Observation of Room-Temperature 5 D 1 Luminescence of an Organoeuropium Complex Encapsulated in Sol-Gel Glass MOHAMMED ZAITOUN* Chemistry Department, Mutah University, P. O Box 7, Karak, Jordan In comparing emissions of the inorganic Eu 3þ salts (chloride or nitrate) to organoeuropium complexes doped into optically transparent sol-gel glass, previous studies have indicated that changes in the local chemical environment by chelation or variation of the ligand or gel matrix compositions were found to leave the main spectral features of Eu 3 þ essentially unchanged; complexation just increases the emission intensity of europium and leads to broadening and splitting of the peaks. In all cases studied and irrespective of the excitation energy, the observable emission peaks result only from relaxations out of the 5 D 0 excited state of Eu 3þ to the first five levels of the 7 F ground manifold. The present research examines the luminescence behavior of EuCl 3 and Eu-TETA (TETA is the macro cycle, 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraaceticacid) doped into a sol-gel host; in addition to emissions from the 5 D 0 , emission from the 5 D 1 excited state of Eu 3þ is observed for the first time. Index Headings: Room temperature; Luminescence; Sol-gel glass; Organo- europium. INTRODUCTION Luminescence complexes of Eu 3 þ emit red light and are used as emitters of electroluminescent devices, photoluminescent labels for fluoroimmunoassays, and dyes for latent fingerprint detection in various fields. These compounds can also be utilized as luminescent markers for shadowing suspects during pursuit in the field of forensic science. The markers must be various types of chemically identifiable ones in order to be used as crime evidence in many cases. 1,2 The unique spectroscopic properties of Eu 3 þ make it a nearly ideal optical probe of local structure. Eu 3 þ spectroscopy has been used to study systems ranging from inorganic glasses to biological systems. 3 Of the chemical methods used to prepare host inorganic materials with improved features, the sol-gel technique appears to be the most versatile. 4,5 The sol-gel involves low- temperature hydrolysis of a suitable metal alkoxide precursor followed by condensation to produce colloidal particles (sol), then gelation to form a wet network of porous metal oxide, and finally drying and shrinkage to form the xerogel (air-dried gel). 6 The sol-gel method is a convenient way to synthesize a host matrix for inorganic, organic, and biomolecules. The substance to be encapsulated (the dopant) is added to the sol after partial hydrolysis of the precursor. As the degree of cross- linking from polycondensation increases, the gel becomes viscous and solidifies. The process continues during aging and the porous matrix is formed around the dopant molecules. The addition of the dopant molecule prior to the gelation process physically traps the dopant in the cross-linked network and ensures the homogeneous distribution of the dopant. 7 The principal advantages for this process are the room-temperature (or lower) processing conditions, chemical inertness, negligible swelling effects, tunable porosity, the ease with which the microstructure of the material can be modified by varying the process parameters, and the high purity of sol-gel derived glasses, making them ideal for many types of applications. 8,9 Glass prepared by melting silica is not a feasible host for most molecules because of the extreme temperatures required. 10 Optical device materials prepared by the sol-gel process are of current technological interest, and sol-gel derived silica glasses doped with rare earth ions are an important class of sol-gel optics, with applications including solid-state lasers and fiber amplifiers. 11 Lanthanide compounds have been encapsulated into a variety of sol-gel matrices and their luminescence properties stud- ied. 12–14 Gels doped with the rare earth metal organic complex were previously shown to possess intense fluorescence characteristics, especially with respect to comparable gels doped with inorganic rare earth (III) salts such as chloride or nitrate. 15–17 In a chelated molecule, the ligand surrounds the rare earth metal ion (RE), separating the RE from nearby OH groups and other RE ions. By separating the RE from these other ions, the RE retains more energy for itself since it no longer loses small bits of energy to these other groups. With more energy for itself, the RE can release bigger blocks of energy at a time, resulting in more fluorescence. 18 The title ligand, TETA (1,4,8,11-tetraazacyclotetradecane- 1,4,8,11-tetraaceticacid), the structure of which is shown in Fig. 1, belongs to a novel class of macro cyclic compounds that contain a metal selective cavity. 19 Potential applications of these molecules include use as reagents for complexometric titrations, 20 models for ion transport in biological systems, 21 and tools in medicine. 22,23 The first reported synthesis of H 4 TETA indicated that TETA is a powerful complexing agent. 24 The molecular dynamics and dynamic calculations, kinetics, and laser-excited luminescence studies were investi- gated for trivalent lanthanide (Ln 3 þ ) complexes with TETA. 25 Eu-TETA crystal structure and the luminescence properties of the complex in solution were reported. 26 Both diamagnetic and paramagnetic nuclear magnetic resonance (NMR) studies of lanthanide complexes of TETA indicated that these complexes have very rigid structure in aqueous solution. 27 Computer- simulated structures are also consistent with those found in the solid state. 28 The potential applications of Eu-TETA are based on the fact that it can absorb ultraviolet light in the organic portion of molecules; then, luminescence is emitted due to the 4f * ! 4f electronic transitions of Eu 3 þ . This produces emissions in the visible region of the spectrum with good efficiency after intramolecular energy transfer from the ligands to Eu 3 þ via an intersystem crossing, as in Fig. 2. 29,30 However, the thermal stability and the mechanical strength of these lanthanide Received 12 September 2005; accepted 6 February 2006. * Current address (on sabbatical): Ministry of Higher Education, Salalah College of Education, P.O. Box 3093, Salalah, 211, Sultanate of Oman. E-mail: zaitoun444@yahoo.com; zaitoun@mutah.edu.jo. 418 Volume 60, Number 4, 2006 APPLIED SPECTROSCOPY 0003-7028/06/6004-0418$2.00/0 Ó 2006 Society for Applied Spectroscopy