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Orientation of (Hetero)aromatic Anions in the LEuH Interlayer and Enhanced Photoluminescence

Shao, Baiyi, Feng, Pingping, Wang, Xinying, Cui, Fangming, Yang, Xiaojing
Journal of physical chemistry 2019 v.123 no.12 pp. 7467-7474
anions, carboxylic acids, electrostatic interactions, energy, europium, hydrogen bonding, hydroxides, ion exchange, nitrates, photoluminescence
Eight kinds of (hetero)aromatic anions terminated with −COO– were intercalated into the interlayer of layered europium hydroxides (LEuHs) as sensitizers for enhancing Eu³⁺ photoluminescence (PL) via an ion exchange with NO₃–. Orientations of the anions in the interlayer were found to be vertical, tilted, or horizontal relative to the LEuH layer. The −COO– group was directly bonded to Eu³⁺ in the mono- or didentate coordination in the vertical and tilted cases, whereas no direct coordination of anions was observed in the horizontal orientation. There are two distinctive factors that determine the orientation of these anions in the interlayer. First, the −COO– group is anchored to the positively charged layer through the electrostatic interaction. For this reason, the orientation of 1,3,5-benzenetricarboxylic acid (BTA) could be vertical as it was deprotonated to BTA²– while horizontal as it was deprotonated to BTA³–. The second important aspect is the key role of the weak interactions between the heteroatoms of the rings and OH–/Eu³⁺ of the LEuH layer, including mainly hydrogen bonds, ionogenic-characterized interactions, and salt bridges. The stronger interactions resulted in more tilted guests. The hydrogen bonds between N–H in pyrrole-2-carboxylic acid and OH– of the layer were strong enough to make the guest horizontally orientating. The organic anion-intercalated LEuHs with vertical and tilted sensitizers showed enhanced PL intensity because of the energy transition effect by the intercalated sensitizers with respect to that of the LEuHs with horizontally oriented sensitizers.