Novel photoluminescent chiral liquid crystalline oligomers containing lanthanide ions

Abstract

Novel photoluminescent chiral liquid crystalline oligomers with lanthanide ions (Ln-LCOs) exhibiting excellent liquid crystalline properties and unique fluorescence properties were prepared using poly(methylhydrogeno)siloxane, cholesteric LC M₁, carboxyl-containing LC M₂ and anhydrous lanthanide chlorides. The chemical structures and liquid-crystalline behaviors of Ln-LCOs were characterized by various experimental techniques. The introduction of low content of lanthanide ions endowed the oligomers with significant luminescence properties. All the oligomers were chiral. The lanthanide ions did not change the liquid crystalline textures of the oligomers which were confirmed by X-ray diffraction. Fourier transform infrared imaging study indicated that the lanthanide ions were evenly distributed in the oligomers, which avoided the fluorescence quenching caused by the aggregation of lanthanide ions. A binary complex model with lanthanide ions acting as central ions and chiral liquid crystalline oligomers acting as ligands was established to express the interaction and distribution of the components. The Ln-LCOs showed reversible mesomorphic phase transitions, wide mesophase temperature ranges, and high thermal stabilities. The thermogravimetric analysis results displayed that the decomposition temperatures (5% weight loss) of all Ln-LCOs were greater than 300 °C. The Ln-LCOs can emit red light or green light when being excited. Luminescence intensities of the Ln-LCOs gradually increased with an increase of lanthanide ions from 1 to 4 mol%. The temperature dependence of luminescent intensity was studied, where the fluorescence intensities of Ln-LCOs decreased monotonically with an increase of temperature in liquid crystalline phases.

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1. Introduction

Lanthanide luminescent complexes are fascinating materials and they have received much attention due to their exhibiting the excellent property of emitting particularly efficient strong narrow-width spectra in the visible light region^1,2 with potential applications in probes for DNA,³ molecular devices,⁴ lasers,⁵ optical storage, sensors^6,7 etc. However, the low thermal stability, high phase inversion temperature and low mechanical strength are major defects which hamper their practical applications.^8,9 In order to overcome these shortcomings, some researches have been committed to solve these problems by combining lanthanide complexes with inert matrices, for instance, liquid crystal (LC) materials,^10–12 polymers,^13,14 organic–inorganic hybrid materials¹⁵ or sol–gel glasses.^16,17 In principle, chiral liquid crystalline oligomers seem to be promising due to their excellent thermal performances, good processabilities, unique optical properties such as selective reflection of light, circular dichroism, thermochromism^18–21 and their potential applications in full-color liquid crystal displays, anisotropic light emitters, nonlinear optical materials, lasers,^22–26 etc. Chiral liquid crystalline oligomers with luminescence properties are fascinating materials. For one thing, the stable silicone backbones strengthen the thermal stabilities of photoluminescent chiral liquid crystalline oligomers and broaden the working temperatures; furthermore, the ordered helical structures of chiral liquid crystalline oligomers improve the uniform distribution of lanthanide ions, which increases the luminous efficiency of such ions. It is possible that luminescent chiral liquid crystalline materials will lead to new technical applications in many fields such as optics, mechanics, electronics, displays and so forth. However, luminescent lanthanide-containing chiral liquid crystalline oligomers (Ln-LCOs) have been described rarely so far. As a consequence, designing and synthesizing multifarious lanthanide-containing liquid crystalline oligomers have extremely great realistic significances.

In this paper, a series of novel Ln-LCOs were synthesized by graft copolymerization and coordination. The schematic illustration of Ln-LCOs was established. The lanthanide ions were distributed in the helical matrixes of LCOs by coordinating with carboxyl. The Ln-LCOs can emit red light or green light when being excited. Luminescence intensities of the Ln-LCOs gradually increased with an increase of lanthanide ion content.

