Amedeo Santoro

Synthesis of Cyclometallated Platinum Complexes with Substituted Thienylpyridines and Detailed Characterization of Their Luminescence Properties

Synthesis of various derivatives of 2-(2-thienyl)pyridine via substituted 3-thienyl-1,2,4-triazines is reported. The final step of the synthesis is a transformation of the triazine ring to pyridine in an aza-Diels-Alder-type reaction. The resulting 5-aryl-2-(2-thienyl)pyridines (HL1-HL4) and 5-aryl-2-(2-thienyl)cyclopenteno[c]pyridines (HL5-HL8) (with aryl = phenyl, 4-methoxyphenyl, 2-naphtyl, and 2-thienyl) were used as cyclometallating ligands to prepare a series of eight luminescent platinum complexes of the type [Pt(L)(acac)] (L = cyclometallating ligand, acac = acetylacetonato). X-ray single crystal structures of three complexes of that series, [Pt(L5)(acac)] = [Pt(5-phenyl-2-(2-thienyl)cyclopenteno[c]pyridine)(acac)], [Pt(L6)(acac)] = [Pt(5-(4-methoxy)-2-(2-thienyl)cyclopenteno[c]pyridine)(acac)], and [Pt(L7)(acac)] = [Pt(5-(2-naphtyl)-2-(2-thienyl) cyclopenteno[c]pyridine)(acac)] were determined. Photoluminescence and electronic absorption spectra of the new [Pt(L)(acac)] complexes are reported. For two representative compounds of that series, [Pt(L4)(acac)] and [Pt(L5)(acac)], a detailed photophysical characterization based on highly resolved emission and excitation spectra, as well as on emission decay properties, was carried out. The studies down to low temperature (T = 1.2 K) and up to high magnetic fields (B = 10 T) allowed us to characterize the three individual substates of the emitting triplet state. In particular, it is shown that the lowest triplet states of [Pt(L4)(acac)] and [Pt(L5)(acac)] are largely ligand-centered (LC) of (3)pi pi* character, which experience only weak spin-orbit couplings to higher lying singlet states.

Emissive metallomesogens based on 2-phenylpyridine complexes of iridium(III).

Preparation of Ir(III) complexes using anisotropic 2,5-di(4-alkoxyphenyl)pyridine ligands leads to emissive, liquid-crystalline complexes containing bound Cl and dimethyl sulfoxide. Using analogous poly(alkoxy) ligands allows the preparation of bis(2-phenylpyridine)iridium(III) acac complexes, which are also mesomorphic. The observation of liquid crystallinity in octahedral complexes of this type is without precedent.

Phosphorescent Mesomorphic Dyads Based on Tetraacetylethane Complexes of Iridium(III)

Complexes of iridium and platinum with aromatic ligands are of interest as emissive materials owing to their high spin–orbit coupling constants, which can promote phosphorescence from triplet states, a process that is otherwise formally forbidden. They are particularly relevant to organic light-emitting diode (OLED) technology, where the combination of charges leads to a surplus of triplet to singlet states, although their application in areas such as bioimaging and sensing have also received recent significant attention. Incorporation of emissive complexes can raise the maximum internal efficiency of an OLED to 100% by promoting triplet emission. 4] Thompson and co-workers have shown that the bis(2phenylpyridine)iridium(III) chromophore is a very effective triplet emitter, the physical properties of which make it suitable for incorporation into devices, and the emission characteristics of which can be readily tuned through substitution of the ligands. To add the property of liquid crystallinity to emissive materials is also of interest, for the ordered state of the molecules in the different mesophases suggests an improved pathway for the transport of charge carriers, whereas certain mesophases offer the possibility of polarized emission. We have reported liquid-crystalline Pt(II) phosphor compounds based on 1,3-di(2-pyridyl)benzene ligands that form columnar phases, whereas those based on extended 2,5-diphenylpyridines show nematic and SmA phases with very high photoluminescent quantum yields. More recently and following a report of a luminescent, ionic, liquid-crystalline complex of Ir(III), we reported the first example of a charge-neutral, luminescent, mesogenic complex of Ir(III) based on a hexacatenar phenylpyridine (ppy) ligand, with acetylacetonate (acac) as an ancillary ligand (1a). In the same study, a related complex (1b) based on a tetracatenar ligand was found to be highly emissive (F = 50%) but not liquid-crystalline, whereas the di-m-chloro dimer, 2, of the same ligand was liquid-crystalline, but scarcely emissive (F< 1%; Figure 1).

