Near-Infrared Luminescence of Lanthanide Calcein and Lanthanide Dipicolinate Complexes Doped into a Silica-PEG Hybrid Material Kris Driesen, Rik Van Deun, Christiane Go ¨rller-Walrand, and Koen Binnemans* Katholieke Universiteit Leuven, Department of Chemistry, Celestijnenlaan 200F, B-3001 Leuven, Belgium Received November 6, 2003. Revised Manuscript Received January 26, 2004 The near-infrared luminescence of lanthanide complexes of 4′,5′-bis[N,N-bis(carboxy- methyl)aminomethyl]fluorescein (calcein) and pyridine-2,6-dicarboxylic acid (dipicolinic acid, dpa) doped in a hybrid sol-gel material was investigated. The silica-poly(ethylene glycol) (silica-PEG) inorganic-organic materials were prepared at a neutral pH. The lanthanide ions are well shielded from the environment by the calcein and dpa ligands, and the complexes are stable in the sol-gel matrix after preparation. The dysprosium and neodymium dipicolinate complexes showed near-infrared luminescence (NIR-luminescence) by direct excitation to the 4f-levels. The ytterbium dipicolinate complex doped in the sol-gel showed NIR-luminescence by excitation of the ligand in the UV region. All other tested lanthanide ions (Ln ) Pr, Sm, Er, Ho) did not show luminescence. Neodymium and ytterbium complexes with calcein show intense NIR-luminescence when the ligand is excited by visible light. The corresponding erbium complex doped in the silica-PEG matrix also showed NIR-luminescence at 1525 nm. No NIR-luminescence could be detected for the other lanthanide complexes doped in the matrix (Ln ) Pr, Sm, Dy, Ho). Introduction The sol-gel process allows the preparation of silica- based or siloxane-based materials at ambient temper- atures. Pure silica glasses doped with lanthanide com- plexes can be made by hydrolysis and polymerization of silicon alkoxides. 1,2 The resulting glasses are trans- parent and have good mechanical properties. However, these materials have some drawbacks, such as the low solubility of lanthanide complexes in the sol-gel matrix at the low pH needed for the hydrolysis reaction. 2,3 One way to overcome these solubility problems is to neutral- ize the solution after hydrolysis and to introduce organic components in the material. The properties of the inorganic-organic hybrid materials depend on the chemical nature of the different constituents. 4,5 Poly- (ethylene glycol) (PEG) can be introduced in a silica matrix by simply mixing PEG with the silica precursors, but no covalent bonds are formed between the PEG chains and the silica backbone. 6,7 The solubility of lanthanide complexes can be improved by performing the sol-gel synthesis in a buffered solution. 8 Recently, much attention has been paid to near- infrared luminescence (NIR-luminescence) of trivalent lanthanide ions, because several lanthanides show luminescence in the telecommunication low-loss NIR- regions of silica. 9,10 Candidates for luminescence in these spectral regions are the trivalent ions of neodymium, praseodymium, samarium, dysprosium, holmium, er- bium, and ytterbium. The observation of NIR-lumines- cence by lanthanide ions in silica sol-gel glasses is hampered by quenching of the excited states via vibronic coupling with the hydroxyl group vibrations (in Si-OH and H 2 O). 11 The concentration of hydroxyl groups is high in wet xerogels. The most common method to decrease the degree of quenching is to calcine the sol- gel materials at high temperatures (>900 °C). This method has successfully been demonstrated for sol-gel- processed glasses, optical fibers, and thin films. 12-14 Different glass compositions (e.g. SiO 2 , SiO 2 -TiO 2 , SiO 2 -Al 2 O 3 , SiO 2 -TiO 2 -P 2 O 5 ) in addition to silica have been used in attempts to enhance the lumines- cence. 11,15-17 A disadvantage of such lanthanide-doped * Corresponding author. Tel: +32-16-327446 Fax: +32-16-327992, E-mail: Koen.Binnemans@chem.kuleuven.ac.be. (1) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: San Diego, CA, 1990. (2) Driesen, K.; Go ¨rller-Walrand, C.; Binnemans, K. 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