Unraveling the Up‐Conversion Mechanism Involved in the Intense Red Emission of a Mononuclear ErIII Complex

Abstract The scarcity of up‐conversion (UC) luminescence studies involving Ln III molecular complexes mainly arises from the challenge of designing a coordination sphere that minimizes the interactions of the metal centers with high‐energy oscillators. By using a combination of reported strategies to reduce these interactions, we synthesized and characterized a mononuclear complex, Er(tta) 3 (bipy) (tta: thenoyltrifluoroacetone; bipy: 2,2′‐bipyridine) ( Er‐TB ). Upon 980 nm laser excitation at room temperature, this complex gives rise to the challenging up‐converted red ( 4 F 9/2 → 4 I 15/2 ) band, besides the bluish‐green ( 2 H 11/2 → 4 I 15/2 ) and green ( 4 S 3/2 → 4 I 15/2 ) ones, as well as a downshifted NIR ( 4 I 13/2 → 4 I 15/2 ) emission. Interestingly, a high intensity of the red emission band and unprecedented luminescence lifetime values (448 µs for green and 775 µs for red emission) were achieved. A detailed analysis of the available data allows us to unravel the probable UC mechanism, which results in a dominant energy transfer up‐conversion (ETU) mechanism via three‐photon absorption, while a cross‐relaxation (CR) process for the intense red emission is proposed. Besides affording a design strategy to achieve UC luminescence in Er III complexes, this study opens the door to the design of other lanthanide‐based molecular UC systems, expanding the scope of these materials in the UC luminescence field.