Highly Distorted Bismuth-Nickel(II) Complex.

The synthesis and characterization of a series of nickel complexes bearing a bismuth-containing pincer ligand are presented herein. In particular, synthesis of a 4-coordinate Bi-Ni(II) complex allows the influence of bismuth on a d8 Ni(II) ion to be investigated. A trigonal-bipyramidal complex, (BiP2)Ni(PPh) (1), possessing an anionic bismuth donor was prepared via the Bi-C bond cleavage of a BiP3 ligand (BiP3 = Bi(o-PiPr2-C6H4)3) mediated by Ni(0). To remove a PPh moiety, compound 1 was treated with MeI to give a 5-coordinate nickel(II) complex (MeBiP2)Ni(PPh)(I) (2), followed by its exposure to heat or UV irradiation, resulting in the formation of a nickel halide complex, (BiP2)Ni(I) (3). The X-ray crystal structure of 2 revealed that the methyl moiety binds to a bismuth site, providing a neutral MeBiP2 ligand, while the iodide anion is bound to the nickel(II) center, displacing one phosphine donor. Because of the methylation on a Bi site, the Bi-Ni bond in 2 is clearly elongated relative to that of 1, which indicates that the bonding interactions between Bi and Ni are substantially different. Interestingly, compound 3 revealing a sawhorse geometry is significantly distorted away from a square-planar structure compared to the previously reported nickel(II) pincer complexes, (NP2)Ni(Cl) and (PP2)Ni(I). Such difference indicates that a bismuth donor can be a structurally influencing cooperative site for a nickel(II) ion, leading to have a Ni(I)-Bi(II) character. Migratory insertion of CO into a Ni-C bond of 1 gives (BiP2)Ni(COPPh) (4), which further leads to an analogous methylated product (MeBiP2)Ni(COPPh)(I) (5) from reaction with MeI. Due to the structural influence of a carbonyl group in each step, the total reaction time from 1 to 3 was dramatically reduced. The bimetallic cooperativity of the complexes and unusual bonding properties presented here highlight the potential of a bismuth-nickel moiety as a new type of heterobimetallic site for the design of bimetallic complexes to facilitate a variety of chemical transformations.

Tailoring Carbene-Metal-Amides for Thermally Activated Delayed Fluorescence: A Computationally Guided Study on the Effect of Cyclic (Alkyl)(amino)carbenes.

Gold-centered carbene-metal-amides (CMAs) containing cyclic (alkyl)(amino)carbenes (CAACs) are promising emitters for thermally activated delayed fluorescence (TADF). Aiming at the design and optimization of new TADF emitters, we report a density functional theory study of over 60 CMAs with various CAAC ligands, systematically evaluating computed parameters in relation to photoluminescence properties. The CMA structures were primarily selected based on experimental synthesis prospects. We demonstrate that TADF efficiency of the CMA materials originates from a compromise between oscillator strength coefficients and exchange energy (ΔEST). The latter is governed by the overlap of HOMO and LUMO orbitals, where HOMO is localized on the amide and LUMO over the Au-carbene bond. The S0 ground and excited T1 states of the CMAs adopt approximately coplanar geometry of carbene and amide ligands, but rotate perpendicular in the excited S1 states, resulting in degeneracy or near-degeneracy of S1 and T1, accompanied by a decrease in the S1-S0 oscillator strength from its maximum at coplanar geometries to near zero at rotated geometries. Based on the computations, promising new TADF emitters are proposed and synthesized. Bright CMA complex (Et2CAAC)Au(carbazolide) is obtained and fully characterized in order to demonstrate that excellent stability and high radiative rates up to 106 s-1 can be obtained for the gold-CMA complexes with small CAAC-carbene ligands.

Organic thermally activated delayed fluorescence material with strained benzoguanidine donor.

Organic thermally activated delayed fluorescence (TADF) materials have been widely investigated due to their impressive electronic properties and applied potential for the third generation of organic light-emitting diodes (OLED). We present organic TADF material (4BGIPN) based on the strained benzoguanidine donor and compare it with the benchmark carbazole-based material (4CzIPN). Extended π-conjugation in 4BGIPN material results in yellow-green luminescence at 512 nm with a fast radiative rate of 5.5 × 10-5 s-1 and a photoluminescence quantum yield of 46% in methylcyclohexane solution. Such a nitrogen-rich 4BGIPN material has a significantly stabilized highest occupied molecular orbital (HOMO) at -6.4 eV while the lowest unoccupied molecular orbital (LUMO) at -4.0 eV, indicating potential suitability for application as the electron transport layer or TADF class III emitter in OLEDs.

