Co(II) Macrocyclic Complexes with Amide-Glycinate Pendants as ParaCEST and Liposomal CEST Agents.

Macrocyclic Co(II) complexes with appended amide-glycinate groups were prepared to develop paramagnetic Co(II) chemical exchange saturation transfer (CEST) agents of reduced overall charge. Complexes with reduced charge and lowered osmolarity are important for their loading into liposomes and to provide complexes that are highly water soluble and well tolerated in animals. Co(L1) has two non-coordinating benzyl groups and two amide-glycinate pendants, whereas Co(L2) has two unsubstituted amide pendants and two amide-glycinate pendants on cyclam (1,4,8,11-tetraazacyclododecane). The 1H NMR spectrum of Co(L1) is consistent with a single cis-pendant isomer with both amide protons in the trans-configuration, as supported by an X-ray crystal structure. Co(L2) has a mixture of different isomers in solution, including the trans-1,4 and 1,8 pendant isomers. The Z-spectrum of Co(L1) shows one highly-shifted CEST peak, whereas Co(L2) exhibits six CEST peaks. Encapsulation of 40 mM Co(L1) in a liposome with osmotically-induced shrinking at 300 mOsm/L produces a liposomal CEST agent with saturation frequency offset of 3 ppm. Addition of the amphiphilic 1,4,7-triazacyclononane-based complex Co(L5) to the liposomal bilayer at 18 mM with Co(L1) encapsulated in the liposome at 50 mM changes the sign and increases the magnitude of the saturation frequency offset to -7.5 ppm at 300 mOsm/L.

Manganese(III) Nitrate Complexes as Bench-Stable Powerful Oxidants.

We report herein a convenient one-pot synthesis for the shelf-stable molecular complex [Mn(NO3)3(OPPh3)2] (2) and describe the properties that make it a powerful and selective one-electron oxidation (deelectronation) reagent. 2 has a high reduction potential of 1.02 V versus ferrocene (MeCN) (1.65 vs normal hydrogen electrode), which is one the highest known among readily available redox agents used in chemical synthesis. 2 exhibits stability toward air in the solid state, can be handled with relative ease, and is soluble in most common laboratory solvents such as MeCN, dichloromethane, and fluorobenzene. 2 is substitutionally labile with respect to the coordinated (pseudo)halide ions enabling the synthesis of other new Mn(III) nitrato complexes also with high reduction potentials ranging from 0.6 to 1.0 V versus ferrocene.

Macrocyclic complexes of Fe(III) with mixed hydroxypropyl and phenolate or amide pendants as T1 MRI probes.

High-spin Fe(III) complexes of 1,4,7-triazacyclononane (TACN) with mixed oxygen donor pendants including hydroxypropyl, phenolate or amide groups are prepared for study as T1 MRI probes. Complexes with two hydroxypropyl pendants and either amide (Fe(TOAB)) or phenolate (Fe(PTOB)) groups are compared to an analog with three hydroxypropyl groups (Fe(NOHP)), in order to study the effect of the third pendant on the coordination sphere as probed by solution chemistry, relaxivity and structural studies. Solution studies show that Fe(PTOB) has two ionizations with the phenol pendant deprotonating with a pKa of 1.7 and a hydroxypropyl pendent with pKa of 6.3. The X-ray crystal structure of [Fe(PTOB)]Br2 features a six-coordinate complex with two bound hydroxypropyl groups, and a phenolate in a distorted octahedral geometry. The Fe(TOAB) complex has a single deprotonation, assigned to a hydroxypropyl group with a pKa value of 7.0. Both complexes are stabilized as high-spin Fe(III) in solution as shown by their effective magnetic moments and Fe(III)/Fe(II) redox potentials of -390 mV and -780 mV versus NHE at pH 7 and 25 °C for Fe(TOAB) and Fe(PTOB) respectively. Both Fe(PTOB) and Fe(TOAB) are kinetically inert to dissociation under a variety of challenges including phosphate/carbonate buffer, one equivalent of ZnCl2, two equivalents of transferrin or 100 mM HCl, or at basic pH values over 24 h at 37 °C. The r1 relaxivity of Fe(TOAB) at 1.4 T, pH 7.4 and 33 °C is relatively low at 0.6 mM-1 s-1 whereas the r1 relaxivity of Fe(PTOB) is more substantial and shows an increase of 2.5 fold to 2.5 mM-1 s-1 at acidic pH. The increase in relaxivity at acidic pH is attributed to protonation of the phenolate group to provide an additional pathway for proton relaxation.

