Coordination of diazines such as quinoxaline to transition metals stabilizes radical anions generated by chemical or electrochemical cathodic reduction. However, even though various sorts of radical anionic diazines have been subjected to spectroscopic investigations in the recent past, reports combining structural, solid-state electron paramagnetic resonance (EPR) and computational investigations of kinetically stable species are still missing. In this study, four radical anions derived from tricarbonylmanganese- and tricarbonylrhenium-bound quinoxaline chelates, embedded within a triple-decker architecture, have been prepared from neutral substrates by chemical reduction over alkaline metals (K, Rb); the electronic structure of the latter metalloorganic paramagnetic salts was investigated by the means of structural X-ray diffraction analysis, electrochemistry, solution and crystal EPR spectroscopy, and density functional theory (DFT). Unprecedented structures of three manganese-bound and one rhenium-bound quinoxaline-derived paramagnetic salts were obtained from solutions of the corresponding radical anions crystallized in the presence of cryptand 222. It is inferred from a comparative study of the structures of anionic and neutral quinoxaline complexes that reduction does not have any significant impact over the coordination mode of the metal centers and over the overall geometry of the triple-decker architecture. The most notable changes in the radical-anionic metalloorganic species, as compared to the neutral parent molecules, comprise a slight hapticity shift of the metal-bound benzyl moiety and a weak intraannular distortion of the quinoxalyl core. Single-crystal EPR experiments carried out with the rhenium and manganese compounds produced the respective anisotropic g tensor, which was found in each case to be essentially located at the quinoxalyl fragment. Computations, carried out using DFT methods (B3LYP-LANL2DZ and Becke-Perdew-TZP), corroborated the features suggested by structural analysis. Single-point calculation using the B3LYP functional and various basis sets [LANL2DZ, 6-31G(d), 6-311+G(d), and 6-311+G(2d,p)] provided us with values of anisotropic g tensors and hyperfine coupling constants consistent with those determined experimentally. It is inferred from this study that the two metal centers bound to the nitrogen atoms of the quinoxalyl core contribute in the lowering of the HOMO-LUMO gap in the neutral species. The triple-decker arrangement, which combines chelation of the metal, steric protection, and encapsulation of the central quinoxalyl core, is a stabilizing factor that provides a long-lived character to the radical-anionic species.

We have investigated the doping mechanism of pentacene with iodine and its impact on the structure and on the electronic properties of single crystals, powders, and thin films in a large range of iodine concentration up to six iodine per pentacene (PEN) molecule (I/PEN = 6). Three regimes of doping have been identified. In the low doping regime I/PEN < 0.05, the pristine pentacene structure of single crystals is maintained. Electron spin resonance (ESR) evidences a Pauli susceptibility, that is, the characteristic fingerprint of delocalized holes in the valence band of pentacene. In the intermediate doping regime (0.1 < I/PEN less than or equal to 2.0), iodine diffuses between the (a,b) planes of the pentacene structure and forms an intercalate. Charge transfer between iodine and pentacene is witnessed by both UV-vis and IR signatures of PEN+ cations and related species, for example, cation dimers (PEN+)(2) and typical Raman signatures of the I-3(-) and I-5(-) species. Spin pairing of pentacene cation radicals is further supported by the observation of a thermally activated behavior of the ESR spin susceptibility. In the heavy doping regime (2 < I/PEN less than or equal to 6), all traces of structural order vanish, indicating that iodine penetrates within the (a,b) planes of the intercalate in a disordered manner, forming an amorphous-like material. This high degree of disorder results in increased charge localization. Most spin/charge species are ESR-silent and only a limited fraction (a few percents) exhibits a Curie-like susceptibility. Because of disorder, the macroscopic conductivity of doped pentacene single crystals does not exceed a few S/cm at 300 K.

The magnetic properties of a series of stable diradicals based on imino nitroxide (IN) and nitronyl nitroxide (NN) are investigated. Both radical fragments are coupled through various diamagnetic conjugated organic spacers made of aromatic rings (4) alternating with either a triple (1x, 3) or a double bond (2x), where x indicates the length of the molecules with respect to the repeating unit. The optimised geometry and the corresponding electronic structure of the whole series (x=0,…,10) have been determined by means of semiempirical calculations based on the NDDO approximation within the AM1 parametrisation scheme. The singlet–triplet splitting has then been studied using configuration interaction within different active spaces. It turns out that the most important parameters are: (a) of geometrical nature (e.g. twist angles between the radical moieties and the planar conjugated spacer); and (b) of topological nature (e.g. the radical substitution at ortho, meta, and para positions of phenyl cycles). The results of the calculations enable us to predict some rules in relation to the experimental observations: (a) for a given value of x, the hierarchy of the exchange coupling within series of derivatives is such as J(R1=R2=NN)>J(R1=IN, R2=NN)>J(R1=R2=IN); (b) whatever the nature of the radical substituent, the singlet is the ground state and the exchange coupling decreases nearly exponentially with the length of the spacer, i.e. as x increases. The exchange coupling is still efficient up to x=8 for R1=R2=IN (experimentally it has been previously found efficient up to x=5 in the para position).

The difference in ability of a quartet molecule to interact with the neighboring solvent molecules is influenced by the molecular surface characteristics of the two diastereomers of the molecule. Therefore, molecular parameters like the surface area and the fractal dimension (see figure) control some of their physicochemical properties, such as their differential chromatographic retention of these two diastereomers, their isomerization equilibrium, and tumbling processes in solid or viscous amorphous matrices.

