Novel luminescent chiral network liquid-crystalline polymers containing Sm(III) ions

Abstract

Novel luminescent chiral network liquid-crystalline polymers containing Sm(III) ions (Sm-LCPs) exhibiting wonderful liquid crystalline properties and unique fluorescence properties were prepared by poly(methylhydrogeno)siloxane (PMHS), chiral liquid crystal monomer M₁, crosslinking agent M₂ and samarium complex M₃. The chemical structures, liquid-crystalline behaviours of Sm-LCPs were characterized by various experimental techniques. The introduction of small amount of samarium ions endowed liquid-crystalline polymers with fascinating fluorescence properties. All Sm-LCPs were chiral, and samarium ions did not marked affect the textures of liquid-crystalline polymers. Fourier transform infrared imaging study exhibited that samarium ions were evenly distributed in polymers. A spiral structural model was established to demonstrate the distribution and interaction of components. All Sm-LCPs showed wide mesophase temperature ranges, reversible mesomorphic phase transitions and high thermal stability. The thermogravimetric analysis (TGA) results displayed that the decomposition temperatures (5% weight loss) of all Sm-LCPs were greater than 297 °C. The Sm-LCPs can emit soft red light when excited. Fluorescence intensity of Sm-LCPs gradually increased with increase in samarium complexes from 0.2 to 1.0 mol%. The temperature dependence of luminescence intensity was studied, where the fluorescence intensity of Sm-LCPs decreased monotonically with increase in temperature.

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1. Introduction

Lanthanide luminescent complexes have made an immense contribution to the rapid development of functional materials. They exhibit excellent property of emitting particularly efficient strong narrow-width spectra in a visible light region,^1–3 and they possess potential applications in biological investigations,⁴ light emission molecular devices,⁵ lasers,⁶ drug delivery and sensors,^7–9 etc. However, the low thermal stability and high phase inversion temperature are major defects which hamper their practical applications.^10,11 Encouraged by overcoming these shortcomings and superior fluorescence materials, some attempts have been made to solve these problems by combining lanthanide complexes with inert matrices – for instance, sol–gel glasses,^12–14 liquid crystal (LC) materials,^15–17 polymers,^18,19 or organic–inorganic hybrid materials.^20,21

In addition to the above-mentioned methods, chiral network liquid crystalline polymers seem to be promising candidates due to their excellent thermal performance, processability, unique optical properties such as thermochromism, selective reflection of light, circular dichroism,^22–26 and their potential applications in full-color liquid crystal displays, nonlinear optical materials, switchable storage devices, lasers, anisotropic light emitters,^27–31 etc. The introduction of fluorescent samarium complexes into the ordered spiral matrices of chiral network LCPs is beneficial for both of them. First, chiral network LCPs endow fluorescent samarium complexes with excellent thermal performance and processability. Second, the ordered spiral structures of chiral network LCPs are conductive to uniform distribution of samarium complexes, which improve the luminous efficiency of samarium ions. Last but not the least, samarium complexes confer self-luminescence property on the chiral network liquid crystalline polymers. It is promising that luminescent chiral network LCPs materials lead to new technical applications in many fields such as mechanics, electronics, optics, displays, etc. As a consequence, developing multifarious luminescent chiral network LCPs is of great practical significance.

In this paper, a series of novel chiral network Sm-LCPs were synthesized by graft copolymerization. A spiral schematic illustration of Sm-LCPs was established to demonstrate the interaction and distribution of components. Sm-LCPs can emit soft red light when excited. Luminescence intensity of Sm-LCPs gradually increased with increase in lanthanide complexes but decreased monotonically with increase in temperature.

2. Experimental

2.1. Materials

Poly(methylhydrogeno)siloxane (PMHS, M_n = 582) are obtained from Jilin Chemical Industry Co. (Jilin City, China); 3-bromopropene, 4-hydroxybenzoic acid, undecylenic acid, isosorbide, sulfurous dichloride, isopropanol, sodium isopropoxide, toluene, benzene, chloroform, ethyl alcohol, tetrahydrofuran (THF) and pyridine were purchased from Shenyang Chemical Co. (China); cholesterol was purchased from Henan Xiayi Medical (China); anhydrous SmCl₃ and 2-thenoyltrifluoroacetone (TTA) was 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 of the synthesised samples were obtained using the KBr method performed on a PerkinElmer Instruments Spectrum One Spectrometer (PerkinElmer, Foster City, CA, USA). ¹H NMR (600 MHz) spectra were measured using a Varian WH-90PFT NMR Spectrometer (Varian Associates, Palo Alto, CA). Elemental analyses (EA) were carried out using an Elementar Vario ELIII (Elementar, Germany). The polarised optical microscopy (POM) study was performed with a Leica DMRX (Leica, 90Wetzlar, Germany) equipped with a Linkam THMSE-600 (Linkam, Surrey, UK) heating stage. Fourier transform infrared imaging was performed using a Spotlight 300 infrared imaging system (PerkinElmer). Specific rotations of samples were measured by WZZ-2S Automatic Polarimeter. The thermal stability of Tb-LCI was measured under a nitrogen atmosphere with a Netzsch TGA 209C thermogravimetric analyser at a heating rate of 10 °C min⁻¹. A HORIBA Jobin Yvon FL3-TCSPC fluorescence spectrophotometer was used for fluorescence measurements. Thermal transition properties were characterised using a Netzsch Instruments DSC 204 (Netzsch, Wittelsbacherstr, Germany) at heating and cooling rates of 10 °C min⁻¹ under a nitrogen atmosphere.

