Superhalogen doping: a new and effective approach to design materials with excellent static and dynamic NLO responses

Abstract

Excess electron generation through doping with alkali and superalkali metals is well known to enhance NLO responses. On the contrary, superhalogen doping is an unexplored dimension. Herein, we report the first ever examples where superhalogen doping alone is introduced as a new and effective approach to impart large NLO responses. Density functional theory (DFT) calculations illustrate that superhalogen (BeF₃ and BeCl₃)-doped cyclic oligofurans (nCF) possess exceptionally high NLO responses (first hyperpolarizability (β₀), hyper-Rayleigh scattering coefficient (β_HRS), electro-optical Pockels effect (EOPE), second harmonic generation (SHG), and nonlinear refractive index (n₂)), which are not trivial for organic compounds. Upon doping with superhalogens, the first hyperpolarizability (β₀) of nCF increases to 3 × 10⁵ a.u. in the BeF₃@6CF complex, whereas the β₀ values of the BeF₃@5CF, BeCl₃@5CF and BeCl₃@6CF complexes are 6 × 10⁴, 3 × 10⁴ and 4 × 10⁴ a.u., respectively. An enormously large third order nonlinear optical response coefficient with an electric field-induced second harmonic generation (ESHG) value of 2.1 × 10⁹ a.u. is observed for the BeCl₃@6CF complex. The remarkable NLO responses of the superhalogen-doped cyclic oligofuran complexes are due to the electron withdrawing nature of the halogen atoms, which are responsible for withdrawing electrons from the oxygen atoms of nCF to create poles. The significant hyperpolarizability (β₀) of the BeF₃@6CF complex is due to the most electronegative nature of fluorine. Furthermore, these results are rationalized through a two-level model. B_vec values are calculated for these complexes because they give more meaningful numbers from an experimental point of view. The stability of the complexes is judged through interaction energies, whereas electronic properties are calculated by chemical reactivity descriptors, the HOMO–LUMO gaps (E_g) and NBO charge transfer analysis. TD-DFT calculations reveal that the maximum absorbance for the BeF₃@6CF complex is shifted to the longest wavelength.