Unraveling the Up-Conversion Mechanism Involved in the Intense Red Emission of a Mononuclear Er^III Complex

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 (⁴F_9/2 →⁴I_15/2) band, besides the bluish-green (²H_11/2 →⁴I_15/2) and green (⁴S_3/2 →⁴I_15/2) ones, as well as a downshifted NIR (⁴I_13/2 →⁴I_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.