Solid Free Radicals Research Articles

Air-stable organic radicals in solid state from a triphenylamine derivative by UV irradiation

Spin-delocalization and steric protection are commonly used and effective strategies to realize stable organic radicals. A bisheptamido-functionalized triarylamines derivative(T1) can enhance the stability of TPA photo-generated radical cations in air by supramolecular self-assembly and “push-pull” dipole structure to expand conjugation, so as to obtain air-stable solid free radicals with a lifetime of more than 60 days, which is based on the best record of the air-stabilized solid TPA radical cations. The preliminary studies demonstrate that T1 may be a promising candidate for organic semiconductor radical materials.

End-On Azido-Bridged Copper Dimers: Spin Population Analysis and Spin Polarization Effect as Exhibited by Valence-Bond/Broken Symmetry, Density Functional Methods

Within the general context of homometallic spin-coupled copper(II) dimers, we define the quantity ΔP2(Cu), the difference of copper squared spin populations as calculated for the high-spin (i.e., triplet) and broken symmetry spin states. In the specific case of an azido-bridged copper(II) dimer, the antiferromagnetic part of the exchange coupling constant is then shown, using density functional (DF), valence bond−broken symmetry (VB−BS) techniques, to be proportional to this quantity. This provides a quantifier of the exchange phenomenon alternative to that usually used, that is, Δ2, squared of the singly occupied molecular orbital splitting in the triplet state. Moreover, spin polarization, through the spin population being delocalized from one copper ion onto the other one, offers the possibility of changing the sign of ΔP2(Cu), thus resulting in a ferromagnetic contribution, for weak magnetic orbital overlap, here found at the VB ground-state level. Phenomenologically, this last effect can be formulated in terms of McConnell's mechanism I for ferromagnetic interaction in solid free radicals (McConnell, H. M. J. Chem. Phys. 1963, 39, 1910). We finally show that the standard copper basis sets commonly used for inorganic chemistry computations may be deficient. This leads to exaggerated spin delocalization and to bad agreement between DF computed (Mulliken) spin populations and those recently measured by polarized neutron experiments (Aebersold, M. A. et al.: J. Am. Chem. Soc. 1998, 120, 5238).

Nuclear relaxation and Overhauser effect in a nearly-one-dimensional Heisenberg system

The spin dynamics in a nearly-one-dimensional Heisenberg system---the solid free radical Tanol (2,2,6,6-tetramethyl-4-piperidinol-1-oxyl)---is studied through dynamical-nuclear-polarization (DNP) experiments (Overhauser effect) and proton spin-lattice relaxation-time (${T}_{1}$) measurements. As the couplings between the electronic spins and the protons have been determined in Tanol, absolute determinations of the values of the electronic-spin-frequency correlation functions at the electronic Larmor frequency (${\ensuremath{\omega}}_{e}$) and at the nuclear Larmor frequency (${\ensuremath{\omega}}_{N}$) can be achieved by performing both DNP and ${T}_{1}$ measurements. The value obtained at ${\ensuremath{\omega}}_{e}$ is in agreement with theoretical calculations in one-dimensional Heisenberg systems using the method of Tahir-Kheli and McFadden. The value obtained at ${\ensuremath{\omega}}_{N}$ together with the frequency dependence of ${T}_{1}$ are explained by introducing a cutoff effect in the spin-diffusion process in one dimension. A derivation is given which expresses the magnitude of the cutoff effect in terms of the interchain couplings. An evaluation of the interchain couplings, using ${T}_{1}$ measurements, is presented. The values of the interchain couplings, which are consistent with both the ${T}_{1}$ measurements and the N\'eel temperature, are ${J}_{1}\ensuremath{\simeq}{J}_{2}\ensuremath{\simeq}{10}^{\ensuremath{-}2}J$, where $J$ is the intrachain exchange ($\frac{J}{k=4.1}$ K), and ${J}_{1}$ and ${J}_{2}$ are the interchain couplings along two axis perpendicular to the chain. However, it cannot be excluded that, in Tanol, the disturbance of the spin-diffusion process is due to finite-length effects.

Formation of Solid Free Radicals by Mechanical Action

THERE is a marked reduction of the molecular weight of nitrocellulose on pulping, for the length of fibres is considerably reduced in the process1,2. Staudinger et al.3 have found a similar effect when polystyrene, cellulose and nitrocellulose are ground in a colloid ball-mill, and have also found that nitrocellulose is partially denitrated when grinding is prolonged. These observations indicate that purely mechanical cutting or breaking action can cleave covalerit bonds.

