Bond dissociation energies for radical dimers derived from highly stabilized carbon-centered radicals

[reaction: see text] The temperature dependence of the dissociation of dimers formed from highly stabilized carbon-centered radicals has been examined. Analysis of the data yields the bond dissociation energy (BDE) for the central head-to-head C-C bond in these compounds. For example, for the dimer derived from 3-phenyl-2-coumaranone, BDE is 23.6 kcal/mol and the C-C bond length 1.596 A, a rather long value for a sigma bond.     

Synthesis and properties of conjugated hybrid tetrathiafulvalene dimers

The synthesis of new hybrid tetrathiafulvalene (TTF) dimers (11a-c) has been carried out by a Wittig-Horner reaction of the respective phosphonate esters (10a-c) with 2-(tetrathiafulvalenylvinyl)-9, 10-anthraquinone (9) prepared by olefination of formyltetrathiafulvalene (7) and the phosphonium salt of anthraquinone 8. Electrochemical studies show that the dimers 11a-c mainly retain the electrochemical properties of both TTF and the pi-extended TTF components, and most importantly, intramolecular electronic interactions between the two moieties are observed by cyclic voltammetry and Osteryoung square wave voltammetry. Semiempirical PM3 calculations reveal an almost planar geometry for the TTF and the benzene ring connected through the vinyl spacer. These compounds can form stable charge-transfer complexes with 2, 3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) showing a stoichiometry of 1:3 (D:A). Attempts to electrocrystallize the dimeric donors with different counteranions are discussed.     

Synthesis and electron-donor ability of the first conjugated pi-extended tetrathiafulvalene dimers

A series of novel conjugated homo (16a,b) and heterodimers (20) of pi-extended tetrathiafulvalenes with p-quinodimethane structures (exTTFs) linked by a conjugative vinyl spacer have been prepared by olefination Wittig-Horner reaction from the corresponding quinones (14, 19) and phosphonates (15a,b). The redox properties, determined by cyclic voltammetry, reveal a strong donor character and the presence of only one four-electron oxidation wave to form the tetracation species at oxidation potential values quite similar to those found for the related monomers. Theoretical calculations (PM3) show a planar central stilbene moiety and highly distorted exTTF units. The electronic spectra support as well as the electrochemical data and theoretical calculation the lack of significant communication between the exTTF units.     

New chiral binaphthyl building blocks: synthesis of the first optically active tetrathiafulvalene dimers

New enantiomerically pure binaphthyl derivatives bearing triphenylphosphine or phosphonate groups have been synthesized and used as building blocks to prepare the first optically active tetrathiafulvalene dimers.     

On the intrinsic optical absorptions by tetrathiafulvalene radical cations and isomers

Gas-phase action spectroscopy shows unambiguously that the low-energy absorptions by tetramethylthiotetrathiafulvalene and tetrathianaphthalene cations in solution phase are due to monomers and not π-dimers.     

Mechanically compliant single crystals of a stable organic radical

Mechanically compliant organic crystals are the foundation of the development of future flexible, light-weight single-crystal electronics, and this requires reversibly deformable crystalline organic materials with permanent magnetism. Here, we report and characterize the first instance of a plastically bendable single crystal of a permanent organic radical, 4-(4′-cyano-2′,3′,4′,5′-tetrafluorophenyl)-1,2,3,5-dithiadiazolyl. The weak interactions between the radicals render single crystals of the β phase of this material exceedingly soft, and the S–N interactions facilitate plastic bending. EPR imaging of a bent single crystal reveals the effect of deformation on the three-dimensional spin density of the crystal. The unusual mechanical compliance of this material opens prospects for exploration into flexible crystals of other stable organic radicals towards the development of flexible light-weight organic magnetoresistance devices based on weak, non-hydrogen-bonded interactions in molecular crystals.

