Electric-field-induced layer-by-layer fabrication of stable second-order nonlinear optical films

BACKGROUND: Second-order nonlinear optical (NLO) films have been made using electric field poling polymer and Langmuir-Blodgett techniques with non-centrosymmetric structures that exhibit relatively high values of nonlinear susceptibility (chi(2)), but the shortcomings of insufficient temporal or mechanical stability have restricted their potential applications. In this study, electric-field-induced layer-by-layer assembly was investigated as an effective technique to prepare low molecular weight chromophoric (LMWC) molecules of high degree of self-ordering and density in NLO films. RESULTS: A new and stable LMWC molecule, 2-({4-[4-(2-carboxy-2-cyanovinyl)-Z-phenylazol-phenyl}-methylamino)-ethyl acid (DRCB), was first designed and synthesized successfully. The chromophore possesses two negative groups, one at each end, and still retains molecular polarity after ionization. DRCB was successfully assembled with polycationic diazoresin using the electric-field-induced layer-by-layer assembly method to construct stable organic second-order NLO multilayer films. Upon UV irradiation, the interaction between multilayers was converted from an electrostatic interaction to covalent bonds. CONCLUSION: Due to the DC electric field effect in the assembly process, in addition to introducing the stable chromophore molecule and the covalent crosslinking structure in the films, the second-order NLO films fabricated using the method described have large second harmonic generation response, good thermal stability and excellent chemical stability, which offer potential advantages for device applications. (C) 2009 Society of Chemical Industry