Nonlinear optical switching in lithiated cyclo[6]carbon: external-electric-field controlled dominance and spectral characteristics

Abstract

Precise modulation of nonlinear optical (NLO) properties in functional materials is crucial for advancing photonic and optoelectronic technologies. Herein, we theoretically designed two stable monolithiated-C₆ configurations (Li@C₆ complexes) with distinct Li positions on the cyclo[6]carbon ring. These configurations exhibit facile room-temperature interconversion due to low free energy barriers (2.37-7.22 kcal/mol) and high-rate constants, with reversible dominance under external electric fields (EEFs). Electronic structure, absorption spectra, and NLO analyses demonstrate that Li positioning governs optical behavior. Li@C₆^OUT displays enhanced absorption intensity, broader spectral range, and stronger Li-directed charge-transfer characteristics compared to Li@C₆^TOPIN. Remarkably, EEF application amplifies first- and second-order hyperpolarizabilities by factors of 16.5/20.4 (Li@C₆^OUT) and 12.4/15.5 (Li@C₆^TOPIN), respectively, while polarizability remains stable. Electron density difference, hole-electron distributions, and hyperpolarizability density analyses corroborate these trends. The pronounced EEF-responsive electronic and NLO variations in Li@C₆ complexes highlight their promise as optically switchable molecules, offering a design strategy for tunable molecular photonic devices.

Keywords: Absorption spectra; Charge-transfer spectrum (CTS); Monolithiated-C(6) complexes; Nonlinear optical (NLO) response; Optical molecular switch.