Beyond p-Hexaphenylenes: Synthesis of Unsubstituted p-Nonaphenylene by a Precursor Protocol.

The synthesis of unsubstituted oligo-para-phenylenes (OPP) exceeding para-hexaphenylene-in the literature often referred to as p-sexiphenyl-has long remained elusive due to their insolubility. We report the first preparation of unsubstituted para-nonaphenylenes (9PPs) by extending our precursor route to poly-para-phenylenes (PPP) to a discrete oligomer. Two geometric isomers of methoxylated syn- and anti-cyclohexadienylenes were synthesized, from which 9PP was obtained via thermal aromatization in thin films. 9PP was characterized via optical, infrared and solid-state 13 C NMR spectroscopy as well as atomic force microscopy and mass spectrometry, and compared to polymeric analogues. Due to the lack of substitution, para-nonaphenylene, irrespective of the precursor isomer employed, displays pronounced aggregation in the solid state. Intermolecular excitonic coupling leads to formation of H-type aggregates, red-shifting emission of the films to greenish. 9PP allows to study the structure-property relationship of para-phenylene oligomers and polymers, especially since the optical properties of PPP depend on the molecular shape of the precursor.

Tetrapodal Diazatriptycene Enforces Orthogonal Orientation in Self-Assembled Monolayers.

Conformationally rigid multipodal molecules should control the orientation and packing density of functional head groups upon self-assembly on solid supports. Common tripods frequently fail in this regard because of inhomogeneous bonding configuration and stochastic orientation. These issues are circumvented by a suitable tetrapodal diazatriptycene moiety, bearing four thiol-anchoring groups, as demonstrated in the present study. Such molecules form well-defined self-assembled monolayers (SAMs) on Au(111) substrates, whereby the tetrapodal scaffold enforces a nearly upright orientation of the terminal head group with respect to the substrate, with at least three of the four anchoring groups providing thiolate-like covalent attachment to the surface. Functionalization by condensation chemistry allows a large variety of functional head groups to be introduced to the tetrapod, paving the path toward advanced surface engineering and sensor fabrication.

Pristine Poly( para-phenylene): Relating Semiconducting Behavior to Kinetics of Precursor Conversion.

We investigated unsubstituted poly( para-phenylene) (PPP), a long-desired prototype of a conjugated polymer semiconductor. PPP was accessed via thermal aromatization of a precursor polymer bearing kinked, solubility-inducing dimethoxycyclohexadienylene moieties. IR spectroscopy and Vis ellipsometry studies revealed that the rate of conversion of the precursor to PPP increases with temperature and decreases with film density, indicating a process with high activation volume. The obtained PPP films were analyzed in thin-film transistors to gain insights into the interplay between the degree of conversion and the resulting p-type semiconducting properties. The semiconducting behavior of PPP was further unambiguously proven through IR and transistor measurements of molybdenum trioxide p-doped films.

A Diketopyrrolopyrrole-Based Dimer as a Blue Pigment.

1,4-Diketopyrrolo[3,4-c]pyrroles (DPPs) constitute a class of hydrogen-bonded pigments covering a color range from yellow-orange to dark violet. We report facile dimerization of a Boc-substituted 3,6-dithienyl-DPP. The formed dimer shows brilliantly blue color as a result of the expansion of the chromophore. The soluble dimer acts as latent pigment and is converted to the corresponding pigment either via thermocleavage or acidic deprotection of the Boc groups. The synthesized pigment is characterized by high migrational stability in a polymer matrix and a clear blue color.

Structure-Property Relationship of Phenylene-Based Self-Assembled Monolayers for Record Low Work Function of Indium Tin Oxide.

Studying the structure-property relations of tailored dipolar phenyl and biphenylphosphonic acids, we report self-assembled monolayers with a significant decrease in the work function (WF) of indium-tin oxide (ITO) electrodes. Whereas the strengths of the dipoles are varied through the different molecular lengths and the introduction of electron-withdrawing fluorine atoms, the surface energy is kept constant through the electron-donating N, N-dimethylamine head groups. The self-assembled monolayer formation and its modification of the electrodes are investigated via infrared reflection absorption spectroscopy, contact angle measurements, and photoelectron spectroscopy. The WF decrease in ITO correlates with increasing molecular dipoles. The lowest ever recorded WF of 3.7 eV is achieved with the fluorinated biphenylphosphonic acid.

Palladium-Catalyzed Dimerization of Bis(2-biphenylyl)acetylene toward Sterically Hindered Acephenanthrylene.

The reaction of 2,2''-(1,2-ethynediyl)bis-1,1'-biphenyl under homogeneous (heterogeneous) catalysis with Pd/C/TMS-Cl results in a dimerization and the additional formation of five- and six-membered rings. The steric demand of the acetylene prevents a [2 + 2 + 2] cycloaddition, but paves the way toward an alternative reaction. By the formation of an unexpected planar acephenanthrylene, the system avoids steric stain. The synthetic scope and requirements are explored, and a reaction mechanism is proposed.

Immobilization Strategies for Organic Semiconducting Conjugated Polymers.

