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

Abstract

A terbium doped yttrium based metal–organic framework [Y_1.8Tb_0.2(CAM)₃(H₂O)₄]·2H₂O, 1, {where H₂CAM = chelidamic acid}, was successfully synthesized using a hydrothermal technique to be used as a phosphor material along with a large Stokes shift and low self-quenching of luminescence. To confirm the structure and phase purity of compound 1, single-crystals of the isomorphous pure yttrium-based compound [Y₂(CAM)₃(H₂O)₄]·2H₂O, 1a, were synthesized under similar experimental conditions. The single-crystal X-ray data of compound 1a confirmed a two-dimensional metal–organic framework formed by the connectivity of the Y³⁺ ion with the carboxylates of chelidamate ligands, while the hydroxyl groups of the chelidamate ligands remain free. The phase purity of compounds 1 and 1a was confirmed by powder X-ray diffraction. Compound 1 was systematically characterized by TGA, IR, SEM, EDX-elemental mapping and BET analysis. The aqueous solution of compound 1 shows a weak visible green emission upon excitation at 280 nm. The emission of compound 1 was utilized to detect the pH response, keeping in mind the favourable presence of free hydroxyl groups. It was observed that the emission intensity increased 4.2 times when the pH was increased from pH 7 to pH 9 along with a sudden jump at pH > 7.5. The luminescence lifetime decay studies also showed a gradual increase in the lifetime of Tb³⁺ centered emission from 0.75 ms (pH 7) to 0.99 ms (pH 9). The luminescence response of compound 1 in the biologically important pH range of 7–9 based on the free hydroxyl groups may be very much useful for real-life applications.