Novel liquid crystals with high fluorescence: Synthesis, mesomorphic and photophysical properties of cholesterol-triazine-BODIPY trimers

Accepted Manuscript Novel liquid crystals with high fluorescence: Synthesis, mesomorphic and photophysical properties of cholesterol-triazine-BODIPY trimers Yuzhi Xiong, Sining Zheng, Ling Zhu, Hongyu Guo, Fafu Yang PII:

S0022-2860(18)30388-0

DOI:

10.1016/j.molstruc.2018.03.093

Reference:

MOLSTR 25032

To appear in:

Journal of Molecular Structure

Received Date: 6 February 2018 Revised Date:

14 March 2018

Accepted Date: 22 March 2018

Please cite this article as: Y. Xiong, S. Zheng, L. Zhu, H. Guo, F. Yang, Novel liquid crystals with high fluorescence: Synthesis, mesomorphic and photophysical properties of cholesterol-triazine-BODIPY trimers, Journal of Molecular Structure (2018), doi: 10.1016/j.molstruc.2018.03.093. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Novel liquid crystals with high fluorescence: Synthesis, mesomorphic and photophysical properties of cholesterol-triazine-BODIPY trimers YUZHI XIONGa, SINING ZHENGa, LING ZHUa, HONGYU GUOa*, AND FAFU

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YANGa,b* a

E-mail: [email protected] b

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College of Chemistry and Chemical Engineering, Fujian Normal University, Fuzhou 350007, P. R. China Tel.: 0086-591-83465225

Fujian Key Laboratory of Polymer Materials, Fuzhou 350007, P. R. China

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Running Title: cholesterol-triazine-BODIPY trimers

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Keywords: Cholesterol; Bodipy; triazine; mesophase ; fluorescence.

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H H

H O O

O

O N

H

H

O

O

N

O

O

O O

O

O

λem = 522 nm

O N

O O H H H

N N

O

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H H H

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F

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F N B N F

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GRAPHICAL ABSTRACT

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Novel cholesterol-triazine-BODIPY trimers 7 bearing two cholesterol units was a hexagonal columnar liquid crystal with strong fluorescence, and the cholesterol-triazine moiety played crucial role for producing liquid crystalline property and enhancing fluorescence.

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Abstract By step-by-step etherification on cyanuric chloride with BODIPY unit and cholesterol units, two novel cholesterol-triazine-BODIPY trimers (compounds 6 and 7 with one and two

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cholesterol unit, respectively) were designed and synthesized in high yields. The investigation on mesomorphic properties suggested that compound 6 with one cholesterol unit was a nematic liquid crystal but compound 7 with two cholesterol units was a hexagonal columnar liquid crystal. Both compounds 6 and 7 exhibited the strong fluorescence with high quantum yields and

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larger Stokes shifts than their precursors. The cholesterol-triazine moiety played crucial role for

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producing liquid crystalline property and enhancing fluorescence.

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Introduction In the past decades, liquid crystalline materials have been paid much attention due to their unique

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and excellent optical application [1-4]. Columnar discotic liquid crystals, as an important subclass of liquid crystals, presented the π-π stacking arrangement in one direction, leading to high 1D charge and/or energy transport with potential application in the fields of organic light-

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emitting diodes, gas sensors, organic photovoltaic cells, organic field-effect transistors, etc [5-9]. Especially, the columnar luminescent liquid crystals attracted much research interests due to the

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effective combination of intrinsic luminescent capability and the supramolecular self-assembly within a mesophase [10, 11], such as perylene liquid crystals with high fluorescence [12-18]. BODIPY (4,4-Difluoro-4-borata-3a,4a-diaza-s-indacene) is a kind of dye with high fluorescence, good photochemical stability and energy- and electron-transfer abilities [19-22].

