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.
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