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-yl]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 UV−vis spectroscopy, contact-angle measurements, X-ray photoelectron spectroscopy, and second harmonic generation (SHG). Relative to carbanion 2, the film UV−vis spectrum exhibits a red-shifted (Δ λ ∼ 170 nm) charge-transfer band, consistent with quaternization of the pyridyl group of 2 by 4 . The nonlinear optical properties of the blue films of 5 were measured by SHG. The average molecular tilt angle of the chromophore was determined to be ∼46°, and ∼ 5.0 × 10 - 8 esu (∼20 pm/V) was estimated at a fundamental wavelength of 1064 nm.