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Tetramethylpyrazine is effectively transformed via controlled N-protonation or N-methylation to afford a series of mono- and dicationic π-acceptors of particular use in the study of charge modulationduring the self-assembly of various anionic donors and π-acid acceptors into ternary synthons and infinite linear chains (molecular wires). Thus, when the charges of the anionic donor (D) and the cationic acceptor (A) are unequal (either as 1:2 or as 2:1), the corresponding crystal structures of the donor/acceptor associates all contain structure motifs of discrete synthons consisting of (A···D···A) or (D···A···D) triads. On the other hand, when the anionic donor and cationic acceptor are of equal charge (either as 1:1 or as 2:2), the corresponding crystal structures all contain infinite chain arrangements: (···D···A···D···A···D···), which are the same as those we previously found in the cocrystallization of various anionic donors with neutralπ-acids. Fully appreciated anion/π interactions are found in all of these solids owing to the absence of aromatic protons in the π-acids, as well as the presence of strong electrostatic anion/cation interactions. It is noteworthy in such associates that halide anions (Cl−, Br−, and I−) show different patterns of donor/acceptor interactions. For example, in the case of the diprotonated tetramethylpyrazinium dication, the chloride donor is π-bonded over the edge of the aromatic ring, whereas bromide and iodide donors are precisely located over the aromatic ring centrosymmetrically. We hope these structural studies will provide additional stimulus for the design of new anion receptors based on the use of charged aromatic acceptors. [ABSTRACT FROM AUTHOR]

Seminal insights provided by the iconic R. S. Mulliken and his “charge-transfer” theory, H. Taube and his “outer/inner-sphere” mechanisms, R. A. Marcus and his “two-state non-adiabatic” theory, and N. S. Hush and his “intervalence” theory are each separately woven into the rich panoramic tapestry constituting chemical research into electron-transfer dynamics, and its mechanistic dominance for the past half century and more. In this Account, we illustrate how the simultaneous melding of all four key concepts allows sharp focus on the charge-transfer character of the critical encounter complex to evoke the latent facet of traditional electron-transfer mechanisms. To this end, we exploit the intervalence (electronic) transition that invariably accompanies the diffusive encounter of electron-rich organic donors (D) with electron-poor acceptors (A) as the experimental harbinger of the collision complex, which is then actually isolated and X-ray crystallographically established as loosely bound π-stacked pairs of various aromatic and olefinic donor/acceptor dyads with uniform interplanar separations of rDA= 3.1 ± 0.2 Å. These X-ray structures, together with the spectral measurements of their intervalence transitions, lead to the pair of important electron-transfer parameters, HDA(electronic coupling element) versus λ T(reorganization energy), the ratio of which generally defines the odd-electron mobility within such an encounter complex in terms of the resonance stabilization of the donor/acceptor assembly [D, A] as opposed to the reorganization-energy penalty required for its interconversion to the electron-transfer state [D +•, A −•]. We recognize the resonance-stabilization energy relative to the intrinsic activation barrier as the mechanistic binding factor, Q= 2 HDA/λ T, to represent the quantitative measure of the highly variable continuum of inner-sphere/outer-sphere interactions that are possible within various types of precursor complexes. First, Q≪ 1 identifies one extreme mechanism owing to slow electron-transfer rates that result from the dominance of the intrinsic activation barrier (λ T) between the encounter and successor complexes. At the other extreme of Q≥ 1, the overwhelming dominance of the resonance stabilization ( HDA) predicts the odd-electron mobility between the donor and acceptor to occur without an activation barrier such that bimolecular electron transfer is coincident with their diffusional encounter. In between lies a potentially infinite set of states, 0 < Q< 1 with opposing attractive and destabilizing forces that determine the location of the bound transition states along the reaction coordinate. Three prototypical potential-energy surfaces evolve as a result of progressively increasing the donor/acceptor bindings ( HDA) extant in the precursor complex (at constant λ T). In these cases, the “outer-sphere” mechanism is limited by the weak donor/acceptor coupling that characterizes the now classical Marcus outer-sphere mechanism. Next, the “inner-sphere” mechanism derives from moderate (localized) donor/acceptor bindings and includes the mechanistic concept of the bridged-activated complex introduced by Taube for a wide variety of ligand-based redox dyads. Finally, the “interior” mechanism is also another subclass of the Taube (inner-sphere) classification, and it lies at the other extreme of very fast electron-transfer rate processes (heretofore unrecognized), arising from the spontaneous annihilation of the donor/acceptor dyad to the delocalized (electron-transfer) complex as it descends barrierlessly into the chemical “black hole” that is rate-limited solely by diffusion. [ABSTRACT FROM AUTHOR]