2. Experimental

2.1. Materials

Poly(methylhydrogeno)siloxane (PMHS, M_n = 582) was obtained from Jilin Chemical Industry Co. (Jilin City, China). 3-Bromopropene, 4-hydroxybenzoic acid, 4,4-dihydroxybiphenyl, hexanedioic acid, isosorbide, terephthalic acid, sulfurous dichloride, N,N-dicyclohexyl carbodiimide (DCC), 4-(N,N-dimethylamino)pyridine (DMAP), toluene, chloroform, ethyl alcohol, sodium isopropoxide, tetrahydrofuran and pyridine were purchased from Shenyang Chemical Co. (China). Cholesterol was purchased from Henan Xiayi Medical (China). Anhydrous TbCl₃ and EuCl₃ were purchased from Sinopharm Chemical Reagent Co. (China). Toluene and tetrahydrofuran were purified by distillation over sodium hydride before use. All other solvents and reagents were purified by standard methods.

2.2. Measurements

Fourier transform infrared (FTIR) spectra were measured with a Nicolet 510 FTIR spectrometer (Nicolet Instruments, Madison, USA). Element analyses were carried out by an Elementar Vario ELIII (Elementar, Germany). ¹H NMR (600 MHz) spectra were measured by a Varian WH-90PFT NMR spectrometer (Varian Associates, Palo Alto, CA). The polarized optical microscopy (POM) study was performed with a Leica DMRX (Leica, Wetzlar, Germany) equipped with a Linkam THMSE-600 (Linkam, Surrey, England) heating stage. Specific rotations of the oligomers were measured by a WZZ-2S automatic polarimeter. X-ray diffraction (XRD) measurements were performed using nickel-filtered Cu-Kα (λ = 1.542 Å) radiation monochromatized with a DMAX-3A Rigaku powder diffractometer (Rigaku, Japan). FTIR imaging was performed using a Spotlight 300 infrared imaging system (PerkinElmer). The thermal transition properties were characterized using a DSC 204 instrument (Netzsch, Wittelsbacherstr, Germany) at heating and cooling rate of 10 °C min⁻¹ under nitrogen atmosphere. The thermal stabilities of the oligomers were measured under nitrogen atmosphere with a Netzsch TGA 209C thermogravimetric analyzer at a heating rate of 10 °C min⁻¹. A HORIBA Jobin Yvon FL3-TCSPC fluorescence spectrophotometer was used for fluorescence measurements.

2.3. The synthesis of M₁ and M₂

The synthesis routes of cholesteric liquid crystalline cholesteryl 4-(allyloxy)benzoate (M₁) and carboxyl-containing liquid crystalline 4-(((6-((6-((4′-((4-(allyloxy)benzoyl)oxy)-[1,1′-biphenyl]-4-yl)oxy)-6-oxohexanoyl)oxy)hexahydrofuro[3,2-b]furan-3-yl)oxy)carbonyl)benzoic acid (M₂) are shown in Fig. 1.

Fig. 1 Synthesis routes of liquid crystalline M₁ and M₂.

2.4. The synthesis of chiral liquid crystalline oligomers

All the oligomers were obtained by the same synthesis routes. The feed ratio and yield are summarized in Table 1. The synthesis of O₁ is taken as an example. Liquid crystalline M₁ (0.52962 g, 0.97 mmol), M₂ (0.02253 g, 0.03 mmol) and PMHS (0.08314 g, 0.1428 mmol) were dissolved in 15 mL of dry, freshly distilled toluene. 0.2 mL of Pt catalyst (0.50 g of hexachloroplatinic acid hydrate dissolved in 100 mL of THF) was added to the stirred solution and heated under nitrogen and anhydrous conditions at 90 °C for 48 h. The reaction was monitored by following the disappearance of the Si–H band at 2166 cm⁻¹ in the FTIR spectra. The complete disappearance of the Si–H band indicated successful incorporation of M₁ and M₂ into the polysiloxane chains. After cooling to room temperature, the mixture was filtered. O₁ was obtained after solvent evaporation and washing with hot anhydrous ethyl alcohol three times and drying at 45 °C in a vacuum oven for 12 h. IR (KBr): 3402 cm⁻¹ (–OH), 2925, 2872 cm⁻¹ (–CH₃, –CH₂–), 1710, 1688 cm⁻¹ (C=O), 1605, 1508 cm⁻¹ (Ar–), 1127–1007 cm⁻¹ (Si–O–Si).

