The Role of Blue‐Emissive 1,8‐Naphthalimidopyridine N‐Oxide in Sensitizing EuIII Photoluminescence in Dimeric Hexafluoroacetylacetonate Complexes

Abstract

Two new fluorescence molecules incorporating coordinated pyridine N‐oxide (PyNO) and the blue‐emitting 1,8‐naphthalimide (NI) chromophores, 4‐(1,8‐naphthalimido)pyridine N‐oxide (4‐NIPyNO) and 3‐(1,8‐naphthalimido)pyridine N‐oxide (3‐NIPyNO), are designed and used as ancillary ligands for constructing mixed‐ligand tris(β‐diketonate) lanthanide complexes, Ln2(hfa)6(4‐NIPyNO)2 and Ln2(hfa)6(3‐NIPyNO)3 (Ln = Eu, Gd; hfa = hexafluoroacetylacetone). Single‐crystal X‐ray diffraction crystallography analyses reveal that all the complexes adopt dimeric dinuclear structures; in each case, two LnIII ions from Ln(hfa)3 moieties are bridged by the oxygen atoms of the N‐oxide groups of 4‐NIPyNO and 3‐NIPyNO. The luminescence quantum yield experiment reveals that the introduction of the ancillary ligands results in the EuIII ions having luminescence enhancement about 13 times greater than that from hydrated Eu(hfa)3(H2O)2. The detailed photophysical determinations are carried out to gain insight into the roles of the NIPyNO ligands in enhancing EuIII luminescence. By investigating the ligand‐centered emission, it is found that both the singlet states (S) and the triplet states (T) are responsible for the energy transfer from the NIPyNO ligands to the metal center; in particular, the energy‐transfer efficiencies of the singlet states reach up to 63 % and 71 % for Eu2(hfa)6(4‐NIPyNO)2 and Eu2(hfa)6(3‐NIPyNO)3, respectively. This auxiliary‐ligand designing strategy enables the use of the blue‐emitting chromophore as a sensitizer of the luminescence from EuIII ions.