Mixed-Metal and Mixed-Ligand Lanthanide Metal-Organic Frameworks Based on 2,6-Naphthalenedicarboxylate: Thermally Activated Sensitization and White-Light Emission

Trivalent lanthanide ions (Ln³⁺) hold an exceptional position in the field of optoelectronic materials due to their atomic-like emission spectra and long luminescence lifetimes. Metal-organic frameworks (MOFs) and coordination polymers are particularly suited as luminescent materials due to their structural diversity and ease of functionalization both at bridging ligands and/or metal centers. In this contribution, we present a series of mixed-metal Ln³⁺/Eu³⁺ (Ln = La, Gd) and mixed-ligand (2,6-naphthalenedicarboxylate (ndc^2-) and 4-aminonaphthalene-2,6-dicarboxylate (andc^2-)) MOFs belonging to three different structural types, with emissions spanning most of the visible region, thereby constituting favorable materials for color tuning and white-light emission. We investigate the thermal stability and photophysical properties of the synthesized materials with regard to their metal and ligand doping levels and structural type, where we discuss excimer and monomer emission. The photophysical study, involving both steady-state and time-resolved luminescence measurements, allows us to discuss the possible energy migration and Eu³⁺ sensitization pathways that take place within these materials following ligand excitation. Low-temperature luminescence studies led us to determine the energies of the ligand-based excited states and investigate their participation in thermally activated energy transfer mechanisms within the studied lattices. We observe emission quantum yields of up to 87% for the Eu³⁺-doped materials, while their ligand- and metal-doped counterparts show decreased quantum yields of up to 17%. Finally, we attempt fine color tuning by carefully adjusting the doping levels to achieve yellow and white-light emission.

Detection and Sorption of Heavy Metal Ions in Aqueous Media by a Fluorescent Zr(IV) Metal-Organic Framework Functionalized with 2-Picolylamine Receptor Groups

Increasing global environmental pollution due to heavy metal ions raises the importance of research on new multifunctional materials for simultaneous detection and removal of these contaminants from water resources. In this study, we report a microporous 8-connected Zr⁴⁺ metal-organic framework (MOF) based on a terephthalate ligand decorated with a chelating 2-picolylamine side group (dMOR-2), which shows highly efficient fluorescence sensing and sorption of heavy metal cations. We demonstrate by detailed fluorescence studies the ability of a water-dispersible composite of dMOR-2 with polyvinylpyrrolidone for real-time detection of Cu²⁺, Pb²⁺, and Hg²⁺ in aqueous media. The limits of detection were found to be below 2 ppb for these species, while the system's performance is not affected by the presence of other potentially competitive ions. In addition, sorption studies showed that a composite of dMOR-2 with calcium alginate (dMOR-2@CaA) is an excellent sorbent for Pb²⁺ and Cu²⁺ ions with capacities of 376 ± 15 and 117 ± 4 mg per gram of dMOR-2@CaA, respectively, while displaying the capability for simultaneous removal of various heavy metal ions in low initial concentrations and in the presence of large excesses of other cationic species. Structural and spectroscopic studies with model ligands analogous to our material's receptor unit showed chelation to the 2-picolylamine moiety to be the main binding mode of metal ions to dMOR-2. Overall, dMOR-2 is shown to represent a rare example of a MOF, which combines sensitive fluorescence detection and high sorption capacity for heavy metal ions.

Highly efficient sorption and luminescence sensing of oxoanionic species by 8-connected alkyl-amino functionalized Zr4+ MOFs

Alkylamino-functionalized Zr4+ MOFs with 8-c nets and easily exchangeable terminal OH−/H2O ligands show remarkable sorption efficiency for toxic and radionuclide-related oxoanions and sensing capability for hexavalent chromium in ppb levels.

