Theoretical insights into double-headed superalkalide ion pairs, Kδ−[AE+@HMHC]Kδ− (AE = Ca and Sr), as stable and high-performance single-pole double-throw mid-far-infrared NLO molecular switches

Abstract

Designing a novel multi-response molecular switch may meet the requirements of higher miniaturization and higher efficiency for next-generation electronic devices. After the doping of two alkalis and an alkaline earth metal atom into hexaazacyclooctadecane (HMHC), our DFT study revealed that the formed double-headed superalkalis of K[AE@HMHC]K (AE = Ca and Sr) exhibited alkalide-alkaline earth-alkalide ion pair interaction with salt-like structures of K^δ−[AE⁺@HMHC]K^δ−. Furthermore, K[AE@HMHC]K can be considered high-performance candidates for single-pole double-throw (SPDT) mid-far-infrared (MF-IR) nonlinear optical (NLO) molecular switches with high stability, owing to the following three features: (1) all isomers (1, 2, and 3) occupy long IR transparent regions (λ > 1800 nm), (2) from the uniform double-headed superalkalide of 1 to top-heavy ones of 2 or 3, the increase in first hyperpolarizability shows 1–2 orders of magnitude enhancement, and (3) application of a suitable external electric field or slight heating can cause them to turn on (2 or 3) or off (1).