Phenylacrylonitrile-bridging triphenylene dimers: the columnar liquid crystals with high ﬂ uorescence in both solid state and solution †

Herein, three phenylacrylonitrile-bridging triphenylene dimers were prepared by condensation and etheri ﬁ cation procedure in moderate yields. They exhibited the ordered hexagonal columnar mesophase based on the strong columnar stacking-induced action of two triphenylene units. A subtle balance between ﬂ uorescence decrease and AIE e ﬀ ect existed upon adding H 2 O to the THF solution, resulting in high ﬂ uorescence for these novel AIE liquid crystals in both solid state and solution for the ﬁ rst time.


Introduction
Organic molecules with columnar mesophases have been paid signicant attention in the past few decades. 1,2 Based on their extended p-stacking arrays for favourable transport of charge carriers along the columnar axis, they have shown broad application prospects in the elds of organic eld-effect transistors, organic light-emitting diodes, organic semiconductors, organic photovoltaic cells, and gas sensors. [3][4][5][6][7] Among all kinds of columnar liquid crystals, the luminescent columnar liquid crystals have attracted signicant research interest due to the effective combination of self-healing characteristics of mesophase and intrinsic luminescence capability. 8,9 For example, using the dyes such as perylene or Bodipy as cores, the luminescent columnar liquid crystals have been prepared successfully. [9][10][11][12][13][14][15][16][17][18][19][20][21] However, although these luminescent liquid crystals exhibited strong emission in solution, they showed weak uorescence in the solid state because of the aggregation-caused quenching (ACQ) effect.
Recently, aggregation-induced emission (AIE) discovered by Tang's group supplied a new strategy to prepare the uorescent liquid crystals. [22][23][24] As a result, some liquid crystalline molecules bearing AIE groups were prepared. [25][26][27][28][29][30][31][32][33][34][35][36] They exhibited good uorescence in the solid state, but they were practically non luminescent in solution, and it was difficult to form columnar mesophase due to the non-coplanar structures of AIE cores against the effective p-p packing in column. Since triphenylene derivatives possess strong columnar-stacking abilities, [37][38][39][40][41][42][43][44][45][46][47] lately, we have prepared the AIE liquid crystals of triphenylenediphenylacrylonitrile derivatives with the ordered hexagonal columnar mesophase based on the columnar stacking-induced action of triphenylene units. 48 Although the strong uorescence in the solid state was observed based on the AIE effect of diphenylacrylonitrile units, the crystals still emitted weakly in good solvents such as THF and CH 2 Cl 2 . Most recently, to attain efficient uorescence in both the solution and solid state, Tang's group has prepared a triphenylamine derivative that possesses not only good emission in solution based on considerable rigidity with limited intramolecular rotation, but also excellent uorescence in the solid state due to the substantial twisting conformation. 49 Inspired by this successful strategy, herein, we report the rst columnar AIE liquid crystals with high uorescence in both the solution and solid state. The columnar mesophase was induced by two triphenylene units, and the uorescence was emitted by the phenylacrylonitrile group. The subtle balance between uorescence decrease and AIE effect was responsible for the high uorescence in both the solution and solid state.