A bifunctional robust metal sulfide with highly selective capture of Pb2+ ions and luminescence sensing ability for heavy metals in aqueous media

A 3-D metal sulfide ion exchanger (MSIE) with extra-framework H+ ions showed a remarkable capability for sorption of Pb2+ ions and highly efficient luminescence sensing properties towards various heavy metal ions.

Development of novel GnRH and Tat48-60 based luminescent probes with enhanced cellular uptake and bioimaging profile

There is a clear need to develop photostable chromophores for bioimaging with respect to the classically utilized green fluorescent dye fluorescein. Along these lines, we utilized a phosphorescent carboxy-substituted ruthenium(ii) polypyridyl [Ru(bipy)2(mcb)]2+ (bipy = 2,2'-bipyridyl and mcb = 4-carboxy-4'-methyl-2,2'-bipyridyl) complex. We developed two luminescent peptide conjugates of the cell-penetrating peptide Tat48-60 consisting of either [Ru(bipy)2(mcb)]2+ or 5(6)-carboxyfluorescein (5(6)-FAM) tethered on the Lys50 of the peptide through amide bond. We confirmed the efficient cellular uptake of both bioconjugates in HeLa cells by confocal microscopy and flow cytometry and proved that the ruthenium-based chromophore possesses enhanced photostability compared to a 5(6)-FAM-based peptide, after continuous laser scanning. Furthermore, we designed and developed a luminescent agent with high photostability, based on the ruthenium core, that could be selectively localized in cancer cells overexpressing the GnRH receptor (GnRH-R). To achieve this, we took advantage of the tumor-homing character of d-Lys6-GnRH which selectively recognizes the GnRH-R. The [Ru(bipy)2(mcb)]2+-d-Lys6-GnRH peptide conjugate was synthesized, and its cellular uptake was evaluated through flow cytometric analysis and live-cell imaging in HeLa and T24 bladder cancer cells as negative and positive controls of GnRH-R, respectively. Besides the selective targeting that the specific conjugate could offer, we also recorded high internalization levels in T24 bladder cancer cells. The ruthenium(ii) polypyridyl peptide-based conjugates we developed is an intriguing approach that offers targeted cell imaging in the Near Infrared region, and simultaneously paves the way for further advancements in the dynamic studies on cellular imaging.

Alkaline earth-organic frameworks with amino derivatives of 2,6-naphthalene dicarboxylates: structural studies and fluorescence properties

Alkaline earth metal ion organic frameworks (AEMOFs) represent a relatively underexplored subcategory of metal-organic frameworks (MOFs). In this contribution, we present the synthesis and structural study of the new MOFs 1-8 based on the alkaline earth ions Mg2+, Ca2+, Sr2+ and Ba2+ and the amino substituted bridging ligands 4-aminonaphthalene-2,6-dicarboxylate (ANDC2-) and 4,8-diaminonaphthalene-2,6-dicarboxylate (DANDC2-). Compounds 1, 5, 6, 7 and 8 constitute rare examples of three-dimensional MOFs which feature square planar M4 secondary building units (SBUs) surrounded by eight bridging ditopic ligands. The underlying topology of MOFs 1, 5, 7 and 8 conforms to the 4-c pcb net which can be simplified to the 8-c bcu net, while 6 adopts the 4-c lta net which simplifies to the 8-c reo net. To the best of our knowledge these are the first examples of MOFs of their structural types formed by linear dicarboxylates instead of trigonal tricarboxylates or tetrahedral tetracarboxylates. Compounds 2, 3 and 4 also feature three dimensional networks with linear rod-shaped SBUs with the Ba2+ MOF 3 displaying an sra rod-net and MOFs 2 and 4 showing very complex rod-nets with so far unique topologies. Fluorescence studies revealed that the free ligands exhibit strong blue-green emission displaying considerable positive solvatochromism thereby pointing towards charge transfer excited states involving the shift of electron density from the amino groups to the aromatic core. Correspondingly, the MOFs display ligand based fluorescence with small differences in emission maxima possibly attributable to the difference in the charge density of the metal ions combined with the different environments around ligands in the crystal structures.

