Optical properties of europium [(2,2'-bipyridine-N,N-dioxide).sub.2] [([NO.sub.3]).sub.3] complex-doped poly(methyl methacrylate).
Rare earth complexes have attracted much attention over the years due to their impressive luminescence properties, arising from f-f transitions. Typically europium salts, terbium salts, and samarium salts, when associated with suitable organic ligands, can absorb light energy and transfer it to the central metal atom to give narrow emission bands in the visible region, usually described as an "antenna effect" (1), (2). The synthesis and the characterization of 2,2'-bipyridine-N,N-dioxide complexes have been studied for many years due to the potential applications related to light-converting molecular devices. In these, the 2,2'-bipyridine-N,N-dioxide (bPy[O.sub.2]) molecule is considered to be a donor molecule towards a suitable metal cation. Although studying the photophysical properties of europium cation--bipyridine dioxide complexes, many researchers reported the occurrence of narrow luminescence effects that have a comparatively longer decay lifetime (3).
Complex molecules can be doped into suitable host matrices to provide practical applications for the fluorescence phenomenon and to improve the thermal and mechanical properties of the resultant composites, including the ease of processing aspects (4-7). Doping can prevent concentration quenching of the europium emission that could otherwise be caused by the aggregation of the molecules of the complex. Polymeric materials offer great potential as host systems for these complexes, on the basis of their thermal and mechanical properties. Rare earth-doped polymers or polymers containing a dispersion of a lanthanide complex (or complexes) have been given much attention due to their potential applications in various technologies (8-10). The simplest method by which one can dope or disperse a rare earth chelate is to blend the polymeric host matrix and the rare earth chelate directly. Sometimes the guest molecules cannot be well-dispersed in the polymeric matrix due to the aggregation effects. It has been reported that poly(methyl methacrylate) (PMMA) is a good host matrix, in that; it is able to ensure better distribution of guest molecules through supra-molecular interactions between the guest and the host system (11-13). The molecular structure of the chelate as well as the interactions between the guest molecules and the host system are important parameters that determine many of the final properties of the material.
No systematic study has yet been reported on the structural and optical characteristics of europium [(2,2'-bipyri-dine-N,N-dioxide).sub.2][([NO.sub.3]).sub.3] complex-doped PMMA system. Therefore, in this work, a complex of europium cation with 2,2'-bipyridine-N,N-dioxide has been synthesized, and characterized using different spectroscopic techniques. This complex was successfully dispersed in a PMMA matrix across a range of different loadings. The optical properties and the structures of the resultant composites were characterized by photoluminescence spectroscopy, luminescent lifetime measurements, UV-visible spectroscopy, X-ray diffraction (XRD), and infrared spectroscopy.
MATERIALS AND METHODS
PMMA (molecular weight 120,000 g [mol.sup.-1]), 2,2'. bipyridine and europium nitrate pentahydrate (99.9% on the metal basis) were obtained from Aldrich, UK. Glacial acetic acid, hydrogen peroxide, acetone, chloroform, and methanol (AR grade) were obtained from Fischer Scientific, UK.
Structural studies of complex-doped, polymeric thin films were carried out using a Perkin--Elmer One Spectrum IR Spectrophotometer, in the reflectance mode. UV-visible absorption spectra were recorded on a Varian Cary 50 probe UV-visible spectrophotometer. Photoluminescence studies and life time measurements were carried out on an Edinburgh FLS920 steady-state and time-resolved fluorescence spectrophotometer. The light source was a microsecond white-light flash lamp and the detector was a visible wavelength photomultiplier tube. XRD analyses were carried out on a Philips X'Pert-MPD XRD system. For the elemental analyses, flash combustion was used, the analysis being carried out on a Thermo flash EA112 unit.
Preparation of 2,2'-Bipyridine-N,N-dioxide
2,2'-Bipyridine-N,N-dioxide was prepared from 2,2'-bipyridine, using a reported procedure (14). Thus, 2,2'-bipyridine (5 mmol) was dissolved in 10 mL of acetic acid to which 4 mL of hydrogen peroxide were added and the mixture heated to 60[degrees]C. The solution was then refluxed for 3 hrs. Four milliliter of hydrogen peroxide was then added and the temperature lowered to 40[degrees]C. Stirring was maintained overnight. The product was precipitated in acetone. The elemental analysis gave C = 64.10; H = 4.35; N = 14.55%. The theoretical composition for 2,2'-bipyridine-N,N-dioxide is [C.sub.10][H.sub.8][N.sub.2][O.sub.2]: C = 63.82; H = 4.28; N = 14.89%.
