Ligand States Research Articles

Measuring the KD of Protein-Ligand Interactions Using Microscale Thermophoresis.

Microscale thermophoresis (MST) has become a widely used technique to determine the KD or EC50 of protein-ligand interactions. The method exploits the tendency of macromolecules to migrate along a thermal gradient (i.e., thermophoresis). Differences in thermophoresis as a function of the liganded state of a macromolecule can be measured and assembled into a binding curve that can be analyzed to yield KD. In this protocol, we outline a simple experiment designed for new MST users, with the goal of using readily available, inexpensive materials to plan, execute, and analyze an MST experiment.

Engineering the ligand states by surface functionalization: a new way to enhance the ferromagnetism of CrI3.

The newly discovered 2D magnetic materials provide new opportunities for basic physics research and device applications. However, their low Curie temperature (TC) is a common weakness. In this paper, by combining magnetic Hamiltonian, Wannier functions and first-principles calculations, we systematically study the magnetic properties of monolayer CrI3 functionalized with halogens. The magnetic exchange coupling (EX) and magnetic anisotropy (MA) are found to increase significantly with X (X = F, Cl and Br) atom adsorption, and increase with increased coverage of X. In the framework of superexchange theory, the enhanced EX can be ascribed to the reduced energy difference and increased hopping strength between Cr d and I p orbitals, due to the states of the I ligand engineered by the X adatom. Besides, the X adatom may provide an additional ferromagnetic superexchange channel. Our results not only give insight into understanding the enhancement of ferromagnetism of CrI3 by atom adsorption, but also propose a promising way to improve the ferromagnetism of 2D magnetic materials.

Binuclear Gadolinium(III) Pivalates with 4,7‐Diphenyl‐1,10‐Phenanthroline: Synthesis, Structure, Thermal Behavior, Magnetic and Photoluminescence Properties

AbstractComplexes [Gd2(piv)6(bath)2]⋅2EtOH (1a⋅2EtOH), [Gd2(piv)6(bath)2]⋅1.5EtOH⋅0.5H2O (1b⋅1.5EtOH⋅0.5H2O), and [Gd2(piv)6(bath)2]⋅H2O (1c⋅H2O), where piv is (CH3)3CCO2− and bath is 4,7‐diphenyl‐1,10‐phenanthroline, which have different molecular and crystal structures, were synthesized and studied by X‐ray diffraction. A unique feature of polymorphs 1a, 1b, and 1c is that they exhibit first‐order phase transitions, namely, structural phase transitions, or melting before thermal decomposition. Magneto‐structural data for binuclear gadolinium pivalates with bath (1), bpy (2), phen (3), and Hpiv (4) ligands and the polymeric complex [Gd(piv)3]n (5) are presented. The complexes were characterized by ESR spectroscopy. The energies of the excited singlet S1 and triplet T1 states of the bath ligand in carboxylates 1a⋅2EtOH, 1b⋅1.5EtOH⋅0.5H2O, and 1c⋅H2O were determined.

Phosphorescent monometallic and bimetallic two-coordinate Au(I) complexes with N-heterocyclic carbene and aryl ligands

A series of mononuclear and binuclear two coordinate Au(I) complexes consisting of a MAC carbene ligand (1,3-bis(2,6-diisopropylphenyl)-5,5-dimethyl-4-keto-tetrahydropyridylidene) and an aryl ligand, including phenyl, biphenyl, naphthyl and anthryl, are reported. The oxidation potential is controlled by the choice of arene ligand, whereas the reduction potential of the complexes is determined by their common MAC carbene ligand. Owing to the heavy atom effect of Au and the aryl ligand-based T1 state, most of these complexes present phosphorescent emission in both solution and polymer film. A high photoluminescence efficiency (ΦPL = 0.84) with a long decay lifetime (τ = 0.36 ms) is obtained for the 1-naphthyl derivative. The complex with the anthryl ligand displays thermally activated delayed fluorescence (TADF) attributed to ISC between the S1 and T2 states, along with locally excited phosphorescence from the T1 state of the anthryl ligand.

