Novel diphenylacrylonitrile-decorated cyclotriveratrylene columnar liquid crystals with high fluorescence

ABSTRACT Although some cyclotriveratrylene (CTV) liquid crystals had been reported, the CTV liquid crystal with high fluorescence in mesophase was not presented up to now. By introducing diphenylacrylonitrile units on CTV skeleton, two CTV derivatives with AIE units (7a and 7b) were prepared for the first time. Their structures and crown conformations were characterised by 1H NMR, 13C NMR, MALDI-MS spectra and elemental analysis. Samples 7a and 7b exhibited the reversible liquid crystalline behaviours with hexagonal columnar mesophase supported by the DSC, POM and XRD analysis. The studies on photophysical properties of samples 7a and 7b suggested the high fluorescence in solid state and mesophase. The absolute fluorescence quantum yield of 7b in mesophase was as high as 0.45. This work not only reported the first CTV liquid crystals with high fluorescence in aggregated state but also provided a good strategy for constructing the novel CTV liquid crystals with excellent fluorescence. GRAPHICAL ABSTRACT

Nevertheless, because of only few examples on CTV liquid crystals, the development of novel CTV columnar liquid crystals and studies on their properties were always expected.
On the other hand, the luminescent liquid crystals had become more and more important roles among various liquid crystals due to the effective combination of intrinsic luminescence property and intermolecular self-assemble within a mesophase [28,29].But luminescence abilities of liquid crystals based on normal CONTACT Fafu Yang yangfafu@fjnu.edu.cnSupplemental data for this article can be accessed online at https://doi.org/10.1080/02678292.2024.2302451.
dyes were usually weakened dramatically in aggregated state due to the aggregation caused quenching (ACQ) effect.In order to solve this problem, lately, the aggregation-induced emission (AIE) were used successfully to prepare luminescent liquid crystal in aggregated states [30][31][32][33][34][35][36][37][38][39][40].For examples, AIE skeleton had been applied to synthesise series fluorescent liquid crystals based on pyrene, perylene and Bodipy cores in aggregated states [41][42][43][44][45][46][47][48].However, the non-coplanar and dissymmetric structure of AIE skeleton (such as diphenylacrylonitrile) was not favourable for constructing columnar liquid crystals.Theoretically, if diphenylacrylonitrile moieties were introduced onto CTV skeleton, it was possible to obtain novel CTV columnar liquid crystals with good fluorescence in aggregated state.To the best of our knowledge, this kind of liquid crystalline molecule was not reported up to now.In this work, we wish to report the design and synthesis of novel diphenylacrylonitrile-decorated cyclotriveratrylene, exhibiting not only the columnar liquid crystal but also the high fluorescence in aggregated state, which was observed for CTV liquid crystals for the first time.

Synthesis and characterization
In order to introduce diphenylacrylonitrile units onto cyclotriveratrylene skeleton, the click chemistry was employed as the coupling step based on its effect and simple synthetic procedure.Also, the aromatic triazole rings produced by click reaction were favourable for enhancing π-π stacking of columnar mesophase.On the other hand, the three diphenylacrylonitrile units and three/six peripheral dodecyl chains onto diphenylacrylonitrile units ensured the symmetric structure for columnar mesophase.The synthetic routes were illustrated as Scheme 1.According to the published procedure, diphenylacrylonitrile derivatives 3a and 3b were prepared in good yields [49].By reacting compounds 3a and 3b with bromochloropropane, compounds 4a and 4b were obtained after column chromatography in yields of 76% and 74%, respectively.On the other hand, compound 6 was synthesised conveniently by using o-methoxyphenol as starting materials according to the published procedure [50].Further treating compound 4a or 4b with 6 in DMF solution in the presence Scheme 1.The synthetic routes of compounds 7a and 7b.
of NaN 3 , CuI and N(C 2 H 5 ) 3 , the click reaction was smoothly carried out and target compounds 7a and 7b were collected after column chromatography in yields of 74% and 78%, respectively.The structures of new compounds were supported by 1 H NMR, 13 C NMR and MALDI-TOF-MS and elemental analysis (see SI).The molecular ion peaks in MS spectra agreed well with the calculating molecular weights of samples 7a and 7b.Their 1 H NMR spectra were also in accordance with their structures.Specially, a pair of doublet for the ArCH 2 Ar of CTV skeleton strongly implied the crown conformations for samples 7a and 7b, which contributed to the bowl-shaped structures.

