have read and agreed to the published version of the manuscript.