Fluorescence properties of polyaryletherketone rare earth-coordinated complexes in near-infrared region

Abstract

Novel rare earth-coordinated polymers were prepared using carboxyl-containing polyaryletherketone (PEK) as the macromolecular ligand and 8-hydroxyquinoline (HQ) as the co-ligand to coordinate with erbium (Er³⁺), neodymium (Nd³⁺) and ytterbium (Yb³⁺), respectively. The FTIR, elemental analysis, inductively coupled plasma, and UV-vis results confirm that the rare earth ions have been attached to the polymer backbone via a coordination bond. The complexes can emit the characteristic near-infrared fluorescence of the corresponding rare earth ions. Moreover, it is found that the synergistic effect of PEK and HQ can significantly enhance the emission intensities. In particular, for PEK-Er³⁺-HQ, the full width at half maximum (FWHM) of the ⁴I_13/2→⁴I_15/2 transition is up to 78 nm. The combination of strong fluorescence emission in the near-infrared range, high glass transition temperature and wide gain bandwidth provides promising potential for optical amplification applications.

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Introduction

Rare earth polymeric complexes are of great value in the materials science field because of their high luminous intensity, long luminous lifetime and extremely sharp emission bands arising from the 4f electronic transitions of rare earth ions. In the past decades, however, studies in this area were mainly focused on the visible range (400–800 nm) for optical display devices, whereas less attention has been paid to the fluorescence materials in the near-infrared range (800–2000 nm).^1–5 Only recently, the importance of rare earth complexes containing erbium (Er³⁺), neodymium (Nd³⁺) and ytterbium (Yb³⁺) ions is being recognized, owing to their increasing potentials in optical amplification (1500 nm emission of Er³⁺ complex), laser systems (1060 nm emission of Nd³⁺ complex), and medical diagnosis (980 nm emission of Yb³⁺ complex).^6–13 A review of the literature reveals that most of the above materials are prepared by doping the rare earth ions into inorganic matrixes such as sol–gel glass,^14–18 but organic polymeric rare earth complexes with fluorescence emitting in the near-infrared region are seldom reported.

Compared to inorganic materials, organic polymers have some intrinsic advantages, such as flexibility, excellent processability and convenient control of polymer properties by varying the molecular structures. Nevertheless, rare earth-doped polymers prepared by the simple mixing of rare earth complexes with a polymer matrix often give rise to serious fluorescence quenching due to the ionic aggregates, even at low rare earth ion content.^19,25 It is desirable that rare earth ions are coordinated with some polymer functional groups. Thus, rare earth ions attached to the polymer backbone via a coordination bond can be well dispersed within the polymer matrix at the molecular level.

Polyaryletherketone (PEK), simultaneously bearing carboxyl and alkyl substitutent groups on the phenylene ring, is a new polymer recently developed in our group.²⁶ The aromatic backbone and bulky alkyl substituents on the phenylene rings result in the high glass transition temperatures (over 210 °C), excellent heat-resistance and high mechanical strength. Meanwhile, the carboxyl groups in the PEK chain provide the sites to coordinate with the rare earth ions, which make it very suitable for utilization as a macromolecular ligand. In the present work, using PEK as a macromolecular ligand and HQ as a synergistic ligand, rare earth coordination polymers (PEK-RE³⁺-HQs, RE³⁺ = Er³⁺, Nd³⁺, Yb³⁺) are prepared and characterized. Their near-infrared fluorescence properties in the near-infrared region are investigated in detail.

Experimental

Materials

Rare earth nitrate hexahydrates (RE(NO₃)₃·6H₂O, RE³⁺ = Er³⁺, Nd³⁺, Yb³⁺) and 8-hydroxyquinoline (HQ) were purchased from Shandong Qingda Fine Chemical Factory and Shenyang Chemical Reagent Factory, respectively, and used without further purification. N,N-Dimethylformamide (DMF) was dried and distilled under reduced pressure just before use. Other chemical reagents were of analytical reagent grade and used as received.