Mesomorphic studies
As the mesomorphic properties of precursors 4a and 4b (or their analogues) had been confirmed [46,51], it was expected that samples 7a and 7b possessed mesomorphic properties.The mesomorphic behaviours of samples 7a and 7b were explored preliminarily by differential scanning calorimeter (DSC).As shown in Figure 1 for the DSC curves of samples 7a and 7b, two thermic peaks were observed for their second heating and first cooling circles.Table 1 summarised the phase transition temperatures as well as thermodynamic data.Sample 7a presented two exothermic peaks at 60.9°C and 89.5°C on second heating, and two endothermic peaks at 56.2°C and 87.6°C upon cooling, respectively.As to sample 7b, the two exothermic peaks appeared at 67.3°C and 95.5°C upon second heating and two endothermic peaks were at 60.9°C and 87.9°C on cooling.These thermic peaks suggested the two phase transition processes upon second heating and cooling, implying the transition of crystalline phase-mesophaseisotropic phase with reversible behaviours.The phase transition temperatures of sample 7b were higher a little than that of 7a, which might be ascribed to the stronger molecular interaction based on the larger molecular weights of 7b.On the other hand, the thermic peaks in DSC curves were broad, which could be attributed to the low crystallisability and the viscous morphology for the large molecular weights of 7a and 7b.All these above analysis supported the reversible liquid crystalline behaviours for samples 7a and 7b.
Samples 7a and 7b were further observed by polarising optical microscopy (POM) to determine the mesomorphic morphology.During the thermic process of heating and cooling, the corresponding phase transition behaviours could be roughly seen at the phase transition  temperatures in Figure 1.By heating the samples to isotropic phase and then slowly cooling to 75°C, the mesomorphic textures appeared gradually.As shown in Figure 2, both sample 7a and 7b exhibited the focalconic textures, suggesting the columnar mesophase for samples 7a and 7b.Moreover, the X-ray diffraction (XRD) analysis was applied to investigate the molecular stacking behaviour of samples 7a and 7b in mesophase.Figure 3 illustrated the XRD traces of samples 7a and 7b at 75°C of mesophase.One can see that, in small-angle region, the obvious reflection peaks were seen at 2.93°, 5. planes for columnar hexagonal mesophase.Moreover, in the wide region, the broad peaks between 17°-28° indicated the d-spacings of 3.18 ~ 5.20 Å, which could be ascribed to the signals for the packing of molten alkyl chains.The small peaks at 26.11° for 7a and 25.36° for 7b implied the distances of 3.41 Å and 3.51 Å, which were assigned for the intracolumnar core-core distance in the hexagonal columnar mesophase.All these XRD data suggested that samples 7a and 7b possessed the ordered self-assembly hexagonal mesophase.Furthermore, based on the above d-spacing, the lattice constant (α) was calculated as 34.79 Å and 39.33 Å for samples 7a and 7b, respectively.These α values were far smaller than the diameter of samples 7a and 7b (~60 Å  estimated by Corey-Pauling-Koltun model), suggesting the folding conformation and/or the interdigitation for the peripheral alkyl chains of samples 7a and 7b in the neighbouring columns.Also, the number (n) of molecules in each disk of the column was estimated by the following formula: where 'α' means the hexagonal lattice parameter, N A represented Avogadro's number and M is the molecular weight.The thickness ([h]) is seen as intracolumnar distance.The density (ρ) is estimated as 1.0 g/cm 3 .
After calculation, the n values were 1.04 for 7a and 1.06 for 7b, respectively.These values closed to 1, indicating one molecule in each disk of columnar mesophase.Therefore, based on the above analysis, the possible molecular stacking model in the hexagonal columnar mesophase was proposed as Figure 4 (using sample 7b as example).

Photophysical properties
The AIE properties for precursors 4a and 4b (or their analogues) had been confirmed [46,51]  reasonable that samples 7a and 7b also possessed good AIE emission.The photophysical behaviours of samples 7a and 7b were explored by UV-vis and fluorescence examination.As shown in Figure S11, sample 7a and 7b possessed the maximum absorption at 345 nm and 357 nm in the absorption scopes of 305-425 nm, respectively.These absorption characteristics were in accordance with that of the normal diphenylacrylonitrile derivatives [47], indicating that these absorptions were ascribed to the diphenylacrylonitrile units.Their UV-Vis absorption spectra in THF-H 2 O mixtures exhibited little change (Figures S12 and S13).Furthermore, the AIE fluorescence properties in THF-H 2 O system with various H 2 O fractions were examined and the results were shown in Figures 5 and 6 for samples 7a and 7b, respectively.It can be seen that the fluorescence was weak for both 7a and 7b when f w <60%.As the water fractions increased from 60% to 95%, the fluorescence intensities of samples 7a and 7b enhanced rapidly and exhibited strong fluorescence in THF-H 2 O system with 95% of H 2 O.Moreover, the fluorescence behaviours of samples 7a and 7b in solid film and mesophase were investigated (Figure 7).The fluorescence spectra and the corresponding fluorescence data were summarised in Table 2.It can be seen that samples 7a and 7b demonstrated strong fluorescence in solid film and mesophase.The absolute fluorescence quantum yields were 0.15, 0.31 and 0.30 for 7a, and 0.34, 0.46 and 0.45 for 7b in THF-H 2 O (5:95), solid film and mesophase, respectively.These results suggested that they were the excellent columnar liquid crystal with high fluorescence in aggregated states.The absolute fluorescence quantum yields of 7b were bigger than that of 7a, which could be explained by that the more alkyl chains were favourable for increasing the rotation restriction of intramolecular aromatic groups.On the other hand, it can be observed that the maximum emission wavelength were 472 nm, 500 nm and 503 nm for 7a, and 475 nm, 511 nm and 515 nm for 7b in THF-H 2 O (5:95), solid film and mesophase, respectively.The obvious red shifts of emission wavelength might suggest the J-aggregation for these samples.The emission wavelength of 7b was larger a little than that of 7a, which might be ascribed the influence of the more alkoxy groups in samples 7b for conformation and molecular stacking behaviours.All these photophysical studies suggested that samples 7a and 7b possessed high fluorescence in various aggregated states and the multiple alkyl chains were favourable for increasing the fluorescence.This kind of CTV liquid crystals with high fluorescence at mesophase was observed for the first time, which supplied a good strategy for design and synthesis of novel calixarene liquid crystals containing excellent fluorescence in all kinds of states.