Protonation State of a Key Histidine Ligand in the Iron-Quinone Complex of Photosystem II as Revealed by Light-Induced ATR-FTIR Spectroscopy.

The iron-quinone complex in photosystem II (PSII) consists of the two plastoquinone electron acceptors, QA and QB, and a non-heme iron connecting them. It has been suggested that nearby histidine residues play important roles in the electron and proton transfer reactions of the iron-quinone complex in PSII. In this study, we investigated the protonation/deprotonation reaction of D1-H215, which bridges the non-heme iron and QB, using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Flash-induced Fe2+/Fe3+ ATR-FTIR difference spectra were measured with PSII membranes in the pH range of 5.0-7.5. In the CN stretching region of histidine, the intensity of a negative peak at 1094 cm-1, which was assigned to the deprotonated anion form of D1-H215, increased as the pH increased. Singular-value decomposition analysis provided a component due to deprotonation of D1-H215 with a pKa of ∼5.5 in the Fe3+ state, whereas no component of histidine deprotonation was resolved in the Fe2+ state. This observation supports the previous proposal that D1-H215 is responsible for the proton release upon Fe2+ oxidation [Berthomieu, C., and Hienerwadel, R. (2001) Biochemistry 40, 4044-4052]. The pH dependence of the 13C isotope-edited bands of the bicarbonate ligand to the non-heme iron further showed that deprotonation of bicarbonate to carbonate does not take place at pH <8 in the Fe2+ or Fe3+ state. These results suggest that the putative mechanism of proton transfer to QBH- through D1-H215 and bicarbonate around Fe2+ functions throughout the physiological pH range.

Facing the Challenges of Borderline Oxidation State Assignments Using State-of-the-Art Computational Methods.

The oxidation state (OS) of metals and ligands in inorganic complexes may be defined by carefully curated rules, such as from IUPAC, or by computational procedures such as the effective oxidation state (EOS) or localized orbital bonding analysis (LOBA). Such definitions typically agree for systems with simple ionic bonding and innocent ligands but may disagree as the boundary between ionic and covalent bonds is approached, or as the role of ligand noninnocence becomes nontrivial, or high oxidation states of metals are supported by heavy dative bonding, and so on. This work systematically compares IUPAC, EOS, and LOBA across a series of complexes where OS assignment is challenging. These systems include high-valent transition metal oxides, transition metal complexes with noninnocent ligands such as dithiolate and nitrosyl, metal sulfur dioxide adducts, and two transition metal carbene complexes. The differences in OS assignment by the three methods are carefully discussed, demonstrating the synergy between EOS and LOBA. In addition, a clarity index for LOBA OS assignments is introduced that provides an indication of whether or not its predictions are close to the ionic-covalent boundary.

From Red to Green Luminescence via Surface Functionalization. Effect of 2-(5-Mercaptothien-2-yl)-8-(thien-2-yl)-5-hexylthieno[3,4-c]pyrrole-4,6-dione Ligands on the Photoluminescence of Alloyed Ag-In-Zn-S Nanocrystals.

A semiconducting molecule containing a thiol anchor group, namely 2-(5-mercaptothien-2-yl)-8-(thien-2-yl)-5-hexylthieno[3,4-c]pyrrole-4,6-dione (abbreviated as D-A-D-SH), was designed, synthesized, and used as a ligand in nonstoichiometric quaternary nanocrystals of composition Ag1.0In3.1Zn1.0S4.0(S6.1) to give an inorganic/organic hybrid. Detailed NMR studies indicate that D-A-D-SH ligands are present in two coordination spheres in the organic part of the hybrid: (i) inner in which the ligand molecules form direct bonds with the nanocrystal surface and (ii) outer in which the ligand molecules do not form direct bonds with the inorganic core. Exchange of the initial ligands (stearic acid and 1-aminooctadecane) for D-A-D-SH induces a distinct change of the photoluminescence. Efficient red luminescence of nanocrystals capped with initial ligands (λmax = 720 nm, quantum yield = 67%) is totally quenched and green luminescence characteristic of the ligand appears (λmax = 508 nm, quantum yield = 10%). This change of the photoluminescence mechanism can be clarified by a combination of electrochemical and spectroscopic investigations. It can be demonstrated by cyclic voltammetry that new states appear in the hybrid as a consequence of D-A-D-SH binding to the nanocrystals surface. These states are located below the nanocrystal LUMO and above its HOMO, respectively. They are concurrent to deeper donor and acceptor states governing the red luminescence. As a result, energy transfer from the nanocrystal HOMO and LUMO levels to the ligand states takes place, leading to effective quenching of the red luminescence and appearance of the green one.

