Observation of a mixed-metal transition in a d8–d10 heterobimetallic Pt–Ag cyanide system: Experimental and theoretical study

Europium(III) Photoluminescence Governed by d8-d10 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 d⁸-d¹⁰ 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 Eu³⁺, [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)(H₂O)₂][M^II(CN)₄]}·[M^I(CN)₂]·H₂O (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^IIM^I} units generating an orange emissive metal-to-metal-to-ligand charge-transfer (MMLCT) state, whose energy is tuned by the applied d⁸-d¹⁰ metal centers. Thanks to these CT states, 1-4 exhibit room temperature red Eu^III photoluminescence enhanced by energy transfer from {M^IIM^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.

Synthesis and Luminescence of Optical Memory Active Tetramethylammonium Cyanocuprate(I) 3D Networks

The structures of three tetramethylammonium cyanocuprate(I) 3D networks [NMe₄]₂[Cu(CN)₂]₂•0.25H₂O (1), [NMe₄][Cu₃(CN)₄] (2), and [NMe₄][Cu₂(CN)₃] (3), (Me₄N = tetramethylammonium), and the photophysics of 1 and 2 are reported. These complexes are prepared by combining aqueous solutions of the simple salts tetramethylammonium chloride and potassium dicyanocuprate. Single-crystal X-ray diffraction analysis of complex 1 reveals {Cu₂(CN)₂(μ₂-CN)₄} rhomboids crosslinked by cyano ligands and D_3h {Cu(CN)₃} metal clusters into a 3D coordination polymer, while 2 features independent 2D layers of fused hexagonal {Cu₈(CN)₈} rings where two Cu(I) centers reside in a linear C_∞v coordination sphere. Metallophilic interactions are observed in 1 as close Cu⋯Cu distances, but are noticeably absent in 2. Complex 3 is a simple honeycomb sheet composed of trigonal planar Cu(I) centers with no Cu^…Cu interactions. Temperature and time-dependent luminescence of 1 and 2 have been performed between 298 K and 78 K and demonstrate that 1 is a dual singlet/triplet emitter at low temperatures while 2 is a triplet-only emitter. DFT and TD-DFT calculations were used to help interpret the experimental findings. Optical memory experiments show that 1 and 2 are both optical memory active. These complexes undergo a reduction of emission intensity upon laser irradiation at 255 nm although this loss is much faster in 2. The loss of emission intensity is reversible in both cases by applying heat to the sample. We propose a light-induced electron transfer mechanism for the optical memory behavior observed.