Band Structure Engineering by Substitutional Doping in Solid-State Solutions of [5-Me-PLY(O,O)]_2B_((1-x))Be_x Radical Crystals

摘要

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, P1-bar, Z = 2; [2]_2Be, P2_1/c, Z = 4) with distinct packing arrangements, yet we are able to isolate crystals of composition [2]_2B_((1-x))Be_x, where x = 0-0.59. The phase transition from the P1-bar to the P2_1/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 Hiickel 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.