From Isolated Porphyrin Ligands to Periodic Al-PMOF: A Comparative Study of the Optical Properties Using DFT/TDDFT

The use of photosensitizers as organic ligands in metal–organic frameworks (MOFs) is a common practice to engineer their UV/vis optical absorption. For instance, MOFs consisting of porphyrin ligands usually inherit their light-harvesting properties and thus follow the Gouterman model in which the low-lying excitations correspond to π → π* transitions. However, the characterization of the excited states of porphyrin ligands in MOFs requires an appropriate description of the periodic crystal including the metal nodes and intermolecular interactions. Here, we investigate the UV/vis absorption properties of porphyrin-MOF Al-PMOF, its two metalated forms, Zn-Al-PMOF and Co-Al-PMOF, and their corresponding isolated porphyrin ligands using density functional theory (DFT)/time-dependent DFT (TDDFT) simulations with Perdew–Burke–Ernzerhof (PBE), PBE0, and CAM-B3LYP functionals. Our results indicate that hybrid functionals are necessary to capture the proper nature of the transitions and the excitonic effects of the optical and fundamental gaps in porphyrin molecules and porphyrin-MOFs that the PBE functional fails to describe. Likewise, the simulations show that a wrong representation of some excitations can be obtained depending on the functional and when the Tamm–Dancoff approximation is used. Finally, our results show that the PBE and PBE0 functionals are not able to capture the gap renormalization when going from the isolated molecules to the periodic crystals. Overall, the nature of the optical transitions, excitonic effects, and gap renormalization are important features to assess in the prediction of optical properties in MOF crystals that require considering proper functionals and approximations to overcome the main failures of DFT/TDDFT calculations.

An Europium (III) Luminophore with Pressure-Sensing Units: Effective Back Energy Transfer in Coordination Polymers with Hexadentate Porous Stable Networks.

A luminescent EuIII coordination polymer with O2 -sensing units under air, EuIII -hcpt (hcpt: 2,3,6,7,10,11-hexakis(4-carboxy-phenyl)triphenylene), is reported. The hexadentate carboxylic acids in hcpt ligands play an important role in the formation of tight-packed three-dimensional networks in EuIII -hcpt, giving hyper thermo-stable structures (decomposition temperature=420 °C). The three-dimensional porous network promotes bright luminescence (4f-4f emission quantum yield=70 %). The emission lifetime of EuIII -hcpt under vacuum (0.86 ms) was twice as large as that under O2 (0.48 ms, 1 atom:101.3 kPa). The Arrhenius analysis of the emission decay profile indicates that the back energy transfer (BEnT) from the emitting level of the europium(III) ion to the excited T1 state of the hcpt ligand should be activated at room temperature. The gradual decrease of emission lifetime is caused by the BEnT process in EuIII -hcpt. Finally, an advanced pressure-sensitive luminophore is demonstrated.

Chemical Evolution of Pt-Zn Nanoalloys Dressed in Oleylamine.

