Design and preparation of zwitterionic organic thin films: self-assembled siloxane-based, thiophene-spaced N-benzylpyridinium dicyanomethanides as nonlinear optical materials

Regioselective alkylation of 1-(4-pyridyl)-2-[5-(dicyanomethanide)thien-2-y l] ethylene sodium salt (2) by a covalently surface-bound benzyl halide (4) produces a polar-ordered zwitterionic thin film (5). Chemisorption of 2 on the coupling layer 4 was monitored by U-V-vis spectroscopy, contact-angle measurements, X-ray photoelectron spectroscopy, and second harmonic generat ion (SHG). Relative to carbanion 2, the film UV-vis spectrum exhibits a red -shifted (Delta lambda similar to 170 nm) charge-transfer band, consistent with quaternization of the pyridyl group of 2 by 4. The nonlinear optical p roperties of the blue films of 5 were measured by SHG. The average molecula r tilt angle of the chromophore was determined to be similar to 46 degrees, and chi ((2))(zzz) similar to 5.0 x 10(-8) esu (similar to 20 pm/V) was es timated at a fundamental wavelength of 1064 nm.