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Some BODIPY liquid crystals were also investigated by introducing soft alkyl chains to BODIPY skeleton [23-32]. However, due to the irregular and asymmetric structures of BODIPY unit, it was difficult to obtain the columnar BODIPY liquid crystal. On the other hand, it was

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well known that cholesterol moiety was a good alkyl unit to construct all kinds of liquid crystal materials [33-38]. Some examples of cholesterol-BODIPY derivatives were also synthesized, but

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no liquid crystalline property was reported due to the defect of BODIPY unit as rigid moiety for mesophase [39-42]. Recently, cyanuric chloride, possessing three Cl groups with stepwise reaction activity [43, 44], was used as a good platform to construct liquid crystals by bridging multiple mesogens onto rigid triazine core. Inspiring by this synthetic strategy, in this paper, the novel cholesterol-triazine-BODIPY trimers were prepared conveniently by grafting sequentially the BODIPY unit and cholesterol units onto cyanuric chloride. Moreover, the studies on 4

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mesomorphic properties revealed that the cholesterol-triazine-BODIPY trimer with one cholesterol unit was a nematic liquid crystal and the one with two cholesterol units was a columnar liquid crystal. These results not only showed the first examples of cholesterol-

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BODIPY liquid crystals, but also suggested the molecular stacking behaviors in mesophase could be tuned by the number of cholesterol units.

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Experimental Materials and Methods

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The chemical reagents were purchased from Aladdin L.L.C. and used directly. TLC analysis was carried on pre-coated glass plates. Column chromatography was purformed on silica gel (200300 mesh). NMR spectra were measured on a Bruker-ARX 600 instrument at 25oC. MS spectra were done on Bruker mass spectrometer. Compound 1 was synthesized by the literature method [45]. Compounds 4 and 5 were prepared by the reported procedures.14 UV-Vis spectra were

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measured on Varian UV-Vis spectrometer. Fluorescence spectra were obtained in a conventional quartz cell (10×10×45 nm) at 25 oC on a Hitachi F-4500 spectrometer, with excitation and emission slits 5 nm wide. The fluorescence quantum yields (ΦF) were measured on Edinburgh

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Instruments FLS920 Fluorescence Spectrometer with a 6-inch integrating sphere. Leica DMRX

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POM with a hot stage (Linkam THMSE 600) was used to examine phase transitions. Thermal analysis of the materials was performed on DSC (Thermal Analysis Q100) at a scanning rate of 10oC/min under N2 atmosphere. XRD data were obtained on SEIFERT-FPM (XRD7), using Cu Kα 1.5406Å as the radiation source with 40 kV, 30 mA power. Synthesis of compound 2 Under N2 atmosphere, a mixture of cyanuric chloride (0.12 g, 0.65 mmol), compound 1 (0.18 g, 0.53 mmol) and NaHCO3 (0.065 g, 0.77 mmol) was stirred in 20 mL of dry acetone at 05

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5 oC for 1.5 h. The TLC detection indicated the disappearance of compound 1. Then the reaction mixture was poured into 100 mL of ice water. The obtained mixture was stirred and the red precipitate emerged gradually. The red precipitate was filtered, washed by ice water and dried

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under vacuum. Compound 2 was obtained as the red solid in yield of 88%. 1H NMR (400 MHz, CDCl3) δ (ppm): 1.46 (s, 6H, CH3), 2.56 (s, 6H, CH3), 6.01 (s, 2H, ArH), 7.31 (d, J = 8.0 Hz, 2H, ArH), 7.40 (d, J = 8.0 Hz, 2H, ArH). 13C NMR (100 MHz, CDCl3) δppm: 14.53, 29.72, 121.55,

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122.28, 129.90, 131.33, 133.95, 139.90, 143.03, 151.61, 155.98, 171.25, 173.17; MS(MALDITOF)(m/z) : 489.338 (M+H)+. MS(ESI) (m/z): calcd for C22H18BCl2F2N5O 488.1026 (M+H)+,

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found 488.1028. Synthesis of compound 6

Under nitrogen protection, the mixture of compound 2 (0.26 g, 0.53 mmol), compound 5 (0.29g, 0.53 mmol) and Na2CO3 (0.074 g, 0.7 mmol) was stirred in 30 mL of dry acetone at 25oC

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for 6 h. The reaction mixture was monitored by TLC technique, suggesting the disappearance of strarting materials. Then 80 mL of ice water was added in the reaction mixture. The obtained mixture was continually stirred and the red precipitate appeared. The red precipitate was

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collected and purified by column chromatography (eluent: petroleum ether : CH2Cl2=2:1). Compound 6 was obtained as the red solid in yield of 83%. Compound 6: 1H NMR (400 MHz,

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CDCl3) δ (ppm): 0.68~2.36 (m, 49H, CH3 and H of cholesterol), 2.58 (s, 6H, CH3), 4.59 (s, 2H, OCH2CO), 4.76 (bs, 1H, OCH), 5.43 (s, 1H, CH=C on cholesterol), 6.02 (s, 2H, ArH), 6.94 (d, J = 8.0 Hz, 2H, ArH), 7.11 (d, J = 8.0 Hz, 2H, ArH), 7.33 (d, J = 8.0 Hz, 2H, ArH), 7.38 (d, J = 8.0 Hz, 2H, ArH).

