pH response of a hydroxyl-functionalized luminescent metal–organic framework based phosphor

Abstract

A terbium doped yttrium based metal-organic framework [Y1.8Tb0.2(CAM)3(H2O)4].2H2O, 1{where H2CAM = chelidamic acid}, has been successfully synthesized using hydrothermal technique to use it as phosphor material along with large Stokes shift and low self-quenching of luminescence. To confirm the structure and phase purity of compound 1, the single crystals of isomorphous pure yttrium based compound [Y2(CAM)3(H2O)4].2H2O, 1a, have been synthesized under similar experimental conditions. Single crystal X-ray data of compound 1a confirmed two-dimensional metal-organic framework formed by the connectivity of Y3+ ion with carboxylates of chelidamate ligands. Whereas the hydroxyl groups of the chelidamate ligands remain free. The phase purity ofcompound 1 and 1a were confirmed by powder X-ray diffraction. Compound 1 was systematically characterized by TGA, IR, EDX, SEM-elemental mapping and BET analysis. The aqueous solution of compound 1 shows weak visible green emission upon excitation at 280 nm. The emissions of compound 1 were utilized to check pH response keeping in mind the favourable presence of free hydroxyl groups. It has been observed that the emission intensity increased by 4.2 times from pH 7 to pH 9 along with sudden jump at pH > 7.5. The luminescence life time decay studies also show gradual increase in life time of Tb3+ centered emission from 0.75ms (pH 7) to 0.99 ms (pH 9). Luminescence response of compound 1 in the biologically important pH range 7-9 based on the free hydroxyl groups may be very much useful for real life application.