Band Structure Engineering by Substitutional Doping in Solid-State Solutions of [5-Me-PLY(O,O)]₂B_{(1–<i>x</i>)}Be_<i>x</i> Radical Crystals

We report the substitutional doping of solid-state spiro-bis­(5-methyl-1,9-oxido-phenalenyl)­boron radical ([2]2B) by co-crystallization of this radical with the corresponding spiro-bis­(5-methyl-1,9-oxido-phenalenyl)­beryllium compound ([2]2Be). The pure compounds crystallize in different space groups ([2]2B, P1̅, Z = 2; [2]2Be, P21/c, Z = 4) with distinct packing arrangements, yet we are able to isolate crystals of composition [2]2B(1–x)Bex, where x = 0–0.59. The phase transition from the P1̅ to the P21/c space group occurs at x = 0.1, but the conductivities of the solid solutions are enhanced and the activation energies reduced for values of x = 0–0.25. The molecular packing is driven by the relative concentration of the spin-bearing ([2]2B) and spin-free ([2]2Be) molecules in the crystals, and the extended Hückel theory band structures show that the progressive incorporation of spin-free [2]2Be in the lattice of the [2]2B radical (overall bandwidth, W = 1.4 eV, in the pure compound) leads to very strong narrowing of the bandwidth, which reaches a minimum at [2]2Be (W = 0.3 eV). The results provide a graphic picture of the structural transformations undergone by the lattice, and at certain compositions we are able to identify distinct structures for the [2]2B and [2]2Be molecules in a single crystalline phase.