Lanthanide-based MOFs are promising candidates for the creation of luminescent materials, since REE(III) ions possess predictable, multicolored, narrow-band luminescence with long excited state lifetimes and high quantum yields.
In this research, the structure and properties of coordination compounds of various dimensions based on 1,4-diazabicyclo[2.2.2]octane-N,N’-dioxide (odabco) and Ln3+ (Ln3+ = Tb3+ and Eu3+) cations were studied. Their formulas are as follows: [Ln(odabco)3](NO3)3∙4.5H2O (1Tb; 1Eu); (Hodabco)3[Ln(H2O)2(NO3)4](NO3)2 (2Tb; 2Eu); [Ln(odabco)3]2[Ln(NO3)6](NO3)3∙3H2O (3Tb; 3Eu).
1Ln contain a three-dimensional porous cell [Ln(odabco)3]3+ with a mononuclear octahedral metal center and disordered nitrate anions in the pores. 2Ln contain polymeric organic cations (Hodabco)+ interconnected by hydrogen bonds into chains. Aquanitrate complexes and free nitrates are located in the interchain space as counterions. 3Ln contain a coordination framework close to 2Ln, but they also contain large hexanitrate complexes of lanthanides in their voids. The crystal structures of all compounds were determined by means of single-crystal XRD analysis, and their phase purity and stability were confirmed by means of PXRD, CHNS analysis, thermogravimetric analysis and IR spectroscopy.
Excitation and emission spectra and kinetic decay curves of luminescence were studied for all samples. Sensing properties were revealed for 3Eu, quenching the luminescence of a suspension in ethanol solutions of chromate or dichromate. For the last, LOD was 2.4·10-6 M for a 270 nm excitation wavelength and 2.1·10-7 M for a 395 nm excitation wavelength. 3Tb were also found to be quenching the luminescence for an ethanol solution of chromate and dichromate. LODs were 6.6·10-7 M and 1.1·10-5 M, respectively. The luminescence response at the micromolar concentration range was also found to be fast, with an equilibrium time no longer than 20 min. Therefore, porous MOFs 3Ln with a cationic polymeric structure were found to be effective luminescent sensors for toxic Cr(VI) anions.
