Structure and Property Correlations in Heavy Atom Radical Conductors

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 π-stacked arrays, the four crystal structures do not constitute an isomorphous set; crystals of 1a and 3a belong to the orthorhombic space group P212121, while those of 2a and 4a belong to the monoclinic space group P21/n. The origin of the structural dichotomy can be traced back to the packing of the radicals in the P21/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 σ(298 K) > 10−3 S cm−1. The application of physical pressure increases the conductivity of both compounds, with σ(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 Hückel 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 π-stacks.