Study of the conformational structure and cluster formation in a Langmuir-Blodgett film using second harmonic generation, second harmonic microscopy,and FTIR spectroscopy

Nonlinear second harmonic generation (SHG), second harmonic microscopy (SHM ), and infrared spectroscopy are used to determine the structural and optic al properties of the Langmuir-Blodgett (LB) monolayer assemblies of NLO-act ive, 4-eicosyloxo-(E)-stilbazolium iodide (4-EOSI) on a glass substrate. Th e packing characteristics of the pretransferred interfacial films are deter mined using pi-A isotherm measurements. The molecular coverage of the trans ferred films is determined by ellipsometry. The films formed on both sides of the glass substrate show substantial second harmonic (SH) conversion fro m p-polarized 1064 nm fundamental excitation. The SHG and FTIR measurements imply that the single LB layer on glass is composed of oriented clusters o f 4-EOSI molecules that are laterally discontinuous. Ordered clusters up to 40 mu m in diameter are directly observed using SHM. Subsequent LB transfe rs using the same 4-EOSI molecule or an amphiphile of comparable chain-leng th (eicosanoic acid) fill in the unoccupied vacancies in the first layer. T he magnitude of the dominant element of the nonlinear susceptibility and th e average molecular orientation angle of the chromophore are determined by modeling the characteristic SHG Maker fringes.