The properties of a series of stable diradicals based either on iminonitroxide (IN) or nitronylnitroxide (NN) are described. Both radical fragments are coupled by various phenylene and phenylethynylene (triple bond) bridges. They are characterized by EPR studies both in solution and in glassy state. Magnetic coupling is evidenced at a very large distance between both radical fragments (up to 36 Angstrom). The intramolecular coupling is found to be antiferromagnetic in all cases and in particular for meta- or ortho-substituted phenylene. Semiempirical calculations using configuration interaction show that the linkers with triple bonds induce a stronger conjugated character than the corresponding ones involving a double bond, in agreement with the experimental optical spectra, and that the relevant geometrical parameters are the twist angles; the corresponding barriers of rotation are then evaluated. Calculations show that, for some values of the twist angle, an antiferromagnetic state may be stabilized.

A phenyl diethynyl bridge is used as a magnetic coupling unit (MCU) between two iminonitroxide pi-radicals. The ethynyl moiety is attached to the central phenyl ring either in the para or in the meta position. The magnetic properties of the resulting bis-imino nitroxide diradicals (p-BIN and m-BIN) are investigated both at the molecular scale and in the condensed phases. An intramolecular through bond spin coupling is observed at an unusual long distance (about 20 Angstrom) in both derivatives. However, the ground state is found to be a singlet spin state in both compounds. The results suggest an interplay between the spin polarization and the spin delocalization (pi-conjugation). An intermolecular weak antiferromagnetic spin coupling is occuring in both compounds.

Hemibenzopinacolate radicals may be incorporated in a polymer chain and the latter used as a macromolecular initiator. Kinetics of polymerization are strongly dependent on the substitution of the pinacolate. A large family of substituted (CH3)(3)SiOC(C6H5)(2) radicals has been synthesized and studied by electron spin resonance (ESR) and semiempirical molecular orbital calculations. ESR spectra are well resolved and, except for one derivative, allow an unambiguous determination of the hyperfine coupling splitting (HFS) constants. The ground state properties have been calculated within different approximations, namely MNDO, AM1 and PM3 for the geometry optimization and INDO for spin density. Geometries obtained within AM1 and PM3 approximations are satisfactory. The HFS constants evaluated using McConnell type relations and spin densities are in a quite good agreement with the experimental ones. It turns out that the effect of substitution on kinetics of polymerization cannot be attributed to any drastic change in the charge distribution.

2-Nitrotriptycene is 4 times as efficient as nitrobenzene for second harmonic generation in nonlinear optics. Molecular orbital calculations have been carried out to rationalize this result. A correlation has been established between the characteristics of the optical absorption peaks, the electron distribution of the frontier molecular orbitals, and the magnitude of the hyperpolarizability coefficients beta. The enhancement of the beta-value seems to be due to a large electron transfer (0.9 e(-)) in the interaromatic ring charge transfer. The difference in dipole moment between the excited state and the ground state is consequently unusually large (17 D). The rather small corresponding extinction coefficients, however, lead to a beta-value of about 10 +/- 3 x 10(-30) esu.

Three possible methods of synthesis of the VINPSI2-TCNQ2 1:2 salt have been compared. Single crystals show two columnar stackings which differ in their orientation (114-degrees). TCNQ molecules of both stacks have different ionicity (1/3e- and 2/3e-) linked to the presence of hydrogen bonding between the cations on one type of TCNQ. Intrinsic magnetic and electrical properties of both chains are governed by the opening in the band structure of a narrow gap owing to diads formation in both stacks. The highly charged one is semiconducting (gap = 70 meV) whereas the other one is probably metallic. The typical quasi-one-dimensional magnetic behaviour of both stacks demonstrates that the chains are uncoupled in the temperature range studied.

The low temperature specific heat and the magnetic properties of the stoïchiometric TV, T2V and T3V alloys (T=Ni, Pd, Pt) have been investigated in both the ordered and the disordered states. All alloys are paramagnetic, except the ordered Pt3V alloy. For the non magnetic alloys, a systematic decrease of the electronic specific heat coefficient has been observed when going from the disordered state to the ordered state. This general trend is discussed in the light of recent band structure calculations.

The ordered ground states of a series of binary fcc transition metal alloys have been investigated by a perturbation method which takes account of both diagonal and off-diagonal disorder.

We have studied the stoichiometric compounds of the V-Pt system. The heat capacities change appreciably between the ordered and disordered phase (VPt2 and VPt3) and between two ordered phases with different crystalline structure (VPt). VPt and VPt2 are always paramagnetic whereas VPt3 is ferromagnetic in the ordered phase.

Semiempirical RHF and UHF PPP calculations have been performed on three molecules involved in charge transfer systems. The same set of parameters has been used for the three ionic states of each molecule. Particular attention has been given to the choice of both number and type of configurations taken into account in the Cl treatment following the RHF SCF calculation; it is shown that they have a great influence on the results. Calculations carried out for several experimental molecular geometries showed that their influence is not always negligible. Experimental data have been collected in a critical way. The unique set of parameters gives satisfactory results for the optical spectra of the neutral molecules and monoions but gives a poor agreement for diions. Spin densities appeared to be very sensitive to both the methods of calculations and the variation of parameters.