Concentration quenching and photostability in Eu(dbm)3phen embedded in mesoporous silica nanoparticles
Introduction
Mesoporous systems are ideal systems for hosting different chemical entities at the nanometer scale, from single organic molecules, metal clusters or nanoparticles to proteins and enzymes. A host can be desirable for different reasons, which may involve or not a host-guest interaction, and find applications in a wide variety of fields, from sensoristics to drug delivery. In the present paper, we present the advantages of hosting luminescent molecules, lanthanide conjugated organic complexes in particular, inside the pores of mesoporous nanoparticles, mainly having in mind their application as spectral converters in silicon solar cells, i.e. as down-shifters of the UV part of the solar spectrum to give a red emission, which is more efficiently converted by the cell into electricity.
Lanthanide conjugated organic complexes have been studied for a long time and have been proposed for interesting applications, such as probes for biological systems [1], [2], LED [3], OLEDs [4], laser materials [5], or spectral converters for solar cells [6], [7], [8], [9], [10], [11]. In fact, when properly designed [12], such complexes combine the intense, broad absorption band of the ligands with the intense, sharp atom-like emission of the lanthanide ion, maintaining absorption and emission sufficiently apart to avoid self-absorption (large Stokes-shift). This is obtained by transfer of the energy absorbed by the ligands to the lanthanide ion in a process that is also known as antenna effect. In particular, the class of organic complexes known as β-diketones (1,3-diketones) has been conjugated to rare-earth ions to give very efficient luminophores, known as β-diketonates [13]. Although very efficient, these complexes suffer from two main problems: concentration quenching and photostability. Some of the authors of the present paper have shown [10], for example, that tris(dibenzoylmethane) mono(1,10-phenanthroline)europium(III) (Eu(dbm)3phen) and tris[3-(trifluoromethylhydroxymethilene)-d-camphorate] europium(III) (Eu(tfc)3) in solution suffer from quenching for concentrations as low as a few thousandths of wt%; for higher concentrations the activation of alternative, non-radiative paths for de-excitation of the ions is evidenced by a mismatch between absorption and excitation spectra, and results in a drastic reduction of the quantum yield.
In a very recent paper, Lima et al. [14] have shown a possible mechanism greatly improving photostability based on a trans-to-cis photo-click switch of the complex Eu-(btfa)3(t-bpete)MeOH under UV irradiation. It is argued that this mechanism could be used to design further interesting complexes.
Embedding the complex in a matrix has shown in some cases to somewhat improve both concentration quenching and photostability [15], [16]. Lanthanide complexes have been embedded in polymers [10], [11], [17], [18], ionic liquids [19], silica sol–gels [20], [21], [22], [23], mesoporous materials (SBA [24], [25], [26], [27] and MCM-41 [28], [29], [30], [31]) and nanoparticles, mesoporous [32] or not [33]. In particular, photostability of the lanthanide complex when embedded in mesoporous matrices have been studied in Refs. [24], [27], [30], [31]. Nonetheless, no definitive solution to these problems have been found and further research is needed to develop better matrices and to gain a better understanding of the matrix role in the final optical properties of the material.
In the present study, the β-diketonate Eu(dbm)3phen was embedded in mesoporous silica nanoparticles (MSN), where the pore size was tailored to match the size of the complex. This has allowed us to overcome the concentration quenching that occurs when the complex is dispersed in a solution or in a polymer matrix (non-embedded systems). The protection of the MSN allowed us to enormously increase the complex loading, yielding much larger luminescence intensity. Furthermore, the nanosize of the particles makes them suitable for different applications where transport through cellular systems is important, or dispersion in a solid medium to obtain a transparent material is of interest. Thus, the present material offers the possibility to have a higher efficiency, for example, in bio-imaging and luminescent solar concentrators. In particular, such nanoparticles may be dispersed in the encapsulating polymeric layer of silicon solar cells, to harvest the UV part of the spectrum, which is not efficiently converted into electricity by the solar cell, and transfer this energy in the red region where the cell is more efficient (spectrum conversion).
Last but not least, such nanoparticles might be easily functionalized to add further properties, like magnetic or analytical properties, or they can be used as drug carriers with bio-sensing capabilities. The improvements obtained in the embedded complex are critically discussed by observing changes in the shape of the relevant excitation and absorption bands.
Section snippets
Experimental
All chemicals were used without further purifications: tetraethylorthosilicate, TEOS (Aldrich); cetyltrimethylammonium bromide, CTABr (Aldrich); tris(dibenzoylmethane) mono(1,10-phenanthroline)europium(III), Eu(dbm)3phen (Aldrich); ammonia 28% (Riedel de Haen); ethanol (BDH); and dichloromethane (Carlo Erba).
Structure and morphology
The presence of an ordered mesoporous structure within the nanoparticels is demonstrated by XRPD measurements. In Fig. 1 the comparison of the matrix S1 with two of the impregnated samples is shown. The low-angle X-ray pattern of the pristine silica shows two characteristics diffraction peaks that are indexed as (100) and (110) lattice planes, being typical of a two-dimensional channel system (hexagonal honey comb pattern). Peaks are broader than for typical MCM41 or SBA-15 well-ordered
Conclusions
Embedding the Eu(dbm)3phen complex in mesoporous silica nanoparticles has allowed to partly overcome the mismatch between absorption and excitation bands which limited the amount of complex that could be meaningfully used in non-embedded configurations (solution and polymers). Up to 23 wt% of complex could be loaded into the silica particles with good quantum efficiency, i.e. order of magnitudes more than in the non-embedded samples. This could be highly beneficial to applications like
Acknowledgments
The technical help of Davide Cristofori, Tiziano Finotto and Martina Marchiori is greatly acknowledged.
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