Manifestation of π-π Stacking interactions in luminescence properties and energy transfer in aromatically-derived Tb, Eu and Gd tris(pyrazolyl)borate complexes

Abstract

The three new complexes Tp Py Ln(CH 3 CO 2) 2 (H 2 O) (Ln = Eu (1), Gd(2), or Tb (3)) were prepared and characterized crystallographically. In the crystal lattices of these complexes, separate molecules are connected in infinite chains by π-stacking interactions. Complexes 1 and 3 display intense photoluminescence and triboluminescence (red and green respectively), while compound 3 exhibits electroluminescence commencing at 9 V in an ITO/PVK/3/Al device (ITO = indium-tin oxide, PVK = poly(N-vinylcarbazole)). A series of Eu/Tb-doped Gd compounds was prepared by cocrystallization from mixtures of 1 and 2 or 2 and 3, respectively. It was shown that π-stacking interactions are involved in increasing the efficiency of energy transfer from the gadolinium complex to emitting [Tp Py Eu] 2+ or [Tp Py Tb] 2+ centers, and this energy transfer occurs through hundreds of molecules, resembling the process of energy harvesting in chloroplast stacks.