Synthesis
Because triphenylene-diphenylacrylonitrile derivatives have a good AIE effect but show weak uorescence in solution, 48 in this research, two diphenylacrylonitrile units were integrated into one conjugated system as a luminogen. Due to the extended rigidity of the conjugated structure, the crystals can emit effectively in solution, and the restriction of intramolecular rotation would produce the AIE effect; on the other hand, due to the large rigidity structure of this kind of phenylacrylonitrile moiety, two triphenylene units with strong columnar-induced abilities were bridged on it to ensure the formation of the columnar mesophase. The synthetic routes are illustrated in Scheme 1. By the condensation reaction of 4-hydroxyphenyl acetonitrile with terephthalaldehyde in a NaOH/EtOH system, the phenylacrylonitrile derivative 1 with two terminal hydroxyl groups was prepared in a yield of 78%. Treatment of monohydroxytriphenylene 2 with excess dibromoalkane in a K 2 CO 3 /MeCN system afforded triphenylene derivatives 3a, 3b, and 3c in the yields of 66-80% aer column chromatography. Furthermore, the target compounds 4a, 4b, and 4c were obtained conveniently by the etherication of the compound 1 with the compounds 3a, 3b, and 3c in the K 2 CO 3 / MeCN system, respectively. The yields were as high as 72-84% aer column chromatography.
The structures of the compounds 4a, 4b, and 4c were characterized by the 1 H NMR spectra, 13 C NMR spectra, MALDI-TOF-MS spectra, and HR-MS spectra. They exhibited the corresponding molecular ion peaks in mass spectrometry spectra. The deviations of molecular weights were less than 5 ppm, indicating the accomplishment of the condensation on two hydroxyl groups of compound 1. In the 1 H NMR spectra, the proton signals agreed well with the structures. Especially, the splits of ArH were observed clearly (see ESI †). The 13 C NMR spectra also supported these structures unambiguously.

Mesomorphic properties
The mesomorphic properties of the compounds 4a, 4b, and 4c were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD). The changes of DSC traces for second heating and cooling are presented in Fig. 1. The corresponding phase transition temperatures and enthalpy changes are listed in Table 1. All these samples exhibited the reversible phase transition with two peaks upon cooling and second heating. The phase transition peaks for the sample 4a appeared at 55.2 C and 95.8 C upon cooling and at 64.5 C and 119.2 C upon second heating. Sample 4b showed the phase transition at 18.0 C and 73.2 C on cooling and at 25.9 C and 81.8 C upon second heating. For the sample 4c, the broad thermal peaks at 10.1 C on cooling and at 12.0 C on second heating were observed, and the small peaks at 61.2 C on cooling and at 65.7 C on second heating emerged. These data suggested the thermal process of crystalline phase-mesophase-isotropic phase for samples 4a, 4b, and 4c. Along with an increase in the length of the alkyl bridging chains, the phase transition temperatures decreased orderly with the trend of 4c < 4b < 4a. The broad peaks and the hysteresis phenomena could be ascribed to the viscous morphology of these samples with high molecular weights. All the DSC analyses indicated that although the large rigid structure of two phenylacrylonitrile units were used as bridging spacers, the compounds 4a, 4b, and 4c possessed good liquid crystalline behaviors due to the strong columnar stackinginduced action of two triphenylene units. The longer alkyl bridging chains resulted in the lower phase transition temperatures.
The observation using POM also supported the presence of the mesophase for the samples 4a, 4b, and 4c. At the phase transition temperatures shown in the DSC curves, the corresponding phase transition processes were observed. Just like the compounds 4b and 4c, compound 4a also exhibited the clear uid in mesophase via microscopic observation although its enthalpy of the Col-Iso transition was obviously larger than that of the compounds 4b and 4c; this might be ascribed to the inuence of the short spacer. By cooling slowly from the isotropic phase, the clear textures appeared gradually. Fig. 2 presents the textures of the sample 4a at 75 C and those of the samples 4b and 4c at 40 C. The typical pseudo-confocal conic textures indicated the columnar mesophase for these samples, which were further conrmed by the XRD analysis. Scheme 1 Synthetic routes of target compounds 4a, 4b, and 4c. Fig. 1 The DSC traces of the compounds 4a, 4b, and 4c upon second heating and cooling (scan rate 10 C min À1 ).
The XRD traces for the samples 4a, 4b, and 4c at mesophase were investigated to study the liquid crystalline molecular stacking behavior. The results are illustrated in .74 for the samples 4a, 4b, and 4c, indicating the d-spacings of 3.68Å, 3.66Å, and 3.60Å, respectively, were the typical reections for the intracolumnar p-p interaction of the columnar liquid crystals. All these data suggested that the compounds 4a, 4b, and 4c possessed an ordered hexagonal columnar mesophase, which were in accordance with the POM observation. Furthermore, based on these XRD data, the lattice parameter a for the compounds 4a, 4b, and 4c could be calculated as 35.02Å, 34.56Å, and 34.12Å, respectively. These values were approximately in accordance with the diameter of one triphenylene column. Thus, the possible schematic of the columnar layered molecular arrangement was proposed as the square inserted in Fig. 3, in which the discs resided in separate columns, with the bridge spanning the distance between columns. Combining all these studies of DSC, POM, and XRD analyses, it could be concluded that the unfavorable inuence of the large rigid structure of phenylacrylonitrile for columnar mesophase was compensated by the strong columnar stacking-induced action of two triphenylene units. The compounds 4a, 4b, and 4c possessed a good reversible liquid crystalline behavior with an ordered hexagonal columnar mesophase.

