Conformationally-restricted bicarbazoles with phenylene bridges displaying deep-blue emission and high triplet energies: systematic structure-property relationships

The synthesis is reported of twelve new symmetrical carbazole dimers in which the carbazole units are linked via 1,4-phenylene spacers. There are two distinct series of compounds based on the position on the carbazole ring where the phenylene spacer is attached: this is either at carbazole C(3) (series 1a-1f) or at C(2) (series 2a-2f). The central phenylene ring is substituted with either two methyl, two methoxy or two cyano substituents which impart an intramolecular torsional angle between the phenylene and carbazole rings, thereby limiting the extent of π-conjugation between the carbazole units, and raising the triplet energies of the molecules to ET 2.6-3.0 eV, as determined from their phosphorescence spectra at 80 K. Structure-property relationships were studied by UV-vis and fluorescence spectroscopy, cyclic voltammetry and theoretical calculations. A notable observation is that substitution at the 2-position of carbazole (linear conjugation) exerts control over the position of the HOMO, while substitution at the 3-position of carbazole (meta conjugation) allows greater control over the LUMO. X-ray crystal structures are reported for two of the bicarbazoles. Compound 2d is shown to be a suitable host for the sky-blue emitter FIrpic in PhOLEDs, with improved device performance compared to CBP as host

Thermally Induced Defluorination during a <i>mer</i> to <i>fac</i> Transformation of a Blue-Green Phosphorescent Cyclometalated Iridium(III) Complex

The new homoleptic tris-cyclometalated [Ir­(C^∧N)₃] complexes <i>mer</i>-<b>8</b>, <i>fac</i>-<b>8</b>, and <i>fac</i>-<b>9</b> incorporating γ-carboline ligands are reported. Reaction of 3-(2,4-difluorophenyl)-5<i></i>-(2-ethylhexyl)-pyrido­[4,3-<i>b</i>]­indole <b>6</b> with iridium­(III) chloride under standard cyclometalating conditions gave the homoleptic complex <i>mer</i>-<b>8</b> in 63% yield. The X-ray crystal structure of <i>mer</i>-<b>8</b> is described. The Ir–C and Ir–N bonds show the expected bond length alternations for the differing <i>trans</i> influence of phenyl and pyridyl ligands. <i>mer</i>-<b>8</b> quantitatively isomerized to <i>fac</i>-<b>8</b> upon irradiation with UV light. However, heating <i>mer</i>-<b>8</b> at 290 °C in glycerol led to an unusual regioselective loss of one fluorine atom from each of the ligands, yielding <i>fac</i>-<b>9</b> in 58% yield. <i>fac</i>-<b>8</b> is thermally very stable: no decomposition was observed when <i>fac</i>-<b>8</b> was heated in glycerol at 290 °C for 48 h. The γ-carboline system of <i>fac</i>-<b>8</b> enhances thermal stability compared to the pyridyl analogue <i>fac</i>-Ir­(46dfppy)₃ <b>10</b>, which decomposes extensively upon being heated in glycerol at 290 °C for 2 h. Complexes <i>mer</i>-<b>8</b>, <i>fac</i>-<b>8</b>, and <i>fac</i>-<b>9</b> are emitters of blue-green light (λ_max^em = 477, 476, and 494 nm, respectively). The triplet lifetimes for <i>fac</i>-<b>8</b> and <i>fac</i>-<b>9</b> are ∼4.5 μs at room temperature; solution Φ_PL values are 0.31 and 0.22, respectively

Thermally Induced Defluorination during a <i>mer</i> to <i>fac</i> Transformation of a Blue-Green Phosphorescent Cyclometalated Iridium(III) Complex