Preparation of Europium [(2,2'-Bipyridine-N,N-dioxide).sub.2][([NO.sub.3]).sub.3]
Initially, 2,2'-bipyridine-N,N-dioxide was dissolved in hot methanol. Europium nitrate pentahydrate, dissolved in methanol, was then added drop-wise. The mixed solution was heated at 60[degrees]C for 3 hrs, then cooled and the product precipitated by adding diethyl ether. The microanalysis data was in close agreement with the expected values. Theoretical: Eu[C.sub.20][H.sub.16][O.sub.13][N.sub.7]: C = 33.63; H = 2.26; N = 13.73. Found C = 33.45; H = 2.55; N = 13.20%.
Preparation of Complex-Doped Thin Films
The complex-doped PMMA thin films were prepared by solution casting technique. For this, known amounts of the rare earth complex, europium [(2,2'-bipyridine-N,N-dioxi-de).sub.2][([NO.sub.3]).sub.3], and PMMA were dissolved in chloroform. The amount of PMMA used was 100g/L. A small amount of methanol was added to the PMMA/complex solution to ensure the complete dissolution of chelate. The solution was stirred for 12 hrs and then poured on to a glass mold. The wet film was then carefully stored to allow the solvent to evaporate. The thin films were then kept in an oven at 50[degrees]C for 12 hrs. The film thickness was 100 [micro]m. The loading of the complex in the films was in the range of 0.1, 0.3, 0.5, 1, and 2 wt% with respect to the PMMA.
RESULTS AND DISCUSSION
Infrared Spectroscopy The IR spectra of the 2,2'-bipyridine-N,N-dioxide ligand and the europium complex are shown in Fig. 1. The IR spectrum of the europium complex shows a stretching band at 3090 [cm.sup.-1], indicating C-H stretching from the 2,2'-bipyridine-N,N-dioxide. The N-0 stretching vibration, usually occurring at 1246 [cm.sup.-1], is shifted to a lower value by 34 cm', with a notable splitting of the signal. The N-0 bending vibrations, usually occurring at 836 [cm.sup.-1] and 850 [cm.sup.-1], appear at 834 and 848 [cm.sup.-1], respectively. These shifts arise from changes incurred in the nature of the nitrogen--oxygen bonding as a result of coordination. The formation of the oxygen-metal atom bonds increases the demand for electrons at the donor oxygen atoms and thereby decreases the double bond character of the nitrogen--oxygen bond in the resonance structure of the ligand molecule. These observations show that the chelation of the ligand to the metal is through the oxygen atoms (15). Elemental analysis supports this suggested composition of the complex.
Infrared spectroscopy is a useful tool for the study of supra-molecular interactions in host-guest systems such as PMMA/chelate systems. The IR spectrum of the pure PMMA and that of the complex-doped PMMA are shown in Fig. 2. The peaks that appear at 2994 [cm.sup.-1] and 2949 [cm.sup.-1] correspond to the -[CH.sub.3] and -[CH.sub.2] stretching of PMMA, respectively. These peaks are slightly shifted towards lower frequencies when the europium chelate is incorporated. This observation is attributed to the effect of chelate molecules on the orientation of the host structure. The shifts of these kinds of stretching frequencies have been used to describe the order of hydrocarbon orientation in many systems (16). The band width of the system can be used to describe the microenvironment of the PMMA. In this study, the band width of the doped systems seems to be narrowed upon increasing the loading of the europium chelate. This band width narrowing is shown in the inset of the Fig. 2. This effect implies that the groups in the host system become more ordered with increase in the molar ratio of the guest molecule.