2.3. Synthesis of olefinic monomers

The synthesis of M₁, M₂, M₃ and intermediate UBA were shown in Fig. 1. M₁ was synthesized via a previously published method,³² UBA and M₂ were synthesized via a previously published method.³³ M₁ and M₂ were explained in ESI.†

Fig. 1 Synthesis of olefinic monomers.

Sm(TTA)₃UBA (M₃). Anhydrous samarium chloride (2.56 g, 10.0 mmol) dissolved in 20 mL of anhydrous isopropanol and benzene (1 : 1) was heated to 50 °C for 12 h under N₂ and then added a solution of sodium isopropoxide (2.46 g, 30.0 mmol) in 20 mL of isopropanol. The mixture was refluxed for 4 h to synthesize samarium isopropoxide and a solution of 2-thenoyltrifluoroacetone (6.66 g, 30.0 mmol) dissolved in 30 mL of benzene was added dropwise to the stirred solution. After the solution was refluxed for 2.5 h, a solution of UBA (3.04 g, 10.0 mmol) in 10 mL of THF was added. The reactive mixture was refluxed for 3 h. After cooling to room temperature, the mixture was filtered. The products were obtained after the solvent evaporated and washed three times with cyclohexane and dried at 30 °C in a vacuum for 12 h. Yield: 64%. IR (KBr): 3091 (=CH), 2928–2853 (–CH₂–), 1714 (C=O), 1603, 1572 (Ar–), 1257 (C–O), 1167 (C–F). Anal. calcd for C₄₂H₃₅F₉O₁₀S₃Sm: C 45.15, H 3.16, O 14.32; found: C 45.02, H 3.15, O 14.34.

2.4. Synthesis of luminescent chiral network LCPs

All polymers were obtained by the same synthesis methods as shown in Fig. 2. Polymerization experiments were summarized in Table 1. The synthesis of Sm-P₁ is presented as an example. Liquid crystal monomer M₁ (0.4910 g, 0.898 mmol) was dissolved in 30 mL of dry, freshly distilled toluene, monomer M₂ (0.0719 g, 0.100 mmol), M₃ (0.0022 g, 0.002 mmol) and poly(methylhydrogeno)siloxane (PMHS, 0.0831 g, 0.1428 mmol) dissolved in 30 mL THF were added to the stirred solution, 0.2 mL of a 0.5% hexachloroplatinic THF solution was added to the stirred solution and heated under nitrogen and anhydrous condition at 90 °C for 48 h. The reaction was monitored by following the disappearance of Si–H band at 2166 cm⁻¹ in FTIR spectra. Complete disappearance of Si–H band indicated successful incorporation of monomers into polysiloxane chains. After cooling to room temperature, the solution was poured into 200 mL methanol, after filtration, the products were washed with anhydrous hot ethyl alcohol (three times) and dried at 45 °C in a vacuum oven for 12 h to obtain polymer Sm-P₁. Yield: 82%. IR (KBr): 2927–2855 (–CH₃, –CH₂–), 1742–1727 (C=O), 1605, 1510 (Ar–), 1127–1007 (Si–O–Si).

Fig. 2 Synthesis methods of Sm-LCPs.

Table 1 Feed ratios and specific rotations

Sample	Feed (mmol)				Sm^3+ (mol%)	Specific rotations^a
	PMHS	M1	M2	M3
a Specific rotations were measured when polymers were dissolved in THF (ρ = 0.1 g/100 mL).
P0	0.1428	0.900	0.100	0.000	0	−8.97
Sm-P1	0.1428	0.898	0.100	0.002	0.2	−8.91
Sm-P2	0.1428	0.896	0.100	0.004	0.4	−8.86
Sm-P3	0.1428	0.894	0.100	0.066	0.6	−8.74
Sm-P4	0.1428	0.892	0.100	0.008	0.8	−8.69
Sm-P5	0.1428	0.890	0.100	0.010	1.0	−8.65

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3. Results and discussion

3.1. Optical textures

Fig. 3 displays some representative optical textures which were obtained by POM with hot stage on the first heating cycles.