Electrocatalytic Production of Hydrogen Peroxide Enabled by Post-Synthetic Modification of a Self-Assembled Porphyrin Cube.

Self-assembled metallacyles and cages formed via coordination chemistry have been used as catalysts to enforce 4H+/4e- reduction of oxygen to water with an emphasis on attenuating the formation of hydrogen peroxide. That said, the kinetically favored 2H+/2e- reduction to H2O2 is critically important to industry. In this work we report the synthesis, characterization, and electrochemical benchmarking of a hexa-porphyrin cube which catalyses the electrochemical reduction of molecular oxgyen to hydrogen peroxide. An established sub-component self-assembly approach was used to synthesize the cubic free-base porphryin topologies from 2-pyridinecarboxaldehyde, tetra-4-aminophenylporphryin (TAPP), and Fe(OTf)2 (OTf- = trifluoromethansulfonate). Then, a tandem metalation/transmetallation was used to introduce Co(II) into the porphyrin faces of the cube, and exchange with the Fe(II) cations at the vertices, furnishing a tetrakaideca cobalt cage. Electron paramagnetic resonance studies on a Cu(II)/Fe(II) analogue probed radical interactions which inform on electronic structure. The efficacy and selectivity of the CoCo-cube as a catalyst for hydrogen peroxide generation was investigated using hydrodynamic voltammetry, revealing a higher selectivity than that of a mononuclear Co(II) porphyrin (83% versus ~50%) with orders of magnitude enhancement in standard rate constant (ks = 2.2 × 102 M-1s-1 versus ks = 3 × 100 M-1s-1). This work expands the use of coordination-driven self-assembly beyond ORR to water by exploiting post-synthetic modification and structural control that is associated with this synthetic method.

Investigations of Nanoparticle Suspensions of Prussian Blue and Its Copper Analogue: Amine Functionalization and Electrochemical Studies.

Prussian blue (PB) and its analogues are promising materials for electrochemical energy storage, yet their use in flow-type devices is limited by their lack of redox responsiveness as colloidal suspensions. We have investigated the redox chemistry amine functionalization of PB along with its Cu analogue (CuPBA). No redox response of colloidal PB was observed and suspensions of CuPBA formed films on electrode surfaces with and without applied potentials; the films were redox-active but the material that remained suspended in solution did not participate in redox chemistry. Propylamine (pa), ethylenediamine (en), or tetramethylethylenediamine (TMEDA) were added in an attempt to maintain well dispersed suspensions through nanoparticle surface functionalization. Propylamine modifications resulted in a loss of the CuPBA network and subsequent precipitation of insoluble materials. Coordination of ethylenediamine prompted the formation of Cu and Fe monomers ([Cu(en)2]m+/[Fe(CN)6]n-]) that remained soluble in aqueous electrolytes. In the absence of supporting electrolytes, these monomers formed a one-dimensional (1D) polymeric structure (Cu2Fe-1D). TMEDA modification preserved the CuPBA extended structure with only modest precipitate formation over 30 min. The redox responsiveness of these suspensions depended on conditions; in 1 M KCl, no redox chemistry was observed for the CuPBA. In pH 4 potassium hydrogen phthalate buffer, a signal was observed that was attributed to the Fe centers of CuPBA. Under these conditions, the material precipitated in ∼15 min and the signal was lost. Although the Fe centers in these networks are redox-active, additional work is needed to realize longer-term redox activity and stability. Ligand modifications can alter the properties of these networks but within a given ligand class, e.g., amines, the effects can vary greatly from the deconstruction of the framework to preventing film formation.

Lowering the Symmetry of Cofacial Porphyrin Prisms for Selective Oxygen Reduction Electrocatalysis.