The photoluminescence spectra of PMMA films that contained different loadings of the europium complex are shown in Fig. 3. Upon excitation at 300nm, which is the absorption maximum of the complex, five emission bands were observed, corresponding to [.sup.5][D.sub.0] [right arrow] [.sup.7][F.sub.0,1,2,3,4] transitions, respectively. The bPy[O.sub.2] ligand is excited to the triplet state, which then transfers the energy to the europium cation, as shown in the Fig. 4. Among the emission peaks, the electric dipole-allowed transition or hypersensitive transition, [.sup.5][D.sub.0] to [.sup.7][F.sub.2], is the strongest relative to the other peaks. This is an indication of the lower symmetry around the europium ion. This asymmetric microenvironment would cause polarization of the europium ion under the influence of the electric field provided by the surrounding ligands which, in turn, can increase the probability of electric dipole transition. Thus the intensity is greater compared to other transitions. The emission profile for the hypersensitive transition can be taken as a measure of the symmetry of the microenvironment around the central metal cation. Thus, the hypersensitive transition profile can be affected by the electrical environment around the central metal ion. The profile for the hypersensitive transition is similar at the low concentration, with a shoulder at 625 nm beginning to appear for increasingly greater loadings.
The [.sup.5][D.sub.0] [right arrow] [.sup.7][F.sub.0] transition is very useful for the study of the europium ion environment; both the states being nondegenerate and a single emission can be obtained for each unique site. Figure 5 shows the [.sup.5][D.sub.0] [right arrow] [.sup.7][F.sub.0] transition bands of the europium complex-doped PMMA system, at room temperature. The symmetry and width of the band indicate the nature of the surrounding environment of the metal ion. It is clear from the spectrum that the emission bands are asymmetric. The asymmetry and inhomogeneous broadening of the [.sup.5][D.sub.0] [right arrow] [.sup.7][F.sub.0] transition bands indicate that the complex molecules are not located in a similar environment in the PMMA matrix, as reported for other host guest systems (17). The energy of the nondegenerate transition is related to the interelectronic repulsion parameter of the europium cation. It can be seen from the spectrum that the peak position has been shifted, indicating that there is change in the environment around the central metal ion across each of the various loadings of the complex. This effect could be due to the coordination of solvent molecules, or the coordination of polymer back-bone, to the metal center. These results show that the complex molecules are located in different environments upon increasing the complex loading. This is interesting in terms of potential for the effective tuning of the emission characteristics by changing the composition of the system.
Fluorescence Lifetime Analysis
The decay curves of the [.sup.5][D.sub.0] level of the europium ion in the europium complex-doped PMMA system, with different loadings of complex, are shown in Fig. 6. The decay curves of the complex-doped PMMA can be fitted by use of a biexponential function. The life time values obtained and the quality of fitting ([R.sup.2]) are listed in Table 1. It can be seen from these data that the chelate molecules in PMMA have two lifetimes. This effect indicates that more than one kind of environment exists around the central metal ion (18). This finding is in agreement with the occurrence of inhomogeneous broadening and asymmetric nature of the [.sup.5][D.sub.0][right arrow][.sup.7][F.sub.0] transition band, observed in the emission spectra. The life time values of the system change with increase in the amount of complex in the composite systems, thereby giving an indication of the change in the environment around the metal ion upon increasing the amount of complex (16). This finding is in agreement with the asymmetric spectral profile of the [.sup.5][D.sub.0][right arrow][.sup.7][F.sub.0] transition obtained. These results show the effect of the composition on the optical properties of the complex-loaded PMMA system.
TABLE 1. Luminescence lifetime for the chelates that were doped in PMMA. Sample Lifetimes [[tau].sub.1] [[tau].sub.2] [R.sup.2] ([mu]s) A (PMMA/0.1% chelate) 578 291 0.99964 B (PMMA/0.3% chelate) 333 609 0.99982 C (PMMA/0.5% chelate) 617 340 0.99991 D (PMMA/1% chelate) 401 717 0.99994 E (PMMA/2% chelate) 385 844 0.99990
Spectroscopy The UV-visible absorption spectra of the pure chelate in two different solvent systems, in acetonitrile and in an acetonitrile (ACN)-chloroform mixture, of the bPy[O.sub.2] ligand in chloroform, of the chelate doped-PMMA solutions in acetonitrile-chloroform mixture and of pure PMMA in acetonitrile-chloroform are shown in Fig. 7. The poor solubility of the complex in chloroform is the reason for the use of the mixed solvent. The spectrum of chelate in acetonitrile shows differences in form relative to the spectrum obtained for the complex in the acetonitrile-chloroform mixed solvent. The spectrum of the pure chelate in the acetonitrile-chloroform mixed solvent is similar to the spectrum of chelate/PMMA solution in the same mixed solvent. The absorption spectrum of the complex is similar to that associated with the bands of the bPy[O.sub.2] ligand. The [Eu.sup.3+] complex of 2,2'-bipyridine-N,N-dioxide is chemically and photochemically stable in the solvents used (19). The likelihood of decomposition of the complexes in the solution is reduced compared with the decomposition of complexes having ligands that coordinate through nitrogen atoms. This reduction in the tendency for dissociation of the complex molecules, in solution, is due to the affinity of the metal cation towards the oxygen atom of the bPy[O.sub.2] ligand. According to the hard acid-hard base concept, the affinity of oxygen atom to the central metal cation is greater than it is to nitrogen atoms (20). Thus, the difference in the microenvironment around the central metal ion, in the composite system, is not due to the decomposition of the complex in the solution during the composite fabrication process. It can be due to the possible interaction of the metal cation to the carbonyl group in the polymer back bone.