Dual Emission in a Ligand and Metal Co-Doped Lanthanide-Organic Framework: Color Tuning and Temperature Dependent Luminescence

In this study, we report the luminescence color tuning in the lanthanide metal-organic framework (LnMOF) ([La(bpdc)Cl(DMF)] (1); bpdc^2- = [1,1'-biphenyl]-4,4'-dicarboxylate, DMF = N,N-dimethylformamide) by introducing dual emission properties in a La³⁺ MOF scaffold through doping with the blue fluorescent 2,2'-diamino-[1,1'-biphenyl]-4,4'-dicarboxylate (dabpdc^2-) and the red emissive Eu³⁺. With a careful adjustment of the relative doping levels of the lanthanide ions and bridging ligands, the color of the luminescence was modulated, while at the same time the photophysical characteristics of the two chromophores were retained. In addition, the photophysical properties of the parent MOF (1) and its doped counterparts with various dabpdc^2-/bpdc^2- and Eu³⁺/La³⁺ ratios and the photoinduced energy transfer pathways that are possible within these materials are discussed. Finally, the temperature dependence study on the emission profile of a doped analogue containing 10% dabpdc^2- and 2.5% Eu³⁺ (7) is presented, highlighting the potential of this family of materials to behave as temperature sensors.

Luminescent metal–organic frameworks as chemical sensors: common pitfalls and proposed best practices

In this review we approach the emerging field of sensors based on luminescent metal–organic frameworks from the perspective of the most commonly encountered pitfalls and we suggest best practices so that they can be avoided.

A microporous Mg2+ MOF with cation exchange properties in a single-crystal-to-single-crystal fashion

A microporous Mg²⁺ MOF shows single-crystal-to-single-crystal exchange of Mg²⁺ by Cu²⁺ ions.

Synthesis and analysis of the anticancer activity of Ru(ii) complexes incorporating 2-hydroxymethylidene-indene-1,3-dione ligands

Piano-stool Ru(ii) complexes incorporating 2-hydroxymethylidene-indene-1,3-dione ligands exhibit promising anticancer activity against four human ovarian cancer cell lines.

Correction: Selective capture of hexavalent chromium from an anion-exchange column of metal organic resin-alginic acid composite

Correction for ‘Selective capture of hexavalent chromium from an anion-exchange column of metal organic resin–alginic acid composite’ by Sofia Rapti et al., Chem. Sci., 2016, DOI: 10.1039/c5sc03732h.

Selective capture of hexavalent chromium from an anion-exchange column of metal organic resin-alginic acid composite

We report an anion exchange composite material based on a protonated amine-functionalized metal-organic framework, denoted Metal Organic Resin-1 (MOR-1), and alginic acid (HA). MOR-1-HA material shows an exceptional capability to rapidly and selectively sorb Cr(vi) under a variety of conditions and in the presence of several competitive ions. The selectivity of MOR-1-HA for Cr(vi) is shown to be the result of strong O3CrVI···NH2 interactions. The composite sorbent can be successfully utilized in an ion-exchange column, in contrast to pristine MOR-1 which forms fine suspensions in water passing through the column. Remarkably, an ion exchange column with only 1% wt MOR-1-HA and 99% wt sand (an inert and inexpensive material) is capable of reducing moderate and trace Cr(vi) concentrations to well below the acceptable safety limits for water. The relatively low cost of MOR-1-HA/sand column and its high regeneration capability and reusability make it particularly attractive for application in the remediation of Cr(vi)-bearing industrial waste.

Turn-on luminescence sensing and real-time detection of traces of water in organic solvents by a flexible metal-organic framework

The development of efficient sensors for the determination of the water content in organic solvents is highly desirable for a number of chemical industries. Presented herein is a Mg(2+) metal-organic framework (MOF), which exhibits the remarkable capability to rapidly detect traces of water (0.05-5 % v/v) in various organic solvents through an unusual turn-on luminescence sensing mechanism. The extraordinary sensitivity and fast response of this MOF for water, and its reusability make it one of the most powerful water sensors known.

