20-State Molecular Switch in a Li@C60 Complex

Abstract

A substantial potential advantage of industrial electric and thermoelectric devices utilizing endohedral metallofullerenes (EMFs) is their ability to accommodate metallic moieties inside their empty cavities. Experimental and theoretical studies have elucidated the merit of this extraordinary feature with respect to developing electrical conductance and thermopower. Published research studies have demonstrated multiple state molecular switches initiated with 4, 6, and 14 distinguished switching states. Through comprehensive theoretical investigations involving electronic structure and electric transport, we report 20 molecular switching states that can be statistically recognized employing the endohedral fullerene Li@C₆₀ complex. We propose a switching technique that counts on the location of the alkali metal that encapsulates inside a fullerene cage. The 20 switching states correspond to the 20 hexagonal rings that the Li cation energetically prefers to reside close to. We demonstrate that the multiswitching feature of such molecular complexes can be controlled by taking advantage of the off-center displacement and charge transfer from the alkali metal to the C₆₀ cage. The most energetically favorable optimization suggests 1.2-1.4 Å off-center displacement, and Mulliken, Hirshfeld, and Voronoi simulations articulate that the charge migrates from the Li cation to C₆₀ fullerene; however, the amount of the charge transferred depends on the nature and location of the cation within the complex. We believe that the proposed work suggests a relevant step toward the practical application of molecular switches in organic materials.

Figure 1

Schematic illustration of the Li@C₆₀ complex. Li cation and C₆₀ fullerene cage, which possesses 20 hexagonal and 12 pentagonal rings, black and red numbers (for clarity, few numbers are shown).

Figure 2

(a) Schematic illustration representing the optimum position of the Li cation inside the C₆₀ cage. (b) Deconstructed C₆₀ fullerene cage, which shows the 20 optimum positions of Li (hexagonal rings accommodate Li).

Figure 3

Zero-bias transmission coefficient T(E) of [Li⁺@C₆₀]PF₆^– complex as a function of energy. Twenty orange curves represent the 20 most energetically favorable orientations shown in Table S1. The HOMO resonance is predicted to be pinned near the DFT-predicted Fermi energy; however, the Fermi energy (E_F) is taken in the vicinity of the mid-gap (E – E_F^DFT ∼ mid-gap).

Figure 4

Electrical conductance ratio of [Li⁺@C₆₀]PF₆^– complex as a function of switching states, including a comparison between theory and experiment (adapted with permission from ref. 15. Copyright 2019 Nature Communications, 2022). STM experiment comprises 14 switched states (purple circles), while DFT theory involves 20 states (orange stars), which correspond to the 20 most energetically favorable orientations shown in Table S1. Theoretical values are obtained at E – E_F^DFT ∼ mid-gap.