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C NMR (100 MHz, CDCl3) δppm:173.32, 168.62, 168.05, 155.67, 152.85,

152.05, 145.81, 142.90, 140.65, 139.24, 131.41, 129.26, 129.17, 123.02, 122.27, 121.69, 121.43, 115.88, 115.47, 75.39, 71.79, 56.79, 56.04, 50.03, 42.33, 39.74, 39.53, 37.99, 37.29, 36.92, 36.57, 6

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36.21, 35.80, 31.93, 31.44, 29.70, 28.23, 27.88, 24.30, 23.85, 22.81, 21.10, 19.40, 18.60, 14.33, 11.87; HR-ESI-MS (m/z): calcd for C57H69BClF2N5O5: 987.5052(M+), found 987.5013. Synthesis of compound 7

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Under the protection of N2 atmosphere, the mixture of compound 5 (0.11 g, 0.2 mmol), compound 6 (0.20 g, 0.2 mmol), and Na2CO3 (0.1 g, 0.9 mmol) was refluxed in 30 mL of dry THF for 10 h. The reaction system was monitored by TLC analysis, indicating the strarting

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materials were used out. Then the reaction mixture was poured into 60 mL of ice water. After stirring for another 20 min, the obtained mixture was extracted by 40 mL of CH2Cl2. The organic

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layer was partitioned and concentrated. The residue was further purified by chromatographic column (eluent: petroleum ether : CH2Cl2 =2:1). Compound 7 was collected as red solid in 75% yield. 1H NMR (400 MHz, CDCl3) δ (ppm): 0.68~2.36 (m, 92H, CH3 and H of cholesterol), 2.57 (s, 6H, CH3), 4.56 (s, 4H, OCH2CO), 4.76 (bs, 2H, OCH), 5.36 (s, 2H, CH=C on cholesterol),

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6.01 (s, 2H, ArH), 6.90 (d, J = 8.0 Hz, 4H, ArH), 7.08 (d, J = 8.0 Hz, 4H, ArH), 7.35 (d, J = 7.2 Hz, 2H, ArH), 7.38 (d, J = 8.0 Hz, 2H, ArH).

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C NMR (100 MHz, CDCl3) δppm: 173.90,

173.12, 172.40, 156.41, 155.42, 152.03, 144.78, 143.05, 140.64, 139.69, 132.45, 131.37, 128.80,

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122.68, 122.18, 121.26, 114.72, 74.08, 67.13, 56.51, 55.81, 49.73, 42.00, 39.31, 37.81, 37.10, 36.58, 35.82, 35.69, 31.96, 28.74, 28.26, 28.02, 27.82, 24.53, 24.30, 23.87, 22.86, 22.61, 21.05,

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19.33, 18.75, 14.17, 11.71. HR-ESI-MS (m/z): calcd for C92H120BF2N5O9: 1487.9155(M+), found 1487.9155.

Results and Discussion

Synthesis and characterization

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Scheme 1 exhibited the synthetic routes for cholesterol-triazine-BODIPY trimers 6 and 7. Due to the different reaction activity of the three Cl groups [43, 44], the BODIPY unit and cholesterol units were introduced sequentially by adjusting the reaction condition. According to the reported

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procedure, the hydroxyl BODIPY 1 and cholesterol derivative 5 with phenolic hydroxyl group were prepared beforehand in moderate yields [14, 45]. Then, by the nucleophilic substitution in NaHCO3 / acetone system at 0-5 oC, the triazine derivative with one BODIPY unit 2 was

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synthesized in yield of 88% after precipitation. Subsequently, reacting compound 2 with compound 5 under the condition of Na2CO3 / acetone at room temperature, triazine derivative 6

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with both BODIPY unit and a cholesterol unit was smoothly obtained in yield of 83% after column chromatography. Furthermore, the triazine derivative 7 with one BODIPY unit and two cholesterol units was prepared by refluxing compound 6 with compound 5 in Na2CO3 / THF system. The yield was as high as 75% after column chromatography.