Iron(II) Complexes of Tridentate Indazolylpyridine Ligands: Enhanced Spin-Crossover Hysteresis and Ligand-Based Fluorescence

Reaction of 2,6-difluoropyridine with 2 equiv of indazole and NaH at room temperature affords a mixture of 2,6-bis(indazol-1-yl)pyridine (1-bip), 2-(indazol-1-yl)-6-(indazol-2-yl)pyridine (1,2-bip), and 2,6-bis(indazol-2-yl)pyridine (2-bip), which can be separated by solvent extraction. A two-step procedure using the same conditions also affords both 2-(indazol-1-yl)-6-(pyrazol-1-yl)pyridine (1-ipp) and 2-(indazol-2-yl)-6-(pyrazol-1-yl)pyridine (2-ipp). These are all annelated analogues of 2,6-di(pyrazol-1-yl)pyridine, an important ligand for spin-crossover complexes. Iron(II) complexes [Fe(1-bip)2](2+), [Fe(1,2-bip)2](2+), and [Fe(1-ipp)2](2+) are low-spin at room temperature, reflecting sterically imposed conformational rigidity of the 1-indazolyl ligands. In contrast, the 2-indazolyl complexes [Fe(2-bip)2](2+) and [Fe(2-ipp)2](2+) are high-spin in solution at room temperature, whereas salts of [Fe(2-bip)2](2+) exhibit thermal spin transitions in the solid state. Notably, [Fe(2-bip)2][BF4]2·2MeNO2 adopts a terpyridine embrace lattice structure and undergoes a spin transition near room temperature after annealing, resulting in thermal hysteresis that is wider than previously observed for this structure type (T1/2 = 266 K, ΔT = 16-20 K). This reflects enhanced mechanical coupling between the cations in the lattice through interdigitation of their ligand arms, which supports a previously proposed structure/function relationship for spin-crossover materials with this form of crystal packing. All of the compounds in this work exhibit blue fluorescence in solution under ambient conditions. In most cases, the ligand-based emission maxima are slightly red shifted upon complexation, but there is no detectable correlation between the emission maximum and the spin state of the iron centers.

Oxidation of organoplatinum(II) by coordinated dimethylsulfoxide: metal-metal bonded, dinuclear, liquid-crystalline complexes of platinum(III).

Reaction of [PtCl(2)(dmso)(2)] with 2,5-(dialkoxyphenyl)pyridine in HOAc leads to a dinuclear, acetate-bridged, metal-metal bonded complex of platinum(III); dmso in the presence of acid is found to be responsible for the oxidation. The dimer is analogous structurally to Pd(III) dimers implicated in catalytic acetoxylation. Platinum dimers with longer alkoxy chains are shown to be unique examples of liquid crystals of platinum(III).

Bead-like structures and self-assembled monolayers from 2,6-dipyrazolylpyridines and their iron( ii ) complexes

Drop-casting acetone solutions of [Fe(bpp)2][BF4]2 (bpp = 2,6-di[pyrazol-1-yl]pyridine) onto a HOPG surface affords unusual chain-of-beads nanostructures. The beads in each chain are similar in size, with diameters in the range of 2–6 nm and heights of up to 10 A, which is consistent with them containing between 10–50 molecules of the compound. The beads can be classified into two types, which exhibit different conduction regimes by current-imaging tunnelling spectroscopy (CITS) which appear to correlate with their positions in the chains, and may correspond to molecules containing high-spin and low-spin iron centres. Similarly drop-cast films of the complex on a gold surface contain the intact [Fe(bpp)2][BF4]2 compound by XPS. 4-Mercapto-2,6-di[pyrazol-1-yl]pyridine undergoes substantial decomposition when deposited on gold, forming elemental sulfur, but 4-(N-thiomorpholinyl)-2,6-di[pyrazol-1-yl]pyridine successfully forms SAMs on a gold surface by XPS and ellipsometry.