Table 1 Feed ratio and yield

Sample	Feed				Ln^3+ (mol%)	Yield (%)	Specific rotations^a
	PMHS (mmol)	M1 (mmol)	M2 (mmol)	LnCl3 (mmol)
a Specific rotations were measured when oligomers were dissolved in toluene (ρ = 0.2 g L^−1).
O1	0.1428	0.97	0.03	—	—	—	−7.36
O2	0.1428	0.94	0.06	—	—	—	−5.84
O3	0.1428	0.91	0.09	—	—	—	−4.28
O4	0.1428	0.88	0.12	—	—	—	−3.06
Tb–O1	0.1428	0.97	0.03	0.01	1	86	−7.29
Tb–O2	0.1428	0.94	0.06	0.02	2	80	−5.78
Tb–O3	0.1428	0.91	0.09	0.03	3	82	−4.21
Tb–O4	0.1428	0.88	0.12	0.04	4	79	−2.97
Eu–O1	0.1428	0.97	0.03	0.01	1	81	−7.31
Eu–O2	0.1428	0.94	0.06	0.02	2	79	−5.76
Eu–O3	0.1428	0.91	0.09	0.03	3	83	−4.24
Eu–O4	0.1428	0.88	0.12	0.04	4	78	−2.99

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2.5. The synthesis of Ln-LCOs

The synthesis routes of the Ln-LCOs are shown in Fig. 2. The feed ratio and yield are summarized in Table 1. The synthesis of Tb–O₁ is taken as an example. Anhydrous terbium chloride dissolved in benzene and anhydrous isopropanol (1 : 1) was heated to 50 °C for 12 h under N₂, adding a solution of sodium isopropoxide in isopropanol to the stirred solution. The mixtures were refluxed for 5 h to obtain terbium isopropoxide and then a solution of oligomer O₁ dissolved in toluene was added dropwise to the stirred solution. The reactive mixtures were refluxed for 12 h. After cooling to room temperature, the mixtures were filtered. After the solvent evaporated, the crude products were washed with anhydrous cyclohexane three times and dried at 30 °C in a vacuum oven for 12 h to obtain the target oligomer Tb–O₁. Yield: 86%. IR (KBr): 2925, 2869 cm⁻¹ (–CH₃, –CH₂–), 1728, 1711 cm⁻¹ (C=O), 1605, 1508 cm⁻¹ (Ar–), 1127–1007 cm⁻¹ (Si–O–Si). The absorption bands at 1688 cm⁻¹ (C=O stretching of carboxyl) and 3402 cm⁻¹ (O–H stretching of carboxyl) disappeared; therefore, the Tb–O₁ was successfully synthesized.

Fig. 2 Synthesis routes of Ln-LCOs.

3. Results and discussion

3.1. Liquid-crystalline behaviors of M₁ and M₂

Fig. 3 shows DSC thermograms of liquid crystals M₁ and M₂ in the first heating cycle. M₁ displayed a crystalline melting peak at 112.9 °C (ΔH = 36.95 J g⁻¹) and a mesogenic–isotropic phase transition peak at 238.5 °C (ΔH = 1.361 J g⁻¹) in the heating cycle. M₂ displayed a crystalline melting peak at 155.4 °C (ΔH = 32.58 J g⁻¹) and a mesogenic–isotropic phase transition peak at 245.1 °C (ΔH = 1.965 J g⁻¹) in the heating cycle.

Fig. 3 DSC thermograms of M₁ and M₂.

