N,N'-dimethylpyrazinediium bis(tetrafluoroborate) and N,N'-diethylpyrazinediium bis(tetrafluoroborate): new examples of anion-pi triads.

Crystallization of N,N'-dimethylpyrazinediium bis(tetrafluoroborate), C(6)H(10)N(2)(2+).2BF(4)(-), (I), and N,N'-diethylpyrazinediium bis(tetrafluoroborate), C(8)H(14)N(2)(2+).2BF(4)(-), (II), from dried acetonitrile under argon protection has permitted their single-crystal studies.

Counter-ion modulation of long-distance pi-bonding of the open-shell p-benzoquinone anions.

X-Ray crystallography of dichlorodicyano- or tetrachloro-p-benzoquinone salts with polyether-ligated alkali metals reveals discrete supramolecular complexes: [{M(L)A}(2)]. They show pairs of co-facial p-benzoquinone anions (A(-)) arranged at close interplanar separation: r(pi) approximately 2.9 A characteristic of ion-radical pi-dimers.

Spectroscopic and electrochemical evaluation of salt effects on electron-transfer equilibria between donor/acceptor and ion-radical pairs in organic solvents.

Addition of "inert" tetrabutylammonium hexafluorophosphate (Bu(4)NPF(6)) to a solution of TMDO/DDQ in dichloromethane (where TMDO=2,2,6,6-tetramethylbenzo[1,2-d;4,5-d]bis[1,3]-dioxole, donor, and DDQ=diclorodicyano-p-benzoquinone, acceptor) is accompanied by drastic changes in the electronic spectrum, which are related to the appearance of the DDQ(-.) and TMDO(+.) ion radicals and a decrease in the concentration of the neutral molecules and the charge-transfer complex [TMDO,DDQ].

Fresh look at electron-transfer mechanisms via the donor/acceptor bindings in the critical encounter complex.

Seminal insights provided by the iconic R. S. Mulliken and his "charge-transfer" theory, H.

The spectral elucidation versus the X-ray structure of the critical precursor complex in bimolecular electron transfers: application of experimental/theoretical solvent probes to ion-radical (redox) dyads.

The mechanistic conundrum is commonly posed by the intrinsic structural disconnect between a bimolecular (reactive) intermediate that is fleetingly detected spectroscopically in solution versus that rigorously defined by isolation and X-ray crystallography. We resolve this ambiguity by the combined experimental and theoretical application of the solvent media probe to the transient (1:1) precursor complex in the simplest chemical reaction involving direct adiabatic electron transfer (ET) among various donor/acceptor pairs. Of particular help in our resolution of such an important ET problem is the characterization of the bimolecular precursor complex as Robin-Day class II (localized) or class III (delocalized) from either the solvent-dependent or the solvent-independent response of the diagnostic intervalence absorption bands for the quantitative evaluation of the electronic coupling elements.

Halogen-bonded assembly of hybrid inorganic/organic 3D-networks from dibromocuprate salts and tetrabromomethane.

The critical halogen-bonding motif (CBr...

Tris(thianthrene)2+ bis(dodecamethylcarba-closo-dodecaborate) dichloromethane tetrasolvate: a crossed triple-decker pi-trimer dication.

The title compound, (3C(12)H(8)S(2))(2+).2C(13)H(36)B(11)(-).4CH(2)Cl(2), contains an unusual cation-radical association comprising a pi-trimer dication of crossed thianthrenes.

Reversible interchange of charge-transfer versus electron-transfer states in organic electron transfer via cross-exchanges between diamagnetic (donor/acceptor) dyads.

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.

Continuum of outer- and inner-sphere mechanisms for organic electron transfer. Steric modulation of the precursor complex in paramagnetic (ion-radical) self-exchanges.

Transient 1:1 precursor complexes for intermolecular self-exchange between various organic electron donors (D) and their paramagnetic cation radicals (D+*), as well as between different electron acceptors (A) paired with their anion radicals (A-*), are spectrally (UV-NIR) observed and structurally (X-ray) identified as the cofacial (pi-stacked) associates [D, D+*] and [A-*, A], respectively. Mulliken-Hush (two-state) analysis of their diagnostic intervalence bands affords the electronic coupling elements (HDA), which together with the Marcus reorganization energies (lambda) from the NIR spectral data are confirmed by molecular-orbital computations. The HDA values are found to be a sensitive function of the bulky substituents surrounding the redox centers.

Molecular and electronic structures of the long-bonded pi-dimers of tetrathiafulvalene cation-radical in intermolecular electron transfer and in (solid-state) conductivity.