Synthesis of carboxyl-containing polyaryletherketone (PEK). Carboxyl-containing polyaryletherketone (PEK) was prepared via a solution nucleophilic polycondensation reaction from bis(4-fluorophenyl)ketone with 2′,2′′-diisopropyl-5′,5′′-dimethyl-phenolphthalin according to the procedure described in our previous paper.²⁶ FTIR (KBr, cm⁻¹): 2961, 2927 and 2869 (CH₃), 1726 and 1695 (Ar–COOH), 1655 (Ar–CO–Ar), 1262, 1227 and 1034 (Ar–O–Ar). ¹H NMR (DMSO-d₆, ppm): 12.68 (Ar–COOH), 6.82–7.85 (Ar–H), 6.60 ((Ar)₃–CH), 2.944 (CH of (CH₃)₂CH), 2.065 (Ar–CH₃), 0.899 (CH₃ of (CH₃)₂CH). GPC (THF, PS as internal standard): M_n (66379 g mol⁻¹), and PDI (3.0).

Synthesis of rare earth-coordinated polymer complexes (PEK-RE³⁺-HQs). The synthesis methods of PEK-RE³⁺-HQs (RE³⁺ = Er³⁺, Nd³⁺, Yb³⁺) are similar. The typical preparation procedure for PEK-Er³⁺-HQ is described as follows: 0.1500 g PEK was dissolved in 2.5 ml dry DMF, and then a solution of 0.1153 g Er(NO₃)₃·6H₂O and 0.1089 g HQ in 2.5 ml dry DMF was slowly added under constant stirring. The reaction solution was stirred for 2 h at room temperature. The homogeneous solution obtained was used for the fluorescence measurement. The solutions of PEK-RE³⁺s and RE³⁺-HQs were also prepared under similar conditions for comparison.

The film samples of PEK-RE³⁺-HQs for FTIR, elemental analysis, XRD and TGA measurements were obtained from their DMF solutions, which were cast onto a clean glass plate at 60 °C. After 8 h, the films were transferred to vacuum oven and dried further at 150 °C for 24 h to constant weight.

The powder samples of PEK-RE³⁺-HQs and PEK-RE³⁺s for the UV-Vis absorption measurements were also prepared from their DMF solutions, which were slowly poured into ethanol under constant stirring. After filtration, the powder products were dried at 80 °C under vacuum to constant weight before measurements. The powder samples of RE³⁺-HQs for the FTIR and XRD measurements were also obtained from their DMF solutions by the above method.

Instruments

Fourier transform infrared spectra (FTIR) were recorded on a Nicolet 20XB FT-IR spectrophotometer using KBr pellets at room temperature.

Elemental analyses were performed on an Elementar Vario EL III elemental analyzer. The rare earth ions content was determined by an optimer 2000dv inductively coupled plasma (ICP).

The ultraviolet-visible (UV-Vis) absorption spectra were measured by a UV-550 spectrophotometer at room temperature.

X-ray diffraction (XRD) measurements were recorded on a D/Max-2400 X-ray diffractometer with Cu-Kα₁ radiation (λ = 1.54 Å) over the 2θ range 4 to 60°.

Differential scanning calorimetries (DSC) were run on a NETZSCH DSC204 in the temperature range 50–350 °C at a heating rate of 10 °C min⁻¹ in a nitrogen atmosphere. The glass transition temperatures (T_g) were read at the middle of the change in heat capacity.

The fluorescence excitation and emission spectra of rare earth coordination polymers were measured on a FS 920 steady-state fluorescence spectrofluorometer with a scanning rate of 300 nm min⁻¹ at room temperature. The slit widths for both excitation and emission were set as 15 nm, and the concentrations of DMF solutions of rare earth coordination polymers were 1 × 10⁻³ mol L⁻¹.

