Second-order nonlinear optical properties of trisubstituted Keggin and Wells-Dawson polyoxometalates: density functional theory investigation of the inorganic donor-conjugated bridge-acceptor structure.

The donor-conjugated bridge-acceptor (D-A) model, as a simple molecular scheme, has been successfully used in the development of second-order organic compound, organometallic compound, and metal complex nonlinear optical (NLO) materials. However, for the totally inorganic molecules, the use of this model is still prohibitive. In the present paper, time-dependent density functional theory (TDDFT) was used to investigate the second-order NLO properties of vanadium- and molybdenum-trisubstituted Keggin and Wells-Dawson polyoxometalates (POMs). The results show that these POM clusters possess D-A structures. The oxygen atoms in the cap region and metal (vanadium and molybdenum) atoms in another cap region in these POM clusters can be viewed as the electron donor and acceptor, respectively. The vanadium ion derivatives possess larger second-order NLO responses and dipole moment than molybdenum ions derivatives; thus, the three vanadium atoms in the cap region act as a strong acceptor related to the three molybdenum atoms in cap region in our D-A scheme. The vanadomolybdate with Wells-Dawson structure displays the good second-order NLO response because of the relevant long conjugated bridge and strong acceptor. This D-A model may be an effective approach for optimizing the first hyperpolarizabilities of inorganic POM clusters.