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

Abstract

The mixed-ligand dithiolene complex [Pt(Bz₂pipdt)(dcbdt)] (1) bearing the two ligands Bz₂pipdt = 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 C₂S₂PtS₂C₂ 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 C₂S₂ units exhibit a significant difference in Bz₂pipdt (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 Bz₂pipdt 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 (μβ₀ = −1296 ×10⁻⁴⁸ 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.