We report on the shape, composition (from Pt95Zn5 to Pt77Zn23), and surface chemistry of Pt-Zn nanoparticles obtained by reduction of precursors M2+(acac)2- (M2+: Pt2+ and Zn2+) in oleylamine, which serves as both solvent and ligand. We show first that the addition of phenyl ether or benzyl ether determines the composition and shape of the nanoparticles, which point to an adsorbate-controlled synthesis. The organic (ligand)/inorganic (nanoparticles) interface is characterized on the structural and chemical level. We observe that the particles, after washing with ethanol, are coated with oleylamine and the oxidation products of the latter, namely, an aldimine and a nitrile. After exposure to air, the particles oxidize, covering themselves with a few monolayer thick ZnO film, which is certainly discontinuous when the particles are low in zinc. Pt-Zn particles are unstable and prone to losing Zn. We have strong indications that the driving force is the preferential oxidation of the less noble metal. Finally, we show that adsorption of CO on the surface of nanoparticles modifies the oxidation state of amine ligands and attribute it to the displacement of hydrogen adsorbed on Pt. All the structural and chemical information provided by the combination of electron microscopy and X-ray photoelectron spectroscopy allows us to give a fairly accurate picture of the surface of nanoparticles and to better understand why Pt-Zn alloys are efficient in certain electrocatalytic reactions such as the oxidation of methanol.

Molecular mechanisms underlying menthol binding and activation of TRPM8 ion channel

Menthol in mints elicits coolness sensation by selectively activating TRPM8 channel. Although structures of TRPM8 were determined in the apo and liganded states, the menthol-bounded state is unresolved. To understand how menthol activates the channel, we docked menthol to the channel and systematically validated our menthol binding models with thermodynamic mutant cycle analysis. We observed that menthol uses its hydroxyl group as a hand to specifically grab with R842, and its isopropyl group as legs to stand on I846 and L843. By imaging with fluorescent unnatural amino acid, we found that menthol binding induces wide-spread conformational rearrangements within the transmembrane domains. By Φ analysis based on single-channel recordings, we observed a temporal sequence of conformational changes in the S6 bundle crossing and the selectivity filter leading to channel activation. Therefore, our study suggested a ‘grab and stand’ mechanism of menthol binding and how menthol activates TRPM8 at the atomic level.

Manganese Complex of a Rigidified 15-Membered Macrocycle: A Comprehensive Study.

Owing to the increasing importance of manganese(II) complexes in the field of magnetic resonance imaging (MRI), large efforts have been devoted to find an appropriate ligand for Mn(II) ion encapsulation by providing balance between the seemingly contradictory requirements (i.e., thermodynamic stability and kinetic inertness vs low ligand denticity enabling water molecule(s) to be coordinated in its metal center). Among these ligands, a large number of pyridine or pyridol based open-chain and macrocyclic chelators have been investigated so far. As a next step in the development of these chelators, 15-pyN3O2Ph and its transition metal complexes were synthesized and characterized using established methods. The 15-pyN3O2Ph ligand incorporates both pyridine and ortho-phenylene units to decrease ligand flexibility. The thermodynamic properties, protonation and stability constants, were determined using pH-potentiometry; the solid-state structures of two protonation states of the free ligand and its manganese complex were obtained by single crystal X-ray diffractometry. The results show a seven-coordinate metal center with two water molecules in the first coordination sphere. The longitudinal relaxivity of [Mn(15-pyN3O2Ph)]2+ was found to be 5.16 mM-1 s-1 at 0.49 T (298 K). Furthermore, the r2p value of 11.72 mM-1 s-1 (0.49 T), which is doubled at 1.41 T field, suggests that design of this Mn(II) complex does achieve some characteristics required for contrast imaging. In addition, 17O NMR measurements were performed in order to access the microscopic parameters governing this key feature (e.g., water exchange rate). Finally, manganese complexes of ligands with analogous polyaza macrocyclic scaffold have been investigated as low molecular weight Mn(CAT) mimics. Here, we report the H2O2 disproportionation study of [Mn(15-pyN3O2Ph)]2+ to demonstrate the versatility of this ligand scaffold as well.