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The structures of novel cholesterol-triazine-BODIPY trimers 6 and 7 were confirmed by HRMS and NMR spectra. In their HR-MS spectra, the corresponding molecular ion peaks at 987.5013 and 1487.9155 (the deviations < 5 ppm) certainly supported that the one cholesterol

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unit and two cholesterol units were introduced onto compounds 6 and 7, respectively. Moreover, the proton signals in their 1H NMR spectra were well in accordance with the structures. For

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example, the clear doublets were observed for the protons on aromatic groups (see supported information). The 13C NMR spectra were also in agreement with the structures of compounds 6 and 7. Although the

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C signals for cholesterol unit were complicated to be distinguished, two

singlets for OCH and OCH2 at 60-75 ppm and the signals for carbon atoms on aromatic groups were in consistent with their structures.

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OH OH TFA

+ CHO

DDQ

Et3N BF3-OEt2

THF, rt

N H

N

N B

F Cl

N

+

1

N

Cl

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NaHCO3

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acetone, 0-5oC

Cl

Cl

F 1

N

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Cl

B

N

2

H

H

H

Cl

H

DCM

HO

OH

H

O

4

H

HO

H

Cl

O

2 + 5

Na2CO3 acetone, rt

O

H

O

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O

N

N

F

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O

B N

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6

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5, Na2CO3 THF, refluxing

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H

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Cl

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3

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K2CO3, MeCN

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Scheme 1. The synthetic routes of compounds 6 and 7

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Mesomorphic properties

The mesomorphic properties of samples 6 and 7 were studied by differential scanning calorimetry (DSC) firstly. The results were exhibited in Figure 1 and the corresponding thermodynamic parameters were summarized in Table 1. One can see that both samples 6 and 7 showed two thermic peaks upon cooling and second heating, suggesting that they might possess mesophase during these thermic processes. Compound 6 presented two broad endothermic peaks 9

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at 76.5 oC and 136.8 oC on heating and two exothermic peaks at 72.0 oC and 128.6 oC upon cooling. As to compound 7, two broad endothermic peaks at 71.5 oC and 124.7 oC on heating and two broad exothermic peaks at 60.3 oC and 118.2 oC for cooling were observed. Based on the

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changes of DSC traces, it could be deduced that novel cholesterol-triazine-BODIPY trimers 6 and 7 displayed the phase transitions of solid state-mesophase-isotropic phase upon cooling and second heating, which were further supported by the studies of polarizing optical microscopy

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(POM) and X-ray diffraction (XRD) analysis. On the other hand, the phase transition temperatures of sample 6 with one cholesterol unit were higher than that of sample 7 with two

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cholesterol units, indicating the more cholesterol units were favorable for decreasing the mesophase temperature. Moreover, after calculation, the scopes of mesophase temperature for compounds 6 and 7 were 60.3 oC and 53.2 oC, respectively, implying that both of them possessed

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liquid crystalline behaviors at broad temperature scopes.

Cr

7

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cooling

LC

heating

Iso

6

Cr

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heating flow exo

Cr

Iso

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LC Iso LC cooling Iso heating

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50

100

150

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250

o

Temperature ( C)

Figure 1. The DSC traces of compounds 6 and 7 on second heating and cooling (scan rate 10oC min-1). Cr = crystalline, LC = liquid crystal, Iso = isotropic

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Table 1. Transition temperatures (oC) and enthalpies (kJ·mol-1) of samples 6 and 7

7

T(∆H)

Phase transition[a]

Heating scan

Cooling scan

Cr-LC(LC -Cr)

76.5(15.8)

72.0(11.9)

LC-Iso(Iso-LC)

136.8(8.2)

Cr-LC (LC-Cr)

71.5(16.8)

LC-Iso(Iso-LC)

124.7(15.9)

128.6(7.6) 60.3(7.1)

118.2(8.8)

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[a] Cr=crystalline, LC=liquid crystal, Iso=isotropic

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6

T(∆H)

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Comp.