Electron vs energy transfer in arrays featuring two Bodipy chromophores axially bound to a Sn(IV) porphyrin via a phenolate or benzoate bridge

In this report we describe the synthesis of multichromophore arrays consisting of two Bodipy units axially bound to a Sn(IV) porphyrin center either via a phenolate (3) or via a carboxylate (6) functionality. Absorption spectra and electrochemical studies show that the Bodipy and porphyrin chromophores interact weakly in the ground state. However, steady-state emission and excitation spectra at room temperature reveal that fluorescence from both the Bodipy and the porphyrin of 3 are strongly quenched suggesting that, in the excited state, energy and/or electron transfer might occur. Indeed, as transient absorption experiments show, selective excitation of Bodipy in 3 results in a rapid decay (τ ≈ 2 ps) of the Bodipy-based singlet excited state and a concomitant rise of a charge-separated state evolving from the porphyrin-based singlet excited state. In contrast, room-temperature emission studies on 6 show strong quenching of the Bodipy-based fluorescence leading to sensitized emission from the porphyrin moiety due to a transduction of the singlet excited state energy from Bodipy to the porphyrin. Emission experiments at 77 K in frozen toluene reveal that the room-temperature electron transfer pathway observed in 3 is suppressed. Instead, Bodipy excitation in 3 and 6 results in population of the first singlet excited state of the porphyrin chromophore. Subsequently, intersystem crossing leads to the porphyrin-based triplet excited state.

Ligand-field excited states of hexacyanochromate and hexacyanocobaltate as sensitisers for near-infrared luminescence from Nd(iii) and Yb(iii) in cyanide-bridged d-f assemblies

Crystallisation of [Co(CN)(6)](3-) or [Cr(CN)(6)](3-) with Ln(iii) salts (Ln = Nd, Gd, Yb) from aqueous dmf afforded the cyanide-bridged d/f systems [Ln(dmf)(4)(H(2)O)(3)(micro-CN)Co(CN)(5)] (-, discrete dinuclear species) and {[Cr(CN)(4)(micro-CN)(2)Ln(H(2)O)(2)(dmf)(4)]}(infinity) (-, infinite cyanide-bridged chains with alternating Cr and Ln centres). With Ln = Gd the characteristic long-lived phosphorescence from d-d excited states of the [M(CN)(6)](3-) units was apparent in the red region of the spectrum, with lifetimes of the order of 1 micros, since the heavy atom effect of the Gd(iii) promotes inter-system crossing at the [M(CN)(6)](3-) units to generate the phosphorescent spin-forbidden excited states. With Ln = Yb or Nd however, the d-block luminescence was completely quenched due to fast (>10(8) s(-1)) energy-transfer to the Ln(iii) centre, resulting in the characteristic sensitised emission from Yb(iii) and Nd(iii) in the near-IR region. For both - and -, calculations based on spectroscopic overlap between emission of the donor (Co) and absorption of the acceptor (Ln) suggest that the Dexter energy-transfer mechanism is responsible for the complete quenching that we observe.

Structural and Photophysical Properties of Adducts of [Ru(bipy)(CN)4]2- with Different Metal Cations: Metallochromism and Its Use in Switching Photoinduced Energy Transfer

