Computational Study of Superalkali-Driven Enhancement of Nonlinear Optical Activity in Carbon Nanohoops

Abstract

Nonlinear optical (NLO) materials have garnered considerable attention across various fields due to their versatile applications. In this work, the potential of Li₃O, Na₃O, and K₃O complexed [6]cycloparaphenylene ([6]CPP) and methylene-bridged [6]cycloparaphenylene (MB[6]CPP) nanohoops as promising NLO candidates has been systematically explored using density functional theory. The geometric, electronic, linear, and nonlinear optical properties of these designed complexes have been investigated. All designed nanohoops exhibited excellent thermodynamic stability, with binding energies reaching up to −93.23 kcal/mol. Remarkably, superalkali complexation has significantly reduced the HOMO–LUMO energy gaps (3.21 eV) compared to the pristine systems and primarily due to the formation of HOMO levels, as confirmed by the TDOS and PDOS analyses. The absorption analysis reflects that the superalkali-based CPPs have an absorption maximum in the visible to near-infrared region, 728–1280 nm. Furthermore, the superalkali-complexed nanohoops displayed outstanding NLO responses, with the Li₃O@MB[6]CPP complex showing the highest first hyperpolarizability value of 1.62 × 10⁵ au. Moreover, dynamic first hyperpolarizabilities were also calculated at the four commonly used wavelengths to further validate the results. These findings highlight the potential of superalkali-complexed [6]CPP nanohoops as advanced NLO materials with possible applications across various cutting-edge photonic and optoelectronic technologies.