Mechanically soft crystals are interesting candidates for single crystal electronics. Here, crystals of a stable dithiadiazolyl radical are shown to be plastically bendable and display a change in their spin density in response to mechanical force.     

Resonance stabilized bisdiselenazolyls as neutral radical conductors

An efficient and versatile synthetic route to resonance stabilized bisselenathiazolyl and bisdiselenazolyl radicals 3 and 4 is described. Structural analysis of 3 and 4 confirm that lattice and pi-delocalization energies are sufficient to offset solid-state dimerization of the radicals and that the two selenium-containing radicals are isostructural with the all-sulfur based system 1. Variable temperature conductivity measurements indicate that sequential replacement of sulfur by selenium leads to a progressive increase in conductivity and reduction in thermal activation energy.     

Direct observation of mixed-valence and radical cation dimer states of tetrathiafulvalene in solution at room temperature: association and dissociation of molecular clip dimers under oxidative control

We have observed the mixed-valence and radical cation dimer states of a glycoluril-based molecular clip with tetrathiafulvalene (TTF) sidewalls at low concentration (1 mM) at room temperature. This molecular clip has four consecutive anodic steps in its cyclic voltammogram, which suggests a sequential oxidation of these TTF sidewalls to generate species existing in several distinct charge states: neutral monomers, mixed-valence dimers, radical cation dimers, and fully oxidized tetracationic monomers. The observation of characteristic NIR spectroscopic absorption bands at approximately 1650 and 830 nm in spectroelectrochemistry experiments supports the presence of intermediary mixed-valence and radical cation dimers, respectively, during the oxidation process. The stacking of four TTF radical cations in the dimer led to the appearance of a charge-transfer band at approximately 946 nm. Nanoelectrospray ionization mass spectrometry was used to verify the tricationic state and confirm the existence of other different charged dimers during the oxidation of the molecular clip.     

First fused perpendicular hybrid tetrathiafulvalene (TTF) dimers: a new strategy in pi-extended and rigidified TTF

[structure: see text] The synthesis, theoretical calculations, and crystallographic and electrochemical properties of fused perpendicular tetrathiafulvalene (TTF) dimers incorporating both a TTF unit and a quinonoid pi-extended TTF are described as a new strategy for obtaining pi-extended, rigidified, and sulfur-rich analogues of TTF.     

Ammoniated electron as a solvent stabilized multimer radical anion

The excess electron in liquid ammonia ("ammoniated electron") is commonly viewed as a cavity electron in which the s-type wave function fills the interstitial void between 6 and 9 ammonia molecules. Here we examine an alternative model in which the ammoniated electron is regarded as a solvent stabilized multimer radical anion in which most of the excess electron density resides in the frontier orbitals of N atoms in the ammonia molecules forming the solvation cavity. The cavity is formed due to the repulsion between negatively charged solvent molecules. Using density functional theory calculations, we demonstrate that such core anions would semiquantitatively account for the observed pattern of Knight shifts for 1H and 14N nuclei observed by NMR spectroscopy and the downshifted stretching and bending modes observed by infrared spectroscopy. We speculate that the excess electrons in other aprotic solvents might be, in this respect, analogous to the ammoniated electron, with substantial transfer of the spin density into the frontier N and C orbitals of methyl, amino, and amide groups.     

Phototransformations of azetidine radical cations stabilized in freonic matrices

It has been established that transformations of azetidine radical cations observed in freonic matrices under the action of light with λ = 436 nm (T = 77 K) are associated with C-N bond cleavage which corresponds to the cyclic form yielding a mixture of open distonic C-centered radical cations of the following structure: ·CH₂CH₂CH=NH ₂ ⁺      

Gramicidin conducting dimers in lipid bilayers are stabilized by single-file ionic flux along them

