Inorganic pigments doped with tris(pyrazol-1-yl)borate lanthanide complexes: A photoluminescence study
Introduction
The possibility to visually discriminate between original and retouched areas is an important target when the restoration of works of art is considered. In particular, in the case of pictorial reintegration the use of luminescent dopants potentially allows a facile recognition of the restored parts after many years. Visible-emitting lanthanide complexes with well-designed antenna-ligands are potential candidates as dopants for materials useful for cultural heritage conservation, because these species are often easily detectable with a common Wood lamp.
The use of luminescent lanthanide complexes as dopants in retouching materials for restoration has been little studied, while their application in many frontier technology fields is well known. For example, these compounds are currently studied for novel optical amplifiers and laser materials and for efficient lighting devices and monitors. Moreover, these species are successfully used as luminescent probes for biological and medicinal applications [1], [2]. Luminescence studies in the field of works of art are instead essentially limited to a number of fluorescent organic pigments sometimes containing transition metal elements [3]. Lanthanide ions have been however recently proposed for the synthesis of novel non-toxic or high-temperature resistant pigments [4].
The major advantages of lanthanide complexes as luminescent species are the long emission lifetimes, narrow bandwidths and large Stokes shifts [5]. An intense metal-centered luminescence can be observed if the excitation energy is transferred from the organic ligands to the excited states of the lanthanide ion by intramolecular mechanism. A subsequent radiative decay to the ground state causes an emission dependent upon the electronic structure of the lanthanide ion. The choice of the ligands does not influence only the energy transfer towards the lanthanide ion. One of the problems of luminescent lanthanide complexes is the quench of the radiative emission in the presence of high-frequency oscillators, in particular the O–H bond of water. The ligands around the metal centre should keep water molecules outside the inner coordination sphere to obtain strongly luminescent species of interest for water-based applications such as pictorial reintegration [2].
A number of ligands of interest for the preparation of luminescent lanthanide complexes are based on the pyrazole moiety [6]. In particular, several coordination compounds having general formula Ln(Tp)3 (Tp=tris(pyrazol-1-yl)borate) have been structurally characterized in the past and the electronic structures of the lanthanide centres have been deeply investigated [7]. The antenna-effect from the coordinated Tp ligand to the lanthanide ion has been detected for some visible- and NIR-emitting complexes [7], [8] and a strong luminescence in the visible range has been observed also for water dispersions of Eu(Tp)3 (red emitter) and Tb(Tp)3 (green emitter). The luminescence of these compounds is maintained also if embedded in acrylic-based polymers such as poly(methyl methacrylate) and the pure complexes did not show any decomposition if heated at temperatures up to 300 °C [9].
The luminescent features of Eu(Tp)3 and Tb(Tp)3 and the maintenance of the photoluminescence under different conditions prompted use to study these coordination compounds as potential dopants for pigments of interest for cultural heritage restoration. In this communication we report the preparation of Ln(Tp)3-doped paints based on malachite, Egyptian blue, Ercolano blue and chrome yellow, using arabic gum or acrylic emulsion as binders. Photoluminescence studies and accelerated ageing tests have been carried out on the film paints.
Section snippets
Materials
The reagents used for the preparation of the lanthanide complexes were Aldrich products used without further purifications, with the exception of the anhydrous EuCl3 and TbCl3 salts (Strem). Potassium hydrotris(pyrazol-1-yl)borate (K[Tp]) was prepared following a literature method by reacting potassium borohydride with an excess of pyrazole [10]. The complexes Eu(Tp)3 and Tb(Tp)3 were synthesized with yields higher than 70% by reacting the corresponding Ln(III) chloride with the proper amount
Results and discussion
Eu(Tp)3 and Tb(Tp)3 are air-, moisture- and thermal-stable compounds that can be easily prepared from potassium tris(pyrazol-1-yl)borate and the proper LnCl3 salt [7], [8]. Paints films containing these lanthanide complexes have been prepared by adding weighted amounts of Ln(Tp)3 to mixtures of the proper pigment (malachite (M), Egyptian blue (EG), Ercolano Blue (ER) or chrome yellow (CY)) in the presence of arabic gum (AR) or acrylic emulsion (AC) as binders. These films will be indicated as
Conclusions
In this work the preparation and characterization of several paints doped with visible-emitting photoluminescent lanthanide complexes have been reported. The luminescence of these materials, based on lanthanide f–f transitions and antenna-effect, strongly depends upon the choice of the binder and the presence of pigments. Thermal ageing influences the photoluminescence mainly when acrylic medium is used, but a visible luminescence is maintained also when the coordination sphere of the
Acknowledgements
The Ca’ Foscari University of Venice is gratefully acknowledged for financial support (Ateneo Fund 2012).
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