Prediction of second-order optical nonlinearity of porphyrin–metal–polyoxometalate sandwich compounds

Abstract

The second-order nonlinear optical (NLO) properties of porphyrin–metal–polyoxometalate (por–metal–POM) sandwich structures [(por)M(PW₁₁O₃₉)]⁵⁻ (por = TPP, TPyP, TPPF₂₀, M = Hf; por = TPP, M = Zr) and [(TPP)Hf(XW₁₁O₃₆)]⁶⁻ (X = Si, Ge) are investigated by time-dependent density functional theory (TDDFT). The character of charge-transfer transition indicates that the porphyrin ligand acts as an electron acceptor and the lacunary Keggin-type POM acts as an electron donor. Our results show that this kind of organic–inorganic hybrid compound possesses remarkably large molecular second-order NLO polarizability, ∼100 × 10⁻³⁰ esu, and might be an excellent second-order NLO material. Furthermore, the NLO response can be tuned by the element substituents. The computed β₀ values increase with the auxiliary electron-accepting group on the porphyrin ring (TPPF₂₀ > TPyP > TPP) and a heavy central heteroatom (Ge > Si > P). The present investigation provides important insight into the NLO properties of this class of por–metal–POM sandwich compound.