Results and discussion

Synthesis and characterization of rare earth-coordinated polymers

As shown in Scheme 1, a series of novel rare earth-coordinated polymers (PEK-RE³⁺-HQs, RE³⁺ = Er³⁺, Nd³⁺, Yb³⁺) were synthesized in DMF solution through a co-coordination reaction with carboxyl-containing polyaryletherketone (PEK) and 8-hydroxyquinoline (HQ). Their structures were characterized by the FTIR spectra, elemental analysis, inductively coupled plasma, UV-Vis absorption spectra and XRD methods.

Scheme 1 Synthesis of rare earth coordination polymers (PEK-RE³⁺-HQs).

The FTIR spectra of HQ, Er³⁺-HQ, PEK and PEK-Er³⁺-HQ are illustrated in Fig. 1. For HQ, the characteristic absorption corresponding to the C–O stretching vibration is located at 1093 cm⁻¹. After coordination, the spectra of Er³⁺-HQ and PEK-Er³⁺-HQ show that the band at 1093 cm⁻¹ shifts to 1107 cm⁻¹, and the absorptions due to the Er³⁺-O and Er³⁺-N stretching vibrations appear at 490 cm⁻¹ and 818 cm⁻¹, respectively, suggesting that HQ is coordinated to the Er³⁺ ion.^27,28 Moreover, the comparison between PEK and PEK-Er³⁺-HQ reveals that the characteristic carboxyl absorption for PEK at 1725 and 1696 cm⁻¹ completely disappears in the spectrum of PEK-Er³⁺-HQ. Instead, a new peak at about 1384 cm⁻¹ due to the symmetric stretching vibration (COO⁻) appears, indicative of the formation of a coordination bond between the Er³⁺ ion and the carboxyl group in the PEK backbone. In addition, in the FTIR spectra of PEK-Er³⁺-HQ and Er³⁺-HQ, no adsorption at 460 cm⁻¹ is observed, suggesting that the solvent DMF molecules do not participate in coordination with the Er³⁺ ion.²⁹ The reason can be attributed to the fact that the carboxyl groups of PEK and HQ have strong coordination ability, which can fully exclude coordination of the solvent molecules (DMF) to the Er³⁺ ion. The results of elemental analysis of PEK-Er³⁺-HQ agree well with the calculated ones (Table 1).

Fig. 1 FTIR spectra of (a) HQ, (b) Er³⁺-HQ, (c) PEK, (d) PEK-Er³⁺-HQ.

Table 1 Elemental analysis results of PEK-Er³⁺-HQ

Element	Found (%)	Calc. (%)
C	58.10	58.17
H	4.63	4.01
N	5.86	6.07
Er	12.18	12.09

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PEK can dissolve easily in N,N-dimethylformamide (DMF), N,N-dimethlyacetamide (DMAc), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), tetrahydrofuran (THF) and acetone at room temperature. Relative to the pure PEK, the three rare earth complexes exhibit obviously decreased solubility due to enhancement of the interaction between the polymer chain and the rare earth ion, which makes the diffusion of solvent molecules between the polymer chains more difficult. Nevertheless, PEK-RE³⁺-HQs can still be soluble in DMF, DMAc, DMSO and NMP at room temperature. More importantly, the films of PEK-RE³⁺-HQs can be obtained from their solutions and the thickness of the films can be controlled by changing the concentration of the solutions to meet the requirement of practical applications.

The XRD patterns of PEK-Er³⁺-HQ, Er³⁺-HQ and PEK are compared in Fig. 2. Er³⁺-HQ displays some sharp crystalline peaks, while PEK exhibits a wide hump, indicative of its amorphous structure. However, PEK-Er³⁺-HQ only shows an amorphous hump similar to PEK, and the sharp peaks corresponding to Er³⁺-HQ cannot be detected, suggesting that, for PEK-Er³⁺-HQ, the rare earth ions are well dispersed in the polymer matrix. The other two polymer complexes also give similar results.

Fig. 2 XRD patterns of (a) PEK, (b) PEK-Er³⁺-HQ, and (c) Er³⁺-HQ.