Fig. 4 displays some representative optical textures of M₁ and M₂ which were obtained by POM with a hot stage in the first heating and cooling cycles. When M₁ was heated above 112.9 °C, it became bright and the LC textures appeared. With the temperature rising, the textures became bright in color and oily streak textures of cholesteric phase were seen as shown in Fig. 4a. The oily streak textures disappeared and the sample became isotropic at 238.5 °C. When the melt was cooled, it showed broken focal-conic textures as shown in Fig. 4b. When M₂ was heated above 155.4 °C, the sample began to melt, and the oily streak textures of cholesteric phase gradually appeared as shown in Fig. 4c. The oily streak textures disappeared and the sample became isotropic at 245.1 °C. When the melt was cooled, broken focal-conic textures appeared as shown in Fig. 4d.

Fig. 4 Optical textures of M₁ and M₂: (a) oily streak textures of cholesteric phase on heating to 146 °C; (b) broken focal-conic textures on cooling to 198 °C; (c) oily streak textures of cholesteric phase on heating to 193 °C; (d) broken focal-conic textures on cooling to 202 °C.

3.2. Optical textures of LCOs and Ln-LCOs

Fig. 5 illustrates some representative optical textures of O₂, Eu–O₂ and Tb–O₂. All of the samples showed similar cholesteric phase textures in the first heating and cooling cycles indicating that lanthanide ions did not seriously affect the liquid crystalline textures of the oligomers. For the sample Tb–O₂, when it was heated above 72.3 °C, it became bright and the cholesteric Grandjean textures gradually appeared. With the temperature rising, the textures became bright in color as shown in Fig. 5c and disappeared at 248.7 °C. Similarly, when the melt was cooled to 243.2 °C, the cholesteric Grandjean textures appeared. Liquid crystalline textures of O₂ and Eu–O₂ are shown in Fig. 5a and b respectively. Opticity is one of the important optical properties for chiral liquid crystals. It is the proof of helical structures of chiral liquid crystals. The opticity study indicated that all the oligomers were chiral in accordance with the POM results and the specific rotations of the oligomers are summarized in Table 1. The cholesteric mesophase of LCOs and Ln-LCOs was also confirmed by XRD diffraction. XRD studies could provide more detailed information on liquid crystalline structures. Fig. 6 shows the representative X-ray diffraction curves of O₂, Eu–O₂ and Tb–O₂ at 110 °C. All the samples showed broad peaks in the wide-angle region around 2θ ≈ 18–20°; furthermore, a sharp peak associated with the smectic layers did not appear in the small-angle region. Therefore, LCOs and Ln-LCOs were of cholesteric phase which is in accordance with the POM results.

Fig. 5 Optical textures of O₂, Eu–O₂ and Tb–O₂ (200×): (a) Grandjean textures of O₂ on heating to 198.5 °C; (b) Grandjean textures of Eu–O₂ on heating to 210.5 °C; (c) Grandjean textures of Tb–O₂ on heating to 205.5 °C.

Fig. 6 XRD curves of O₂, Eu–O₂ and Tb–O₂ at 110 °C.

3.3. Distribution of lanthanide ions in Ln-LCOs

Carboxylates exhibit characteristic absorptions at 1419 cm⁻¹. Subsequently, the total infrared absorptions of Ln-LCOs were tested by a FTIR imaging system to analyze the distribution of lanthanide ions in the oligomers. The samples were dissolved in chloroform and anhydrous isopropanol (5 : 1) to prepare the same concentration of each solution. The same volume of each solution was put onto a KBr window, and the sample tested after solvent evaporation. Finally the characteristic infrared absorptions of carboxylates at 1419 cm⁻¹ were marked in the total absorptions of Ln-LCOs; the representative infrared images of Tb–O₁, Tb–O₂, Tb–O₃ and Tb–O₄ are displayed in Fig. 7a–d. The green areas represent weak absorptions at 1419 cm⁻¹, and the yellow and red areas represent strong absorptions at 1419 cm⁻¹. It is obvious that the terbium ions were evenly distributed in the oligomers instead of getting together to form clusters. Furthermore, the content of terbium ions in Tb-LCOs was in the order of Tb–O₄ > Tb–O₃ > Tb–O₂ > Tb–O₁, which was consistent with the theory.

