P-Carborane Conjugation in Radical Anions of Cage-Cage and Cage-Phenyl Compounds

Optical electron transfer (intervalence) transitions in radical anions of p-carborane oligomers attest to delocalization of electrons between two p-carboranes cages or a p-carborane and a phenyl ring. Oligomers of the 12 vertex p-carborane (C₂B₁₀H₁₂) cage, [12], with up to 3 cages were synthesized, as well as p-carboranes with one or two trimethylsilylphenyl groups, [6], attached to the carbon termini. Pulse radiolysis in tetrahydrofuran produced radical anions, determined redox potentials by equilibria and measured their absorption spectra. Density functional theory computations provided critical insight into the optical electron transfer bands and electron delocalization. One case, [6-12-6], showed both Robin-Day class II and III transitions. The class III transition resulted from a fully delocalized excess electron across both benzene rings and the central p-carborane, with an electronic coupling H_ab = 0.46 eV between the cage and either benzene. This unprecedented finding shows that p-carborane bridges are not simply electron withdrawing insulators. In other cases with more than ∼¹/₂ of the excess electron localized on a [12], large cage distortions were triggered, producing a partially open cage with a nido-like structure. This resulted in class II transitions with similar H_ab but massive reorganization energies. The computations also predicted delocalization in radical cations, but complexities in cation formation allowed only tentative experimental support of the predictions. The results with anions provide clear evidence for carborane conjugation that might be exploited in molecular wire materials, which are classically composed of all π-conjugated molecules.