Emergence of the super antenna effect in mixed crystals of ytterbium and lutetium complexes showing near-infrared luminescence

Abstract

The synthesis of luminescent molecular crystalline materials requires a good understanding of the luminescence properties of crystals in which many molecules are densely packed. Previously, we studied the near-infrared (NIR) luminescence of a trivalent ytterbium (Yb(III)) complex with a Schiff base ligand, tris[2-(5-methylsalicylideneimino)ethyl]amine (H₃L). Herein, we extended our study on the Yb complex (YbL) to enhance and understand its solid-state luminescence via mixed crystallization with the lutetium complex (LuL). We prepared (YbL)_x(LuL)_1−x mixed crystals (x = 0.01, 0.05, 0.1, 0.2, 0.3, 0.5, and 0.7) and studied their NIR luminescence properties. The NIR luminescence intensity per Yb(III) ion for (YbL)_0.01(LuL)_0.99 was determined to be two orders of magnitude larger than that for YbL. The excitation spectral shape of (YbL)_0.01(LuL)_0.99 was different from the absorption spectral shape of YbL but similar to that of LuL. We attribute this observation to the emergence of an intermolecular energy-migration path. In the mixed crystals, LuL molecules acted as a light-harvesting super antenna for Yb(III) luminescence. Decay measurements of the NIR luminescence for (YbL)_x(LuL)_1−x with x > 0.2 showed mono-exponential decay, while (YbL)_x(LuL)_1−x with x < 0.1 showed a grow-in component, which reflected the lifetime of the intermediate state for energy migration. The decay lifetime values tended to increase with decreasing x, suggesting that Yb(III) isolation resulted in a reduction in concentration quenching. We propose that the luminescence enhancement in the highly Yb-diluted conditions was mainly caused by an increase in the super antenna effect.