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Turning on lanthanide luminescence via nanoencapsulation
Duncan, Alexandra K.
Elam, Ashley R.
de Bettencourt-Dias, Ana
Medley, Colin D.
Smith, Joshua E.
Werner, Eric J.
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The incorporation of luminescent lanthanide (Ln) complexes into discrete silica-based nanoparticles invites a variety of applications ranging from new optical materials to molecular imaging or bioassay work. Enhancement of the photophysical properties of the complexes resulting from encapsulation within such particles further increases the utility of these hybrid systems, particularly if intense Ln-centered emission can be generated from combining relatively simple precursors via straightforward means. We report here the encapsulation of readily available macrocyclic Eu(III) chelates by discrete, monodisperse SiO2 nanoparticles and the resultant dramatic effect on metal-derived luminescence. The tetraiminodiphenolate (TIDP) motif chosen for this study is easily synthesized and incorporated into the nanoparticle matrix under ambient conditions. The free complex exhibits primarily weak ligand-derived emission at room temperature, typical for these compounds, and displays intense metal-centered luminescence from the Eu only when cooled to 77 K. Upon encapsulation by the nanoparticles, however, Eu-derived luminescence is visibly 搕urned on� at room temperature yielding strong emission peaks characteristic of Eu(III) with a corresponding enhancement factor of 6 � 106. The similar ligand singlet and triplet excited state energies determined for the free complex (20,820 and 17,670 cm-1, respectively) versus the encapsulated complex (20,620 and 17,730 cm-1) indicate that the encapsulation does not affect the ligand to metal energy transfer process appreciably. Instead, a detailed analysis of the metal-centered emission and ligand singlet and triplet emission bands for the free and encapsulated complexes reveals that the enhanced metal emission is due to the rigid environment afforded by the silica matrix affecting vibrationally mediated energy transfer. Further, the metal-centered emission lifetimes in methanol versus deuterated methanol were used to determine a decrease in the number of coordinated solvent molecules upon encapsulation, changing from an average 3.3 to 2.1 bound methanol molecules, reducing the known quenching effect due to nearby OH vibrations.