The new homoleptic tris-cyclometalated [Ir­(C^∧N)₃] complexes <i>mer</i>-<b>8</b>, <i>fac</i>-<b>8</b>, and <i>fac</i>-<b>9</b> incorporating γ-carboline ligands are reported. Reaction of 3-(2,4-difluorophenyl)-5<i></i>-(2-ethylhexyl)-pyrido­[4,3-<i>b</i>]­indole <b>6</b> with iridium­(III) chloride under standard cyclometalating conditions gave the homoleptic complex <i>mer</i>-<b>8</b> in 63% yield. The X-ray crystal structure of <i>mer</i>-<b>8</b> is described. The Ir–C and Ir–N bonds show the expected bond length alternations for the differing <i>trans</i> influence of phenyl and pyridyl ligands. <i>mer</i>-<b>8</b> quantitatively isomerized to <i>fac</i>-<b>8</b> upon irradiation with UV light. However, heating <i>mer</i>-<b>8</b> at 290 °C in glycerol led to an unusual regioselective loss of one fluorine atom from each of the ligands, yielding <i>fac</i>-<b>9</b> in 58% yield. <i>fac</i>-<b>8</b> is thermally very stable: no decomposition was observed when <i>fac</i>-<b>8</b> was heated in glycerol at 290 °C for 48 h. The γ-carboline system of <i>fac</i>-<b>8</b> enhances thermal stability compared to the pyridyl analogue <i>fac</i>-Ir­(46dfppy)₃ <b>10</b>, which decomposes extensively upon being heated in glycerol at 290 °C for 2 h. Complexes <i>mer</i>-<b>8</b>, <i>fac</i>-<b>8</b>, and <i>fac</i>-<b>9</b> are emitters of blue-green light (λ_max^em = 477, 476, and 494 nm, respectively). The triplet lifetimes for <i>fac</i>-<b>8</b> and <i>fac</i>-<b>9</b> are ∼4.5 μs at room temperature; solution Φ_PL values are 0.31 and 0.22, respectively

A Selective Transformation of Enals into Chiral γ‑Amino Alcohols

A one-pot synthesis of chiral amino alcohols from α,β-unsaturated aldehydes is reported which circumvents competitive 1,2- versus 1,4-boryl addition, by means of using a sterically hindered amine-derived imine. In addition to the complete chemoselectivity, modification of the Cu(I) catalyst with readily available chiral diphosphines, such as (<i>R</i>)-DM-BINAP, gave the 1,4-boryl addition products with high levels of asymmetric induction

Spectroscopic and Structural Characterization of the CyNHC Adduct of B₂pin₂ in Solution and in the Solid State

The Lewis base adduct of B₂pin₂ and the NHC (1,3-bis­(cyclohexyl)­imidazol-2-ylidene), which was proposed to act as a source of nucleophilic boryl groups in the β-borylation of α,β-unsaturated ketones, has been isolated, and its solid state structure and solution behavior was studied. In solution, the binding is weak, and NMR spectroscopy reveals a rapid exchange of the NHC between the two boron centers. DFT calculations reveal that the exchange involves dissociation and reassociation of NHC rather than an intramolecular process

Structural Versatility of Pyrene-2-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolane) and Pyrene-2,7-bis(4,4,5,5-tetramethyl-[1,3,2]dioxaborolane)

Three polymorphs of pyrene-2,7-bis­(Bpin)₂ (<b>1</b>) and two of pyrene-2-(Bpin) (<b>2</b>), where Bpin = 4,4,5,5-tetramethyl-[1,3,2]­dioxaborolane, two different 1:1 co-crystals of <b>1</b> with toluene, and co-crystals of hexafluorobenzene (HFB) with <b>1</b> (of highly unusual 2:1 composition) and <b>2</b> (of usual 1:1 composition) were isolated, studied by X-ray diffraction and differential scanning calorimetry, and described using Hirshfeld surfaces and two-dimensional fingerprint plots. Centrosymmetric phases β- and γ-<b>1</b> have densities respectively lower and higher than the chiral α-<b>1</b>; α- and β-<b>2</b> have different packing modes, both with <i>Z</i>′ = 3. Compound <b>1</b> is prone to form channel host–guest structures, for example, α- and β-<b>1·</b>PhMe and <b>1</b>·2HFB. The drastically different stabilities of α- and β-<b>1·</b>PhMe are discussed. The complex <b>2·</b>HFB has a mixed-stack packing motif. The structural versatility of <b>1</b> and <b>2</b> is explained by synthon frustration between structurally incongruent pyrene and Bpin moieties

Structural versus Electrical Functionalization of Oligo(phenylene ethynylene) Diamine Molecular Junctions

We explore both experimentally and theoretically the conductance and packing of molecular junctions based on oligo­(phenyleneethynylene) (OPE) diamine wires, when a series of functional groups are incorporated into the wires. Using the scanning tunnelling microscopy break-junction (STM BJ) technique, we study these compounds in two environments (air and 1,2,4-trichlorobenzene) and explore different starting molecular concentrations. We show that the electrical conductance of the molecular junctions exhibits variations among different compounds, which are significant at standard concentrations but become unimportant when working at a low enough concentration. This shows that the main effect of the functional groups is to affect the packing of the molecular wires, rather than to modify their electrical properties. Our theoretical calculations consistently predict no significant changes in the conductance of the wires due to the electronic structure of the functional groups, although their ability to hinder ring rotations within the OPE backbone can lead to higher conductances at higher packing densities