The Palladium Way to N-Heteroacenes.

Novel synthetic methodologies allow increasingly efficient access to known organic materials, as well as the preparation of otherwise inaccessible species. Pd-catalyzed coupling of aromatic dihalides to ortho-diaminoarenes furnishes embedded stable N,N'-dihydropyrazines expediently and in often excellent yields. The embedded N,N'-dihydropyrazines can then be oxidized by MnO2 to give substituted azatetracenes, azapentacenes, azahexacenes, and azaheptacenes, which are soluble, processable, and stable. This powerful Pd-catalyzed methodology allows the preparation of azaacenes, including diaza-, tetraaza- and hexaazaacenes. In combination with a suitable Pd precursor, Buchwald-type biarylphosphines have been shown to give excellent results. Activated dihalides such as 2,3-dihaloquinoxalines are coupled easily under simplified conditions, whereas 2,3-dibromoacenes require more stringent conditions and advanced catalyst precursors. Pd catalysts effect the assembly of azaacenes with otherwise difficult to obtain substitution patterns. High yields and flexibility make this method most attractive.

Halogenated Symmetrical Tetraazapentacenes: Synthesis, Structures, and Properties.

We herein describe the synthesis and property evaluation of several brominated and chlorinated tetraazapentacenes. The targets were obtained by thermal condensation of 2,5-dihydroxyquinone with 4,5-dichloro-, 2,6-dichloro-, and 4,5-dibromo-1,2-phenylenediamine, followed by oxidation with hot acidic dichromate. Double alkynylation, reductive deoxygenation, and subsequent oxidation using MnO2 furnishes the target compounds. Absorption spectra, electrochemistry, and single crystal structures of the targets are reported. The 1,4,8,11-tetrachlorotetraazapentacene (1,4,8,11-tetrachloroquinoxalino[2,3-b]phenazine) carrying its chlorine atoms in the peri-positions packs in a herringbone type arrangement, while the isomer (2,3,9,10-tetrachloroquinoxalino[2,3-b]phenazine, with the chlorine atoms in the east and west positions) packs in one-dimensional stacks. In all cases, the reduction potentials and the calculated LUMO-positions are decreased by the introduction of the halogen atoms.

Unveiling Singlet Fission Mediating States in TIPS-pentacene and its Aza Derivatives.

Femtosecond pump-depletion-probe experiments were carried out in order to shed light on the ultrafast excited-state dynamics of triisopropylsilylethynyl (TIPS)-pentacene and two nitrogen-containing derivatives, namely, diaza-TIPS-pentacene and tetraaza-TIPS-pentacene. Measurements performed in the visible and near-infrared spectral range in combination with rate model simulations reveal that singlet fission proceeds via the extremely short-lived intermediate (1)TT state, which absorbs in the near-infrared spectral region only. The T1 → T3 transition probed in the visible region shows a rise time that comprises two components according to a consecutive reaction (S1 → (1)TT → T1). The incorporation of nitrogen atoms into the acene structure leads to shorter dynamics, but the overall triplet formation follows the same kinetic model. This is of particular importance, since experiments on tetraaza-TIPS-pentacene allow for investigation of the triplet state in the visible range without an overlapping singlet contribution. In addition, the pump-depletion-probe experiments show that the triplet absorption in the visible (T1 → T3) and near-infrared (T1 → T2) regions occurs from the same initial state, which was questioned in previous studies. Furthermore, an additional ultrafast transfer between the excited triplet states (T3 → T2) is identified, which is also in agreement with the rate model simulation. By applying depletion pulses, which are resonant with higher vibrational levels, we gain insight into internal vibrational energy redistribution processes within the triplet manifold. This additional information is of great relevance regarding the study of loss channels within these materials.

Substituted Tetraaza- and Hexaazahexacenes and their N,N'-Dihydro Derivatives: Syntheses, Properties, and Structures.

The palladium-catalyzed coupling of a substituted o-diaminoanthracene and a substituted o-diaminophenazine to substituted 2,3-dichloroquinoxalines furnishes 10 differently substituted N,N'-dihydrotetraaza- or -hexaazahexacenes with the quinoxaline group of the azaacenes carrying fluorine, chlorine, or nitro groups. The N,N'-dihydrotetraazahexacenes with hydrogen, chlorine, and fluorine subtituents are oxidized to azaacenes, whereas only the parent N,N'-dihydrohexaazahexacenes, with hydrogen substituents, are oxidized by MnO2. The resultant azaacenes are characterized by their optical and spectroscopic data. In addition, single-crystal X-ray structures have been obtained for the parent tetraazahexacenes and their difluoro-substituted derivatives. The di- and tetrachloro derivatives of the N,N'-dihydrohexaazahexacene have also been structurally characterized.

Soluble diazaiptycenes: materials for solution-processed organic electronics.

The synthesis and characterization of soluble azaiptycenes is reported. Optical and physical properties were studied and compared with those of the structurally consanguine azaacenes. Electrochemical experiments and quantum-chemical calculations revealed the electronic structure of the iptycene derivatives. Their crystallization behavior was examined. A highly fluorescent amorphous diazatetracene derivative was integrated into a simple organic light-emitting diode, showing enhanced performance compared with that of previously reported, structurally similar tetracenes.

A persistent diazaheptacene derivative.