Fig. 7 IR imaging of Tb–O₁ (a), Tb–O₂ (b), Tb–O₃ (c) and Tb–O₄ (d).

Combining the analysis results of POM, opticity, XRD and FTIR imaging, a binary complex model with lanthanide ions acting as central ions and chiral liquid crystalline oligomers acting as ligands was established to express the interaction and distribution of the components as shown in Fig. 8. The lanthanide ions were evenly distributed in the helical matrixes of LCOs by coordinating with carboxyl in chiral mesogens. The Ln-LCOs emit red light or green light when being excited.

Fig. 8 Schematic illustration of Ln-LCOs. The cyan rods represent the cholesteric mesogens, the yellow rods represent carboxyl-containing cholesteric mesogens, the blue lines represent the backbones of liquid crystalline oligomers, and the brown spheres represent the lanthanide ions which can form coordinate bonds with the carboxyl in cholesteric mesogens.

3.4. Thermal properties

For the LCOs and Ln-LCOs, Table 2 shows the phase transition temperatures obtained in the second heating cycles and decomposition temperatures. The effects of lanthanide ions on phase transition temperatures and decomposition temperatures are shown in Fig. 9. The quantized values of glass transition temperatures (T_g) and mesophase–isotropic phase transition temperatures were obtained from the software included with the Netzsch instrument. All the LCOs and Ln-LCOs displayed glass transitions and mesophase–isotropic phase transitions when they were heated from −8 °C to 295 °C in the second heating cycles suggesting that lanthanide ions did not change the liquid crystalline state of the oligomer systems. Some representative DSC curves of oligomers are illustrated in Fig. 10. As seen from Fig. 9, the T_g values of LCOs and Ln-LCOs increased with an increase of M₂. Furthermore, the T_g values of Ln-LCOs were higher than those of LCOs for the same feed ratio. This result indicated that lanthanide ions introduced into helical matrixes of LCOs by coordination bonds imposed additional constraints on segment motions of oligomer chains, which might be expected to raise the glass transition temperatures.

Table 2 DSC, POM and TGA results of LCOs and Ln-LCOs

Sample	DSC		ΔT^a	POM		TGA
	Tg (°C)	Ti (°C)		Ti1^b (°C)	Ti2^c (°C)	T5%^d (°C)
a Mesophase temperature ranges (Ti − Tg).b Temperature at which the birefringence disappeared completely.c Temperature at which the mesophase occurred.d Temperature at which 5% weight loss occurred.
O1	69.4	229.8	160.4	232.7	227.6	298.4
O2	70.1	232.0	161.9	235.4	230.7	302.6
O3	70.6	233.6	163.0	237.2	232.8	305.8
O4	71.1	235.2	164.1	239.3	234.3	309.3
Tb–O1	71.8	247.1	175.3	252.4	246.8	304.7
Tb–O2	72.3	244.6	172.3	248.7	243.2	310.9
Tb–O3	73.0	239.7	166.7	243.5	237.5	316.5
Tb–O4	73.7	236.6	162.9	239.7	235.3	323.5
Eu–O1	71.6	248.2	176.6	253.7	247.3	304.1
Eu–O2	72.1	244.9	172.8	250.2	243.8	310.3
Eu–O3	72.8	240.4	167.6	244.6	238.3	315.7
Eu–O4	73.4	237.2	163.8	240.3	236.5	322.6

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Fig. 9 Effects of lanthanide ions on phase transition temperatures and decomposition temperatures.

Fig. 10 DSC thermograms of O₃, Tb–O₃ and Eu–O₃ in the second heating cycles.

