Luminescent Stability of Hybrids Based on Different Borate Glass Matrix’s and Organic Metal Complexes

The stability of the luminescent properties of new hybrid materials based on 8-oxyquinoline metal (Li, Rb, Sr) complexes and Eu complex with phenanthroline and low-melting Pb-based inorganic glass matrixes under conditions of prolonged exposure under ambient conditions and heating above the glass transition temperature of the matrix’s has been investigated.


I. INTRODUCTION
Hybrid materials (HM) are widely used in photonics as active and passive elements as well as integral optics components [1]. One of the problems which can be solved by the creation of HM is protection of organic phosphors from environmental conditions (oxygen, water vapor, UV light, etc.) which lead to the destruction of metal-organic complexes and their loss of luminescence properties [2]. The possibility of synthesis of photoluminescent HM based on 8-hydroxyquinoline [3]- [6] and phenantroline [7]- [8] metal organic phosphors and inorganic low melting glasses as a matrix using a non-aqueous technique has been demonstrated. Glassy boron oxide, borate and borosilicate glasses were used as boron-based matrix. For preparation of a volume HM a glass matrix should have both a low melting temperature and low melt viscosity. In the present research we have synthesized the HM using the melt of fluoroborate and lead borosilicate glasses.
One of the main requirements to HM's is their stability to adverse environmental conditions. We have found out that the B 2 O 3 -based HM's have not been sufficiently stable at prolonged exposure [7], while the HM's based on the glass matrix with the composition of 80PbF 2 -20B 2 O 3 have been stable enough not only at prolonged exposure [7], but also when heated to the glass transition temperatures of the matrix [8].
In the present research we have analyzed two different low melting glass matrixes for HM' in respect of improvement of their stability towards air, humidity and heating. The first glass matrix with the composition of 80(mol.%)PbF 2 -20(mol.%)B 2 O 3 (matrix M1) have the lowest operating temperatures in PbF 2 -B 2 O 3 system. These glasses are stable and widely applied in photonics [9]- [10]. The second matrix with the composition of 62(mol.%)PbO-26(mol.%)B 2 O 3 -12(mol.%)SiO 2 (matrix M2) is a eutectic composition in the PbO-B 2 O 3 -SiO 2 quazi ternary system with the lowest-melting temperature of 484°C [11].
As an organic part of HM we used 8-hydroxyquinoline complexes (Mq n ) of metals (M), and metal-phenanthroline complexes of REE. These complexes are successfully used in OLED technology as emitting materials [12].

Synthesis of hybrid materials
Metal organic complexes synthesis. 8-Hydroxyquinolinol was purchased from Sigma-Aldrich and additionally purified by high vacuum sublimation. All solvents were procured from Merck.
The syntheses have been conducted in 2-propanol solution at pH = 9-10 and 25-27°C (depending on the metal ion) and continuous mixing for 1 h by the reaction of 8-hydroxyquinoline with metal ions as described in [13]. We obtained precipitates of Mq n , which were filtered, dried under vacuum and finally purified by high vacuum sublimation (p = 10 -5 Torr).
Synthesis of glass matrices. The glasses have been synthesized using PbF 2 , PbO, SiO 2 and H 3 BO 3 preparations. All reagents were purchased from Sigma-Aldrich and were as pure as 99.99 wt%. B 2 O 3 has been produced by vacuum dehydrolization of H 3 BO 3 with further melting in a Pt crucible. The final B 2 O 3 had 99.999 wt% purity according to ICP-MS analysis (Fig.1a). The batches were 50 g in weight. Glasses have been synthesized at 850-900°C during 0.25-0.5 h in closed corundum crucibles. The melt has been poured into a steel mold heated to 200°С.
Synthesis of HM's. Synthesis of HM's has also been carried out in corundum crucibles. Pre-synthesized glass (5 g) has been melted at 800°C then cooled up to 600°C and powdered organometallic phosphors (0.10 -0.15 wt%) was dropped into the melt. Then the mixture has been stirring during 10-20 seconds. The samples have been obtained by rapid casting into a steel mold.

