1,2,3,4-Tetrafluorobiphenylene: A Prototype Janus-Headed Scaffold for Ambipolar Materials.

The title compound was synthesized by Ullmann cross-coupling in low yield as the first representative of [n]phenylene containing hydrocarbon and fluorocarbon rings. Stille/Suzuki-Miyaura cross-coupling reactions, as well as substitution of fluorine in suitable starting compounds, failed to give the same product. The geometric and electronic structures of the title compound were studied by X-ray diffraction, cyclic voltammetry and density functional theory calculations, together with Hirshfeld surface and reduced density gradient analyses. The crystal structure features head-to-tail π-stacking and other fluorine-related secondary bonding interactions. From the nucleus-independent chemical shifts descriptor, the four-membered ring of the title compound is antiaromatic, and the six-membered rings are aromatic. The Janus molecule is highly polarized; and the six-membered fluoro- and hydrocarbon rings are Lewis π-acidic and π-basic, respectively. The electrochemically-generated radical cation of the title compound is long-lived as characterized by electron paramagnetic resonance, whereas the radical anion is unstable in solution. The title compound reveals electrical properties of an insulator. On expanding its molecular scaffold towards partially fluorinated [n]phenylenes (n≥2), the properties presumably can be transformed into those of semiconductors. In this context, the title compound is suggested as a prototype scaffold for ambipolar materials for organic electronics and spintronics.

Crystal Structure and Predominant Defects in CdS Quantum Dots Fabricated by the Langmuir-Blodgett Method.

The crystal structure and shape of the CdS quantum dots (QDs) obtained by the Langmuir-Blodgett method were studied by transmission electron microscopy, extended X-ray absorption fine structure spectroscopy (EXAFS), and ultraviolet spectroscopy. X-ray photoelectron spectroscopy (XPS) and stationary photoluminescence spectroscopy (PL) methods were used to determine the dominant surface defects. Initially synthesized QDs within the Langmuir-Blodgett film of fatty behenic acid have a cubic structure and oblate spheroid shape, while free-standing QDs obtained after the matrix evaporation have a wurtzite structure and sphere-like shape. QDs within the matrix demonstrate a wide PL band centered at 2.3 eV corresponding to defect-assisted radiative recombination; after the matrix annealing and passivation of the QD surface in an ammonia atmosphere, the PL spectrum demonstrates a high-intensity band-edge peak together with a low-intensity defect-assisted shoulder. It was established that sulfur (VS) vacancies are the dominating defects. A model of simultaneous band-edge and defect-assisted recombination through the VS level was proposed.