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Figure 2. Mesomorphic textures of compounds 6 and 7 at 100 oC (×400) under POM on cooling.

Furthermore, the mesomorphic textures of compounds 6 and 7 were investigated under POM

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observation. The phase transitions of solid state-mesophase-isotropic phase were observed for compounds 6 and 7 on both heating and cooling. The phase transition temperatures also agreed with the thermic peaks of DSC traces approximately. As cooling slowly from the isotropic phase, the orderly mesomorphic textures could be distinguished gradually. One can see that sample 6 showed the schlieren texture (Figure 2), indicating the nematic mesophase for sample 6.

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However, sample 7 exhibited the focal-conic and band texture, suggesting the columnar

o

4.54

Intensity

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o

20.16

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o

7.87 o 9.08

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mesophase for sample 7. These textures were further confirmed by XRD analysis.

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60

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Scattering angle 2θ

(degree)

Figure 3. XRD traces of samples 6 and 7 at 100 oC.

The XRD traces of samples 6 and 7 at 100 oC for their mesophases were displayed in

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Figure 3. It can be seen that only the broad halo at 2θ = 13~25o for the molten cholesterol and alkyl chains of sample 6 was observed, and no other obvious peak appeared, indicating the nematic mesophase for sample 6, which was in accordance with the previous texture analysis.

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However, sample 7 presented completely different XRD trace from that of sample 6. At the small-angle region, three reflections at 4.54o, 7.87o and 9.08o were observed for sample 7

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obviously. Based on Bragg’s formula d = λ/(2Sinθ), the corresponding d-spacings were 19.45, 11.22 and 9.73 Å after calculation. The ratios of these d-spacings were consistent with 1:1/√3:1/2 for the [100], [110] and [200] planes of hexagonal columnar mesophase. Furthermore, the broad halo at 2θ = 13~25o for sample 7 indicated the very short correlation length of the molten cholesterol and alkyl chains. The small reflection at 20.16o (d-spacing of 4.4 Å) could be identified as the typical intracolumnar distance of π-π interactions of ordered hexagonal 12

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columnar mesophase. The above XRD analysis suggested that sample 6 with one cholesterol unit had the nematic mesophase but sample 7 with two cholesterol units possessed the hexagonal columnar mesophase. The difference of mesophase for samples 6 and 7 could be explained by

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their structures. Sample 6 with one cholesterol unit was a rodlike structure, preferring to the nematic mesophase. But sample 7 with two cholesterol units showed higher molecular symmetry than sample 6, which might be favorable for columnar molecular stacking. The possible

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schematic representation of sample 7 in hexagonal columnar mesophase was proposed in Figure 4. Based on all the analysis of DSC, POM and XRD data, it could be concluded that novel

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cholesterol-triazine-BODIPY trimers 6 and 7 exhibited good liquid crystalline behaviors and two cholesterol units on triazine skeleton were favorable for not only decreasing the mesophase temperature, but also producing hexagonal columnar mesophase.

H H H

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d100

d110

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O

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O O

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N

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Figure 4. The proposed schematic representation of sample 7 in hexagonal mesophase

Photophysical properties

The photophysical properties of novel cholesterol-triazine-BODIPY trimers 6 and 7 were investigated and their precursors 1 and 2 were used for data comparison. Their absorption spectra and fluorescence spectra in toluene with concentrations of 1.0×10-6 M were exhibited in Figure 5 and 6. The photophysical data for samples 6, 7 and their precursors 1 and 2 were summarized in 13

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Table 2. One can see that samples 6 and 7 showed similar absorption spectra, but the absorbance of sample 6 was larger than that of sample 7. Their maximum absorption wavelengths were at about 505 nm, which were in accordance with the precursors 1 and 2. These results might

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suggest the non coplanar structure of BODIPY unit and triazine group, leading to little influence on the absorption spectra of BODIPY skeleton. However, the fluorescence spectra of samples 6 and 7 exhibited different changes as compared with their precursors 1 and 2. It can be seen that

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the maximum emission wavelength exhibited the order of 1 (511 nm) < 2 (516 nm) < 6 (521 nm) < 7 (522 nm). These phenomena could be explained by the intramolular charge transfer (ICT)

