The luminescence, molecular and electronic structure, and excited state energetics of tris complexes of 4-phenylethynyl-2,6-pyridinedicarboxylic acid with Eu(III) and Tb(III) prepared in sol–gel J. Sokolnicki a , J. Legendziewicz a, * , G. Muller b , J.P. Riehl b, * a Faculty of Chemistry, University of Wroclaw, 14 F. Joliot-Curie Street, 50-383 Wroclaw, Poland b Department of Chemistry, University of Minnesota Duluth, Duluth, MN 55812, USA Available online 16 February 2005 Abstract The incorporation of high symmetry racemic lanthanide complexes into sol–gels has recently been demonstrated to be a useful technique to eliminate racemization, and thus allow one the ability to probe individual enantiomers by circularly polarized excita- tion (CPE) followed by circularly polarized luminescence (CPL). Reduced CPL from sol–gels containing high concentrations of lan- thanide complexes has also led to an interpretation of the results in terms of racemization due to excited state energy transfer. This paper is a continuation of our previous studies on the diminished differential excited states concentration of a photoenriched racemic mixture of Euðdipicolinate ¼ DPAÞ 3 3 incorporated in a sol–gel. The high resolution emission, emission excitation spectra, decay times and CPL of the Eu(III) and Tb(III) complexes of the 4-phenyl-ethynyl-dipicolinic acid at 300 and 77 K in solution and incor- porated into silica sol–gel are reported. The kind of experiments mentioned above are inherently sensitive to the nature of the local environment of the complexes in the sol–gel, and this is the focus of the results presented here. This information is critical in the continued development of sol–gel based sensors. Comparison of the CPL, total luminescence, and excitation results with spectra obtained from aqueous solution are interpreted in terms of local and bulk structural properties of the sol–gel system. The mecha- nism of intra- and intermolecular energy transfer in Eu(III) and Tb(III) systems is also analyzed. Ó 2005 Elsevier B.V. All rights reserved. 1. Introduction Luminescent lanthanide complexes having strong absorption chromophores have attracted considerable attention in recent efforts to produce effective lantha- nide-based luminescent sensors. This is due to the remarkably strengthened luminescence intensity of the rare earth ions that is possible through indirect excita- tion of the emitting state as compared to the weak par- ity-forbidden intraconfigurational f ! f absorption. This phenomenon is often referred to as the ‘‘antenna effect’’ [1–14]. The increased luminescence efficiency and sensitivity that may be obtained is especially impor- tant in the design of lanthanide-based luminescence probes of biological structure [2,6–11], and light conver- sion molecular device (LCMD) [1,3,12–16]. There has also been an increased effort to incorporate luminescent lanthanide complexes into solid matrices for the purpose of developing useful sensors. For this latter purpose, the sol–gel technique offers the possibility to prepare an inorganic matrix possessing extremely good optical, thermal and chemical stability [14,16]. A high-quality optical component can be obtained in the formation of a sol–gel if special care is taken during the drying and 0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.01.010 * Corresponding authors. Tel.: +48 71 3204 300; fax: +48 71 3282 348. E-mail address: jl@wchuwr.chem.uni.wroc.pl (J. Legendziewicz). www.elsevier.com/locate/optmat Optical Materials 27 (2005) 1529–1536