Cofacial porphyrin catalysts for the Oxygen Reduction Reaction (ORR) formed via coordination-driven self-assembly have so far been limited to designs with fourfold symmetry, where four molecular clips bridge two porphyrin sites. We have synthesized six PynPhm (Py = pyridyl, Ph = phenyl) metalloporphyrin prisms (Co2+, Zn2+) bridged by molecular clips containing two Rh3+ centers. Four of these structures are lower symmetry, with the Py3Ph and Py2Ph2 prisms containing three and two molecular clips, respectively. The Co2+ species were evaluated for their ORR activity. Cyclic and hydrodynamic voltammetry studies of heterogeneous catalyst inks in aqueous media revealed marked differences in selectivity from ∼5% (Py3Ph) to ∼37% (Py2Ph2) for the formation of H2O2. The single-crystal X-ray structure of the Zn2 Py2Ph2 prism shows an offset between the porphyrin faces. This structural feature may be responsible for the change in selectivity, consistent with previous studies of covalently tethered cofacial porphyrins that have shown that geometry is a critical determinant of two-electron/two-proton versus four-electron/four-proton pathways. Extraction of standard rate constants ks for the ORR revealed a cofacial enhancement of ∼2 orders of magnitude over mononuclear Co2+ tetrapyridyl porphyrin. Even though all the prisms described here use the same molecular clip, the resultant structures, and thus the reactivity for the ORR, differ significantly based on the number and orientation of pyridyl donor groups on the porphyrins, highlighting how coordination-driven self-assembly can be used to rapidly tune dinuclear catalysts.

Altering the solubility of metal-organic polyhedra via pendant functionalization of Cp3Zr3O(OH)3 nodes.

The chemistry of zirconium-based metal-organic polyhedra (ZrMOPs) is often limited by their poor solubilities. Despite their attractive features-including high yielding and facile syntheses, predictable topologies, high stability, and tunability-problematic solubilities have caused ZrMOPs to be under-studied and under-applied. Although these cages have been synthesized with a wide variety of carboxylate-based bridging ligands, we explored a new method for ZrMOP functionalization via node-modification, which we hypothesized could influence solubility. Herein, we report ZrMOPs with benzyl-, vinylbenzyl-, and trifluoromethylbenzyl-pendant groups decorating cyclopentadienyl moieties. The series was characterized by 1H/19F NMR, high-resolution mass spectrometry, infrared spectroscopy, and single-crystal X-ray diffraction. The effects of node functionalities on ZrMOP solubility were quantified using inductively coupled plasma mass spectrometry. Substitution caused a decrease in water solubility, but for certain organic solvents, e.g. DMF, solubility could be enhanced by ∼20×, from 16 µM for the unfunctionalized cage to 310 µM for the vinylbenzyl- and trifluoromethylbenzyl-cages.

The rigidity of self-assembled cofacial porphyrins influences selectivity and kinetics of oxygen reduction electrocatalysis.

We report the electrocatalytic Oxygen Reduction Reaction on a rigid Co(II) porphyrin prism scaffold bridged by Ag(I) ions. The reactivity of this scaffold differs significantly from previous prism catalysts in that its selectivity is similar to that of monomer (∼35% H2O) yet it displays sluggish kinetics, with an order of magnitude lower ks of ∼0.5 M-1 s-1. The deleterious cofacial effect is not simply due to metal-metal separation, which is similar to our most selective prism catalysts. Instead we conclude the structural rigidity is responsible for these differences.

Metal-Organic Polyhedron with Four Fe(III) Centers Producing Enhanced T1 Magnetic Resonance Imaging Contrast in Tumors.

A metal-organic polyhedron (MOP) with four paramagnetic Fe(III) centers was studied as a magnetic resonance imaging (MRI) probe. The MOP was characterized in solution by using electron paramagnetic resonance (EPR), UV-visible (UV-vis) spectroscopies, Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry, and in the solid state with single-crystal X-ray diffraction. Water proton T1 relaxation properties were examined in solution and showed significant enhancement in the presence of human serum albumin (HSA). The r1 relaxivities in the absence and presence of HSA were 8.7 mM-1 s-1 and 21 mM-1 s-1, respectively, per molecule (2.2 mM-1 s-1 and 5.3 mM-1 s-1 per Fe) at 4.7 T, 37 °C. In vivo studies of the iron MOP show strong contrast enhancement of the blood pool even at a low dose of 0.025 mmol/kg with prolonged residence in vasculature and clearance through the intestinal tract of mice. The MOP binds strongly to serum albumin and shows comparable accumulation in a murine tumor model as compared to a covalently linked Gd-HSA contrast agent.

Tuning the Reactivity of Cofacial Porphyrin Prisms for Oxygen Reduction Using Modular Building Blocks.

