Enhanced luminescence of delaminated layered europium hydroxide (LEuH) composites with sensitizer anions of coumarin-3-carboxylic acid

Abstract

The organic compound of coumarin-3-carboxylic acid (CCA), deprotonated beforehand by NaOH, and the 1-octane sulfonic acid anion (OS) were co-intercalated into the gallery of the layered europium hydroxide (LEuH) via an ion exchange method. Different molar ratios of CCA/OS and NaOH/CCA gave rise to the composites of CCA_1−xOS_x-LEuH (x = 0.8–1.0) showing different emission intensities. In formamide (FM), all composites were delaminated and the formed colloidal suspensions exhibited enhanced red luminescence of Eu³⁺ in comparison with the OS-LEuH without CCA. Also, the red emissions of the composites were different from the violet emission (421 nm) of free CCA⁻ and blue emission (471 nm) of CCA²⁻ anions in different deprotonation states. The energy levels of CCA and Eu³⁺ were analyzed to explain the sensitization effect for Eu³⁺ luminescence. The fluorescence lifetimes of CCA_0.2OS_0.8-LEuH-1 : 1, CCA_0.02OS_0.98-LEuH-1 : 1, CCA_0.2OS_0.8-LEuH-1 : 2, and CCA_0.02OS_0.98-LEuH-1 : 2 were determined to be 0.705, 0.704, 0.699 and 0.638 ms, respectively, indicating significantly longer lifetimes. The PL quantum yields of ∼10% demonstrate the excellent luminescence properties of the as-prepared CCA_1−xOS_x-LEuH composites. This is the first report on the sensitized luminescence properties of layer Eu³⁺ ions in LRH composites in the delaminated state. The intriguing red luminescence of delaminated LEuH composites offers a promising approach to achieve efficient luminescent film materials.