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42269-B3
Rare Earth Calixarene Complexes in a Sol-Gel Matrix: Synthesis and Luminescence
To enhance the fluorescence of rare earth ions in silica matrices, we are investigating the incorporation of rare earth ion (RE3+) calixarene complexes into silica sol gels. The calixarene ligands will not only reduce hydroxyl and cluster quenching of the rare earth in the gel by surrounding the rare earth ion, but may also serve to enhance fluorescence through the so-called antenna effect. This effect allows excitation of the rare earth ion fluorescence at a wavelength where neither the ligand nor the rare earth ion absorb significantly.
In this second year of the proposed project, we have continued our efforts to synthesize and characterize substituted calixarene ligands and complexing them with terbium and europium, two highly fluorescent rare earth ions. In addition we have begun to work to better solubilize the complexes so that they may be homogenously incorporated into silica-base sol gels.
We have chosen to begin our ligand syntheses by targeting calixarene ligands derivatized on the lower rim of the calix[4]arene parent ligand. In particular, we have continued to targeted phosphoryl, acetic acid ethyl ester, and bipyridyl deriviatives on the lower rim of the calixarene ring (where R = (CH2)3PO(OEt)2, CH2COOEt, and CH2-bipyridyl). These are challenging ligands to synthesize, especially with the aim of tetrasubstitution on the lower rim. We have successfully replicated the synthesis of the tetraacetic acid ester derivative and are in the process of scaling-up and purifying the di-substituted bipyridyl ligand. The synthesized ligands are characterized by 1H NMR, IR, and UV-vis spectroscopy and compared to the literature.
We also continue to seek the best reaction conditions for the complexation of the rare earth ions onto the calixarene ligands. Last year we made the observation that triethylamine accomplished the deprotonation of the hydroxy groups on the tetra-t-butylcalix[4]arene parent ligand (available commercially) and have continued to use this method. We also continue to use RE(NO3)3⋅xDMSO as the starting material in place of the commercially available RE(NO3)3⋅6H2O to obtain the complexed products. Successful synthesis products are characterized by through UV-vis and fluorescence spectroscopy.
Dimethylformamide (DMF) has proven to be the best co-solvent for the preparation of silica-based sol gels prepared from tetraethylorthosilcate (TEOS) and containing the rare earth complexes (as well as other rare earth complexes). We have found so far that a pH of about 3 along with brief sonication of the sol will allow the acidic conditions to form the gels and can produce a homogeneous gel. The gels, which are formed at 40 ˚C, are aged at 60 ˚C over a period of 2-4 days. The temperature is then slowly raised from 60 ˚C to 90 ˚C over a period of 48 hours and the gels dried for 24 hours at 90 ˚C. Cracking can be extensive and the rate of heating as well as the amounts of solvents (DMF and water) will be varied in the next stage of the project to ameliorate the cracking with the goal of forming monolithic samples.
In addition to the work on calixarene-complexed rare earth ions, we have also initiated an investigation of the synthesis and incorporation of porphyrin- and crown ether-complexed rare earth ions into silica sol gels. Just started this summer, in the first months of this work we have focused on the synthesis and characterization of some complexes published in the literature and examining their solubilities in the TEOS sol-gel precursor. These ligands may prove interesting in that, because of their planar structure, they do not completely surround the rare earth ion, but leave open coordination sites that might show intermediate levels of fluorescence in the gels compared to either uncomplexed rare earth ions (which show no florescence until the gel has been annealed to glass at 800 ˚C) of from complexes containing the thenoyltrifluoroacetonate (tta) with phenanthroline ligands (which show fluorescence in gels dried only at 90 ˚C).
This project has, to date, included the work of six undergraduate students over three summers and one senior project student. Both students working on the project this past summer are senior chemistry majors who are continuing their work now during their year-long senior project. This grant has allowed me to continue to pursue a new direction in my research program that parallels my continuing collaborative work with my physics colleague on the spectroscopy of uncomplexed rare earth ions in sol-gel matrices.