Structural Basis of Functional Transitions in Mammalian NMDA Receptors

Excitatory neurotransmission meditated by glutamate receptors including N-methyl-D-aspartate receptors (NMDARs) is pivotal to brain development and function. NMDARs are heterotetramers composed of GluN1 and GluN2 subunits, which bind glycine and glutamate, respectively, to activate their ion channels. Despite importance in brain physiology, the precise mechanisms by which activation and inhibition occur via subunit-specific binding of agonists and antagonists remain largely unknown. Here, we show the detailed patterns of conformational changes and inter-subunit and -domain reorientation leading to agonist-gating and subunit-dependent competitive inhibition by providing multiple structures in distinct ligand states at 4Å or better. The structures reveal that activation and competitive inhibition by both GluN1 and GluN2 antagonists occur by controlling the tension of the linker between the ligand-binding domain and the transmembrane ion channel of the GluN2 subunit. Our results provide detailed mechanistic insights into NMDAR pharmacology, activation, and inhibition, which are fundamental to the brain physiology.

Electronically Asynchronous Transition States for C-N Bond Formation by Electrophilic [CoIII(TAML)]-Nitrene Radical Complexes Involving Substrate-to-Ligand Single-Electron Transfer and a Cobalt-Centered Spin Shuttle.

The oxidation state of the redox noninnocent tetra-amido macrocyclic ligand (TAML) scaffold was recently shown to affect the formation of nitrene radical species on cobalt(III) upon reaction with PhI=NNs [van LeestN. P.; J. Am. Chem. Soc.2020, 142, 552−56331846578]. For the neutral [CoIII(TAMLsq)] complex, this leads to the doublet (S = 1/2) mono-nitrene radical species [CoIII(TAMLq)(N•Ns)(Y)] (bearing an unidentified sixth ligand Y in at least the frozen state), while a triplet (S = 1) bis-nitrene radical species [CoIII(TAMLq)(N•Ns)2]– is generated from the anionic [CoIII(TAMLred)]– complex. The one-electron-reduced Fischer-type nitrene radicals (N•Ns–) are formed through single (mono-nitrene) or double (bis-nitrene) ligand-to-substrate single-electron transfer (SET). In this work, we describe the reactivity and mechanisms of these nitrene radical complexes in catalytic aziridination. We report that [CoIII(TAMLsq)] and [CoIII(TAMLred)]– are both effective catalysts for chemoselective (C=C versus C–H bonds) and diastereoselective aziridination of styrene derivatives, cyclohexane, and 1-hexene under mild and even aerobic (for [CoIII(TAMLred)]–) conditions. Experimental (Hammett plots; [CoIII(TAML)]-nitrene radical formation and quantification under catalytic conditions; single-turnover experiments; and tests regarding catalyst decomposition, radical inhibition, and radical trapping) in combination with computational (density functional theory (DFT), N-electron valence state perturbation theory corrected complete active space self-consistent field (NEVPT2-CASSCF)) studies reveal that [CoIII(TAMLq)(N•Ns)(Y)], [CoIII(TAMLq)(N•Ns)2]–, and [CoIII(TAMLsq)(N•Ns)]– are key electrophilic intermediates in aziridination reactions. Surprisingly, the electrophilic one-electron-reduced Fischer-type nitrene radicals do not react as would be expected for nitrene radicals (i.e., via radical addition and radical rebound). Instead, nitrene transfer proceeds through unusual electronically asynchronous transition states, in which the (partial) styrene substrate to TAML ligand (single-) electron transfer precedes C–N coupling. The actual C–N bond formation processes are best described as involving a nucleophilic attack of the nitrene (radical) lone pair at the thus (partially) formed styrene radical cation. These processes are coupled to TAML-to-cobalt and cobalt-to-nitrene single-electron transfer, effectively leading to the formation of an amido-γ-benzyl radical (NsN––CH2–•CH–Ph) bound to an intermediate spin (S = 1) cobalt(III) center. Hence, the TAML moiety can be regarded to act as a transient electron acceptor, the cobalt center behaves as a spin shuttle, and the nitrene radical acts as a nucleophile. Such a mechanism was hitherto unknown for cobalt-catalyzed hypovalent group transfer and the more general transition-metal-catalyzed nitrene transfer to alkenes but is now shown to complement the known concerted and stepwise mechanisms for N-group transfer.

