Photophysical Properties of Ruthenium(II) Tris(2,2′-Bipyridine) and Europium(III) Hexahydrate Salts Assembled into Sol-Gel Materials Wilhelm R. Glomm,* ,† Sondre Volden, † Johan Sjo ¨blom, † and Mikael Lindgren ‡ Ugelstad Laboratory, Department of Chemical Engineering, and Department of Physics, Norwegian UniVersity of Science and Technology (NTNU), N-7491 Trondheim, Norway ReceiVed April 19, 2005. ReVised Manuscript ReceiVed August 18, 2005 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. 1. Introduction The exceptional properties of organosilicon compounds to form siloxane polymers remain the basis for the sol-gel technique. 1 Hydrolysis and condensation of monomeric silicon alkoxide precursors upon addition of water can be described by the following three equations. Hydrolysis: Water condensation: Alcohol condensation: where R is an alkyl group C x H 2x+1 . The pH of the reaction environment highly affects the outcome. Low pH values yield fast hydrolysis and slow condensation, resulting in a three-dimensional gel. High pH yields slow hydrolysis rates and rapid condensation, resulting in a suspension of particles, in most cases with a monodis- perse particle size distribution. In 1992, Sjo ¨blom 2 et al. showed that hydrolysis and condensation of monomeric silicon alkoxide precursors into three-dimensional gels can occur with water replaced by hydrated metal saltssin this case copper nitrate was used. A subsequent study by the same group 3 determined the hydrolysis and condensation rates of tetramethyl orthosili- cate (TMOS) in alcohol solutions of Ca(NO 3 ) 2 ‚4H 2 O and Ni(NO 3 ) 2 ‚6H 2 O. Here, the reactions were monitored as a function of time using FT-IR spectroscopy, and multiple linear regression was used to calculate the rate constants from the time evolution of spectra. The authors concluded that both the hydrolysis and condensation rate constants were proportional to the amount of hydration water and that the identity of the metal in the salts had a pronounced effect on the overall reaction rate. This latter observation was attributed to differences in the dissociation states in the two metal complexes studied here. Selected rare earth and transition metal ions have lumi- nescence properties that make them useful as optical probes of the sol-gel process, structure and properties of den- dritic encapsulation and energy transfer, 4,5 as well as for luminescence and lasing applications. 6,7 Ruthenium polypy- ridyl complexes, and particularly Ru(II) tris(2,2′-bipyridine) * Corresponding author. Fax: +47 73 59 40 80. E-mail: wilhelm.robert.glomm@chemeng.ntnu.no. † Department of Chemical Engineering. ‡ Department of Physics. (1) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: San Diego, CA, 1990. (2) Sjoblom, J.; Skodvin, T.; Selle, M. H.; Saeten, J. O.; Friberg, S. E. J. Phys. Chem. 1992, 96, 8578. (3) Oye, G.; Libnau, F. O.; Sjoblom, J.; Friberg, S. E. Colloids Surf., A 1997, 123, 329. (4) Pitois, C.; Hult, A.; Lindgren, M. J. Lumin. 2005, 111, 265. (5) Kawa, M.; Frechet, J. M. J. Chem. Mater. 1998, 10, 286. (6) Weber, M. J. J. Non-Cryst. Solids 1990, 123, 208. ≡Si-OR + H 2 O a ≡Si-OH + ROH (1) ≡Si-OH + ≡Si-OH a ≡Si-O-Si≡ + H 2 O (2) ≡Si-OR + ≡Si-OH a ≡Si-O-Si≡ + ROH (3) 5512 Chem. Mater. 2005, 17, 5512-5520 10.1021/cm050825d CCC: $30.25 © 2005 American Chemical Society Published on Web 09/29/2005