Fig. 3 Optical textures (200×): (a) oily streak texture of M₁ on heating to 126 °C; (b) Grandjean texture of P₀ on heating to 142.5 °C; (c) Grandjean texture of Sm-P₁ on heating to 138.5 °C; (d) Grandjean texture of Sm-P₃ on heating to 145.5 °C; (e) Grandjean texture of Sm-P₅ on heating to 135.5 °C.

Optical textures of liquid crystal monomer M₁ were studied. POM results showed that M₁ exhibited enantiotropic oily streak texture and focal conic texture of cholesteric phase during the heating and cooling cycles. The oily streak texture of M₁ is shown in Fig. 3(a). Fig. 3(b)–(e) illustrates some representative optical textures of P₀, Sm-P₁, Sm-P₃ and Sm-P₅, all polymers showed similar cholesteric phase textures indicating that samarium complexes did not marked affect the liquid crystalline textures of polymers. Take sample Sm-P₁ as an example, when it was heated above 46.7 °C, eyesight became bright and cholesteric Grandjean texture gradually appeared. With the temperature rising, Grandjean texture became bright in color as shown in Fig. 3(b) and disappeared at 182.2 °C. Similarly, when the melt was cool, the cholesteric Grandjean texture appeared again.

Opticity is one of the important optical properties of chiral liquid crystalline materials. It is the proof of chiral liquid crystalline materials. The opticity study showed that all polymers were chiral in accordance with the POM results. The specific rotations are summarized in Table 1.

3.2. Distribution of samarium ions in Sm-LCPs

Carboxylates exhibit characteristic absorptions at 1419 cm⁻¹. Subsequently, the total infrared absorptions of Sm-LCPs were tested by Fourier transform infrared imaging system to analyse the distribution of samarium ions in polymers. The samples were dissolved in anhydrous isopropanol and chloroform (1 : 5) to prepare the same concentration of solution. Same volume of solution was put onto KBr window, test the sample after the solvent evaporation. Finally the characteristic infrared absorptions of carboxylates at 1419 cm⁻¹ were marked in the total absorptions of Sm-LCPs, the infrared imaging of Sm-P₂ and Sm-P₅ are displayed in Fig. 4. Green areas represent weak absorptions at 1419 cm⁻¹, yellow and red areas represent strong absorptions at 1419 cm⁻¹. It is obvious that the samarium ions were evenly distributed in polymers instead of forming clusters, furthermore, the content of samarium ions in Sm-LCPs was in the order of Sm-P₅ > Sm-P₄ > Sm-P₃ > Sm-P₂ > Sm-P₁, – consistent with our theory.

Fig. 4 IR imaging of Sm-P₂ (a) and Sm-P₅ (b).

Combining the analysis results of POM, opticity and Fourier Transform Infrared Imaging, a spiral structural model of Sm-LCPs was established to demonstrate the interaction and distribution of components as shown in Fig. 5. Samarium complexes are evenly distributed in spiral matrices of LCPs. Sm-LCPs can emit soft red light when excited.

Fig. 5 Spiral schematic illustration of Sm-LCPs. The blue rods represent the chiral cholesteric mesogens, the yellow rods represent TTA, the pink rods represent UBA, the pink lines represent crosslinking agents, the light blue lines represent the backbones of liquid crystalline polymers, the brown spheres represent samarium ions which form coordinate bonds with TTA and UBA.

3.3. Thermal properties

For P₀ and Sm-LCPs, the quantised values of glass transition temperatures (T_g) and mesophase–isotropic phase transition temperatures (T_i) were obtained from the dedicated software of the Netzsch instrument. Table 2 shows decomposition temperatures (T_d) and phase transition temperatures. DSC curves are illustrated in Fig. 6(a), the effects of samarium complexes on phase transition temperatures are shown in Fig. 6(b). All Sm-LCPs displayed obvious glass transitions and mesophase–isotropic phase transitions, suggesting that samarium complexes did not change the liquid crystalline state. As seen from Table 2 and Fig. 6, glass transition temperature (T_g) values of Sm-LCPs decreased with increase in samarium complexes ranging from 47.4 to 43.3 °C due to the decrease of rigid mesogens and increase of flexible segments. T_i value of Sm-LCPs decreased with an increase concentration of samarium complex units (0.2–1.0 mol%), the reason for the decreased T_i is that the samarium complex units affected the regularity of chain segments and the mobility, orientation of chiral liquid crystalline mesogenic units. All Sm-LCPs displayed wide mesophase temperature ranges (ΔT) as shown in Table 2.

