Optical and magnetic properties of lanthanide(III) complexes with quercetin-5′-sulfonic acid in the solid state and silica glass
Graphical abstract
Introduction
Since the first announce on the application of the lanthanide complexes as light conversion molecular devices (LCMDs) [1], the interest in their optical properties has grown considerably. Hundreds papers reporting the results of the studies on the physicochemical properties of these materials were analyzed in three fundamental reviews on this topic [[2], [3], [4]]. The LCMD term replaced the idea of ‘antenna effect’ denoting the absorption, energy transfer, emission sequence involving ligand absorption and lanthanide ion emission, thus overcoming the very small absorption coefficient of lanthanide ion. In these processes chelating organic ligands often act as a photosensitizers because they can play a double role in formation of the lanthanide complexes: they can be coordinated to a single metal ion as well as they can bind Ln(III) ions forming a polymeric system. Such an organic ligand contains a light-absorbing unit acting as antenna chromophore and a tightly chelating unit. This sensitization is significantly more efficient than direct excitation of the central ion because absorption coefficients of organic chromphores are many times higher in magnitude than those of lanthanide ions.
Several theories were proposed to explain photosensitization and energy transfer processes. They can proceed in two pathways involving triplet [[5], [6], [7]] or singlet states [8,9] of a the ligand. Because, generally, intra-molecular charge transfer (ICT) in the organic molecules competes with the intersystem crossing process (ISC), both these processes are usually considered possible in luminescence of lanthanide complexes. The ICT process of most organic chromophores dominates the ISC process because the rate of the former process is faster than that of the latter one.
Energy transfer can proceed via dipolar or multipolar interactions between sensitizer and lanthanide ion called Förster mechanism [10] or via an exchange Dexter mechanism [11]. Malta [12] compared several processes occurring in non-radiative energy transfer to lanthanide ions. According to these calculations the lanthanide accepting levels are excited depending on (1) the energy gap between the singlet or the triplet state of the sensitizer and lanthanide ion levels should not be too large; (2) fulfilling the respective selection rules, and (3) significant matrix elements of the orbital overlap. In these calculations the energy transfer via triplet states of the sensitizer was taken into account.
‘ The above cited studies indicated that some ligands show a special predisposition for to mediation in the ligand-to-metal energy transfer. In the present paper we would like to study the possibility of the energy transfer in the quercetin lanthanide(III) complexes because these compounds have been widely used in medicine, pharmacology and optics since the discovery of their antitumor properties and citotoxicity against cell cultures [13,14], application in fluorimetric determination of nucleic acids [15] and an efficient contrast enhancer in magnetic resonance imaging [16]. Their ligand and several other flavonoids exhibit biochemical and pharmaceutical properties including anti-allergic, anti-viral, anti-bacterial, anti-mutagenic, anti-carcinogenic, anti-neoplastic, anti-thrombotic and anti-oxidant activities [[17], [18], [19], [20], [21], [22]].
The quercetin lanthanide complexes were firstly synthesized by Kopacz and Nowak [22,23]. In our previous work, the IR and Raman spectra of these complexes were recorded and analyzed based on DFT chemical calculations using the B3LYP/LANL2DZ quantum chemical approach [24]. The present work is a continuation of these investigations. The electron absorption spectra of [Ln(QSA)3 × 6H2O complexes, where Ln = La, Pr, Nd, Sm. Eu, Gd, Dy, Tb, Ho, Er and n = 13–14, were measured. In the present paper the femtosecond spectroscopy were applied for the complexes in polycrystalline state and in silica glass. The aim of this work is to answer the question: whether the ligand-to-metal energy transfer occurs in these complexes under very strong and short femtosecond laser pulses. The chosen complexes with Ln = Dy, Er, Gd, Sm were also studied using EPR technique. Undertaking these studies we expected that QSA ligand can play the role of an “antenna” capable of absorbing electromagnetic radiation energy and then transferring it to the lanthanide ion, which in turn can emits it in characteristic spectral ranges. The probability of this energy transfer could be forecast from energy levels of the ligand singlet and triplet states obtained from theoretical quantum chemical calculations. The time-dependent density functional theory was applied to determine the electronic excited states of the model Eu(QSA)3(H2O) molecular system.
Section snippets
Synthesis and structure of the studied complexes
Synthesis of the studied complexes were carried out by the modified method described by Kopacz, Nowak et al. [22,23]. Lanthanide nitrates or chlorates and QSA as the ligand were used. The chemical composition of the studied complexes was described by the formula Ln(C15H9O10S)3 x nH2O where n = 12,13 or 14 depending on the type of lanthanide ion. Before the physicochemical studies, these materials were stored at 80 °C up to moment in which the weight of the samples stabilized at the composition
Structure and optimization of the geometrical parameters
Considering the results obtained from the elemental chemical analysis and quantum DFT calculations the composition and structure of the studied complexes were determined. Their formula is described in the form [Ln(C15H9O10S)3(H2O)](H2O)5 consisting the [Ln(QSA)3(H2O)] coordination polyhedron and five crystallization water units. The structure of the lanthanide polyhedron was determined on the basis of quantum chemical calculations carried out for the europium derivative. It is shown in Fig. 1.
Conclusions
Taking into account the obtained in this work experimental and theoretical data the following conclusions could be proposed:
Lanthanide ions form with quercetin sulfonic acid [Ln(C15H9O10S)3(H2O)](H2O)5 complexes built of [Ln(QSA)3(H2O)] central unit and five crystalline water molecules. The chemical composition of the studied compounds was confirmed by elementary CHS analysis. The quantum chemical calculations performed for the [Eu(QSA)3(H2O)] molecular system show that its coordination
CRediT authorship contribution statement
P. Godlewska: Writing - review & editing, Visualization, Conceptualization. J. Hanuza: Writing - original draft. E. Kucharska: Formal analysis, Visualization. P. Solarz: Investigation. S. Roszak: Investigation, Visualization, Formal analysis. S.M. Kaczmarek: Investigation, Writing - original draft. G. Leniec: Writing - original draft, Investigation. M. Ptak: Investigation. M. Kopacz: Resources. K. Hermanowicz: Investigation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The present work was supported by the Polish National Centre of Science under the grant No. 2014/15/B/ST5/04730.
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