Structural Characterization of Quaterphenyl Cation Radical: X-ray Crystallographic Evidence of Quinoidal Charge Delocalization in Poly-\u3cem\u3ep\u3c/em\u3e-phenylene Cation Radicals

Quaterphenyl derivative (QP) containing tert-butyl solubilizing groups at the terminal positions yields a stable cation radical salt that was isolated, and its structure was established by X-ray crystallography. The crystal structure of neutral QP and its cation radical (QP+•SbCl6-) provides unequivocal evidence for the quinoidal stabilization of the cationic charge or polaron by smoothing out the torsional motion of the interconnected p-phenylene rings. Such an observation of stabilization of the cationic charge in a poly-p-phenylene (PPP) derivative forms the basis for the noted high conductivities in PPP oligomers in their doped state

Hexabenzo[4.4.4]propellane: A Helical Molecular Platform for the Construction of Electroactive Materials

Helical hexabenzo[4.4.4]propellane (a relative of hexaphenylethane) and its derivatives are synthesized and their structures are established by X-ray crystallography. Isolation and X-ray crystallographic characterization of a robust trication-radical salt of hexamethoxypropellane derivative confirms that its framework is stable toward oxidative (aliphatic) C−C bond cleavage. It is also demonstrated that propellane can be easily brominated at the 4,4‘-positions of the biphenyl linkages for its usage as a molecular platform for the preparation of electroactive materials

A Versatile Synthesis of Electroactive Stilbenoprismands for Effective Binding of Metal Cations

A versatile synthesis of a new class of polyaromatic receptors (stilbenoprismands) containing a Δ-shaped cavity similar to that of the π-prismand together with an intimately coupled electroactive stilbenoid moiety was accomplished via an efficient intramolecular McMurry coupling reaction. The presence of the Δ-shaped cavity in stilbenoprismands allows an efficient binding of a single silver cation as probed by 1H NMR spectroscopy. Electron-rich stilbenoprismands undergo a ready oxidation to their highly robust cation−radical and dicationic salts. X-ray structure determination of a representative dicationic stilbenoprismand showed that the charges were largely localized on the tetraarylethylene moiety, which results in a twisting of the ethylenic C═C bond by ∼35°. Moreover, the electronic coupling among the stilbenoid and π-prismand moieties in various stilbenoprismands was briefly probed by optical methods

Structural Characterization of Quaterphenyl Cation Radical: X-ray Crystallographic Evidence of Quinoidal Charge Delocalization in Poly-\u3cem\u3ep\u3c/em\u3e-phenylene Cation Radicals

Quaterphenyl derivative (QP) containing tert-butyl solubilizing groups at the terminal positions yields a stable cation radical salt that was isolated, and its structure was established by X-ray crystallography. The crystal structure of neutral QP and its cation radical (QP+•SbCl6-) provides unequivocal evidence for the quinoidal stabilization of the cationic charge or polaron by smoothing out the torsional motion of the interconnected p-phenylene rings. Such an observation of stabilization of the cationic charge in a poly-p-phenylene (PPP) derivative forms the basis for the noted high conductivities in PPP oligomers in their doped state

From Static to Dynamic: Electron Density of HOMO at Biaryl Linkage Controls the Mechanism of Hole Delocalization

In order to extend the physical length of hole delocalization in a molecular wire, chromophores of increasing size are often desired. However, the effect of size on the efficacy and mechanism of hole delocalization remains elusive. Here, we employ a model set of biaryls to show that with increasing chromophore size, the mechanism of steady-state hole distribution switches from static delocalization in biaryls with smaller chromophores to dynamic hopping, as exemplified in the largest system, tBuHBC2 (i.e., “superbiphenyl”), which displays a vanishingly small electronic coupling. This important finding is analyzed with the aid of Hückel molecular orbital and Marcus–Hush theories. Our findings will enable the rational design of the novel molecular wires with length-invariant redox/optical properties suitable for long-range charge transfer

A Polyaromatic Receptor with an Ethereal Fence that Directs K\u3csup\u3e+\u3c/sup\u3e for Effective Cation−π Interaction

