Mixed-ligand Pt(II) dithione-dithiolato complexes: influence of the dicyanobenzodithiolato ligand on the second-order NLO properties

The mixed-ligand dithiolene complex [Pt(Bz 2pipdt)(dcbdt)] (1) bearing the two ligands Bz 2pipdt = 1,4-dibenzyl-piperazine-3,2- dithione and dcbdt = dicyanobenzodithiolato, has been synthesized, characterized and studied to evaluate its second-order optical nonlinearity. The dithione/dithiolato character of the two ligands gives rise to an asymmetric distribution of the charge in the molecule. This is reflected by structural data showing that in the C 2S 2PtS 2C 2 dithiolene core the four sulfur atoms define a square-planar coordination environment of the metal where the Pt-S bond distances involving the two ligands are similar, while the C-S bond distances in the C 2S 2 units exhibit a significant difference in Bz 2pipdt (dithione) and dcbdt (dithiolato). 1 shows a moderately strong absorption peak in the visible region, which can be related to a HOMO-LUMO transition, where the dcbdt ligand (dithiolato) contributes mostly to the HOMO, and the Bz 2pipdt one (dithione) mostly to the LUMO. Thus this transition has ligand-to-ligand charge transfer (CT) character with some contribution of the metal and undergoes negative solvatochromism and molecular quadratic optical nonlinearity (\u3bc\u3b2 0 = -1296 710 -48 esu), which was determined by the EFISH (electric-field-induced second-harmonic generation) technique and compared with the values of similar complexes on varying the dithiolato ligand (mnt = maleonitriledithiolato, dmit = 2-thioxo-1,3-dithiole-4, 5-dithiolato). Theoretical calculations help to elucidate the role of the dithiolato ligands in affecting the molecular quadratic optical nonlinearity of these complexes