Orientation-induced molecular rectification and nonlinear optical properties of a squaraine derivative

Self- assembled monolayers (SAMs) obtained via the chemisorption of 2,4-bis-[N-methyl-N-(3-acetylsulfanylpropyl)-aminophenyl]-squaraine on gold are disordered with the molecules adopting vertical alignments via a single C3H6-S-Au anchoring group and horizontal alignments via two C3H6-S-Au groups located at opposite ends of the donor-acceptor-donor chromophore. The films exhibit asymmetric current-voltage characteristics with a rectification ratio of ca. 6 at +/- 1 V, this unexpected behaviour being induced by different molecular orientations at the interfaces with the electrodes: the donor groups are adjacent both top and bottom whereas the long axis of the D-A-D moiety is only in contact with the gold-coated substrate. The higher forward bias current invariably relates to electron flow from the substrate electrode adjacent to the horizontally aligned chromophores to the contact adjacent to the terminal donors of the vertically aligned molecules. In contrast, Langmuir-Blodgett (LB) films of the squaraine exhibit symmetrical I-V curves and aggregation-induced second-harmonic generation (SHG) with an effective second-order susceptibility of ca. 80 pm V-1 at 1.064 mum.