Cocrystallization of coordinative and inorganic lanthanide centers showing dual emission via linked or unlinked antenna

Abstract

In this paper, we report the design and synthesis of two pyridyl-substituted tripodal ligands, 3-TPyMNTB (tris(((pyridin-3-ylmethyl)benzimidazol-2-yl)methyl)amine) and 4-TPyMNTB (tris(((pyridin-4-ylmethyl)benzimidazol-2-yl)methyl)amine). Reaction of these two ligands with Eu(III)/Tb(III) picrate salts gives mononuclear complexes [Eu(3-TPyMNTB)(pic)₃]·2H₂O (1) and [Tb(3-TPyMNTB)(pic)₃]·2H₂O (2), respectively. While reaction with Eu(III)/Tb(III) nitrate salts leads to cocrystallization of a coordinative motif and an inorganic salt in one single-crystal, namely, [Eu(3-TPyMNTB)(NO₃)₃]·0.5[Eu(NO₃)₃(H₂O)₄]·3CH₃CN (3), [Tb(3-TPyMNTB)(NO₃)₃]·0.5[Tb(NO₃)₃(H₂O)₄]·3CH₃CN·1.5C₂H₅OH (4), [Eu(4-TPyMNTB)(NO₃)₃]·[Eu(NO₃)₃(H₂O)₃]·3CH₃CN·0.5H₂O (5) and [Tb(4-TPyMNTB)(NO₃)₃]·[Tb(NO₃)₃(H₂O)₃]·3CH₃CN·0.5H₂O (6). In these cocrystals, the organic ligand-coordinated motif [Ln(3(4)-TPyMNTB)(NO₃)₃] and the nitrate anion-bonded motif [Ln(NO₃)₃(H₂O)_n] (Ln = Eu(III) or Tb(III), n = 3 or 4) provide two Eu(III)/Tb(III) centers which coexist in the same crystal lattice. Of most interest, both coordinative and inorganic centers can be excited by the absorption of the organic ligand, that is, the double luminescent centers in the cocrystalline systems can be sensitized by the same antenna source. Therefore, both Dexter and Förster mechanisms are proposed to be involved in the emission process, in which the energy transfer is conducted either through direct bond linkages or through space from unlinked organic antennae.