Europium(III) Photoluminescence Governed by d 8 -d 10 Heterometallophilic Interactions in Trimetallic Cyanido-Bridged Coordination Frameworks.

We report an efficient pathway toward sensitization of red room temperature Eu ^III emission by the charge-transfer (CT) states related to d ⁸ -d ¹⁰ heterometallophilic interactions achieved by the simultaneous application of tetracyanidometallates of Pt ^II /Pd ^II and dicyanidometallates of Au ^I /Ag ^I in the construction of a trimetallic d-d-f assembly. The combination of Eu ³⁺ , [M ^II (CN) ₄ ] ^2- (M = Pt, Pd), and [M ^I (CN) ₂ ] ^- (M = Au, Ag) ions along with 4,4'-bipyridine N , N '-dioxide (4,4'-bpdo) results in four novel isostructural 2D {[Eu ^III (4,4'-bpdo)(H ₂ O) ₂ ][M ^II (CN) ₄ ]}·[M ^I (CN) ₂ ]·H ₂ O (M ^II /M ^I = Pt/Au, 1 ; Pt/Ag, 2 ; Pd/Au, 3 ; Pd/Ag, 4 ) coordination networks. They are built of hybrid coordination layers, based on cyanido-bridged {Eu ^III [M ^II (CN) ₄ ]} _n square grids coexisting with metal-organic {Eu ^III (4,4'-bpdo)} _n chains, with the further attachment of [M ^I (CN) ₂ ] ^- ions through metallophilic {M ^II -M ^I } interactions. This results in dinuclear {M ^II M ^I } units generating an orange emissive metal-to-metal-to-ligand charge-transfer (MMLCT) state, whose energy is tuned by the applied d ⁸ -d ¹⁰ metal centers. Thanks to these CT states, 1 - 4 exhibit room temperature red Eu ^III photoluminescence enhanced by energy transfer from {M ^II M ^I } units, with the additional role of 4,4'-bpdo also transferring the energy to lanthanides. These donor CT states lying in the visible range successfully broaden the available efficient excitation range up to 500 nm. The overall emission quantum yield ranges from 8(1)% for 4 to 15(2)% for 1 , with the intermediate values for 2 and 3 relatively high among the reported Eu ^III -based compounds with tetracyanido- and dicyanidometallates. We found that the sensitization efficiency is equally high for all compounds because of the similar energies of the CT states, while the main differences are related to the observed emission lifetimes ranging from ca. 80 μs for 4 to 120-130 μs for 2 and 3 to ca. 180 μs for 1 . This phenomenon was correlated with the energies of the vibrational states, e.g., cyanide stretching vibrations, responsible for nonradiative deactivation of Eu ^III excited states, which are the highest for the Pd/Ag pair of 4 and the lowest for the Pt/Au pair in 1 . Thus, the heaviest pair of Pt ^II /Au ^I cyanide metal complexes is proven to be the best candidate for the sensitization of room temperature Eu ^III luminescence.