A quantum mechanical study of the second-order nonlinear optical properties of aryldiazenido-substituted hexamolybdates: a surprising charge transfer

The second-order polarizabilities, transition moments and density of states of aryldiazenido hexamolybdates derivatives were investigated by density functional theory (DFT). System 2 [Mo₆O₁₈(N ₂C₆H₅)]^3- has a considerably large second-order polarizability, 14.50 × 10^-30 esu and it is larger than that of system 1 [Mo₆O₁₈(N₂C ₆H₄NO₂)]^3- due to the absence of nitro group in the aryldiazenido ligand. The aryldiazenido ligand acts as an electron donor and the polyanion acts as an electron acceptor. The substitution of an amino (-NH₂) group in the ortho/para positions on the aryldiazenido segment leads to a substantially higher nonlinear optical (NLO) response. The introduction of an electron donor (-NH₂) in the Ortho, meta, para and ortho/para positions on the aryldiazenido ligand significantly enhances the secondorder polarizabilities of aryldiazenido hexamolybdates in comparison to the electron acceptor (-NO₂) as in system 1, because the electron-donating ability was reasonably enhanced when the electron donor is attached to the aryldiazenido ligand. Furthermore, orbital analysis shows that incorporation of another phenyl (aromatic) ring in the aryldiazenido ligand leads to a maximum NLO response by reverting the direction and degree of charge transfer (CT), which might result from the C=C π-conjugated bridge. System 8 [Mo₆O₁₈ (N₂C₄H₁₁)] ^3- possesses a strikingly large and conspicuous static second-order polarizability (β_veo) computed to be 210.21×10 ^-30 esu. The NLO response can be tuned by subtle changes in the aryldiazenido segment; the present investigation provides important insight into the NLO properties of (aryldiazenido) hexamolybdate derivatives.