Graphyne allotropes of carbon are fascinating materials, and their electronic properties are predicted to rival those of the “wonder material” graphene. One allotrope of graphyne, having rectangular symmetry rather than hexagonal, stands out as particularly attractive, namely 6,6,12-graphyne. It is currently an insurmountable challenge, however, to design and execute a synthesis of this material. Herein, we present synthesis and electronic properties of molecules that serve as model compounds. These oligomers, so-called radiaannulenes, are prepared by iterative acetylenic coupling reactions. Systematic optical and redox studies indicate the effective conjugation length of the radiaannulene oligomers is nearly met by the length of the trimer. The HOMO-LUMO gap suggested by the series of oligomers is still, however, higher than that expected for 6,6,12-graphyne from theory, which predicts two nonequivalent distorted Dirac cones (no band gap). Thus, the radiaannulene oligomers present a suitable length in one dimension of a sheet, but should be expanded in the second dimension to provide a unique representation of 6,6,12-graphyne.
Extended tetrathiafulvalenes with central diindenothienoacene cores were prepared and studied for their redox and spectroelectrochemical properties, which depended strongly on the orientation of the thiophene rings. The cations undergo remarkably strong associations, rendering them attractive as redox-controllable tectons in supramolecular chemistry, and in one case crystals were grown by electrocrystallization.
Two tetraceno[2,1,12,11-opqra]tetracene-extended tetrathia-fulvalenes were prepared and found to undergo reversible conversion into their planar polycyclic aromatic hydrocarbons (PAHs) upon electrochemical oxidation - at potentials probing the best valence bond representations.
Accurate predictions of redox potentials through a simple methodology employing methods that are all implemented in standard electronic structure packages.
Development of molecules
that can switch between redox states with
paired and unpaired electrons is important for molecular electronics
and spintronics. In this work, a selection of redox-active indenofluorene-extended
tetrathiafulvalenes (IF-TTFs) with thioacetate end groups was prepared
from a readily obtainable dibromo-functionalized IF-TTF building block
using palladium-catalyzed cross-coupling reactions, such as the Suzuki
reaction. The end groups served as electrode anchoring groups for
single-molecule conductance studies, and the molecules were subjected
to mechanically controlled break-junction measurements with gold contacts
and to low-bias charge transport measurements in gated three-terminal
electromigration junctions. The neutral molecules showed clear conductance
signatures, and somewhat surprisingly, we found that a meta–meta anchoring configuration gave a higher conductance than a para–meta configuration. We explain this behavior
by “through-space” coupling between the gold electrode
and the phenyl on which the anchoring group is attached. Upon charging
the molecule in a gated junction, we found reproducibly a Kondo effect
(zero-bias conductance) attributed to a net spin. Ready generation
of radical cations was supported by cyclic voltammetry measurements,
revealing stepwise formation of radical cation and dication species
in solution. The first oxidation event was accompanied by association
reactions as the appearance of the first oxidation peak was strongly
concentration dependent.
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