Heiner Detert

Soluble oligo(phenylenevinylene)s with electron withdrawing substituents for the use in light emitting diodes

Poly(p-phenylenevinylene) (PPV) and its derivatives are one of the most widely used classes of phosphors in light emitting diodes. Central problems are the imbalanced charge carrier injection and transport properties due to the high lying conduction band. A series of good soluble oligo(phenylenevinylene)s (OPV) with five benzene rings and a variety of electron withdrawing and accepting substituents have been prepared and may serve as model compounds for PPV with increased electron affinity. The central ring of the OPVs is substituted with two octyloxy groups to ensure good solubility and the lateral styrene units carry further chromophoric groups of different electronic character. The synthesis was performed via Horner olefination and Knoevenagel condensation. The electronic spectra of these OPVs are dominated by the main chromophore, further auxochrome groups on the lateral rings (m- and p-positions) shift absorption and emission spectra only slightly to longer wavelengths. Significant bathochrome shifts are observed for OPVs with cyano-groups on the terminal vinylene segments. The OPVs are photochemical stable versus irradiation with NUV whereas MUV and traces of acids cause decomposition of the chromophore.

Thermotropic Properties and Molecular Packing of Discotic Tristriazolotriazines with Rigid Substituents

Tristriazolotriazines with a threefold dialkoxyaryl substitution have been prepared by Huisgen reaction of cyanuric chloride and the corresponding tetrazoles. Although these dyes show a negative or inverted solvatochromism of the UV/Vis absorption, their fluorescence is strongly positive solvatochromic. These discotic fluorophores are also emissive in their solid state and in their broad liquid-crystalline mesophase. The structural study indicates that the thermotropic properties and organization of these systems can be well tuned by the steric demand of the aryl groups. Depending on the substituents, the compounds showed either a pure crystalline phase or a highly complex helical superstructure with a characteristic liquid-crystalline phase at elevated temperatures. Changing the steric demand of the attached aryls allowed controlling the discs arrangement within the columnar helix, which is of great importance for the molecular orbital overlap.

Practical Synthesis of Vinyl-Substitutedp-Phenylenevinylene Oligomers and Their Triethoxysilyl Derivatives

Luminescent semiconducting organic compounds are widely used as active layers in electro-optical devices. Apart from conjugated polymers, monodisperse oligomers also represent attractive materials. The synthesis of stilbenoid oligomers with polymerizable end groups is presented. Oligo(phenylenevinylene)s with terminal vinyl groups 17–19 are prepared in good yields by Horner–Emmons olefinations or by the Heck reaction of the iodo-substituted oligomers 15, 16 with compressed ethene. Triethoxysilyl groups can be linked via rigid 1,2-vinylene units to the chromophores 26–30, either in the direct reaction of 14, 24 with silanes 21, 22 or by cross-metathesis of 17–19 with the vinylsilanes 21, 22 using Grubbs catalyst.

Synthesis and Electronic Spectra of Substitutedp-Distyrylbenzenes for the Use in Light-Emitting Diodes

The influence of substitution on the absorption and Luminescence spectra of oligo(phenylenevinylene)s has been studied using distyrylbenzene (DSB) as a model compound. The degree, character, and pattern of substitution was varied systematically, altering the electronic properties of the DSB, the wavelength of the emitted light could be tuned over a range of 100 nm. The syntheses of 6b—h were performed by twofold Wittig Horner-olefinations of bisphoshonates 1a, b with substituted benzaldehydes 2a—i, 6ivia Heck-reaction of the dibromosulfonylbenzene 3, 6k by Siegrist-reaction of 4 with N-phenylbenzaldimine and the Knoevenagel-reaction of benzyl cyanide with 5 led to 6l.

Stilbenoid phosphors with increased electron affinity: sulfones as electron accepting groups

Stilbenoid compounds are one of the preferred classes of luminescent materials for the use in light emitting diodes (LEDs). A problem connected with the use of poly(phenylenevinylene) (PPV) in LEDs is the unbalanced charge carrier transport, electron injection from aluminium and electron transport properties are only poor. The substitution with electron accepting groups like nitriles is a suitable way to lower the LUMO level of the chromophore. The electron withdrawing strength of sulfones is comparable to cyanides and the second binding site is useful to attach solubilising side chains. A two-step synthesis of model compounds and polymers from ethene and sulfonyl dibromobenzene and bromo aromatics is described. The electronic spectra of the model compounds and the polymers are discussed.