Unravelling the Chemistry of the [Cu(4,7-Dichloroquinoline)2Br2]2 Dimeric Complex through Structural Analysis: A Borderline Ligand Field Case

Large dark prismatic crystals (P 1 ¯ ) consisting of closely packed centrosymmetric [Cu(4,7-dichloroquinoline)2]2Br4 binuclear units are formed when 4,7-dichloroquinoline (DCQ, C9H5NCl2) binds copper(II). Cu2+ adopts a strongly distorted square pyramidal coordination geometry, perturbed by electrostatic interactions with two axial μ–Br ligands acting as highly asymmetric bridges. It is shown that, as electronic states of ligands are higher in energy than the metal ones, antibonding orbitals bear significant ligand-like character and electronic charge is partially transferred from inner-sphere coordinated halogen atoms to copper. Overall, the title compound sits on the Hoffman’s border between main group and transition chemistry, with non-negligible contributions of the ligands to the frontier orbitals. The relative energy placement of metal and ligand states determines an internal redox process, where the metal is slightly reduced at the expense of partial oxidation of the bromide ligands. In fact, the crystal structure is partially disordered due to the substitution of some penta-coordinated Cu(II) centers with tetra-coordinated Cu(I) ions. The geometry of the complex is rationalized in terms of electrostatic-driven distortions from an ideal octahedral prototype. Implications on the reactivity of Cu(II)–quinoline complexes are discussed.

Numerical evaluation of polyethylene glycol ligand conjugation to gold nanoparticle surface using ToF-SIMS and statistical analysis.

Nanoparticles (NPs) are substances between 1 and 100 nm in size. They have been the subject of numerous studies because of their potential applications in a wide range of fields such as cosmetics, electronics, medicine, and food. For biological applications of nanoparticles, they are usually coated with a substance capable of preventing agglomeration of the nanoparticles and nonspecific binding and exhibiting water-solubility characteristics with specific immobilized (bio)molecules. In order to evaluate the chemical properties of the surface-modified nanoparticles for bioapplications, including drug delivery, a simple and reliable method for the analysis of the presence of the surface chemicals and the ligand states of the nanoparticles is necessary. In this study, the authors numerically evaluated the extent of polyethylene glycol (PEG) ligand conjugation on AuNPs by concurrently adopting a microliquid inkjet printing system for sampling of the PEGylated AuNPs solution and ToF-SIMS imaging together with statistical analysis. The statistical correlation values calculated from the signals of PEG and Au measured by ToF-SIMS imaging on the sample spots made by a microliquid inkjet printing system showed better reproducibility and improved correlation values compared to the pipet spotting. Their improved method will be useful to evaluate ligand-conjugated nanoparticles for quality control of each conjugation process.

Deciphering the role of cationic substitution towards highly stable polyanionic cathodes

For the majority of intercalation compounds as electrode materials, ion substitution offers a facile but sensible pathway toward modifying the intrinsic properties thereby boosting unexpected electrochemical performances. For polyanionic compounds, however, the underlying mechanism and site tuning of ion-doping are not well put forward owing to subtle structural variations and complex charge compensation. Herein, taking Li3V2(PO4)3 as a model, we firstly theoretically predict the lattice site preference and impurity-related defect complexes for Mg dopant under various synthesis conditions. Furthermore, the Mg-doping experimental evidence verifies that the substituted sites rely heavily on the relative abundance of lithium and the charge imbalance is mainly counterbalanced by vacant or interstitial Li, rather than hole/electron polarons. Compared to Li-site doping, V-site substituted by Mg2+ is preferable to form under Li-rich conditions, which causes cell volume expansion and simultaneously decreases the VO6 octahedral distortion, realizing a highly stable structure with improved kinetics. Detailed studies on the site-dependent electronic/ionic properties elucidate the role of cation substitution in tuning electrochemical behavior, intrinsically through the energetic movement of ligand states. Specifically, selective substitution induces negatively charged anions surrounding the dopants, lowering the ligand p band center, and thus maintaining structural/interfacial stability. Our results build a clear connection between the doping site engineering and intrinsic properties for designing high-power long-term electrode materials.

