Prediction of robustly large molecular second-order nonlinear optical properties of terpyridine-substituted hexamolybdates: Structural modelling towards a rational entry to NLO materials

We have explored an innovative, versatile, and novel molecular hybrid containing polyoxometalate (POM) cluster linked with terpyridine ligand via π-bridged donor–acceptor (D–A) configuration. The dipole polarizabilities, density of states, and second-order nonlinear optical (NLO) properties of terpyridine-substituted hexamolybdates have been investigated by using time-dependent density functional response theory (TDDFT). This class of organic–inorganic hybrid compounds possesses a robustly large molecular second-order NLO response, especially [Mo6O18(N4C25H16I2)]2− (system 5) and [Mo6O17(N4C25H16(CN)2)(N4C25H16(CN)2)]2− (system 10) with the static second-order polarizability (βvec) computed to be 1209.25 × 10−30 esu and 1622.67 × 10−30 esu respectively. Thus, these systems have the possibility to be excellent second-order nonlinear optical materials. Analysis of the major contributions to the βvec value suggests that the charge transfer (CT) from POM-cluster to terpyridine ligand (D–A) along the z-axis plays the key role in the NLO response, POM-cluster (hexamolybdates) acts as a donor (D) whereas terpyridine ligand acts as an acceptor (A) in all the studied systems. The computed βvec values increase by the incorporation of electron acceptors (halogen = F, Cl, Br and I) at the terminus of terpyridine ligand. Furthermore, substitution of trifluoromethoxy (–OCF3), trifluoromethyl (–CF3), and cyanide (–CN) at the end of terpyridine ligand respectively enhances the optical nonlinearity. Orbital analysis shows that the degree of CT between POM and terpyridine segment was increased in 2D and organometallic/POM hybrid systems. The present investigation provides important and thought provoking insight into the robustly large NLO properties of terpyridine-substituted hexamolybdates.