Similarly, the mesophase–isotropic phase transition temperatures (T_i) of Ln-LCOs were higher than those of LCOs for the same feed ratio. For the Ln-LCOs, heating to the isotropic state needed additional energy to contort the oligomer backbones from the anisotropic state at crosslinking formed by lanthanide ions and carboxyl in chiral mesogens. However, the T_i values decreased slightly with an increase of lanthanide ions in the Ln-LCO systems. This result indicated that chemical crosslinking could prevent the motions and orientations of mesogenic molecules which was not conductive to the mesogenic orientations. T_i values were in the order of Ln–O₁ > Ln–O₂ > Ln–O₃ > Ln–O₄ > O₄–O₁. Both Tb-LCOs and Eu-LCOs displayed wide mesophase temperature ranges (ΔT) as shown in Table 2.

The thermal stabilities of the Ln-LCOs play an important role in determining their working temperature limits and environmental conditions. The thermal stabilities of oligomers are related to their thermal decomposition temperatures (T_d). Temperature for 5 wt% loss is always used as T_d to estimate thermal stability of an oligomer. Some representative thermogravimetric analysis (TGA) curves of Ln-LCOs are illustrated in Fig. 11. The T_d of all Ln-LCOs was above 300 °C showing obviously good thermal stabilities, which indicated that the stable silicone backbones of chiral LCOs prominently strengthened the thermal stabilities of photoluminescent chiral Ln-LCOs and broadened the working temperatures. Furthermore, for all samples, the decomposition temperatures were in the order of Ln–O₄ > Ln–O₃ > Ln–O₂ > Ln–O₁ > O₄–O₁ which indicated that T_d increased with the introduction of lanthanide ions.

Fig. 11 TGA curves of Tb-LCOs.

3.5. Luminescence properties of the Ln-LCOs

The Ln-LCOs in LC phases were obtained by quick freezing in liquid nitrogen vitrified mesophase. The luminescence properties of Ln-LCOs in LC states were examined. The typical emission spectra of Tb–O₁ and Tb–O₃ are presented in Fig. 12 and the emission spectra of Eu–O₁ and Eu–O₃ are presented in Fig. 13. The detailed luminescence data are shown in Tables 3 and 4. The excitation spectrum wavelength of Tb-LCOs was 282 nm obtained by monitoring the maximum emission line ⁵D₄–⁷F₅ at 545 nm. In the emission spectra of Tb–O₁, the bands near 489 nm, 545 nm, 582 nm and 619 nm were assigned to the ⁵D₄ → ⁷F_J transitions with J = 6, 5, 4 and 3 respectively. Characteristic narrow-width green emissions were observed in all Tb-LCOs, which indicated that effective energy transfers took place. The excitation spectrum wavelength of Eu-LCOs was 376 nm. The emission lines of Eu–O₁ near 580 nm, 591 nm, 615 nm, 649 nm and 696 nm were assigned to the ⁵D₀ → ⁷F_J transitions with J = 0–4 respectively. Characteristic narrow-width red emissions were observed in all Eu-LCOs.

Fig. 12 Emission spectra of Tb–O₁ and Tb–O₃ at room temperature.

Fig. 13 Emission spectra of Eu–O₁ and Eu–O₃ at room temperature.

Table 3 Luminescence data of the Tb-LCOs

Sample	Excitation bands (nm)	Emission bands (nm)	Relative intensities^a
a Relative intensities were obtained by the calculation of the integral area of the same emission bands.
Tb–O1	282	489, 545, 582, 619	85.6, 227.9, 14.0, 7.1
Tb–O2	282	488, 545, 583, 620	155.6, 414.3, 25.5, 12.7
Tb–O3	282	489, 545, 582, 620	230.9, 727.3, 36.0, 16.1
Tb–O4	282	489, 544, 582, 619	307.9, 969.7, 48.0, 21.44

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Table 4 Luminescence data of the Eu-LCOs

