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Calixarene molecules are originally known as inclusion compounds in host-guest chemistry, which can bind various metal ions and organic molecules. We investigated the binding property of calixarene polymer against trivalent europium (Eu3+) complex which possesses excellent luminescent characteristics. An interaction, i.e. coordination structure, is established between calixarene polymer and Eu3+ complex, which results in an effective suppression of concentration quenching of Eu3+ complex and enhancement of luminescent intensity in high concentration.
©2012 Optical Society of America
1. Introduction
In recent years, much attention has been paid to photonic polymers because of their high transparency, processability, refractive index tunability. Therefore, a lot of photonic polymers have been developed for optical device applications such as plastic optical fiber, polymer optical waveguide, and so on [1–3]. Moreover, functional photonic polymers have been expected for future active optical devices [4,5].
Polymerizable macrocyclic molecules such as calixarene and calixresorcinarene polymers have been extensively developed as new type of photoresists for photo- or electron-beam lithography [6]. These calixarene polymers are expected to possess good thermal stability due to their molecular structure containing many phenol rings. Thus, we are interested in an application of the calixarene polymers to thermally stable optical waveguide materials [7], and three-dimension optical waveguide has been demonstrated by two-photon photopolymerization [8].
Calixarene molecules are also originally famous as inclusion compounds in host-guest chemistry field, which can bind various metal ions and organic molecules [9–11]. Thus, it is important to investigate the binding property of calixarene polymers against certain guest molecule for future active polymer devices. Europium complex are expected in optical application such as plastic optical fiber amplifiers and lasers, due to their high fluorescence intensity and high quantum yield. However, there is a problem in the europium complexes with polymer materials, i.e. concentration quenching [12]. To avoid this phenomenon a homogeneous distribution of rare ions is an indispensable condition [13–15]. If an interaction between europium ions and macrocyclic polymer is established, the ions will be distributed homogeneously with appropriate separation distance from each other. Consequently, aggregation and interaction between neighboring ions could be suppressed even at high doping concentration.
In this paper, we demonstrate our study on binding of trivalent europium (Eu3+) ions to calixarene-based polymer containing oxetane groups, and correlated change in optical properties due to this binding. By avoiding aggregation of Eu3+ ions we observe enhancement of luminescent intensity in high concentration
2. Material preparation
Figure 1 shows chemical structure of materials used in this experiment. Europium tris-(thenoyltrifluoro-acetonate) anhydrous (Eu(TTA)3) (ACROS organics, Inc.) was used as luminescent material. A calixarene monomer, 2,8,14,20-tetramethyl-4,6,10,12,16,18,22,24-octakis(3-ethyl-3-oxetanylethoxy)- calix(4)resorcinarene (CRAOX), which contains oxetane groups as polymerizable groups was used as a matrix. Polymerizable calixarene has several merits; high thermal stability, inclusion property of functional guest materials, blocking effect of aggregation of guest materials by its bulky structure. A co-monomer, oxetanemethacrylate (OXMA), which is able to copolymerize with CRAOX for the purpose of controlling refractive index and improving film quality of co-polymers was also used. Luminescent polymeric material, Eu(TTA)3 doped CRAOX-OXMA, was prepared by thermal curing as follows. Firstly, Eu(TTA)3, CRAOX, OXMA and phthalic anhydride (used as a curing agent) were dissolved in chloroform. The solution was then stirred at room temperature for 1h, and spin-casted on appropriate substrate. After a soft-baking at 60°C, a thermal curing process was carried out on a hot plate at 120°C for 10 h. For comparison, Eu(TTA)3 doped polystyrene (PS) was also prepared by adding Eu(TTA)3 to PS, since there expects no interaction between two materials.
The thermal stability of the calixarene copolymer was examined by thermogravimetry analysis and shown in Fig. 2 . T5wt% of calixarene copolymer was higher than 300 °C, indicating their good thermal stability. Differential scanning calorimetory measurement was also carried out, though no significant glass transition temperature was observed. This means the crosslinking reaction of calixarene copolymer was proceeded effectively. Moreover, rather than pure calixarene, film quality remarkably improved without any crack and visible scattering by using copolymer system (CRAOX-OXMA).
