Structure and Property Correlations in Heavy Atom Radical Conductors

Abstract

The synthesis and solid-state characterization of the resonance-stabilized heterocyclic thia/selenazyl radicals 1a-4a is described. While all the radicals crystallize in undimerized slipped pi-stacked arrays, the four crystal structures do not constitute an isomorphous set; crystals of 1a and 3a belong to the orthorhombic space group P2(1)2(1)2(1), while those of 2a and 4a belong to the monoclinic space group P2(1)/n. The origin of the structural dichotomy can be traced back to the packing of the radicals in the P2(1)/n structure, which maximizes intermolecular Se-Se' contacts. There are marked differences in the transport properties of the two groups. Variable temperature conductivity measurements reveal high, but activated, conductivity for the monoclinic pair (2a/4a), with sigma(298 K) > 10(-3) S cm(-1). The application of physical pressure increases the conductivity of both compounds, with sigma(298 K) at 5 GPa reaching 0.5 S cm(-1) for 2a and 2 S cm(-1) for 4a. Variable-temperature magnetic susceptibility measurements indicate strong antiferromagnetic (AFM) coupling for the monoclinic pair 2a and 4a, the behavior of which has been modeled in terms of a molecular-field modified 1D Heisenberg chain of AFM coupled S = 1/2 centers. Extended Huckel theory band structure calculations and density functional theory first principles methods have been used to develop a qualitative understanding of the conductive and magnetic properties of radicals of the type 1-4 as a function of the degree and direction of slippage of the radical pi-stacks.

Magnetic Ordering and Anisotropy in Heavy Atom Radicals

“…31 Results of this approach are illustrated in Figure 5, which shows the variations in calculated J π for sterically unhindered model radicals 8D (R 1 /R 2 = H), as a function of translation along x and y (computed at the (U)B3LYP/ 6-31G(d,p) level). 21 The resulting two-dimensional (2D) energy surface shows regions of both FM (green) and AFM (blue) exchange, which may be related qualitatively to the expected pairwise overlap integral and hence t ij 00 . All four radicals 8A−D produce qualitatively similar profiles, although the magnitude of J π (be it +ve or −ve) is enhanced with increasing selenium content.…”

Hydrogen bond-promoted metallic state in a purely organic single-component conductor under pressure

“…In general, the neutral-radical system falls into the Mott insulating states, due to the intrinsic half-filled nature of the valence band consisting of the SOMO. Nevertheless, the relatively high electrical conductivity at room-temperature, s n B10 À 6 -10 À 1 S cm À 1 , has been observed in several types of neutral-radical-based single-component molecular conductors [19][20][21][22][23][24][25][26][27][28][29][30][31][32] . So far, among the purely organic single-component systems, the metallic behaviour has been exclusively achieved under high physical pressure for some neutral-radical conductors 32,33 .…”

Spin-orbit effects in heavy-atom organic radical ferromagnets

“…8,9 The other mixed S/Se system 3 also orders, although as a spin-canted antiferromagnet, with T N = 14 K. 7 The increase in ordering temperature with Se incorporation can be partly understood in terms of an enhancement of the isotropic exchange interactions occasioned by more diffuse magnetic orbitals. 10 The increase in H c has been associated with an enhancement of anisotropic exchange interactions resulting from spin-orbit effects. 11 In order to explore the role of spin-orbit effects in this series, we present a theoretical framework for understanding the magnitude and symmetry of the anisotropic exchange terms.…”