Table 2 DSC and TG results of polymers

Sample	DSC		ΔT^a	TG
	Tg (°C)	Ti (°C)		T5%^b (°C)
a Mesophase temperature ranges (Ti–Tg).b Temperature at which 5% weight loss occurred.
P0	47.4	183.7	136.3	314.5
Sm-P1	46.7	182.2	135.5	310.5
Sm-P2	45.9	180.8	134.9	305.1
Sm-P3	44.8	179.6	134.8	308.3
Sm-P4	44.1	177.5	133.4	303.7
Sm-P5	43.3	175.3	132.0	297.6

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Fig. 6 (a) DSC thermograms of P₀ and Sm-LCPs. (b) The effects of samarium complexes on phase transition temperatures.

The thermal stability of polymers which is related to molecular structure and molecular weight is important to estimate their working temperature limits and environmental conditions. Temperature for 5 wt% loss is always used as T_d to estimate thermal stability of a polymer. Some representative TGA curves are illustrated in Fig. 7. T_d of all Sm-LCPs took place above 297 °C showing good thermal stability, which indicated that the stable silicone backbones of chiral network LCPs prominently strengthened the thermal stability of samarium complexes and broadened the working temperatures.

Fig. 7 TGA curves of P₀, Sm-P₁, Sm-P₂ and Sm-P₅.

3.4. Luminescence properties of Sm-LCPs

Sm-LCPs in LC phases were obtained by rapid freezing in a liquid nitrogen-vitrified mesophase. Luminescence properties of Sm-LCPs in LC states were examined. The typical emission spectra of Sm-P₂ and Sm-P₄ are presented in Fig. 8. Excitation spectrum wavelength of M₃ and Sm-LCPs was 397 nm, the emission lines near 564 nm, 600 nm and 647 nm were assigned to the ⁴G_5/2 → ⁶H_J/2 (J = 5, 7, 9) transitions. The black dot on the color coordinates in Fig. 8 represents characteristic narrow-width soft red emission. For all samples, fluorescence intensity gradually increased with increase in samarium complexes. When Sm³⁺ (mol%) = 1.0%, Sm-LCPs did not show fluorescence quenching, due to samarium complexes being evenly distributed in the ordered spiral structure of chiral network liquid crystalline polymers, which increased the luminous efficiency of samarium ions.

Fig. 8 Emission spectra of Sm-P₂ and Sm-P₄ at room temperature.

The temperature dependence of fluorescence intensity was measured to investigate the thermal stability of photoluminescence in the LC phases. Fig. 9 shows the emission spectra of Sm-P₃ at different temperatures. The fluorescence intensity of Sm-P₃ decreased monotonically with increase in temperature in the scope of study. The possible reason may be that, with the temperature increased, the molecular liquidity increased resulting in higher fluorescence quenching efficiency in the liquid crystalline phase when excited. Consistent with our theory, fluorescence intensity as a function of temperature fits well the well-known thermal activation function:³⁴

I(T) = I₀/[1 + α exp(−E_A/K_BT)]

where I₀ is emission intensity at 0 K, E_A is thermal activation energy, K_B is Boltzmann's constant, α is proportional coefficient and T is absolute temperature. In any given Sm-LCPs, the value of E_A is the same. It is possible that the fluorescence intensity of chiral network Sm-LCPs can be quantitatively adjusted by accurately controlling the temperature.

Fig. 9 Temperature-dependent emission spectra of Sm-P₃.

4. Conclusions

In this paper, a series of novel luminescent chiral network liquid crystalline polymers containing samarium ions were synthesized and characterized. All Sm-LCPs are chiral. They displayed similar cholesteric Grandjean textures, which indicated that the introduction of small amount of samarium complexes did not change the liquid crystalline state of polymers. A spiral structural model was established to demonstrate that samarium complexes were uniformly distributed in the spiral matrices of chiral network LCPs. The fluorescent chiral network LCPs showed wide mesophase temperature ranges and obviously good thermal stability. All chiral network Sm-LCPs can emit soft red light when excited. Fluorescence intensity of chiral network Sm-LCPs gradually increased with increase in samarium complexes from 0.2 to 1.0 mol%. The ordered spiral structure of chiral network liquid crystalline polymers improved the uniform distribution of samarium complexes, which increased the luminous efficiency. Fluorescence intensity of chiral network Sm-LCPs decreased with increase in temperature in the liquid crystalline phase. It is feasible that the fluorescence intensity of chiral network Sm-LCPs can be quantitatively adjusted by accurately controlling the temperature. The combination of chiral liquid crystalline properties and fluorescence properties allows the potential to fabricate multi-functional 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, ¹H NMR (600 MHz) spectra of M₁ and M₂. See DOI: 10.1039/c6ra07388c

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