3. Spectroscopic results and analysis
3.1 Chemical shifts of CRAOX
Table 1 shows chemical shift of CRAOX in CDCl3 measured by 1H-NMR spectra. The spectra was notably changed after Eu(TTA)3 doping. These positional changes of the chemical shift (δ) are caused by de-shielding effect called Lanthanide Induce Shift (LIS). Since europium ion is paramagnetic, in the presence of external magnetic field, the ion produces induced magnetic field and increases the local magnetic field. Consequently, a proton (1H) located nearby may sense strong local magnetic field and become more de-shielded.
Table 1. Chemical Shift of CRAOX Measured by 1H-NMR Spectra
The proton peaks of oxetane groups of pure CRAOX, H-4, in Fig. 1, appeared at 3.62 ppm and at 3.83 ppm as indicated in Table 1. In Eu(TTA)3 doped CRAOX, those protons were noticeably upfield shifted where the δ of H-4 was displaced to around 4.8 ppm. This fact suggests the interaction of the trivalent europium ion with the oxygen atoms of the oxetane rings in CRAOX. The trivelent lanthanide ion, a kind of lewis acid, has strong preference for negatively charged atom such as fluorine, oxygen and nitrogen [16]. According to these facts, appearance of δ shifting in Table 1 can be assigned to pseudocontact shift originating from the coordination of europium ion to oxygen atom of oxetane ring in CRAOX. This interaction is expected to improve their solubility against CRAOX. Consequently, complex aggregation and interaction between neighboring ions could be suppressed even at high doping concentration. On the other hand, no positional change of chemical shift was observed in 1H-NMR spectra of Eu(TTA)3 doped PS, suggests that no interaction exists between Eu(TTA)3 and PS.
3.2 Luminescence properties of the Eu(TTA)3/PS and Eu(TTA)3/CRAOX-OXMA
The Eu(TTA)3 doped PS and CRAOX-OXMA films were excited by a 375nm diode laser (5 mW/cm2) and the emission spectra are shown in Fig. 3(a) . The luminescence peaks, which are considered from 5D0 → 7FJ transitions, and the corresponding J values are indicated in the Fig. 3. According to the Judd–Ofelt theory, luminescence can be originated from magnetic dipole transition or/ and forced electric dipole transitions [17,18]. A hypersensitive I0-2 (5D0 → 7F2) line peak at 610nm, which is a typical emission peak of β-diketonate europium complexes, was observed. The hypersensitive transition is usually associated with large value of Judd–Ofelt intensity parameter Ω2, which caused by the absence of inversion symmetry in coordination structure. In the case of low symmetry and highly anisotropic ligand/crystal field, the intensity of this hypersensitive peak is much larger than that of other transition lines. Moreover, because the maximum number of this ligand field splitting for J manifold is (2J + 1), energy level splitting into maximum of five Stark levels in this hypersensitive line can be expected. Figure 3(b) shows the curve fitting of the hypersensitive line of Eu(TTA)3/CRAOX–OXMA with three Gaussian functions. The fitting result clarifies that the peak is composed of three peaks (with their spectral shape parameters as indicated in Table 2 ), suggesting three Stark levels for 7F2 level. In this case, the ligand field in Eu(TTA)3/CRAOX-OXMA are not able to completely remove the degeneracy of 7F2 level. Nevertheless the ligand field in Eu(TTA)3/CRAOX-OXMA exists with much lower symmetry than that in Eu(TTA)3/PS. This may also relate to extensive distortion of coordination structure in Eu(TTA)3/CRAOX-OXMA rather than in Eu(TTA)3/PS. The splitting of this I0-2 line therefore indicates a binding of Eu(TTA)3 to CRAOX.