Photophysical properties
The photophysical behaviors of the samples 4a, 4b, and 4c were studied to explore their luminescence properties. Their UV-vis absorption spectra in series of solvents with increased polarities are shown in Fig. S13-S15. † It can be seen that the l abs of the samples 4a, 4b, and 4c were at about 390 nm in toluene and exhibited a small blue shi with an increase of solvent polarities to 384 nm in DMF nally. The uorescence spectra of the samples 4a, 4b, and 4c in toluene, CH 2 Cl 2 , CHCl 3 , THF, and DMF are illustrated in Fig. S16-S18 † and 4 for 4b as the representative sample. With an increase of solvent polarity, their emission peaks presented obvious red shis and the uorescence intensity decreased correspondingly. However, the spectral changes were small in toluene, CH 2 Cl 2 , CHCl 3 , and THF, except for the DMF solution. These spectral changes might be ascribed to the intramolecular charge transfer effect and/or low solubility in highly polar solvents.
Furthermore, the uorescence behaviors of the samples 4a, 4b, and 4c in THF/H 2 O mixtures with different fractions of H 2 O were investigated. All these samples have good solubility in THF and are sparingly solubility in the THF/H 2 O mixture. Their uorescence spectra in the THF/H 2 O mixture and the plots of changes for emission peak intensity are exhibited in Fig. S19-S24. † Fig. 5 illustrates the spectra and plot for sample 4b as the representative sample. It can be observed that being different from the typical AIE molecules with little emission in pure THF solution, samples 4a, 4b, and 4c possess strong emission due to considerable rigidity with limited intramolecular rotation of large conjugated phenylacrylonitrile moiety. With the addition Fig. 2 The mesophase textures observed using POM on cooling at 75 C for 4a and 40 C for 4b and 4c (Â400).  of water to the THF solution, the emission intensity reduced rapidly. As the water contents attained 20-30%, the samples 4a, 4b, and 4c still maintained moderate uorescence although the emission intensities were at the lowest values among various contents of water. Combining the abovementioned absorption and uorescence investigation in various solvents, the decrease of uorescence in the THF/H 2 O mixture might be attributed to the intramolecular charge transfer effect and/or the low solubility in highly polar solvents. The addition of water increased the polarity of solvents and led to both stronger charge separation and dipole moment for the samples 4a, 4b, and 4c and low solubility in the THF/H 2 O mixture; this resulted in the decrease of uorescence intensity correspondingly. On the other hand, the phenylacrylonitrile unit is a typical AIE unit, which produces gradually enhanced uorescence with the addition of water to the THF/H 2 O mixture. Thus, when the contents of water were higher than 50% for 4a and 30% for 4b and 4c, the uorescence intensities increased dramatically. When the contents of water approached 90%, the emissions were even stronger than those in the pure THF solution. Moreover, with the addition of water, the uorescence peaks of the samples 4a, 4b, and 4c became broader and exhibited a slight red shi. These phenomena suggested that the strong AIE process occurred when water was added to the solutions. The increased uorescence based on the AIE process made up well for the previous uorescence decrease. These results also implied a subtle balance between uorescence decrease and AIE effect while adding water to the THF solution. This balance resulted in the strong uorescence for the samples 4a, 4b, and 4c in both the pure THF solution and THF/H 2 O mixture with high contents of water although a relatively low uorescence emerged at moderate contents of water. This kind of balance between uorescence decrease and AIE phenomenon was similar to the recently reported previous studies on uorescent probe, fumaronitrile-based uorogen, and piezouorochromic molecule. [50][51][52] Moreover, the uorescence spectra in powder were obtained (Fig. S25 †), and the uorescence quantum yields (F F ) are summarized in Table 2. It can be seen that the samples 4a, 4b, and 4c possess high uorescence with the F F values around 20% in both the pure THF solution and THF/H 2 O mixture with 90% of water. The F F values in powder also attained around 9%, indicating that the samples could emit uorescence well in the solid state. Furthermore, the uorescence emission in the solid state and mesophase of the compound 4a were observed using a handheld 365 nm UV lamp as the excitation source. The uorescence images in the solid state (26 C before heating), mesophase (90 C), and solid state (30 C aer cooling) are exhibited in Fig. S26. † It can be seen that the compound 4a emits good uorescence in both the solid state and mesophase. The uorescence wavelength in the mesophase showed obvious blue-shi in comparison with that in the solid state; this is a normal phenomenon for the thermochromism of uorescent liquid crystals. 34 Based on all the abovementioned analysis of photophysical data, it could be summarized that the samples 4a, 4b, and 4c possessed good uorescence emission in pure THF solution, THF/H 2 O mixture, and solid state.