Conclusions
In conclusion, two CTV derivatives with AIE units 7a and 7b were prepared for the first time by introducing diphenylacrylonitrile units on CTV skeleton.The structures and crown conformations of 7a and 7b were confirmed by 1 H NMR, 13 C NMR, MALDI-MS spectra and elemental analysis.The DSC, POM and XRD analysis suggested the reversible liquid crystalline behaviours for samples 7a and 7b with hexagonal columnar mesophase.The studies on photophysical properties indicated that samples 7a and 7b possessed high fluorescence in various aggregated states and the multiple alkyl chains were favourable for increasing the fluorescence.The absolute fluorescence quantum yield of 7b in mesophase was as high as 0.45.This kind of CTV liquid crystals with high fluorescence in aggregated states were reported for the first time, supplying a good example for constructing the bowl CTV liquid crystals with excellent fluorescence.

Experimental
The chemical reagents were supplied by commercial suppliers (Aladdin Reagent Co., Ltd) and used without further purification.MeCN was purified to dryness according to standard anhydrous methods.Anhydrous K 2 CO 3 was obtained by roasting at 500°C for 12 h.TLC analysis was performed on pre-coated glass plates.Silica gel (200-300 mesh) was used for column chromatography.NMR spectra were measured on a Bruker-ARX 600 instrument, using CDCl 3 as solvent.Tetramethylsilane (TMS) as internal standard to determine the chemical shifts.MS spectra were collected on MALDI-TOF mass spectrometer.UV-Vis spectra were done on Varian UV-Vis spectrometer.Fluorescence spectra were examined on a Hitachi F-4500 spectrometer.The fluorescence absolute yields (Φ F ) were determined on an Edinburgh Instruments FLS920 Fluorescence Spectrometer containing a 6-inch integrating sphere.Compounds 3a and 3b were synthesised by the published procedures [46].Compound 6 was prepared by the published literature [47].

Synthesis of compounds 4a and 4b
Under the protection of N 2 atmosphere, the mixture of compound 3a (or 3b) (0.

Synthesis of compounds 7a and 7b
The mixture of cyclotriveratrylene derivative 6 (0.26 g, 0.5 mmol), 4a or 4b (1.7 mmol), NaN 3 (0.35 g, 5 mmol), CuI (0.19 g, 1 mmol) and 4 drops of triethylamine was stirred in 20 mL of DMF at 95°C for 24 h.The reaction process was monitored by the TLC technique.After cooling, the obtained mixture was partitioned by 50 mL of distilled water and 60 mL of CHCl 3 .Then the organic layer was separated and concentrated under reduced pressure.The residue was further purified by column chromatography (eluent: CH 2 Cl 2 : ethyl acetate = 3 : 1).Compounds 7a and 7b were collected as yellow powder in yield of 74% and 78%, respectively.

Figure 1 .
Figure 1.(Colour online) The DSC curve of compounds 7a and 7b for the circle of second heating and cooling (scan rate 5°C min −1 ).

Figure 2 .
Figure 2. (Colour online) The mesomorphic textures for 7a and 7b at 75°C on cooling from isotropic state under the observation of POM (magnify 200 times).
λ ex = 360 nm in THF-H 2 O (5:95) (1.0 × 10 −5 M); b absolute fluorescence quantum yield for a; c λ ex = 360 nm in films at r.t.; d absolute fluorescence quantum yield for c; e λ ex = 360 nm in mesophase at 75°C; f absolute fluorescence quantum yield for e (The sample on plate was heated at 75°C for 0.5 h and then examined by fluorescence at r.t.rapidly to simulate the detection process of liquid crystal phase).

Figure 7 .
Figure 7. (Colour online) Fluorescence spectra of samples 7a and 7b in solid film and mesophase.
, it was

Table 2 .
The photophysical data of samples 7a and 7b.