We show in this paper how the 3MLCT luminescence of [Ru(bipy)(CN)4]2-, which is known to be highly solvent-dependent, may be varied over a much wider range than can be achieved by solvent effects, by interaction of the externally directed cyanide ligands with additional metal cations both in the solid state and in solution. A series of crystallographic studies of [Ru(bipy)(CN)4]2- salts with different metal cations Mn+ (Li+, Na+, K+, mixed Li+/K+, Cs+, and Ba2+) shows how the cyanide/Mn+ interaction varies from the conventional "end-on" with the more Lewis-acidic cations (Li+, Ba2+) to the more unusual "side-on" interaction with the softer metal cations (K+, Cs+). The solid-state luminescence intensity and lifetime of these salts is highly dependent on the nature of the cation, with Cs+ affording the weakest luminescence and Ba2+ the strongest. A series of titrations of the more soluble derivative [Ru(tBu2bipy)(CN)4]2- in MeCN with a range of metal salts showed how the cyanide/Mn+ association results in a substantial blue-shift of the 1MLCT absorptions, and 3MLCT energies, intensities, and lifetimes, with the complex varying from essentially non-luminescent in the absence of metal cation to showing strong (phi = 0.07), long-lived (1.4 micros), and high-energy (583 nm) luminescence in the presence of Ba2+. This modulation of the 3MLCT energy, over a range of about 6000 cm-1 depending on the added cation, could be used to reverse the direction of photoinduced energy transfer in a dyad containing covalently linked [Ru(bipy)3]2+ and [Ru(bipy)(CN)4]2- termini. In the absence of a metal cation, the [Ru(bipy)(CN)4]2- terminus has the lower 3MLCT energy and thereby quenches the [Ru(bipy)3]2+-based luminescence; in the presence of Ba2+ ions, the 3MLCT energy of the [Ru(bipy)(CN)4]2- terminus is raised above that of the [Ru(bipy)3]2+ terminus, resulting in energy transfer to and sensitized emission from the latter.

Luminescent Pt(II)(bipyridyl)(diacetylide) chromophores with pendant binding sites as energy donors for sensitised near-infrared emission from lanthanides: structures and photophysics of Pt(II)/Ln(III) assemblies

The complexes [Pt(bipy){CC-(4-pyridyl)}(2)] (1) and [Pt(tBu(2)bipy){CC-(4-pyridyl)}(2)] (2) and [Pt(tBu(2)-bipy)(CC-phen)(2)] (3) all contain a Pt(bipy)(diacetylide) core with pendant 4-pyridyl (1 and 2) or phenanthroline (3) units which can be coordinated to {Ln(diketonate)(3)} fragments (Ln = a lanthanide) to make covalently-linked Pt(II)/Ln(III) polynuclear assemblies in which the Pt(II) chromophore, absorbing in the visible region, can be used to sensitise near-infrared luminescence from the Ln(III) centres. For 1 and 2 one-dimensional coordination polymers [1Ln(tta)(3)](infinity) and [2Ln(hfac)(3)](infinity) are formed, whereas 3 forms trinuclear adducts [3{Ln(hfac)(3)}(2)] (tta=anion of thenoyl-trifluoroacetone; hfac=anion of hexafluoroacetylacetone). Complexes 1-3 show typical Pt(II)-based (3)MLCT luminescence in solution at approximately 510 nm, but in the coordination polymers [1Ln(tta)(3)](infinity) and [2Ln(hfac)(3)](infinity) the presence of stacked pairs of Pt(II) units with short PtPt distances means that the chromophores have (3)MMLCT character and emit at lower energy ( approximately 630 nm). Photophysical studies in solution and in the solid state show that the (3)MMLCT luminescence in [1Ln(tta)(3)](infinity) and [2Ln(hfac)(3)](infinity) in the solid state, and the (3)MLCT emission of [3{Ln(hfac)(3)}(2)] in solution and the solid state, is quenched by Pt-->Ln energy transfer when the lanthanide has low-energy f-f excited states which can act as energy acceptors (Ln=Yb, Nd, Er, Pr). This results in sensitised near-infrared luminescence from the Ln(III) units. The extent of quenching of the Pt(II)-based emission, and the Pt-->Ln energy-transfer rates, can vary over a wide range according to how effective each Ln(III) ion is at acting as an energy acceptor, with Yb(III) usually providing the least quenching (slowest Pt-->Ln energy transfer) and either Nd(III) or Er(III) providing the most (fastest Pt-->Ln energy transfer) according to which one has the best overlap of its f-f absorption manifold with the Pt(II)-based luminescence.

