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

The second-order nonlinear optical (NLO) properties of porphyrin-metal-polyoxometalate (por-metal-POM) sandwich structures [(por)M(PW(11)O(39))](5-) (por = TPP, TPyP, TPPF(20), M = Hf; por = TPP, M = Zr) and [(TPP)Hf(XW(11)O(36))](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, approximately 100 x 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 beta(0) values increase with the auxiliary electron-accepting group on the porphyrin ring (TPPF(20) > 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.