Effects of gamma radiation on the photoluminescence properties of polycarbonate matrices doped with terbium complex

Abstract

Luminescent films containing terbium complex [Tb(acac)₃(H₂O)₃] (acac=acetylacetonate) doped into a polycarbonate (PC) matrix were prepared and irradiated at low-dose gamma radiation with ratio of 5 and 10 kGy. The PC polymer was doped with 5% (w/w) of the Tb³⁺ complex. The thermal behavior was investigated by utilization of differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA). Changes in thermal stability due to the addition of doping agent into the polycarbonate matrix. Based on the emission spectra of PC:5% Tb(acac)₃ film were observed the characteristic bands arising from the ⁵D₄→⁷F_J transitions of Tb³⁺ ion (J=0–6), indicating the ability to obtain the luminescent films. Doped samples irradiated at low dose of gamma irradiation showed a decrease in luminescence intensity with increasing of the dose.

Introduction

Trivalent rare-earth (RE³⁺) complexes have singular spectroscopic properties due to the long decay time of luminescence, very narrow band emission lines, large Stokes shift between excitation and emission of the RE³⁺ ion, (Arnaud and Georges, 2003; Bunzli and Piguet, 2002; Evans et al., 2006; Lehn, 1990; Malta et al., 1998; Uekawa et al., 1998). In addition, the high luminescence emission efficiency of the RE³⁺ complexes is attributed to the high absorption coefficient of ligands and efficient energy transfer from the triplet (T) state of the ligand to energy levels of the RE³⁺ ion (Brito et al., 1999; Teotonio et al., 2004). Acetylacetonate anion (acac⁻) is an excellent antenna that acts as bidentate chelate protecting the terbium ion. There is a possibility to increase the luminescence efficiency of this β-diketonate by partial substitution of coordinated water molecules by other ligands that operate as co-sensitizer in the energy transfer of rare earths systems (Parra et al., 2003).

Usually, the luminescent Tb³⁺ complexes present the energy transferred ligand to metal ion from the triplet state T of ligand to the excited energy ⁵D₄ level of terbium ion, showing the luminescent transitions to the ground state mainfold ⁷F_J (J=0–6) (Teotonio et al., 2004; Bunzli and Piguet, 2002; Parra et al., 2004). In general, the most intense transition of Tb³⁺ ion is the energy level ⁵D₄→⁷F₅, corresponding to a green emission band near 546 nm.

The hard acid character of rare-earth ions has a strong affinity for the compounds that contain fluoride, oxygen and nitrogen as donor atoms make possible utilize as dopant in the host polycarbonate (PC) matrices. Recent research has developed luminescent materials doped into a polymeric matrix (Parra et al., 2003, Parra et al., 2005, Parra et al., 2006; Soares-Santos et al., 2003). Polycarbonate has excellent optical properties, such as transparency and ratio of refractive indices, low water absorption and high glass transition temperature, around 145 °C. The carbonyl groups present in polycarbonate can coordinate the rare-earth complex, a property that makes polycarbonate an interesting matrix for this purpose.

In this present work, we report the preparation and characterization of terbium complex doped in polycarbonate matrix. The films were irradiated with low doses of gamma radiation (5 and 10 kGy). The doping effect of the [Tb(acac)₃(H₂O)₃] complex into the PC film has also investigated. Photoluminescence behavior of the doped system indicates the interaction of the Tb³⁺ ions with the polymeric system.