Anthracene as a sensitiser for near-infrared luminescence in complexes of Nd(iii), Er(iii) and Yb(iii): an unexpected sensitisation mechanism based on electron transfer

The ligand L(1), which contains a chelating 2-(2-pyridyl)benzimidazole (PB) unit with a pendant anthacenyl group An connected via a methylene spacer, (L(1) = PB-An), was used to prepare the 8-coordinate lanthanide(III) complexes [Ln(hfac)(3)(L(1))] (Ln = Nd, Gd, Er, Yb) which have been structurally characterised and all have a square antiprismatic N(2)O(6) coordination geometry. Whereas free L(1) displays typical anthracene-based fluorescence, this fluorescence is completely quenched in its complexes. The An group in L(1) acts as an antenna unit: in the complexes [Ln(hfac)(3)(L(1))] (Ln = Nd, Er, Yb) selective excitation of the anthracene results in sensitised near-infrared luminescence from the lanthanide centres with concomitant quenching of An fluorescence. Surprisingly, the anthracene fluorescence is also quenched even in the Gd(III) complex and in its Zn(II) adduct in which quenching via energy transfer to the metal centre is not possible. It is proposed that the quenching of anthracene fluorescence in coordinated L(1) arises due to intra-ligand photoinduced electron-transfer from the excited anthracene chromophore (1)An* to the coordinated PB unit generating a short-lived charge-separated state [An(.+)-PB(.-)] which collapses by back electron-transfer to give (3)An*. This electron-transfer step is only possible upon coordination of L(1) to the metal centre, which strongly increases the electron acceptor capability of the PB unit, such that (1)An* --> PB PET is endoergonic in free L(1) but exergonic in its complexes. Thus, rather than a conventional set of steps ((1)An* -->(3)An* --> Ln), the sensitization mechanism now includes (1)An* --> PB photoinduced electron transfer to generate charge-separated [An(.+)-PB(.-)], then back electron-transfer to generate (3)An* which finally sensitises the Ln(III) centre via energy transfer. The presence of (3)An* in L(1) and its complexes is confirmed by nanosecond transient absorption studies, which have also shown that the (3)An* lifetime in the Nd(III) complex matches the rise time of Nd-centred near-infrared emission, confirming that the final step of the sequence is (3)An* --> Ln(III) energy-transfer.

Luminescent complexes of Re(I) and Ru(II) with appended macrocycle groups derived from 5,6-dihydroxyphenanthroline: cation and anion binding

A range of ligands in which a macrocyclic unit is fused to a 1,10-phenanthroline unit has been prepared starting from 5,6-dihydroxyphenanthroline. The ligands are L1 in which the pendant ligand is 18-crown-6; L2, in which the pendant ligand is benzo-24-crown-8; and L(3), in which the macrocycle contains two carboxamide units. Ligands L1 and L2 can bind Group 1 and 2 metal cations in their crown-ether cavities; L3 contains two H-bond (amide) donors and is suitable for anion-binding. Luminescent complexes of the form [Ru(bipy)2L]2+, [ReL(CO)3Cl] and [RuL(CN)4]2- were prepared and some were structurally characterised; their interactions with various guest species were investigated by luminescence and NMR spectroscopy. For complexes with the crown ethers (L1 and L2), binding of K+ was rather weak, but the electrostatic effect due to the charge on the host complex was clear with [RuL1(CN)4]2- binding K+ more strongly than [Ru(bipy)2L1]2+. Binding to the pendant crown ethers was much stronger with Ba2+, and both [ReL1(CO)3Cl] and [ReL2(CO)3Cl] showed substantial luminescence quenching in MeCN on addition of Ba2+ ions, with binding constants of 4.5 x 10(4) M(-1) for [ReL1(CO)3Cl]/Ba2+ and 1.3 x 10(5) M(-1) for [ReL2(CO)3Cl]/Ba2+. Complexes [Ru(bipy)2L3]2+ and [ReL3(CO)3Cl], due to their H-bond donor sites, showed binding of dihydrogenphosphate to the macrocycle. Whereas [ReL3(CO)3Cl] showed 1 : 1 binding with (H2PO4)- in dmso with a binding constant of 65 M(-1), [Ru(bipy)2L3]2+ showed 1 : 2 binding, with microscopic association constants of ca. 1 x 10(6) and 1.6 x 10(6) M(-1) in MeCN. The fact that K2 > K1 suggests a cooperative interaction whereby binding of the first anion makes binding of the second one easier to an extent which overcomes electrostatic effects, and a model for this is proposed which also accounts for the substantial increase in luminescence from [Ru(bipy)2L3]2+ (5-fold enhancement) when the second (H2PO4)- anion binds. Both [Ru(bipy)2L3]2+ and [ReL3(CO)3Cl] undergo complete luminescence quenching and a change in colour to near-black in the presence of (anhydrous) fluoride in MeCN, probably due to deprotonation of the carboxamide group. These changes are however irreversible on a long timescale and lead to slow decomposition.