Semiconductivity of organic substances. Part 11.—Electrical properties of substituted allyl radicals

The electrical properties of the stable solid free radicals of the type α,γ bisdiphenylene-β-R-phenyl allyl radicals have been examined. They behave as semiconductors with energy gaps in the range 1.5–1.7 eV whether as single crystals, films or compressed powders. The radicals also exhibit photoconduction. In common with other free radicals, the photocurrent in surface cells follows the electronic absorption spectrum, except for the lowest energy band where the photocurrent is extremely small.

Spin Excitations in Ionic Molecular Crystals

Most organic molecular crystals can be classified rather sharply into one of two groups, ionic or nonionic. Most strongly paramagnetic molecular crystals-the free radicals-also fall into one of these two groups. Although solid free radicals were at one time characterized as being prob­ ably only worth a footnote (1), this can certainly no longer be said to be true of the ionic free radicals, which have been found to exhibit magnetic and electrical properties as remarkable as any known in inorganic solid state physics, except possibly for superconductors. The present review gives a brief account of the paramagnetic properties of ionic molecular crystals. In the opinion of the authors, some of these properties are now understood quite well from a theoretical point of view. It is to be hoped that our discussion below will stimulate further definitive experimental and theoretical work in this area. An earlier review (2) covers many of the same subjects, but from a more qualitative point of view. In the present review, we proceed, roughly, from the well-understood and the well-documented to the tentatively understood and the insufficiently documented. In Section I, the magnetic properties of ionic free radicals are reviewed briefly, and the salient features of the paramagnetic resonance spectra of triplet excitons are presented. In Section II, the purely electronic properties of triplet excitons are reviewed. In Section III, the effects of exciton-phonon interactions are discussed. These two sections demonstrate that triplet exciton theory leads to a detailed qualitative-and in some aspects even semiquantitative--understanding of a class of ionic free radicals. In the last two sections, we consider the role of spin excitations in two areas that are considerably less tractable. The first area considered, in Section IV, is that of phase transitions in ionic molecular crystals. The second, in Section V, is the nature of charge-transfer molecular 1 The literature review for this chapter was completed during September 1965. 2 This review has benefited greatly by support from the National Science Foun­ dation (GP 3430), the Atomic Energy Commission (Contract AT(04-3)-326 PA No. 11], and by facilities provided by the Advanced Research Projects Agency through the Center for Materials Research at Stanford University. 3 Nato Postdoctoral Fellow, 1964-65. Present address: Instituto di Chimica Fisica. Via Loredan 4, Padova, Italy. • N.S.F. Postdoctoral Fellow, 1965-66. present address: Department of Chemis­ try, Princeton University, Princeton, New Jersey.

Pseudospin Approach to Linear Antiferromagnetic Chains

The generalized linear antiferromagnetic chain with two spins per unit cell $J>0$, and $a, b, d, e>~0$, ${\mathcal{H}}_{G}=\ensuremath{\Sigma}\stackrel{\frac{N}{2}}{j=1}J{a{{S}_{2j}}^{z}{{S}_{2j+1}}^{z}+b({{S}_{2j}}^{x}{{S}_{2j+1}}^{x}+{{S}_{2j}}^{y}{{S}_{2j+1}}^{y})+d{{S}_{2j}}^{z}{{S}_{2j\ensuremath{-}1}}^{z}+e({{S}_{2j}}^{x}{{S}_{2j\ensuremath{-}1}}^{x}+{{S}_{2j}}^{y}{{S}_{2j\ensuremath{-}1}}^{y})},$ is solved in the pseudospin approximation. The pseudospin method is compared with the exactly soluble special case of the Heisenberg-Ising antiferromagnet ($a=b=1+\ensuremath{\delta}$, $d=1\ensuremath{-}\ensuremath{\delta}$, $e=0$) and good agreement is obtained even for a sensitive property like the long-range order. It is suggested that the pseudospin approach provides a single, systematic approximation scheme, not only for describing the magnetic properties of a class of solid free radicals, but also as a check for widely used many-body techniques.

Semiconductivity of organic substances. Part 10.—Electrical properties of galvinoxyl, α,α-diphenyl-β-picryl hydrazyl and related solid free radicals

The semiconductivity and photoconductivity of the solid free radicals galvinoxyl (2,6-di-tert-butyl-α(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene) p-tolyloxy radical) and DPPH (α,α-diphenyl-β-picryl hydrazyl) and related compounds have been examined. Their behaviour is similar to that of diamagnetic poly-cyclic hydrocarbons. They show semi-conducting properties with energy gaps in the range 1.4–1.6 eV whether as single crystals, films or compressed powders. In sandwich cells the photocurrent follows the absorption spectrum, except for the lowest energy band where no significant photocurrent is observed.

