Jens U. Engelhart

Stable Hexacenes through Nitrogen Substitution

The stabilization of larger acenes is a demanding task both synthetically as well as conceptually to be solved in different ways. Neither unsubstituted hexacene nor its higher homologues are stable, but their existence can be demonstrated in suitable matrixes. In the case of pentacene, two strategically attached TIPS-ethynyl groups suffice to fully stabilize and solubilize this material (TIPS = triisopropylsilyl). In larger acenes, however, two TIPS-ethynyl groups do not provide enough stabilization to furnish long-term persistent representatives. Anthony et al. demonstrated that even sterically encumbered hexacenes (with two tris(trimethylsilyl)silylethynyl substituents) react in solution under butterfly dimerization with a half-life of around 20 minutes. Only the introduction of four more aryl groups in lateral positions increases the stability of higher acenes so far that the Wudl and Chi groups could obtain persistent heptacene derivatives with a half-life of up to one week in solution. But even here, formation of endoperoxides is observed after some time. The laterally attached phenyl groups lead to isolation of the p systems with respect to their next neighbors, as evidenced by single-crystal structure analysis. We demonstrate herein that nitrogen atoms introduced into the acene skeleton give persistent disubstituted heterohexacenes, which are stable even when stored for longer periods of time. The palladium-catalyzed coupling of 1 with 2 in the presence of the ligand L gave the tetrazaacene 3 after oxidation with MnO2 in good yields (Scheme 1). [8, 9] The dichlorobenzoquinoxaline 4 also couples in good yields to give 5 (Scheme 2) However, attempts to oxidize 5 by MnO2, IBX = 2-iodoxybenzoic acid, N-bromosuccinimide (NBS), potassium chromate, pyridinium chlorochromate (PCC), or Cu(OAc)2 were fruitless. Difficult-to-separate product mixtures formed, but not the desired acene. This behavior was not entirely unexpected, as Kummer and Zimmermann had already unsuccessfully attempted to oxidize 6, readily obtained by co-melting of diaminonaphthalene and dihydroxyanthracene at 220 8C, using chloroanil or PbO2. Azahexacenes remained unknown. To maximize the shielding effect of the TIPS groups, it might be better to attach them in the center of the molecule. Consequently, 7a,b were coupled to 2 (Scheme 3). The Pdcatalyzed coupling works very well in the presence of L and furnishes 8a,b in good to excellent yields (92 %, 56%). Both are dehydrogenated by MnO2 into the azaacens 9 a,b in 56% and 74% yield, but it is not clear why this reaction does not work for 5. The heteroacenes 9a and 9b are greenish black crystalline powders, stable under laboratory conditions both as solids and in solutions, which display a green-yellow color. By NMR spectroscopy we could not detect endoperoxide formation or dimerization of 9 to butterfly cycloadducts. In our case, triisopropylsilyl and even the triethylsilyl groups are sufficient to stabilize and solubilize the hexacene skeleton, but 9b is considerably less soluble than 9a. To extend this chemistry, we prepared 7c in a multistep synthesis starting from diaminoScheme 1. Palladium-catalyzed synthesis of 3. dba = dibenzylideneacetone.

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.

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.

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.

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.

The effect of tuning the microstructure of TIPS-tetraazapentacene on the performance of solution processed thin film transistors

We report a comprehensive study of the symmetrical 6,13-bis(triisopropylsilylethynyl)tetraazapentacene (TIPS-TAP) used as an electron transporting material in organic field-effect transistors. We study the optical, electronic, structural and morphological properties of thin films of TIPS-TAP as deposited by spin-coating and zone-casting techniques. Depending on the solution processing conditions and procedures we find a variety of microstructures for TIPS-TAP ranging from highly polycrystalline to well-aligned crystalline films. Field-effect transistors are fabricated in two different architectures to evaluate the charge transport properties of TIPS-TAP in such films, and bias-stress experiments reveal a good electric stability of TIPS-TAP. The extracted electron mobilities vary over several orders of magnitude depending on the resulting morphology of the active layer reaching a maximum of 0.42 cm2 V−1 s−1 for uniaxial aligned crystallites in zone-cast transistors.

Electron-transporting phenazinothiadiazoles with engineered microstructure

Novel triisopropylsilyl-(TIPS)-alkynylated phenazinothiadiazoles were prepared by condensation of ortho-quinones and an alkynylated 5,6-diamino-2,1,3-benzothiadiazole. The targets show head-to-head dimerisation in the solid state, which, according to calculated transfer integrals should favor charge transport. The compounds form polycrystalline thin films when spin cast from solution. Two derivatives are attractive electron transporting materials with an average electron mobility μe in thin film transistors (TFT) of 0.07 cm2 V−1 s−1.

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.

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.

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.