The DSC measurements for both PEK and PEK-Er³⁺-HQ were conducted at a heating rate of 10 °C min⁻¹ in a N₂ atmosphere. As illustrated in Fig. 1S (ESI†), PEK shows quite a high glass transition temperature (T_g) at 230 °C because of its rigid aromatic backbone. After coordination with the rare earth ion, for PEK-3-Er³⁺-HQ, the former glass transition disappears, and no obvious T_g can be detected in the whole measurement temperature range. The possible reason may be due to the enhanced interchain interaction caused by the introduction of rare earth ions, leading to a significantly increased T_g beyond the measurement range. The high glass transition temperature is especially favorable for optical amplification application of PEK-Er³⁺-HQ.

UV-Vis absorptions and near-infrared emissions of rare earth-coordinated polymers

The UV-Vis absorption spectra of PEK and PEK-Er³⁺-HQ are presented in Fig. 3. Relative to PEK, the absorption bands of PEK-Er³⁺-HQ are apparently red-shifted, which is caused by the decrease in electron density around PEK due to the coordination bond between the Er³⁺ ion and the oxygen atom of the carboxyl group in PEK. The weak peaks at 488, 522 and 653 nm are attributed to the ⁴I_15/2→⁴F_7/2, ⁴I_15/2→²H_11/2 and ⁴I_15/2→⁴F_9/2 transitions of Er³⁺ ions, respectively.

Fig. 3 UV-Vis absorption spectra of (a) PEK and (b) PEK-Er³⁺-HQ.

The emission spectrum of PEK-Er³⁺-HQ is shown in Fig. 4. Under excitation at 392 nm, a broad emission band at 1527 nm, attributed to the typical ⁴I_13/2→⁴I_15/2 transition of the Er³⁺ ion appears, which is just located in the desired wavelength range for optical-fiber telecommunication. Moreover, the full width at half maximum (FWHM) of PEK-Er³⁺-HQ is up to 78 nm, much wider than other Er³⁺-containing materials, such as Er³⁺-doped phosphate glass (40 nm).³⁰ The above reason can be attributed to the structure of PEK, which simultaneously contains multiple alkyl substituents and a bulky side group. As a result, the resultant PEK-Er³⁺-HQ backbone becomes seriously twisted, and the chain packing is very disordered. The previous study has revealed that the electron transition of a rare earth ion is very sensitive to the coordination environment.³¹ The disordered coordination environment of the Er³⁺ ion can disturb the energy transfer of the ligands, and thus lead to the broadened FWHM of PEK-Er³⁺-HQ. The wide gain bandwidth is desirable for optical amplification applications.

Fig. 4 Emission spectrum of PEK-Er³⁺-HQ.

The replacement of Er³⁺ with Nd³⁺ (Fig. 5) makes the emission bands of PEK-Nd³⁺-HQ shift toward the shorter wavelengths appearing at 898, 1060 and 1328 nm, corresponding to the ⁴F_3/2→⁴I_9/2, ⁴F_3/2→⁴I_11/2 and ⁴F_3/2→⁴I_13/2 transitions, respectively. Among them, the emission intensity at 1060 nm due to the ⁴F_3/2→⁴I_11/2 transition is the strongest, suggesting that PEK-Nd³⁺-HQ is suitable for the laser materials corresponding to the 1060 nm wavelength.^6–13

Fig. 5 Emission spectrum of PEK-Nd³⁺-HQ.