Separation and Investigation of a Stable Solid Free Radical of Chlorpromazine

A solid stable free radical of chlorpromazine was prepared and characterized. The free radical nature of this compound was verified by electron spin resonance and ultraviolet spectral studies. The molecular weight and melting point of the free radical were determined. A solid stable free radical of chlorpromazine was prepared and characterized. The free radical nature of this compound was verified by electron spin resonance and ultraviolet spectral studies. The molecular weight and melting point of the free radical were determined.

Phonon-Coupled Interactions between Paramagnetic Excitons

It is shown that exciton—phonon coupling in solid free radicals leads to a spin-independent repulsion between paramagnetic excitons. This repulsion may make a significant contribution to the activation energy for spin-exchange line broadening that is observed in the paramagnetic resonance of a number of solid free radicals.

Ferromagnetism in Solid Free Radicals

Ferromagnetism in Solid Free Radicals

Theory of Paramagnetic Excitons in Solid Free Radicals

A linear chain of S=½ molecules with alternating intermolecular exchange integrals is used as a model to investigate the properties of paramagnetic excitons in solid free radicals from a theoretical point of view. The spin exchange Hamiltonian for the problem is H=Σν(JS2ν·S2ν+1+J′S2ν·S2ν−1),when J>J′>0. In the limiting case that J≫J′, the exciton bandwidth is simply J′. For this case we have calculated (a) the fine-structure splitting of the paramagnetic resonance of isolated (noninteracting) excitons, (b) wave functions for a free exciton gas of arbitrary excitation density, (c) the magnetic susceptibility of the exciton gas, and (d) first-order exchange interactions between excitations. In the case that thermal energies are much larger than the exciton bandwidth, exchange interactions between excitations are simply proportional to the excitation concentration, but at temperatures corresponding to thermal energies less than the bandwidth, there is a ``Fermi hole'' type repulsion between the excitons that reduces the exchange interaction. In fact, the excitons behave somewhat like Fermi particles, except their number is not constant (depending on the temperature), and only one exciton may occupy a given momentum state, irrespective of spin. Also, when the exciton bandwidth is comparable to or larger than kT, it is predicted that the exciton fine-structure splitting will be temperature dependent. When J′ becomes comparable to J, then the elementary exciton takes on a complicated, distributed spin structure.

Paramagnetic Excitons in Solid Free Radicals

It is suggested that the low-temperature paramagnetism of many aromatic free-radical solids may be due to triplet excitons, and that the formation of these exciton states rather than other low-temperature magnetic states is due to low-temperature crystal lattice structures that favor a singlet pairing of electrons on neighboring molecules. As an example it is shown that the paramagnetic resonance spectra of certain TCNQ charge-transfer complexes observed by Chesnut and Phillips may be interpreted in terms of triplet excitons.

Organic Solids and Heterogeneous Catalysis. Electron Transfer in αα‐Diphenyl‐β‐picryl‐hydrazyl

AbstractThe unpaired electrons of the solid free radical αα'‐diphenyl‐β‐picryl‐hydrazyl (D.P.P.H.) convert parahydrogen at low temperatures. The mechanism is the paramagnetic physical mechanism, as absence of the hydrogen‐deuterium exchange reaction rules out dissociative chemisorption of hydrogen molecules by the free valencies. This confirms earlier work. In extension of this, the mobility of the free radical electrons in the solid are determined by A.C. and D.C. conductance measurements. Hydrogen, nitrogen and oxygen do not effect this conductance and are therefore probably not chemisorbed. Hydrogen in the presence of a palladium film lowers the A.C. conductance of a D.P.P.H. film indicating covalency formation with the H atom, a partial electron transfer H ← D.P.P.H. Hydrogen sulphide raises the D.C. conductance of D.P.P.H. powder, suggesting a lowering of intercrystalline barriers by an electron transfer H2S → D.P.P.H. A film of D.P.P.H. deposited on a film of Al or Pd raises the D.C. resistance of the latter, suggesting electron transfer (Al or Pd) ← D.P.P.H.

Solid Free Radical as Catalyst for Ortho—Para Hydrogen Conversion1a

Solid Free Radical as Catalyst for Ortho—Para Hydrogen Conversion^1a