Fig. 3 (a) Emission spectra of Eu(TTA)3 which is embedded in PS(solid line) or CRAOX-OXMA (dash line) excited at 375 nm, and (b) curve fitting of the Eu(TTA)3 emission spectra embedded in CRAOX-OXMA with three Gaussian functions.
Table 2. Emission Spectra Shape Parameters of Eu(TTA)3 Embedded in CRAOX-OXMA
3.3 Concentration quenching of the Eu(TTA)3/PS and Eu(TTA)3/CRAOX-OXMA
The UV-Vis absorption spectra of Eu(TTA)3/PS with various Eu(TTA)3 concentration are shown in Fig. 4(a) and their normalized spectra are shown in Fig. 4(b). Figures 4(c) and 4(d) are the spectra and normalized spectra of Eu(TTA)3/CRAOX-OXMA respectively. The peak around 345 nm is considered from the absorption of TTA ligands. For Eu(TTA)3/PS system by increasing europium concentration absorption, the peak wavelength shifts approximate 9nm to longer wavelength, indicating that there exists concentration quenching of Eu3+ complex. On the other hand, for the Eu(TTA)3/CRAOX-OXMA system, the peak wavelength is independent of Eu3+ complex concentration up to 11.6 wt%. This suggests that the concentration quenching of Eu3+ complex is suppressed.
Fig. 4 UV-Vis absorption spectra of (a) Eu(TTA)3/PS and (c) Eu(TTA)3/CRAOX-OXMA for various Eu(TTA)3 concentration. (b) and (d) show normalized peak of (a) and (c) respectively.
The emission intensity dependence of Eu(TTA)3/PS and Eu(TTA)3/CRAOX-OXMA on the Eu(TTA)3 content is shown in Fig. 5 . In the Eu(TTA)3/PS, the emission intensity increases with the increase of Eu(TTA)3 content and reaches its maximum around 5 wt % and then exhibits typical emission concentration quenching on further increasing europium content. This quenching phenomenon may be caused by the deactivation of the 5D0 or 5D1 state through electrostatic multipolar interaction or by the exciton migration via the Forster dipole-dipole mechanism in solid complexes. When the content of Eu(TTA)3 doped in PS matrix is small, the probability of energy migration via diffusional collision of the Eu3+ complex should be small. But at high content of the complex, the aggregates of the complex occur in the solid matrix. Under this condition, the exciton migration process may be a dominant effect accounting for emission concentration quenching.
Fig. 5 The emission intensity dependence of Eu(TTA)3/PS and Eu(TTA)3/CRAOX-OXMA on the Eu(TTA)3 concentration.
In striking contrast, the emission intensity of Eu(TTA)3/CRAOX-OXMA increases with increasing Eu(TTA)3 content up to 13 wt%. The higher concentration of Eu(TTA)3 in CRAOX-OXMA matrix results approximately 8 times higher photoluminescent intensity than that of Eu(TTA)3/PS system. As mentioned above, the Eu(TTA)3 are uniformly dispersed in the CRAOX-OXMA and are surrounded by the polymer chain. This special conformation reduces the ligand interaction and decreases the exciton migration, which results in the decrease of the probability of the emission concentration quenching.
4. Conclusions
The binding property of calixarene polymer against trivalent europium (Eu3+) complex was investigated. The interaction, i.e. coordination bond, between europium complex and calixarene polymer was verified from 1H-NMR spectra and UV-vis spectra measurement. This interaction suppresses the exciton migration of europium complex, which results in the decrease of the probability of the emission concentration quenching. Enhancement of luminescent intensity from Eu(TTA)3/CRAOX-OXMA in high concentration was observed successfully. Considering high emission characteristics derived from suppression of concentration quenching, high thermal stability, good film and device processability, Europium complex doped calixarene copolymer is useful for future compact optical waveguide-type active polymer devices.
Acknowledgments
The authors wish to thank Professors Hiroto Kudo and Tadatomi Nishikubo for their technical support for calixarene polymer synthesis.
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