Intrazeolite Photochemistry. 20. Characterization of Highly Luminescent Europium Complexes inside Zeolites

Abstract

Four organometallic complexes of Eu3+ with 2,2‘-bipyridine (bipy), benzoyltrifluoroacetone (btfa), 1,10-phenanthroline (phen), and dipicolinic acid (dpic) have been prepared in Y, mordenite, and ZSM-5 zeolites by adsorbing the ligand into Eu3+-doped zeolites. Different samples were obtained varying the Eu3+ content and the Eu3+:ligand ratio. Formation of the complexes and their stoichiometry have been assessed by elemental chemical analysis (combustion and atomic absorption) and thermogravimetry-differential scanning calorimetry as well as IR and diffuse reflectance (DR) spectroscopies. The distinctive feature of DR spectra of the solids is the presence of a band of medium intensity in the 500 nm region that is absent for the same complexes in solution. All the samples, including those that do not contain any organic ligand, exhibit room-temperature emission upon laser excitation, although luminescence intensity notably increases upon complexation. The emission lifetimes are in the millisecond time scale and do not follow simple first-order kinetics. Concerning the emission decay, the following observations were made: (i) the complex luminescence is much longer lived inside the zeolites than in solution; (ii) the emission lifetime follows the order dpic > phen > bipy > bfta; (iii) lifetime increases with the amount of the ligand incorporated; (iv) the decays are more rapid as the Eu3+ content of the zeolite increases; and (v) emission becomes longer lived in the order ZSM-5 > mordenite > Y. These facts have been rationalized as the result of an increase in the conformational rigidity experienced by the complex that impedes alternative nonemissive deactivation pathways in zeolite media. Dehydration of the solids and subsequent D2O rehydration does not alter the decay of the uncomplexed Eu3+−Y samples but increases the lifetime of the Eu−btfa complex inside Y zeolite by a factor of 2. This is indicative that emitting Eu3+ are not solvated (probably located inside the sodalite cages and hexagonal prisms), while the Eu−btfa complex is coordinated with a single H2O molecule.