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