Photophysical Tuning of σ-SiH Copper-Carbazolide Complexes To Give Deep-Blue Emission.

A series of σ-SiH copper complexes with different carbazole derivatives have been synthesized and characterized that adopt a neutral SiHP2 ligand (SiHP2 = (2-iPr2PC6H4)2SiHMe) and present photophysical properties. A previously reported copper complex (SiHP2)Cu(carbazolide), and its derivatives showed that tuning of the emission properties is possible by incorporating various substituents on the carbazolide moiety. Newly synthesized copper complexes (2-6) having 3,6-dichlorocarbazolide, 3,6-dibromocarbazolide, 1-fluorocarbazolide, 3,6-dimethylcarbazolide, and 3,6-diphenylcarbazolide show a range of λmax values of emission from 418 to 511 nm. Detailed analysis supports that their emission bands originate from excited states with Cu metal-ligand charge transfer (MLCT) and/or ligand-centered (LC) π-π* transitions. Substitution of a methyl or trifluoromethyl group at the 1-position of the carbazolide moiety was also investigated to regulate the structural tuning of the copper emitters. From the X-ray crystallographic data of (SiHP2)Cu(1-methylcarbazolide) (7) and (SiHP2)Cu(1,3-di(trifluoromethyl)carbazolide) (8), unusual structural features, arising from the interaction of a SiH moiety with CH3 and CF3, respectively, were recognized. Such interaction forces the carbazolide moiety to tilt, while the copper geometry remains consistent with the other complexes. In the case of 8, a SiH···F3C interaction locks the carbazolide moiety in place, restricting its orbital overlapping with a copper-based orbital, according to the theoretical analysis by using density functional theory (DFT) computations. Thus, the unusual tilting results in deep-blue emission with a λmax of 430 nm.

Phosphorescent Carbene-Gold-Arylacetylide Materials as Emitters for Near UV-OLEDs.

A series of carbene-gold-acetylide complexes [(BiCAAC)AuCC]n C6 H5- n (n = 1, Au1; n = 2, Au2; n = 3, Au3; BiCAAC = bicyclic(alkyl)(amino)carbene) have been synthesized in high yields. Compounds Au1-Au3 exhibit deep-blue to blue-green phosphorescence with good quantum yields up to 43% in all media. An increase of the (BiCAAC)Au moieties in gold complexes Au1-Au3 increases the extinction coefficients in the UV-vis spectra and stronger oscillator strength coefficients supported by theoretical calculations. The luminescence radiative rates decrease with an increase of the (BiCAAC)Au moieties. The time-dependent density functional theory study supports a charge-transfer nature of the phosphorescence due to the large (0.5-0.6 eV) energy gap between singlet excited (S1 ) and triplet excited (T1 ) states. Transient luminescence study reveals the presence of both nonstructured UV prompt-fluorescence and vibronically resolved long-lived phosphorescence 428 nm. Organic light-emitting diodes (OLED) are fabricated by physical vapor deposition with 2,8-bis(diphenylphosphoryl)dibenzo[b,d]furan (PPF) as a host material with complex Au1. The near-UV electroluminescence is observed at 405 nm with device efficiency of 1% while demonstrating OLED device lifetime LT50 up to 20 min at practical brightness of 10 nits, indicating a highly promising class of materials to develop stable UV-OLEDs.

Deep-Blue and Fast Delayed Fluorescence from Carbene-Metal-Amides for Highly Efficient and Stable Organic Light-Emitting Diodes.

Linear gold complexes of the "carbene-metal-amide" (CMA) type are prepared with a rigid benzoguanidine amide donor and various carbene ligands. These complexes emit in the deep-blue range at 424 and 466 nm with 100% quantum yields in all media. The deep-blue thermally activates delayed fluorescence originates from a charge transfer state with an excited state lifetime as low as 213 ns, resulting in fast radiative rates of 4.7 × 106 s-1. The high thermal and photo-stability of these carbene-metal-amide (CMA) materials enabled the authors to fabricate highly energy-efficient organic light-emitting diodes (OLED) in host-guest architectures. Deep-blue OLED devices with electroluminescence at 416 and 457 nm with practical external quantum efficiencies of up to 23% at 100 cd m-2 with excellent color coordinates CIE (x; y) = 0.16; 0.07 and 0.17; 0.18 are reported. The operating stability of these OLEDs is the longest reported to date (LT50 = 1 h) for deep-blue CMA emitters, indicating a high promise for further development of blue OLED devices. These findings inform the molecular design strategy and correlation between delayed luminescence with high radiative rates and CMA OLED device operating stability.

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.