Gramicidin D was incorporated in a biomimetic membrane consisting of a lipid bilayer tethered to a mercury electrode via a hydrophilic spacer, and its behavior was investigated in aqueous 0.1 M KCl by potential-step chronocoulometry and electrochemical impedance spectroscopy. The impedance spectra, recorded from 0.1 to 1 x 10(5) Hz over a potential range of 0.7 V, were fitted to a series of RC meshes, which were related to the different substructural elements of the biomimetic membrane. These impedance spectra were compared with those obtained by incorporating valinomycin, under otherwise identical conditions. The potential dependence of the stationary currents reported on bilayer lipid membranes by Bamberg and L?uger (Bamberg, E.; L?uger, P. J. Membrane Biol. 1973, 11, 177-194) as well as those extracted from potential-step chronocoulometric measurements was interpreted by relating the increase in gramicidin dimerization to a progressive increase in single-file K+ flux along the dimeric channels. An analogous approach was adopted in explaining the difference between the impedance spectra obtained with gramicidin D and those obtained with valinomycin. It is concluded that gramicidin has a low tendency to form dimers in the absence of ionic flux.     

Structure and bonding of proximity-enforced main-group dimers stabilized by a rigid naphthyridine diimine ligand

The development of ligands capable of effectively stabilizing highly reactive main-group species has led to the experimental realization of a variety of systems with fascinating properties. In this work, we computationally investigate the electronic, structural, energetic, and bonding features of proximity-enforced group 13–15 homodimers stabilized by a rigid expanded pincer ligand based on the 1,8-naphthyridine (napy) core. We show that the redox-active naphthyridine diimine (NDI) ligand enables a wide variety of structural motifs and element-element interaction modes, the latter ranging from isolated, element-centered lone pairs (e.g., E = Si, Ge) to cases where through-space π bonds (E = Pb), element-element multiple bonds (E = P, As) and biradical ground states (E = N) are observed. Our results hint at the feasibility of NDI-E₂ species as viable synthetic targets, highlighting the versatility and potential applications of napy-based ligands in main-group chemistry.     

Synthesis and radical coupling of pyridine-bridged pi-extended tetrathiafulvalene (TTF)-type donors and push-pull analogues

A new family of pi-extended TTF analogues (3a-c) and D-pi-A chromophores (5a-c), in which the electroactive units (1,3-dithiole rings and 2,2-dicyanovinyl groups) are connected through a pyridine bridge with a meta substitution pattern, is reported. The redox behavior of these compounds has been investigated by cyclic voltammetry and theoretical calculations performed at the B3P86/6-31G** level. Unlike many pi-extended TTF derivatives, the 1,3-dithiole rings in compounds 3a-c do not behave independently and two oxidation processes are observed with an anodic separation ranging from 50 to 150 mV. Calculations show that electrons are equally extracted from both dithiole rings. A biradical structure is predicted for the dication state due to the near-degeneracy of the HOMO and HOMO - 1 orbitals. The presence of both donor (D) and acceptor (A) fragments in conjugates results in irreversible oxidation and reduction processes associated with the 1,3-dithiole ring and with the 2,2-dicyanovinyl moiety, respectively. An electrochemical-chemical-electrochemical (ECE) process takes place for all the compounds reported. The chemical process implies the dimerization of the radical cation for compounds 5 and the oligomerization of the biradical dication for compounds 3. The ECE process therefore generates new neutral dimeric (5) or oligomeric (3) species that incorporate the TTF vinylogue core.     

New chiral binaphthyl building blocks: synthesis of the first optically active tetrathiafulvalene and 11,11,12,12-tetracyano-9, 10-anthraquinodimethane dimers

New enantiomerically pure binaphthyl derivatives bearing triphenylphosphine or phosphonate groups have been synthesized and used as building blocks to prepare the first optically active TTF and TCAQ dimers. Due to the restricted rotation of the two naphthalene rings, binaphthyl derivatives are ideal candidates to be used as nonplanar spacers between electroactive units in the search for materials with enhanced dimensionality. The electronic absorption spectra of dimers in which the electroactive unit is in conjugation with the naphthalene fragment reveal the presence of intramolecular photoinduced electron-transfer process from the TTF to the naphthalene unit in 5 and 6 and from the naphthalene moiety to the TCAQ unit in 7. Electrochemical studies on the new dimers show the redox potentials of TTF and TCAQ units as well as the oxidation wave for the naphthalene moiety and reveal no significant electronic interaction between the two electroactive units, which is in agreement with the results obtained from theoretical calculations at the PM-3 level which indicate that the angle between rings in the binaphthyl systems ranges from 75 to 80 degrees.     