Ruthenium(II) Complex Containing a Redox-Active Semiquinonate Ligand as a Potential Chemotherapeutic Agent: From Synthesis to In Vivo Studies.

Chemotherapy remains one of the dominant treatments to cure cancer. However, due to the many inherent drawbacks, there is a search for new chemotherapeutic drugs. Many classes of compounds have been investigated over the years to discover new targets and synergistic mechanisms of action including multicellular targets. In this work, we designed a new chemotherapeutic drug candidate against cancer, namely, [Ru(DIP)2(sq)](PF6) (Ru-sq) (DIP = 4,7-diphenyl-1,10-phenanthroline; sq = semiquinonate ligand). The aim was to combine the great potential expressed by Ru(II) polypyridyl complexes and the singular redox and biological properties associated with the catecholate moiety. Experimental evidence (e.g., X-ray crystallography, electron paramagnetic resonance, electrochemistry) demonstrates that the semiquinonate is the preferred oxidation state of the dioxo ligand in this complex. The biological activity of Ru-sq was then scrutinized in vitro and in vivo, and the results highlight the promising potential of this complex as a chemotherapeutic agent against cancer.

When Like Destabilizes Like: Inverted Solvent Effects in Apolar Nanoparticle Dispersions.

We report on the colloidal stability of nanoparticles with alkanethiol shells in apolar solvents. Small-angle X-ray scattering and molecular dynamics simulations were used to characterize the interaction between nanoparticles in linear alkane solvents ranging from hexane to hexadecane, including 4 nm gold cores with hexadecanethiol shells and 6 nm cadmium selenide cores with octadecanethiol shells. We find that the agglomeration is enthalpically driven and that, contrary to what one would expect from classical colloid theory, the temperature at which the particles agglomerate increases with increasing solvent chain length. We demonstrate that the inverted trend correlates with the temperatures at which the ligands order in the different solvents and show that the inversion is due to a combination of enthalpic and entropic effects that enhance the stability of the ordered ligand state as the solvent length increases. We also explain why cyclohexane is a better solvent than hexadecane despite the two having very similar solvation parameters.

Synthesis and investigation of enhanced luminescence of Ln(III)-complexes containing fluorinated β-diketone and oxygen donor ancillary ligands for efficient advanced displays

2,2-dimethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octanedione (fod) fluorinated β-diketone had been used to prepare a series of Ln3+ (Eu or Tb) ternary complexes with monodentate ancillary ligands such as aqua, urea, triphenylphosphineoxide (TPPO) and pyridine-N-oxide (PNO). Prepared ternary materials Eu(fod)3.(aqua)2 (1), Tb(fod)3.(aqua)2 (2), Eu(fod)3.(urea)2 (3), Tb(fod)3.(urea)2 (4), Eu(fod)3.(TPPO)2 (5), Tb(fod)3.(TPPO)2 (6), Eu(fod)3.(PNO)2 (7), Tb(fod)3.(PNO)2 (8) were illustrated through various structural and photo-physical spectroscopic practices. Emission measurement showed characteristics strong emission in red aa well as green spectral region accredited to the electric dipole 5D0→7F2 and 5D4→7F5 hypersensitive transitions of Eu3+ and Tb3+ ions respectively. An inclusive comparative investigation of enhancement features in the luminescence intensity had been carried out to determine the role of ancillary ligands. Improved luminosity was achieved as auxiliary ligands not only substituted the solvent molecules in coordination sphere but also indirectly sensitize metal ion through transfer of energy to emissive level of respective Ln3+ ion from triplet state of ligand. Decay curve specified uniformity in chemical environment around central metal ion. Efficient red and green light materials could be successfully used for fabricating OLEDs.