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 and other macrocyclic ligand complexes into silica sol gels. The ligands chosen are those that will reduce hydroxyl quenching by encapsulating the rare earth ions completely within the ligands so as to isolate the ion from OH groups present in the sol gel environment and that will serve as antennae, absorbing UV light and transferring it to the rare earth ion and therefore enhance the luminescence of the sample.
In this year of the project, we have continued our efforts to synthesize and characterize substituted calixarene ligands with the aim of complexing them with europium and comparing the fluorescence of the solid complexes with that of the complexes imbedded into a sol gel matrix Both aspects of this project have been challenging. The synthesis of our targeted calixarene derivatives has proven difficult due to the formation of many side prodicts. Thus, the separation of the mixture of products and confirmation that we have synthesized the ligand has been slow. Nonetheless, we achieved success this past summer with the synthesis of a bis-benzimidizolylpyridine-derived calixarene. We are now trying various co-solvents to incorporate the europium complex into the water based tetraorthosilcate (TEOS) sol gel matrix.
Last year we found that co-solvents were important to the successful incorporation of rare earth complexes into the gels, yet the cracking could be more extensive as these solvents evaporated in the gelling and ageing processes for the gels. We have built up extensive experience with ethanol and dimethylformamide as co-solvents coupled with detailed heating protocols which result in minimal cracking of the gels.
In addition to the work on calixarene-complexed rare earth ions, we synthesized crown ether and pyridine-2,6-dicarboxylic acid bis-[(1-naphthalen-1-yl-ethyl)-amide] complexes of europium and placed the complexes into TEOS-derived sol gels aged to 90ūC. We found that for the 18-crown-6 complex, the best co-solvent for the incorporation of the complex was ethanol. The use of ethanol, however, substantially slowed down the gelation process, but resulted in very slightly cloudy, yet crack free, monoliths after processing over 40 days. The fluorescence spectrum showed decreased fluorescence over the pure complex as might be expected since the crown ether will not completely isolate the rare earth ion from hydroxyl quenching due to open coordination sites. In addition, the use of an small amount of acid to catalyze the gelation of TEOS may have caused a substantial portion of the rare earth ion to be released from the coordination sphere of the crown ether ligand.
We have also found that europium complexes with pyridine-2,6-dicarboxylic acid bis-[(1-naphthalen-1-yl-ethyl)-amide] as a ligand were quite easily incorporated into TEOS-based silica gels. Because we suspected that the presence of an acid catalyst for the formation of the gels would be detrimental for this ligand because of several possible protonation sites, we prepared gels both with and without the acid catalyst. Indeed, gel s prepared with acid did not fluoresce when exposed to UV light (both short and long wavelength) while those that were prepared without the addition of trace amounts of acid did fluoresce. We are continuing this study to optimize the processing of the sols to obtain monolithic gels and will be studying the luminescence properties of these gels over the next year.
This project to date has included the work of eight different undergraduate students over four summers and four other senior project students. All of the students working on the project this past year were either senior chemistry majors or students who had just completed their first two years of chemistry courses at Hamilton College and are potential chemistry or chemical physics majors. This grant has allowed me to continue to pursue this 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.