3,7,11-Tris{4-[(1R,3S,4S)-neomenth­yl­oxy]phen­yl}tri[1,2,4]triazolo[4,3-a:4′,3′-c:4′′,3′′-e][1,3,5]triazine–chloro­form–ethanol (1/1/1)

The title compound, C54H69N9O3·CHCl3·C2H5OH, was prepared by a threefold nucleophilic substitution of p-neomenthyloxyphenyltetrazole on cyanuric chloride followed by threefold cycloelimination of nitrogen and ring closure. The central tristriazolotriazine is roughly planar with a maximum deviation of 0.089 (7) Å but the adjacent benzene rings are twisted out of this plane. N—C—C—C torsion angles of −80.2 (9), 159.3 (7) and 50.6 (10)° destroy the formal C3 symmetry. Cavities are found between the phenoxy residues: one is occupied by a chloroform molecule, another by ethanol forming a hydrogen bond to a triazole ring while two isopropyl groups point into the third void. One methyl group and the chlorofrm molecule are disorderd and were refined using a split model.

Monoclinic polymorph of 2,5-bis[4-(dimethyl­amino)­styr­yl]-3,6-dimethyl­pyrazine

The title compound, C26H30N4, was prepared by condensation of tetramethylpyrazine and dimethylaminobenzaldehyde and crystallizes from chloroform/methanol in two different forms. Block-shaped crystals belong to the monoclinic crystal system and plates to the triclinic system. The two crystal forms differ in the arrangement of the centrosymmetric molecules, which have nearly identical geometries. In the monoclinic crystals reported here, planar molecules [maximum deviation = 0.062 (2) Å], with a transoid arrangement of the (E)-styryl units and completely planarized dimethylamino groups [sum of the C—N bond angles = 359.9 (2)°], form layers connected via H–π-stacking. The dihedral angle between the central and pendant rings is 1.30 (8)°. The triclinic polymorph contains two half molecules, both completed by crystallographic inversion symmetry.

Light-Induced Alkylation of (Hetero)aromatic Nitriles in a Transition-Metal-Free C–C-Bond Metathesis

A light-induced C-C-σ-bond metathesis was achieved through transition-metal-free activation of an unstrained C(sp3)-C(sp3)-σ-bond in 1-benzyl-1,2,3,4-tetrahydroisoquinolines. A photoredox-mediated single-electron oxidation of these precursor amines yield radical cations which undergo a homolytic cleavage of a C(sp3)-C(sp3)-σ-bond rather than the well-known α-C-H-scission. The resulting carbon-centered radicals are used in the ipso-substitution of (hetero)aromatic nitriles proceeding through another single-electron transfer-mediated C-C-bond cleavage and formation.

2,5-Bis[4-(dimethyl-amino)-phen-yl]-3,6-dimethyl-pyrazine.

The title compound, C22H26N4, was prepared from p-dimethylaminopropiophenone in six steps. The molecule has no crystallographic symmetry. The dihedral angles between the pyrazine ring and the phenyl rings are 35.81 (6) and 37.11 (8)°. The dimethylamino groups are essentially planar (sum of the bond angles at N = 359.3 and 359.9°) and nearly coplanar with the adjacent aromatic ring [dihedral angles = 5.54 (11) and 7.40 (3)°]. This effect and the short aniline C—N bonds can be rationalised in terms of charge transfer from the amino groups to the central pyrazine ring.

5-Methyl-3-(3-methyl­phen­yl)-7-phenyl-1,2,4-triazolo[4,3-c]pyrimidine

The title compound, C19H16N4, is one of the few known 3,7-diaryl-1,2,4-triazolo[4,3-c]pyrimidines. The triazolopyrimidine unit is essentially planar (r.m.s. deviation = 0.048 Å). The phenyl ring and the heterocyclic core subtend a dihedral angle of only 15.09 (6)°, whereas the m-tolyl ring is twisted by 71.80 (6)° out of the plane of the triazole ring. Two C—H⋯N hydrogen bonds and π–π stacking interactions [centroid–centroid distance = 3.7045 (8) Å] stabilize the crystal packing.