Luminescent sequence-dependent materials through a step by step assembly of RE1–1,4-benzendicarboxylate–RE2 (REx = Y3+, Eu3+ and Tb3+) architectures on a silica surface

Abstract

We present an approach toward sequence-dependent materials based on a stepwise assembly on a silica surface of rare earth cations (RE: Y³⁺, Eu³⁺ and Tb³⁺) and the 1,4-benzenedicarboxylate (terephthalate, T²⁻) ligand, organized in an ordered sequence RE¹–T–RE². The first RE was grafted to silica through a reaction of its N,N-dibutylcarbamato derivative, [RE¹(O₂CNBu₂)₃], with the surface silanols. The residual carbamato groups in the metal coordination sphere were then substituted by HT⁻ through a reaction with H₂T in pyridine. The Brønsted acidity of the pendant COOH groups was successively exploited for the anchorage of the second metal ion (RE¹ = Eu, RE² = Tb; RE¹ = Y, RE² = Tb). The choice of pyridine as the solvent, able to dissolve both H₂T and the secondary product [NH₂Bu₂]₂[T]·H₂T, was crucial for achieving success with this method. The emission properties of the materials were modulated by modifying the order of the synthetic steps. Metal complexes and materials were characterized by IR, EDX, ICP and photoluminescence. We used Tb³⁺ to Eu³⁺ energy transfer (ET) to study the RE³⁺ spatial distribution and intermetal distances to prove the formation of a RE¹–T–RE² sequence. Photoluminescence was used as a multi-tool technique to characterize the functional properties of the materials and to obtain structural information supporting the sequence growth.