We assembled eight cofacial porphyrin prisms using MTPyP (M = Co(II) or Zn(II), TPyP = 4-tetrapyridylporphyrin) and functionalized ruthenium-based "molecular clips" using coordination-driven self-assembly. Our approach allows for the rapid synthesis of these architectures in isolated yields as high as 98% for the assembly step. Structural and reactivity studies provided a deeper understanding of the role of the building blocks on the oxygen reduction reaction (ORR). Catalytic efficacy was probed by using cyclic and hydrodynamic voltammetry on heterogeneous catalyst inks in aqueous media. The reported prisms showed outstanding selectivity (>98%) for the kinetically hindered 4e-/4H+ reduction of O2 to H2O over the kinetically more accessible 2e-/2H+ reduction to H2O2. Furthermore, we have demonstrated significant cofacial enhancement in the observed catalytic rate constant ks (∼5 orders of magnitude) over the mononuclear analogue. We conclude that the steric bulk of the clip plays an important role in the structural dynamics of these prisms, which in turn modulates the ORR reactivity with respect to selectivity and kinetics.

A hemilabile manganese(I)-phenol complex and its coordination induced O-H bond weakening.

The known compound K[(PO)2Mn(CO)2] (PO = 2-((diphenylphosphino)methyl)-4,6-dimethylphenolate) (K[1]) was protonated to form the new Mn(i) complex (HPO)(PO)Mn(CO)2 (H1) and was determined to have a pKa approximately equal to tetramethylguanidine (TMG). The reduction potential of K[1] was determined to be -0.58 V vs. Fc/Fc+ in MeCN and allowed for an estimation of an experimental O-H bond dissociation free energy (BDFEO-H) of 73 kcal mol-1 according to the Bordwell equation. This value is in good agreement with a corrected DFT computed BDFEO-H of 68.0 kcal mol-1 (70.3 kcal mol-1 for intramolecular H-bonded isomer). The coordination of the protonated O-atom in the solid-state H1 was confirmed using FTIR spectroscopy and X-ray crystallography. The phenol moiety is hemilabile as evident from computation and experimental results. For instance, dissociation of the protonated O-atom in H1 is endergonic by only a few kcal mol-1 (DFT). Furthermore, [1]- and other Mn(i) compounds coordinated to PO and/or HPO do not react with MeCN, but H1 reacts with MeCN to form H1+MeCN. Experimental evidence for the solution-bound O-atoms of H1 was obtained from 1H NMR and UV-vis spectroscopy and by comparing the electronic spectra of bona fide 16-e- Mn(i) complexes such as [{PNP}Mn(CO)2] (PNP = -N{CH2CH2(PiPr2)}2) and [(Me3SiOP)(PO)Mn(CO)2] (Me3Si1). Compound H1 is only meta-stable (t1/2 0.5-1 day) and decomposes into products consistent with homolytic O-H bond cleavage. For instance, treatment of H1 with TEMPO resulted in formation of TEMPOH, free ligand, and [MnII{(PO)2Mn(CO)2}2]. Together with the experimental and calculated weakened BDFEO-H, these data provide strong evidence for the coordination and hemilability of the protonated O-atom in H1 and represents the first example of the phenolic Mn(i)-O linkage and a rare example of a "soft-homolysis" intermediate in the bond-weakening catalysis paradigm.

Exploring Inner-Sphere Water Interactions of Fe(II) and Co(II) Complexes of 12-Membered Macrocycles To Develop CEST MRI Probes.

Several paramagnetic Co(II) and Fe(II) macrocyclic complexes were prepared with the goal of introducing a bound water ligand to produce paramagnetically shifted water 1H resonances and for paramagnetic chemical exchange saturation transfer (paraCEST) applications. Three 12-membered macrocycles with amide pendent groups including 1,7-bis(carbamoylmethyl)-1,4,7,10-tetraazacyclodocane (DCMC), 4,7,10-tris(carbamoylmethyl)-,4,7,10-triaza-12-crown-ether (N3OA), and 4,10-bis(carbamoylmethyl)-4,10-diaza-12-crown-ether (NODA) were prepared and their Co(II) complexes were characterized in the solid state and in solution. The crystal structure of [Co(DCMC)]Br2 featured a six-coordinated Co(II) center with distorted octahedral geometry, while [Co(NODA)(OH2)]Cl2 and [Co(N3OA)](NO3)2 were seven-coordinated. The analogous Fe(II) complexes of NODA and NO3A were successfully prepared, but the complex of DCMC oxidized rapidly to the Fe(III) form. Similarly, [Fe(NODA)]2+ oxidized over several days, forming crystals of the Fe(III) complex isolated as the µ-O bridged dimer. Magnetic susceptibility values and paramagnetic NMR spectra of the Fe(II) complexes of NODA and N3OA, as well as Co(II) complexes of DCMC, NODA, and N3OA, were consistent with high spin complexes. CEST peaks ranging from 60 ppm to 70 ppm, attributed to NH groups of the amide pendents, were identified. Variable-temperature 17O NMR spectra of Co(II) and Fe(II) NODA complexes were consistent with rapid exchange of the water ligand with bulk water. Notably, the Co(II) and Fe(II) complexes presented here produced substantial paramagnetic shifts of bulk water 1H resonances, independent of having an inner-sphere water.