Tetrathiafulvalene (TTF) as the prototypical electron donor for solid-state (electronics) applications is converted to the unusual cation-radical salt, TTF+* CB- (where CB- is the non-coordinating closo-dodecamethylcarboranate), for crystallographic and spectral analyses. Near-IR studies establish the spontaneous self-association of TTF+* to form the diamagnetic [TTF+,TTF+] dication and to also undergo the equally rapid cross-association with its parent donor to form the mixed-valence [TTF+*,TTF] cation-radical. The latter, most importantly, represents the first (dyad) member of a series of p-doped tetrathiafulvalene (stacked) arrays, and the thorough scrutiny of its electronic structure with the aid of Mulliken-Hush (two-state) analysis of the diagnostic (intervalence) NIR band reveals Robin-Day Class II behavior.

Quinones as electron acceptors. X-ray structures, spectral (EPR, UV-vis) characteristics and electron-transfer reactivities of their reduced anion radicals as separated vs contact ion pairs.

Successful isolation of a series of pure (crystalline) salts of labile quinone anion radicals suitable for X-ray crystallographic analysis allows for the first time their rigorous structural distinction as "separated" ion pairs (SIPs) vs "contact" ion pairs (CIPs). The quantitative evaluation of the precise changes in the geometries of these quinones (Q) upon one-electron reduction to afford the anion radical (Q-*) is viewed relative to the corresponding (two-electron) reduction to the hydroquinone (H2Q) via the Pauling bond-length/bond-order paradigm. Structural consequences between such separated and contact ion pairs as defined in the solid state with those extant in solution are explored in the context of their spectral (EPR, UV-vis) properties and isomerization of tightly bound CIPs.

The question of aromaticity in open-shell cations and anions as ion-radical offsprings of polycyclic aromatic and antiaromatic hydrocarbons.

Arene cation-radicals and anion-radicals result directly from the one-electron oxidation and reduction of many aromatic hydrocarbons, yet virtually nothing is known of their intrinsic (thermodynamic) stability and hence "aromatic character". Since such paramagnetic ion radicals lie intermediate between aromatic (Hückel) hydrocarbons with 4n + 2-electrons and antiaromatic analogues with 4n-electrons, we can now address the question of pi-delocalization in these odd-electron counterparts. Application of the structure-based "harmonic oscillator model of aromaticity" or the HOMA method leads to the surprising conclusion that the aromaticity of these rather reactive, kinetically unstable arene cation and anion radicals (as measured by the HOMA index) is actually higher than that of their (diamagnetic) parent-contrary to conventional expectations.

Very fast electron migrations within p-doped aromatic cofacial arrays leading to three-dimensional (toroidal) pi-delocalization.

The charge-resonance phenomenon originally identified by Badger and Brocklehurst lies at the core of the basic understanding of electron movement and delocalization that is possible within p-doped aromatic (face-to-face) arrays. To this end, we now utilize a series of different aryl-donor groups (Ar) around a central platform to precisely evaluate the intramolecular electron movement among these tethered redox centers. As such, the unique charge-resonance (intervalence) absorption bands observed upon the one-electron oxidation or p-doping of various hexaarylbenzenoid arrays (Ar6C6) provide quantitative measures of the reorganization energy (lambda) and the electronic coupling element (H(ab)) that are required for the evaluation of the activation barrier (deltaG(ET)) for electron-transfer self-exchange according to Marcus-Hush theory.

Crystallographic view of fluidic structures for room-temperature ionic liquids: 1-butyl-3-methylimidazolium hexafluorophosphate.

Shock-induced crystallization of the supercooled ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, C8H15N2+.PF6-, allows for the first time precise X-ray diffraction analysis directly pertinent to the fluid state. This intermediate-chain-length structure shows features of both short- and long-chain analogs.

Characterizing the dimerizations of phenalenyl radicals by ab initio calculations and spectroscopy: sigma-bond formation versus resonance pi-stabilization.

Electronic-structure calculations for the self-association of phenalenyl radical (P*) predict the formation of dimeric species (sigma-P2) in which both moieties are connected by a sigma-bond with rP-P approximately 1.59 A and bond dissociation enthalpy of DeltaH(D) approximately 16 kcal mol(-1). Such an unusually weak sigma-bond is related to the loss of aromatic stabilization energy of approximately 34 kcal mol(-1) per phenalenyl moiety, largely owing to rehybridization.

Intermolecular electron-transfer mechanisms via quantitative structures and ion-pair equilibria for self-exchange of anionic (dinitrobenzenide) donors.

Definitive X-ray structures of "separated" versus "contact" ion pairs, together with their spectral (UV-NIR, ESR) characterizations, provide the quantitative basis for evaluating the complex equilibria and intrinsic (self-exchange) electron-transfer rates for the potassium salts of p-dinitrobenzene radical anion (DNB(-)). Three principal types of ion pairs, K(L)(+)DNB(-), are designated as Classes S, M, and C via the specific ligation of K(+) with different macrocyclic polyether ligands (L). For Class S, the self-exchange rate constant for the separated ion pair (SIP) is essentially the same as that of the "free" anion, and we conclude that dinitrobenzenide reactivity is unaffected when the interionic distance in the separated ion pair is r(SIP) > or =6 Angstroms.