We describe the synthesis of characterizable diazaheptacene derivatives. Diazaheptacenes need four silylethynyl protecting groups to be isolable. TIPS-ethynyl groups are not bulky enough to allow stabilization. Four Si(sec-Bu)3-ethynyl groups symmetrically attached to the acene core sufficiently protect the formed diazaheptacene from dimerization through Diels-Alder reaction. It was characterized by NMR and UV-vis spectroscopies and cyclic voltammetry.

Acceleration of Singlet Fission in an Aza-Derivative of TIPS-Pentacene.

The influence of the carbon to nitrogen substitution on the photoinduced dynamics of TIPS-pentacene was investigated by ultrafast transient absorption measurements on spin-coated thin films in the visible and in the near-infrared spectral region. A global target analysis was performed to provide a detailed picture of the excited-state dynamics. We found that the chemical modification has a high impact on the triplet formation and leads to shorter dynamics; hence it speeds up the singlet fission process. A faster relaxation from the singlet into the triplet manifold implies a higher efficiency because other relaxation channels are avoided. The air-stable aza-derivatives have the potential to exceed the energy conversion efficiency of TIPS-pentacene.

Partially fluorinated tetraazaacenes by nucleophilic aromatic substitution.

We report the sodium hydride-mediated reactions of a diethynylated diaminophenazine with perfluorobenzene, perfluoronaphthalene, and two octafluoroanthracene derivatives. In all of the cases, an N,N-dihydropyrazine ring is formed, and partially fluorinated tetraazapentacenes, tetraazahexacenes, and tetraazaheptacenes (in their respective N,N-dihydro forms) are easily prepared. In the case of the dihydrotetraazapentacenes and -hexacenes, oxidation with manganese dioxide is possible to give the desired, fully unsaturated tetraazaacenes; two molecules of the azahexacene undergo a Diels-Alder reaction in which an alkyne substituent in the conserved hexacene unit works as the dienophile while the tetraazahexacene participates as the diene to give an unsymmetrical dimer. All of the coupling targets were investigated by NMR and UV-vis spectroscopies, and several single-crystal structures of the N,N-dihydrotetraazaacenes and also that of the tetrafluorotetraazaacene were obtained.

Large N-heteroacenes: new tricks for very old dogs?

Azaacenes have been known for a very long time, either as N,N'-dihydro compounds or in their oxidized form as 4n+2π systems, but only recently have processable and charcterizable derivatives been sought. In the last three years synthetic routes to large N-heteroacenes have been developed. In particular, the Pd-catalyzed coupling of aromatic diamines with activated aromatic dihalogenides has enabled simple access to numerous new azaacenes. Since 2010, azapentacene and stabile oligoazahexacene have been synthesized, as well as a symmetrical tetraazapentacene, which acts as an excellent electron-transport material for thin-film transistors.

From thia- to selenadiazoles: changing interaction priority.

The synthesis, optical, and electrochemical properties as well as solid-state structures of a series of alkynylated, benzannulated selenadiazoles are reported. This set of compounds is compared to the lighter homologue series, the thiadiazoles. The selenadiazoles show head-to-head dimerization in the solid state, while packing of the thiadiazoles was dominated by the steric bulk of the side groups. The Se-N interaction is a supramolecular motif that should drive the effective self-assembly and modulate charge transport when these compounds are used as thin films in devices.

Reactions of large tetraaza-N,N'-dihydroacenes: formation of unexpected adducts and an unstable tetraazahexacene.

We report the reaction of several N,N'-dihydrotetraazaacenes with oxalylic chloride to give unusual adducts, which were characterized by single-crystal X-ray structures.

Synthesis and optical properties of diaza- and tetraazatetracenes.

A series of functionalized diaza- and tetraazatetracenes was synthesized, either by condensation of an aromatic diamine with an ortho-quinone/diethyloxalate followed by chlorination with POCl(3) to give diazatetracenes or by palladium-catalyzed coupling of a phenylenediamine with various 2,3-dichloroquinoxalines to give tetraazatetracenes (after oxidation with MnO(2)). Representative examples included halogenated and nitrated derivatives. The optical properties of these azatetracenes were discussed with respect to their molecular structures and substitution patterns. The diazatetracenes and tetraazatetracenes formed two different groups that had significantly different electronic structures and properties. Furthermore, 1,2,3,4-tetrafluoro-6,11-bis((triisopropylsilyl)ethynyl)benzo[b]phenazine was synthesized, which is the first reported fluorinated diazatetracene. Single-crystal X-ray analysis of this compound is reported.

Large azaacenes: pyridine rings reacting like carbonyl groups.

N,N'-Dihydropentaazapentacenes and -hexacenes displaying terminal pyridine rings are surprisingly easily oxidized by MnO(2) into their corresponding pyridones.

Solvent-free aromatic C-H functionalisation/halogenation reactions.

The solvent-free, palladium-catalysed reaction of anilides with CuCl(2) in the presence or absence of copper acetate yields ortho-chlorinated anilides in good to excellent yields, even on a large scale (100 mmol). By contrast, the equivalent reactions with copper bromide, either solvent free or in 1,2-dichloroethane, in the presence or absence of palladium, under air or inert conditions, gave the products of simple electrophilic bromination. Mechanistic studies highlighted the involvement of palladacyclic intermediates, one of which was characterised crystallographically, which undergo subsequent reaction with copper(II) chloride to yield the chlorinated anilide products.