We have designed and synthesized a HAB-based receptor with six ethereal oxygens on one face of the central benzene ring by a trimerization of a diarylacetylene in which the ethereal oxygens are tied together with a tetramethylene bridge. This unique amphiphilic receptor allows an efficient binding of a single potassium cation by a synergistic interaction with the polar ethereal fence and with the central benzene ring via cation−π interaction. Furthermore, the ready accessibility of this unique receptor with a bipolar binding pocket will allow the exploration of its usage for developing efficient sensing devices for various metal cations

Electron Transfer Prompted Ejection of a Tightly Bound K\u3csup\u3e+\u3c/sup\u3e from the Ethereal Cavity of a Hexaarylbenzene-Based Receptor

Synthesis of a pair of rotamers (9u/9s) of a hexaarylbenzene derivative containing six (cofacially arranged) electroactive 2,5-dimethoxytolyl groups is described. The toroidal electronic stabilization due to the circular arrangement of aryl groups in 9u/9s leads to the observation of multiple (reversible) oxidation waves and lowering of their Eox1 by ∼250 mV relative to model compounds. The binding of K+ to symmetrical rotamer 9s was monitored by an electrochemical method and further confirmed by X-ray crystallography

Terphenyl Crowns: a New Family of Receptors Containing Ethereal Canopies that Direct Potassium Cation onto Benzenoid Platforms for Cation–π Interactions

We have synthesized three simple and versatile terphenyl crowns (TC) receptors containing ethereal canopies that direct a potassium cation for efficient cation–π interactions as established by 1H NMR spectroscopy and by isolation and X-ray crystallography of their K+ salts

Molecular Recognition of NO/NO\u3csup\u3e+\u3c/sup\u3e via Multicenter (Charge-Transfer) Binding to Bridged Diarene Donors. Effect of Structure on the Optical Transitions and Complexation Thermodynamics

Bridged diarenes form very strong [1:1] complexes with nitrosonium/nitric oxide in which the NO moiety is optimally sandwiched in the cleft between a pair of cofacial aromatic rings which act as a molecular “Venus flytrap”. The spectral features of these associates are generally similar to those for [1:1] and [2:1] nitrosonium complexes with mononuclear alkyl-substituted benzenes, and they are appropriately described within the LCAO molecular-orbital methodology and the Mulliken (charge-transfer) formulation of donor/acceptor electronic transitions. The thermodynamics study indicates that the efficient binding is determined by (i) the close matching of the donor/acceptor redox potentials and (ii) the ability of bridged diarenes for multicentered interactions with a single NO moiety. The best fit of the electronic and structural parameters is provided by a calixarene host that allows the interacting centers to be arranged in a manner similar to those extant in [2:1] nitrosonium complexes with analogous (nonbridged) aromatic donors; this results in its very strong noncovalent binding with nitrosonium/nitric oxide with the formation constant of KB ≈ 108 M-1 and free-energy change of −ΔG° = 45 kJ mol-1. Such strong, selective, and reversible bindings of nitrosonium/nitric oxide by (cofacial) aromatic centers thus provide the basis for the development of efficient NO sensors/absorbents and also suggest their potential relevance to biochemical systems

Silver(I) Complexation of (Poly)aromatic Ligands. Structural Criteria for Depth Penetration into \u3cem\u3ecis\u3c/em\u3e-Stilbenoid Cavities

Silver(I) complexes with aromatic donors are thoroughly analyzed (with aid of the Cambridge Crystallographic Database) to identify the basic structural factors inherent to the bonding of an arene ligand. Most strikingly, the distance parameter d (which simply measures the normal separation of Ag from the mean aromatic plane) is singularly invariant at d = 2.41 ± 0.05 Å for all silver/arene complexes, independent of the hapticity (η1 or η2), hybridization, or multiple coordination. As such, a systematic series of stilbenoid ligands has been successfully designed to precisely modulate the penetration of silver(I) into the ligand cleft, and a multicentered poly(arene) ligand (X) designed to form a one-dimensional assembly of Ag/arene units. Simply stated, the depth penetration of silver(I) into the aromatic cavities of various cis-stilbenoid donors can be precisely predicted with a single parameter γ that measures the separation of the two cofacial aryl groups comprising the cleft. This simple geometric consideration must be taken into account in any successful design of novel (poly)aromatic ligands for silver(I) complexation to constitute new molecular architectures