A new strategy for achieving white-light emission of lanthanide complexes: effective control of energy transfer from blue-emissive fluorophore to Eu(iii) centres

Abstract

Two novel bifunctional 1,8-naphthalimide-based ligands incorporating coordinated carboxyl groups and blue-emitting 1,8-naphthalimide, 2-(1,8-naphthalimido)ethanoic acid (HL¹) and 3-(1,8-naphthalimido)propanoic acid (HL²) were designed for synthesis of white-light-emitting lanthanide complexes. Self-assembly of ligands with lanthanide ions was used to form two series of 1D coordination polymers: {Ln(L¹)₃(CH₃OH)(H₂O)}_n (Ln = Eu³⁺ (1), Gd³⁺ (2)) and {[Ln(L²)₃(H₂O)]·H₂O}_n (Ln = Eu³⁺ (3), Gd³⁺ (4)). Single-crystal X-ray crystallography analyses reveal the complexes feature 1D chain structure. In 1 and 3, partial energy transfer from the 1,8-naphthalimide moieties to the metal centres results in sensitized Eu(III)-based emission in addition to the ligand-based blue/green luminescence. Through changing the length of the linkers between the chromophore and the metal centre, energy transfer efficiency of ligands to Eu is effectively controlled. In 1, the emission spectrum is mainly dominated by the red light of the Eu(III) ions. However, the red light emission is suppressed in 3 due to the decrease of chromophore-to-Eu energy transfer efficiency. This effective adjustment balances the three primary colours, thus resulting in white-light emission in 3. This result opens up a new synthetic strategy for white-light-emitting materials.