Conclusions
In conclusion, three phenylacrylonitrile-bridging triphenylene dimers 4a, 4b, and 4c were designed and synthesized in moderate yields. The studies on mesomorphic property explored that although the large rigid structure of diphenylacrylonitrile units was introduced, the compounds 4a, 4b, and 4c exhibited the ordered hexagonal columnar mesophase based on the strong columnar stacking-induced action of two triphenylene units. Moreover, they exhibited high uorescence in both the solid state and solution. The subtle balance between uorescence decrease and AIE effect was proposed to be responsible for these interesting uorescence phenomena. The compounds 4a, 4b, and 4c were the rst columnar AIE liquid crystals with high uorescence in both the solution and solid state. This  study supplies a new method for the design and synthesis of the novel uorescent liquid crystals.

General
All chemical reagents, organic solvents, and inorganic reagents were purchased from Aladdin Reagent Co. Ltd. They were puri-ed by the standard anhydrous methods before use. TLC analysis was conducted on pre-coated glass plates. Column chromatography was carried out using silica gel (200-300 mesh). NMR spectra were obtained in CDCl 3 at 26 C using a Bruker-ARX 400 instrument. Chemical shis were obtained in ppm with tetramethylsilane (TMS) as the internal standard. MS spectra were obtained using a Bruker mass spectrometer. The compounds 3a and 3b were synthesized according to the reported literature. 53 Compound 3c was prepared by the literature method. 38 UV-vis spectra were obtained using the Varian UV-vis spectrometer. Fluorescence spectra were obtained using a conventional quartz cell (10 Â 10 Â 45 mm) via a Hitachi F-4500 spectrometer. The excitation and emission slits were 5 nm wide. The uorescence absolute F F values were obtained using an Edinburgh Instruments FLS920 Fluorescence Spectrometer with a 6-inch integrating sphere. A polarized optical microscopy (Leica DMRX) image was obtained using a hot stage (Linkam THMSE 600) to investigate the phase transitions. Thermal analysis of the materials was carried out using a differential scanning calorimeter (DSC) (Thermal Analysis Q100) under a N 2 atmosphere at a scanning rate of 10 C min À1 . XRD experiments were conducted by SEIFERT-FPM (XRD7) using Cu Ka 1.5406Å as the radiation source with 40 kV and 30 mA power.
Synthetic procedure for the compound 1

Conflicts of interest
There are no conicts to declare.