Electric-field-induced layer-by-layer fabrication of second-order nonlinear optical films with high thermal stability

A promising methodology for fabricating highly thermally stable organic second-order nonlinear optical (NLO) thin films containing low molecular weight chromophoric (LMWC) molecules was sustained by electric-field-induced layer-by-layer assembly. The new LMWC molecule but-2-enedioic acid mono-[2-({4-[4-(2-carboxy-2-cyano-vinyl)-phenylazo]-phenyl}-methyl-amino)-ethyl] ester (BCPE) was first designed and synthesized successfully, which possesses two negative groups at both ends and still retains the molecular polarity after ionization. Under a do electric field, the density of the oriented chromophores BCPE deposited on the substrate is increased largely, and upon UV irradiation, accompanied with the transition of the ionic bonds between the layers to covalent bonds, the resulting NLO films possess excellent environmental stability and high thermal stability (the resulting films can retain 92% of the polar order up to 200 C), which may rival the properties of the latest poled polymer films for electro-optic devices.