Orbital Engineering Directed Electronic Structure Studies: Design of Pseudo 𝜋� Aromatic Molecules, Borophenes and Borophites

Abstract

The many ways of using three-membered rings as a design element in molecules and borophenes (2D sheets of boron) will be presented. The underlying design principles require constructing and manipulating molecular orbitals and band structure with chemical intuition using "Orbital Engineering" [1]. The out-of-plane (π) delocalization in an aromatic system is not confined to the overlap of standard p-, d-, and f- orbitals. We present an alternative by choosing σ* Fragment Molecular Orbitals (FMO) with π symmetry, named pseudo π* FMOs [2,3]. The all-bonding combination of these in cyclic systems creates pseudo π* aromaticity. Therefore, we utilize our orbital engineering principles to design stable pseudo π* aromatic systems, starting from the smallest aromatic ring, cyclopropenium cation (C3H3+). We stabilize a three-membered Si2B ring involving two pseudo π* FMOs based on Si and one pure p- orbital based on B, which is synthesized by the Roesky group [3]. We extend this concept to the saturated three-, four-, five- and six-membered rings of Silicon involving only pseudo π* FMOs. Polycondensation of all-boron analogs of C3H3+ such as B3H5 and B3H6+ leads to borophene sheets with varying hole density, with the three-membered B3 ring as a continuum. Based on this, we present a generalized electron counting rule for borophenes with any hole density [4]. Knowing the electronic requirement, we propose various ways to stabilize both electron-deficient and rich borophenes. In electron-deficient borophenes, such as β12 sheet with a hole density of 1/6, we suggest hydrogenation and metal doping as the ways of electron donation, detaching the 2D layer from the metal surface on which theses are generated [4,5]. On the other hand, for electron-rich borophenes, the excess electrons are localized by creating interlayer bonds between two layers [6]. Interlayer bonding can be used as a design element to construct multilayers of borophenes: the number and nature of which change based on hole density on the borophene sheets and the number of layers [7]. We also examine the potential existence of layered boron materials: borophites, similar to graphite, where van der Waals forces stack borophene layers [8,9]. However, unlike graphite, in borophites, the constituent layers can be a monolayer, a bilayer, or even a multilayer. Quantitative studies on these will be presented.