三苯甲基自由基及其二聚体稳定性的理论研究

采用量子化学AM1 MO方法优化了三苯甲基自由基及其两种二聚体的构型，并计算了反应物和二聚物的生成焓、活化能以及它们的电荷布居，计算结果表明，三苯甲基自由基是一种较稳定的自由基，在这个自由基中存在明显的共轭效应，分子中电子云呈平均化分布．它的二聚体1，4—环己二烯衍生物分子中的5个苯基排布在环己二烯环平面的两端，明显降低了苯基之间的空间排斥力，使二聚物—环己二烯衍生物分子的稳定性大大高于六苯乙烷．因此，三苯甲基自由基的二聚体是1，4—环己二烯衍生物而不是六苯基乙烷，得到了与实验事实一致的结果。     

Stable pi-dimer of a tetrathiafulvalene cation radical encapsulated in the cavity of cucurbit[8]uril

The first stable pi-dimer of a tetrathiafulvalene (TTF) cation radical encapsulated in the cavity of cucurbit[8]uril has been isolated at room temperature and fully characterized; it shows absorption bands at 400, 540 and 760 nm, characteristic of the TTF cation radical dimer.     

Explicit and implicit solvation of radical ions: the cycloreversion of CPD dimers

The electron transfer catalyzed cycloreversion of cyclobutane pyrimidine dimers is the key step in repair of light-induced DNA lesions catalyzed by the enzyme CPD photolyase. The formation of the CPD radical anion was found to be strongly solvent dependent due to a specific hydrogen bond that stabilizes the valence bound state over the dipole bound state of the additional electron. The effect of solvation on the vertical and adiabatic electron affinity of uracil and uracil dimers as well as on the mechanism of the cycloreversion of the uracil dimer radical anion is explored for three model systems that include explicit solvent molecules at the B3LYP/6-311++G^∗∗/B3LYP/6-31+G^∗ level of theory. The second solvation shell is described using the implicit C-PCM solvation model. These calculations indicate an effectively barrierless mechanism. These results are in agreement with the available experimental data for the reaction energies and isotope effects. It is also shown that a single hydrogen bond donor is a sufficient minimal model for the first solvation shell by adequately describing the stabilization of the valence bound state of the radical anion through hydrogen bonding. The relationship of these model systems with the enzymatic reaction catalyzed by DNA photolyase is also discussed.     

Molecular modeling studies of the reactions of phenoxy radical dimers: pathways to dibenzofurans

The B3LYP hybrid density functional computational technique was applied to describe the sequence of phenoxy radicals coupling reactions leading to the formation of dibenzofurans. Reaction kinetic parameters were estimated for key reactions. Aromatization of bis-keto dimers of phenoxy radicals followed by intermediate dehydration or dehydroxylation was demonstrated to be a strongly stereoselective process. While the S,S-diastereomer of the ortho-C//ortho-C keto dimer forms (o,o')-dihydroxybiphenyl, a known dibenzofuran intermediate, via inter-ring hydrogen transfer reaction, the less stable R,S-stereoisomer can easily be transformed into another 5-hydroxyl-4,5-cyclohexadiene-2,3-benzofuran intermediate that provides an energetically more favorable pathway for formation of dibenzofuran. The possible channels of radical-chain processes that convert these intermediates to dibenzofuran and polychlorinated dibenzofurans are discussed.