Measurement techniques
Purity Measurements. Powder preparations were consequently dissolved in H 2 O 2 (1 mL), then HNO 3 (1 mL, 1%) using microwave decomposition procedure (Speedwave Four, Berghof GmbH) in DAC-100 TFM-PTFE autoclaves. Impurity measurements were carried out by an inductively coupled plasma mass-spectrometer NexION 300D (Perkin-Elmer Inc.) in the KED mode. The powder preparations purity (trace metals) is presented in Fig.1. All metal-organic complexes were at least as pure as 99.96%. And their purity corresponds to the purity of inorganic reagents. Luminescence measurements. All luminescence measurements have been carried out at room temperature. A Fluorolog FL3-22 spectrofluorimeter (Horiba Jobin Yvon, USA) with double grating excitation and emission monochromators has been used for luminescent measurements in a wavelength range of 390-700 nm with a 0.1 nm step. The emission were studied at excitation by the pulsed diode laser (λ = 377 nm, ∆τ = 1.5 ns).

III. RESULTS AND DISCUSSION
Analysis of resistance of HM's based on 8-oxyquinoline with M1 towards the room humidity has demonstrated high performance of luminescent properties after 3-6 months exposure (Fig. 2  a, b). The difference in PL spectra of as-synthesized and annealed HM's could be explained by a chemical exchange reaction proceeding between metal complex and the matrix. As a result of the reaction a spatial glass net is forming. In the net the organic ligands form the new complexes with matrix cations (Pb, Al, B).  3 (Phen)+M2 (f), compared with the PL spectra of the initial metal organic complexes (dash lines) and HM's exposed at RT during 6 months (2, blue line) and HM's annealed over glassy temperatures (3, red line). In the case of HM's based on Eu(NTA) 3 (Phen) in PL spectra besides a wide band shifted towards a short wave region comparing to Meq n spectra we observed narrow lines attributed to f-f transitions on the Eu 3+ ion (Fig. 2 c).
Formation of complexes of Pb with organic ligands which demonstrate a wide PL band with 520-530 nm maximum has been described in [14].
The HM's based on the M2 matrix have performed less stability. It should be noted that the absolute values of luminescence intensities of HM in the M1 matrix are higher comparing to the M2 matrix with the same amount of the phosphor added into the matrix at HM's synthesis. At heating and air exposure in the case of HM based on M1 we fixed not only the PL spectrum outline preservation, but also the constancy of the absolute values of intensities. The PL intensities increased just when an appreciable crystallization of the matrix took place In the case of the HMs based on M2 matrix absolute intensity markedly increased with exposure under air and at heating. Probably, in the case of more reactive M1 matrix, in which Pb 2+ is surrounded by a movable Fions, the Pb-based stable complexes were formed directly during HM's synthesis. Inside the more inert M2 matrix the Pb 2+ ion is surrounded by the O 2ions forming a stable PbO 4 pyramid, whose structure is specific for the glasses containing PbO at concentrations higher than 50 mol% [15]. The organic ligands are not completely reacted with Pb 2+ ions in the matrix and remain as nonluminescent fragments. During prolonged exposure or heating the HM's based on the M2 matrix the reaction of lead-organic complex formation starts to proceed at a significant rate. It results to an increase in luminescence intensity with luminescence maximum near 530 nm. Such an increase in intensity suggests that HM's based on the M2 matrix may be promising materials for laser-stimulated luminescence [5]. The color of luminescence of HM's varied from blue to yellow-green (Table 1).

IV. CONCLUSION
New fluorescent hybrid materials based on 8-hydroxyquinoline and phenantroline metal organic phosphors and inorganic low melting lead glasses have been synthesized. It has been shown that HM's based on the PbF 2 -B 2 O 3 matrix exhibited high stability of luminescent properties both in prolonged exposure at ambient conditions and at heating, resulting from the formation of stable lead-metal complexes. HM's based on PbO-B 2 O 3 -SiO 2 matrix demonstrated less stability because this case the reaction of formation of lead organic complex proceeded slowly and there was a need in heating or prolonged exposure under air.