Second-Order Nonlinear Optical Response of Electron Donor–Acceptor Hybrids Formed between Corannulene and Metallofullerenes

Abstract

A charge transfer (CT) complex was formed between C₂₀H₁₀ and Li⁺@C₆₀ in the ground state by the concave–convex π–π CT interaction. Herein, the structures, binding interactions, electronic absorption spectra, and first hyperpolarizabilities of a series of Li⁺ and Li atom in contact with C₆₀ have been explored with density functional theory methods. It is found that independent of the doping position, doping Li atom can significantly narrow the wide gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) (<i>E</i>_gap = 2.82 eV) of the pure C₂₀H₁₀/C₆₀ in the range of 0.86–0.99 eV. Among them, the Li-doped outer isomer C₂₀H₁₀/LiC₆₀ can exhibit the intriguing n-type characteristic, where a high energy level containing the excess electron is introduced as the new HOMO orbital in the original gap of C₂₀H₁₀/C₆₀. Furthermore, the diffuse excess electron also brings C₂₀H₁₀/C₆₀ the considerable first hyperpolarizability, which is 3.53 × 10^–29 esu. When Li⁺ and Li were encapsulated into the C₆₀ cage, inner complexes C₂₀H₁₀/Li⁺@C₆₀ and C₂₀H₁₀/Li@C₆₀ of enhanced static first hyperpolarizabilities (5.39 and 2.13 × 10^–29 esu, respectively) are also delivered due to that encapsulated Li⁺@C₆₀ and Li@C₆₀ show enhanced electron acceptability as compared to pristine C₆₀, leading to more obvious intermolecular CT transitions. From a certain point of view, such systems can be considered as high-performance NLO materials that combine the basic characteristics of a classical donor–acceptor superstructure and systems with cations and easily polarizable excess electrons