Pi and sigma double conjugations in boronyl polyboroene nanoribbons: B(n)(BO)2- and B(n)(BO)2 (n = 5-12)

A series of boron dioxide clusters, B(x)O2(-) (x = 7-14), have been produced and investigated using photoelectron spectroscopy and quantum chemical calculations. The dioxide clusters are shown to possess elongated ladder-like structures with two terminal boronyl (BO) groups, forming an extensive series of boron nanoribbons, B(n)(BO)2(-) (n = 5-12). The electron affinities of B(n)(BO)2 exhibit a 4n periodicity, indicating that the rhombic B4 unit is the fundamental building block in the nanoribbons. Both π and σ conjugations are found to be important in the unique bonding patterns of the boron nanoribbons. The π conjugation in these clusters is analogous to the polyenes (aka polyboroenes), while the σ conjugation plays an equally important role in rendering the stability of the nanoribbons. The concept of σ conjugation established here has no analogues in hydrocarbons. Calculations suggest the viability of even larger boronyl polyboroenes, B16(BO)2 and B20(BO)2, extending the boron nanoribbons to ~1.5 nm in length or possibly even longer. The nanoribbons form a new class of nanowires and may serve as precursors for a variety of boron nanostructures.