Synthesis, structure, and photoluminescent behaviour of molecular lanthanide–2-thiophenecarboxylate–2,2′:6′,2′′-terpyridine materials

Abstract

The preparation of a lanthanide series, with the exception of promethium, incorporating 2,2′:6′,2′′-terpyridine (terpy) and 2-thiophenecarboxylate (TC) functionality has been achieved via hydrothermal reaction conditions. The structures of the crystalline products were determined using single crystal X-ray diffraction and bulk purity was confirmed via powder X-ray diffraction and elemental analysis. The series is comprised of four structure types wherein the effects of the lanthanide contraction are manifested in changes in the coordination number and nuclearity of the complexes. From lanthanum through holmium an isomorphous series of symmetric dimers of the general formula [Ln₂(terpy)₂(TC)₄(μ-TC)₂(H₂O)₂]·nH₂O (Ln-1, Ln = La–Ho, excluding Pm) was isolated. Using holmium oxide resulted in the formation of Ho-1 whereas use of the holmium chloride precursor resulted in the formation of [Ho₂(terpy)₂(TC)₃(μ-TC)₂(H₂O)₂]TC (Ho-2), that consists of an asymmetric dimer. Beyond Ho, Er–Lu were found to form compounds of the general formula [Ln(terpy)(TC)₃(H₂O)], the structures of which consist of monomeric units. While Er adopts Er-3, Tm–Lu adopt another structure type (Ln-4, where Ln = Tm, Yb, and Lu), the latter arising from differences in intramolecular hydrogen bonding interactions as well as π–π stacking interactions. The luminescent behaviour of the solid samples was examined and showed ligand-sensitized metal-based visible (Sm, Eu Tb, Dy) and NIR (Nd, Sm, and Yb) emission, with lifetimes up to hundreds of microseconds for select samples. Quantum yields of the visible light emitters have been measured and are additionally presented.