Design of nickel donor-acceptor dithiolenes for 2nd order nonlinear optics: an experimental and computational study

The structures and properties, including second-order non-linear optical properties in solution and in polymethylmethacrylate (PMMA) films, of new nickel heteroleptic donor-acceptor dithiolene complexes are reported here ([Ni(Bn(2)pipdt)L1-4], 1-4). The acceptor dithione system (Bn(2)pipdt = 1,4-dibenzylpiperazine-2,3-dithione) is invariant, whereas the donor dithiolate moiety varies in 1 to 4 (i-mnt = 2,2-dicyanoethylene-1,1-dithiolate) (L1), sqdt = 1,2-dithiosquarate (L2), ddmedt = 1,2-dicarbomethoxyethylene dithiolate (L3), and dddmedt = 5,6-bis(methoxycarbonyl)-1,4-dithiine-2,3-bis(thiolate) (L4). Molecular structural characterization of 1, 3 and 4 points out that the metal center exhibits a square-planar geometry bound by the dithione ligand Bz(2)pipdt, and by three different dithiolate-type ligands. Complexes 1-4 are characterized in the visible region by a peak of moderately strong intensity, which undergoes negative solvatochromism. The molecular quadratic optical nonlinearities were determined in DMF solution by the electric field induced second harmonic generation technique, working with a 1907 nm incident wavelength. The mu beta(1907) values (-680 (1), -855 (2), -1080 (3); and -1180 (4)) (10(-48) esu) were measured, showing that 1-4 exhibit negative second-order polarizabilities whose values depend on the donor ligand. While 1-4 exhibit similar thermolytic behavior, when embedded in a PMMA poled matrix, only 3 maintains second-order NLO properties in the solid state. A good NLO response with a value of the three nonzero coefficients of the second-order susceptibility tensor chi(2)33, chi(2)31, and chi(2)15 = 2.20 +/- 0.44 pm V-1, 0.56 +/- 0.12 pm V-1 and 0.42 +/- 0.08 pm V-1, respectively, was found. According to computational studies, the mu beta(1907) sequence is mainly related to the donor capability of the dithiolate ligands, which affects the HOMO-LUMO energy gap (2.05 (1), 1.80 (2), 1.56 (3), and 1.36 (4) eV), and it can thus modulate the position of the low-energy absorption and related properties in the series.