Photophysical Properties of Ruthenium(II) Tris(2,2‘-Bipyridine) and Europium(III) Hexahydrate Salts Assembled into Sol−Gel Materials

A series of luminescent sol−gel-encapsulated Ru(bpy) 3 Cl 2 ·6H 2 O and EuCl 3 ·6H 2 O mixtures with Zn(NO 3 ) 2 ·6H 2 O were assembled and characterized in terms of their steady-state and time-dependent photophysical properties. UV−vis absorption, steady-state emission, and FT-IR spectra were measured for the materials both in rigid and fluid media. Time-resolved luminescence measurements were also performed in order to determine radiative decay times. The samples described in this study were prepared without the addition of excess water. This was achieved by allowing the hydrolysis and condensation reactions to only consume hydration water, thus utilizing the metal salts as reactants rather than passive dopants in the system. By using this approach, the amount of hydroxyl quenchers is minimized, which can be expected to yield luminescent materials with higher luminescence quantum yields than a conventional sol−gel entrapment procedure. The emission bands of both chromophores studied here, Ru(bpy) 3 2+ and Eu 3+, were found to exhibit higher emission intensities, hypsochromic shifts in the emission bands, and increased decay times upon sol-to-gel conversion, which can be attributed to rigidochromism. In the case of sol−gel-encapsulated Ru(bpy) 3 2+, the complexes are thought to be surrounded by solvent molecules that interact with the silanol groups of the gel network. Thus, the Franck−Condon excited state of the complex is relaxed to a lesser extent, giving rise to the observed hypsochromic shift of the luminescence associated with the materials upon sol-to-gel conversion. A similar mechanism is proposed to be in effect for the Eu 3+ -functionalized materials.