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effect. Due to alkoxy, phenoxy and triazine group were introduced onto BODIPY unit, their electron donating capabilities enhanced the ICT effect, and the more cholesterol units resulting in stronger ICT effect. Thus, the stoke shifts of samples 6 and 7 were obvious larger than precursors 1 and 2, which were favorable for reducing the self-absorption in fluorescent emission. On the

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other hand, both the fluorescence intensities and fluorescence quantum yields of compounds 2, 6 and 7 were higher than that of precursor 1. Moreover, by adding 0.5 mL of corresponding solutions of toluene (1.0×10-3 M with compounds 1, 2, 6, and 7, respectively) on quartz glass, the

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corresponding solid films of compounds 1, 2, 6, and 7 were prepared after natural vaporization of solvents. As shown in Table 2, samples 6 and 7 displayed the solid fluorescence, while their

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precursors 1 and 2 showed no fluorescence in solid films. These results could be explained by that the introduction of triazine group and cholesterol units on BODIPY unit produced large steric effect and then reduced the aggregation fluorescence quenching. The fluorescence in liquid crystalline phase by heating the solid films was also investigated, but little emission was observed due to the strong aggregation fluorescence quenching of the ordered π-π stacking in mesophase. It was interesting that sample 6 with one cholesterol unit exhibited the little stronger 14

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fluorescence intensity and higher fluorescence quantum yield than sample 7 with two cholesterol units. This fact might be ascribed to that, by comparing with sample 6, sample 7 possessed higher molecular symmetry and stronger self-assembly aggregation ability (as analyzed in liquid

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crystalline section). Combing the above photophysical analysis, it could be summarized that novel cholesterol-triazine-BODIPY trimers 6 and 7 had high fluorescence with large Stokes

0.08

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1

0.06

Absorbance

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shifts. The triazine group and cholesterol units were favorable for good fluorescent performance.

6

0.04

7

2

0.02

0.00

350

400

450

500

550

600

650

700

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Wavelength (nm)

Figure 5. Adsorption spectra of samples 6, 7 and precursors 1 and 2 in toluene (10-6 M)

5500 5000

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4500 4000

6

3000 2500

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Intensity

3500

7

2000

2

1500

1

1000 500 0

-500 480

500

520

540

560

580

600

Wavelength (nm)

Figure 6. Fluorescence spectra of samples 6, 7 and precursors 1 and 2 in toluene (10-6 M). All of compounds were excited at 470 nm. 15

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Table 2. Absorption and fluorescence data for samples 6 and 7 in toluene at 298K λem

Stokes shift

ФF

ФF

(nm)

(nm)

(nm)

(toluene)

(films)

6

504

521

17

0.98

0.08

7

505

522

17

0.95

0.06

1

503

511

8

0.94

none

2

505

516

11

0.96

none

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Conclusions

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λabs

Samples

In conclusion, the novel cholesterol-triazine-BODIPY trimers 6 and 7 were designed and synthesized in good yields by orderly introducing BODIPY unit and cholesterol units onto cyanuric chloride. The cholesterol-triazine-BODIPY trimer with one cholesterol unit was a

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nematic liquid crystal but the one with two cholesterol units was a hexagonal columnar liquid crystal. The studies on photophysical properties indicated that compounds 6 and 7 possessed the

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strong fluorescence intensity with higher quantum yields and larger Stokes shifts than their precursors. This research not only showed the first examples of cholesterol-BODIPY liquid

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crystals, but also implied that the introduction of cholesterol unit was favorable for both liquid crystalline behavior and excellent fluorescence, which provides a good strategy on design and synthesis of novel Bodipy liquid crystal with high fluorescence.

Supplementary information (SI) All additional information pertaining to characterization of the new compounds using NMR spectra and MS technique are given in the supporting information. 16

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Acknowledgments Financial support from the National Natural Science Foundation of China (No:

innovation program of FJNU (2018) were greatly acknowledged.

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The first examples of cholesterol-BODIPY liquid crystals 6 and 7 were prepared. The mesophase and fluorescence were tuned by cholesterol-triazine moiety. Both compounds 6 and 7 exhibited strong fluorescence with high quantum yields. Compound 6 with one cholesterol unit was a nematic liquid crystal. Compound 7 with two cholesterol units was a hexagonal columnar liquid crystal.