"Separated" versus "contact" ion-pair structures in solution from their crystalline states: dynamic effects on dinitrobenzenide as a mixed-valence anion.

Qualitative structural concepts about dynamic ion pairs, historically deduced in solution as labile solvent-separated and contact species, are now quantified by the low-temperature isolation of crystalline (reactive) salts suitable for direct X-ray analysis. Thus, dinitrobenzenide anion (DNB(-)) can be prepared in the two basic ion-paired forms by potassium-mirror reduction of p-dinitrobenzene in the presence of macrocyclic polyether ligands: L(C) (cryptand) and L(E) (crown-ethers). The crystalline "separated" ion-pair salt isolated as K(L(C))(+)//DNB(-) is crystallographically differentiated from the "contact" ion-pair salt isolated as K(L(E))(+)DNB(-) by their distinctive interionic separations.

Intermolecular pi-to-pi bonding between stacked aromatic dyads. Experimental and theoretical binding energies and near-IR optical transitions for phenalenyl radical/radical versus radical/cation dimerizations.

The high symmetry and stability of phenalenyl systems, both as the planar pi-radical (P*) and as the pi-cation (P+), are desirable characteristics of prototypical aromatic donor/acceptor pairs that encourage their use as (binary) models for the study of intermolecular interactions extant in stacked molecular arrays. Thus, quantitative ESR spectroscopy of the paramagnetic P* identifies its spontaneous self-association to the diamagnetic P2, previously characterized as the stacked pi-dimer by X-ray crystallography. Likewise, the rapid cross-association of P* with the closed-shell P+ leads to the stacked pi-dimer cation P2*+ with the "doubled" ESR spectrum diagnostic of complete (odd) electron delocalization.

Crystallographic distinction between "contact" and "separated" ion pairs: structural effects on electronic/ESR spectra of alkali-metal nitrobenzenides.

The classic nitrobenzene anion-radical (NB(-*) or nitrobenzenide) is isolated for the first time as pure crystalline alkali-metal salts. The deliberate use of the supporting ligands 18-crown-6 and [2.2.

Donor-acceptor (electronic) coupling in the precursor complex to organic electron transfer: intermolecular and intramolecular self-exchange between phenothiazine redox centers.

Intermolecular electron transfer (ET) between the free phenothiazine donor (PH) and its cation radical (PH*+) proceeds via the [1:1] precursor complex (PH)(2)*+ which is transiently observed for the first time by its diagnostic (charge-resonance) absorption band in the near-IR region. Similar intervalence (optical) transitions are also observed in mixed-valence cation radicals with the generic representation: P(br)P*+, in which two phenothiazine redox centers are interlinked by p-phenylene, o-xylylene, and o-phenylene (br) bridges. Mulliken-Hush analysis of the intervalence (charge-resonance) bands afford reliable values of the electronic coupling element H(IV) based on the separation parameters for (P/P*+) centers estimated from some X-ray structures of the intermolecular (PH)(2)*+ and the intramolecular P(br)P*+ systems.

Stable (long-bonded) dimers via the quantitative self-association of different cationic, anionic, and uncharged pi-radicals: structures, energetics, and optical transitions.

Unusual dimers with wide interplanar separations, that is, very long bonds with d(D) approximately 3.05 A, are common to the spontaneous self-association of various organic pi-radicals in solution and in the crystalline solid state, independent of whether they are derived from negatively charged anion radicals of planar electron acceptors (TCNE-*, TCNQ-*, DDQ-*, CA-*), positively charged biphenylene cation-radical (OMB+*), or neutral phenalene radical (PHEN*). All dimeric species are characterized by intense absorption bands in the near-IR region that are diagnostic of the charge-transfer transitions previously identified with intermolecular associations of various electron-donor/acceptor dyads.

The charge-transfer motif in crystal engineering. Self-assembly of acentric (diamondoid) networks from halide salts and carbon tetrabromide as electron-donor/acceptor synthons.

Unusual strength and directionality for the charge-transfer motif (established in solution) are shown to carry over into the solid state by the facile synthesis of a series of robust crystals of the [1:1] donor/acceptor complexes of carbon tetrabromide with the electron-rich halide anions (chloride, bromide, and iodide). X-ray crystallographic analyses identify the consistent formation of diamondoid networks, the dimensionality of which is dictated by the size of the tetraalkylammonium counterion. For the tetraethylammonium bromide/carbon tetrabromide dyad, the three-dimensional (diamondoid) network consists of donor (bromide) and acceptor (CBr(4)) nodes alternately populated to result in the effective annihilation of centers of symmetry in agreement with the sphaleroid structural subclass.