Structural and Photophysical Properties of Coordination Networks Combining [Ru(bipy)(CN)₄]^2- Anions and Lanthanide(III) Cations: Rates of Photoinduced Ru-to-Lanthanide Energy Transfer and Sensitized Near-Infrared Luminescence

Co-crystallization of K2[Ru(bipy)(CN)4] with lanthanide(III) salts (Ln = Pr, Nd, Gd, Er, Yb) from aqueous solution
affords coordination oligomers and networks in which the [Ru(bipy)(CN)4]2- unit is connected to the lanthanide
cation via Ru−CN−Ln bridges. The complexes fall into two structural types:  [{Ru(bipy)(CN)4}2{Ln(H2O)m}{K(H2O)n}]·xH2O (Ln = Pr, Er, Yb; m = 7, 6, 6, respectively), in which two [Ru(bipy)(CN)4]2- units are connected to a single
lanthanide ion by single cyanide bridges to give discrete trinuclear fragments, and [{Ru(bipy)(CN)4}3{Ln(H2O)4}2]·xH2O (Ln = Nd, Gd), which contain two-dimensional sheets of interconnected, cyanide-bridged Ru2Ln2 squares. In
the Ru−Gd system, the [Ru(bipy)(CN)4]2- unit shows the characteristic intense 3metal-to-ligand charge transfer
luminescence at 580 nm with τ = 550 ns; with the other lanthanides, the intensity and lifetime of this luminescence
are diminished because of a Ru → Ln photoinduced energy transfer to low-lying emissive states of the lanthanide
ions, resulting in sensitized near-infrared luminescence in every case. From the degree of quenching of the
Ru-based emission, Ru → Ln energy-transfer rates can be estimated, which are in the order Yb (kEnT ≈ 3 × 106
sec-1, the slowest energy transfer) < Er < Pr < Nd (kEnT ≈ 2 × 108 sec-1, the fastest energy transfer). This order
may be rationalized on the basis of the availability of excited f−f levels on the lanthanide ions at energies that
overlap with the Ru-based emission spectrum. In every case, the lifetime of the lanthanide-based luminescence is
short (tens/hundreds of nanoseconds, instead of the more usual microseconds), even when the water ligands on
the lanthanide ions are replaced by D2O to eliminate the quenching effects of OH oscillators; we tentatively ascribe
this quenching effect to the cyanide ligands.