Zwitterionic mixed-carbene coinage metal complexes: Synthesis, structures and photophysical studies

A series of three zwitterionic mixed-ligand bis(carbene) complexes of copper (1), silver (2) and gold (3) have been synthesized, based on a combination of ethyl-substituted cyclic (alkyl)(amino)carbene (Et2CAAC) and an anionic methyl-malonate-derived N-heterocyclic carbene (maloNHC). The crystal structures confirm the linear two-coordinate geometry without close intermolecular contacts. The compounds show blue-white phosphorescence consistent with a wide HOMO-LUMO energy gap (2.87–3.07 eV) estimated by cyclic voltammetry. The excited state lifetimes of crystalline powders decrease in the series Cu > Ag > Au, from 400 µs for copper 1 to 50 µs for the gold complex 3. DFT calculations indicate a large change in the transition dipole moment on excitation, of up to 16 D

Are Methylaluminoxane Activators Sheets?

Density functional theory calculations on neutral sheet models for methylaluminoxane (MAO) indicate that these structures, containing 5-coordinate and 4-coordinate Al, are likely precursors to ion-pairs seen during the hydrolysis of trimethylaluminum (Me3Al) in the presence of donors such as octamethyltrisiloxane (OMTS). Ionization by both methide ([Me](-)) and [Me2Al](+) abstraction, involving this donor, were studied by polarizable continuum model calculations in fluorobenzene (PhF) and o-difluorobenzene (DFB) media. These studies suggest that low MW, 5-coordinate sheets ionize by [Me2Al](+) abstraction, while [Me](-) abstraction from Me3Al-OMTS is the likely process for higher MW 4-coordinate sheets. Further, comparison of anion stabilities per mole of aluminoxane repeat unit (MeAlO)(n), suggest that anions such as [(MeAlO)(7)(Me3Al)(4)Me](-)=[7,4](-) are especially stable compared to higher homologues, even though their neutral precursors are unstable.Peer reviewe

Synthesis and Photophysical Properties of Au(III)-Ag(I) Aggregates

Cyclometallated gold (III) complexes of the type (C^N^C)AuX [HC^N^CH = 2,6-bis(4-ButC6H4)pyrazine; 2,6-bis(4-ButC6H4)pyridine, or 2,6-bis(4-ButC6H4)4-Butpyridine; X = CN, CH(COMe)2 or CH(CN)2] have been used as building blocks for the construction of the first family of AuIII/AgI aggregates. The crystal structures of these aggregates reveal the formation of complex architectures in which the Ag+ cations are stabilized by the basic centers present on each of the Au precursors. The photophysical properties of these aggregates are reported. Compared to mononuclear pincer complexes, a general red-shift and an increase in the emission intensity are observed. In agreement with DFT calculations the lowest energy absorption and the emission are assigned to 1IL(C^N^C) and 3IL(C^N^C) transitions dominated by the HOMO and the LUMO orbitals

Organic persistent room temperature phosphorescence enabled by carbazole impurity

The molecular design of metal-free organic phosphors is essential for realizing persistent room-temperature phosphorescence (pRTP) despite its spin-forbidden nature. A series of halobenzonitrile–carbazoles has been prepared following a one-pot nucleophilic substitution protocol involving commercially available and laboratory-synthesized carbazoles. We demonstrate how halo- and cyano-substituents affect the molecular geometry in the crystal lattice, resulting in tilt and/or twist of the carbazole with respect to the phenyl moiety. Compounds obtained from the commercially available carbazole result in efficient pRTP of organic phosphors with a high quantum yield of up to 22% and a long excited state lifetime of up to 0.22 s. Compounds obtained from the laboratory-synthesized carbazole exhibit thermally activated delayed fluorescence with an excited state lifetime in the millisecond range. In-depth photophysical studies reveal that luminescence originates from the mixed locally excited state (3LE, nπ*)/charge transfer state

Highly photoluminescent copper carbene complexes based on prompt rather than delayed fluorescence

Linear two-coordinate copper complexes of cyclic (alkyl)(amino)-carbenes (CAAC)CuX (X = halide) show photoluminescence with solid-state quantum yields of up to 96%; in contrast to previously reported Cu photoemitters the emission is independent of temperature over the range T = 4 – 300 K and occurs very efficiently by prompt rather than delayed fluorescence, with lifetimes in the sub-nanosecond range

Carbene-Metal-Amide Polycrystalline Materials Feature Blueshifted Energy yet Unchanged Kinetics of Emission

The nature of carbene-metal-amide (CMA) photoluminescence in the solid state is explored through spectroscopic and quantum-chemical investigations on a representative Au-centered molecule. The crystalline phase offers well-defined coplanar geometries-enabling the link between molecular conformations and photophysical properties to be unravelled. We show that a combination of restricted torsional distortion and molecular electronic polarization blue shift the charge-transfer emission by around 400 meV in the crystalline versus the amorphous phase, through energetically raising the less-dipolar S1 state relative to S0. This blue shift brings the lowest charge-transfer states very close to the localized carbazole triplet state, whose structured emission is observable at low temperature in the polycrystalline phase. Moreover, we discover that the rate of intersystem crossing and emission kinetics are unaffected by the extent of torsional distortion. We conclude that more coplanar triplet equilibrium conformations control the photophysics of CMAs

Synthesis and photophysical properties of linear gold(I) complexes based on a CCC carbene.

From PubMed via Jisc Publications RouterPublication status: aheadofprintThe reaction between allenylpyridine (L1) and (Me S)AuCl resulted in the quantitative formation of the (Indolizy)gold chloride complex 1 (Indolizy = indolizin-2-ylidene). The reaction of 1 with carbazole in the presence of KO Bu affords the corresponding (Indolizy)Au(Cz) complex 2. Both compounds show high air- and temperature stability. The crystal structure of 2 confirmed the linear co-planar geometry. Complex 1 shows an intense low energy absorption of mixed character in the UV-vis spectrum, ascribed to intraligand and (M + Hal)L charge transfer processes, and exhibits bright yellow phosphorescence with an excited state lifetime of 62.8 μs in the crystal and a luminescence quantum yield up to 65%. On the other hand, the carbazolate complex 2 in a polystyrene matrix shows bright red delayed fluorescence at 617 nm with a sub-microsecond excited state lifetime and a quantum yield of 21.6%

Towards Controlling the Metallocene/Methylaluminoxane Catalyzed Olefin Polymerization Process by a Computational Approach

We describe and compare the proposed mechanisms of ethene polymerization by the metallocene/methylaluminoxane (MAO) catalyst in terms of quantum chemical calculations. In combination with the Cp2ZrMe2 precatalyst, we employ two models for MAO, produced by hydrolysis of trimethylaluminum (TMA). The both MAOs contain associated TMA as a key ingredient for cocatalytic activity. The TMA association/dissociation equilibrium in the MAOs controls the mechanism of catalyst activation, and suggests preference for catalyst activation via [AlMe2]+ abstraction from the MAO by the precatalyst rather than via Lewis-acidic abstraction of the leaving group from the precatalyst by the MAO. Solvent interactions increase the relative concentration of Lewis-acidic sites. Chlorination of MAO facilitates the catalytic processes. Studies as a function of precatalyst structure reproduce the general experimental observations of the easier catalyst activation by zirconocenes than by hafnocenes, and the positive effects of adding a dimethylsilyl bridge and replacing the cyclopentadienyl with an indenyl ligand. The study provides a starting point for rational control of the behavior of the metallocene/MAO catalyst