Theoretical studies on oxidation-switchable second-order nonlinear optical responses of Metallosalen-Keggin polyoxometalate derivatives

Abstract

To explore the effect of one-electron oxidation on nonlinear optical (NLO) responses, the frontier molecular orbitals, second-order NLO responses and electronic transition properties of three metallosalen–polyoxometalate compounds {SiW₁₁O₃₉[O(SiR)₂M]}⁴⁻ (R = (CH₂)₃N–CH(2-OPh); M = Pd, Ni and Co) (M-salen–POM) and their one-electron oxidized species were systematically investigated by density functional theory (DFT). Fukui function analysis shows that the M-salen is the oxidized center in M-salen–POM compounds. In addition, four functionals have been taken to confirm the switching behavior of the NLO properties by the one-electron oxidation. The β_tot value of oxo-Pd-salen–POM is 98 times larger than that of Pd-salen–POM, indicating that the first hyperpolarizability is enhanced by one-electron oxidation. Time dependent (TD) DFT calculations show that the charge transfer from POM to M-salen, and intramolecular charge transfer within M-salen in oxidized species are beneficial to reduce the transition energy and improve the static first hyperpolarizabilities. The M-salen–POM may be a potential class of switchable nonlinear optical materials by one-electron oxidized processes.