Figure 8 shows the XRD patterns of the pure complex, the pure bPy[O.sub.2] ligand and the complex loaded PMMA compositions. No sharp diffraction peaks were found in the complex/PMMA systems that contained amounts of the complex, up to I wt%. Sharp diffraction peaks appear for PMMA compositions that contained 2 wt% of the complex showing elements of the formation of crystalline. Some of the peaks are interestingly different from those of the pure complex and the pure ligand. These differences in the diffraction peaks can arise from the formation of different crystalline states of the complex in the PMMA matrix.
It can be seen from the XRD results that the structural characteristics of the complex are different when the complex is incorporated into the PMMA matrix, due to interactions between the polymer and the complex. This change in the structural characteristics results in different photo- luminescent behavior in complex-loaded PMMA systems. There is considerable potential for the occurrence of interaction between the central metal cation and the carbonyl group of the PMMA, as reported for many other systems (21). This change in coordination environment around central metal ion leads to shifts in the peak position of [.sup.5][D.sub.0][right arrow][.sup.7][F.sub.0] emission band. The IR and XRD results confirm the influence of complex on the structure of PMMA matrix. The Full width at half maximum (FWHM) of the first diffraction halo is increased upon the inclusion of the complex, an effect that can be attributed to interactions between the host and guest molecules.
Europium [(2,2'-bipyridine-N,N-dioxide).sub.2][([NO.sub.3]).sub.3] complex-doped PMMA thin films were developed by a solution based casting technique. The IR and photo-physical measurements that were carried out on the complex indicate the occurrence of coordination of bPy[O.sub.2] molecule to the central metal atom through the oxygen atoms. The fluorescence analyses of complex-doped PMMA thin films suggest that the microenvironment around the central metal ion changes with composition. This observation is supported by XRD analysis. The doping of the complex into the polymer matrix causes structural changes in the polymer matrix, as evidenced by the IR and XRD analysis. The europium complex gives good luminescence properties when incorporated into the PMMA polymer matrix. The optical properties of the resulting composites can be tuned by changing the composition of the composites. The doped system has potential applications in lasing, in luminescent light conversion molecular devices and in other optoelectronic devices.
The authors also acknowledge the support obtained from Government of India and Government of UK through UKIERI programme (UK-INDIA Education and Research Initiative), funded by British Council.
Correspondence to: R. Puthiyottil; e-mail: firstname.lastname@example.org
Contract grant sponsors: Governments of India, UKIERI programme (UK-India Education Research Initiative), The British Council.
Published online in Wiley Online Library (wileyonlinelibrary.com).
[c] 2012 Society of Plastics Engineers
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R. Puthiyottil, (1) S. Varghese, (2) U. Gopalakrishnapanicker, (1) J.T. Guthrie (3)
(1) Department of Chemistry, National Institute of Technology Calicut, Kerala 673601, India
(2) School of Nano Science and Technology, National Institute of Technology Calicut, Kerala 673601, India
(3) Department of Colour Science, School of Chemistry, University of Leeds, Leeds LS2 9JT, UK
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|Author:||Puthiyottil, R.; Varghese, S.; Gopalakrishnapanicker, U.; Guthrie, J.T.|
|Publication:||Polymer Engineering and Science|
|Date:||Jan 1, 2013|
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