The self-assembly of various anions (e.g., thiocyanate, nitrite/nitrate, sulfite/sulfate, tetrahalometallates, etc.) with prototypical π-acids (consisting of cyano- and nitrosubstituted pyrazine and benzene, as well as tetracyanoethylene) occurs rapidly and selectively to yield a series of novel one-dimensional structures. The wire-like molecular chains all consist of parallel stacks of π-acids and alternate anions of different sizes and shapes that establish the dihedral angles α and φ sufficient to define these unique structures. Analogy of such linear arrays to nanoscopic wires is reinforced by the protective sheath of countercations that are completely arrayed around the linear cores. The critical feature of these efficient self-assemblies is shown to derive from anion/π-recognitions via charge-transfer forces between the electron-rich anions acting as electron donors and the electron-poor π-acids acting as electron acceptors to spontaneously generate synthons according to Mulliken theory. [ABSTRACT FROM AUTHOR]

The choice of appropriate electron donors (D) and acceptors (A) allows for the first time the simultaneous observation of Mulliken charge-transfer states, D,A, that can coexist in reversible equilibrium with electron-transfer states, D•,A-•, for various diamagnetic organic redox dyads. The theoretical analysis based on the (two-state) Mulliken−Hush analysis of the intervalence optical transition, together with the spectral identification of the transient ion-radical pairs of D•and A-•, leads to the construction of the unusual potential-energy surface consisting of a single minimum without any reorganizational barrier for electron-transfer cross-exchanges with driving forces close to the isergonic limit. The mechanistic implications of this direct demonstration of the facile charge-transfer/electron-transfer interchange are discussed. [ABSTRACT FROM AUTHOR]

Three polymorphs of a model carcinogen 4-nitroquinoline-N-oxide were crystallized from its solutions in various organic solvents using either (i) slow cooling or evaporation (form I), (ii) moderate rate of cooling (form II) or (iii) flash cooling (form III). These polymorphs are characterized by distinct crystal morphologies, powder diffractions and IR spectra. Single-crystal X-ray analysis of forms I and II revealed essentially the same geometries of 4-nitroquinoline-N-oxide molecules (with the exception of the nitrogroup characteristics), and distinct patterns of multiple hydrogen bonds, electrostatic interactions and π-stacking in their crystal structures. [ABSTRACT FROM AUTHOR]

The facile reduction of tetracyanopyrazine (TCP) by tetrakis(dimethylamino)ethylene (TDAE) leads to black crystals of the donor/acceptor salt: TDAE2+·(TCP)22−·2CH3CN in which the vertical anionic TCPstacks consisting of distinct π-(TCP)22−units are surrounded by TDAEdications. The monomers within these supramolecular π-(TCP)22−complexes are arranged co-facially at interplanar separation of ∼3.17 characteristic for ion-radical π-dimers and show unusual ∼30° rotation relative to each other. The structural scrutiny of the π-dimers together with the X-ray crystallographic analysis of the neutral tetracyanopyrazine acceptor, its charge-transfer complexes, and the monomeric anion-radical TCP− reveals the unique “quasi-quinonoidal” distortion of the π-bonded tetracyanopyrazine moieties. In contrast to the molecular bending and intermolecular charge-transfer that are commonly observed in various ion-radical π-dimers and in conventional donor/acceptor complexes, the specific electron density redistribution and bond length alternation in π-bonded (planar) tetracyanopyrazine moieties (relative to the neutral molecule and isolated anion-radical) can only be accommodated by the bonding molecular orbital (HOMO) of the (TCP)22−dimer that is notably different from the corresponding SOMO of the monomeric TCP−. [ABSTRACT FROM AUTHOR]

The critical halogen-bonding motif (CBr⋯BrCu) is responsible for the successful synthesis of three-dimensional networks of unusual 4,5-connected dodecahedron cells (each containing the encapsulated cation) via the self-assembly of tetra-n-propylammonium dibromocuprate(i) and tetrabromomethane as prototypical donor/acceptor dyads. [ABSTRACT FROM AUTHOR]