Tuning the Activity of Heterogeneous Cofacial Cobalt Porphyrins for Oxygen Reduction Electrocatalysis through Self-Assembly.

We report a suite of coordination-driven self-assembled prisms for heterogeneous electrocatalytic oxygen reduction (ORR) differing in the molecular clips linking two porphyrin faces in a cofacial arrangement. ORR activities and selectivities of monomeric CoTPyP along with cofacial prisms Ox-Co, Oxa-Co, and Benzo-Co were probed using cyclic voltammetry and rotating ring-disk techniques. All species were immobilized as heterogeneous catalysts on glassy carbon electrodes using a Nafion ink method. The selectivities of Ox-Co, Oxa-Co, and Benzo-Co prisms towards H2 O as determined by RRDE were 87, 97, and 75 %, respectively. The current density of the Oxa-Co plateaus at five times that of Pt/C when normalized per Co/Pt. The high synthetic yield (79 %), competitive overpotential (η ≈800 mV) and high selectivity (%H2 O ≈97 %) of the Oxa-Co highlights how self-assembly can be used to address multi-electron multi-proton transformations using polynuclear catalysts.

Highly Emissive Self-Assembled BODIPY-Platinum Supramolecular Triangles.

Light-emitting supramolecular coordination complexes (SCCs) have been widely studied for applications in the chemical and biological sciences. Herein, we report the coordination-driven self-assembly of two highly emissive platinum(II) supramolecular triangles (1 and 2) containing BODIPY-based bridging ligands. The metallacycles exhibit favorable anticancer activities against HeLa cells (IC50 of 6.41 and 2.11 µM). The characteristic ∼570 nm fluorescence of the boron dipyrromethene (BODIPY) moieties in the metallacycles permits their intracellular visualization using confocal microscopy. Additionally, the BODIPY fluorophore is an excellent photodynamic agent, making the metallacycles as ideal therapeutics for photodynamic therapy (PDT) and chemotherapy. In vitro studies demonstrate that the combination indexes against HeLa cells are 0.56 and 0.48 for 1 and 2, respectively, confirming their synergistic anticancer effect. More importantly, these SCCs also exhibit superior anticancer efficacy toward cisplatin-resistant A2780cis cell line by combining PDT and chemotherapy, showing promise in overcoming drug resistance. This study exploits a multicomponent approach to self-assembled metallacages that enables design of effective theranostic agents wherein the platinum acceptors are toxic chemotherapeutics and the BODIPY donors are imaging probes and photosensitizers. Since each piece may be independently tuned, i.e., Pt(II) polypyridyl fragment swapped for Pt(II) phosphine, the activity may be optimized without a total redesign of the system.

Synthesis, Characterization, and Catalytic Studies of Dinuclear d8 Metal-Phosphazane Complexes.

Herein we report the synthesis and characterization of two diphosphazane (P-N-P) ligands, along with their corresponding novel palladium and platinum complexes. Compounds were characterized by FTIR and NMR spectroscopy, single crystal X-ray diffraction (SC-XRD), and powder X-ray diffraction (PXRD)), as well as mass spectrometry. The Pd(II) complex of the p-phenyldiphosphazane was shown to effectively and efficiently catalyze Suzuki-Miyaura cross-coupling reactions between a variety of substrates. Yields were as high as 96%, with reaction times as short as 15 min at room temperature and open to air. No additional supporting ligands, such as triphenylphosphine, were needed. The work reported here expands the use of phosphazane ligands to support catalytic centers and provides an understanding of phosphazane metal-ligand bonding interactions (specifically diphosphazanes).

An FeIII Azamacrocyclic Complex as a pH-Tunable Catholyte and Anolyte for Redox-Flow Battery Applications.