The excitation spectra of PEK-Yb³⁺-HQ, PEK-Yb³⁺ and Yb³⁺-HQ, by monitoring the emission of the Yb³⁺ ions at 980 nm, are shown in Fig. 6. The inset is the enlarged view of the excitation spectrum of PEK-Yb³⁺, in which the broad band centered at 350 nm was attributed to the π→π^* electronic transition of PEK, and the sharp peak at 490 nm was associated to the 4f–4f transitions of the Yb³⁺ ion. The excitation intensity of the former is higher than that of the latter, implying that the fluorescence emission of the PEK-Yb³⁺ is mainly sensitized by the absorption of the organic ligand rather than that of Yb³⁺ ion. This deduction can be further proved by the excitation spectra of PEK-Yb³⁺-HQ and Yb³⁺-HQ (Fig. 6a and Fig. 6b), in which the excitation peaks corresponding to the 4f–4f transitions of Yb³⁺ ion can hardly be observed. The maximum excitation wavelengths of PEK-Yb³⁺-HQ and Yb³⁺-HQ are located at 392 and 388 nm, respectively. On the other hand, PEK-Yb³⁺-HQ displays a stronger and broader excitation band than PEK-Yb³⁺ and Yb³⁺-HQ. The reason can be due to the fact that, relative to PEK-Yb³⁺ and Yb³⁺-HQ, PEK-Yb³⁺-HQ has the greater π-electronic system, owing to the simultaneous introduction of macromolecular ligand PEK and small molecule HQ, which is favorable for the absorption of the ultraviolet light.

Fig. 6 Excitation spectra of (a) PEK-Yb³⁺-HQ, (b) Yb³⁺-HQ, and (c) PEK-Yb³⁺.

In order to further investigate the synergistic effect of PEK and HQ on the fluorescence properties of polymeric rare earth complexes, the comparative emission spectra of PEK-Yb³⁺-HQ, PEK-Yb³⁺ and Yb³⁺-HQ are measured and illustrated in Fig. 7. The inset presents the enlarged emission spectrum of PEK-Yb³⁺. It can be seen that all the complexes emit the characteristic fluorescence of the Yb³⁺ ion at about 980 nm, corresponding to the ⁴F_5/2→²F_7/2 transition, but their emission intensities are different. Among them, the emission intensity of PEK-Yb³⁺ is the lowest, due to the lack of effective energy transfer within the complex system. In contrast, the simultaneous coordination of PEK and HQ to the Yb³⁺ ion leads to the PEK-Yb³⁺-HQ sample having the strongest fluorescence. The reason is attributed to the fact that, firstly, the coordination numbers of the Yb³⁺ ions in the PEK-Yb³⁺-HQ system can be much more easily satisfied than in either the PEK-Yb³⁺ or the Yb³⁺-HQ system and, secondly, the macromolecular ligand PEK is quite rigid, which makes the coordination structure of PEK-Yb³⁺-HQ more stable, thereby effectively inhibiting the non-radiative transition of rare earth ions.

Fig. 7 Emission spectra of (a) PEK-Yb³⁺-HQ, (b) Yb³⁺-HQ, and (c) PEK-Yb³⁺.

Conclusions

A series of novel rare earth coordination polymers (PEK-RE³⁺-HQs, RE³⁺ = Er³⁺, Nd³⁺, Yb³⁺) with near-infrared fluorescence emission were successfully prepared, in which the RE³⁺ ions were attached onto the polymer backbone via coordination bonds, and surrounded by both PEK and low molecular weight ligand HQ, and thus distributed homogeneously in the polymer matrix. The rare earth coordination polymers could dissolve in the common solvents at room temperature and were easily cast into uniform and smooth films, which is important in future practical applications. PEK-Er³⁺-HQ, PEK-Nd³⁺-HQ and PEK-Yb³⁺-HQ emit intense fluorescence in the near-infrared region and the strongest emission peaks are located at 1527, 1060 and 978 nm, respectively. The synergistic effects of PEK and HQ could significantly improve their emission intensities. These rare earth coordination polymers with excellent near-infrared fluorescence properties and good film-forming properties showed potential applications in the optical communication, laser and bio-analysis fields.

Acknowledgements

The Program for New Century Excellent Talents in University of China (Grant No. NCET-06-0280), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (Grant No. 2005-546), and the Supporting Program for Optoelectronic Materials of Dalian University of Technology is gratefully acknowledged. We are grateful to Prof. Chunying Duan for the fluorescence measurements.

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Footnote

† Electronic Supplementary Information (ESI) available. See DOI: 10.1039/c2ra21336b

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