This Review details synthetic routes toward and properties of insoluble polymeric organic semiconductors obtained through desolubilization strategies. Typical applications include fixation of donor-acceptor bulk-heterojunction morphologies in organic photovoltaic cells, cross-linking of charge transport materials and active emitters in light emitting diodes or similar devices, and immobilization of morphologies in field effect transistors. A second important application is the structuring of organic semiconductors, using them as photoresists. After desolubilization, removal of the nonirradiated resist leads to elevated, micron-sized features of the semiconductor. In this Review, different strategies for desolubilization are covered. By photochemical or thermal cleavage of solubility-mediating groups such as esters, sulfonium salts, amides, ethers, and acetals or by retro-Diels-Alder reactions, volatile elimination products and the insoluble semiconductor are formed. In another case, desolubilization is achieved by cross-linking via functional groups present in the polymer side chains including vinyl, halide, silicone, boronic acid, and azide functionalities, which polymerize thermally or photochemically. Alternatively, small molecular additives such as photoacids, oligothiols, or oligoazides result in network formation in combination with compatible functional groups present in the immobilizable polymers. Advantages and disadvantages of the respective methods are discussed.

Chemical Tongues and Noses Based upon Conjugated Polymers.

We report the uses of conjugated polymers in multisensory applications and in chemical and optoelectronic tongues. We look at the potential of single polymers to discriminate multiple analytes and into small libraries of conjugated polymers that represent sensors. These small libraries combine several barely selective, promiscuous sensor elements and react with the analytes in a fairly non-selective fashion by change of color, emission wavelength, or emission intensity. In such optoelectronic noses and tongues, response of a single element is not specific or particularly useful at all, but the response pattern after the combination of several sensor elements is often specific for an analyte and allows discrimination and identification without any problem. These types of tongues and noses are well suited for quality control of foodstuff, beverages, and biological species such as proteins or cells. The discriminative process is often not well understood but it is powerful, particularly if the obtained data are analyzed by sophisticated statistical methods, i.e., linear discriminant analysis and/or principal component analysis. This added layer of analysis extracts the hidden information/patterns out of the data and allows visualization of the results.

Ultra-Narrow Low-Bandgap Graphene Nanoribbons from Bromoperylenes-Synthesis and Terahertz-Spectroscopy.

Soluble ultra-narrow armchair graphene nanoribbons (AGNRs) with length of ≈20 nm and completely fused cores exceeding 5 nm in length, pronounced near-infrared (NIR) absorption up to 1400 nm, and a low band gap (≈0.9 eV) are synthesized from 3,9(3,10)-dibromoperylene and 3,4,9,10-tetrabromoperylene. Since the monomers are obtained from perylene carboxyanhydrides, a remarkable link between dye and graphene chemistry is established. The two-step protocol for the AGNR synthesis by aryl-aryl coupling and subsequent cyclodehydrogenation is compared with a direct Yamamoto polymerization. Ultrafast photoconductivity analysis by optical pump-terahertz (THz) probe spectroscopy of the longest AGNRs excited in the NIR regime (800 nm) reveal their high intrinsic charge carrier mobility.

A New Solution to an Old Problem: Synthesis of Unsubstituted Poly(para-phenylene).

Unsubstituted and structurally well-defined poly(para-phenylene) (PPP) has been long-desired as an organic semiconductor prototype of conjugated polymers. To date, several attempts to synthesize unsubstituted, pristine, high-molecular-weight PPP have failed. Here we solved this synthetic problem by a versatile precursor route. Suzuki polymerization of kinked disubstituted 1,4-dimethoxycyclohexadienylene monomers yielded a well-soluble, nonaromatic precursor polymer. Its solubility allowed processing by spin-coating into nanometer-thick films. Subsequent additive-free thermal treatment induced aromatization and led to exclusively para-connected, highly fluorescent PPP with a length of about 75 phenylene units.

Versatile colorant syntheses by multiple condensations of acetyl anilines with perylene anhydrides.

We report a key step forward in rylene chemistry: the transformation of rylenes into novel chromophore families. The imidization of rylene anhydrides with 2-acetyl anilines could be controlled by the choice of the solvent, thus causing a transformation into either a 4-hydroxyquinoline (4-HQ) or a 4-oxoquinoline (4-OQ) unit. The 4-OQ motif contains an aminoenone group formed by intramolecular aldol condensation and is the first vinylogous rylene imide. The concept of vinylogy was further developed by utilizing 2,6-diacetyl aniline leading to an 3a-aza-1,6-phenalenedione-extended rylene skeleton fully embracing the nitrogen atom. By functionalization of the aminoenone motifs, for example, malononitrile addition at the carbonyl groups, the optical and electronic properties could be further tuned.

Toward perylene dyes by the Hundsdiecker reaction.

An efficient method to synthesize 3,4,9,10-tetrabromoperylenes is reported under optimized Hunsdiecker conditions. Various octasubstituted perylenes were obtained by reaction of 1,6,7,12-tetrachloro-3,4,9,10-tetrabromoperylene with phenol, trimethylsilyl chloride, cooper cyanide, or sulfur via metal-catalyzed couplings or nucleophilic substitutions. These new perylenes show completely different optical and redox properties, thus opening a facile way to develop new chromophophore structures.