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 ¹³C 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.

Transformation from helical to layered supramolecular organization of asymmetric perylene diimides via multiple intermolecular hydrogen bonding

The solid-state supramolecular organization of asymmetric perylene diimide is transformed from helical to layered self-assembly after thermal annealing.

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.

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.

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.

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.