Second-Order Nonlinear Optical Properties of Transition-Metal-Trisubstituted Polyoxometalate−Diphosphate Complexes: A Donor−Conjugated Bridge−Acceptor Paradigm for Totally Inorganic Nonlinear Optical Materials

Abstract

To date, the most widely used second-order nonlinear optical (NLO) materials are the totally inorganic crystals. However, the small photoelectric coefficients of inorganic NLO materials are the bottleneck in practical applications. The donor−conjugated bridge−acceptor (D−A) model, which is successfully used in the development of organic second-order NLO materials, is still prohibitive in totally inorganic molecules. In the present paper, time-dependent density functional (TDDFT) has been employed to investigate the second-order NLO properties of a series of transition-metal-trisubstituted polyoxometalates (POMs)−diphosphate clusters. We find that these totally inorganic POM clusters possess D−A structure, and the large static first hyperpolarizability can be effectively designed based on this D−A model. The results show that the substituted transition metal centers can be viewed as electron acceptor, and the POM cluster serves as both electron donor and conjugated bridge. The three vanadium atoms derivative of 30-molybdobipyrophosphate POM cluster displays large static first hyperpolarizability by ∼700 × 10−30 esu, and it is ∼70 times as large as that of typical organic NLO molecule p-nitroaniline (PNA) according to LB94/TZP calculations. Thus, this POM cluster seems to be promising totally inorganic materials for application in nonlinear optics.