ABSTRACT
A representative, low-temperature gas-phase reaction mechanism synthesizing polyacenes via ring annulation exemplified by the formation of pentacene (C22 H14 ) along with its benzo[a]tetracene isomer (C22 H14 ) is unraveled by probing the elementary reaction of the 2-tetracenyl radical (C18 H11 . ) with vinylacetylene (C4 H4 ). The pathway to pentacene-a prototype polyacene and a fundamental molecular building block in graphenes, fullerenes, and carbon nanotubes-is facilitated by a barrierless, vinylacetylene mediated gas-phase process thus disputing conventional hypotheses that synthesis of polycyclic aromatic hydrocarbons (PAHs) solely proceeds at elevated temperatures. This low-temperature pathway can launch isomer-selective routes to aromatic structures through submerged reaction barriers, resonantly stabilized free-radical intermediates, and methodical ring annulation in deep space eventually changing our perception about the chemistry of carbon in our universe.
ABSTRACT
The thermal and photochemical syntheses of 2,3,9,10-tetrafluoropentacene (F4PEN) from 6,13-etheno bridged precursors were investigated computationally and experimentally. A computational study of the retro-Diels-Alder reaction to give 2,3,9,10-tetrasubstituted pentacenes and pyridazine revealed a linear correlation between barrier height and substituent constant (σp) indicative of an electronic effect that could diminish the yield of electron-poor 2,3,9,10-tetrasubstituted pentacenes in this reaction. The photochemical route from the corresponding bridged α-diketone yields F4PEN, which was characterized photophysically, electrochemically, and structurally. The compound crystallizes in a herringbone motif with quite short intermolecular F-F contacts that are, however, only very weakly bonding according to computations. The electrochemical and photophysical data show that the HOMO-LUMO gap of F4PEN is increased compared to that of PEN. This is due to an increase of the oxidation potential of F4PEN by 0.18 V in combination with an essentially unchanged reduction potential. The radical cation and dication of F4PEN could be generated in oxidizing solvents and characterized by optical spectroscopy and ESR or NMR, respectively. Both charged F4PEN species persist for days in solution.
ABSTRACT
The synthesis of a hexa-peri-hexabenzocoronene (HBC) with a central borazine core is described. The solid-state structure of this BN-doped HBC (BN-HBC) is isotypic with that of the parent HBC. Scanning tunneling microscopy shows that BN-HBC lies flat on Au(111) in a two-dimensional pattern.