Syntheses and Crystal Structures of Dinuclear Complexes Containing d-Block and f-Block Luminophores. Sensitization of NIR Luminescence from Yb(III), Nd(III), and Er(III) Centers by Energy Transfer from Re(I)− and Pt(II)−Bipyrimidine Metal Centers

Mononuclear complexes [Re(bpym)(CO)(3)Cl] and [Pt(bpym)(CC-C(6)H(4)CF(3))(2)] (bpym = 2,2'-bipyrimidine), in which one of the bipyrimidine sites is vacant, have been used as "complex ligands" to prepare heterodinuclear d-f complexes in which a lanthanide tris(1,3-diketonate) unit is attached to the secondary bipyrimidine site to evaluate the ability of d-block chromophores to act as antennae for causing sensitized near-infrared (NIR) luminescence from adjacent lanthanide(III) centers. The two sets of complexes so prepared are [Re(CO)(3)Cl(mu-bpym)Ln(fod)(3)] (abbreviated as Re-Ln; where Ln = Yb, Nd, Er) and [(F(3)C-C(6)H(4)-CC)(2)Pt(mu-bpym)Ln(hfac)(3)] (abbreviated as Pt-Ln; where Ln = Nd, Gd). Members of both series have been structurally characterized; the metal-metal separation across the bipyrimidine bridge is approximately 6.3 A in each case. In these complexes, the (3)MLCT (MLCT = metal to ligand charge-transfer) luminescences of the mononuclear [Re(bpym)(CO)(3)Cl] and [Pt(bpym)(CC-C(6)H(4)CF(3))(2)] complexes are quenched by energy transfer to those lanthanides (Ln = Yb, Nd, Er) that have low-lying f-f states capable of NIR luminescence; as a result, sensitized NIR luminescence is seen from the lanthanide center following excitation of the d-block unit. In the solid state, quenching of the luminescence from the d-block chromophore is complete, indicating efficient d --> f energy transfer, as a result of the short metal-metal separation across the bipyrimidine bridge. In a CH(2)Cl(2) solution, partial dissociation of the dinuclear complexes into the mononuclear units occurs, with the result that some (3)MLCT luminescence is observed from mononuclear [Re(bpym)(CO)(3)Cl] or [Pt(bpym)(CC-C(6)H(4)CF(3))(2)] present in the equilibrium mixture. Solution UV-vis and luminescence titrations, carried out by the addition of portions of Ln(fod)(3)(H(2)O)(2) or Ln(hfac)(3)(H(2)O)(2) to the d-block complex ligands, indicate that binding of the lanthanide tris(1,3-diketonate) unit at the secondary bipyrimidine site to give the d-f dinuclear complexes occurs with an association constant of ca. 10(5) M(-)(1).