A reversible Fe3+ /Fe2+ redox couple of an azamacrocyclic complex is evaluated as an electrolyte with a pH-tunable potential range for aqueous redox-flow batteries (RFBs). The FeIII complex is formed by 1,4,7-triazacyclononane (TACN) appended with three 2-methyl-imidazole donors, denoted as Fe(Tim). This complex exhibits pH-sensitive redox couples that span E1/2 (Fe3+ /Fe2+ )=317 to -270 mV vs. NHE at pH 3.3 and pH 12.8, respectively. The 590 mV shift in potential and kinetic inertness are driven by ionization of the imidazoles at various pH values. The Fe3+ /Fe2+ redox is proton-coupled at alkaline conditions, and bulk electrolysis is non-destructive. The electrolyte demonstrates high charge/discharge capacities at both acidic and alkaline conditions throughout 100 cycles. Given its tunable redox, fast electrochemical kinetics, exceptional stability/cyclability, this complex is promising for the design of aqueous RFB catholytes and anolytes that utilize the earth-abundant element iron.

Increasing phosphorescent quantum yields and lifetimes of platinum-alkynyl complexes with extended conjugation.

The emission of platinum-alkynyl complexes with terminal pyridyl moieties changes upon simple alkylation reactions. Due to growing interest in photovoltaics, photocatalysis, and light-emitting devices, understanding the nature of these changes is important to develop simple synthetic pathways for the rational design of photophysically active molecules. Herein, the choice of ligand isomer, methylation, and Pt-coordination environment on phosphorescent quantum yields, lifetimes, and associated radiative and non-radiative rate constants of eight organometallic complexes were studied. Single-crystal X-ray diffraction experiments and computational studies provide evidence for stabilization of metallo-cumulene resonance forms whose increased rigidities manifest in the observed photophysical changes. This effect is more pronounced for 4-ethynylpyridyl complexes over 3-ethynylpyridyl variants since the metallo-cumulene form shifts electron density to the electronegative N-atom at the para site. Furthermore, the use of σ-donating N-heterocyclic carbenes to complete the Pt-coordination environment enhanced the quantum yield of phosphorescence as high as 39% (λmax = 512 nm) with a lifetime of 21.2 µs.

Phosphorescent Decanuclear Bimetallic Pt6M4 (M = Zn, Fe) Tetrahedral Cages.

Coordination-driven self-assembly delivers discrete, nanoscopic architectures that may preserve or enhance the physicochemical properties of their parent building blocks. Herein, we report the syntheses, characterization, and photophysical properties of two tetrahedral cages, [ZnII4L6](PF6)8 (C1) and [FeII4L6](OTf)8 (C2), where L = PtII(PEt3)2(C≡C-bpy)2 (PEt3 = triethylphosphine; C≡C-bpy = 5-ethynyl-2,2'-bipyridine) and OTf = trifluoromethanesulfonate. C1 and C2 were assembled in isolated yields of 72% and 81% by treating 2 equiv of Zn(NO3)2·6H2O or Fe(OTf)2 with 3 equiv of L, respectively. Both cages were fully characterized by NMR, electrospray ionization mass spectrometry, and single-crystal X-ray diffraction (SCXRD). The local D3 symmetry at each polypyridyl metal node raises the possibility of a number of isomeric cages; however, only the homochiral enantiomers (ΔΔΔΔ and ΛΛΛΛ) are formed based on 1H NMR and SCXRD. C1 exhibits phosphorescence centered at 545 nm with a quantum yield of 10% in N2-degassed acetonitrile at 25 °C. The quantum yield of C2 is significantly lower due to a nonradiative relaxation from 5MC (MC = metal-centered) states introduced by the FeII nodes.

Photophysical Enhancement of Triplet Emitters by Coordination-Driven Self-Assembly.

The quantum yields of organic fluorophores used as donors in coordination-driven self-assembly often suffer from the heavy atom effect of nearby metal sites. Here, the role of intersystem crossing from a deactivating process to one that delivers emissive triplet states was reversed. A phosphorescent trans bis-N-heterocyclic carbene platinum(II) compound, Pt(dhim)2 (C≡C-4-py)2 (D1; dhim=1,3-dihexyl-2-H-imidazol-2-ylidene), was used along with other linear donors 4,4'-bipyridine (D2) and 1,4-bis(4-pyridyl ethynyl)benzene (D3) in self-assembly reactions with Pt(dtbpy)X2 acceptors (dtbpy=4,4'-di-tert-butyl-2,2'-bipyridine) to afford three metallacycles. Photophysical investigations revealed that, although the building blocks used to construct M1 have relatively low quantum yields (Φ=1.2 and <1 % for D1 and 2, respectively), the metallacycle has a quantum yield of 14 %. This increase reflects a change in radiative rate constant from 3.6×104  s-1 for D1 to 2.1×105  s-1 for M1.