Unraveling the Coulombic Forces in Electronically Decoupled Bichromophoric Systems during Two Successive Electron Transfers.

Coulombic forces are vital in modulating the electron transfer dynamics in both synthetic and biological polychromophoric assemblies, yet quantitative studies of the impact of such forces are rare, as it is difficult to disentangle electrostatic forces from simple electronic coupling. To address this problem, the impact of Coulombic interactions in the successive removal of two electrons from a model set of spirobifluorenes, where the interchromophoric electronic coupling is nonexistent, is quantitatively assessed. By systematically varying the separation of the bifluorene moieties using model compounds, ion pairing, and solvation, these interactions, with energies up to about 0.4 V, are absent at distances greater than about 9 Å. These findings can be (quantitatively) applied for the design of polychromophoric assemblies, whereby the redox properties of donors and/or acceptors can be tuned by judicious positioning of the charged groups to control the electron-transfer dynamics.

Charge delocalization in self-assembled mixed-valence aromatic cation radicals.

The spontaneous assembly of aromatic cation radicals (D(+•)) with their neutral counterpart (D) affords dimer cation radicals (D(2)(+•)). The intermolecular dimeric cation radicals are readily characterized by the appearance of an intervalence charge-resonance transition in the NIR region of their electronic spectra and by ESR spectroscopy. The X-ray crystal structure analysis and DFT calculations of a representative dimer cation radical (i.e., the octamethylbiphenylene dimer cation radical) have established that a hole (or single positive charge) is completely delocalized over both aromatic moieties. The energetics and the geometrical considerations for the formation of dimer cation radicals is deliberated with the aid of a series of cyclophane-like bichromophoric donors with drastically varied interplanar angles between the cofacially arranged aryl moieties. X-ray crystallography of a number of mixed-valence cation radicals derived from monochromophoric benzenoid donors established that they generally assemble in 1D stacks in the solid state. However, the use of polychromophoric intervalence cation radicals, where a single charge is effectively delocalized among all of the chromophores, can lead to higher-order assemblies with potential applications in long-range charge transport. As a proof of concept, we show that a single charge in the cation radical of a triptycene derivative is evenly distributed on all three benzenoid rings and this triptycene cation radical forms a 2D electronically coupled assembly, as established by X-ray crystallography.

ortho-Phenylenes: unusual conjugated oligomers with a surprisingly long effective conjugation length.

ortho-Phenylenes represent a fundamental but relatively unexplored class of conjugated molecular architecture. We have developed a robust synthetic approach to monodisperse o-phenylene oligomers which we have demonstrated by synthesizing a homologous series up to the dodecamer. The o-phenylenes exhibit complex conformational behavior but are biased toward a specific 2-fold-symmetric conformation which we believe corresponds to a stacked helix. Surprisingly, the series exhibits long-range delocalization, as measured by bathochromic shifts in UV/vis spectra. Although the overall magnitude of the shifts is modest (but comparable to some other classes of conjugated materials), the effective conjugation length of the series is approximately eight repeat units. The oligomers also exhibit an unusual hypsochromic shift in their fluorescence spectra with increasing length. The origin of these trends is discussed in the context of conformational analysis and DFT calculations of the frontier molecular orbitals for the series.

Probing the arenium-ion (protontransfer) versus the cation-radical (electron transfer) mechanism of Scholl reaction using DDQ as oxidant.

DDQ/H(+) system readily oxidizes a variety of electron donors with oxidation potential as high as approximately 1.7 V to the corresponding cation radicals. A re-examination of the controversial arenium-ion versus cation-radical mechanisms for Scholl reaction using DDQ/H(+) together with commonly utilized FeCl(3) as oxidants led us to demonstrate that the reaction proceeds largely via a cation-radical mechanism. The critical experimental evidence in support of a cation-radical pathway for the Scholl reaction includes the following: (i) There is no reaction in Scholl precursors in a mixture of dichloromethane and various acids (10% v/v). (ii) The necessity to use powerful oxidants such as ferric chloride (FeCl(3)) or DDQ/H(+) for Scholl reactions is inconsistent with the arenium-ion mechanism in light of the fact that aromatization of the dihydro intermediates (formed via arenium-ion mechanism) can be easily accomplished with rather weak oxidants such as iodine or air. (iii) Various Scholl precursors with oxidation potentials 1.7 V vs SCE do not react. (iv) Finally, the feasibility of the dicationic intermediate, formed by loss of two electrons, has been demonstrated by its generation from a tetraphenylene derivative using DDQ/H(+) as an oxidant.

A facile synthesis of elusive alkoxy-substituted hexa-peri-hexabenzocoronene.

Oxidative cyclodehydrogenation of hexakis(4-alkoxyphenyl)benzene produces a quantitative yield of an indenofluorene derivative rather than the expected alkoxy-substituted hexa-peri-hexabenzocoronene (HBC). The structure of the unexpected indenofluorene was established by X-ray crystallography. The mechanistic considerations for the formation of the indenofluorene derivative led us to devise an alternative synthesis of elusive alkoxy-substituted HBC-a potentially important, disk-shaped structure for the preparation of liquid crystalline materials for practical applications in the emerging areas of molecular electronics and nanotechnology.

Synthesis, electronic properties, and X-ray structural characterization of tetrarylbenzo[1,2-b:4,5-b']difuran cation radicals.

Electroactive tetraarylbenzo[1,2- b:4,5- b']difuran (BDF) and model diarylbenzofuran derivatives are synthesized and their structures are established by X-ray crystallography. Isolation and X-ray crystallographic characterization of the robust cation-radical salts of BDF derivatives confirm that a single charge in the BDFs is stabilized largely by the benzodifuran and coplanar alpha-aryl groups lying on the longitudinal axis. These findings suggest that the linear arrays of BDFs may allow the construction of molecular wires suitable for long-range electron transport.