A lithium cycloparaphenylene as an emerging second-order non-linear optical molecular switch

Non-linear optical (NLO) materials are essential in laser technology and optical information science, and finding high-quality NLO materials is a prominent topic in materials science. Here, we theoretically investigate the doping of a lithium atom into [10]cycloparaphenylene ([10]CPP) with unique π structure. Two stable configurations, Li-[10]CPP and [10]CPP-Li, are obtained. Both configurations exhibit significantly improved second-order NLO properties than [10]CPP alone, with high first hyperpolarizabilities. Notably, [10]CPP-Li displays a greater first hyperpolarizability than Li-[10]CPP. Interestingly, the energy of Li-[10]CPP is lower in the gas phase than [10]CPP-Li, while the opposite is observed in water. This indicates that Li[10]CPP exhibits promising potential as a water-triggered second-order NLO molecular switch. There is one-electron transfer and non-covalent cation-π interaction between Li and [10]CPP in both Li-[10]CPP and [10]CPP-Li. Moreover, Li-[10]CPP and [10]CPP-Li show favorable thermodynamic and dynamic stabilities, making them ideal for experimental synthesis and potential applications in organic optical devices.