Sample	Excitation bands (nm)	Emission bands (nm)	Relative intensities^a
a Relative intensities were obtained by the calculation of the integral area of the same emission bands.
Eu–O1	376	580, 591, 615, 649, 696	12.9, 74.3, 305.1, 6.1, 5.7
Eu–O2	376	579, 590, 616, 649, 697	19.2, 127.5, 526.4, 6.5, 7.3
Eu–O3	376	579, 589, 616, 650, 697	26.0, 173.0, 714.4, 8.82, 9.92
Eu–O4	376	578, 590, 616, 648, 696	33.1, 220.2, 909.3, 11.2, 12.6

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For all samples, the luminescence intensities gradually increased with an increase of lanthanide ions accordingly from Ln–O₁ to Ln–O₄. When Ln³⁺ content = 4 mol%, the Ln-LCOs did not show fluorescence quenching due to the Ln³⁺ being evenly distributed in the ordered helical structures of the chiral liquid crystalline oligomers, which increased the luminous efficiency of lanthanide ions. The introduction of oligomers was beneficial to the increase of luminescence intensity. Firstly, the oligomers coordinated to the Ln³⁺ ions with carboxyl which can satisfy the coordination number and so replace water molecules, reducing the non-radiation deactivation resulting from hydroxyl stretching. Secondly, the oligomers increased the absorption cross-section, so increasing the luminescence intensity.

In order to research the thermal stability of photoluminescence in the LC phases, it is necessary to measure the temperature dependence of fluorescence intensity. Fig. 14 exhibits the emission spectra of Tb–O₂ at different temperatures. It was apparent that the fluorescence intensities of Tb–O₂ decreased monotonically with an increase of temperature in the scope of the study. Consistent with theory, the fluorescence intensity as a function of temperature fits well the famous thermal activation function:²⁷

I(T) = I₀/[1 + α exp(−E_A/K_BT)]

where I₀ is emission intensity at 0 K, α is a proportionality coefficient, E_A is thermal activation energy, K_B is Boltzmann's constant and T is absolute temperature. For the same Ln-LCOs, the value of E_A is the same. It is possible that the fluorescence intensities of the photoluminescent chiral Ln-LCOs can be quantitatively adjusted by accurately controlling the temperature.

Fig. 14 Temperature-dependent emission spectra of Tb–O₂.

4. Conclusions

In this paper, a series of novel photoluminescent chiral liquid crystalline oligomers containing lanthanide ions were synthesized and characterized. All the Ln-LCOs are chiral. They displayed similar cholesteric Grandjean textures confirmed by X-ray diffraction, which indicated that the introduction of low content of lanthanide ions did not change the liquid crystalline states of the oligomers. A binary complex model was established to express that lanthanide ions were uniformly distributed in the helical matrixes of LCOs by coordinating with carboxyl in chiral mesogens. The T_i and T_d of Ln-LCOs were higher than those of LCOs for the same feed ratio. The Ln-LCOs showed wide mesophase temperature ranges and obviously good thermal stabilities. The Ln-LCOs can emit red light or green light when being excited. With an increase of lanthanide ion content from 1 to 4 mol%, the luminescence intensities of Ln-LCOs gradually increased. The ordered helical structures of chiral liquid crystalline oligomers improved the uniform distribution of lanthanide ions, which increased the luminous efficiency of the lanthanide ions. The fluorescence intensities of the Ln-LCOs decreased with an increase of temperature in the LC phases. It is feasible that the fluorescence intensities of Ln-LCOs can be quantitatively adjusted by accurately controlling the temperature. The combination of liquid crystalline properties and fluorescence properties of Ln-LCOs makes them possible candidates for the fabrication of multifunctional optical materials.

Acknowledgements

The authors are grateful to the National Key Technology Support Program of China (contract grant number 2008BAL55B03) and the Science and Technology Department of Liaoning Province for financial support of this work.

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Footnote

† Electronic supplementary information (ESI) available: Fourier transform infrared (FTIR) spectra of the samples, synthesis procedures of M₁ and M₂, ¹HNMR (600 MHz) spectra of M₁ and M₂. See DOI: 10.1039/c5ra15655f

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