Bonding and Mobility of Alkali Metals in Helicenes

Abstract

In this work, we analyze the interactions of alkali metal cations with [6]- and [14]helicene and the cation mobility of therein. We found that the distortion of the carbon skeleton is the reason that some of the structures which are local minima for the smallest cations are not energetically stable for K⁺ , Rb⁺ , and Cs⁺ . Also, the most favorable complexes are those where the cation is interacting with two rings forming a metallocene-like structure, except for the largest cation Cs⁺ , where the distortion provoked by the size of the cation destabilizes the complex. As far as mobility is concerned, the smallest cations, particularly Na⁺ , are the ones that can move most efficiently. In [6]helicene, the mobility is limited by the capture of the cation forming the metallocene-like structure. In larger helicenes, the energy barriers for the cation to move are similar both inside and outside the helix. However, complexes with the cation between two layers are more energetically favored so that the movement will be preferred in that region. The bonding analysis reveals that interactions with no less than 50 % of orbital contribution are taking place for the series of E⁺ -[6]helicene. Particularly, the complexes of Li⁺ show remarkable orbital character (72.5-81.6 %).

Keywords: alkali metals; bonding; helicenes; mobility; non-bonding interactions.