Intercalation of coumaric acids into layered rare-earth hydroxides: controllable structure and photoluminescence properties

Abstract

Organic compounds of ortho-coumaric acid (abbr. o-CMA) and para-coumaric acid (abbr. p-CMA) are intercalated into the layered rare-earth hydroxides (LRHs, R = Eu, Gd) via the ion exchange method. The two organics having the same phenolic hydroxyl and carboxyl groups located at different positions demonstrate variant intercalation structures. The CMA anion guests within the LRH gallery indicate monolayered or bi-layered arrangements depending on their own characteristic features and deprotonation degrees. The combination of o-/p-CMA molecules with LEuH/LGdH layers generates hybrid materials exhibiting versatile luminescence properties. In the solid state, for LEuH layers, the luminescence of layer Eu³⁺ and interlayer o-CMA is co-quenched; however, for LGdH layers, green emissions (∼520 nm) are observed, both of which are different from the cyan emission (475 nm) of free o-CMA anions. When dispersed in formamide (FM), o-CMA–LEuH composites exhibit weak luminescence, in sharp contrast to o-CMA–LGdH composites displaying green emissions (495–520 nm) with markedly enhanced intensity. The p-CMA–LGdH composites display blue emission (457 nm) quite different from the green emission (499 nm) for free p-CMA in FM, compared with the unobservable emission within LEuH layers. In addition, co-intercalation of the surfactant OS (1-octane sulfonic acid sodium) with o-CMA anions into LEuH produces composites showing desirable blue emission or violet emission, due to the change of the microenvironment of organic guests. The energy transfer between layer Eu³⁺ and interlayer CMA was proposed to account for the co-quenching or blue shift. This work offers a beneficial approach to fabricate organic–inorganic photofunctional materials with a controllable structure and tunable fluorescence properties.